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Zusammenfassung:
We discovered 2.8 s pulsations in the X-ray emission of the ultraluminous X-ray source (ULX) M51 ULX-7 within the UNSEeN project, which was designed to hunt for new pulsating ULXs (PULXs) with XMM-Newton. The pulse shape is sinusoidal, and large variations of its amplitude were observed even within single exposures (pulsed fraction from less than 5% to 20%). Source M51 ULX-7 is variable, generally observed at an X-ray luminosity between 1039 and 1040 erg s−1, located in the outskirts of the spiral galaxy M51a at a distance of 8.6 Mpc. According to our analysis, the X-ray pulsar orbits in a 2-d binary with a projected semi-major axis aXsini≃ 28 lt-s. For a neutron star (NS) of 1.4 M⊙, this implies a lower limit on the companion mass of 8 M⊙, placing the system hosting M51 ULX-7 in the high-mass X-ray binary class. The barycentric pulse period decreased by ≃0.4 ms in the 31 d spanned by our May -- June 2018 observations, corresponding to a spin-up rate P˙≃−1.5×10−10s s−1. In an archival 2005 XMM-Newton exposure, we measured a spin period of ∼3.3 s, indicating a secular spin-up of P˙sec≃−10−9 s s−1, a value in the range of other known PULXs. Our findings suggest that the system consists of an OB giant and a moderately magnetic (dipole field component in the range 1012 G ≲Bdip≲1013G) accreting NS with weakly beamed emission (1/12≲b≲1/4).
