(2017) show that the typical donor of NS ULXs is a red giant star with a mass of ∼1.0 M ⊙. Utilizing the StarTrack EPS code, Wiktorowicz et al. And the distribution of the NS ULX population was demonstrated in the donor mass-orbital period plane (see their Figs. They suggest that NS ULXs may contribute significantly to the whole ULX population. 2014), Shao & Li (2015) first studied the population of NS ULXs by the use of both evolutionary population synthesis (EPS) and a detailed binary evolution method. For example, after the discovery of the first NS ULX (i.e., M 82 X-2, Bachetti et al. 2015, 2017, 2019), as a highly super-Eddington mass transfer rate is unlikely to reach through stellar wind. In order to account for the ultra-high luminosity of NS ULXs, mass transfer through Roche-lobe overflow (RLOF) via an accretion disk has always been assumed in previous theoretical studies ( Shao & Li 2015 Fragos et al.
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2020), or a cyclotron resonance feature, for example, M 51 ULX8 ( Brightman et al. 2019), and M 51 ULX-7 ( Rodríguez Castillo et al. 2019b), NGC 1313 ULX-2 ( Sathyaprakash et al. Several NS ULXs have been identified so far and they are characterized by regular pulses with periods of ∼1 s, that is, M 82 X-2 ( Bachetti et al. Strikingly, ULXs can also harbor an accreting neutron star (NS), of which the Eddington limit L Edd is much lower (i.e., around 2 × 10 38 erg s −1 for a 1.4 M ⊙ NS 1). 2018) demonstrate that most if not all ULXs are instead stellar-mass binary systems, proposed due to super-Eddington accretion ( Begelman 2002) and/or geometric beaming ( King et al. Although suggested to be intermediate mass (10 2 − 10 5 M ⊙) BHs ( Colbert & Mushotzky 1999) in early studies as the accretor in ULXs, growing pieces of evidence ( Gladstone et al. Ultraluminous X-ray sources (ULXs) are non-nuclear, point-like objects with apparent luminosities of L X > 10 39 erg s −1, exceeding the Eddington limit for a 10 M ⊙ black hole (BH, Fabbiano 1989, see Kaaret et al. Key words: stars: evolution / X-rays: binaries / stars: neutron / methods: statistical / binaries: close We also present the current distributions of binary parameters of wind-fed NS ULXs, which may be further testified by future high-resolution optical and X-ray observations of these populations. Moreover, the two NS ULXs can be well reproduced in the WRLOF paradigm, which significantly enriches our understanding of the nature of ULXs and the population. Sources with massive (∼15−40 M ⊙) (super)giant donors dominate wind-fed NS ULXs in the very low metallicities, while sources in near solar cases are dominated by a red supergiant with a lower mass M 2 < 10 M ⊙ instead. The number of NS ULXs with (super)giant donors can be enhanced greatly, by one or two orders of magnitude, depending on the metallicity adopted. We find that the population of wind-fed NS ULXs in the WRLOF mode is distinct in numbers and binary parameters from that in the traditional Bondi-Hoyle-Lyttleton mode, and it is strongly metallicity dependent. By using an evolutionary population synthesis method, we modeled the population of NS ULXs with (super)giant donors, taking the WRLOF accretion mode into account. In this work, we test if the two sources could fit into the WRLOF paradigm. School of Physics, Xi’an Jiaotong University, Xi’an 710049, PR ChinaĮ-mail: recently, wind Roche-lobe overflow (WRLOF) has been suggested as a possible mass transfer mechanism for ultra-luminous X-ray sources (ULXs) and, to date, two neutron-star (NS) ULXs (i.e., NGC 7793 P13 and NGC 300 ULX-1) are remarkable and hard to understand in the current, usual RLOF picture. Zhao-Yu Zuo, Hao-Tian Song and Han-Chen Xue Astronomical objects: linking to databases.Including author names using non-Roman alphabets.Suggested resources for more tips on language editing in the sciences
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