GCN Circular 8901
Subject
SGR activity seen by BAT during ICSP Ionospheric Events
Date
2009-02-12T01:13:04Z (16 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
David Palmer, on behalf of the Swift-BAT team, reports:
S.K Chakrabarti, et al. (GCN #8881) report disturbances in the
ionosphere observed by ICSP as measured by 18.2 kHz radio
propagation. Some of these were correlated with reported bursts from
AXP 1E1547.0-5408 (== SGR J1550-5418) and it was suggested that the
other disturbances indicated unreported activity.
Swift-BAT has seen over 800 bursts from this source, but a complete
catalog is still being prepared. However, the times of the ICSP
events can be examined to see how reliably VLF measurements of
ionospheric disturbances can determine SGR activity in a 'blind search'.
Of the 9 ionospheric events in GCN #8881 which did not correspond to
reported SGR events, all fortuitously occurred at times when the SGR
was above the horizon as seen by BAT, and five occurred when the SGR
was in BAT's Field of View. The BAT 15-100 keV count rate light
curves were examined with 64 ms resolution within one minute of the
times of these ICSP events to search for correlated SGR bursts. the
results are given in the following table:
Time of ICSP event Exposure
(UT from GCN #8881) cm^2 Detection
2009-01-21T19:50:57 1413 None
2009-01-21T21:16:50 0 None
2009-01-21T21:35:01 1300 None
2009-01-21T23:19:56 1361 None
2009-01-21T23:56:07 0 Possibly real[1]
2009-01-22T00:32:32 0 None
2009-01-22T05:41:22 4364 Coincidence[2]
2009-01-22T05:56:02 4364 Coincidence[3]
2009-01-22T09:34:09 0 None
[1] Double burst at T-10 seconds, peaking at 3 kcount/s. Due to the
orientation of the spacecraft, the SGR could illuminate the back of
the BAT detector only through the body of the Swift spacecraft, so
this would correspond to a much more intense burst than the comparable
count rate from a source in the FOV. If this is an SGR burst, it
marks the earliest known detection during this episode. (Previous
earliest was Fermi-GBM Trigger 254278434, at 2009-01-22T00:53:52.)
[2] Strong burst at T+15 (~100kcounts/s), midsize burst (~20 kcount/s)
at T+32, weaker bursts (<5 kcount/s) at T-42,T+8,T+11.
[3] Weak burst (5k/s) at T-8
The 2009-01-21T23:56:07 disturbance [1] is the most compelling one for
blind detection of activity. The non-detections by BAT on 2009-01-21
in those cases where the SGR location was in the FOV indicate that
those ionospheric disturbances were unrelated to the SGR. The burst
activity around the 2009-01-22T05:41:22 [2] and 2009-01-22T05:56:02
[3] disturbances is likely to be coincidental, considering the large
amount of activity occurring around that time: during the 40 minute
BAT observation from 5:37-6:17, there were 6 bursts comparable to or
stronger than the 05:41:37 strong burst (which came after the
ionospheric disturbance), and at least 32 bursts total seen by BAT.
From this we conclude that ionospheric disturbances are not a
reliable measure of SGR bursts in the absence of high energy
confirmation.