GCN Circular 8850
Subject
GRB 090118: no afterglow candidate
Event
Date
2009-01-24T19:23:07Z (17 years ago)
From
Jochen Greiner at MPI <jcg@mpe.mpg.de>
A. Rossi (Tautenburg Obs.), T. Kruehler, J. Greiner and F. Olivares
(all MPE Garching), report on behalf of the GROND team:
We report on further analysis and new observations of the optical afterglow
candidate of GRB 090118 (Copete et al. 2009, GCN #8825).
We re-analyzed our first epoch images (Olivares et al. 2009, GCN Circ. 8826),
taken at about 11 hrs after the GRB. No source is seen in our NIR images at
the position of the X-ray afterglow (Rowlinson et al. 2008, GCN Circ. 8828),
giving upper limits (Vega system) of
J > 20.6
H > 19.6
K > 18.7
calibrated against 2MASS field stars.
The XRT position (Rowlinson et al. 2009, GCN Circ. 8828) is outside of the
field-of-view of the g'r'i'z' detectors, so no statement can be made on
the optical brightness during our first epoch.
We observed the field of GRB 090118 again with GROND on 2009-01-22 at a
mid-time of 01:30 UTC, about 100 hours after the burst. We clearly detect
the candidate mentioned by Updike and Hartmann (2009, GCN Circ. 8829) in
our optical filters, but not in the NIR:
g'= 23.46 +- 0.11 (AB)
r'= 23.27 +- 0.10 (AB)
i'= 22.95 +- 0.12 (AB)
z'= 22.25 +- 0.14 (AB)
J > 21.5 (Vega)
H > 20.5 (Vega)
K > 19.6 (Vega)
No correction for the foreground extinction of E(B-V)=0.135 mag
has been applied.
Comparing our g'r'i'z' values with the J and H detections of Updike & Hartmann
(2009a; GCN Circ. 8829) and Minowa et al. (2009; GCN Circ. 8832) about two
days earlier, we find that the spectral energy distribution is smooth, thus
supporting the non-fading nature of this object as indicated already by the
results of Updike & Hartmann (2009b; GCN Circ. 8831) and Minowa et al.
(2009; GCN Circ. 8832).
The detection in g'r'i'z' implies that the object, whatever its nature,
is at a redshift smaller than 3.5.
This excludes the possibility that this object is at high redshift and
exhibited a long plateau phase. The plateau phase would have to last more
than 3 days in the NIR range, while at X-rays a steady decline has been
seen (Rowlinson & Page 2009, GCN Circ. 8840). We conclude that this object
is very likely not the optical/NIR afterglow of GRB 090118. Instead, the
object's SED is consistent with a low-redshift galaxy.