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

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

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%). 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 aX sin i  28 lt-s. For a neutron star (NS) of 1.4 Me, this implies a lower limit on the companion mass of 8 Me, 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   - ´ - - P 1.5 10 s s 10 1. In an archival 2005 XMM-Newton exposure, we measured a spin period of ∼3.3 s, indicating a secular spin-up of   - - - Psec 10 s s 9 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   Bdip 1013 G) accreting NS with weakly beamed emission (1 12 1 4  b ).

Citation

Rodríguez Castillo, G., Israel, G., Belfiore, A., Bernardini, F., Esposito, P., Pintore, F., De Luca, A., Papitto, A., Stella, L., Tiengo, A., Zampieri, L., Bachetti, M., Brightman, M., Casella, P., D’Agostino, D., Dall’Osso, S., Earnshaw, H., Fürst, F., Haberl, F., Harrison, F., …Wolter, A. (2020). 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. Astrophysical Journal, 895(1), Article 60. https://doi.org/10.3847/1538-4357/ab8a44

• Journal Article Type: Article
• Acceptance Date: Apr 15, 2020
• Online Publication Date: May 26, 2020
• Publication Date: 2020-05
• Deposit Date: Jun 24, 2020
• Publicly Available Date: Jun 24, 2020
• Journal: Astrophysical Journal
• Print ISSN: 0004-637X
• Electronic ISSN: 1538-4357
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 895
• Issue: 1
• Article Number: 60
• DOI: https://doi.org/10.3847/1538-4357/ab8a44
• Public URL: https://durham-repository.worktribe.com/output/1262079