GCN Circular 8881

ICSP VLF observation of the signatures of SGR/AXP 1E1547.0-5408 bursts

Sandip K. Chakrabarti (S. N. Bose Centre and ICSP), Sushanta K. Mondal (ICSP),
Sudipta Sasmal (ICSP) and Debashis Bhowmick (ICSP)


A number of recent satellite observations have reported the outbursts of
anomalous X-Ray Soft gamma repeater AXP 1E1547.0-5408 which reportedly 
started from January 22nd, 2009 (GCN #8833, 8835, 8837, 8838). We looked at
the ionospheric data of Indian Centre for Space Physics (ICSP) obtained by
receiving Very Low Frequency (VLF) signal from the VTX station at 18.2KHz 
for the 22nd of January (IST), 2009 (21st Jan. 18:30 UT to 22nd Jan. 18:30 
UT) using a Gyrator-II receiver. We found statistically significant 
signatures of short bursts which either suddenly amplified the signal 
or suddenly attenuated the signal. Each event lasted for a few seconds 
as the recombination time is usually longer than the burst duration. We
observe 30 significant signatures out of which 9 were not reported by 
others and these events mostly took pace in late 21st January, 2009 (UT)
before other satellite results were reported. However, the timings of the
remaining 21 events match with the satellite observations. A list and the
light curves of all the major events seen by us in the 22nd January (IST) 
can be found in http://www.bose.res.in/~chakraba/axpsgr.html.

Since both the amplifications and the attenuations were seen, often one
after the other, each event could tell us about the energy spectrum in 
each burst as the upper ionosphere would be primarily affected by 
soft-X-rays and the lower ionosphere would be primarily affected by the
hard-X-rays. The highly tangential source illumination of the ionosphere 
as seen from ICSP (22d34m N, 88d21m E) was probably advantageous. 
We encourage other VLF observers to look into such signatures and 
particularly the latitude dependence.