GCN Circular 8937

A.P. Beardmore, K. L. Page, P.A. Evans (U. Leicester), C. Pagani, J. Kennea,
D.N. Burrows (PSU) report on behalf of the Swift-XRT team :

The Swift-XRT began observing the field of the short GRB 090305
(Beardmore et al. GCN Circ. 8932
Not found
 
 
) in photon counting mode at
2009-03-05T05:21:37, 103.4 s after the trigger.  The first orbit of
data lasted only 90 s before the spacecraft slewed to another target
because of an observing constraint.

The default data processing does not reveal any X-ray source during
the first orbit (103.4 to 193.5 s post trigger), either within the BAT
refined error circle (Krimm et al. GCN Circ. 8936
Not found
 
 
) or at the location
of the optical afterglow (Cenko et al. GCN Circ. 8933
Not found
 
 
) - the latter
occurring close to a bad-column on the XRT CCD.  However, by relaxing
the default screening criteria we find 8 counts in a 10 pixel radius
circle extraction region at the location of the optical afterglow,
compared with a predicted background level of 1.0 count (where an
annulus with radii 15 to 100 pixels was used to sample the local
background level), implying a greater than 99.99 percent confidence
detection (Kraft, Burrows, and Nousek 1991, Ap. J., 374, 344). The
clustering of the counts, their proximity to the location of the
optical afterglow and their detection significance allow us to
conclude the afterglow was detected by the XRT at this time. The
estimated count rate is 0.112 +0.051 -0.041 count/s (where a
systematic error of 5 percent has been included in the 1 sigma error
estimate to account for the PSF correction uncertainty).  Applying a
canonical GRB count to observed flux conversion factor of 5e-11 erg
cm^-2 count^-1 (for a photon index of 1.98, Evans et al. 2009, MNRAS,
submitted. arXiv:0812.3662, http://www.swift.ac.uk/xrt_spectra), we
estimate the observed 0.3-10 keV flux was (5.5 +2.6 -2.0)e-12 
erg cm^-2 s^-1.

A further 2.05 ks of data was taken in orbit two, from 3.92 to 5.97 ks
after the trigger, where the optical afterglow position was located
away from the XRT CCD bad column. This showed the source was no longer
detected and a 3 sigma upper limit to the count rate of 7.0e-3 count/s
was obtained.  Using all of the available data after the first orbit
(i.e. presently an exposure of 12.64 ks from 3.92 to 46.1 ks post
trigger) reduces the 3 sigma upper limit to 1.7e-3 count/s.  The X-ray
data suggest the source faded with a decay slope of at least ~0.8.

This circular is an official product of the Swift-XRT team.