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VERITAS Detects VHE Gamma-Ray Emission from LS I +61 303 Near Periastron

ATEL # 2948; Rene A. Ong (UCLA), for the VERITAS Collaboration
on 17 Oct 2010; 0:04 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Rene Ong (

Subjects: Gamma Ray, Request for Observations, Binaries

LS I +61 303 is a HMXB system composed of a Be star and a compact object of unknown nature (Casares et al. 2005, MNRAS, 360, 1105). Its elliptical orbit has a period of 26.5 days, with periastron occurring at phase 0.225 and apastron at phase 0.775. LS I +61 303 has previously been detected as a VHE (E>100 GeV) gamma-ray source, with significant emission only observed near apastron passage, around phases 0.5-0.8 (Albert et al. 2006, Science, 312, 1771; Acciari et al. 2008, ApJ, 679, 1427). No VHE emission at any phase has been reported since summer 2008. Observations with the VERITAS gamma-ray observatory between Oct 7 and Oct 12 2010, corresponding to the orbital phase range of 0.05 to 0.23, reveal evidence for VHE gamma-ray emission with a statistical significance of greater than 5 standard deviations. The total exposure was 7.7 hours, and the measured flux is approximately 5% of the Crab Nebula flux above 300 GeV. VERITAS will continue to monitor this source and hard X-ray observations with Swift have also been scheduled. Since the detection of VHE emission from LS I +61 303 during these orbital phases is not expected, we encourage observations at all wavelengths.

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