Swift XRT spectrum of transient X-ray source at Sgr A*'s position

ATel #5011; Tom Dwelly, Gabriele Ponti, Frank Haberl (MPE), Maica Clavel (APC Paris)
on 26 Apr 2013; 10:29 UT
Credential Certification: Gabriele Ponti (ponti@iasfbo.inaf.it)

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We report a preliminary X-ray spectral analysis of the recent X-ray outburst observed from the vicinity of SgrA* (ATELs #5006, #5008, #5009).

We have combined data from Swift XRT observations 00091712004, 00035650239, 00091736019, 00554491000, 00554491991 and 00554491001, carried out between 2013-04-24T17:34:14 and 2013-04-26T00:14:40, and totalling 7.5 ks. The 2-10 keV flux observed in each of these Swift visits was similar to that reported by Degenaar et al. (ATEL #5006).

For each visit we extracted an X-ray spectrum from a 22 arcsec radius region centred on the catalogued position of SgrA*. Background counts were extracted from a nearby 100 arcsec radius region free of bright sources (total ~430 counts). The individual spectra from each visit, and their respective response matrices, were combined to produce a single source spectrum, which was then binned to give a minimum of 20 counts per bin. The total number of counts in the spectrum is ~1400.

We used XSPEC to fit this spectrum over the 2-10 keV range with a model consisting of an absorbed power law. The best fitting power law photon index is 3.6+-0.4 (90% confidence) and the best fitting column density is (14+-2)x10^22 cm^-2. This model gives a good fit to the data (Chi^2 61.2 for 60 d.o.f.). The 2-10 keV absorbed flux is 2.2x10^-11 erg cm^-2 s^-1 (equivalent to a luminosity of 1.6x10^35 erg s^-1 for a distance of 8 kpc).

We added an unresolved emission line to the model and refitted the spectrum. The best fit line energy is 6.5+-0.1 keV with intensity (1.1+-0.6)x10^-4 phot cm^-2 s^-1. Adding the line marginally improves the fit, with a new Chi^2 of 53.2 for 58 d.o.f. We have examined a Swift-XRT spectrum from the same region using data from before the bright flare (total of 29 ks) which shows a line at the same energy, but with a slightly lower intensity (0.21+-0.15)x10^-4 phot cm^-2 s^-1.

Further observations are encouraged to determine the nature of this transient event.