[vsnet-grb-info 13656] Fermi GRB 130702A: Swift XRT and UVOT observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  14973
SUBJECT: Fermi GRB 130702A: Swift XRT and UVOT observations
DATE:    13/07/03 19:01:40 GMT
FROM:    Andrea Melandri at INAF-OAB  <andrea.melandri at brera.inaf.it>

P. D'Avanzo (INAF-OAB), B. Porterfield, D. N. Burrows, M. Siegel  
(PSU), A. Melandri (INAF-OAB), P. A. Evans (Univ. Leicester) report on  
behalf of the Swift team:

The Fermi GBM and LAT instruments detected GRB 130702A at 00:05:23 UT  
on July 2, 2013 (Fermi trigger 394416326; Cheung et al. 2013, GCN Circ  
14971). Followup by the Palomar Transient Factory (PTF) located a  
possible optical counterpart, iPTF13bxl (Singer et al. 2013, GCN Circ.  
14967).  A subsequent TOO observation by Swift located a coincident X- 
ray counterpart, making this a likely counterpart of GRB 130702A  
(Singer et al. 2013, GCN Circ 14971). Here we report the results of  
the analysis of Swift XRT and UVOT data for this object.

We have analysed 3.4 ks of XRT data for this object,  from 89.1 ks to  
95.7 ks after the Fermi/GBM trigger.  The data are entirely in Photon  
Counting (PC) mode.  Using 3411 s of PC mode data and 5 UVOT images,  
we find an enhanced XRT position (using the XRT-UVOT alignment and  
matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =  
217.31166, +15.77388 which is equivalent to:

RA (J2000): 14h 29m 14.80s
Dec(J2000): +15d 46' 26.0"

with an uncertainty of 1.5 arcsec (radius, 90% confidence). This  
position is consistent with the optical counterpart reported by Singer  
et al. 2013 (GCN Circ. 14967; see also Guidorzi et al. 2013, GCN Circ.  
14968).

The light curve displays variability consistent with a fading  
behaviour. More observations are scheduled to confirm whether the  
source is fading.

A spectrum formed from the PC mode data can be fitted with an absorbed  
power-law with a photon spectral index of 1.99 (+/-0.16). The best- 
fitting absorption column is  6.3 (+3.4, -3.1) x 10^20 cm^-2, in  
excess of the Galactic value of 1.7 x 10^20 cm^-2 (Kalberla et al.  
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion  
factor deduced from this spectrum  is 3.6 x 10^-11 (4.2 x 10^-11) erg  
cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column:	     6.3 (+3.4, -3.1) x 10^20 cm^-2
Galactic foreground: 1.7 x 10^20 cm^-2
Excess significance: 2.4 sigma
Photon index:	     1.99 (+/-0.16)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00032876.


The Swift/UVOT began settled observations of the field of GRB 130702A  
89111 s after the GBM detection.  A source consistent with the P48  
position (Singer et al. 2013, GCN Circ. 14967) is detected in the  
initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  14:29:14.77 = 217.31155 (deg.)
    Dec (J2000) = +15:46:26.5  =  15.77403 (deg.)
with an estimated uncertainty of 0.50 arc sec. (radius, 90% confidence).

Preliminary detections using the UVOT photometric system (Breeveld et  
al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)           Mag

white            89232        95967          3414         18.26 +/- 0.04

The magnitudes in the table are not corrected for the Galactic  
extinction due to the reddening of E(B V) = 0.04 in the direction of  
the burst (Schlegel et al. 1998).


This circular is an official product of the Swift team.