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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. and Israel, G. L. and Belfiore, A. and Bernardini, F. and Esposito, P. and Pintore, F. and De Luca, A. and Papitto, A. and Stella, L. and Tiengo, A. and Zampieri, L. and Bachetti, M. and Brightman, M. and Casella, P. and D'Agostino, D. and Dall'Osso, S. and Earnshaw, H. P. and Fürst, F. and Haberl, F. and Harrison, F. A. and Mapelli, M. and Marelli, M. and Middleton, M. J. and Pinto, C. and Roberts, T. P. and Salvaterra, R. and Turolla, R. and Walton, D. J. and 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). Art. No. 60. ISSN 1538-4357. doi:10.3847/1538-4357/ab8a44.

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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 a_X sin I ≃ 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 Ṗ ≃−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 Ṗ_(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 ≲ B_(dip) ≲10¹³ G) accreting NS with weakly beamed emission (1/12 ≲ b ≲ 1/4).

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Israel, G. L.0000-0001-5480-6438
Esposito, P.0000-0003-4849-5092
Tiengo, A.0000-0002-6038-1090
Bachetti, M.0000-0002-4576-9337
Brightman, M.0000-0002-8147-2602
Earnshaw, H. P.0000-0001-5857-5622
Fürst, F.0000-0003-0388-0560
Harrison, F. A.0000-0003-2992-8024
Middleton, M. J.0000-0002-8183-2970
Pinto, C.0000-0003-2532-7379
Roberts, T. P.0000-0001-8252-6337
Walton, D. J.0000-0001-5819-3552
Alternate Title: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
Additional Information:© 2020 The American Astronomical Society. Received 2019 June 11; revised 2020 April 9; accepted 2020 April 15; published 2020 May 26. We thank Dr. Norbert Schartel for granting DPS XMM-Newton time. The scientific results reported in this article are based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and NASA. G.R. acknowledges the support of high-performance computing resources awarded by CINECA (MARCONI and GALILEO), under the ISCRA initiative and the INAF–CIENCA MoU, and the computing centers of INAF–Osservatorio Astronomico di Trieste and Osservatorio Astrofisico di Catania, under the coordination of the CHIPP project, for the availability of computing resources and support. This work has also made use of observations made with the NASA/ESA Hubble Space Telescope and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA), and the Canadian Astronomy Data Centre (CADC/NRC/CSA). This work has also made use of data from the ESA mission Gaia, processed by the Gaia Data Processing and Analysis Consortium (DPAC). We acknowledge funding in the framework of the project ULTraS ASI–INAF contract No. 2017-14-H.0, project ASI–INAF contract I/037/12/0, and PRIN grant 2017LJ39LM. F.B. is funded by the European Unions Horizon 2020 research and innovation program under Marie Skłodowska Curie grant agreement No. 664931. F.F. and C.P. acknowledge support from ESA Research Fellowships. This research was supported by high-performance computing resources at New York University Abu Dhabi. H.P.E. acknowledges support under NASA contract NNG08FD60C. T.P.R. acknowledges support from STFC as part of consolidated grant ST/K000861/1. We thank the anonymous referee for the helpful comments on the manuscript. A.P. acknowledges financial support from grants ASI/INAF I/037/12/0, ASI/INAF 2017-14-H.0 (PI: Belloni) and from INAF "Sostegno allaricerca scientifica main streams dell'INAF," Presidential Decree 43/2018 (PI: Belloni).
Group:Astronomy Department, Space Radiation Laboratory
Funding AgencyGrant Number
Agenzia Spaziale Italiana (ASI)2017-14-H.0
Agenzia Spaziale Italiana (ASI)I/037/12/0
Istituto Nazionale di Astrofisica (INAF)2017LJ39LM
Marie Curie Fellowship664931
Science and Technology Facilities Council (STFC)ST/K000861/1
Istituto Nazionale di Astrofisica (INAF)43/2018
Subject Keywords:Neutron stars ; Stellar accretion ; X-ray sources ; Stellar magnetic fields ; X-ray binary stars ; Pulsars
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Neutron stars (1108); Stellar accretion (1578); X-ray sources (1822); Stellar magnetic fields (1610); X-ray binary stars (1811); Pulsars (1306)
Record Number:CaltechAUTHORS:20190619-080458696
Persistent URL:
Official Citation:G. A. Rodríguez Castillo et al 2020 ApJ 895 60
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:96512
Deposited By: Tony Diaz
Deposited On:19 Jun 2019 18:37
Last Modified:16 Nov 2021 17:21

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