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Swift BAT detection of a SGR-like burst from LS I+ 61 303

ATEL # 1715; Guillaume Dubus (LAOG), Berrie Giebels (LLR)
on 11 Sep 2008; 22:28 UT
Password Certification: Berrie Giebels (

Subjects: X-ray, Gamma Ray, >GeV, Binaries, Gamma-Ray Bursts, Pulsars, Soft Gamma-ray Repeaters

The Swift BAT triggered on a burst in the direction of the gamma-ray binary LS I +61 303 (V615 Cas) on Sept. 10, 2008 at 12:52:21 UT (de Pasquale et al. GCN 8209). Observations by the Swift XRT (Evans et al. GCN 8211) starting 921s after the BAT trigger detect LS I +61 303 at a flux level (9.2e-12 erg/cm^2/s) and with a spectrum (Nh=6e21 cm^-2, photon index 2.0) consistent with its habitual state (Leahy et al. 1997, Greiner & Rau 2001, Albert et al. 2008).

The Swift team further reports (Barthelmy et al. GCN 8215) that (1) the BAT source is localized with 2.2 arcminute accuracy (90% containment) and is 1.5 arcminute away from LS I+61 303 ; (2) the burst was single peaked with a duration of 0.23 seconds (T90) ; (3) the best fit spectrum is a blackbody kT=7.5 keV ; (4) the burst fluence was 1.6e-8 erg cm^-2. The BAT team analysis is available here.

The burst location, lightcurve, duration, fluence and spectra are fully consistent with a soft gamma-ray repeater / anomalous X-ray pulsar short duration burst originating from LS I +61 303. At a distance of 2.3 kpc (Gregory et al. 1979), the total energy released was 1e37 ergs, in line with this interpretation (e.g. Woods & Thompson 2006). This is the first manifestation of magnetar-like activity in a high mass X-ray binary. The nature of the compact object in orbit around the Be companion has been debated and is of major interest to interpret the high energy gamma-ray emission from this system (Mirabel 2006, Science, 312, 1759).

Based on the above, we conclude LS I +61 303 is most likely to host a young, highly magnetized pulsar.

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R. E. Rutledge , Editor
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