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 (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 day binary with a projected semimajor axis $a_X\sin i\simeq$ 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 days spanned by our 2018 May–June observations, corresponding to a spin-up rate $\dot{P}\simeq -1.5\times {10}^{-10}s{s}^{-1}$. In an archival 2005 XMM-Newton exposure, we measured a spin period of ∼3.3 s, indicating a secular spin-up of ${\dot{P}}_{\sec }\simeq -{10}^{-9}s{s}^{-1}$, 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 10¹² G $\lesssim B_{dip}\lesssim {10}^{13}$ G) accreting NS with weakly beamed emission ($1/12\lesssim b\lesssim 1/4$).