[vsnet-grb-info 6827] Fermi LAT/GBM short-hard GRB 081024B: Possible TLS Afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8422
SUBJECT: Fermi LAT/GBM short-hard GRB 081024B: Possible TLS Afterglow
DATE:    08/10/28 01:22:45 GMT
FROM:    Alexander Kann at TLS Tautenburg  <kann at 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.