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
The X-ray lightcurve of S1 can be found at:
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
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:
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.