[vsnet-grb-info 4006] GRB 050915B: optical afterglow
GCN Circulars
gcncirc at capella.gsfc.nasa.gov
Tue Nov 14 07:50:38 JST 2006
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 at 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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