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

Journal
Date Issued
2020
Author(s)
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Belfiore, A.
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Esposito, P.
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Tiengo, A.
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Brightman, M.
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D’Agostino, D.
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Dall’Osso, S.
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Earnshaw, H. P.
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Fürst, F.
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Haberl, F.
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Harrison, F. A.
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Mapelli, M.
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Middleton, M.
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Roberts, T. P.
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Turolla, R.
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Walton, D. J.
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DOI
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 aXsinI≃ 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 P˙≃−1.5×10−10s s−1 . In an archival 2005 XMM-Newton exposure, we measured a spin period of ∼3.3 s, indicating a secular spin-up of P˙sec≃−10−9s 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 ≲Bdip≲1013 G) accreting NS with weakly beamed emission ( 1/12≲b≲1/4 ).
Volume
895
Issue
1
Start page
60
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2020ApJ...895...60R
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open.access
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