GCN Circular 506

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
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) 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
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) 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
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) 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.


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