Spin-down rate and inferred dipole magnetic field of the soft gamma-ray repeater SGR 1627-41

Using Chandra data taken in 2008 June, we detected pulsations at 2.594 39(4) s in the soft gamma-ray repeater SGR 1627-41. This is the second measurement of the source spin period and allows us to derive for the first time a long-term spin-down rate of (1.9 +/- 0.4) x 10(-11) s s(-1). From this value, we infer for SGR 1627-41 a characteristic age of similar to 2.2 kyr, a spin-down luminosity of similar to 4 x 10(34) erg s(-1) (one of the highest among sources of the same class), and a surface dipole magnetic field strength of similar to 2 x 10(14) G. These properties confirm the magnetar nature of SGR 1627-41; however, they should be considered with caution since they were derived on the basis of a period derivative measurement made using two epochs only, and magnetar spin-down rates are generally highly variable. The pulse profile, double-peaked and with a pulsed fraction of 13 +/- 2 per cent in the 2-10 keV range, closely resembles that observed by XMM-Newton in 2008 September. Having for the first time a timing model for this soft gamma-ray repeater (SGR), we also searched for a pulsed signal in archival radio data collected with the Parkes radio telescope 9 months after the previous X-ray outburst. No evidence for radio pulsations was found, down to a luminosity level similar to 10-20 times fainter (for a 10 per cent duty cycle and a distance of 11 kpc) than the peak luminosity shown by the known radio magnetars.