Experimental studies of black holes: status and future prospects

Author(s)

Genzel, Reinhard, Eisenhauer, Frank, Gillessen, Stefan

Abstract

More than a century ago, Albert Einstein presented his general theory of gravitation (GR) to the Prussian Academy of Sciences. One of the predictions of the theory is that not only particles and objects with mass, but also the quanta of light, photons, are tied to the curvature of space-time, and thus to gravity. There must be a critical compactness, above which photons cannot escape. These are black holes (henceforth BH). It took fifty years after the theory was announced before possible candidate objects were identified by observational astronomy. And another fifty years have passed, until we finally have in hand detailed and credible experimental evidence that BHs of 10 to 10^10 times the mass of the Sun exist in the Universe. Three very different experimental techniques, but all based on Michelson interferometry or Fourier-inversion spatial interferometry have enabled the critical experimental breakthroughs. It has now become possible to investigate the space-time structure in the vicinity of the event horizons of BHs. We briefly summarize these interferometric techniques, and discuss the spectacular recent improvements achieved with all three techniques. Finally, we sketch where the path of exploration and inquiry may go on in the next decades.

Figures

Relationship between inferred central mass (from stellar and gas dynamics) $M_{\bullet}$ and bulge mass in local Universe massive galaxies (ellipticals, or disks with classical bulges). There clearly is a correlation between these two components, with a best fit $\frac{M_{\bullet}}{M_{\rm bulge}}=(4.9 \pm 0.6)\times 10^{-3}\times \left( \frac{M_{\rm bulge}}{10^{11}\,M_{\odot}}\right)^{0.14 \pm 0.08}$ (adapted from Fig.~18 of \citealt{kormendy2013}).

Relationship between inferred central mass (from stellar and gas dynamics) $M_{\bullet}$ and bulge mass in local Universe massive galaxies (ellipticals, or disks with classical bulges). There clearly is a correlation between these two components, with a best fit $\frac{M_{\bullet}}{M_{\rm bulge}}=(4.9 \pm 0.6)\times 10^{-3}\times \left( \frac{M_{\rm bulge}}{10^{11}\,M_{\odot}}\right)^{0.14 \pm 0.08}$ (adapted from Fig.~18 of \citealt{kormendy2013}).


Summary of the different components: stars (old giants, red and blue super-giants), cold (20--200 K) molecular/neutral gas and dust, ionized ($10^{4}$ K) and hot (1--$10\times10^{6}$ K) gas, and their distributions on sub-parsec, to 10 parsec scale in the Galactic Center (adapted from \citealt{genzel2010}). The cross in the center of the images marks the location of the compact, non-thermal radio source SgrA*, probably a MBH of 4.3 million solar masses \citep{genzel2010}. \textbf{Top right:} Largest scales of the SgrA region, with the HII region SgrA WEST and the supernova remnant SgrA EAST (presumably an explosion of one or several massive O/Wolf--Rayet star(s) $\sim$20--40,000 years ago. Outside of this region are two giant molecular clouds at `+20' and `+50' km/s LSR velocity. \textbf{Top left and center:} zoom in onto SgrA WEST, which harbors the center of a dense ($\rho_{*} > 10^{6}\, M_{\odot}\, \mathrm{pc}^{-3}$) cluster of old, and young, massive stars. The central 1.5-pc diameter region is filled with ionized gas streamers (bottom left), hot X-ray emitting gas (bottom center), and the most massive, recently formed O, Wolf--Rayet and B-stars (bottom right). Winds and UV-radiation from these stars and the MBH have created a lower density `cavity' relatively devoid of dense molecular gas and dust (average hydrogen density $n_{H}\sim10^{3..4.5}\, \mathrm{cm}^{-3}$). The central cavity in turn is surrounded by a rotating, clumpy `circum-nuclear' ring of warm dust and dense, molecular gas (HCN and other high excitation gas components are found here, and the molecular hydrogen density is $n_{\rm H2}\sim10^{5-6}\, \mathrm{cm}^{-3}$, \citealt{becklin1982, ho1995}). Gas is streaming in and out of the central region in form of clumpy, tidally disrupted `streamers', such as the `northern' and `eastern' arms and the `bar' (cf.\ \citealt{oort1977, lo1983, genzel1987, ho1991, genzel1994, melia2001, genzel2010, morris2012}).

Summary of the different components: stars (old giants, red and blue super-giants), cold (20--200 K) molecular/neutral gas and dust, ionized ($10^{4}$ K) and hot (1--$10\times10^{6}$ K) gas, and their distributions on sub-parsec, to 10 parsec scale in the Galactic Center (adapted from \citealt{genzel2010}). The cross in the center of the images marks the location of the compact, non-thermal radio source SgrA*, probably a MBH of 4.3 million solar masses \citep{genzel2010}. \textbf{Top right:} Largest scales of the SgrA region, with the HII region SgrA WEST and the supernova remnant SgrA EAST (presumably an explosion of one or several massive O/Wolf--Rayet star(s) $\sim$20--40,000 years ago. Outside of this region are two giant molecular clouds at `+20' and `+50' km/s LSR velocity. \textbf{Top left and center:} zoom in onto SgrA WEST, which harbors the center of a dense ($\rho_{*} > 10^{6}\, M_{\odot}\, \mathrm{pc}^{-3}$) cluster of old, and young, massive stars. The central 1.5-pc diameter region is filled with ionized gas streamers (bottom left), hot X-ray emitting gas (bottom center), and the most massive, recently formed O, Wolf--Rayet and B-stars (bottom right). Winds and UV-radiation from these stars and the MBH have created a lower density `cavity' relatively devoid of dense molecular gas and dust (average hydrogen density $n_{H}\sim10^{3..4.5}\, \mathrm{cm}^{-3}$). The central cavity in turn is surrounded by a rotating, clumpy `circum-nuclear' ring of warm dust and dense, molecular gas (HCN and other high excitation gas components are found here, and the molecular hydrogen density is $n_{\rm H2}\sim10^{5-6}\, \mathrm{cm}^{-3}$, \citealt{becklin1982, ho1995}). Gas is streaming in and out of the central region in form of clumpy, tidally disrupted `streamers', such as the `northern' and `eastern' arms and the `bar' (cf.\ \citealt{oort1977, lo1983, genzel1987, ho1991, genzel1994, melia2001, genzel2010, morris2012}).


Tests of the MBH paradigm and GR in the Galactic Center using individual stellar orbits

Tests of the MBH paradigm and GR in the Galactic Center using individual stellar orbits


Evidence for the concentration of the mass of $4.3\times 10^6 M_\odot$ (as determined from stellar orbits) within few gravitational radii. Left: The flaring NIR emission of SgrA* revolves on a time scale compatible with the Keplerian period at the observed radius. The polarization of the NIR emission rotates on the same time scale. Middle: The EHT-image of SgrA* shows a silhouette as expected for an MBH of $4.3\times 10^6 M_\odot$. Right: The SED of SgrA*.

Evidence for the concentration of the mass of $4.3\times 10^6 M_\odot$ (as determined from stellar orbits) within few gravitational radii. Left: The flaring NIR emission of SgrA* revolves on a time scale compatible with the Keplerian period at the observed radius. The polarization of the NIR emission rotates on the same time scale. Middle: The EHT-image of SgrA* shows a silhouette as expected for an MBH of $4.3\times 10^6 M_\odot$. Right: The SED of SgrA*.


\textbf{Top:} Schematic of the structures around a luminous, rapidly accreting extragalactic active galactic nucleus $(\dot{M} > 0.01 \times \dot{M}_{\max, \rm Eddignton})$, with a super-massive $(>10^{8\ldots10}\,M_{\odot})$ BH at its center (e.g. \citealt{osmer2004, netzer2015}). The SMBH is surrounded by a hot accretion disk. Generalizing our current GRAVITY results (Figure 3, \citealt{gravity2018c, gravityplus2024}, and references therein) on its outer side are self-gravitating ionized clouds, the BLR in virial equilibrium and rotating around the SMBH. This central region in turn is surrounded by a dusty molecular region (the ``Torus''), and ionized clouds on 100 pc-10 kpc scale (the narrow-line region, NLR). Image credit: Claudio Ricci. \textbf{Bottom:} GRAVITY spectro-astrometry of the broad P$\alpha$ line in the $z=0.16$ Quasar 3C 273 \citep{schmidt1963}. The left panel shows the observed line profile and the inferred interferometric phase gradient across the line, with a measurement accuracy of about 1$\mu$as (500 AU, or 0.0024 pc at 550 Mpc distance). The 2D spectro-astrometry model (bottom middle panel) shows that this phase gradient extends over 50 micro-arcsec, approximately perpendicular to the direction of the known radio jet (black line). The model yields a MBH mass of $2.6\times10^{8}\, M_{\odot}$, surrounded by a thick rotating gas disk of 0.18 parsec (46$\mu$as) diameter (\citealt{gravity2018c}, cf.\ \citealt{netzer2020}).

\textbf{Top:} Schematic of the structures around a luminous, rapidly accreting extragalactic active galactic nucleus $(\dot{M} > 0.01 \times \dot{M}_{\max, \rm Eddignton})$, with a super-massive $(>10^{8\ldots10}\,M_{\odot})$ BH at its center (e.g. \citealt{osmer2004, netzer2015}). The SMBH is surrounded by a hot accretion disk. Generalizing our current GRAVITY results (Figure 3, \citealt{gravity2018c, gravityplus2024}, and references therein) on its outer side are self-gravitating ionized clouds, the BLR in virial equilibrium and rotating around the SMBH. This central region in turn is surrounded by a dusty molecular region (the ``Torus''), and ionized clouds on 100 pc-10 kpc scale (the narrow-line region, NLR). Image credit: Claudio Ricci. \textbf{Bottom:} GRAVITY spectro-astrometry of the broad P$\alpha$ line in the $z=0.16$ Quasar 3C 273 \citep{schmidt1963}. The left panel shows the observed line profile and the inferred interferometric phase gradient across the line, with a measurement accuracy of about 1$\mu$as (500 AU, or 0.0024 pc at 550 Mpc distance). The 2D spectro-astrometry model (bottom middle panel) shows that this phase gradient extends over 50 micro-arcsec, approximately perpendicular to the direction of the known radio jet (black line). The model yields a MBH mass of $2.6\times10^{8}\, M_{\odot}$, surrounded by a thick rotating gas disk of 0.18 parsec (46$\mu$as) diameter (\citealt{gravity2018c}, cf.\ \citealt{netzer2020}).


\textbf{Top left:} GRAVITY spectro-astrometry of the broad H$\alpha$ line in the $z=2.32$ quasar J0920, with the new capabilities of `GRAVITY-wide' to fringe-track on a bright star up to 25'' from the phase center \citep{gravityplus2024}. The data and modeling of this distant quasar (distance 17,700 Mpc, 2.89 Gyr after the Big Bang) are qualitatively quite similar to those of the 33 times closer 3C 273, with a linear velocity gradient, indicating predominantly rotation in a 0.33 pc diameter (40 micro-arcsec) thick disk rotating around a $3.2\times10^{8}\,M_{\odot}$ central mass (top right and bottom left). In contrast to moderately luminous local AGN, 3C~273 and J0920, both optically very luminous (i.e. high Eddington ratio accretion) MBHs, seem to have a smaller broad-line region, and contain a smaller fraction of the overall baryonic mass of the galaxy than predicted by scaling relations (bottom right).

\textbf{Top left:} GRAVITY spectro-astrometry of the broad H$\alpha$ line in the $z=2.32$ quasar J0920, with the new capabilities of `GRAVITY-wide' to fringe-track on a bright star up to 25'' from the phase center \citep{gravityplus2024}. The data and modeling of this distant quasar (distance 17,700 Mpc, 2.89 Gyr after the Big Bang) are qualitatively quite similar to those of the 33 times closer 3C 273, with a linear velocity gradient, indicating predominantly rotation in a 0.33 pc diameter (40 micro-arcsec) thick disk rotating around a $3.2\times10^{8}\,M_{\odot}$ central mass (top right and bottom left). In contrast to moderately luminous local AGN, 3C~273 and J0920, both optically very luminous (i.e. high Eddington ratio accretion) MBHs, seem to have a smaller broad-line region, and contain a smaller fraction of the overall baryonic mass of the galaxy than predicted by scaling relations (bottom right).


Top: Inspiral, merger/plunge-in and ring-down of a SBH binary \citep{abbott2016a}. Bottom: `Spectroscopy' of a SBH inspiral. Mid and right: The Kerr metric gives a unique relation-ship between mass, and the orbital $l=2$ mode frequency $\omega_{R}$ near the plunge in, and the imaginary frequency $\omega_i$ expressing the decay time of this mode, $t_d \sim1/\omega_i$ \citep{cardoso2019}. Given the short duration of a $2\times30\, M_{\odot}$ inspiral, a single inspiral like GW150914 does not yield enough SNR to determine these frequencies with sufficient accuracy with the current aLIGO sensitivity, and a stacking of about 30 such inspirals would be required (simulation by \citealt{brito2018}). The small yellow star is the true input value injected into the simulation.

Top: Inspiral, merger/plunge-in and ring-down of a SBH binary \citep{abbott2016a}. Bottom: `Spectroscopy' of a SBH inspiral. Mid and right: The Kerr metric gives a unique relation-ship between mass, and the orbital $l=2$ mode frequency $\omega_{R}$ near the plunge in, and the imaginary frequency $\omega_i$ expressing the decay time of this mode, $t_d \sim1/\omega_i$ \citep{cardoso2019}. Given the short duration of a $2\times30\, M_{\odot}$ inspiral, a single inspiral like GW150914 does not yield enough SNR to determine these frequencies with sufficient accuracy with the current aLIGO sensitivity, and a stacking of about 30 such inspirals would be required (simulation by \citealt{brito2018}). The small yellow star is the true input value injected into the simulation.


Data of the first BH-binary inspiral, GW150914, as seen by the Hanford and Livingston antennas of aLIGO, and the derived source properties |\cite{abbott2016a, abbott2016b}.

Data of the first BH-binary inspiral, GW150914, as seen by the Hanford and Livingston antennas of aLIGO, and the derived source properties |\cite{abbott2016a, abbott2016b}.


Current status of the SBH-SBH and SBH-NS inspirals observed by aLIGO and aLIGO+Virgo+KAGRA after GWTC-3 (Fig.~\ref{fig:A1c}, right panel, \citealt{abbott2016a, abbott2016b, abbott2022}). Compilation of the inferred inspiral masses of all events seen at the end of the GWTC-3 run with LIGO, Virgo and KAGRA \cite{abbott2023}. Image courtesy of \url{https://www.ligo.caltech.edu/image/ligo20211107a}.

Current status of the SBH-SBH and SBH-NS inspirals observed by aLIGO and aLIGO+Virgo+KAGRA after GWTC-3 (Fig.~\ref{fig:A1c}, right panel, \citealt{abbott2016a, abbott2016b, abbott2022}). Compilation of the inferred inspiral masses of all events seen at the end of the GWTC-3 run with LIGO, Virgo and KAGRA \cite{abbott2023}. Image courtesy of \url{https://www.ligo.caltech.edu/image/ligo20211107a}.


Constraints on the contribution of various mass components of baryonic and dark fermionic matter (fermion ball) to the central mass density (left) and rotation velocity (right) at different radii $R$ (adapted from \citealt{arguelles2019, arguelles2023}).

Constraints on the contribution of various mass components of baryonic and dark fermionic matter (fermion ball) to the central mass density (left) and rotation velocity (right) at different radii $R$ (adapted from \citealt{arguelles2019, arguelles2023}).


Current status and future improvements in the quality of experimental studies of the BH paradigm. The central table lists the constraints (here in the compactness parameter $\epsilon$, where $\epsilon=0$ is a Kerr BH) achieved so far by the different techniques discussed in the text are in white color, while the expected further improvements in the future are in yellow. Current state of the art sets limits in $\epsilon$ of a few tenths. The very faint stationary near-infrared emission of SgrA* can in principle be interpreted as a strong evidence for the absence of a surface of the source, and thus in favor of an event horizon. However, this argument relies on the emission be isotropic and not strongly affected by gravitational lensing. Detection with GRAVITY+ of the Lense--Thirring precession of a star with $R_{\rm peri}< 10 \,\mu{\rm as}$ would yield a spin determination of the MBH in the Galactic Center, and together with other stars yield a limit of $\epsilon\sim0.1$. Higher quality measurements of the photon-ring ($n\geq1$) in SgrA* with ngEHT, or space VLBI (together with the priors from GRAVITY+ would reach $\epsilon<0.1$. The same limit could be reached with timing of a Galactic Center pulsar within an arcsecond of SgrA*. Still better limits could then come from a combination of all three techniques in the next ten years. Detailed 100 m/s spectroscopy with MICADO@ELT in the 2030s of a late type star in a close peri-approach to SgrA* might achieve $\epsilon\sim0.03$. Finally gravitational wave analysis of an inspiral of a stellar BH into a MBH (EMRI) would reach $\epsilon\sim0.0001$ with LISA data in 2+ decades.

Current status and future improvements in the quality of experimental studies of the BH paradigm. The central table lists the constraints (here in the compactness parameter $\epsilon$, where $\epsilon=0$ is a Kerr BH) achieved so far by the different techniques discussed in the text are in white color, while the expected further improvements in the future are in yellow. Current state of the art sets limits in $\epsilon$ of a few tenths. The very faint stationary near-infrared emission of SgrA* can in principle be interpreted as a strong evidence for the absence of a surface of the source, and thus in favor of an event horizon. However, this argument relies on the emission be isotropic and not strongly affected by gravitational lensing. Detection with GRAVITY+ of the Lense--Thirring precession of a star with $R_{\rm peri}< 10 \,\mu{\rm as}$ would yield a spin determination of the MBH in the Galactic Center, and together with other stars yield a limit of $\epsilon\sim0.1$. Higher quality measurements of the photon-ring ($n\geq1$) in SgrA* with ngEHT, or space VLBI (together with the priors from GRAVITY+ would reach $\epsilon<0.1$. The same limit could be reached with timing of a Galactic Center pulsar within an arcsecond of SgrA*. Still better limits could then come from a combination of all three techniques in the next ten years. Detailed 100 m/s spectroscopy with MICADO@ELT in the 2030s of a late type star in a close peri-approach to SgrA* might achieve $\epsilon\sim0.03$. Finally gravitational wave analysis of an inspiral of a stellar BH into a MBH (EMRI) would reach $\epsilon\sim0.0001$ with LISA data in 2+ decades.


{\bf GRAVITY.} See text. Image credits: ESO/GRAVITY$^{(+)}$ collaborations.

{\bf GRAVITY.} See text. Image credits: ESO/GRAVITY$^{(+)}$ collaborations.


{\bf Event Horizon Telescope.} See text. Image credits: EHT collaboration.

{\bf Event Horizon Telescope.} See text. Image credits: EHT collaboration.


{\bf Ground-based gravitational-wave interferometry.} See text. Image credits: LIGO/Virgo/KAGRA collaborations.

{\bf Ground-based gravitational-wave interferometry.} See text. Image credits: LIGO/Virgo/KAGRA collaborations.


References
  • Abbott BP, Abbott R, Abbott TD, et al (2016a) GW151226: Observation of
  • Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence.
  • Phys. Rev. Lett. 116(24):241103. https://doi.org/10.1103/PhysRevLett.116.241103, arXiv:1606.04855 [gr-qc]
  • Abbott BP, Abbott R, Abbott TD, et al (2016b) Astrophysical Implications of the
  • Binary Black-hole Merger GW150914. ApJ 818(2):L22. https://doi.org/10.3847/
  • 2041-8205/818/2/L22, arXiv:1602.03846 [astro-ph.HE]
  • Abbott R, Abbott TD, Abraham S, et al (2020) Properties and Astrophysical Implications of the 150 M⊙ Binary Black Hole Merger GW190521. ApJ 900(1):L13. https://doi.org/10.3847/2041-8213/aba493, arXiv:2009.01190 [astro-ph.HE]
  • Abbott R, Abbott TD, Acernese F, et al (2022) Search for intermediate-mass black hole binaries in the third observing run of Advanced LIGO and Advanced Virgo.
  • A&A 659:A84. https://doi.org/10.1051/0004-6361/202141452, arXiv:2105.15120
  • [astro-ph.HE]
  • Abbott R, Abbott TD, Acernese F, et al (2023) Population of Merging Compact
  • Binaries Inferred Using Gravitational Waves through GWTC-3. Physical Review
  • X 13(1):011048. https://doi.org/10.1103/PhysRevX.13.011048, arXiv:2111.03634
  • Abramowicz MA, Kluźniak W, Lasota JP (2002) No observational proof of the black-hole event-horizon. A&A 396:L31–L34. https://doi.org/10.1051/0004-6361:
  • 20021645, arXiv:astro-ph/0207270 [astro-ph]
  • Alexander DM, Hickox RC (2012) What drives the growth of black holes? New A Rev.
  • 56(4):93–121. https://doi.org/10.1016/j.newar.2011.11.003, arXiv:1112.1949 [astroph.GA]
  • Alexander T (2005) Stellar processes near the massive black hole in the Galactic center [review article]. Phys. Rep. 419(2-3):65–142. https://doi.org/10.1016/j.physrep.
  • 2005.08.002, arXiv:astro-ph/0508106 [astro-ph]
  • Alexander T (2017) Stellar Dynamics and Stellar Phenomena Near a Massive Black
  • Hole. ARA&A 55(1):17–57. https://doi.org/10.1146/annurev-astro-091916-055306, arXiv:1701.04762 [astro-ph.GA]
  • Almheiri A, Marolf D, Polchinski J, et al (2013) Black holes: complementarity or firewalls? Journal of High Energy Physics 2013:62. https://doi.org/10.1007/
  • JHEP02(2013)062, arXiv:1207.3123 [hep-th]
  • Amaro-Seoane P (2019) Extremely large mass-ratio inspirals. Phys. Rev. D
  • 99(12):123025. https://doi.org/10.1103/PhysRevD.99.123025, arXiv:1903.10871
  • [astro-ph.GA]
  • Amaro-Seoane P, Audley H, Babak S, et al (2017) Laser Interferometer Space
  • Antenna. arXiv e-prints arXiv:1702.00786. https://doi.org/10.48550/arXiv.1702.
  • 00786, arXiv:1702.00786 [astro-ph.IM]
  • Amaro-Seoane P, Andrews J, Arca Sedda M, et al (2023) Astrophysics with the Laser
  • Interferometer Space Antenna. Living Rev Relativ 26:2. https://doi.org/10.1007/ s41114-022-00041-y, arXiv:2203.06016 [gr-qc]
  • Antonini F (2014) On the Distribution of Stellar Remnants around Massive Black
  • Holes: Slow Mass Segregation, Star Cluster Inspirals, and Correlated Orbits. ApJ
  • 794(2):106. https://doi.org/10.1088/0004-637X/794/2/106, arXiv:1402.4865 [astroph.GA]
  • Argüelles CR, Krut A, Rueda JA, et al (2019) Can fermionic dark matter mimic supermassive black holes? Int J Mod Phys D 28(14):1943003. https://doi.org/10.
  • 1142/S021827181943003X, arXiv:1905.09776 [astro-ph.GA]
  • Argüelles CR, Becerra-Vergara EA, Rueda JA, et al (2023) Fermionic Dark Matter:
  • Physics, Astrophysics, and Cosmology. Universe 9(4):197. https://doi.org/10.3390/ universe9040197, arXiv:2304.06329 [astro-ph.GA]
  • Argüelles CR, Rueda JA, Ruffini R (2024) Baryon-induced Collapse of Dark Matter Cores into Supermassive Black Holes. ApJ 961(1):L10. https://doi.org/10.3847/
  • 2041-8213/ad1490, arXiv:2312.07461 [astro-ph.GA]
  • Arkani-Hamed N, Dimopoulos S, Dvali G (1998) The hierarchy problem and new dimensions at a millimeter. Phys Lett B 429(3):263–272. https://doi.org/https:// doi.org/10.1016/S0370-2693(98)00466-3
  • Ayzenberg D, Blackburn L, Brito R, et al (2024) Fundamental Physics Opportunities with the Next-Generation Event Horizon Telescope. Living Rev Relativ 27. arXiv:2312.02130 [astro-ph.HE]
  • Babak S, Gair J, Sesana A, et al (2017) Science with the space-based interferometer
  • LISA. V. Extreme mass-ratio inspirals. Phys. Rev. D 95(10):103012. https://doi. org/10.1103/PhysRevD.95.103012, arXiv:1703.09722 [gr-qc]
  • Baganoff FK, Maeda Y, Morris M, et al (2003) Chandra X-Ray Spectroscopic Imaging of Sagittarius A* and the Central Parsec of the Galaxy. ApJ 591(2):891–915. https:
  • //doi.org/10.1086/375145, arXiv:astro-ph/0102151 [astro-ph]
  • Barausse E, Berti E, Hertog T, et al (2020) Prospects for fundamental physics with
  • LISA. Gen Relativ Gravit 52(8):81. https://doi.org/10.1007/s10714-020-02691-1, arXiv:2001.09793 [gr-qc]
  • Bardeen JM (1973) Timelike and null geodesics in the Kerr metric. In: Black Holes
  • (Les Astres Occlus). Gordon and Breach, New York, pp 215–239
  • Bardeen JM, Press WH, Teukolsky SA (1972) Rotating Black Holes: Locally Nonrotating Frames, Energy Extraction, and Scalar Synchrotron Radiation. ApJ
  • 178:347–370. https://doi.org/10.1086/151796
  • Bardeen JM, Carter B, Hawking SW (1973) The four laws of black hole mechanics.
  • Commun Math Phys 31(2):161–170. https://doi.org/10.1007/BF01645742
  • Becerra-Vergara EA, Argüelles CR, Krut A, et al (2020) Geodesic motion of S2 and
  • G2 as a test of the fermionic dark matter nature of our Galactic core. A&A 641:A34. https://doi.org/10.1051/0004-6361/201935990, arXiv:2007.11478 [astro-ph.GA]
  • Becerra-Vergara EA, Argüelles CR, Krut A, et al (2021) Hinting a dark matter nature of Sgr A* via the S-stars. MNRAS 505(1):L64–L68. https://doi.org/10.1093/ mnrasl/slab051, arXiv:2105.06301 [astro-ph.GA]
  • Becklin EE, Gatley I, Werner MW (1982) Far-infrared observations of Sagittarius
  • A - The luminosity and dust density in the central parsec of the Galaxy. ApJ
  • 258:135–142. https://doi.org/10.1086/160060
  • Bekenstein JD (1975) Statistical black-hole thermodynamics. Phys. Rev. D
  • 12(10):3077–3085. https://doi.org/10.1103/PhysRevD.12.3077
  • Bentz MC, Peterson BM, Netzer H, et al (2009) The Radius-Luminosity Relationship for Active Galactic Nuclei: The Effect of Host-Galaxy Starlight on Luminosity Measurements. II. The Full Sample of Reverberation-Mapped AGNs. ApJ 697(1):160–
  • 181. https://doi.org/10.1088/0004-637X/697/1/160, arXiv:0812.2283 [astro-ph]
  • Bentz MC, Walsh JL, Barth AJ, et al (2010) The Lick AGN Monitoring Project:
  • Reverberation Mapping of Optical Hydrogen and Helium Recombination Lines. ApJ
  • 716(2):993–1011. https://doi.org/10.1088/0004-637X/716/2/993, arXiv:1004.2922
  • [astro-ph.CO]
  • Bentz MC, Denney KD, Grier CJ, et al (2013) The Low-luminosity End of the RadiusLuminosity Relationship for Active Galactic Nuclei. ApJ 767(2):149. https://doi. org/10.1088/0004-637X/767/2/149, arXiv:1303.1742 [astro-ph.CO]
  • Berti E, Yagi K, Yunes N (2018) Extreme gravity tests with gravitational waves from compact binary coalescences: (I) inspiral-merger. Gen Relativ Gravit 50(4):46. https:
  • //doi.org/10.1007/s10714-018-2362-8, arXiv:1801.03208 [gr-qc]
  • Blandford R, Meier D, Readhead A (2019) Relativistic Jets from
  • Active Galactic Nuclei. ARA&A 57:467–509. https://doi.org/10.1146/ annurev-astro-081817-051948, arXiv:1812.06025 [astro-ph.HE]
  • Active Galactic Nuclei. ARA&A 57:467–509. https://doi.org/10.1146/ annurev-astro-081817-051948, arXiv:1812.06025 [astro-ph.HE]
  • Blandford RD (1999) Relativistic Accretion. In: Sellwood JA, Goodman J (eds)
  • Astrophysical Discs - an EC Summer School, ASP Conference Series, vol 160.
  • Astronomical Society of the Pacific, p 265, astro-ph/9902001
  • Blandford RD, Begelman MC (1999) On the fate of gas accreting at a low rate on to a black hole. MNRAS 303(1):L1–L5. https://doi.org/10.1046/j.1365-8711.1999.
  • 02358.x, arXiv:astro-ph/9809083 [astro-ph]
  • Blandford RD, McKee CF (1982) Reverberation mapping of the emission line regions of Seyfert galaxies and quasars. ApJ 255:419–439. https://doi.org/10.1086/159843
  • Blandford RD, Znajek RL (1977) Electromagnetic extraction of energy from Kerr black holes. MNRAS 179:433–456. https://doi.org/10.1093/mnras/179.3.433
  • Boehle A, Ghez AM, Schödel R, et al (2016) An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis. ApJ 830(1):17. https://doi.org/10.3847/
  • 0004-637X/830/1/17, arXiv:1607.05726 [astro-ph.GA]
  • Bousso R (2002) The holographic principle. Reviews of Modern Physics 74(3):825–874. https://doi.org/10.1103/RevModPhys.74.825, arXiv:hep-th/0203101 [hep-th]
  • Bower GC, Broderick A, Dexter J, et al (2018) ALMA Polarimetry of Sgr A*: Probing the Accretion Flow from the Event Horizon to the Bondi Radius. ApJ 868(2):101. https://doi.org/10.3847/1538-4357/aae983, arXiv:1810.07317 [astro-ph.HE]
  • Brito R, Buonanno A, Raymond V (2018) Black-hole spectroscopy by making full use of gravitational-wave modeling. Phys. Rev. D 98(8):084038. https://doi.org/10.
  • 1103/PhysRevD.98.084038, arXiv:1805.00293 [gr-qc]
  • Broderick AE, Loeb A (2006) Imaging optically-thin hotspots near the black hole horizon of Sgr A* at radio and near-infrared wavelengths. MNRAS 367(3):905–916. https://doi.org/10.1111/j.1365-2966.2006.10152.x, arXiv:astro-ph/0509237 [astroph]
  • Broderick AE, Loeb A, Narayan R (2009) The Event Horizon of Sagittarius A*. ApJ 701(2):1357–1366. https://doi.org/10.1088/0004-637X/701/2/1357, arXiv:0903.1105 [astro-ph.HE]
  • Buonanno A, Cook GB, Pretorius F (2007) Inspiral, merger, and ring-down of equalmass black-hole binaries. Phys. Rev. D 75(12):124018. https://doi.org/10.1103/
  • PhysRevD.75.124018, arXiv:gr-qc/0610122 [gr-qc]
  • Carballo-Rubio R, Filippo FD, Liberati S, et al (2023) Constraints on thermalizing surfaces from infrared observations of supermassive black holes. J. Cosmology
  • Astropart. Phys. 2023(11):041. https://doi.org/10.1088/1475-7516/2023/11/041, arXiv:2306.17480 [astro-ph.HE]
  • Cardoso V, Pani P (2019) Testing the nature of dark compact objects: a status report. Living Rev Relativ 22:4. https://doi.org/10.1007/s41114-019-0020-4, arXiv:1904.05363 [gr-qc]
  • Carr BJ (1975) The primordial black hole mass spectrum. ApJ 201:1–19. https://doi. org/10.1086/153853
  • Carr BJ, Hawking SW (1974) Black holes in the early Universe. MNRAS 168:399–416. https://doi.org/10.1093/mnras/168.2.399
  • Carter B (1971) Axisymmetric Black Hole Has Only Two Degrees of Freedom.
  • Phys. Rev. Lett. 26(6):331–333. https://doi.org/10.1103/PhysRevLett.26.331
  • Christodoulou D (1970) Reversible and irreversible transformations in black-hole physics. Phys Rev Lett 25:1596–1597. https://doi.org/10.1103/PhysRevLett.25.
  • 1596, URL https://link.aps.org/doi/10.1103/PhysRevLett.25.1596
  • Christodoulou D, Ruffini R (1971) Reversible Transformations of a Charged Black
  • Hole. Phys. Rev. D 4(12):3552–3555. https://doi.org/10.1103/PhysRevD.4.3552
  • Colpi M, Danzmann K, Hewitson M, et al (2024) LISA Definition Study
  • Report. arXiv e-prints arXiv:2402.07571. https://doi.org/10.48550/arXiv.2402.
  • 07571, arXiv:2402.07571 [astro-ph.CO]
  • Crawford MK, Genzel R, Harris AI, et al (1985) Mass distribution in the galactic centre. Nature 315(6019):467–470. https://doi.org/10.1038/315467a0
  • Davies R, Hörmann V, Rabien S, et al (2021) MICADO: The Multi-Adaptive
  • Optics Camera for Deep Observations. The Messenger 182:17–21. https://doi.org/
  • 10.18727/0722-6691/5217, arXiv:2103.11631 [astro-ph.IM]
  • Dexter J, Tchekhovskoy A, Jiménez-Rosales A, et al (2020) Sgr A* near-infrared flares from reconnection events in a magnetically arrested disc. MNRAS 497(4):4999–5007. https://doi.org/10.1093/mnras/staa2288, arXiv:2006.03657 [astro-ph.HE]
  • Do T, Ghez AM, Morris MR, et al (2009) A Near-Infrared Variability Study of the Galactic Black Hole: A Red Noise Source with NO Detected Periodicity. ApJ 691(2):1021–1034. https://doi.org/10.1088/0004-637X/691/2/1021, arXiv:0810.0446 [astro-ph]
  • Do T, Hees A, Ghez A, et al (2019a) Relativistic redshift of the star S0-2 orbiting the
  • Galactic Center supermassive black hole. Science 365(6454):664–668. https://doi. org/10.1126/science.aav8137, arXiv:1907.10731 [astro-ph.GA]
  • Do T, Witzel G, Gautam AK, et al (2019b) Unprecedented Near-infrared Brightness and Variability of Sgr A*. ApJ 882(2):L27. https://doi.org/10.3847/2041-8213/ ab38c3, arXiv:1908.01777 [astro-ph.GA]
  • Dodds-Eden K, Porquet D, Trap G, et al (2009) Evidence for X-Ray Synchrotron
  • Emission from Simultaneous Mid-Infrared to X-Ray Observations of a Strong
  • Sgr A* Flare. ApJ 698(1):676–692. https://doi.org/10.1088/0004-637X/698/1/676, arXiv:0903.3416 [astro-ph.GA]
  • Dodds-Eden K, Sharma P, Quataert E, et al (2010) Time-Dependent Models of Flares from Sagittarius A*. ApJ 725(1):450–465. https://doi.org/10.1088/0004-637X/725/
  • 1/450, arXiv:1005.0389 [astro-ph.GA]
  • Dodds-Eden K, Gillessen S, Fritz TK, et al (2011) The Two States of Sgr A* in the Near-infrared: Bright Episodic Flares on Top of Low-level Continuous Variability. ApJ 728(1):37. https://doi.org/10.1088/0004-637X/728/1/37, arXiv:1008.1984
  • Eckart A, Genzel R (1996) Observations of stellar proper motions near the Galactic
  • Centre. Nature 383(6599):415–417. https://doi.org/10.1038/383415a0
  • Eckart A, Baganoff FK, Schödel R, et al (2006a) The flare activity of Sagittarius A*.
  • New coordinated mm to X-ray observations. A&A 450(2):535–555. https://doi.org/
  • 10.1051/0004-6361:20054418, arXiv:astro-ph/0512440 [astro-ph]
  • Eckart A, Schödel R, Meyer L, et al (2006b) Polarimetry of near-infrared flares from
  • Sagittarius A*. A&A 455(1):1–10. https://doi.org/10.1051/0004-6361:20064948, arXiv:astro-ph/0610103 [astro-ph]
  • Einstein A (1916) Die Grundlage der allgemeinen Relativitätstheorie. Ann Phys
  • 354(7):769–822. https://doi.org/10.1002/andp.19163540702
  • Eisenhauer F, Monnier JD, Pfuhl O (2023) Advances in Optical/Infrared Interferometry. ARA&A 61:237–285. https://doi.org/10.1146/annurev-astro-121622-045019, arXiv:2303.00453 [astro-ph.IM]
  • Evans FA, Rasskazov A, Remmelzwaal A, et al (2023) Constraints on the Galactic centre environment from Gaia hypervelocity stars III: insights on a possible companion to Sgr A*. MNRAS 525(1):561–576. https://doi.org/10.1093/mnras/stad2273, arXiv:2304.12169 [astro-ph.GA]
  • Event Horizon Telescope Collaboration, Akiyama K, Alberdi A, et al (2019) First
  • M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black
  • Hole. ApJ 875(1):L1. https://doi.org/10.3847/2041-8213/ab0ec7, arXiv:1906.11238
  • Event Horizon Telescope Collaboration, Akiyama K, Algaba JC, et al (2021a)
  • First M87 Event Horizon Telescope Results. VII. Polarization of the Ring. ApJ
  • 910(1):L12. https://doi.org/10.3847/2041-8213/abe71d, arXiv:2105.01169 [astroph.HE]
  • Event Horizon Telescope Collaboration, Akiyama K, Algaba JC, et al (2021b)
  • First M87 Event Horizon Telescope Results. VIII. Magnetic Field Structure near
  • The Event Horizon. ApJ 910(1):L13. https://doi.org/10.3847/2041-8213/abe4de, arXiv:2105.01173 [astro-ph.HE]
  • Event Horizon Telescope Collaboration, Akiyama K, Alberdi A, et al (2022a) First
  • Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive
  • Black Hole in the Center of the Milky Way. ApJ 930(2):L12. https://doi.org/10.
  • 3847/2041-8213/ac6674
  • Event Horizon Telescope Collaboration, Akiyama K, Alberdi A, et al (2022b) First
  • Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole. ApJ 930(2):L16. https://doi.org/10.3847/
  • 2041-8213/ac6672
  • Fabian AC (2012) Observational Evidence of Active Galactic Nuclei Feedback. ARA&A 50:455–489. https://doi.org/10.1146/annurev-astro-081811-125521, arXiv:1204.4114 [astro-ph.CO]
  • Fabian AC, Iwasawa K (2000) Broad Fe-K lines from Seyfert Galaxies. Adv Space Res
  • 25(3-4):471–480. https://doi.org/10.1016/S0273-1177(99)00782-6
  • Falcke H, Markoff S (2000) The jet model for Sgr A*: Radio and X-ray spectrum.
  • A&A 362:113–118. https://doi.org/10.48550/arXiv.astro-ph/0102186, arXiv:astroph/0102186 [astro-ph]
  • Falcke H, Melia F, Agol E (2000) Viewing the Shadow of the Black Hole at the
  • Galactic Center. ApJ 528(1):L13–L16. https://doi.org/10.1086/312423, arXiv:astroph/9912263 [astro-ph]
  • Ferrarese L, Merritt D (2000) A Fundamental Relation between Supermassive Black
  • Holes and Their Host Galaxies. ApJ 539(1):L9–L12. https://doi.org/10.1086/
  • 312838, arXiv:astro-ph/0006053 [astro-ph]
  • Gebhardt K, Bender R, Bower G, et al (2000) A Relationship between Nuclear Black
  • Hole Mass and Galaxy Velocity Dispersion. ApJ 539(1):L13–L16. https://doi.org/
  • 10.1086/312840, arXiv:astro-ph/0006289 [astro-ph]
  • Genzel R, Townes CH (1987) Physical conditions, dynamics, and mass distribution in the center of the galaxy. ARA&A 25:377–423. https://doi.org/10.1146/annurev.aa.
  • 25.090187.002113
  • Genzel R, Hollenbach D, Townes CH (1994) The nucleus of our Galaxy. Rep Prog
  • Phys 57(5):417–479. https://doi.org/10.1088/0034-4885/57/5/001
  • Genzel R, Eckart A, Ott T, et al (1997) On the nature of the dark mass in the centre of the Milky Way. MNRAS 291(1):219–234. https://doi.org/10.1093/mnras/291.1.219
  • Genzel R, Schödel R, Ott T, et al (2003) Near-infrared flares from accreting gas around the supermassive black hole at the Galactic Centre. Nature 425(6961):934–
  • 937. https://doi.org/10.1038/nature02065, arXiv:astro-ph/0310821 [astro-ph]
  • Genzel R, Eisenhauer F, Gillessen S (2010) The Galactic Center massive black hole and nuclear star cluster. Rev Mod Phys 82(4):3121–3195. https://doi.org/10.1103/
  • RevModPhys.82.3121, arXiv:1006.0064 [astro-ph.GA]
  • Ghez AM, Klein BL, Morris M, et al (1998) High Proper-Motion Stars in the Vicinity of Sagittarius A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy. ApJ 509(2):678–686. https://doi.org/10.1086/306528, arXiv:astroph/9807210 [astro-ph]
  • Ghez AM, Duchêne G, Matthews K, et al (2003) The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy’s Central Black Hole:
  • A Paradox of Youth. ApJ 586(2):L127–L131. https://doi.org/10.1086/374804, arXiv:astro-ph/0302299 [astro-ph]
  • Ghez AM, Salim S, Weinberg NN, et al (2008) Measuring Distance and Properties of the Milky Way’s Central Supermassive Black Hole with Stellar Orbits. ApJ
  • 689(2):1044–1062. https://doi.org/10.1086/592738, arXiv:0808.2870 [astro-ph]
  • Giacconi R (2003) Nobel Lecture: The dawn of X-ray astronomy. Rev Mod Phys
  • 75(3):995–1010. https://doi.org/10.1103/RevModPhys.75.995
  • Giacconi R, Gursky H, Paolini FR, et al (1962) Evidence for X rays from sources outsideo the solar system. Phys. Rev. Lett. 9(11):439–443. https://doi.org/10.1103/
  • PhysRevLett.9.439
  • Gillessen S, Eisenhauer F, Trippe S, et al (2009) Monitoring Stellar Orbits Around the Massive Black Hole in the Galactic Center. ApJ 692(2):1075–1109. https://doi. org/10.1088/0004-637X/692/2/1075, arXiv:0810.4674 [astro-ph]
  • Gillessen S, Plewa PM, Eisenhauer F, et al (2017) An Update on Monitoring Stellar
  • Orbits in the Galactic Center. ApJ 837(1):30. https://doi.org/10.3847/1538-4357/ aa5c41, arXiv:1611.09144 [astro-ph.GA]
  • Gillessen S, Plewa PM, Widmann F, et al (2019) Detection of a Drag Force in
  • G2’s Orbit: Measuring the Density of the Accretion Flow onto Sgr A* at 1000
  • Schwarzschild Radii. ApJ 871(1):126. https://doi.org/10.3847/1538-4357/aaf4f8
  • Gondolo P, Silk J (1999) Dark Matter Annihilation at the Galactic Center.
  • Phys. Rev. Lett. 83(9):1719–1722. https://doi.org/10.1103/PhysRevLett.83.1719, arXiv:astro-ph/9906391 [astro-ph]
  • GRAVITY Collaboration, Abuter R, Accardo M, et al (2017) First light for GRAVITY: Phase referencing optical interferometry for the Very Large
  • Telescope Interferometer. A&A 602:A94. https://doi.org/10.1051/0004-6361/
  • 201730838, arXiv:1705.02345 [astro-ph.IM]
  • GRAVITY Collaboration, Abuter R, Amorim A, et al (2018a) Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole. A&A 615:L15. https://doi.org/10.1051/0004-6361/201833718, arXiv:1807.09409 [astro-ph.GA]
  • GRAVITY Collaboration, Abuter R, Amorim A, et al (2018b) Detection of orbital motions near the last stable circular orbit of the massive black hole SgrA*. A&A
  • 618:L10. https://doi.org/10.1051/0004-6361/201834294, arXiv:1810.12641 [astroph.GA]
  • GRAVITY Collaboration, Sturm E, Dexter J, et al (2018c) Spatially resolved rotation of the broad-line region of a quasar at sub-parsec scale. Nature 563(7733):657–660. https://doi.org/10.1038/s41586-018-0731-9, arXiv:1811.11195 [astro-ph.GA]
  • GRAVITY Collaboration, Abuter R, Amorim A, et al (2019a) A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty. A&A 625:L10. https://doi.org/10.1051/0004-6361/201935656, arXiv:1904.05721 [astro-ph.GA]
  • GRAVITY Collaboration, Amorim A, Bauböck M, et al (2019b) Test of the Einstein Equivalence Principle near the Galactic Center Supermassive Black Hole.
  • Phys. Rev. Lett. 122(10):101102. https://doi.org/10.1103/PhysRevLett.122.101102, arXiv:1902.04193 [astro-ph.GA]
  • GRAVITY Collaboration, Abuter R, Amorim A, et al (2020a) The flux distribution of Sgr A*. A&A 638:A2. https://doi.org/10.1051/0004-6361/202037717, arXiv:2004.07185 [astro-ph.GA]
  • GRAVITY Collaboration, Abuter R, Amorim A, et al (2020b) Detection of the
  • Schwarzschild precession in the orbit of the star S2 near the Galactic centre massive black hole. A&A 636:L5. https://doi.org/10.1051/0004-6361/202037813, arXiv:2004.07187 [astro-ph.GA]
  • GRAVITY Collaboration, Bauböck M, Dexter J, et al (2020c) Modeling the orbital motion of Sgr A*’s near-infrared flares. A&A 635:A143. https://doi.org/10.1051/
  • 0004-6361/201937233, arXiv:2002.08374 [astro-ph.HE]
  • GRAVITY Collaboration, Jiménez-Rosales A, Dexter J, et al (2020d) Dynamically important magnetic fields near the event horizon of Sgr A*. A&A 643:A56. https:
  • //doi.org/10.1051/0004-6361/202038283, arXiv:2009.01859 [astro-ph.HE]
  • GRAVITY Collaboration, Abuter R, Amorim A, et al (2021) Constraining particle acceleration in Sgr A⋆ with simultaneous GRAVITY, Spitzer, NuSTAR, and Chandra observations. A&A 654:A22. https://doi.org/10.1051/0004-6361/202140981, arXiv:2107.01096 [astro-ph.HE]
  • GRAVITY Collaboration, Abuter R, Aimar N, et al (2022a) Deep images of the Galactic center with GRAVITY. A&A 657:A82. https://doi.org/10.1051/0004-6361/
  • 202142459, arXiv:2112.07477 [astro-ph.GA]
  • GRAVITY Collaboration, Abuter R, Aimar N, et al (2022b) Mass distribution in the
  • Galactic Center based on interferometric astrometry of multiple stellar orbits. A&A
  • 657:L12. https://doi.org/10.1051/0004-6361/202142465, arXiv:2112.07478 [astroph.GA]
  • GRAVITY+ Collaboration, Abuter R, Allouche F, et al (2022) First light for GRAVITY Wide. Large separation fringe tracking for the Very Large
  • Telescope Interferometer. A&A 665:A75. https://doi.org/10.1051/0004-6361/
  • 202243941, arXiv:2206.00684 [astro-ph.IM]
  • GRAVITY Collaboration, Abuter R, Aimar N, et al (2023a) Polarimetry and astrometry of NIR flares as event horizon scale, dynamical probes for the mass of Sgr A*. A&A 677:L10. https://doi.org/10.1051/0004-6361/202347416, arXiv:2307.11821 [astro-ph.GA]
  • GRAVITY Collaboration, Straub O, Bauböck M, et al (2023b) Where intermediatemass black holes could hide in the Galactic Centre. A full parameter study with the S2 orbit. A&A 672:A63. https://doi.org/10.1051/0004-6361/202245132, arXiv:2303.04067 [astro-ph.GA]
  • GRAVITY+ Collaboration, Abuter R, Allouche F, et al (2024) A dynamical measure of the black hole mass in a quasar 11 billion years ago. Nature https://doi.org/10.
  • 1038/s41586-024-07053-4, arXiv:2401.14567 [astro-ph.GA]
  • GRAVITY Collaboration, Widmann F, Haubois X, et al (2024) Polarization analysis of the VLTI and GRAVITY. A&A 681:A115. https://doi.org/10.1051/0004-6361/
  • 202347238, arXiv:2311.03472 [astro-ph.IM]
  • Greene JE, Seth A, den Brok M, et al (2013) Using Megamaser Disks to Probe Black
  • Hole Accretion. ApJ 771(2):121. https://doi.org/10.1088/0004-637X/771/2/121, arXiv:1304.4254 [astro-ph.CO]
  • Greene JE, Seth A, Kim M, et al (2016) Megamaser Disks Reveal a Broad Distribution of Black Hole Mass in Spiral Galaxies. ApJ 826(2):L32. https://doi.org/10.3847/
  • 2041-8205/826/2/L32, arXiv:1606.00018 [astro-ph.GA]
  • Greene JE, Strader J, Ho LC (2020) Intermediate-Mass Black Holes. ARA&A 58:257–
  • 312. https://doi.org/10.1146/annurev-astro-032620-021835, arXiv:1911.09678
  • Gültekin K, Cackett EM, Miller JM, et al (2009) The Fundamental Plane of Accretion onto Black Holes with Dynamical Masses. ApJ 706(1):404–416. https://doi.org/10.
  • 1088/0004-637X/706/1/404, arXiv:0906.3285 [astro-ph.HE]
  • Haller JW, Rieke MJ, Rieke GH, et al (1996) Stellar Kinematics and the Black Hole in the Galactic Center. ApJ 456:194. https://doi.org/10.1086/176640
  • Häring N, Rix HW (2004) On the Black Hole Mass-Bulge Mass Relation. ApJ
  • 604(2):L89–L92. https://doi.org/10.1086/383567, arXiv:astro-ph/0402376 [astroph]
  • Hasinger G (2020) Illuminating the dark ages: cosmic backgrounds from accretion onto primordial black hole dark matter. J. Cosmology Astropart. Phys. 2020(7):022. https://doi.org/10.1088/1475-7516/2020/07/022, arXiv:2003.05150 [astro-ph.CO]
  • Hawking SW (1974) Black hole explosions? Nature 248(5443):30–31. https://doi.org/
  • 10.1038/248030a0
  • Hees A, Do T, Ghez AM, et al (2017) Testing General Relativity with Stellar Orbits around the Supermassive Black Hole in Our Galactic Center.
  • Phys. Rev. Lett. 118(21):211101. https://doi.org/10.1103/PhysRevLett.118.211101, arXiv:1705.07902 [astro-ph.GA]
  • Hees A, Do T, Roberts BM, et al (2020) Search for a Variation of the Fine
  • Structure Constant around the Supermassive Black Hole in Our Galactic Center.
  • Phys. Rev. Lett. 124(8):081101. https://doi.org/10.1103/PhysRevLett.124.081101, arXiv:2002.11567 [astro-ph.GA]
  • Ho PTP (1995) Molecular gas surrounding the galactic center. In: Winnewisser G, Pelz GC (eds) The Physics and Chemistry of Interstellar Molecular Clouds, Lecture
  • Notes in Physics, vol 459. Springer, Berlin, Heidelberg, pp 33–40, https://doi.org/
  • 10.1007/BFb01020
  • Ho PTP, Ho LC, Szczepanski JC, et al (1991) A molecular gas streamer feeding the
  • Galactic Centre. Nature 350(6316):309–312. https://doi.org/10.1038/350309a0
  • Israel W (1967) Event horizons in static vacuum space-times. Phys Rev 164:1776–1779. https://doi.org/10.1103/PhysRev.164.1776
  • Issaoun S, Johnson MD, Blackburn L, et al (2019) The Size, Shape, and Scattering of Sagittarius A* at 86 GHz: First VLBI with ALMA. ApJ 871(1):30. https://doi. org/10.3847/1538-4357/aaf732, arXiv:1901.06226 [astro-ph.HE]
  • Johannsen T (2013) Photon Rings around Kerr and Kerr-like Black Holes.
  • ApJ 777(2):170. https://doi.org/10.1088/0004-637X/777/2/170, arXiv:1501.02814
  • Johannsen T (2016) Sgr A* and general relativity. Class Quantum
  • Gravit 33(11):113001. https://doi.org/10.1088/0264-9381/33/11/113001, arXiv:1512.03818 [astro-ph.GA]
  • Johannsen T, Wang C, Broderick AE, et al (2016) Testing General Relativity with
  • Accretion-Flow Imaging of Sgr A∗
  • . Phys. Rev. Lett. 117(9):091101. https://doi.org/
  • 10.1103/PhysRevLett.117.091101, arXiv:1608.03593 [astro-ph.HE]
  • Johnson MD, Lupsasca A, Strominger A, et al (2020) Universal interferometric signatures of a black hole’s photon ring. Science Advances 6(12):eaaz1310. https:
  • //doi.org/10.1126/sciadv.aaz1310, arXiv:1907.04329 [astro-ph.IM]
  • Jovanović P, et al (2024) Improvement of graviton mass constraints using GRAVITY’s detection of Schwarzschild precession in the orbit of S2 star around the Galactic
  • Center. Phys Rev D arXiv:2305.13448 [astro-ph.GA]
  • Kaspi S, Smith PS, Netzer H, et al (2000) Reverberation Measurements for 17 Quasars and the Size-Mass-Luminosity Relations in Active Galactic Nuclei. ApJ 533(2):631–
  • 649. https://doi.org/10.1086/308704, arXiv:astro-ph/9911476 [astro-ph]
  • Kerr RP (1963) Gravitational field of a spinning mass as an example of algebraically special metrics. Phys Rev Lett 11:237–238. https://doi.org/10.1103/PhysRevLett.
  • 11.237
  • Kormendy J (2004) The Stellar-Dynamical Search for Supermassive Black Holes in
  • Galactic Nuclei. In: Ho LC (ed) Coevolution of Black Holes and Galaxies. Cambridge
  • University Press, pp 1–20, astro-ph/0306353
  • Kormendy J, Ho LC (2013) Coevolution (Or Not) of Supermassive Black
  • Holes and Host Galaxies. ARA&A 51(1):511–653. https://doi.org/10.1146/ annurev-astro-082708-101811, arXiv:1304.7762 [astro-ph.CO]
  • Holes and Host Galaxies. ARA&A 51(1):511–653. https://doi.org/10.1146/ annurev-astro-082708-101811, arXiv:1304.7762 [astro-ph.CO]
  • Kormendy J, Kennicutt JRobert C. (2004) Secular Evolution and the Formation of
  • Pseudobulges in Disk Galaxies. ARA&A 42(1):603–683. https://doi.org/10.1146/ annurev.astro.42.053102.134024, arXiv:astro-ph/0407343 [astro-ph]
  • Krabbe A, Genzel R, Eckart A, et al (1995) The Nuclear Cluster of the Milky Way:
  • Star Formation and Velocity Dispersion in the Central 0.5 Parsec. ApJ 447:L95. https://doi.org/10.1086/309579
  • Lacroix T (2018) Dynamical constraints on a dark matter spike at the Galactic centre from stellar orbits. A&A 619:A46. https://doi.org/10.1051/0004-6361/201832652, arXiv:1801.01308 [astro-ph.GA]
  • Lacy JH, Townes CH, Geballe TR, et al (1980) Observations of the motion and distribution of the ionized gas in the central parsec of the Galaxy. II. ApJ 241:132–146. https://doi.org/10.1086/158324
  • Laplace PS (1795) Exposition du système du monde, vol 2. Imprimerie du Cercle
  • Social, Paris, URL https://gallica.bnf.fr/ark:/12148/bpt6k1050382f
  • Linial I, Sari R (2022) Stellar Distributions around a Supermassive Black Hole: Strongsegregation Regime Revisited. ApJ 940(2):101. https://doi.org/10.3847/1538-4357/ ac9bfd, arXiv:2206.14817 [astro-ph.GA]
  • Lo KY, Claussen MJ (1983) High-resolution observations of ionized gas in central
  • 3 parsecs of the Galaxy: possible evidence for infall. Nature 306(5944):647–651. https://doi.org/10.1038/306647a0
  • Lu W, Kumar P, Narayan R (2017) Stellar disruption events support the existence of the black hole event horizon. MNRAS 468(1):910–919. https://doi.org/10.1093/ mnras/stx542, arXiv:1703.00023 [astro-ph.HE]
  • Luminet JP (1979) Image of a spherical black hole with thin accretion disk. A&A
  • 75:228–235
  • Lynden-Bell D (1969) Galactic Nuclei as Collapsed Old Quasars. Nature
  • 223(5207):690–694. https://doi.org/10.1038/223690a0
  • Lynden-Bell D, Rees MJ (1971) On quasars, dust and the galactic centre. MNRAS
  • 152:461. https://doi.org/10.1093/mnras/152.4.461
  • Magorrian J, Tremaine S, Richstone D, et al (1998) The Demography of Massive Dark
  • Objects in Galaxy Centers. AJ 115(6):2285–2305. https://doi.org/10.1086/300353, arXiv:astro-ph/9708072 [astro-ph]
  • Maiolino R, Scholtz J, Witstok J, et al (2023) A small and vigorous black hole in the early Universe. arXiv e-prints https://doi.org/10.48550/arXiv.2305.12492, arXiv:2305.12492 [astro-ph.GA]
  • Maldacena J (1998) The large N limit of superconformal field theories and supergravity. Adv Theor Math Phys 2:231. https://doi.org/10.4310/ATMP.1998.v2.n2. a1
  • Maoz E (1995) A Stringent Constraint on Alternatives to a Massive Black Hole at the
  • Center of NGC 4258. ApJ 447:L91. https://doi.org/10.1086/309574, arXiv:astroph/9503113 [astro-ph]
  • Marrone DP, Moran JM, Zhao JH, et al (2007) An Unambiguous Detection of Faraday
  • Rotation in Sagittarius A*. ApJ 654(1):L57–L60. https://doi.org/10.1086/510850, arXiv:astro-ph/0611791 [astro-ph]
  • Matsumoto T, Chan CH, Piran T (2020) The origin of hotspots around Sgr A*: orbital or pattern motion? MNRAS 497(2):2385–2392. https://doi.org/10.1093/ mnras/staa2095, arXiv:2004.13029 [astro-ph.HE]
  • Mazur PO, Mottola E (2004) Gravitational vacuum condensate stars. Proceedings of the National Academy of Science 101(26):9545–9550. https://doi.org/10.1073/pnas.
  • 0402717101, arXiv:gr-qc/0407075 [gr-qc]
  • McClintock JE, Remillard RA (2006) Black hole binaries. In: Lewin W, van der Klis
  • M (eds) Compact Stellar X-ray Sources. Cambridge University Press, Cambridge, p 157–214, https://doi.org/10.1017/CBO9780511536281.005
  • McConnell NJ, Ma CP (2013) Revisiting the Scaling Relations of Black Hole Masses and Host Galaxy Properties. ApJ 764(2):184. https://doi.org/10.1088/0004-637X/
  • 764/2/184, arXiv:1211.2816 [astro-ph.CO]
  • McGinn MT, Sellgren K, Becklin EE, et al (1989) Stellar Kinematics in the Galactic
  • Center. ApJ 338:824. https://doi.org/10.1086/167239
  • Melia F, Falcke H (2001) The Supermassive Black Hole at the Galactic Center.
  • ARA&A 39:309–352. https://doi.org/10.1146/annurev.astro.39.1.309, arXiv:astroph/0106162 [astro-ph]
  • Merritt D (2010) The Distribution of Stars and Stellar Remnants at the Galactic Center. ApJ 718(2):739–761. https://doi.org/10.1088/0004-637X/718/2/739, arXiv:0909.1318 [astro-ph.GA]
  • Merritt D, Alexander T, Mikkola S, et al (2010) Testing properties of the Galactic center black hole using stellar orbits. Phys. Rev. D 81(6):062002. https://doi.org/
  • 10.1103/PhysRevD.81.062002, arXiv:0911.4718 [astro-ph.GA]
  • Michell J (1784) On the Means of Discovering the Distance, Magnitude, &c. of the
  • Fixed Stars, in Consequence of the Diminution of the Velocity of Their Light, in
  • Case Such a Diminution Should be Found to Take Place in any of Them, and Such
  • Other Data Should be Procured from Observations, as Would be Farther Necessary for That Purpose. Phil Trans R Soc London Ser I 74:35–57. https://doi.org/10.
  • 1098/rstl.1784.0008
  • Michelson A, Morley E (1887) On the Relative Motion of the Earth and the
  • Luminiferous Ether. Am J Sci 34(203):333–345. https://doi.org/10.2475/ajs.s3-34.
  • 203.333
  • Miyoshi M, Moran J, Herrnstein J, et al (1995) Evidence for a black hole from high rotation velocities in a sub-parsec region of NGC4258. Nature 373(6510):127–129. https://doi.org/10.1038/373127a0
  • Moran JM (2008) The Black-Hole Accretion Disk in NGC 4258: One of Nature’s Most
  • Beautiful Dynamical Systems. In: Bridle AH, Condon JJ, Hunt GC (eds) Frontiers of
  • Astrophysics: A Celebration of NRAO’s 50th Anniversary, ASP Conference Series, vol 395. Astronomical Society of the Pacific, p 87, https://doi.org/10.48550/arXiv.
  • 0804.1063, 0804.1063
  • Morris M, Serabyn E (1996) The Galactic Center Environment. ARA&A 34:645–702. https://doi.org/10.1146/annurev.astro.34.1.645
  • Morris MR, Meyer L, Ghez AM (2012) Galactic center research: manifestations of the central black hole. Research in Astronomy and Astrophysics 12(8):995–1020. https://doi.org/10.1088/1674-4527/12/8/007, arXiv:1207.6755 [astro-ph.GA]
  • Morris MS, Thorne KS (1988) Wormholes in Spacetime and Their Use for Interstellar
  • Travel. Am J Phys 56:395. https://doi.org/10.1119/1.15620
  • Munyaneza F, Tsiklauri D, Viollier RD (1999) Dynamics of the Star S0-1 and the
  • Nature of the Compact Dark Object at the Galactic Center. ApJ 526(2):744–751. https://doi.org/10.1086/308026, arXiv:astro-ph/9903242 [astro-ph]
  • Nandra K, George IM, Mushotzky RF, et al (1997) ASCA Observations of Seyfert 1
  • Galaxies. II. Relativistic Iron Kα Emission. ApJ 477(2):602–622. https://doi.org/
  • 10.1086/303721, arXiv:astro-ph/9606169 [astro-ph]
  • Netzer H (2013) The Physics and Evolution of Active Galactic Nuclei. Cambridge
  • University Press, https://doi.org/10.1017/CBO9781139109291
  • Netzer H (2015) Revisiting the Unified Model of Active Galactic Nuclei.
  • ARA&A 53:365–408. https://doi.org/10.1146/annurev-astro-082214-122302, arXiv:1505.00811 [astro-ph.GA]
  • Netzer H (2020) Testing broad-line region models with reverberation mapping. MNRAS 494(2):1611–1621. https://doi.org/10.1093/mnras/staa767, arXiv:2003.07660 [astro-ph.GA]
  • Newman ET, et al (1965) Metric of a Rotating, Charged Mass. J Math Phys 6:918. https://doi.org/10.1063/1.1704351
  • Olivares H, Younsi Z, Fromm CM, et al (2020) How to tell an accreting boson star from a black hole. MNRAS 497(1):521–535. https://doi.org/10.1093/mnras/staa1878, arXiv:1809.08682 [gr-qc]
  • Oort JH (1977) The galactic center. ARA&A 15:295–362. https://doi.org/10.1146/ annurev.aa.15.090177.001455
  • Oppenheimer JR, Snyder H (1939) On Continued Gravitational Contraction. Phys
  • Rev 56(5):455–459. https://doi.org/10.1103/PhysRev.56.455
  • Osmer PS (2004) The evolution of quasars. In: Ho LC (ed) Coevolution of BHs and
  • Galaxies, Carnegie Observatories Astrophysics Series, vol 1. Cambridge University
  • Press, pp 324–340
  • Özel F, Psaltis D, Narayan R, et al (2010) The Black Hole Mass Distribution in the
  • Galaxy. ApJ 725(2):1918–1927. https://doi.org/10.1088/0004-637X/725/2/1918, arXiv:1006.2834 [astro-ph.GA]
  • Penrose R (1963) Asymptotic properties of fields and space-times. Phys Rev Lett
  • 10:66–68. https://doi.org/10.1103/PhysRevLett.10.66
  • Penrose R (1965) Gravitational collapse and space-time singularities. Phys Rev Lett
  • 14:57–59. https://doi.org/10.1103/PhysRevLett.14.57
  • Peterson BM (2014) Measuring the Masses of Supermassive Black Holes.
  • Space Sci. Rev. 183(1-4):253–275. https://doi.org/10.1007/s11214-013-9987-4
  • Ponti G, George E, Scaringi S, et al (2017) A powerful flare from Sgr A* confirms the synchrotron nature of the X-ray emission. MNRAS 468(2):2447–2468. https:
  • //doi.org/10.1093/mnras/stx596, arXiv:1703.03410 [astro-ph.HE]
  • Portegies Zwart SF, Boekholt TCN, Heggie DC (2023) Punctuated chaos and the unpredictability of the Galactic Centre S-star orbital evolution. MNRAS
  • 526(4):5791–5799. https://doi.org/10.1093/mnras/stad2654, arXiv:2308.14817
  • Psaltis D (2024) Black Holes in Classical General Relativity and Beyond. In: Haiman Z
  • (ed) The Encyclopedia of Cosmology, Set 2: Frontiers in Cosmology, Volume 3: Black
  • Holes. World Scientific, Singapore, p 1–25, https://doi.org/10.1142/9789811282676
  • 0001, 2304.09984
  • Psaltis D, Johannsen T (2011) Sgr A*: The Optimal Testbed of Strong-Field Gravity. J Phys Conf Ser 283:012030. https://doi.org/10.1088/1742-6596/283/1/012030, arXiv:1012.1602 [astro-ph.HE]
  • Psaltis D, Wex N, Kramer M (2016) A Quantitative Test of the No-hair Theorem with
  • Sgr A* Using Stars, Pulsars, and the Event Horizon Telescope. ApJ 818(2):121. https://doi.org/10.3847/0004-637X/818/2/121, arXiv:1510.00394 [astro-ph.HE]
  • Quataert E (2004) A Dynamical Model for Hot Gas in the Galactic Center. ApJ
  • 613(1):322–325. https://doi.org/10.1086/422973, arXiv:astro-ph/0310446 [astro-ph]
  • Quataert E, Gruzinov A (2000) Constraining the Accretion Rate onto Sagittarius
  • A* Using Linear Polarization. ApJ 545(2):842–846. https://doi.org/10.1086/317845, arXiv:astro-ph/0004286 [astro-ph]
  • Rees MJ (1984) Black Hole Models for Active Galactic Nuclei. ARA&A 22:471–506. https://doi.org/10.1146/annurev.aa.22.090184.002351
  • Remillard RA, McClintock JE (2006) X-Ray Properties of Black-Hole Binaries.
  • ARA&A 44(1):49–92. https://doi.org/10.1146/annurev.astro.44.051905.092532, arXiv:astro-ph/0606352 [astro-ph]
  • Ressler SM, Quataert E, Stone JM (2018) Hydrodynamic simulations of the inner accretion flow of Sagittarius A* fuelled by stellar winds. MNRAS 478(3):3544–3563. https://doi.org/10.1093/mnras/sty1146, arXiv:1805.00474 [astro-ph.HE]
  • Ressler SM, Quataert E, Stone JM (2020a) The surprisingly small impact of magnetic fields on the inner accretion flow of Sagittarius A* fueled by stellar winds. MNRAS 492(3):3272–3293. https://doi.org/10.1093/mnras/stz3605, arXiv:2001.04469 [astro-ph.HE]
  • Ressler SM, White CJ, Quataert E, et al (2020b) Ab Initio Horizon-scale Simulations of Magnetically Arrested Accretion in Sagittarius A* Fed by Stellar Winds. ApJ
  • 896(1):L6. https://doi.org/10.3847/2041-8213/ab9532, arXiv:2006.00005 [astroph.HE]
  • Reynolds CS (2021) Observational Constraints on Black Hole Spin. ARA&A 59:117–
  • 154. https://doi.org/10.1146/annurev-astro-112420-035022, arXiv:2011.08948
  • Rezzolla L, Most ER, Weih LR (2018) Using Gravitational-wave Observations and
  • Quasi-universal Relations to Constrain the Maximum Mass of Neutron Stars. ApJ
  • 852(2):L25. https://doi.org/10.3847/2041-8213/aaa401, arXiv:1711.00314 [astroph.HE]
  • Robinson DC (1975) Uniqueness of the Kerr Black Hole. Phys. Rev. Lett. 34(14):905–
  • 906. https://doi.org/10.1103/PhysRevLett.34.905
  • Rosa JaL, Garcia P, Vincent FH, et al (2022) Observational signatures of hot spots orbiting horizonless objects. Phys Rev D 106:044031. https://doi.org/10.1103/
  • PhysRevD.106.044031
  • Ruffini R, Argüelles CR, Rueda JA (2015) On the core-halo distribution of dark matter in galaxies. MNRAS 451(1):622–628. https://doi.org/10.1093/mnras/stv1016, arXiv:1409.7365 [astro-ph.GA]
  • Sadeghian L, Ferrer F, Will CM (2013) Dark-matter distributions around massive black holes: A general relativistic analysis. Phys. Rev. D 88(6):063522. https://doi. org/10.1103/PhysRevD.88.063522, arXiv:1305.2619 [astro-ph.GA]
  • Saglia RP, Opitsch M, Erwin P, et al (2016) The SINFONI Black Hole Survey:
  • The Black Hole Fundamental Plane Revisited and the Paths of (Co)evolution of
  • Supermassive Black Holes and Bulges. ApJ 818(1):47. https://doi.org/10.3847/
  • 0004-637X/818/1/47, arXiv:1601.00974 [astro-ph.GA]
  • Schmidt M (1963) 3C 273 : A Star-Like Object with Large Red-Shift. Nature
  • 197(4872):1040. https://doi.org/10.1038/1971040a0
  • Schneider R, Valiante R, Trinca A, et al (2023) Are we surprised to find SMBHs with JWST at z ≥ 9? MNRAS 526(3):3250–3261. https://doi.org/10.1093/mnras/ stad2503, arXiv:2305.12504 [astro-ph.GA]
  • Schödel R, Ott T, Genzel R, et al (2002) A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way. Nature 419(6908):694–696. https://doi.org/10.1038/nature01121, arXiv:astro-ph/0210426 [astro-ph]
  • Schwarzschild K (1916) On the gravitational field of a mass point according to
  • Einstein’s theory. Sitzungsber Preuss Akad Wiss, Phys Math Kl 1916:189–196. https://doi.org/10.1023/A:1022971926521, transl. reprinted in Gen Relativ Gravit
  • 35:951 (2003)
  • Serabyn E, Lacy JH (1985) NE II observations of the galactic center: evidence for a massive black hole. ApJ 293:445–458. https://doi.org/10.1086/163250
  • Shakura NI, Sunyaev RA (1973) Black holes in binary systems. Observational appearance. A&A 24:337–355
  • Susskind L (1995) The world as a hologram. J Math Phys 36(11):6377–6396. https:
  • //doi.org/10.1063/1.531249, arXiv:hep-th/9409089 [hep-th]
  • Tanaka Y, Nandra K, Fabian AC, et al (1995) Gravitationally redshifted emission implying an accretion disk and massive black hole in the active galaxy MCG-6-30-15.
  • Nature 375(6533):659–661. https://doi.org/10.1038/375659a0
  • Thompson RA, Moran JM, Swenson Jr. GW (2017) Interferometry and Synthesis in Radio Astronomy, 3rd edn. Springer, Cham, https://doi.org/10.1007/
  • 978-3-319-44431-4
  • Torres DF, Capozziello S, Lambiase G (2000) Supermassive boson star at the galactic center? Phys. Rev. D 62(10):104012. https://doi.org/10.1103/PhysRevD.62.104012, arXiv:astro-ph/0004064 [astro-ph]
  • Tsiklauri D, Viollier RD (1998) Dark Matter Concentration in the Galactic Center. ApJ 500(2):591–595. https://doi.org/10.1086/305753, arXiv:astro-ph/9805273
  • [astro-ph]
  • Vestergaard M, Osmer PS (2009) Mass Functions of the Active Black Holes in Distant
  • Quasars from the Large Bright Quasar Survey, the Bright Quasar Survey, and the
  • Color-selected Sample of the SDSS Fall Equatorial Stripe. ApJ 699(1):800–816. https://doi.org/10.1088/0004-637X/699/1/800, arXiv:0904.3348 [astro-ph.CO]
  • Vestergaard M, Fan X, Tremonti CA, et al (2008) Mass Functions of the Active Black
  • Holes in Distant Quasars from the Sloan Digital Sky Survey Data Release 3. ApJ
  • 674(1):L1. https://doi.org/10.1086/528981, arXiv:0801.0243 [astro-ph]
  • Vincent FH, Meliani Z, Grandclément P, et al (2016) Imaging a boson star at the
  • Galactic center. Class Quantum Gravit 33(10):105015. https://doi.org/10.1088/
  • 0264-9381/33/10/105015, arXiv:1510.04170 [gr-qc]
  • Viollier RD, Trautmann D, Tupper GB (1993) Supermassive neutrino stars and galactic nuclei. Phys Lett B 306(1-2):79–85. https://doi.org/10.1016/0370-2693(93)
  • 91141-9
  • Waisberg I, Dexter J, Gillessen S, et al (2018) What stellar orbit is needed to measure the spin of the Galactic centre black hole from astrometric data? MNRAS
  • 476(3):3600–3610. https://doi.org/10.1093/mnras/sty476, arXiv:1802.08198 [astroph.GA]
  • Wheeler JA (1968) Our universe: the known and the unknown. American Scientist
  • 56(1):1–20
  • Wielgus M, Moscibrodzka M, Vos J, et al (2022) Orbital motion near Sagittarius A∗
  • .
  • Constraints from polarimetric ALMA observations. A&A 665:L6. https://doi.org/
  • 10.1051/0004-6361/202244493, arXiv:2209.09926 [astro-ph.HE]
  • Will CM (2008) Testing the General Relativistic “No-Hair” Theorems Using the Galactic Center Black Hole Sagittarius A*. ApJ 674(1):L25. https://doi.org/10.1086/
  • 528847, arXiv:0711.1677 [astro-ph]
  • Will CM, Naoz S, Hees A, et al (2023) Constraining a Companion of the Galactic Center Black Hole Sgr A*. ApJ 959(1):58. https://doi.org/10.3847/1538-4357/ad09b3, arXiv:2307.16646 [astro-ph.GA]
  • Witzel G, Eckart A, Bremer M, et al (2012) Source-intrinsic Near-infrared Properties of Sgr A*: Total Intensity Measurements. ApJS 203(2):18. https://doi.org/10.1088/
  • 0067-0049/203/2/18, arXiv:1208.5836 [astro-ph.HE]
  • Witzel G, Martinez G, Hora J, et al (2018) Variability Timescale and Spectral Index of Sgr A* in the Near Infrared: Approximate Bayesian Computation Analysis of the
  • Variability of the Closest Supermassive Black Hole. ApJ 863(1):15. https://doi.org/
  • 10.3847/1538-4357/aace62, arXiv:1806.00479 [astro-ph.HE]
  • Witzel G, Martinez G, Willner SP, et al (2021) Rapid Variability of Sgr A* across the Electromagnetic Spectrum. ApJ 917(2):73. https://doi.org/10.3847/1538-4357/ ac0891, arXiv:2011.09582 [astro-ph.HE]
  • Wollman ER, Geballe TR, Lacy JH, et al (1977) Ne II 12.8 micron emission from the galactic center. II. ApJ 218:L103–L107. https://doi.org/10.1086/182585
  • Yuan F, Narayan R (2014) Hot Accretion Flows Around Black Holes. ARA&A 52:529–
  • 588. https://doi.org/10.1146/annurev-astro-082812-141003, arXiv:1401.0586 [astroph.HE]
  • Yuan F, Quataert E, Narayan R (2003) Nonthermal Electrons in Radiatively Inefficient
  • Accretion Flow Models of Sagittarius A*. ApJ 598(1):301–312. https://doi.org/10.
  • 1086/378716, arXiv:astro-ph/0304125 [astro-ph]
  • Zakharov AF, Nucita AA, de Paolis F, et al (2007) Apoastron shift constraints on dark matter distribution at the Galactic Center. Phys. Rev. D 76(6):062001. https:
  • //doi.org/10.1103/PhysRevD.76.062001, arXiv:0707.4423 [astro-ph]
  • Zhang YP, Zeng YB, Wang YQ, et al (2022) Motion of test particle in rotating boson star. Phys Rev D 105:044021. https://doi.org/10.1103/PhysRevD.105.044021