TITLE: GCN CIRCULAR NUMBER: 8901 SUBJECT: SGR activity seen by BAT during ICSP Ionospheric Events DATE: 09/02/12 01:13:04 GMT FROM: David Palmer at LANL 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.