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Register a new userX-ray hiccups from SgrA* observed by XMM-Newton. The second brightest flare and three moderate flares caught in half a day
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Added by
Delphine Porquet Dr.
Publication date
2008
fields
Physics
and research's language is
English
Authors
D. Porquet
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[truncated] In Spring 2007, we observed SgrA* with XMM with a total exposure of ~230ks. We have performed timing and spectral analysis of the new X-ray flares detected during this campaign. To study the range of flare spectral properties, in a consistent manner, we have also reprocessed, using the same analysis procedure and the latest calibration, archived XMM data of previously reported rapid flares. The dust scattering was taken into account during the spectral fitting. We also used Chandra archived observations of the quiescent state of SgrA* for comparison. On April 4, 2007, we observed for the first time within a time interval of ~1/2 day, an enhanced incidence rate of X-ray flaring, with a bright flare followed by three flares of more moderate amplitude. The former event represents the second brightest X-ray flare from Sgr A* on record. This new bright flare exhibits similar light-curve shape (nearly symmetrical), duration (~3ks) and spectral characteristics to the very bright flare observed in October 3, 2002. The measured spectral parameters of the new bright flare, assuming an absorbed power law model taken into account dust scattering effect, are N_H=12.3(+2.1,-1.8)e22 cm-2 and Gamma~2.3+/-0.3 calculated at the 90% c.l. The spectral parameter fits of the sum of the three following moderate flares, while lower, are compatible within the error bars with those of the bright flares. The column density found, for a power-law, during the flares is at least two times higher than the value expected from the (dust) visual extinction toward SgrA* (AV~25 mag). However, our fitting of the SgrA* quiescent spectra obtained with Chandra shows that an excess of column density is already present during the non-flaring phase. The two brightest X-ray flares observed so far from SgrA* exhibited similar soft spectra.
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We report the high S/N observation on October 3, 2002 with XMM-Newton of the brightest X-ray flare detected so far from SgrA* with a duration shorter than one hour (~ 2.7 ks). The light curve is almost symmetrical with respect to the peak flare, and no significant difference between the soft and hard X-ray range is detected. The overall flare spectrum is well represented by an absorbed power-law with a soft photon spectral index of Gamma=2.5+/-0.3, and a peak 2-10 keV luminosity of 3.6 (+0.3-0.4) x 10^35 erg/s, i.e. a factor 160 higher than the Sgr A* quiescent value. No significant spectral change during the flare is observed. This X-ray flare is very different from other bright flares reported so far: it is much brighter and softer. The present accurate determination of the flare characteristics challenge the current interpretation of the physical processes occuring inside the very close environment of SgrA* by bringing very strong constraints for the theoretical flare models.
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Stellar dynamics indicate the presence of a super massive 3-4x10^6 Msun solm black hole at the Galactic Center. It is associated with the variable radio, near-infrared, and X-ray counterpart Sagittarius A* (SgrA*). The goal is the investigation and understanding of the physical processes responsible for the variable emission from SgrA*. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatorys Very Large Telescope (July 2005, May 2007) and the ACIS-I instrument aboard the Chandra X-ray Observatory (July 2005). We find that for the July 2005 flare the variable and polarized NIR emission of SgrA* occurred synchronous with a moderately bright flare event in the X-ray domain with an excess 2 - 8 keV luminosity of about 8x10^33erg/s. We find no time lag between the flare events in the two wavelength bands with a lower limit of less than 10 minutes. The May 2007 flare shows the highest sub-flare to flare contrast observed until now. It provides evidence for a variation in the profile of consecutive sub-flares. We confirm that highly variable and NIR polarized flare emission is non-thermal and that there exists a class of synchronous NIR/X-ray flares. We find that the flaring state can be explained via the synchrotron self-Compton (SSC) process involving up-scattered sub-millimeter photons from a compact source component. The observations can be interpreted in a model involving a temporary disk with a short jet. In the disk component the flux density variations can be explained due to spots on relativistic orbits around the central super massive black hole (SMBH). The profile variations for the May 2007 flare are interpreted as a variation of the spot structure due to differential rotation within the disk.
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