GCN Circular 5782

Pall Jakobsson (U. Hertfordshire), Johan P. U. Fynbo,
Brian L. Jensen , Jens Hjorth, Maximilian D. Stritzinger
(DARK, NBI), Andrew Levan (U. Hertfordshire), Nial Tanvir
(U. Leicester), Paul Vreeswijk, Cedric Ledoux (ESO),
Houri Ziaeepour (UCL-MSSL) and Mathew Smith (U. Portsmouth)
report on behalf of a larger collaboration:

We have performed follow-up observations of the GRB 061004
optical afterglow candidate (S1) reported in Jakobsson et al.
(GCN 5698
Not found
 
 
). The S1 flux is roughly constant between the
three epochs of observations:


Tel/inst   Date (UT)    Delta-t   Exp. time     Magnitude
                         (days)    (s)
--------------------------------------------------------------
VLT/FORS1  Oct  5.366    0.539    2*300         22.55 +/- 0.09
VLT/FORS1  Oct  7.344    2.517    2*300 + 120   22.82 +/- 0.07
NTT/EMMI   Oct 15.352   10.525    3*500         22.73 +/- 0.20


In addition, S1 is 3.9" from the edge of the revised XRT error
circle (Racusin et al. GCN 5774
Not found
 
 
). The most likely conclusion
is that S1 is not associated with the GRB.

There is no evidence for other sources inside the revised XRT
error circle. The limiting magnitude of our first epoch
observation is R > 25.1 (2 sigma). The corresponding upper limit
on beta_OX is 0.42, rendering the burst dark according to the
definition proposed by Jakobsson et al. (2004, ApJ, 617, L21).

Considering the brightness of S1, it is unlikely that it is a
normal galaxy if z ~ 3.3 (GCN 5698
Not found
 
 
). The relatively low S/N of
the VLT spectrum makes it difficult to reach a firm conclusion
on the reality of absorption features (apart from the feature
around 5230 A). The lack of broad emission lines and the
marginal evidence for optical variability might indicate that
S1 is a blazar. An alternative scenario is that S1 is at a lower
redshift. The 5230 A feature could be the Balmer/4000 A break,
indicating a redshift of z ~ 0.3.

We thank Kim Page for providing us with the afterglow X-ray flux.