Discovery of a 2.8 s pulsar in a 2 d orbit High-Mass X-ray Binary powering the Ultraluminous X-ray source ULX-7 in M51

Understand

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%).
• M51 ULX-7 is a variable source, 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-d binary with a projected semi-major axis $a_\mathrm{X} \sin i \simeq$ 28 lt-s.