scholarly journals Short-term variability of Sgr A*

2006 ◽  
Vol 2 (S238) ◽  
pp. 195-200
Author(s):  
M. R. Morris ◽  
S. D. Hornstein ◽  
A. M. Ghez ◽  
J. R. Lu ◽  
K. Matthews ◽  
...  
Keyword(s):  
Time Scale ◽  
Near Infrared ◽  
Accretion Rate ◽  
Outer Boundary ◽  
Radial Distance ◽  
Emission Region ◽  
Sagittarius A ◽  
Dynamical Time ◽  
Galactic Black Hole ◽  
Sgr A

AbstractObservations of Sgr A* over the past 4 years with the Keck Telescope in the near-infrared, coupled with millimeter and submillimeter observations, show that the 3.7×106M⊙Galactic Black Hole, Sagittarius A*, displays continuous variability at all these wavelengths, with the variability power concentrated on characteristic time scales of a few hours, and with a variability fraction that increases with wavelength. We review the observations indicating that the few-hour time scale for variability is reproduced at all accessible wavelengths. Interpreted as a dynamical time, this time scale corresponds to a radial distance of 2 AU, or ∼25 Schwarzschild radii. Searches for quasi-periodicities in the near-infrared data from the Keck Telescope have so far been negative. One interpretation of the character of these variations is that they result from a recurring disk instability, rather than from variations in the mass accretion rate flowing through the outer boundary of the emission region. However, neither a variable accretion rate nor a mechanism associated with a jet can presently be ruled out.

scholarly journals Spectral and imaging properties of Sgr A* from high-resolution 3D GRMHD simulations with radiative cooling

2020 ◽  
Vol 499 (3) ◽  
pp. 3178-3192
Author(s):  
D Yoon ◽  
K Chatterjee ◽  
S B Markoff ◽  
D van Eijnatten ◽  
Z Younsi ◽  
...  
Keyword(s):  
Accretion Rate ◽  
Dynamical Evolution ◽  
Radiative Properties ◽  
Spectral Energy ◽  
Radiative Cooling ◽  
Galactic Centre ◽  
Accretion Flow ◽  
Sagittarius A ◽  
Sgr A ◽  
The Impact

ABSTRACT The candidate supermassive black hole in the Galactic Centre, Sagittarius A* (Sgr A*), is known to be fed by a radiatively inefficient accretion flow (RIAF), inferred by its low accretion rate. Consequently, radiative cooling has in general been overlooked in the study of Sgr A*. However, the radiative properties of the plasma in RIAFs are poorly understood. In this work, using full 3D general–relativistic magnetohydrodynamical simulations, we study the impact of radiative cooling on the dynamical evolution of the accreting plasma, presenting spectral energy distributions and synthetic sub-millimetre images generated from the accretion flow around Sgr A*. These simulations solve the approximated equations for radiative cooling processes self-consistently, including synchrotron, bremsstrahlung, and inverse Compton processes. We find that radiative cooling plays an increasingly important role in the dynamics of the accretion flow as the accretion rate increases: the mid-plane density grows and the infalling gas is less turbulent as cooling becomes stronger. The changes in the dynamical evolution become important when the accretion rate is larger than $10^{-8}\, M_{\odot }~{\rm yr}^{-1}$ ($\gtrsim 10^{-7} \dot{M}_{\rm Edd}$, where $\dot{M}_{\rm Edd}$ is the Eddington accretion rate). The resulting spectra in the cooled models also differ from those in the non-cooled models: the overall flux, including the peak values at the sub-mm and the far-UV, is slightly lower as a consequence of a decrease in the electron temperature. Our results suggest that radiative cooling should be carefully taken into account in modelling Sgr A* and other low-luminosity active galactic nuclei that have a mass accretion rate of $\dot{M} \gt 10^{-7}\, \dot{M}_{\rm Edd}$.

scholarly journals The infrared K-band identification of the DSO/G2 source from VLT and Keck data

2013 ◽  
Vol 9 (S303) ◽  
pp. 269-273
Author(s):  
A. Eckart ◽  
M. Horrobin ◽  
S. Britzen ◽  
M. Zamaninasab ◽  
K. Mužić ◽  
...  
Keyword(s):  
Fast Moving ◽  
Near Infrared ◽  
Dust Cloud ◽  
Continuum Emission ◽  
Proper Motions ◽  
Infrared Excess ◽  
Sagittarius A ◽  
Band Emission ◽  
Sgr A ◽  
Data Result

AbstractA fast moving infrared excess source (G2) which is widely interpreted as a core-less gas and dust cloud approaches Sagittarius A* (Sgr A*) on a presumably elliptical orbit. VLT Ks-band and Keck K′-band data result in clear continuum identifications and proper motions of this ∼19m Dusty S-cluster Object (DSO). In 2002-2007 it is confused with the star S63, but free of confusion again since 2007. Its near-infrared (NIR) colors and a comparison to other sources in the field speak in favor of the DSO being an IR excess star with photospheric continuum emission at 2 microns than a core-less gas and dust cloud. We also find very compact L′-band emission (<0.1″) contrasted by the reported extended (0.03″ up to ∼0.2″ for the tail) Brγ emission. The presence of a star will change the expected accretion phenomena, since a stellar Roche lobe may retain a fraction of the material during and after the peri-bothron passage.

scholarly journals Event horizon scale emission models for Sagittarius A*

2013 ◽  
Vol 9 (S303) ◽  
pp. 298-302
Author(s):  
J. Dexter
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Event Horizon ◽  
Near Infrared ◽  
Sagittarius A ◽  
Long Baseline ◽  
Black Hole Accretion ◽  
Emission Models ◽  
Infrared Interferometry ◽  
Sgr A

AbstractVery long baseline interferometry observations at millimeter wavelengths have detected source structure in Sgr A* on event horizon scales. Near-infrared interferometry will achieve similar resolution in the next few years. These experiments provide an unprecedented opportunity to explore strong gravity around black holes, but interpreting the data requires physical modeling. I discuss the calculation of images, spectra, and light curves from relativistic MHD simulations of black hole accretion. The models provide an excellent description of current observations, and predict that we may be on the verge of detecting a black hole shadow, which would constitute the first direct evidence for the existence of black holes.

scholarly journals Sgr A* near-infrared flares from reconnection events in a magnetically arrested disc

2020 ◽  
Vol 497 (4) ◽  
pp. 4999-5007 ◽  
Author(s):  
J Dexter ◽  
A Tchekhovskoy ◽  
A Jiménez-Rosales ◽  
S M Ressler ◽  
M Bauböck ◽  
...  
Keyword(s):  
Black Hole ◽  
Magnetic Flux ◽  
Linear Polarization ◽  
Near Infrared ◽  
Broad Class ◽  
Emission Region ◽  
Polarization Angle ◽  
Continuous Rotation ◽  
General Relativistic ◽  
Sgr A

ABSTRACT Large-amplitude Sgr A* near-infrared (NIR) flares result from energy injection into electrons near the black hole event horizon. Astrometry data show continuous rotation of the emission region during bright flares, and corresponding rotation of the linear polarization angle. One broad class of physical flare models invokes magnetic reconnection. Here, we show that such a scenario can arise in a general relativistic magnetohydrodynamic simulation of a magnetically arrested disc. Saturation of magnetic flux triggers eruption events, where magnetically dominated plasma is expelled from near the horizon and forms a rotating, spiral structure. Dissipation occurs via reconnection at the interface of the magnetically dominated plasma and surrounding fluid. This dissipation is associated with large increases in NIR emission in models of Sgr A*, with durations and amplitudes consistent with the observed flares. Such events occur at roughly the time-scale to re-accumulate the magnetic flux from the inner accretion disc, ≃10 h for Sgr A*. We study NIR observables from one sample event to show that the emission morphology tracks the boundary of the magnetically dominated region. As the region rotates around the black hole, the NIR centroid and linear polarization angle both undergo continuous rotation, similar to the behaviour seen in Sgr A* flares.

scholarly journals Diameter and Proper Motion of Sgr A∗

1996 ◽  
Vol 169 ◽  
pp. 193-198
Author(s):  
D. C. Backer
Keyword(s):  
Energy Source ◽  
Accretion Rate ◽  
Model Parameters ◽  
Quiescent State ◽  
Massive Black Hole ◽  
General Hypothesis ◽  
Sagittarius A ◽  
Nonthermal Radio ◽  
Sgr A ◽  
Ultimate Fate

The best hypothesis for the energy source of the compact, nonthermal radio source in the center of our galaxy, Sagittarius A∗ (Sgr A∗), is accretion onto a massive black hole from winds emanating from nearby luminous stars. The hole mass, accretion rate, and ultimate fate of accreted matter are uncertain. In this report I give a summary of recent critical observations. The interpretation of these results supports this general hypothesis, and begins to place constraints on model parameters. If so, then Sgr A∗ is a miniature version of extragalactic AGNs in a quiescent state.

scholarly journals Near infrared variability of Sgr A* - spectral index measurements

2013 ◽  
Vol 9 (S303) ◽  
pp. 274-282 ◽  
Author(s):  
G. Witzel ◽  
M. Morris ◽  
A. Ghez ◽  
L. Meyer ◽  
E. Becklin ◽  
...  
Keyword(s):  
Spectral Index ◽  
Near Infrared ◽  
Time Lag ◽  
Time Variable ◽  
Time Variability ◽  
Data Set ◽  
Sagittarius A ◽  
State Process ◽  
Sgr A ◽  
High Cadence

AbstractWe discuss observations of Sagittarius A* with NACO@VLT in K-band and recent synchronous observations with NIRC2@Keck II and OSIRIS@Keck I in L′-band and H-band, respectively. The variability of Sagittarius A* in the near infrared is a continuous one-state process that can be described by a pure red-noise process having a timescale of a few hours. We describe this process and its properties in detail. Our newest observations with the Keck telescopes represent the first truly synchronous high cadence data set to test for time variability of the spectral index within the near infrared. We discovered a time-variable spectral index that might be interpreted as a time lag of the L′-band with respect to the H-band.

scholarly journals Continuation of the X-ray monitoring of Sgr A*: the increase in bright flaring rate confirmed

2020 ◽  
Vol 636 ◽  
pp. A25
Author(s):  
E. Mossoux ◽  
B. Finociety ◽  
J.-M. Beckers ◽  
F. H. Vincent
Keyword(s):  
Black Hole ◽  
Near Infrared ◽  
Milky Way ◽  
Detection Efficiency ◽  
Previous Method ◽  
Arrival Times ◽  
X Ray ◽  
Sagittarius A ◽  
Block Algorithm ◽  
Sgr A

Context. The supermassive black hole Sagittarius A* (Sgr A*) is located at the dynamical center of the Milky Way. In a recent study of the X-ray flaring activity from Sgr A* using Chandra, XMM-Newton, and Swift observations from 1999 to 2015, it has been argued that the bright flaring rate has increased from 2014 August 31 while the faint flaring rate decreased from around 2013 August. Aims. We tested the persistence of these changes in the flaring rates with new X-ray observations of Sgr A* performed from 2016 to 2018 (total exposure of 1.4 Ms). Methods. We reprocessed the Chandra, XMM-Newton, and Swift observations from 2016 to 2018. We detected 9 flares in the Chandra data and 5 flares in the Swift data that we added to the set of 107 previously detected flares. We computed the intrinsic distribution of flare fluxes and durations corrected for the sensitivity bias using a new method that allowed us to take the error on the flare fluxes and durations into account. From this intrinsic distribution, we determined the average flare detection efficiency for each Chandra, XMM-Newton, and Swift observation. After correcting each observational exposure for this efficiency, we applied the Bayesian blocks algorithm on the concatenated flare arrival times. As in the above-mentioned study, we also searched for a flux and fluence threshold that might lead to a change in flaring rate. We improved the previous method by computing the average flare detection efficiencies for each flux and fluence range. Results. The Bayesian block algorithm did not detect any significant change in flaring rate of the 121 flares. However, we detected an increase by a factor of about three in the flaring rate of the most luminous and most energetic flares that have occurred since 2014 August 30. Conclusions. The X-ray activity of Sgr A* has increased for more than four years. Additional studies about the overall near-infrared and radio behavior of Sgr A* are required to draw strong results on the multiwavelength activity of the black hole.

scholarly journals Dynamically important magnetic fields near the event horizon of Sgr A*

2020 ◽  
Vol 643 ◽  
pp. A56
Author(s):  
◽  
A. Jiménez-Rosales ◽  
J. Dexter ◽  
F. Widmann ◽  
M. Bauböck ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Black Hole ◽  
Near Infrared ◽  
Emission Region ◽  
Compact Region ◽  
Background Magnetic Field ◽  
Field Geometry ◽  
Sgr A ◽  
The One ◽  
Linear Polarisation

We study the time-variable linear polarisation of Sgr A* during a bright near-infrared flare observed with the GRAVITY instrument on July 28, 2018. Motivated by the time evolution of both the observed astrometric and polarimetric signatures, we interpret the data in terms of the polarised emission of a compact region (“hotspot”) orbiting a black hole in a fixed, background magnetic field geometry. We calculated a grid of general relativistic ray-tracing models, created mock observations by simulating the instrumental response, and compared predicted polarimetric quantities directly to the measurements. We take into account an improved instrument calibration that now includes the instrument’s response as a function of time, and we explore a variety of idealised magnetic field configurations. We find that the linear polarisation angle rotates during the flare, which is consistent with previous results. The hotspot model can explain the observed evolution of the linear polarisation. In order to match the astrometric period of this flare, the near horizon magnetic field is required to have a significant poloidal component, which is associated with strong and dynamically important fields. The observed linear polarisation fraction of ≃30% is smaller than the one predicted by our model (≃50%). The emission is likely beam depolarised, indicating that the flaring emission region resolves the magnetic field structure close to the black hole.

scholarly journals Observations of NIR polarized light from Sagittarius A*

2013 ◽  
Vol 9 (S303) ◽  
pp. 283-287
Author(s):  
B. Shahzamanian ◽  
A. Eckart ◽  
M. Zamaninasab ◽  
G. Witzel ◽  
N. Sabha
Keyword(s):  
Black Hole ◽  
Adaptive Optics ◽  
Near Infrared ◽  
Polarized Light ◽  
Time Interval ◽  
Physical Conditions ◽  
Supermassive Black Hole ◽  
Sagittarius A ◽  
Linearly Polarized ◽  
Sgr A

AbstractWe present a brief overview of results obtained from near-infrared polarized observations of Sgr A*, which is associated with the supermassive black hole at the center of the Milky Way. The observations have been carried out using the NACO adaptive optics instrument at the VLT UT4 in the infrared Ks-band from 2004 to 2012. Several polarized flares have been observed in this time interval which allow us to determine the statistical properties of NIR linearly polarized light from Sgr A*. Linear polarization at 2.2 μm and its variations can help us to constrain the physical conditions of the accretion process around this supermassive black hole.

scholarly journals NIR triggered observations of Sgr A* at 43 GHz

2016 ◽  
Vol 11 (S322) ◽  
pp. 52-53
Author(s):  
C. Rauch ◽  
E. Ros ◽  
T. P. Krichbaum ◽  
A. Eckart ◽  
J. A. Zensus ◽  
...  
Keyword(s):  
Time Delay ◽  
Near Infrared ◽  
Position Angle ◽  
Angular Distance ◽  
Quiescent State ◽  
Adiabatic Expansion ◽  
Core Component ◽  
Sagittarius A ◽  
Expected Values ◽  
Sgr A

AbstractThe compact radio and near-infrared (NIR) source Sagittarius A* has been observed in the context of two NIR triggered global VLT and VLBA campaigns at 43 GHz (7 mm) on May 16-18 2012 and October 4 2014. While on October 4 2014 Sgr A* remained in a quiescent state, a NIR flare on May 17 2012 is accompanied by an increase in flux density of 0.22 Jy at 7 mm delayed by 4.5±0.5 h. Additionally, Sgr A* seems to develop a weak secondary radio off-core component of 0.02 Jy at a position angle of 140° and an angular distance of 1.5 mas shortly before the peak of the flare. This spatial extension and the time delay are in the range of expected values for events casually connected by adiabatic expansion.

Sign in / Sign up

Export Citation Format

Share Document