TITLE: GCN GRB OBSERVATION REPORT NUMBER: 506 SUBJECT: GRB 991216 RXTE ASM Observations DATE: 99/12/21 19:31:40 GMT FROM: Robin Corbet at NASA-GSFC Robin Corbet (GSFC/USRA) and Don Smith (MIT) report for the RXTE All-Sky Monitor team at GSFC and MIT: Using the RXTE ASM we have extracted an X-ray light curve for the position of the optical afterglow from GRB991216 (Uglesich et al., GCN 472) and we find evidence suggesting a detection of the X-ray afterglow (2-12 keV) at only one hour after the burst. Observations of this location were obtained as part of the normal ASM sky-monitoring program. Standard ASM observations ("dwells") are 90 seconds long, and the ASM rotates between dwells such that a large fraction (~80%) of the sky is observed over 90 minutes (Levine et al. 1996, ApJ, 469, L33). The first ASM dwell that covered the location of GRB991216 was obtained 0.99 hours after the peak of the burst (Kippen et al. GCN 463) and a total of 7 dwells were obtained during the next 11 minutes. From these 7 dwells we derive a mean flux of 32 +/- 8 mCrab (1 sigma error). A second sequence of 10 dwells covering this location was obtained starting 2.57 hours after the burst peak in an interval of about 15 minutes. This second sequence yields a mean flux of 12 +/- 4 mCrab. For comparison, we note that two other later clusters of dwells, centered on times of ~5.8 hours and ~7.5 hours after the burst peak, yield mean fluxes of 4 +/- 4 and 0.3 +/- 4 mCrab respectively. Takeshima et al. (GCN 478) report a power-law fit to the two X-ray afterglow measurements made with the RXTE PCA at 4 and 11 hours after the burst. An extrapolation of this fit to earlier times predicts fluxes at 1.1 and 2.7 hours after the burst peak of 42 and 10 mCrab respectively. These are completely consistent with the mean ASM fluxes given above. The average spectrum of the afterglow during the first ASM sequence seems to be slightly steeper than that of the Crab nebula, with a spectral index of 1.8 +/- 0.3. This measurement is consistent with the spectral index of 2.1 determined from the PCA (ibid.). While GRB afterglows are generally faint and thus difficult for the RXTE ASM to study, due to its modest collecting area and short observation times, we believe that this unusually bright afterglow has indeed been detected. These observations will require more detailed analysis, but the indications are that the RXTE ASM is providing a measurement of the X-ray afterglow light curve at times which have previously not been studied. This message may be cited.