Study of the X-ray activity of Sagittarius A* during the 2011 XMM-Newton campaign

Context. At the dynamical center of the Milky Way, there is the closest supermassive black hole: Sgr A*. Its non-flaring luminosity is several orders of magnitude lower than the Eddington luminosity, but flares can be observed in the infrared and X-rays. This flaring activity can help us to understand radiation mechanisms from Sgr A*.
Aims: Our aim is to investigate the X-ray flaring activity of Sgr A* and to constrain the physical properties of the X-ray flares and their origin.
Methods: In March and April 2011, we observed Sgr A* with XMM-Newton with a total exposure of ≈226 ks in coordination with the 1.3 mm Very-Long-Baseline Interferometry array. We performed timing analysis of the X-ray emission from Sgr A* using a Bayesian-blocks algorithm to detect X-ray flares observed with XMM-Newton. Furthermore, we computed X-ray smoothed light curves observed in this campaign in order to have better accuracy on the position and the amplitude of the flares.
Results: We detected two X-ray flares on March 30 and April 3, 2011, which for comparison have a peak detection level of 6.8 and 5.9σ in the XMM-Newton/EPIC (pn+MOS1+MOS2) light curve in the 2-10 keV energy range with a 300 s bin. The former is characterized by two sub-flares: the first one is very short (~458 s) with a peak luminosity of L^unabs_2-10~keV ~9.4 × 10³⁴ erg s^-1, whereas the second one is longer (~1542 s) with a lower peak luminosity (L^unabs_2-10~keV ~6.8 × 10³⁴ erg s^-1). The comparison with the sample of X-ray flares detected during the 2012 Chandra XVP campaign favors the hypothesis that the 2011 March 30 flare is a single flare rather than two distinct subflares. We model the light curve of this flare with the gravitational lensing of a simple hotspot-like structure, but we cannot satisfactorily reproduce the large decay of the light curve between the two subflares with this model. From magnetic energy heating during the rise phase of the first subflare and assuming an X-ray photons production efficiency of 1 and a magnetic field of 100 G at 2 r_g, we derive an upper limit to the radial distance of the first subflare of 100⁺¹⁹_-29 r_g. We use the decay phase of the first subflare to estimate a lower limit to the radial distance of 4 r_g from synchrotron cooling in the infrared.
Conclusions: The X-ray emitting region of the first subflare is located at a radial position of 4-100⁺¹⁹_-29 and has a corresponding radius of 1.8-2.87 ± 0.01 in r_g unit for a magnetic field of 100 G at 2 r_g.

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