Pulsating ULXs: large pulsed fraction excludes strong beaming

Abstract

The recent discovery of pulsating ultraluminous X-ray sources (ULXs) shows that the apparent luminosity of accreting neutron stars can exceed the Eddington luminosity by a factor of 100s. The relation between the actual and apparent luminosity is a key ingredient in theoretical models of ULXs, but it is still under debate. A typical feature of the discovered pulsating ULXs is a large pulsed fraction (PF). Using Monte Carlo simulations, we consider a simple geometry of accretion flow and test the possibility of simultaneous presence of a large luminosity amplification due the geometrical beaming and a high PF. We argue that these factors largely exclude each other and only a negligible fraction of strongly beamed ULX pulsars can show PF above 10 per cent. Discrepancy between this conclusion and current observations indicates that pulsating ULXs are not strongly beamed and their apparent luminosity is close to the actual one.

The impact of precession on the observed population of ULXs

“…Determining the underlying ULX demographic presently relies on detecting unambiguous indicators for the presence of a neutron star such as pulsations or a CRSF. However, it has been proposed that many NS ULXs with high accretion rates may not exhibit pulsations King et al (2017), that large pulse fractions may be absent in the presence of strong beaming (Mushtukov et al 2021b), and CRSFs may not fall within the accessible X-ray energy range or may be diluted (see Mushtukov et al 2017a). An independent and simple method to constrain the nature of the underlying population in ULXs such as the one we have explored here is therefore of value (and joins others such as observing the evolution of quasi-periodic oscillations, see Middleton et al 2019b).…”

“…Under the assumption that geometrical beaming acts to some extent across the entire ULX population (i.e. ignoring the presence of very strong magnetic fields -see King & Lasota 2019 but also Mushtukov et al 2021a), the proportion of neutron stars and black holes within the ULX population has been analytically estimated by Middleton & King (2017), while estimates leveraging binary population synthesis have also recently been explored (Wiktorowicz et al 2019). Both studies predict that, whilst NS systems almost certainly dominate the entire intrinsic population of ULXs, observationally the populations of NS and BH ULXs may be comparable (particularly for host regions with low metallicity), although this may be in conflict with spectral similarities between the brightest ULXs (typically > a few × 10 39 erg s −1 ) and those systems confirmed to harbour neutron stars (Pinto et al 2017;Walton et al 2018).…”

Chandra Probes the X-Ray Variability of M51 ULX-7: Evidence of Propeller Transition and X-Ray Dips on Orbital Periods

“…(King et al 2017), would need to be revisited in the context of ULXPs. Small beaming is also consistent with the detection of pulsations in ULXPs, as large beaming factors would otherwise result in very small pulsed fractions (Mushtukov et al 2021). The above suggests that strong beaming is not needed for ULXPs, and this should be considered in the framework of ULX population synthesis (e.g., Misra et al 2020;Abdusalam et al 2020;Kuranov et al 2020) and perhaps gravitational wave progenitors, which are thought to go through a ULX phase (Marchant et al 2017).…”

The Chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1

“…In a possible alternative scenario, the increasingly brighter hard tail, from SSUL to SUL, could be produced by an increase in the accretion rate which boosts the emission from the inner super-Eddington regions, followed by further scattering through the outer disc/wind (Walton et al 2020;Gúrpide et al 2021). This has been invoked to explain the lower pulse fraction at higher X-ray fluxes of some ULXs (Mushtukov et al 2021;Robba et al 2021).…”