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Is the SGR-like burst from LS I +61 303?

ATEL # 1731; Nanda Rea (University of Amsterdam) & Diego F. Torres (ICREA & IEEC-CSIC)
on 23 Sep 2008; 15:29 UT
Password Certification: Nanda Rea (N.Rea@sron.nl)

Subjects: X-ray, Request for Observations, Binaries, Neutron Stars, Pulsars, Soft Gamma-ray Repeaters, Transients

We report here on possible other candidates which might be responsible of the SGR-like burst detected from the direction of the high mass X-ray binary LS I +61 303.

On 2008 September 10th, Swift-BAT triggered on a short SGR-like burst from the direction of LS I +61 303 (de Pasquale et al. GCN 8209). The first BAT on-board location of the burst had an uncertainty of 3 arcmin radius, which has been subsequently refined to 2.2 arcminutes (all at 90% confidence level; Barthelmy et al. GCN 8215), centered 88 arcseconds away from the sub-arcsecond accurate position of LS I +61 303.

Swift-XRT did not detect the burst because started observing ~921s after the BAT trigger (Evans et al. GCN 8211), but clearly detect LS I +61 303 (being the brightest source in the field) at a flux level and a spectrum consistent with what was previously observed by other missions and Swift-XRT itself (Leahy et al. 1997, Greiner & Rau 2001, Sidoli et al. 2006, Esposito et al. 2008). Hence, unfortunately Swift-XRT could not refine the positional accuracy of the burst itself.

We analyzed an archival Chandra ACIS-I observation of LS I +61 303 performed on 2006 April 7th (~50ks; Paredes et al. 2007), where we detect many faint sources in the field of view. See the Chandra image here. In that image, green ellipses are the 3 sigma positional errors, centered on all the sources detected at a signal-to-noise > 3 (contact N. Rea for the complete list of the source positions and count rates). The localization of the burst provided by Swift is also marked.

On the basis of this SGR-like burst detected by Swift-BAT it has been very recently proposed that LS I +61 303 hosts a magnetar in a binary orbit with the massive star V615 Cas (Dubus & Giebels Atel# 1715), which would represent the first case of a magnetar in a binary system.

Although this possibility cannot be ruled out, we would like to draw the attention on the large number of faint X-ray sources in the ACIS-I field that might be the quiescent counterpart to a new transient magnetar close to LS I +61 303.

Chandra ACIS-I Fielf of View of LS I +61 303


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