{
  "bibcode": "2008GCN..8422....1K",
  "body": "We observed the LAT error circle of the Fermi-LAT (Omodei, GCN 8407) and \nFermi-GBM (Connaughton and Briggs, GCN 8408) detected short/hard GRB \n081024B with the TLS Tautenburg 1.34m Schmidt telescope in full frame mode \n(field of view 42' x 42'), covering the complete error circle to about 2 \nsigma confidence level. We obtained 24 x 300 sec images in the Rc band \nunder good observing conditions, at mid-time 0.88644 days after the GRB.\n\nWe report the following analysis of the three XRT sources (Guidorzi et \nal., GCN 8410), of which source 1 and 3 seem to remain constant (Guidorzi \n& Margutti, GCN 8416), whereas source 2 may be fading and thus represent \nthe X-ray afterglow of GRB 081024B:\n\nSource 1: We find a relatively bright stellar object which is clearly \nvisible in the DSS.\n\nSource 3: We find a moderately faint stellar object which is under the DSS \nlimit.\n\nSource 2: We detect all four objects reported by Cenko & Kasliwal (GCN \n8417). Source O3 is close to the detection limit, implying that it is ~ Rc \n> 23.5, similar to that of the P200 image.\n\nWe use several nearby, isolated USNOB1.0 stars to determine the image zero \npoint. We find that the zero point error is 0.04 magnitudes from four \nstars, better than the pessimistic estimation (0.3 magnitudes) of Cenko & \nKasliwal (which also includes bandpass differences). Photometry was done \nusing a 7 pixel aperature (matched to the typical seeing) on sources O1 \nand O4 (which are reasonably isolated). For sources O2 (close to a bright \nstar) and O3 (close to O2), we used a 3 pixel aperature and used two \nbright, isolated stars to compute the aperature correction (a factor of \n2).\n\nWe derive the following magnitudes for the four sources (in brackets, we \ngive the magnitudes of Cenko & Kasliwal):\n\nO1 (bright, stellar):\tRc = 20.55 +/- 0.04\t(20.6)\n\nO2 (extended in P200):\tRc = 22.25 +/- 0.10\t(22.6)\n\nO3 (faint):\t\tRc = 23.48 +/- 0.34\t(23.4)\n\nO4 (faint, stellar):\tRc = 22.53 +/- 0.16\t(22.6)\n\nAll sources except O2 agree well with the P200 magnitudes obtained 0.36 \ndays later, to within +/- 0.08 magnitudes despite the multiple differences \n(comparison stars, bandpass, etc.). Only source O2 is brighter, by ~ 0.35 \nmagnitudes. While the source is close to a bright (~ magnitude of O1) \nstar, which may contaminate the photometry even in the reduced aperature, \nO2 is clearly brighter in our image than O4, whereas they have identical \nmagnitudes (as reported by Cenko & Kasliwal) in the P200 images. Since \nCenko & Kasliwal report this source to be extended, this may indicate that \nwe are seeing the optical afterglow as well as the underlying host galaxy \nof GRB 081024B. Still, caution is advised.\n\nThis GRB is of special interest. It is the first Fermi-detected short/hard \nGRB reported so far, and possibly the first one with a detection at >> 1 \nGeV (the very bright GRB 930131, the \"Superbowl Burst\", was only detected \nto ~ 1 GeV by CGRO EGRET, Sommer et al. 1994, ApJL, 422, L63), and, \nsimilar to GRB 930131 and also the AGILE-Grid-detected long GRB 080514B \n(Guiliani et al. 2008, A&A, in press), emission is seen for several \nseconds longer than at lower energies. Therefore, additional deep \nfollow-up and spectroscopy of extended source O2 is highly encouraged. \nFurther observations at TLS are planned (to perform image subtraction) in \ncase we ever see the stars again.\n\nThis message may be cited.",
  "circularId": 8422,
  "createdOn": 1225156965000,
  "email": "kann@tls-tautenburg.de",
  "subject": "Fermi LAT/GBM short-hard GRB 081024B: Possible TLS Afterglow",
  "submitter": "Alexander Kann at TLS Tautenburg  <kann@tls-tautenburg.de>",
  "eventId": "GRB 081024B"
}