scholarly journals The Dark Mass Concentration at the Galactic Center

1997 ◽  
Vol 163 ◽  
pp. 637-646
Author(s):  
F. Melia
Keyword(s):  
Black Hole ◽  
Large Scale ◽  
Galactic Center ◽  
Line Emission ◽  
Concentrated Mass ◽  
Massive Black Hole ◽  
Central Mass ◽  
Kinematic Studies ◽  
Sgr A ◽  
Large Scale Flow

AbstractStellar kinematic studies indicate the presence of a concentrated central mass at just under 2 × 106M⊙, in close agreement with the mass deduced from gas velocities measured with the [Ne II] line. Although this mass is most likely a black hole, it may be dominated by a tightly concentrated cluster of stellar remnants. If Sgr A*, a point radio source coincident with this central mass, is a massive black hole embedded in a region with strong gaseous outflows, as suggested by the observation of He I, Brα and Brγ line emission, it is accreting from its environment via the Bondi-Hoyle process. We discuss the consequences of this activity, including the expected mass and angular momentum accretion rate onto the black hole, and the resulting observable characteristics. The latest infrared images of this region appear to rule out the possibility that this large scale flow settles down into a standard α-disk at small radii. We discuss some possible scenarios that might account for this, including strong advection in the disk or the presence of a massive, fossilized disk. Not all of the gas affected in this way by Sgr A*’s strong gravitational field becomes bound. Some of it is redirected into a focused flow that in turn interacts with other coherent gas structures near the black hole. We suggest that the mini-cavity (to the south-west of Sgr A*) may be formed as a result of this activity, and argue that the characteristics of the mini-cavity lend some observational support for the presence of a concentrated mass near Sgr A*. We show, however, that as far as the mini-cavity is concerned, this concentrated mass need not be in the form of a point mass, but may instead be a highly concentrated cluster of stellar remnants.

scholarly journals Detection of the Schwarzschild precession in the orbit of the star S2 near the Galactic centre massive black hole

2020 ◽  
Vol 636 ◽  
pp. L5 ◽  
Author(s):  
◽  
R. Abuter ◽  
A. Amorim ◽  
M. Bauböck ◽  
J. P. Berger ◽  
...  
Keyword(s):  
Black Hole ◽  
Reference Frame ◽  
Adaptive Optics ◽  
Galactic Centre ◽  
Massive Black Hole ◽  
Central Mass ◽  
Black Hole Candidate ◽  
Beam Combiner ◽  
Direct Measurements ◽  
Sgr A

The star S2 orbiting the compact radio source Sgr A* is a precision probe of the gravitational field around the closest massive black hole (candidate). Over the last 2.7 decades we have monitored the star’s radial velocity and motion on the sky, mainly with the SINFONI and NACO adaptive optics (AO) instruments on the ESO VLT, and since 2017, with the four-telescope interferometric beam combiner instrument GRAVITY. In this Letter we report the first detection of the General Relativity (GR) Schwarzschild Precession (SP) in S2’s orbit. Owing to its highly elliptical orbit (e = 0.88), S2’s SP is mainly a kink between the pre-and post-pericentre directions of motion ≈±1 year around pericentre passage, relative to the corresponding Kepler orbit. The superb 2017−2019 astrometry of GRAVITY defines the pericentre passage and outgoing direction. The incoming direction is anchored by 118 NACO-AO measurements of S2’s position in the infrared reference frame, with an additional 75 direct measurements of the S2-Sgr A* separation during bright states (“flares”) of Sgr A*. Our 14-parameter model fits for the distance, central mass, the position and motion of the reference frame of the AO astrometry relative to the mass, the six parameters of the orbit, as well as a dimensionless parameter fSP for the SP (fSP = 0 for Newton and 1 for GR). From data up to the end of 2019 we robustly detect the SP of S2, δϕ ≈ 12′ per orbital period. From posterior fitting and MCMC Bayesian analysis with different weighting schemes and bootstrapping we find fSP = 1.10 ± 0.19. The S2 data are fully consistent with GR. Any extended mass inside S2’s orbit cannot exceed ≈0.1% of the central mass. Any compact third mass inside the central arcsecond must be less than about 1000 M⊙.

scholarly journals Observing a black hole event horizon: (sub)millimeter VLBI of Sgr A*

2009 ◽  
Vol 5 (S261) ◽  
pp. 271-276 ◽  
Author(s):  
Vincent L. Fish ◽  
Sheperd S. Doeleman
Keyword(s):  
Black Hole ◽  
Event Horizon ◽  
Strong Evidence ◽  
Galactic Center ◽  
Strong Field ◽  
Variable Component ◽  
Massive Black Hole ◽  
Black Hole Candidate ◽  
Long Baseline ◽  
Sgr A

AbstractVery strong evidence suggests that Sagittarius A*, a compact radio source at the center of the Milky Way, marks the position of a super massive black hole. The proximity of Sgr A* in combination with its mass makes its apparent event horizon the largest of any black hole candidate in the universe and presents us with a unique opportunity to observe strong-field GR effects. Recent millimeter very long baseline interferometric observations of Sgr A* have demonstrated the existence of structures on scales comparable to the Schwarzschild radius. These observations already provide strong evidence in support of the existence of an event horizon. (Sub)Millimeter VLBI observations in the near future will combine the angular resolution necessary to identify the overall morphology of quiescent emission, such as an accretion disk or outflow, with a fine enough time resolution to detect possible periodicity in the variable component of emission. In the next few years, it may be possible to identify the spin of the black hole in Sgr A*, either by detecting the periodic signature of hot spots at the innermost stable circular orbit or parameter estimation in models of the quiescent emission. Longer term, a (sub)millimeter VLBI “Event Horizon Telescope” will be able to produce images of the Galactic center emission to the see the silhouette predicted by general relativistic lensing. These techniques are also applicable to the black hole in M87, where black hole spin may be key to understanding the jet-launching region.

scholarly journals Observations of the gas cloud G2 in the Galactic center

2013 ◽  
Vol 9 (S303) ◽  
pp. 254-263
Author(s):  
S. Gillessen ◽  
R. Genzel ◽  
T. K. Fritz ◽  
F. Eisenhauer ◽  
O. Pfuhl ◽  
...  
Keyword(s):  
Black Hole ◽  
Galactic Center ◽  
Accretion Rate ◽  
Tidal Effects ◽  
Massive Black Hole ◽  
Radial Orbit ◽  
Sgr A ◽  
The Many ◽  
Gas Cloud ◽  
Luminosity Evolution

AbstractIn 2011, we discovered a compact gas cloud (“G2”) with roughly three Earth masses that is falling on a near-radial orbit toward the massive black hole in the Galactic center. The orbit is well constrained and pericenter passage is predicted for early 2014. Our data beautifully show that G2 gets tidally sheared apart due to the massive black hole's force. During the next months, we expect that in addition to the tidal effects, hydrodynamics get important, when G2 collides with the hot ambient gas around Sgr A*. Simulations show that ultimately, the cloud's material might fall into the massive black hole. Predictions for the accretion rate and luminosity evolution, however, are very difficult due to the many unknowns. Nevertheless, this might be a unique opportunity in the next years to observe how gas feeds a massive black hole in a galactic nucleus.

scholarly journals Constraining jet physics in weakly accreting black holes

2006 ◽  
Vol 2 (S238) ◽  
pp. 145-149
Author(s):  
Sera Markoff
Keyword(s):  
Black Hole ◽  
Large Scale ◽  
Galactic Center ◽  
Mass Scale ◽  
Compact Objects ◽  
Jet Physics ◽  
Accretion Rates ◽  
Test Population ◽  
Multi Wavelength ◽  
Sgr A

AbstractOutflowing jets are observed in a variety of astronomical objects such as accreting compact objects from X-ray binaries (XRBs) to active galactic nuclei (AGN), as well as at stellar birth and death. Yet we still do not know exactly what they are comprised of, why and how they form, or their exact relationship with the accretion flow. In this talk I focus on jets in black hole systems, which provide the ideal test population for studying the relationship between inflow and outflow over an extreme range in mass and accretion rate.I present several recent results from coordinated multi-wavelength studies of low-luminosity sources. These results not only support similar trends in weakly accreting black hole behavior across the mass scale, but also suggest that the same underlying physical model can explain their broadband spectra. I discuss how comparisons between small- and large-scale systems are revealing new information about the regions nearest the black hole, providing clues about the creation of these weakest of jets. Furthermore, comparisons between our Galactic center nucleus Sgr A* and other sources at slightly higher accretion rates can elucidate the processes which drive central activity, and pave the way for new tests with upcoming instruments.

scholarly journals A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty

2019 ◽  
Vol 625 ◽  
pp. L10 ◽  
Author(s):  
◽  
R. Abuter ◽  
A. Amorim ◽  
M. Bauböck ◽  
J. P. Berger ◽  
...  
Keyword(s):  
Black Hole ◽  
Near Infrared ◽  
Galactic Center ◽  
Accurate Determination ◽  
Massive Black Hole ◽  
Geometric Distance ◽  
Significance Level ◽  
Beam Combiner ◽  
Infrared Interferometry ◽  
Sgr A

We present a 0.16% precise and 0.27% accurate determination of R0, the distance to the Galactic center. Our measurement uses the star S2 on its 16-year orbit around the massive black hole Sgr A* that we followed astrometrically and spectroscopically for 27 years. Since 2017, we added near-infrared interferometry with the VLTI beam combiner GRAVITY, yielding a direct measurement of the separation vector between S2 and Sgr A* with an accuracy as good as 20 μas in the best cases. S2 passed the pericenter of its highly eccentric orbit in May 2018, and we followed the passage with dense sampling throughout the year. Together with our spectroscopy, in the best cases with an error of 7 km s−1, this yields a geometric distance estimate of R0 = 8178 ± 13stat. ± 22sys. pc. This work updates our previous publication, in which we reported the first detection of the gravitational redshift in the S2 data. The redshift term is now detected with a significance level of 20σ with fredshift = 1.04 ± 0.05.

scholarly journals Forming misaligned stellar disks around a massive black hole: cloud infall in the Galactic center

2013 ◽  
Vol 9 (S303) ◽  
pp. 245-247
Author(s):  
William Lucas ◽  
Ian Bonnell ◽  
Melvyn Davies ◽  
Ken Rice
Keyword(s):  
Black Hole ◽  
Angular Momentum ◽  
Galactic Center ◽  
Massive Black Hole ◽  
Hydrodynamic Simulations ◽  
Large Spread ◽  
Smoothed Particle ◽  
The Creation ◽  
Sgr A ◽  
Smoothed Particle Hydrodynamic

AbstractThe innermost parsec around Sgr A* has been found to play host to two disks or streamers of O and W-R stars. They are misaligned by an angle approaching 90°. That the stars are approximately coeval indicates that they formed in the same event rather than independently. We have performed smoothed particle hydrodynamic simulations of the infall of a single prolate cloud towards a massive black hole. As the cloud is disrupted, the large spread in angular momentum can, if conditions allow, lead to the creation of misaligned gas disks. In turn, stars may form within those disks. We are now investigating the origins of these clouds in the Galactic center (GC) region.

scholarly journals 7.8. Particle cascades in Sgr A∗: the possibility of observing their γ-ray signature

1998 ◽  
Vol 184 ◽  
pp. 307-308
Author(s):  
Sera Markoff ◽  
Fulvio Melia ◽  
Ina Sarcevic
Keyword(s):  
Black Hole ◽  
Particle Physics ◽  
Galactic Center ◽  
Theoretical Models ◽  
High Energy ◽  
Massive Black Hole ◽  
Inverse Compton Scattering ◽  
Black Hole Candidate ◽  
Γ Ray ◽  
Sgr A

The recent detection of a γ-ray flux from the direction of the Galactic center by EGRET on the Compton GRO raises the question of whether this is a point source (possibly coincident with the massive black hole candidate Sgr A∗) or a diffuse emitter. Using the latest experimental particle physics data and theoretical models, we have examined in detail the γ-ray spectrum produced by synchrotron, inverse Compton scattering and mesonic decay resulting from the interaction of relativistic protons with hydrogen accreting onto a point-like object. Such a distribution of high-energy baryons may be expected to form within an accretion shock as the inflowing gas becomes supersonic. This scenario is motivated by hydrodynamic studies of Bondi-Hoyle accretion onto Sgr A∗, which indicate that many of its radiative characteristics may ultimately be associated with energy liberated as this plasma descends down into the deep potential well. Earlier attempts at analyzing this process concluded that the EGRET data are inconsistent with a massive point-like object (Mastichiadis & Ozernoy, 1994). Our results demonstrate that a more careful treatment of the physics of p-p scattering suggests that a ~ 106M⊙ black hole may be contributing to this high-energy emission.

scholarly journals On the origin of young stars at the Galactic center

2013 ◽  
Vol 9 (S303) ◽  
pp. 238-241
Author(s):  
Ann-Marie Madigan ◽  
Oliver Pfuhl ◽  
Yuri Levin ◽  
Stefan Gillessen ◽  
Reinhard Genzel ◽  
...  
Keyword(s):  
Black Hole ◽  
Galactic Center ◽  
Stellar Populations ◽  
Massive Black Hole ◽  
B Stars ◽  
Orbital Parameters ◽  
Original Distribution ◽  
Orbital Configuration ◽  
Early Type Stars ◽  
Sgr A

AbstractThe center of our Galaxy is home to a massive black hole, Sgr A*, and a nuclear star cluster containing stellar populations of various ages. While the late type stars may be too old to have retained memory of their initial orbital configuration, and hence formation mechanism, the kinematics of the early type stars should reflect their original distribution. In this contribution we present a new statistic which uses directly-observable kinematic stellar data to infer orbital parameters for stellar populations, and is capable of distinguishing between different origin scenarios. We use it on a population of B-stars in the Galactic center that extends out to large radii (∼0.5 pc) from the massive black hole. We find that the high K-magnitude population (≲15 M⊙) form an eccentric distribution, suggestive of a Hills binary-disruption origin.

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