Ongoing X-ray activity from Sgr A*

ATel #5008; Tom Dwelly, Gabriele Ponti (MPE)
on 25 Apr 2013; 21:07 UT
Credential Certification: Gabriele Ponti (ponti@iasfbo.inaf.it)

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Following ATEL 5006 of Degenaar et al. who reported a very high flux level from Sgr A*, we triggered a pre-approved Swift ToO observation. We report here the measurements from the first of these Swift-XRT data (Swift OBSID 00035650239).
Sgr A* was observed by Swift in two visits, at MJD=56407.6099 for 342s and at MJD=56407.6654 for 639s. Both visits were made with the XRT in photon counting mode. An automated script (developed at the University of Southampton) was used to retrieve the data from the Swift quick-look site and to extract X-ray counts from a 22 arcsec radius circular aperture, centred on the radio coordinates of Sgr A*. The instrumental background contribution was estimated by extracting counts from a circular region 100 arcsec in radius placed on a region free of bright point sources lying ~3 arcmin from Sgr A*. The raw 2-10keV count-rates in the source aperture were 0.18 and 0.21 cts/s in the first and second visits respectively, corresponding to 2-10 keV fluxes of (2.1+-0.3) and (2.4+-0.2)x10^-11 erg cm^-2 s^-1. These fluxes are derived from the measured 2-10 keV count-rate taking account of vignetting, bad pixels and the fraction of source photons expected to be enclosed in the target aperture. There is expected to be a small amount of residual diffuse emission within the source aperture, contributing ~2-3x10^-12 erg cm^-2 s^-1 to the source flux.
A visual inspection of a 200s binned light curve shows no evidence for variation within each visit.