GCN Circular 3034
Subject
GRB 050215b: XRT confirmation of IR afterglow candidate
Event
Date
2005-02-18T18:35:51Z (21 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:
We confirm that the fading IR object discovered by Tanvir et al. 2005 (GCN
3031) is the afterglow of GRB 050215b.
We have discovered an error in the astrometry for previously-reported XRT
positions on the field of GRB050215b. This affects both the position
reported by Page et al. 2005 (GCN 3027) and some of the information in Goad
et al. 2005 (GCN 3032). We recommend that those GCNs not be referenced,
and that this Circular be referenced instead for XRT observations of this
burst.
The corrected XRT position for the afterglow of GRB 050215b is
RA(J2000) = 11 37 47.7 , Dec(J2000) = +40 47 44.0
The estimated uncertainty in this position is 6 arcseconds radius. We note
that this corrected error circle now includes the UKIRT counterpart
candidate (Tanvir et al. 2005, GCN 3031). Further, the new XRT position
puts the USNO/2MASS object mentioned in GCN 3032 outside of the XRT error
circle. We can therefore definitely confirm this object as the afterglow
of GRB 050215b.
We can also now definitely say that the XRT counterpart is fading, but with
a fairly flat decay curve (slope of -0.76 +/- 0.26).
The Swift XRT has continued to monitor the X-ray source (now designated SWIFT
J113747.7+404744) 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 J113747.7+404744 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 J113747.7+404744 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).
Further Swift observations of this field are planned through the weekend.
[GCN OPS NOTE (18feb05): Per author's request some typos were corrected:
In lines 42 and 55, "J1137477+404744" was changed to "J113747.7+404744".
In line 49 "J113746.1+404754" was changed to "J113747.7+404744".]