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

G. A. Rodríguez Castillo, G. L. Israel, A. Belfiore, F. Bernardini, P. Esposito, F. Pintore, A. De Luca, A. Papitto, L. Stella, A. Tiengo, L. Zampieri, M. Bachetti, M. Brightman, P. Casella, D. D'Agostino, S. Dall'Osso, H. P. Earnshaw, F. Fürst, F. Haberl, F. A. HarrisonM. Mapelli, M. Marelli, M. Middleton, C. Pinto, T. P. Roberts, R. Salvaterra, R. Turolla, D. J. Walton, A. Wolter

    Research output: Contribution to journalArticlepeer-review


    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 28 lt-s. For a neutron star (NS) of 1.4 M o˙, this implies a lower limit on the companion mass of 8 M o˙, 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. In an archival 2005 XMM-Newton exposure, we measured a spin period of ∼3.3 s, indicating a secular spin-up of, 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 G) accreting NS with weakly beamed emission ().

    Original languageEnglish (US)
    Article number60
    JournalAstrophysical Journal
    Issue number1
    StatePublished - May 20 2020

    ASJC Scopus subject areas

    • Astronomy and Astrophysics
    • Space and Planetary Science


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