GCN Circular 8422
Subject
Fermi LAT/GBM short-hard GRB 081024B: Possible TLS Afterglow
Event
Date
2008-10-28T01:22:45Z (18 years ago)
From
Alexander Kann at TLS Tautenburg <kann@tls-tautenburg.de>
We observed the LAT error circle of the Fermi-LAT (Omodei, GCN 8407) and
Fermi-GBM (Connaughton and Briggs, GCN 8408) detected short/hard GRB
081024B with the TLS Tautenburg 1.34m Schmidt telescope in full frame mode
(field of view 42' x 42'), covering the complete error circle to about 2
sigma confidence level. We obtained 24 x 300 sec images in the Rc band
under good observing conditions, at mid-time 0.88644 days after the GRB.
We report the following analysis of the three XRT sources (Guidorzi et
al., GCN 8410), of which source 1 and 3 seem to remain constant (Guidorzi
& Margutti, GCN 8416), whereas source 2 may be fading and thus represent
the X-ray afterglow of GRB 081024B:
Source 1: We find a relatively bright stellar object which is clearly
visible in the DSS.
Source 3: We find a moderately faint stellar object which is under the DSS
limit.
Source 2: We detect all four objects reported by Cenko & Kasliwal (GCN
8417). Source O3 is close to the detection limit, implying that it is ~ Rc
> 23.5, similar to that of the P200 image.
We use several nearby, isolated USNOB1.0 stars to determine the image zero
point. We find that the zero point error is 0.04 magnitudes from four
stars, better than the pessimistic estimation (0.3 magnitudes) of Cenko &
Kasliwal (which also includes bandpass differences). Photometry was done
using a 7 pixel aperature (matched to the typical seeing) on sources O1
and O4 (which are reasonably isolated). For sources O2 (close to a bright
star) and O3 (close to O2), we used a 3 pixel aperature and used two
bright, isolated stars to compute the aperature correction (a factor of
2).
We derive the following magnitudes for the four sources (in brackets, we
give the magnitudes of Cenko & Kasliwal):
O1 (bright, stellar): Rc = 20.55 +/- 0.04 (20.6)
O2 (extended in P200): Rc = 22.25 +/- 0.10 (22.6)
O3 (faint): Rc = 23.48 +/- 0.34 (23.4)
O4 (faint, stellar): Rc = 22.53 +/- 0.16 (22.6)
All sources except O2 agree well with the P200 magnitudes obtained 0.36
days later, to within +/- 0.08 magnitudes despite the multiple differences
(comparison stars, bandpass, etc.). Only source O2 is brighter, by ~ 0.35
magnitudes. While the source is close to a bright (~ magnitude of O1)
star, which may contaminate the photometry even in the reduced aperature,
O2 is clearly brighter in our image than O4, whereas they have identical
magnitudes (as reported by Cenko & Kasliwal) in the P200 images. Since
Cenko & Kasliwal report this source to be extended, this may indicate that
we are seeing the optical afterglow as well as the underlying host galaxy
of GRB 081024B. Still, caution is advised.
This GRB is of special interest. It is the first Fermi-detected short/hard
GRB reported so far, and possibly the first one with a detection at >> 1
GeV (the very bright GRB 930131, the "Superbowl Burst", was only detected
to ~ 1 GeV by CGRO EGRET, Sommer et al. 1994, ApJL, 422, L63), and,
similar to GRB 930131 and also the AGILE-Grid-detected long GRB 080514B
(Guiliani et al. 2008, A&A, in press), emission is seen for several
seconds longer than at lower energies. Therefore, additional deep
follow-up and spectroscopy of extended source O2 is highly encouraged.
Further observations at TLS are planned (to perform image subtraction) in
case we ever see the stars again.
This message may be cited.