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

Abstract

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. For a neutron star (NS) of 1.4 $M_{\odot }$, this implies a lower limit on the companion mass of 8 $M_{\odot }$, placing the system hosting M51 ULX-7 in the high-mass X-ray binary class. The barycentric pulse period decreased by $\simeq$0.4 ms in the 31 d spanned by our May -- June 2018 observations, corresponding to a spin-up rate $\dot{P}\simeq -1.5\times 10^{-10}{\text{s s}}^{-1}$. In an archival 2005 XMM-Newton exposure, we measured a spin period of $\sim$3.3 s, indicating a secular spin-up of ${\dot{P}}_{\sec }\simeq -10^{-9}{\text{ s s}}^{-1}$, 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 $10^{12}$ G $\lesssim B_{\mathrm{dip}}\lesssim 10^{13}$G) accreting NS with weakly beamed emission ($1/12\lesssim b\lesssim 1/4$).

arXiv commentAccepted for publication in The Astrophysical Journal

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