GCN Circular 17544

Alessandro Maselli (INAF-IASFPA), Phil Evans (U. Leicester),
David Burrows (PSU), Sergio Campana (INAF-OAB),
Neil Gehrels (GSFC), Jamie Kennea (PSU), Amy Lien (GSFC/UMBC),
Daniele Malesani (DARK/NBI), Craig. B. Markwardt (GSFC),
Frank Marshall (GSFC), Julian Osborne (U. Leicester),
David. M. Palmer (LANL), Kim L. Page (U Leicester),
Rhaana L. C. Starling (U Leicester)
report on behalf of the Swift team:

Swift continues to observe the field of BAT trigger 633105
(aka GRB 150301C) with sky location in M31. To avoid further
naming confusion, this object will be referred as Swift J0045.2+4151
hereafter. The XRT continues to detect only a single source within
the BAT error circle at a position coincident with the previously
known XMM-Newton source, [PFH2005] 622 (Pietsch et al. 2005).
For the rest of this circular we assume that the X-rays detected by
XRT are from [PFH2005] 622.

The XRT has acquired four observations so far. The source
(which we presume is [PFH2005] 622) was detected in the first snapshot,
(T0+107s to T0+720s), in which we detect 8 events in 614 seconds,
with a mean background expectation of 0.34 counts.  This results in a rate
of 1.48 (+0.6, -0.5) e-2 counts/s (1 sigma errors).

In the 2nd epoch (T0+4 ks to T0+80 ks), we detect 3 events in 7.2 ks, with
an expected background of 1.25 events, giving a 3 sigma upper limit of
< 0.002 counts/s.

In the 3rd epoch (T0+171 ks to T0+206 ks), we find 5 events in 4.8 ks with a
mean background expectation of 0.64 events, giving a 3 sigma detection
of 1.2 (+0.8, -0.6) e-3 counts/s (1 sigma errors).

All measurements use a source radius of 9 pixels (21 arcseconds),
a background measurement from an annulus around the source region, and
the Bayesian method of Kraft, Burrows, and Nousek (1991, ApJ, 374, 344) to obtain
the 1-sigma confidence intervals (for detections) or 3-sigma upper limits (for non-detections).

Therefore, after a detection in the initial XRT observation, the source
faded below the XRT detection limit and may have brightened again
in the 3rd epoch.

We cannot conclude firmly at this point whether the source
[PFH2005] 622 is associated with the BAT trigger or not.
More observations at all wavelengths are encouraged.

A table and a light curve that summarize current XRT observations
of [PFH2005] 622 are available at:
http://swift.gsfc.nasa.gov/results/BATbursts/633105/xrt/atel_20150304.html

[GCN OPS NOTE(05mar15), Per author's request, the last line in the 5th paragraph
was changed from "the 3 sigma confidence regions and upper limits."
to "the 1-sigma confidence intervals (for detections) or 3-sigma upper limits (for non-detections).".
And the places in the 2nd & 4th paragraphs that said "(1 sigma errors)"
were changed to "(1 sigma errors)".]