{
  "bibcode": "2009GCN..8901....1P",
  "body": "David Palmer, on behalf of the Swift-BAT team, reports:\n\nS.K Chakrabarti, et al. (GCN #8881) report disturbances in the  \nionosphere observed by ICSP as measured by 18.2 kHz radio  \npropagation.  Some of these were correlated with reported bursts from  \nAXP 1E1547.0-5408 (== SGR J1550-5418) and it was suggested that the  \nother disturbances indicated unreported activity.\n\nSwift-BAT has seen over 800 bursts from this source, but a complete  \ncatalog is still being prepared.  However, the times of the ICSP  \nevents can be examined to see how reliably VLF measurements of  \nionospheric disturbances can determine SGR activity in a 'blind search'.\n\nOf the 9 ionospheric events in GCN #8881 which did not correspond to  \nreported SGR events, all fortuitously occurred at times when the SGR  \nwas above the horizon as seen by BAT, and five occurred when the SGR  \nwas in BAT's Field of View.  The BAT 15-100 keV count rate light  \ncurves were examined with 64 ms resolution within one minute of the  \ntimes of these ICSP events to search for correlated SGR bursts.  the  \nresults are given in the following table:\n\nTime of ICSP event  Exposure\n(UT from GCN #8881)   cm^2  Detection\n2009-01-21T19:50:57  1413   None\n2009-01-21T21:16:50     0   None\n2009-01-21T21:35:01  1300   None\n2009-01-21T23:19:56  1361   None\n2009-01-21T23:56:07     0   Possibly real[1]\n2009-01-22T00:32:32     0   None\n2009-01-22T05:41:22  4364   Coincidence[2]\n2009-01-22T05:56:02  4364   Coincidence[3]\n2009-01-22T09:34:09     0   None\n\n[1] Double burst at T-10 seconds, peaking at 3 kcount/s. Due to the  \norientation of the spacecraft, the SGR could illuminate the back of  \nthe BAT detector only through the body of the Swift spacecraft, so  \nthis would correspond to a much more intense burst than the comparable  \ncount rate from a source in the FOV.  If this is an SGR burst, it  \nmarks the earliest known detection during this episode.  (Previous  \nearliest was Fermi-GBM Trigger 254278434, at 2009-01-22T00:53:52.)\n\n[2] Strong burst at T+15 (~100kcounts/s), midsize burst (~20 kcount/s)  \nat T+32, weaker bursts (<5 kcount/s) at T-42,T+8,T+11.\n\n[3] Weak burst (5k/s) at T-8\n\nThe 2009-01-21T23:56:07 disturbance [1] is the most compelling one for  \nblind detection of activity.  The non-detections by BAT on 2009-01-21  \nin those cases where the SGR location was in the FOV indicate that  \nthose ionospheric disturbances were unrelated to the SGR.  The burst  \nactivity around the 2009-01-22T05:41:22 [2] and 2009-01-22T05:56:02  \n[3] disturbances is likely to be coincidental, considering the large  \namount of activity occurring around that time: during the 40 minute  \nBAT observation from 5:37-6:17, there were 6 bursts comparable to or  \nstronger than the 05:41:37 strong burst (which came after the  \nionospheric disturbance), and at least 32 bursts total seen by BAT.\n\n From this we conclude that ionospheric disturbances are not a  \nreliable measure of SGR bursts in the absence of high energy  \nconfirmation.",
  "circularId": 8901,
  "createdOn": 1234401184000,
  "email": "palmer@lanl.gov",
  "subject": "SGR activity seen by BAT during ICSP Ionospheric Events",
  "submitter": "David Palmer at LANL  <palmer@lanl.gov>"
}