GCN Circular 3032
Subject
GRB 050215b: Swift XRT Source Variability
Date
2005-02-17T21:39:55Z (20 years ago)
From
David Burrows at PSU/Swift <dxb15@psu.edu>
M. Goad, K. Page, J. P. Osborne, P. O'Brien (U. Leicester), D. C.
Morris, J. A. Kennea, D. N. Burrows, J. L. Racusin, J. E. Hill, M. M.
Chester, J. A. Nousek (PSU), P. Giommi, M. Capalbi, M. Perri, F. Tamburelli
(ASDC), P. Romano, G. Tagliaferri, S. Campana, A. Moretti, C. Pagani, G.
Chincarini (INAF-OAB), G. Cusumano, V. Mangano, V. La Parola
(INAF-IASF/Palermo), A. A. Wells (U. Leicester), B. Zhang (U. Nevada), T.
Sakamoto, L. Angelini, N. Gehrels (GSFC), W. Voges (MPE), L. Cominsky
(Sonoma State U.), M. Tripicco (GSFC-SSAI) report on behalf of the Swift
XRT team:
The Swift XRT has continued to monitor the X-ray source (designated SWIFT
J113746.1+404754) discussed in GCN 3027 (Page et al. 2005) in the field of
GRB 050215b. We now have data over two epochs, indicating a decaying light
curve for this object.
First epoch:
We obtained 5155 seconds of data in Photon-Counting mode (our most
sensitive observing mode) from six orbits beginning at 04:09:52 UT on
2005-02-15. The average count rate for SWIFT J113746.1+404754 in the first
two orbits is 0.0231 +/- 0.0050 cps (0.2-10 keV). The average count rate
for this source over orbits 3-6 is 0.0108 +/- 0.0018 cps, consistent with a
decreasing rate.
Second epoch:
We obtained 1640 seconds on SWIFT J113746.1+404754 between 01:36:37 UT and
09:39:42 UT on 2005-02-17 in Photon-Counting mode. The source is
undetectable in this second epoch, with a 95% confidence upper limit on the
count rate of 0.0018 cps (0.2-10 keV, calculated using the method of Kraft,
Burrows, and Nousek 1993, ApJ, 1991, 374, 344).
The fading count rate suggests that this source could be the X-ray
afterglow of GRB 050215b, although it is extremely weak in the first epoch
(a few times 10^-13 ergs/cm**2/s at about 95 minutes post-burst). However,
we cannot rule out other possibilities. We note that there is an optical
source (R=17.3, B=18.5) in the USNO catalog at the edge of the XRT error
circle of the detected X-ray source reported in GCN 3027. If we assume
that this object is the optical counterpart of our X-ray source, the
optical-to-X-ray ratio is alpha_ox~1.35 during our first epoch, compatible
with a radio-quiet AGN. Optical spectroscopy of this USNO object would
help determine its nature, and could eliminate it from consideration as a
possible counterpart of SWIFT J113746.1+404754.
We do not detect any X-ray object in either epoch at the position of the
UKIRT afterglow candidate (Tanvir et al. 2005, GCN 3031). We place 95%
upper limits on the XRT count rate from the UKIRT afterglow candidate of
0.0006 cps in the first epoch and 0.0018 cps in the second epoch (0.2-10
keV), within a 20 arcsecond radius circle of the UKIRT position.
Further Swift observations of this field are planned through the weekend.