Rapid Variability of Sgr A* across the Electromagnetic Spectrum
G. Witzel, G. Martinez, S. P. Willner, E. E. Becklin, 4 H. Boyce, T. Do, A. Eckart, G. G. Fazio, A. Ghez, M. A. Gurwell, D. Haggard, R. Herrero-Illana, J. L. Ho… 展开作者
G. Witzel, G. Martinez, S. P. Willner, E. E. Becklin, 4 H. Boyce, T. Do, A. Eckart, G. G. Fazio, A. Ghez, M. A. Gurwell, D. Haggard, R. Herrero-Illana, J. L. Hora, Z. Li, J. Liu, N. Marchili, Mark R. Morris, Howard A. Smith, M. Subroweit, J. A. Zensus
备注：
Accepted by ApJS, June 3, 2021. The journal version of figure 21 is animated. The animation can also be found here: https://doi.org/10.17617/1.kctx3s25. This version (version 2) has been revised according to the referee's suggestions and includes optimized figures and some changes and corrections of the text; no scientific conclusions have changed
Sagittarius A* (Sgr A*) is the variable radio, near-infrared (NIR), and X-ray source associated with accretion onto the Galactic center black hole. We have analyzed a comprehensive submillimeter (including new observations simultaneous with NIR monitoring), NIR, and 2-8 keV dataset. Submillimeter variations tend to lag those in the NIR by $\sim$30 minutes. An approximate Bayesian computation (ABC) fit to the X-ray first-order structure function shows significantly less power at short timescales in the X-rays than in the NIR. Less X-ray variability at short timescales combined with the observed NIR-X-ray correlations means the variability can be described as the result of two strictly correlated stochastic processes, the X-ray process being the low-pass-filtered version of the NIR process. The NIR--X-ray linkage suggests a simple radiative model: a compact, self-absorbed synchrotron sphere with high-frequency cutoff close to NIR frequencies plus a synchrotron self-Compton scattering component at higher frequencies. This model, with parameters fit to the submillimeter, NIR, and X-ray structure functions, reproduces the observed flux densities at all wavelengths, the statistical properties of all light curves, and the time lags between bands. The fit also gives reasonable values for physical parameters such as magnetic flux density $B\approx13$ G, source size $L \approx2.2R_{S}$, and high-energy electron density $n_{e}\approx4\times10^{7}$ cm$^{-3}$. An animation illustrates typical light curves, and we make public the parameter chain of our Bayesian analysis, the model implementation, and the visualization code.
Significant X-ray variability and flaring has been observed from Sgr A* but is poorly understood from a theoretical standpoint. We perform GRMHD simulations that take into account a population of non-thermal electrons with energy distributions and injection rates that are motivated by PIC simulations of magnetic reconnection. We explore the effects of including these non-thermal electrons on the predicted broadband variability of Sgr A* and find that X-ray variability is a generic result of localizing non-thermal electrons to highly magnetized regions, where particles are likely to be accelera…
Sagittarius A* (Sgr A*) is the variable radio, near-infrared (NIR), and X-ray source associated with accretion onto the Galactic center black hole. We present an analysis of the most comprehensive NIR variability dataset of Sgr A* to date: eight 24-hour epochs of continuous monitoring of Sgr A* at 4.5 $μ$m with the IRAC instrument on the Spitzer Space Telescope, 93 epochs of 2.18 $μ$m data from Naos Conica at the Very Large Telescope, and 30 epochs of 2.12 $μ$m data from the NIRC2 camera at the Keck Observatory, in total 94,929 measurements. A new approximate Bayesian computation method for fi…
We present the first systematic analysis of the X-ray variability of Sgr A* during the Chandra X-ray Observatory's 2012 Sgr A* X-ray Visionary Project (XVP). With 38 High Energy Transmission Grating Spectrometer (HETGS) observations spaced an average of 7 days apart, this unprecedented campaign enables detailed study of the X-ray emission from this supermassive black hole at high spatial, spectral and timing resolution. In 3 Ms of observations, we detect 39 X-ray flares from Sgr A*, lasting from a few hundred seconds to approximately 8 ks, and ranging in 2-10 keV luminosity from ~1e34 erg/s to…
We present the first detection from the Spitzer Space Telescope of 4.5 micron variability from Sgr A*, the emitting source associated with the Milky Way's central black hole. The >23 hour continuous light curve was obtained with the IRAC instrument in 2013 December. The result characterizes the variability of Sgr A* prior to the closest approach of the G2 object, a putative infalling gas cloud that orbits close to Sgr A*. The high stellar density at the location of Sgr A* produces a background of ~250 mJy at 4.5 microns in each pixel with a large pixel-to-pixel gradient, but the light curve fo…
We explore the variability properties of long, high cadence GRMHD simulations across the electromagnetic spectrum using an efficient, GPU-based radiative transfer algorithm. We focus on both disk- and jet-dominated simulations with parameters that successfully reproduce the time-averaged spectral properties of Sgr A* and the size of its image at 1.3mm. We find that the disk-dominated models produce short timescale variability with amplitudes and power spectra that closely resemble those inferred observationally. In contrast, jet-dominated models generate only slow variability, at lower flux le…
The observable quantities that carry the most information regarding the structures of the images of black holes in the interferometric observations with the Event Horizon Telescope are the closure phases along different baseline triangles. We use long time span, high cadence, GRMHD+radiative transfer models of Sgr A$^*$ to investigate the expected variability of closure phases in such observations. We find that, in general, closure phases along small baseline triangles show little variability, except in the cases when one of the triangle vertices crosses one of a small regions of low visibilit…
Continuously time variable sources are often characterized by their power spectral density and flux distribution. These quantities can undergo dramatic changes over time if the underlying physical processes change. However, some changes can be subtle and not distinguishable using standard statistical approaches. Here, we report a methodology that aims to identify distinct but similar states of time variability. We apply this method to the Galactic supermassive black hole, where 2.2 um flux is observed from a source associated with SgrA*, and where two distinct states have recently been suggest…
We use a model of the molecular gas distribution within ~100 pc from the center of the Milky Way (Kruijssen, Dale & Longmore) to simulate time evolution and polarization properties of the reflected X-ray emission, associated with the past outbursts from Sgr A*. While this model is too simple to describe the complexity of the true gas distribution, it illustrates the importance and power of long-term observations of the reflected emission. We show that the variable part of X-ray emission observed by Chandra and XMM from prominent molecular clouds is well described by a pure reflection model, pr…
We report new infrared measurements of the supermassive black hole at the Galactic Center, Sgr A*, over a decade that was previously inaccessible at these wavelengths. This enables a variability study that addresses variability timescales that are ten times longer than earlier published studies. Sgr A* was initially detected in the near-infrared with adaptive optics observations in 2002. While earlier data exists in form of speckle imaging (1995 - 2005), Sgr A* was not detected in the initial analysis. Here, we improved our speckle holography analysis techniques. This has improved the sensitiv…
We report new observations with the Very Large Array, Atacama Large Millimeter Array, and Submillimeter Array at frequencies from 1.0 to 355 GHz of the Galactic Center black hole, Sagittarius A*. These observations were conducted between October 2012 and November 2014. While we see variability over the whole spectrum with an amplitude as large as a factor of 2 at millimeter wavelengths, we find no evidence for a change in the mean flux density or spectrum of Sgr A* that can be attributed to interaction with the G2 source. The absence of a bow shock at low frequencies is consistent with a cross…
Synthesis imaging of the black hole in the center of the Milky Way, Sgr A*, with the Event Horizon Telescope (EHT) rests on the assumption of a stationary image. We explore the limitations of this assumption using high-cadence GRMHD simulations of Sgr A*. We employ analytic models that capture the basic characteristics of the images to understand the origin of the variability in the simulated visibility amplitudes. We find that, in all simulations, the visibility amplitudes for baselines oriented perpendicular to the spin axis of the black hole typically decrease smoothly over baseline lengths…
We performed the observation of the flux densities of Sgr A* at 90 and 102 GHz on 6 April 2005 using the Nobeyama Millimeter Array in order to detect the time lag between these frequencies. We constructed light curves covering a few hour with 1 min bin, and the Intra-Day Variability, which had a rising phase and intensity peak, of Sgr A* is clearly seen at both frequencies. We calculated the z-transformed discrete correlation function between the light curves of Sgr A* at 90 and 102 GHz. The derived time lag of the flares at these frequencies was approximately zero, contrary to our expectation…
We present results of our dense spectroscopic monitoring of Nova V5668 Sgr. Starting on March 19 in 2015, only a few days after discovery, we have obtained a series of spectra with the TIGRE telescope and its HEROS echelle spectrograph which offers a resolution of R = 20,000 and covers the optical wavelength range from 3800 to 8800 Å. We performed a line identification of the discernible features for four spectra which are representative for the respective phases in the light curve evolution of that nova. By simultaneously analysing the variations in the visual light curve and the correspondin…
For a decade now, evidence has accumulated that giant molecular clouds located within the central molecular zone of our Galaxy reflect X-rays coming from past outbursts of the supermassive black hole Sgr A*. However, the number of illuminating events as well as their ages and durations are still unresolved questions. We aim to reconstruct parts of the history of Sgr A* by studying this reflection phenomenon in the molecular complex Sgr C and by determining the line-of-sight positions of its main bright substructures. Using observations made with XMM-Newton and Chandra between 2000 and 2014, we…
AIMS: A few well studied cataclysmic variables (CVs) have shown discrete characteristic frequencies of fast variability; the most prominent ones are around log(f/Hz) $\simeq$ -3. Because we still have only small number statistics, we obtained a new observation to test whether this is a general characteristic of CVs, especially if mass transfer occurs at a high rate typical for dwarf nova in outbursts, in the so called "high state". METHODS: We analyzed optical Kepler data of the quiescent nova and intermediate polar V4743 Sgr. This system hosts a white dwarf accreting through a disk in the hig…
The near-infrared (NIR) and X-ray emission of Sagittarius A* shows occasional bright flares that are assumed to originate from the innermost region of the accretion flow. We identified $25$ $4.5 μm$ and $24$ X-ray flares in archival data obtained with the \textit{Spitzer} and \textit{Chandra} observatories. With the help of general relativistic ray-tracing code, we modeled trajectories of ``hot spots'' and studied the light curves of the flares for signs of the effects of general relativity. Despite their apparent diversity in shape, all flares share a common, exponential impulse response, a c…
We present deep Hubble Space Telescope near-infrared (NIR) observations of the magnetar SGR 1935+2154 from June 2021, approximately 6 years after the first HST observations, a year after the discovery of fast radio burst like emission from the source, and in a period of exceptional high frequency activity. Although not directly taken during a bursting period the counterpart is a factor of ~1.5 to 2.5 brighter than seen at previous epochs with F140W(AB) = $24.65\pm0.02$ mag. We do not detect significant variations of the NIR counterpart within the course of any one orbit (i.e. on minutes-hour t…
The electromagnetic counterpart to the Galactic center supermassive black hole, Sgr A*, has been observed in the near-infrared for over 20 years and is known to be highly variable. We report new Keck Telescope observations showing that Sgr A* reached much brighter flux levels in 2019 than ever measured at near-infrared wavelengths. In the K$^\prime$ band, Sgr A* reached flux levels of $\sim6$ mJy, twice the level of the previously observed peak flux from $>13,000$ measurements over 130 nights with the VLT and Keck Telescopes. We also observe a factor of 75 change in flux over a 2-hour time spa…
We performed the observation of the flux densities of SgrA* at 90 and 102GHz in order to detect the time lag between these frequencies using the Nobeyama Millimeter Array, which was previously reported at lower frequencies. We detected a radio flare during the observation period on 6 April 2005 and calculated the z-transformed discrete correlation function between the light curves. The time lag between these frequencies was not detected. If the expanding plasma model which explains the time lag at lower frequencies is valid, the light curve at 90GHz would be delayed with respect to the one at …
General relativistic magnetohydrodynamic (GRMHD) simulations of accretion disks and jets associated with supermassive black holes show variability on a wide range of timescales. On timescales comparable to or longer than the gravitational timescale $t_G=GM/c^3$, variation may be dominated by orbital dynamics of the inhomogeneous accretion flow. Turbulent evolution within the accretion disk is expected on timescales comparable to the orbital period, typically an order of magnitude larger than $t_G$. For Sgr A*, $t_G$ is much shorter than the typical duration of a VLBI experiment, enabling us to…
Monitoring of Sagittarius A* from X-ray to radio wavelengths has revealed structured variability --- including X-ray flares --- but it is challenging to establish correlations between them. Most studies have focused on variability in the X-ray and infrared, where variations are often simultaneous, and because long time series at sub-millimeter and radio wavelengths are limited. Previous work on sub-mm and radio variability hints at a lag between X-ray flares and their candidate sub-millimeter or radio counterparts, with the long wavelength data lagging the X-ray. However, there is only one pub…
The accretion flow around the Galactic Center black hole Sagittarius A* (Sgr A*) is expected to have an electron temperature that is distinct from the ion temperature, due to weak Coulomb coupling in the low-density plasma. We present four two-temperature general relativistic radiative magnetohydrodynamic (GRRMHD) simulations of Sgr A* performed with the code KORAL. These simulations use different electron heating prescriptions, motivated by different models of the underlying plasma microphysics. We compare the Landau-damped turbulent cascade model used in previous work with a new prescription…
We report the results of multi-epoch observations of Sgr A* with VLBI Exploration of Radio Astrometry (VERA) at 43 GHz, carried out from 2004 to 2008. We detected a time variation of flux at 11 % level and intrinsic size at 19 %. In addition, comparisons with previous Very Long Baseline Array (VLBA) results shows that Sgr A* underwent the flaring event at least longer than 10 days in May 2007. The intrinsic size of Sgr A* remained unchanged within 1 $σ$ level from the size before/after the flaring event, indicating that the brightness temperature of Sgr A* was increased. The flaring event occu…
We present a comprehensive data description for Ks-band measurements of Sgr A*. We characterize the statistical properties of the variability of Sgr A* in the near-infrared, which we find to be consistent with a single-state process forming a power-law distribution of the flux density. We discover a linear rms-flux relation for the flux-density range up to 12 mJy on a timescale of 24 minutes. This and the power-law flux density distribution implies a phenomenological, formally non-linear statistical variability model with which we can simulate the observed variability and extrapolate its behav…
Emission from Sgr A* is highly variable at both X-ray and infrared (IR) wavelengths. Observations over the last ~20 years have revealed X-ray flares that rise above a quiescent thermal background about once per day, while faint X-ray flares from Sgr A* are undetectable below the constant thermal emission. In contrast, the IR emission of Sgr A* is observed to be continuously variable. Recently, simultaneous observations have indicated a rise in IR flux density around the same time as every distinct X-ray flare, while the opposite is not always true (peaks in the IR emission may not be coinciden…
Extended emission is a mystery in short gamma-ray bursts (SGRBs). By making time resolved spectral analyses of brightest nine events observed by ${\it Swift}$ XRT, we obviously classify the early X-ray emission of SGRBs into two types. One is the extended emission with exponentially rapid decay, which shows significant spectral softening during hundreds seconds since the SGRB trigger and is also detected by ${\it Swift}$-BAT. The other is a dim afterglow only showing power-law decay over $10^4$ s. The correlations between the temporal decay and spectral indices of the extended emissions are in…
The Event Horizon Telescope (EHT) observed the compact radio source, Sagittarius A* (Sgr A*), in the Galactic Center on 2017 April 5-11 in the 1.3 millimeter wavelength band. At the same time, interferometric array data from the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array were collected, providing Sgr A* light curves simultaneous with the EHT observations. These data sets, complementing the EHT very-long-baseline interferometry, are characterized by a cadence and signal-to-noise ratio previously unattainable for Sgr A* at millimeter wavelengths, and they allow for …
Sgr A*, the supermassive black hole (SMBH) at the center of our Milky Way Galaxy, is known to be a variable source of X-ray, near-infrared (NIR), and submillimeter (submm) radiation and therefore a prime candidate to study the electromagnetic radiation generated by mass accretion flow onto a black hole and/or a related jet. Disentangling the power source and emission mechanisms of this variability is a central challenge to our understanding of accretion flows around SMBHs. Simultaneous multiwavelength observations of the flux variations and their time correlations can play an important role in…
In 2019, Sgr A* - the supermassive black hole in the Galactic Center - underwent unprecedented flaring activity in the near infrared (NIR), brightening by up to a factor of 100 compared to quiescent values. Here we report ALMA observations of Sgr A*'s continuum variability at 1.3 mm (230 GHz) - a tracer of the accretion rate - conducted one month after the brightest detected NIR flare and in the middle of the flaring activity of 2019. We develop an innovative light curve extraction technique which (together with ALMA's excellent sensitivity) allows us to obtain the light curves which are simul…
Variable emission from Sgr~A*, the luminous counterpart to the super-massive black hole at the center of our Galaxy, arises from the innermost portions of the accretion flow. Better characterization of the variability is important for constraining models of the low-luminosity accretion mode powering Sgr~A*, and could further our ability to use variable emission as a probe of the strong gravitational potential in the vicinity of the $4\times10^{6}\mathrm{M}_{\odot}$ black hole. We use the \textit{Herschel} Spectral and Photometric Imaging Receiver (SPIRE) to monitor Sgr~A* at wavelengths that a…
Together, the Fermi-LAT and HESS have revealed the presence of an unusual GeV-TeV source coincident with Sgr A* at the Galactic center. Its high-energy emission appears to be bimodal, hinting at an energizing process more sophisticated than mere shock acceleration. It has been suggested that this may be evidence of strong, rapid variability, as required if Sgr A*'s emission were responsible for the fluorescent X-ray echos detected in nearby molecular clouds. In this {\it Letter}, however, we show that stochastic acceleration in a more realistic two-phase environment surrounding the central bla…
We present radio continuum light curves of the magnetar SGR J1745$-$2900 and Sgr A* obtained with multi-frequency, multi-epoch Very Large Array observations between 2012 and 2014. During this period, a powerful X-ray outburst from SGR J1745$-$2900 occurred on 2013-04-24. Enhanced radio emission is delayed with respect to the X-ray peak by about seven months. In addition, the flux density of the emission from the magnetar fluctuates by a factor of 2 to 4 at frequencies between 21 and 41 GHz and its spectral index varies erratically. Here we argue that the excess fluctuating emission from the ma…
Aims. We report on simultaneous observations and modeling of mid-infrared (MIR), near-infrared (NIR), and submillimeter (submm) emission of the source Sgr A* associated with the supermassive black hole at the center of our Galaxy. Our goal was to monitor the activity of Sgr A* at different wavelengths in order to constrain the emitting processes and gain insight into the nature of the close environment of Sgr A*. Methods. We used the MIR instrument VISIR in the BURST imaging mode, the adaptive optics assisted NIR camera NACO, and the sub-mm antenna APEX to monitor Sgr A* over several nights in…
On 28 May 2008, the Swift satellite detected the first reactivation of SGR 1627-41 since its discovery in 1998. Following this event we began an observing campaign in near infrared wavelengths to search for a possible counterpart inside the error circle of this SGR, which is expected to show flaring activity simultaneous to the high energy flares or at least some variability as compared to the quiescent state. For the follow-up we used the 0.6m REM robotic telescope at La Silla Observatory, which allowed a fast response within 24 hours and, through director discretionary time, the 8.2m Very La…
Galactic Center (GC) molecular cloud Sgr B2 is the best manifestation of an X-ray reflection nebula (XRN) reprocessing a past giant outburst from the supermassive black hole Sgr A*. Alternatively, Sgr B2 could be illuminated by low-energy cosmic ray electrons (LECRe) or protons (LECRp). In 2013, NuSTAR for the first time resolved Sgr B2 hard X-ray emission on sub-arcminute scales. Two prominent features are detected above 10 keV - a newly emerging cloud G0.66-0.13 and the central 90" radius region containing two compact cores Sgr B2(M) and Sgr B2(N) surrounded by diffuse emission. It is inconc…
Here we present stringent low-frequency 185MHz limits on coherent radio emission associated with a short gamma-ray burst (SGRB). Our observations of the short GRB 180805A were taken with the upgraded Murchison Widefield Array (MWA) rapid-response system, which triggered within 20s of receiving the transient alert from Swift, corresponding to 83.7s post-burst. The SGRB was observed for 30m, resulting in a 3sigma persistent flux density upper-limit of 40.2mJy/beam. Transient searches were conducted at the Swift position of this GRB on 0.5s, 5s, 30s, and 2m timescales, resulting in 3sigma limits …
The compact radio source at the center of our Galaxy, Sagittarius A* (Sgr A*), is the subject of intensive study as it provides a close-up view of an accreting supermassive black hole. Sgr A* provides us with a prototype of a low-luminosity active galactic nucleus (LLAGN), but interstellar scattering and the resolution limits of our instruments have limited our understanding of the emission sites in its inner accretion flow. The temporal variability of Sgr A* can help us understand whether we see a plasma outflow or inflow in the region close to the black hole. In this work, we look at a compr…
Soft gamma-ray repeaters and anomalous X-ray pulsars are a small (but growing) group of X-ray sources characterized by the emission of short bursts and by a large variability in their persistent flux. They are believed to be magnetars, i.e. neutron stars powered by extreme magnetic fields 1E14-1E15 G). We found evidence for a magnetar with a low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of soft gamma-ray repeaters. New X-ray observations show that its dipolar magnetic field cannot be greater than 8E12 G, well in the range of ordinary radio pulsar…
The recent discovery of the G2 cloud of dense, ionized gas on a trajectory toward Sgr A*, the black hole at the dynamical center of the Galaxy, offers a unique opportunity to observe an accretion event onto a massive black hole as well as to probe its immediate environment. Simulations and models predict increased X-ray and radio variability resulting from increased accretion driven by drag on an atmosphere of hot, X-ray emitting gas surrounding Sgr A*. Here, we present X-ray and radio light curves of the emission resulting from the potential encounter of the G2 cloud with a relativistic jet f…
Active Galactic Nuclei (AGN) are known to show significant variability over a wide frequency range. We review observational results on the variability characteristics of blazars in the very high energy (VHE) domain, focusing on recent findings of rapid VHE variability and evidence for an underlying multiplicative driving process in PKS 2155-304. We explore a physical scenario where the variability is assumed to arise due to accretion disk fluctuations transmitted to the jet, and discuss its implications for the central powerhouse.
Measuring the Black Hole and Accretion Parameters of Sagittarius A* from EHT Observations using a Semi-Analytic Model
University of Maryland, College Park（马里兰大学帕克分校）；Black Hole Initiative at Harvard University（哈佛大学黑洞倡议）；Department of Physics, Harvard University（哈佛大学物理系）
Nonthermal hotspot orbiting the supermassive black hole Sgr A*
Instituto de Astronomía, Universidad Nacional Autónoma de México（墨西哥国立自治大学天文研究所）；Instituto Nacional de Astrofísica, Óptica y Electrónica（墨西哥国家天文学、光学和电子学研究所）；Tsung-Dao Lee Institute, Shanghai Jiao Tong University（上海交通大学李政道研究所）；School of Physics and Astronomy, Shanghai Jiao Tong University（上海交通大学物理与天文学院）
We examine the low angular momentum flow model for Sgr A* using two-dimensional hydrodynamical calculations based on the parameters of the specific angular momentum and total energy estimated in the recent analysis of stellar wind of nearby stars around Sgr A*. The accretion flow with the plausible parameters is non-stationary and an irregularly oscillating shock is formed in the inner region of a few tens to a hundred and sixty Schwarzschild radii. Due to the oscillating shock, the luminosity and the mass-outflow rate are modulated by several per cent to a factor of 5 and a factor of 2-7, res…
Variability of Sagittarius A* at 3 GHz on minute-scale with MeerKAT
Max-Planck-Institut für Radioastronomie（马克斯·普朗克射电天文研究所）；Argelander Institute for Astronomy, University of Bonn（波恩大学阿尔冈德天体物理研究所）；Deutsches Zentrum für Astrophysik（德国天体物理中心）；Thüringer Landessternwarte（图林根州立天文台）；Instituto de Astrofísica de Andalucía-CSIC（安达卢西亚天体物理研究所（CSIC））；Center for Astrophysics | Harvard & Smithsonian（哈佛史密森尼天体物理中心）；South African Radio Astronomy Observatory（南非射电天文观测站）
The Event Horizon Telescope (EHT) is a millimeter-wavelength, very-long-baseline interferometry (VLBI) experiment that is capable of observing black holes with horizon-scale resolution. Early observations have revealed variable horizon-scale emission in the Galactic Center black hole, Sagittarius A* (Sgr A*). Comparing such observations to time-dependent general relativistic magnetohydrodynamic (GRMHD) simulations requires statistical tools that explicitly consider the variability in both the data and the models. We develop here a Bayesian method to compare time-resolved simulation images to v…
The centre of our Galaxy harbours a 4 million solar mass black hole that is unusually quiet: its present X-ray luminosity is more than 10 orders of magnitude less than its Eddington luminosity. The observation of iron fluorescence and hard X-ray emission from some of the massive molecular clouds surrounding the Galactic Centre has been interpreted as an echo of a past flare. Alternatively, low-energy cosmic rays propagating inside the clouds might account for the observed emission, through inverse bremsstrahlung of low energy ions or bremsstrahlung emission of low energy electrons. Here we rep…
We analyze the two brightest Chandra X-ray flares detected from Sagittarius A*, with peak luminosities more than 600 x and 245 x greater than the quiescent X-ray emission. The brightest flare has a distinctive double-peaked morphology --- it lasts 5.7 ksec ($\sim 2$ hours), with a rapid rise time of 1500 sec and a decay time of 2500 sec. The second flare lasts 3.4 ksec, with rise and decay times of 1700 sec and 1400 sec. These luminous flares are significantly harder than quiescence: the first has a power law spectral index $Γ= 2.06\pm 0.14$ and the second has $Γ= 2.03\pm 0.27$, compared to $Γ…
Infrared Line Diagnostics Fail to Constrain Sgr A*'s UV Output
McGill University（麦吉尔大学）；Trottier Space Institute at McGill（麦吉尔大学特罗蒂尔太空研究所）；University of Toronto（多伦多大学）；Canadian Institute for Theoretical Astrophysics（加拿大理论天体物理研究所）；Max Planck Institute for Radioastronomy（马克斯·普朗克射电天文研究所）
Over the last decade, X-ray observations of Sgr A* have revealed a black hole in a deep sleep, punctuated roughly once per day by brief flares. The extreme X-ray faintness of this supermassive black hole has been a long-standing puzzle in black hole accretion. To study the accretion processes in the Galactic Center, Chandra (in concert with numerous ground- and space-based observatories) undertook a 3 Ms campaign on Sgr A* in 2012. With its excellent observing cadence, sensitivity, and spectral resolution, this Chandra X-ray Visionary Project (XVP) provides an unprecedented opportunity to stud…
The fluorescent X-ray emission from neutral iron in the molecular clouds (Sgr B) indicates that the clouds are being irradiated by an external X-ray source. The source is probably associated with the Galactic central black hole (Sgr A*), which triggered a bright outburst one hundred years ago. We suggest that such an outburst could be due to a partial capture of a star by Sgr A*, during which a jet was generated. By constraining the observed flux and the time variability ($\sim$ 10 years) of the Sgr B's fluorescent emission, we find that the shock produced by the interaction of the jet with th…
Sagittarius A* exhibits regular variability in its multiwavelength emission, including daily X-ray flares and roughly continuous near-infrared (NIR) flickering. The origin of this variability is still ambiguous since both inverse Compton and synchrotron emission are possible radiative mechanisms. The underlying particle distributions are also not well constrained, particularly the non-thermal contribution. In this work, we employ the GPU-accelerated general relativistic magnetohydrodynamics (GRMHD) code H-AMR perform a study of flare flux distributions, including the effect of particle acceler…
