Discovery of 47-s pulsations in the X-ray source 1RXS J225352.8+624354

We report on the discovery of pulsations at a period of ∼47 s in the persistent X-ray source 1RXS J225352.8+624354 (1RXS J2253) using five Chandra observations performed in 2009. The signal was also detected in Swift and ROSAT data, allowing us to infer over a 16-yr baseline an average, long-term period increasing rate of ≈17 ms per year and therefore to confirm the signal as the spin period of an accreting, spinning-down neutron star. The pulse profile of 1RXS J2253 (∼50-60 per cent pulsed fraction) is complex and energy independent (within the statistical uncertainties). The 1-10 keV Chandra spectra are well fit by an absorbed power-law model with Γ ∼ 1.4 and observed flux of (2-5) × 10^-12 erg cm^-2 s^-1. The source was also detected by INTEGRAL in the 17-60 keV band at a persistent flux of ∼6 × 10^-12 erg cm^-2 s^-1, implying a spectral cut off around 15 keV. We also carried out optical spectroscopic follow-up observations of the Two-Micron All-Sky Survey counterpart at the Nordic Optical Telescope. This made it possible to first classify the companion of 1RXS J2253 as a B0-1III-Ve (most likely a B1Ve) star at a distance of about 4-5 kpc (favouring an association with the Perseus arm of the Galaxy). The latter finding implies an X-ray luminosity of ∼3 × 10³⁴ erg s^-1, suggesting that 1RXS J2253 is a new member of the sub-class of low-luminosity long-orbital-period persistent Be/X-ray pulsars in a wide and circular orbit (such as X Persei).