CONSTRAINING THE FLARING REGION FOR SAGITTARIUS A* BY 1.3MM VLBI MEASUREMENTS

Recent VLBI observations have resolved Sagittarius A* at horizon scales. The event horizon telescope is expected to provide increasingly good images of the region around the Schwarzschild radius [Formula: see text] of Sgr A* soon. A number of authors have recently pointed out the possibility that nonperturbative quantum gravitational phenomena could affect the space surrounding a black hole. Here, we point out that the existence of a region around [Formula: see text], where these effects should be maximal.

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The reflection of two past outbursts of Sagittarius A* observed by Chandra during the last decade

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314000891 ◽

2013 ◽

Vol 9 (S303) ◽

pp. 344-348

Author(s):

M. Clavel ◽

R. Terrier ◽

A. Goldwurm ◽

M. R. Morris ◽

G. Ponti ◽

...

Keyword(s):

Black Hole ◽

Galactic Center ◽

Line Of Sight ◽

High Resolution Data ◽

The Past ◽

Sagittarius A ◽

Cloud Parameters ◽

Angular Scale ◽

Sgr A ◽

First Time

AbstractThe supermassive black hole at the Galactic center, Sagittarius A* has experienced periods of higher activity in the past. The reflection of these past outbursts is observed in the molecular material surrounding the black hole but reconstructing its precise lightcurve is difficult since the distribution of the clouds along the line of sight is poorly constrained.Using Chandra high-resolution data collected from 1999 to 2011 we studied both the 6.4 keV and the 4–8 keV emission of the region located between Sgr A* and the Radio arc, characterizing its variations down to 15″ angular scale and 1-year time scale. The emission from the molecular clouds in the region varies significantly, showing either a 2-year peaked emission or 10-year linear variations. This is the first time that such fast variations are measured. Based on the cloud parameters, we conclude that these two behaviors are likely due to two distinct past outbursts of Sgr A* during which its luminosity rose to at least 1039 erg s−1.

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Search for time lag in intra-day variability of Sgr A*

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314000842 ◽

2013 ◽

Vol 9 (S303) ◽

pp. 322-323

Author(s):

A. Miyazaki ◽

S. S. Lee ◽

B. W. Sohn ◽

T. Jung ◽

M. Tsuboi ◽

...

Keyword(s):

Black Hole ◽

Galactic Center ◽

Time Lag ◽

Light Curves ◽

Time Lags ◽

Significant Time ◽

Sagittarius A ◽

Receiver System ◽

Sgr A ◽

Plasma Blobs

AbstractWe searched the time lag between the intra-day variables (IDVs) of Sagittarius A* at 22, 43, and 86 GHz bands using the Korean VLBI Network (KVN). The time lags between the IDV flare peaks at 22 and 43 GHz are reported, and they suggest that the flare emissions come from adiabatically expanding plasma blobs, ejected close to the Galactic center black hole. We searched the time lags between light curves at 90 and 102 GHz using the Nobeyama Millimeter Array, but could not find significant time lags. In order to detect the diversity of the time lags of Sgr A* flares, we performed observations of Sgr A* in the 22, 43, and 86 GHz bands using the KVN in the winter of 2013. Because the receiver system of KVN can observe Sgr A* in these three bands simultaneously, the KVN is very useful to detect the time lags of Sgr A* flares.

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Event horizon scale emission models for Sagittarius A*

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314000775 ◽

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.

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Near infrared variability of Sgr A* - spectral index measurements

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314000738 ◽

2013 ◽

Vol 9 (S303) ◽

pp. 274-282 ◽

Cited By ~ 2

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.

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

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937136 ◽

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.

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Simulations of imaging the event horizon of Sagittarius A* from space

Astronomy and Astrophysics ◽

10.1051/0004-6361/201732423 ◽

2019 ◽

Vol 625 ◽

pp. A124 ◽

Cited By ~ 18

Author(s):

Freek Roelofs ◽

Heino Falcke ◽

Christiaan Brinkerink ◽

Monika Mościbrodzka ◽

Leonid I. Gurvits ◽

...

Keyword(s):

General Relativity ◽

Black Hole ◽

Event Horizon ◽

Signal To Noise Ratio ◽

Satellite Orbits ◽

Tests Of General Relativity ◽

Sagittarius A ◽

Long Baseline ◽

Sgr A ◽

Long Baselines

Context. It has been proposed that Very Long Baseline Interferometry (VLBI) at submillimeter waves will allow us to image the shadow of the black hole in the center of our Milky Way, Sagittarius A* (Sgr A*), and thereby test basic predictions of the theory of general relativity. Aims. This paper presents imaging simulations of a new Space VLBI (SVLBI) mission concept. An initial design study of the concept has been presented in the form of the Event Horizon Imager (EHI). The EHI may be suitable for imaging Sgr A* at high frequencies (up to ∼690 GHz), which has significant advantages over performing ground-based VLBI at 230 GHz. The concept EHI design consists of two or three satellites in polar or equatorial circular medium-Earth orbits (MEOs) with slightly different radii. Due to the relative drift of the satellites along the individual orbits over the course of several weeks, this setup will result in a dense spiral-shaped uv-coverage with long baselines (up to ∼60 Gλ), allowing for extremely high-resolution and high-fidelity imaging of radio sources. Methods. We simulated observations of general relativistic magnetohydrodynamics (GRMHD) models of Sgr A* for the proposed configuration and calculate the expected noise based on preliminary system parameters. On long baselines, where the signal-to-noise ratio may be low, fringes could be detected assuming that the system is sufficiently phase stable and the satellite orbits can be reconstructed with sufficient accuracy. Averaging visibilities accumulated over multiple epochs of observations could then help improving the image quality. With three satellites instead of two, closure phases could be used for imaging. Results. Our simulations show that the EHI could be capable of imaging the black hole shadow of Sgr A* with a resolution of 4 μas (about 8% of the shadow diameter) within several months of observing time. Conclusion. Our preliminary study of the EHI concept shows that it is potentially of high scientific value. It could be used to measure black hole shadows much more precisely than with ground-based VLBI, allowing for stronger tests of general relativity and accretion models.

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Statistical and theoretical studies of flares from Sagittarius A⋆

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316012540 ◽

2016 ◽

Vol 11 (S322) ◽

pp. 31-38

Author(s):

Ya-Ping Li ◽

Qiang Yuan ◽

Q. Daniel Wang ◽

P. F. Chen ◽

Joseph Neilsen ◽

...

Keyword(s):

Black Hole ◽

Solar Flares ◽

Galactic Center ◽

Theoretical Models ◽

Spatial Dimension ◽

X Ray ◽

Sagittarius A ◽

Multi Wavelength ◽

Self Organized Criticality ◽

Sgr A

AbstractMulti-wavelength flares have routinely been observed from the supermassive black hole, Sagittarius A⋆ (Sgr A⋆), at our Galactic center. The nature of these flares remains largely unclear, despite many theoretical models. We study the statistical properties of the Sgr A⋆ X-ray flares and find that they are consistent with the theoretical prediction of the self-organized criticality system with the spatial dimension S = 3. We suggest that the X-ray flares represent plasmoid ejections driven by magnetic reconnection (similar to solar flares) in the accretion flow onto the black hole. Motivated by the statistical results, we further develop a time-dependent magnetohydrodynamic (MHD) model for the multi-band flares from Sgr A⋆ by analogy with models of solar flares/coronal mass ejections (CMEs). We calculate the X-ray, infrared flare light curves, and the spectra, and find that our model can explain the main features of the flares.

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Observations of NIR polarized light from Sagittarius A*

Proceedings of the International Astronomical Union ◽

10.1017/s174392131400074x ◽

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.

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