Spin-down rate and inferred dipole magnetic field of the soft gamma-ray repeater SGR1627-41

Using Chandra data taken in 2008 June, we detected pulsations at 2.59439(4) s in the soft gamma-ray repeater SGR1627-41. 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) × 10^-11ss^-1. From this value, we infer for SGR1627-41 a characteristic age of ~2.2 kyr, a spin-down luminosity of ~4 × 10³⁴ erg s^-1 (one of the highest among sources of the same class), and a surface dipole magnetic field strength of ~2 × 10¹⁴ G. These properties confirm the magnetar nature of SGR1627-41 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 ~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.