Discovery of a 2.8 s Pulsar in a 2 Day Orbit High-mass X-Ray Binary Powering the Ultraluminous X-Ray Source ULX-7 in M51

We discovered 2.8 s pulsations in the X-ray emission of the ultraluminous X-ray source 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 10(39) and 10(40) 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 day binary with a projected semimajor axis a(X) sin i similar or equal to 28 lt-s. For a neutron star (NS) of 1.4 M-circle dot, this implies a lower limit on the companion mass of 8 M-circle dot, placing the system hosting M51 ULX-7 in the high-mass X-ray binary class. The barycentric pulse period decreased by similar or equal to 0.4 ms in the 31 days spanned by our 2018 May-June observations, corresponding to a spin-up rate (P) over dot similar or equal to -1.5 x 10(-10) s s(-1). In an archival 2005 XMM-Newton exposure, we measured a spin period of similar to 3.3 s, indicating a secular spin-up of (P) over dot(sec) similar or equal to -10(-9) s s(-1), a value in the range of other known PULXs. Our findings suggest that the system consists of a massive donor, possibly an OB giant or supergiant, and a moderately magnetic (dipole field component in the range 10(12) G less than or similar to B-dip less than or similar to 10(13) G) accreting NS with weakly beamed emission (1/12 less than or similar to b less than or similar to 1/4).