Chandra discovery of 3.2s X-ray pulsations from SGR 1935+2154

ATel #6370; Gian Luca Israel (INAF-OAR), Nanda Rea (CSIC-IEEC; U. Amsterdam), Francesco Coti Zelati (U. Insubria; INAF-OAB; U. Amsterdam), Paolo Esposito (INAF-IASF), Marta Burgay (INAF-OAC), Sandro Mereghetti (INAF-IASF), Andrea Possenti (INAF-OAC), Andrea Tiengo (IUSS Pavia), on behalf of a larger collaboration
on 5 Aug 2014; 16:59 UT
Credential Certification: GianLuca Israel (gianluca@mporzio.astro.it)

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Following the Swift-BAT detection of a short and relatively soft burst from less than 1 degree from the Galactic plane (GRB 140705A; GCN Circ. 16520, 16521, 16522 and 16530), and similar to those usually displayed by Soft Gamma-ray Repeaters, we have activated our Chandra (ID 15875; PI: Rea) ToO program.

Chandra observed SGR 1935+2154 on the 15th and 28th of July 2014 for 9ks (ACIS-S, TE-mode 1/8 sub-array, sampling time of 0.44s) and 75ks (ACIS-S CC-mode, sampling time of 2.85ms). During the 28th July dataset we detected highly significant, >10sigma, pulsations at a period of 3.24498(1) s (1 sigma c.l.). Folding with this period the first, shorter, Chandra observation we could detect the signal at a lower confidence level.

Several ToO Swift pointings, for a total exposure time of about 55ks, were also analysed covering the time interval between the BAT trigger and the first Chandra observation. An a posteriori search for the 3.2 s pulsations reveals the presence of the signal albeit at a very low confidence level in some of the pointings. We thank the Chandra team for the prompt scheduling of these observations.