TITLE: GCN CIRCULAR NUMBER: 6265 SUBJECT: GRB 070406: Swift XRT team further analysis DATE: 07/04/10 21:10:25 GMT FROM: Kim Page at U.of Leicester E. Troja (U. Leicester/INAF-IASFPa), K. L. Page (U. Leicester), J. Racusin (PSU), S. McBreen (MPE), J. P. Osborne (U. Leicester), V. La Parola (INAF-IASFPa), D. N. Burrows (PSU) and N. Gehrels (NASA/GSFC) report on behalf of the Swift XRT team: We have performed further analysis on Swift XRT observations of the field of GRB 070406 (Cummings et al., GCN Circ. 6247). Our data set consists of 56 ks of XRT observations taken in Photon Counting (PC) mode. Thirteen serendipitous X-ray sources are detected with S/N > 3 with the WAVDETECT algorithm, five of which have nearby optical counterparts in the SDSS catalogue. We match these sources to obtain a best fit mean frame shift, carefully accounting for several instrumental factors including exposure map correction, and additional hot pixel removal. The resulting astrometric corrected position of the X-ray source (S1), proposed as the afterglow candidate (Troja et al., GCN Circ. 6255), is: RA (J2000) = 13h 15m 51.59s Dec(J2000) = +16d 30' 46.6" with an estimated error radius of 4.7'' (90% containment). This position lies 1.9 arcsec from the position quoted in Butler et al. (GCN Circ. 6263) and 1.7 arcsec from the bright optical source reported by Kann (GCN Circ. 6256). The X-ray lightcurve of S1 can be found at: http://www.ifc.inaf.it/~troja/grb070406_s1.gif It displays a steady behavior with a fairly constant count rate of 1.5e-03 cts/s. It seems unlikely that S1 is the X-ray counterpart of GRB 070406, as noted by Butler et al. (GCN Circ. 6263), and it is more likely associated with AGN activity (Berger et al., GCN Circ. 6262). Examining the observations performed between 2 d and 3.5 d after the burst, another faint source (S2) is detected within the refined BAT error circle (Krimm et al., GCN Circ. 6261). Its astrometric corrected position is: RA (J2000) = 13h 15m 43.42s Dec(J2000) = +16d 31' 09.3" with an estimated error radius of 5.7'' (90% containment). This position lies 92 arcsec from the BAT refined position given in Krimm et al. (GCN Circ. 6261). According to the SDSS catalogue, two galaxies lie 3.7 and 13.6 arcsec from the X-ray position of S2, respectively. A third object, classified as a star, is 1.3 arcsec from the X-ray source. The X-ray lightcurve of S2 can be found at: http://www.ifc.inaf.it/~troja/grb070406_s2.gif It shows some evidence of fading, but due to the faintness of the source we are unable to clearly state whether the source is decaying. A Kolmogorov-Smirnov test, performed on the arrival times of 20 source photons, gives a probability of 10% that the source S2 is steady. The 10% probability of constancy is much less than a two sigma detection of variability; therefore the source may be consistent with being constant. Swift follow-up observations will be performed to confirm its variability. This Circular is an official product of the Swift XRT Team.