We report on the discovery of pulsations at a period of \sim 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 \approx 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 ( \sim 50–60 % 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 \Gamma \sim 1.4 and observed flux of ( 2– 5 ) \times 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 \sim 6 \times 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 2MASS 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 \sim 3 \times 10 ^ { 34 } 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 ) .