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Period derivative of the Soft Gamma-ray Repeater SGR 1627-41

ATEL # 2068; P. Esposito, A. Tiengo, S. Mereghetti, A. De Luca (INAF-IASF Milano), G. L. Israel (INAF-O.A.Roma), D. Gotz (CEA-Saclay), N. Rea (U. Amsterdam), R. Turolla (U. Padova), S. Zane (MSSL/UCL), P. Romano (INAF-IASF Palermo), M. Burgay, A. Possenti (INAF-O.A.Cagliari)
on 6 Jun 2009; 12:51 UT
Password Certification: Paolo Esposito (paoloesp@iasf-milano.inaf.it)

Subjects: X-ray, Neutron Stars, Pulsars, Soft Gamma-ray Repeaters

After nearly a decade of quiescence, the soft gamma-ray repeater SGR 1627-41 reactivated on 2008 May 28 with a bursting episode (Esposito et al. 2008, MNRAS, 390, L34). On 2008 September 27-28 we performed an XMM-Newton target of opportunity observation of the source and discovered its long-sought spin period at 2.594578(6) s (Esposito et al. 2009, ApJ, 690, L105).

We searched in the archival Chandra data of SGR 1627-41 taken on 2008 June 03 (obsID 9126, ATEL #1564 by Woods et al.) for the presence of a periodic signal over a period range extrapolated from our XMM-Newton data, assuming a period derivative dP/dt < 10-9 s s-1.

A significant signal was found in a Z22-periodogram at 2.59439 0.00004 s. The probability of this peak to appear by chance in the Chandra data, taking into account the number of trials (1177), is 6 10-5, that is a 4-sigma detection.

The lightcurve folded at the best-fit period is double-peaked and closely resembles that observed with XMM-Newton. The root mean square pulsed fraction is (13 2)% in the 2-10 keV energy range.

The Chandra and XMM-Newton data, separated by about 114 days, imply a long-term spin-down rate dP/dt = (1.9 0.4) 10-11 s s-1. This value can be used to infer a surface dipole magnetic field strength of 2 1014 G, confirming the magnetar nature of SGR 1627-41.


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