GCN Circular 8991
J. Mao & R. Margutti (INAF-OAB) report on behalf of the Swift-XRT team: We have analysed 2 ks of XRT data for GRB 090313 (Mao et al. GCN Circ. 8980), from 26.8 ks to 28.6 ks after the BAT trigger. All the data were taken in Photon Counting (PC) mode. Using 2038 s of XRT Photon Counting mode data and 1 UVOT image, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 198.40130, 8.09730 which is equivalent to: RA (J2000): 13 13 36.30 Dec (J2000): +08 05 50.4 with an uncertainty of 1.8 arcsec (radius, 90% confidence). The optical candidate (Chornock et al. GCN 8979 and Updike et al. GCN 8983) lies within the XRT error circle. The lightcurve can be modeled as a single power-law with an index around 5.4. Given the present data set the value of this parameter is affected by a large uncertainty. The PC mode spectrum can be fit with an absorbed simple power-law, with a photon index of 2.12 � 0.33 and an absorbing column density of 1.29 � 0.97 e21 cm-2, in excess of the Galactic value of 2.10e20 cm-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.1e-11 (5.9e-11) erg cm-2 count-1. Given the very uncertain power-law decay index, no firm prediction of the lightcurve can be made. The results of the automatic XRT analysis are available online at http://www.swift.ac.uk/xrt_products/00346386. This circular is an official product of the Swift-XRT team.