[vsnet-grb-info 8486] GRB 090904A: afterglow confirmation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  10000
SUBJECT: GRB 090904A: afterglow confirmation
DATE:    09/10/08 05:16:08 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani at 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