[vsnet-grb-info 16270] GRB 150305A: Swift XRT confirms weak INTEGRAL event as a new GRB

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17551
SUBJECT: GRB 150305A: Swift XRT confirms weak INTEGRAL event as a new GRB
DATE:    15/03/08 18:12:17 GMT
FROM:    Rhaana Starling at U of Leicester  <rlcs1 at star.le.ac.uk>

R. Starling (U. Leicester) reports:

On 2015-03-05T09:49:19 UT INTEGRAL recorded a 7.6 sigma event at 
RA, Dec 269.78943, -41.62041 degrees (err 3.39 arcmin) and issued WEAK 
Alert 6905. We triggered a Swift ToO to observe this position.

Swift observations began at 1.8E4 s after the INTEGRAL trigger with 
exposure time 3ks (obsID33663). Two sources were present in the error circle.
One of these is consistent with the ROSAT source 1RXS J175914.5-423529, 
which is likely associated with dwarf nova V728 CrA.

The second is a new source, with enhanced XRT position of
RA, Dec (J2000) = 17h 59m 02.60s, -42$B!k(B 39$B!l(B 49.5$B!l!l(B (269.76082, -42.66374 deg) 
with an uncertainty of 3.3$B!l!l(B (radius, 90% confidence).

In this first observation the new source had a count rate 0.035 ct/s.
Further Swift observations beginning 1.1E5s post-burst with exposure time 
3.7ks showed that this source had a count rate of approx. 0.007 ct/s, having 
faded with a decay rate of alpha ~ -0.7. 

The spectrum of the new source can be described with an absorbed power law.

The UVOT data were taken in UVM2 and UVW1 filters. A preliminary analysis 
does not reveal any UV counterpart to the new XRT source.

We conclude that the INTEGRAL WEAK event was a real GRB and that the 
fading source discovered by Swift XRT is the afterglow.

We thank the Swift team for carrying out these ToOs, and acknowledge use
of the UK Swift Science Data Centre in our analysis.