[vsnet-grb-info 5505] Refined Swift-XRT analysis of the transient in NGC 2770

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7170
SUBJECT: Refined Swift-XRT analysis of the transient in NGC 2770 
DATE:    08/01/11 21:01:20 GMT
FROM:    Kim Page at U.of Leicester  <kpa at star.le.ac.uk>

K.L. Page (U Leicester), D. Burrows (PSU), P.T. O'Brien (U Leicester), 
A.M. Soderberg (Princeton U), E. Berger (Princeton U), S. Immler 
(NASA/UMCP/GSFC), A. Cucchiara (PSU), D. Fox (PSU) and N. Gehrels 
(NASA/GSFC) report on behalf of the Swift team:

Following the initial detection of an X-ray transient in NGC 2770 on 
2008-01-09 (GCN Circ. 7159, ATel 1353 - Berger & Soderberg; ATel 1355 - 
Kong & Maccarone), a Swift Target of Opportunity observation was performed 
between 2008-01-10 and 2008-01-11. 

As previously mentioned in GCN Circ. 7159, the X-ray data are seen to rise 
at the beginning of the XRT observation. The light-curve then decays 
steeply for the following 700-800 seconds, with a power-law index of 3.3 
+/- 0.4. Between the end of this first orbit of data and the start of the 
next (a gap of approximately 5 ks), the decay flattens off, and the 
count-rate remains close to constant [around (2.5-3)e-3 count s^-1] for at 
least 180 ks.

There is spectral evolution during the first orbit, with the data 
softening for the first ~400 seconds, before slightly hardening again. The 
time-averaged spectrum can be well modelled by an absorbed power-law, with 
Gamma = 2.1 +/- 0.2, and NH = (6.1 +1.3/-1.1)e21 cm^-2.

This field was also observed on 2008-01-06, before the transient was 
detected. Although there may be another faint, nearby source, the upper 
limit on the count-rate for this transient is ~7e-4 count s^-1 (0.3-10 
keV) at this early time.