TITLE:   GCN CIRCULAR
NUMBER:  6550
SUBJECT: GRB 070616: Swift XRT refined circular
DATE:    07/06/17 10:52:11 GMT
FROM:    Rhaana Starling at U of Leicester  <rlcs1@star.le.ac.uk>

R.L.C. Starling (U. Leicester) reports, on behalf of the Swift XRT team:

We have analysed the first eight orbits of Swift XRT data (18.6 ks of data 
out to T0+5.8E4s) for GRB 070616 (Starling et al., GCN Circ. 6542).

Using 1.1 ks of overlapping XRT PC mode data and UVOT V-band data, we
obtain an astrometrically corrected X-ray position (using the USNO-B1 
catalogue) of 

RA(J2000) = 02 08 36.70
Dec(J2000)= +56 56 43.9

with an error of 2.3 arcseconds (radius, 90% containment).
This is 3.7 arcsec from the initially reported XRT position
and 6.4 arcsec from the BAT position (GCN Circ. 6542), and 6.3 arcsec from 
the possible optical transient reported in De Ugarte Postigo et al. (GCN
Circ. 6548). The error circle overlaps with the USNO-B1.0 source noted in 
GCN Circ. 6542.

The lightcurve began with a constant flux up to T0+536s, and then decayed 
with alpha=-4.6 until about T0+1000s. A steep decay follows this, though
there are not enough data to accurately measure its slope. 
After T0+1400s the decay continues with a shallower slope of alpha=-1.2.
Several small flares appear on top of the continuum emission up to 
T0+1000s, some of which may be simultaneous with BAT peaks.

The PC mode spectrum including 14.6 ks of data from the second orbit 
onwards is well fit with a power law of photon index
Gamma=2.4+0.2,-0.1 (90%) absorbed only by the large Galactic column of 
3.4E21 cm^-2.
The 0.3-10 keV observed (unabsorbed) flux is 2.2E-12 (4.0E-12) erg/cm^2/s,
corresponding to a count rate of 4.9E-02 count/s.

The predicted fluxes at T0+24h and T0+48h are 2.3E-13 and 9.9E-14 
erg/cm^2/s respectively, assuming the decay continues with alpha=-1.2.

This is an official product of the Swift XRT team.