The INTEGRAL SPI veto system observed 25 mysterious bursts
The detectors of the gamma-ray spectrometer SPI on-board INTEGRAL
are shielded by the anti-coincidence veto system (ACS) against
charged particles and background radiation. This shield also serves
as an exceptional gamma-ray and hard X-ray burst detector: the huge
detection area for photons with energies between 100 keV and more
than 50 MeV make SPI-ACS substantially more sensitive to short hard
bursts than any other instrument in space. The SPI-ACS is collecting
one of the largest samples of the cosmological Gamma-Ray Bursts (GRBs),
but the data were also found to be very useful for studying bursts
from extremely magnetized neutron stars (magnetars): Soft Gamma
Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs)
From noon April 19 to early morning April 21, the SPI-ACS recorded
18 mostly short (50 ms to 8 s) bursts at a significance level of
5-10 sigma (peak count-rate of 1x104 - 1x105
counts/s per 50 ms time bin). Another 7 bursts were detected on
May 5 and 7. The highest peak countrate recorded by the ACS was
1.5x105 counts/s (detection significance up to 50 sigma).
SPI-ACS light curves of some of the brightest events are shown in
the picture.
Judging from the temporal properties, this episode resembled the
activity of a magnetar. Magnetars are known to occasionally produce
outbursts, when 10's to 100's bursts are emitted within a few days.
Because no spatial information is available from the SPI-ACS, the
bursting activity cannot firmly be associated with any known source.
However, two magnetars were reported to be active around this time:
SGR1806-20 and 1E 2259+586 (ATel #4080, GCN #13237, GCN #13280);
both were favorably located for the SPI-ACS at the time of the bursts.
But, magnetar bursts were always observed to be relatively soft
(with a spectrum peaking around 10's of keV at most), and hence
brighter in other burst detectors than in the SPI-ACS. Amazingly,
not a single burst of the two clusters observed from late April to
early May has been detected by any other instrument, despite sufficient
exposure. It is likely, that the spectra of the bursts were very hard,
not only harder than normal magnetar bursts but also harder than typical
cosmological gamma-ray bursts (peaking at about 1 MeV). Unfortunately,
the spectrum cannot be measured with the SPI-ACS detector.
As far as it is known, the SPI-ACS detectors were operating nominally
during these observations, and during nearly 10 years of operation, no
instrumental effects had been identified which could explain the data.
