TITLE:   GCN CIRCULAR
NUMBER:  5821
SUBJECT: GRB 050915B: optical afterglow
DATE:    06/11/13 22:50:34 GMT
FROM:    Daniele Malesani at Niels Bohr Inst,Dark Cosmology Center  <malesani@astro.ku.dk>

D. Malesani (SISSA/ISAS & NBI-DARK), S. Covino (INAF/OABr), S. 
Piranomonte (INAF/OAR), L.A. Antonelli (INAF/OAR), G. Chincarini (Univ. 
Milano-Bicocca & INAF-OABr), P. D'Avanzo (INAF/OABr), L. Stella 
(INAF/OAR), and G. Tagliaferri (INAF/OABr), report on behalf of the 
MISTICI collaboration:

We observed the field of one-year-old GRB 050915B (Falcone et al., GCNs 
3987, 389) with the ESO-VLT UT2 equipped with FORS1. Observations 
started on 2005 Sep 16.01065 UT (2.87 hr after the GRB) and consisted in 
a series of 24 R-band images lasting 1 minute each. The field was 
observed again, in the R band, with the ESO-NTT equipped with the SuSI2 
imager, with mean time 2005 Oct 1.0261 UT (15.13 days after the GRB). 
The field is very crowded and reddened due to the low Galactic latitude 
(b = -6.6 deg).

Inside the XRT error circle (Moretti et al. 2006, A&A, 448, L9) there 
are three sources, but only two are consistent with the 
boresight-corrected position (Butler 2006, astro-ph/0611031 + priv. 
comm.), which has a 90% error radius of 1.4". One of them is clearly 
fading between the VLT and NTT exposures, being undetectable in the NTT 
images. Its coordinates (J2000) are:

alpha = 14:36:26.17
delta = -67:24:32.5

It is remarkable, however, that no fading is seen for this source over 
the course of the VLT observation (2.87-3.66 hr after the GRB). 
PSF-matched profile photometry revealed that the source was constant 
(<R> = 21.68 +- 0.03) to within ~0.1 mag, with no trend for decline. In 
the SuSI2 image (15 days after the burst), it has R > 23.4 (3-sigma 
upper limit).

The other object consistent with the X-ray position has coordinates

alpha = 14:36:26.39
delta = -67:24:31.6

and is constant between the FORS1 and SuSI2 exposures at R = 21.2. This 
source, which is just 1.5" away from the afterglow, might be consistent 
with source "B" reported by Cobb & Bailyn. (GCN 3994).

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