On 06 April 2002, the ASM detected a hard, brief, bright (peak
5-12 flux ~2 Crab) X-ray flare from a celestial location initially
thought to be inconsistent with any known X-ray source. We
tentatively identified the burst as a GRB. However, we have found
evidence for episodes of previous outbursts from
this location, and have therefore reclassified this event as a rapid
X-ray transient akin to CI Cam or V4641 Sgr.
The latest event appears to have be a single flare (the ASM
scanned off the source before the flux went to zero), beginning at
around 2002/04/06 18:18:15 (UTC). The flare was detected in two of
the ASM scanning shadow cameras in two successive dwells. A light
curve for the event is shown in 1-s bins in the below figure:
Here is the light curve in the three ASM bands, in 9-s bins,
along with the hardness ratio between the C and B bands. The
hardening of the spectrum is clear.
Using the strongest detection in each SSC, the source can be localized
to within a parallelogram (with 90% confidence) centered on the
coordinates (J2000.0):
| 285.441443 | 1.437693 |
| 19h 01m 45.95s | 1°26’15.7” |
With the following corners:
| 285.4381 | 1.4150 |
| 285.3389 | 1.4563 |
| 285.4472 | 1.4595 |
| 285.5416 | 1.4201 |
As shown in the figure, below, the thick lines represent the
localizations from the second dwell, and the thin lines are the error
boxes derived from the second dwell. The corners above define the
parallelogram that contains the intersection of all four of these
error boxes. Each of the two smallest error boxes is about 3’
across, and since they cross at a 24° angle, that makes each
side roughly 7’ long.
This position greatly refines our localization reported in GCN
#1132 and appears to rule out the counterpart candidate
proposed by Durig & McDermott in GCN
#1138.
In the summer of 1999, we performed a search through all the archived
ASM data up to that time for evidence of rapid X-ray transients that
might be Gamma-Ray Bursts. still under construction
|