GCN Circular 8901

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.