Most supermassive black holes (SMBHs) are accreting at very low levels and are difficult to distinguish from the galaxy centers where they reside. Our own Galaxy's SMBH provides an instructive ...exception, and we present a close-up view of its quiescent x-ray emission based on 3 megaseconds of Chandra observations. Although the x-ray emission is elongated and aligns well with a surrounding disk of massive stars, we can rule out a concentration of low-mass coronally active stars as the origin of the emission on the basis of the lack of predicted iron (Fe) Kα emission. The extremely weak hydrogen (H)—like Fe Kα line further suggests the presence of an outflow from the accretion flow onto the SMBH. These results provide important constraints for models of the prevalent radiatively inefficient accretion state.
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 data set of Sgr A* to date: eight 24 hr epochs of continuous monitoring of Sgr A* at 4.5 m with the IRAC instrument on the Spitzer Space Telescope, 93 epochs of 2.18 m data from Naos Conica at the Very Large Telescope, and 30 epochs of 2.12 m data from the NIRC2 camera at the Keck Observatory, in total 94,929 measurements. A new approximate Bayesian computation method for fitting the first-order structure function extracts information beyond current fast Fourier transformation (FFT) methods of power spectral density (PSD) estimation. With a combined fit of the data of all three observatories, the characteristic coherence timescale of Sgr A* is minutes (90% credible interval). The PSD has no detectable features on timescales down to 8.5 minutes (95% credible level), which is the ISCO orbital frequency for a dimensionless spin parameter a = 0.92. One light curve measured simultaneously at 2.12 and 4.5 m during a low flux-density phase gave a spectral index s = 1.6 0.1 ( ). This value implies that the Sgr A* NIR color becomes bluer during higher flux-density phases. The probability densities of flux densities of the combined data sets are best fit by log-normal distributions. Based on these distributions, the Sgr A* spectral energy distribution is consistent with synchrotron radiation from a non-thermal electron population from below 20 GHz through the NIR.
Small angle scattering by dust grains causes a significant contribution to the total interstellar extinction for any X-ray instrument with sub-arcminute resolution (Chandra, Swift, XMM–Newton). ...However, the dust-scattering component is not included in the current absorption models: phabs, TBabs, and TBnew. We simulate a large number of Chandra spectra to explore the bias in the spectral fit and N
H measurements obtained without including extinction from dust scattering. We find that without incorporating dust scattering, the measured N
H will be too large by a baseline level of 25 per cent. This effect is modulated by the imaging resolution of the telescope, because some amount of unresolved scattered light will be captured within the aperture used to extract point source information. In high-resolution spectroscopy, dust scattering significantly enhances the total extinction optical depth and the shape of the photoelectric absorption edges. We focus in particular on the Fe–L edge at 0.7 keV, showing that the total extinction template fits well to the high-resolution spectrum of three X-ray binaries from the Chandra archive: GX 9+9, XTE J1817-330, and Cyg X-1. In cases where dust is intrinsic to the source, a covering factor based on the angular extent of the dusty material must be applied to the extinction curve, regardless of angular imaging resolution. This approach will be particularly relevant for dust in quasar absorption line systems and might constrain clump sizes in active galactic nuclei.
Starting in 2012, we began an unprecedented observational program focused on the supermassive black hole in the center of our Galaxy, Sgr A*, utilizing the High Energy Transmission Grating ...Spectrometer (HETGS) instrument on the Chandra X-Ray Observatory. Here we report an observation performed on 2012 February 9 wherein we detected what are the highest peak flux and fluence flare ever observed from Sgr A*. The flare, which lasted for 5.6 ks and had a decidedly asymmetric profile with a faster decline than rise, achieved a mean absorbed 2-8 keV flux of (8.5 + or - 0.9) x 10 super(-12) erg cm super(-2) s super(-1). We find good agreement among the fitted spectral slopes (Gamma ~ 2) and X-ray absorbing columns (N sub(H) ~ 15 x 10 super(22) cm super(-2)) for all three of these events, resolving prior differences (which are most likely due to the combined effects of pileup and spectral modeling) among Chandra and XMM-Newton observations of Sgr A* flares.
Emission from Saggitarius A* is highly variable at both X-ray and infrared (IR) wavelengths. Observations over the last ∼20 yr 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 coincident with an X-ray flare). Characterizing the behavior of these simultaneous X-ray/IR events and measuring any time lag between them can constrain models of Sgr A*'s accretion flow and the flare emission mechanism. Using 100+ hours of data from a coordinated campaign between the Spitzer Space Telescope and the Chandra X-ray Observatory, we present results of the longest simultaneous IR and X-ray observations of Sgr A* taken to date. The cross-correlation between the IR and X-ray light curves in this unprecedented data set, which includes four modest X-ray/IR flares, indicates that flaring in the X-ray may lead the IR by approximately 10-20 min with 68% confidence. However, the 99.7% confidence interval on the time-lag also includes zero, i.e., the flaring remains statistically consistent with simultaneity. Long-duration and simultaneous multi-wavelength observations of additional bright flares will improve our ability to constrain the flare timing characteristics and emission mechanisms, and must be a priority for Galactic Center observing campaigns.
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 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 obtaining a better understanding of possible emission mechanisms and their origin. This paper presents observations of two flares that both apparently violate the previously established patterns in the relative timing of submillimeter/NIR/X-ray flares from Sgr A*. One of these events provides the first evidence of coeval structure between NIR and submillimeter flux increases, while the second event is the first example of the sequence of submillimeter/X-ray/NIR flux increases all occurring within ∼1 hr. Each of these two events appears to upend assumptions that have been the basis of some analytic models of flaring in Sgr A*. However, it cannot be ruled out that these events, even though unusual, were just coincidental. These observations demonstrate that we do not fully understand the origin of the multiwavelength variability of Sgr A* and show that there is a continued and important need for long-term, coordinated, and precise multiwavelength observations of Sgr A* to characterize the full range of variability behavior.
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. With 38 High Energy Transmission Grating ...Spectrometer 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 ~10 super(34) erg s super(-1) to 2 x 10 super(35) erg s super(-1). Despite tentative evidence for a gap in the distribution of flare peak count rates, there is no evidence for X-ray color differences between faint and bright flares. Our preliminary X-ray flare luminosity distribution dN/dL is consistent with a power law with index -19 super(+0.3) sub(-0.4); this is similar to some estimates of Sgr A*'s near-IR flux distribution. The observed flares contribute one-third of the total X-ray output of Sgr A* during the campaign, and as much as 10% of the quiescent X-ray emission could be comprised of weak, undetected flares, which may also contribute high-frequency variability. We argue that flares may be the only source of X-ray emission from the inner accretion flow.
The magnetar SGR J1745−2900, discovered at a distance of parsecs from the Milky Way central black hole, Sagittarius A , represents the closest pulsar to a supermassive black hole ever detected. ...Furthermore, its intriguing radio emission has been used to study the environment of the black hole, as well as to derive a precise position and proper motion for this object. The discovery of SGR J1745−2900 has led to interesting debates about the number, age, and nature of pulsars expected in the Galactic center region. In this work, we present extensive X-ray monitoring of the outburst of SGR J1745−2900 using the Chandra X-ray Observatory, the only instrument with the spatial resolution to distinguish the magnetar from the supermassive black hole (2 4 angular distance). It was monitored from its outburst onset in 2013 April until 2019 August, collecting more than 50 Chandra observations for a total of more than 2.3 Ms of data. Soon after the outburst onset, the magnetar emission settled onto a purely thermal emission state that cooled from a temperature of about 0.9-0.6 keV over 6 yr. The pulsar timing properties showed at least two changes in the period derivative, increasing by a factor of about 4 during the outburst decay. We find that the long-term properties of this outburst challenge current models for the magnetar outbursts.
Context We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the Sgr A* counterpart associated with the massive 3-4 x 106 M black hole at the Galactic Center. Aims. We ...investigate the physical processes responsible for the variable emission from Sgr A*. Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope* and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA** on Mauna Kea, Hawaii, and the Very Large Array*** in New Mexico. Results. We detected one moderately bright flare event in the X-ray domain and 5 events at infrared wavelengths. The X-ray flare had an excess 2-8 keV luminosity of about 33 x 1033 erg/s. The duration of this flare was completely covered in the infrared and it was detected as a simultaneous NIR event with a time lag of 10 min. Simultaneous infrared/X-ray observations are available for 4 flares. All simultaneously covered flares, combined with the flare covered in 2003, indicate that the time-lag between the NIR and X-ray flare emission is very small and in agreement with a synchronous evolution. There are no simultaneous flare detections between the NIR/X-ray data and the VLA and SMA data. The excess flux densities detected in the radio and sub-millimeter domain may be linked with the flare activity observed at shorter wavelengths. Conclusions. We find that the flaring state can be explained with a synchrotron self-Compton (SSC) model involving up-scattered sub-millimeter photons from a compact source component. This model allows for NIR flux density contributions from both the synchrotron and SSC mechanisms. Indications for an exponential cutoff of the NIR/MIR synchrotron spectrum allow for a straightforward explanation of the variable and red spectral indices of NIR flares.
We report the first time series of broadband infrared color measurements of Sgr A*, the variable emission source associated with the supermassive black hole at the Galactic center. Using the laser ...and natural guide star adaptive optics systems on the Keck II Telescope, we imaged Sgr A* in multiple near-infrared broadband filters with a typical cycle time of similar to 3 minutes during four observing runs (2005-2006), two of which were simultaneous with Chandra X-ray measurements. In spite of the large range of dereddened flux densities for Sgr A* (2-30 mJy), all of our near-infrared measurements are consistent with a constant spectral index of alpha = -0.6 plus or minus 0.2 (F sub(v) alpha square root ). Furthermore, this value is consistent with the spectral indices observed at X-ray wavelengths during nearly all outbursts, which is consistent with the synchrotron self-Compton model for the production of the X-ray emission. During the coordinated observations, one infrared outburst occurs less than or equal to 36 minutes after a possibly associated X-ray outburst, while several similar infrared outbursts show no elevated X-ray emission. A variable X-ray to IR ratio and constant infrared spectral index challenges the notion that the infrared and X-ray emission are connected to the same electrons. We, therefore, posit that the population of electrons responsible for both the IR and X-ray emission are generated by an acceleration mechanism that leaves the bulk of the electron energy distribution responsible for the infrared emission unchanged, but has a variable high-energy cutoff. Occasionally a tail of electrons unk1 GeV is generated, and it is this high-energy tail that gives rise to the X-ray outbursts. One possible explanation for this type of variation is from the turbulence induced by a magnetorotational instability, in which the outer scale length of the turbulence varies and changes the high-energy cutoff.
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