Wind-Powered Ultraluminous X-Ray Sources

Draft version August 18, 2021 Preprint typeset using LATEX style emulateapj v. 12/16/11 WIND-POWERED ULTRALUMINOUS X-RAY SOURCES Grzegorz Wiktorowicz1,2,3, Jean-Pierre Lasota3,4, Krzysztof Belczynski3, Youjun Lu1,2, Jifeng Liu1,2,5, Krystian Iłkiewicz3,6,7 1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China 2 School of Astronomy & Space Science, University of the Chinese Academy of Sciences, Beijing 100012, China 3 Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland 4 Institut d’Astrophysique de Paris, CNRS et Sorbonne Université, UMR 7095, 98bis Bd Arago, 75014 Paris, France 5 WHU-NAOC Joint Center for Astronomy, Wuhan University, Wuhan, China 6 Centre for Extragalactic Astronomy, Department of Physics, University of Durham, South Road, Durham, DH1 3LE, UK 7 Department of Physics and Astronomy, Box 41051, Science Building, Texas Tech University, Lubbock, TX 79409-1051, USA Draft version August 18, 2021 ABSTRACT Although ultraluminous X-ray sources (ULX) are important for astrophysics because of their ex- treme apparent super-Eddington luminosities, their nature is still poorly known. Theoretical and observational studies suggest that ULXs could be a diversified group of objects that are composed of low- mass X-ray binaries, high-mass X-ray binaries and marginally also systems containing intermediate- mass black holes. Observational data on the ULX donors could significantly boost our understanding of these systems, but only a few have been detected. There are several candidates, mostly red supergiants (RSGs), but surveys are typically biased toward luminous near-infrared objects. In ULXs harbouring RSGs matter accreted onto the compact body would have to be provided by the stellar wind of the companion because a Roche-lobe overflow could be unstable for relevant mass-ratios. Here we present a comprehensive study of the evolution and population of wind-fed ULXs, and we provide a theoretical support for the link between RSGs and ULXs. Assuming a minimal model of stellar-wind emission, our estimated upper limit on contribution of wind-fed ULX to the overall ULX population is 75–96% for young (< 100 Myr) star forming environments, 49–87% for prolonged ∼ ∼ constant star formation (e.g., disk of Milky Way), and . 1% for environments in which star formation ceased long time (> 2 Gyr) ago. We demonstrate that some wind-fed ULXs (up to 6%) may evolve into merging double compact objects (DCOs). We demonstrate that, the exclusion of wind-fed ULXs from population studies of ULXs might have lead to systematic errors in their conclusions. Subject headings: stars: black holes, gravitational waves, binaries: general, X-ray: binaries, methods: numerical, methods: statistical, astronomical databases: miscellaneous 1. INTRODUCTION erences therein), but their luminosities are always less 39 1 than 3 10 erg s− (e.g., Tsygankov et al. 2017; Chan- Ultraluminous X-ray sources (ULXs) are defined as × point-like off-nuclear X-ray sources with luminosities ex- dra et al. 2020). ceeding 39 1 (for a recent review see Kaaret et al. The choice of the threshold ULX luminosity at 10 erg s− 39 1 2017). Therefore, they are the brightest X-ray sources 10 erg s− is rather unfortunate because it fails to sep- that are neither supernovae (SNe) nor active galactic nu- arate the “standard” X-ray binaries (XRBs) from the clei. After their identification at the end of the previous “generic” ULX. Indeed, at least five transient galac- 39 1 century, it was thought for a long time that ULXs con- tic XRBs reach luminosities superior to 10 erg s− tain intermediate-mass (M 102–104 M ) black holes (Tetarenko et al. 2016). Middleton et al. (2015) suggest 39 1 (IMBHs Colbert & Mushotzky∼ 1999) that are accret- that one should take 3 10 erg s− as the minimum ing at rates close to or below their Eddington values. luminosity defining ULXs× whose nature is “contentious” The dissenting view of King et al. (2001)–who pointed (i.e., that could contain either stellar-mass accretors or arXiv:2103.02026v2 [astro-ph.HE] 17 Aug 2021 out that there are no viable evolutionary scenarios lead- IMBHs). The idea was to make sure that if the accretor ing to the formation of such systems and suggested that is a black hole (BH), then such a floor value would make most ULXs are stellar-mass binaries that are accreting at sure that the apparent luminosity is super-Eddington. super-Eddington rates–was largely ignored until Bachetti However, this suggestion does not take into account that et al. (2014) discovered that the source M82 ULX-2 con- it is now known that stellar evolution can produce BHs tains an X-ray pulsar. This observation and the subse- with masses & 30 M (Belczynski et al. 2010b; Abbott quent observations of five other pulsating ULXs (PULXs) et al. 2016), and up to 50 M under favorable conditions by Israel et al. (2017), Fürst et al. (2017), Carpano et al. (e.g. Belczynski 2020). Therefore, only sources with lu- 39 1 (2018), Sathyaprakash et al. (2019), Rodríguez Castillo minosities larger than 6 10 erg s− , for example, et al. (2020) led to a change of paradigm and it is now can be expected to be∼ (apparently)× super-Eddington if universally believed that most (if not all) ULXs con- they contain a stellar-mass BH1. tain stellar-mass accretors. PULXs have been also observed during giant outbursts of Be-X-ray binary stars 1 The limit may even be higher for He-rich accretion and most (one in the Galaxy, see King & Lasota 2020, and ref- massive BHs (e.g. Belczynski 2020) 2 Since the isotropic luminosity of an accreting body can in the majority of ULXs. For example, Wiktorowicz et al. be, at best, a few times its Eddington value (see, e.g. (2017) showed that the majority of ULXs will harbour Shakura & Sunyaev 1973; Poutanen et al. 2007), ob- M < 3 M MS donors. Such stars are only observable in served luminosities L LEdd, where LEdd is the Ed- the vicinity of the Sun (even HST is constrained to the dington luminosity, are only apparently super-Eddington Milky Way in their case). Hyper-luminous X-ray sources, and must be due to radiation beamed toward the ob- which are typically defined as ULXs with apparent lu- 41 1 server (King et al. 2001; Abramowicz 2005). For exam- minosities above 10 erg s− , are still viable candidates ple, in the model developed by King (2009) radiation to harbour massive donors (& 10 M ; e.g. Wiktorowicz is collimated for L & 3LEdd. One should realise that et al. 2015). the beaming of apparently super-Eddington luminosities Several ULXs possess donor candidates spatially that is unavoidable for neutron star (NS) accretors. In the are coincident with X-ray sources. Most of them were de- case of NS ULXs, it has been speculated that magnetar- tected through observations in the near-infrared (NIR) strength magnetic field (B > 1013 G), by increasing the spectral band. In this band, red supergiants (RSG) value of the threshold luminosity, would make the emis- should easily overcome NIR emission from an accretion sion effectively sub-critical (cf Tong 2015; Dall’Osso et al. disk (Copperwheat et al. 2005). Therefore, these stars 2015; Ekşi et al. 2015; Mushtukov et al. 2015). However, were of particular interest in most of the surveys (e.g. this hypothesis is contradicted by observations in several Heida et al. 2014; López et al. 2017, 2020). Although ULXs of cyclotron resonance scattering feature (CRSF), blue supergiants were, at first, perceived as viable candi- which corresponds to magnetic fields B 1011 1012 dates for donors in ULXs because of their prevalence in G (Brightman et al. 2018; Middleton et∼ al. 2019;− Wal- star forming regions, where most of the ULXs are being ton et al. 2018), as well as by the absence of magne- found, it was later realised that the optical companions tars in binary systems, which is consistently explained might be outshone by the emission from an irradiated by their formation scenarios invoking the destruction of accretion disk (e.g. Sutton et al. 2014) unless the sep- the parent binary system (see King & Lasota 2019, and aration is very large (Copperwheat et al. 2007). This references therein). The importance of beaming in the bias toward high-luminosity NIR companions might be context of the ULX origin and observed populations was the reason why the majority of the detected companion recently analyzed by Wiktorowicz et al. (2019a). candidates are RSGs (e.g. Heida et al. 2019). This is Since the universal presence of extremely strong mag- further complicated by the fact that we cannot measure netic fields in NS ULXs is very unlikely and is impossible distances to ULXs and RSGs directly and the distance 39 1 in BH ULXs, the observed luminosities & 3 10 erg s− to the coinciding galaxy is used as a proxy. for NU LXs (we allow for the existence× of NSs with Even if RSG donors form only a small fraction of the masses 2:5 M , Abbott et al. 2020), and larger than intrinsic ULX population, most of these binary compan- ≈39 1 6 10 erg s− in general, cannot be intrinsic and must ions cannot be Roche-lobe (RL) filling stars. For exam- be∼ beamed,× as predicted by King et al. (2001). This re- ple, Wiktorowicz et al. (2017) estimated that only < 1% 2 _ _ of all ULXs can be accompanied by supergiants if RLOF quires high accretion rates typically m_ M=MEdd & 9, 8 ≡ accretion is concerned exclusively. In binaries contain- where M_ Edd = 2:0 10− M?(0:1/η) M =yr, with η be- × ing a compact object and a RSG companion, the mass ing the accretion efficiency and M? the accretor mass ratio is typically high (q = M =M > 3), especially in solar units.

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