Detection of bursting activity with INTEGRAL/SPI-ACS, possibly from 1E 2259+586 or SGR 1806-20

ATel #4101; V. Savchenko (ISDC/University of Geneva, Switzerland), S. Mereghetti (INAF/IASF-Milano, Italy), C. Ferrigno, E. Bozzo, T. J.-L. Courvoisier (ISDC/University of Geneva, Switzerland), D. Goetz (Cea/Saclay, France), J. Borkowski (CAMK, Torun, Poland), A. von Kienlin, A. Rau, X. Zhang (MPE-Garching, Germany), V. Beckmann (APC Paris, France)
on 8 May 2012; 10:21 UT
Credential Certification: Carlo Ferrigno (Carlo.Ferrigno@unige.ch)

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We report on a possible detection of bursting activity (tentatively from the magnetars 1E 2259+586 or SGR 1806-20) with the SPI Anti-Coincidence System (ACS) on-board INTEGRAL.

From 2012-04-19T12:03:10 to 2012-04-21 06:28:49 UTC, the ACS detected 25 short (from 50ms to 8s) bursts at a significance level of 5-10 sigma (peak count-rate of 1×10⁴ - 1×10⁵ counts/s on 50 ms time bin). Other 7 bursts were detected from 2012-05-05T06:04:47 to 2012-05-07T16:25:07 UTC. The highest peak count-rate recorded by the ACS was 1.5×10⁵ counts/s (detection significance up to 50 sigma).

A rough conversion between the ACS count-rate and fluence is 1×10^-10 erg/cm² in the 75keV-1MeV range per one ACS count (Viganò & Mereghetti, 2009, arXiv:0912.5329).

As no spatial information are available from the ACS, we cannot firmly associate this activity with any known source. At the best of our knowledge, two magnetars were recently reported to be active: SGR1806-20 and 1E 2259+586 (ATel #4080, GCN #13237, GCN #13280). The events that we detected have not been reported by other GRB monitoring experiments. If these are non detections, it might be due to the intrinsic faintness or (more likely) a peculiar spectral shape of these events.

We stress that no instrumental issues that could give rise to similar effects have ever been observed from the ACS in 10 years of operations.

The ACS light curves of all the bursts are available at:

SPI ACS bursts