GCN Circular 10000
Subject
GRB 090904A: afterglow confirmation
Event
Date
2009-10-08T05:16:08Z (17 years ago)
From
Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst <malesani@astro.ku.dk>
Daniele Malesani (DARK/NBI), Pall Jakobsson (Univ. Iceland), and Ilya
Ilyin (AIP), report on behalf of a larger collaboration:
We observed again the field of GRB 090904A (Perri et al., GCN 9879) with
the NOT equipped with StanCam. Observations were carried out in the R
band, for a total exposure time of 30 min, with a mean time September
6.218 UT (2.176 days after the GRB trigger).
The object identified in our previous epoch of imaging (Malesani et al.,
GCN 9884) has clearly faded and is hardly detectable, if at all. A faint
source is visible at its position, with magnitude R = 24.0 +- 0.3. If
real, the inferred decay slope is alpha = 0.55 +- 0.12, assuming F(t)
propto t^-alpha. The value of alpha is shallower than usually found at
late times, and may indicate some host contribution or an unusual light
curve.
The position of the transient is consistent with the latest available
UVOT-enhanced XRT position (Goad et al., GCN 9885; see also
http://www.swift.ac.uk/xrt_positions/index.php). We thus identify the
object as the optical afterglow of GRB 090904A.
A finding chart comparing the two epochs can be found at the following URL:
http://www.astro.ku.dk/~malesani/GRB/090904A/GRB090904A_finder_NOT.jpg