EXTraS discovery of an 1.2-s X-ray pulsar in M 31

摘要: During a search for coherent signals in the X-ray archival data of XMM-Newton, we discovered modulation at 1.2 s 3XMM J004301.4+413017 (3X J0043), source lying direction an external arm M 31. This short period indicates neutron star (NS). Between 2000 and 2013, position 3X J0043 was imaged by public XMM-Newton observations 35 times. The analysis these allowed us to detect orbital 1.27 d study long-term properties source. emission pulsar rather hard (most spectra are described power law with $\Gamma < 1$) and, assuming distance 31, 0.3-10 keV luminosity variable, from $\sim$$3\times10^{37}$ $2\times10^{38}$ erg s$^{-1}$. optical shows that, while is likely associated globular cluster counterpart $V\gtrsim22$ outside cannot be excluded. Considering our findings, there two main viable scenarios J0043: peculiar low-mass binary, similar 4U 1822-37 or 1626-67, intermediate-mass binary resembling Her X-1. Regardless exact nature system, first accreting NS 31 which spin has been detected.