{
  "bibcode": "1999GCN...506....1C",
  "body": "Robin Corbet (GSFC/USRA) and Don Smith (MIT) report for the RXTE\nAll-Sky Monitor team at GSFC and MIT:\n\nUsing the RXTE ASM we have extracted an X-ray light curve for the\nposition of the optical afterglow from GRB991216 (Uglesich et al., GCN\n472) and we find evidence suggesting a detection of the X-ray afterglow\n(2-12 keV) at only one hour after the burst.\n\nObservations of this location were obtained as part of the normal ASM\nsky-monitoring program.  Standard ASM observations (\"dwells\") are 90\nseconds long, and the ASM rotates between dwells such that a large\nfraction (~80%) of the sky is observed over 90 minutes (Levine et al.\n1996, ApJ, 469, L33).\n\nThe first ASM dwell that covered the location of GRB991216 was obtained\n0.99 hours after the peak of the burst (Kippen et al. GCN 463) and a\ntotal of 7 dwells were obtained during the next 11 minutes. From these\n7 dwells we derive a mean flux of 32 +/- 8 mCrab (1 sigma error).  A\nsecond sequence of 10 dwells covering this location was obtained\nstarting 2.57 hours after the burst peak in an interval of about 15\nminutes.  This second sequence yields a mean flux of 12 +/- 4 mCrab.\nFor comparison, we note that two other later clusters of dwells,\ncentered on times of ~5.8 hours and ~7.5 hours after the burst peak,\nyield mean fluxes of 4 +/- 4 and 0.3 +/- 4 mCrab respectively.\n\nTakeshima et al. (GCN 478) report a power-law fit to the two X-ray\nafterglow measurements made with the RXTE PCA at 4 and 11 hours after\nthe burst. An extrapolation of this fit to earlier times predicts\nfluxes at 1.1 and 2.7 hours after the burst peak of 42 and 10 mCrab\nrespectively. These are completely consistent with the mean ASM fluxes\ngiven above.  The average spectrum of the afterglow during the first\nASM sequence seems to be slightly steeper than that of the Crab nebula,\nwith a spectral index of 1.8 +/- 0.3.  This measurement is consistent\nwith the spectral index of 2.1 determined from the PCA (ibid.).\n\nWhile GRB afterglows are generally faint and thus difficult for the\nRXTE ASM to study, due to its modest collecting area and short\nobservation times, we believe that this unusually bright afterglow has\nindeed been detected.  These observations will require more detailed\nanalysis, but the indications are that the RXTE ASM is providing a\nmeasurement of the X-ray afterglow light curve at times which have\npreviously not been studied.\n\n\nThis message may be cited.",
  "circularId": 506,
  "createdOn": 945804700000,
  "email": "corbet@bastet.gsfc.nasa.gov",
  "subject": "GRB 991216 RXTE ASM Observations",
  "submitter": "Robin Corbet at NASA-GSFC  <corbet@bastet.gsfc.nasa.gov>",
  "eventId": "GRB 991216"
}