RXTE QUICK RESPONSE OBSERVATIONS OF AFTERGLOWS FROM GRBS

T. TAKESHIMA(1,2), F. MARSHALL, A. VALINIA, J. SWANK(1), R. CORBET(1,2),
AND C. ROBINSON(3)

(1) NASA/GSFC
(2) USRA
(3) NASA/GSFC)

The detection of an x-ray afterglow from GRB 970228 with SAX about 8
hours after the burst raised a new question: "How does the afterglow
evolve shortly after a burst?"  A rapid determination of the GRB
position makes it easier to perform follow-up observations in various
wavelengths.  Preparations for quick response GRB observations with
the Rossi X-ray Timing Explorer (RXTE) began in early March, 1997, in
collaboration with the Burst and Transient Source Experiment (BATSE)
team.

By the end of June, RXTE performed two rapid response observations
within about four hours after the burst detections (GRB 970603 and GRB
970616).  During the observation of the BATSE error region of GRB
970616, a faint new source was detected by the Proportional Counter
Array (PCA), the main instrument on RXTE.  The source position
obtained with RXTE overlaps the annulus determined by the
interplanetary network (Ulysses and BATSE).  The PCA source count rate
was 4.5 -/+ 1.6 counts per second in the 2-10 keV range (1.1e-11
erg/s/cm2).  A follow-up observation made with RXTE about 27 hours after
the detection failed to detect the source, showing it to be transient.

With seven hours being the quickest RXTE response time for ToOs in
normal operation, a more rapid response time demands special
procedures.  Since the PCA is not an imaging detector, detecting faint
new sources requires scanning observations.  Scans covering a large
sky region are somewhat time consuming and the PCA is not as sensitive
as an imaging detector, we therefore only chase well-localized, bright
bursts.

We will present our procedures and strategy for quick response
observations of GRBs and describe the results.

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