* Astronomy

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RE: Sgr A*

Blobrana · L

Title: Fifteen years of XMM-Newton and Chandra monitoring of Sgr A*: Evidence for a recent increase in the bright flaring rate
Author: G. Ponti, B. De Marco, M. R. Morris, A. Merloni, T. Munoz-Darias, M. Clavel, D. Haggard, S. Zhang, K. Nandra, S. Gillessen, K. Mori, J. Neilsen, N. Rea, N. Degenaar, R. Terrier, A. Goldwurm

We present a study of the X-ray flaring activity of Sgr A* during all the 150 XMM-Newton and Chandra observations pointed at the Milky Way center over the last 15 years. This includes the latest XMM-Newton and Chandra campaigns devoted to monitoring the closest approach of the very red Br-Gamma emitting object called G2. The entire dataset analysed extends from September 1999 through November 2014. We employed a Bayesian block analysis to investigate any possible variations in the characteristics (frequency, energetics, peak intensity, duration) of the flaring events that Sgr A* has exhibited since their discovery in 2001. We observe that the total bright-or-very bright flare luminosity of Sgr A* increased between 2013-2014 by a factor of 2-3 (~3.5 sigma significance). We also observe an increase (~99.9% significance) from 0.27±0.04 to 2.5±1.0 day^-1 of the bright-or-very bright flaring rate of Sgr A*, starting in late summer 2014, which happens to be about six months after G2's peri-center passage. This might indicate that clustering is a general property of bright flares and that it is associated with a stationary noise process producing flares not uniformly distributed in time (similar to what is observed in other quiescent black holes). If so, the variation in flaring properties would be revealed only now because of the increased monitoring frequency. Alternatively, this may be the first sign of an excess accretion activity induced by the close passage of G2. More observations are necessary to distinguish between these two hypotheses.

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ATel 6017: NRAO VLA service monitoring observations of Sgr A*

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Title: Submillimetre Quasi-Periodic Oscillations in Magnetically Choked Accretion Flows Models of Sgr A*
Authors: Roman V. Shcherbakov, Jonathan C. McKinney

High-frequency quasi-periodic oscillations (QPOs) appear in general-relativistic magnetohydrodynamic simulations of magnetically choked accretion flows around rapidly rotating black holes (BHs). We perform polarised radiative transfer calculations with our ASTRORAY code in order to explore the manifestations of these QPOs for Sgr A*. We construct a simulation-based model of a radiatively inefficient accretion flow and find model parameters by fitting the mean observed polarized source spectrum. The simulated QPOs have a total sub-mm flux amplitude of under 5% and a linearly polarised flux amplitude of up to 2%. The oscillation period T ~100M ~35 min corresponds to the rotation period of the BH magnetosphere that produces a trailing spiral in resolved disk images. The total flux signal is statistically significant over noise for all tested frequencies 87 GHz, 230 GHz, and 857 GHz and inclination angles 10°, 37°, and 80°. The non-detection in the 230 GHz Sub-Millimetre Array light curve is consistent with a low signal level and low sampling rate. The possible presence of such magnetospheric QPOs in Sgr A* will be better tested with Atacama Large Millimetre Array.

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Title: Swift XRT spectrum of transient X-ray source at Sgr A*'s position
Authors: Tom Dwelly, Gabriele Ponti, Frank Haberl (MPE), Maica Clavel (APC Paris)

We report a preliminary X-ray spectral analysis of the recent X-ray outburst observed from the vicinity of SgrA*

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Title: Constraining the Accretion Flow in Sgr A* by General Relativistic Dynamical and Polarised Radiative Modelling
Authors: Roman V. Shcherbakov, Robert F. Penna, Jonathan C. McKinney

We briefly summarise the method of simulating Sgr A* polarised sub-mm spectra from the accretion flow and fitting the observed spectrum. The dynamical flow model is based on three-dimensional general relativistic magneto hydrodynamic simulations. Fully self-consistent radiative transfer of polarised cyclo-synchrotron emission is performed. We compile a mean sub-mm spectrum of Sgr A* and fit it with the mean simulated spectra. We estimate the ranges of inclination angle theta=42-75deg, mass accretion rate Mdot=(1.4-7.0)*10^{-8}Msun/yr, and electron temperature Te=(3-4)*10^{10}K at 6M. We discuss multiple caveats in dynamical modelling, which must be resolved to make further progress.

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Title: Chandra-HETGS Observations of the Brightest Flare Seen from Sgr A*
Authors: M. A. Nowak, J. Neilsen, S. B. Markoff, F. K. Baganoff, D. Porquet, N. Grosso, Y. Levin, J. Houck, A. Eckart, H. Falcke, L. Ji, J. M. Miller, Q. D. Wang

Starting in 2012, we began an unprecedented observational program focused on the supermassive black hole in the center of our Galaxy, Sgr A*, utilising the High Energy Transmission Gratings Spectrometer (HETGS) instrument on the Chandra X-ray Observatory. These observations will allow us to measure the quiescent X-ray spectra of Sgr A* for the first time at both high spatial and spectral resolution. The X-ray emission of Sgr A*, however, is known to flare roughly daily by factors of a few to ten times over quiescent emission levels, with rarer flares extending to factors of greater than 100 times quiescence. Here were report an observation performed on 2012 February 9 wherein we detected what is 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±0.9)X10^{-12} erg cm^{-2} s^{-1}. The peak flux was 2.5 times higher, and the total 2-10 keV emission of the event was approximately 10^{39} erg. Only one other flare of comparable magnitude, but shorter duration, has been observed in Sgr A* by XMM-Newton in 2002 October. We perform spectral fits of this Chandra observed flare, and compare our results to the two brightest flares ever observed with XMM-Newton. We find good agreement among the fitted spectral slopes (Gamma~2) and X-ray absorbing columns (N_H~15X10^{22} cm^{-2}) for all three of these events, resolving prior differences (which are most likely due to the combined effects of pileup and spectral modelling) among Chandra and XMM-Newton observations of Sgr A* flares. We also discuss fits to the quiescent spectra of Sgr A*.

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Title: The X-ray lightcurve of Sgr A* over the past 150 years inferred from Fe-Ka line reverberation in Galactic Centre molecular clouds
Authors: R. Capelli, R. S. Warwick, D. Porquet, S. Gillessen, P. Predehl

We examine the temporal and spectral properties of nine Fe-Ka bright molecular clouds within about 30 pc of Sgr A*, in order to understand and constrain the primary energising source of the Fe fluorescence. Significant Fe-Ka variability was detected, with a spatial and temporal pattern consistent with that reported in previous studies. The main breakthrough that sets our paper apart from earlier contributions on this topic is the direct measurement of the column density and the Fe abundance of the MCs in our sample. We used the EW measurements to infer the average Fe abundance within the clouds to be 1.6 ±0.1 times solar. The cloud column densities derived from the spectral analysis were typically of the order of 10^{23} cm^{-2}, which is significantly higher than previous estimates. This in turn has a significant impact on the inferred geometry and time delays within the cloud system. Past X-ray activity of Sgr A* is the most likely source of ionisation within the molecular clouds in the innermost 30 pc of the Galaxy. In this scenario, the X-ray luminosity required to excite these reflection nebulae is of the order of 10^{37}-10^{38} erg s^{-1}, significantly lower than that estimated for the Sgr B2 molecular cloud. Moreover, the inferred Sgr A* lightcurve over the past 150 years shows a long-term downwards trend punctuated by occasional counter-trend brightening episodes of at least 5 years duration. Finally, we found that contributions to the Fe fluorescence by X-ray transient binaries and cosmic-ray bombardment are very likely, and suggest possible ways to study this latter phenomenon in the near future.

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Title: Detection of pulsar beams deflected by the black hole in Sgr A*: effects of black hole spin
Authors: Sourabh Nampalliwar, Richard H. Price, Teviet Creighton, Fredrick A. Jenet

Some Galactic models predict a significant population of radio pulsars close to the our galactic center. Beams from these pulsars could get strongly deflected by the supermassive black hole (SMBH) believed to reside at the galactic center and reach the Earth. Earlier work assuming a Schwarzschild SMBH gave marginal chances of observing this exotic phenomenon with current telescopes and good chances with future telescopes. Here we calculate the odds of observability for a rotating SMBH. We find that the estimates of observation are not affected by the SMBH spin, but a pulsar timing analysis of deflected pulses might be able to provide an estimate of the spin of the central black hole.

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An international team of astronomers, including scientists from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, has obtained the closest views ever of what is believed to be a super-massive black hole at the center of the Milky Way galaxy.

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