[vsnet-grb-info 13215] GRB 130502B: possible TNG counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  14552
SUBJECT: GRB 130502B: possible TNG counterpart
DATE:    13/05/04 09:19:10 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani at dark-cosmology.dk>

D. Malesani (DARK/NBI), P. D'Avanzo (INAF/OABr), S. Covino (INAF/OABr), 
A. Melandri (INAF/OABr), V. D'Elia (INAF/OAR and ASI/ASDC), S. Campana 
(INAF/OABr), D. Fugazza (INAF/OABr), G. Tagliaferri (INAF/OABr), B. Blu, 
L. Di Fabrizio (INAF/FGG), C. P. Padilla-Torres (INAF/FGG), report on 
behalf of the CIBO collaboration:

We observed the field of the LAT-detected GRB 130502B (Von Kienlin & 
Younes, GCN 14530; Kocevski et al., GCN 14532; Golenetskii et al., GCN 
14542) with the TNG equipped with the DOLoRes imager. Observations were 
carried out starting on 2013 May 3.872 UT (1.54 days after the trigger), 
for a total of 30 min on source in the R band, at a high airmass of 
around 2.2. The seeing was 1.2".

No bright object is detected inside the X-ray error circle (Melandri & 
Immler, GCN 14540; see also http://www.swift.ac.uk/xrt_positions/), down 
to a 3-sigma limit of R = 23.7 (calibration based on several nearby USNO 
stars). However, a faint flux enhancement is seen consistent with the 
XRT position, at coordinates (J2000):

RA = 04:27:02.82
Dec = +71:03:38.5

A finding chart is shown at 
http://www.astro.ku.dk/~malesani/GRB/130502B/GRB130502B_finder.jpg

We estimate the magnitude of this marginally detected source to be R = 
24.7 +- 0.5. At the moment, we have no information about its 
variability, and it could be either the GRB counterpart or its host galaxy.

Last, independently of the reality of the object, this GRB can be 
classified as dark. Considering the limit R > 23.7, and a Galactic 
extinction A_V = 0.57 (Schlafly & Finkbeiner 2011, ApJ, 737, 103), the 
optical-to-X-ray spectral index is beta_OX < 0.47 (if the object is 
real, then beta_OX = 0.34 +- 0.07). The presence of significant excess 
column density in the X-ray spectrum (Melandri & Immler, GCN 14540; see 
also https://www.swift.ac.uk/team/xrt_spectra/00020266/) suggests an 
extinguished event.