Pulsing and non-pulsing ULXs: the iceberg emerges

Abstract

We show that ultraluminous X-ray sources (ULXs) with coherent X-ray pulsing (PULXs) probably have neutron-star spin axes significantly misaligned from their central accretion discs. Scattering in the funnels collimating their emission and producing their apparent super-Eddington luminosities is the most likely origin of the observed correlation between pulse fraction and X-ray photon energy. Pulsing is suppressed in systems with the neutron-star spin closely aligned to the inner disc, explaining why some ULXs show cyclotron features indicating strong magnetic fields, but do not pulse. We suggest that alignment (or conceivably, field suppression through accretion) generally occurs within a fairly short fraction of the ULX lifetime, so that most neutron-star ULXs become unpulsed. As a result we further suggest that almost all ULXs actually have neutron-star accretors, rather than black holes or white dwarfs, reflecting their progenitor high-mass X-ray binary and supersoft X-ray source populations.

Pulsating ULXs: large pulsed fraction excludes strong beaming

“…2, NGC 7793 P13 and NGC 300 ULX1, and ∼100 for NGC 5907 ULX1, see Table 1 in King & Lasota 2020). The first one is a large observed PF, which is expected to be reduced in systems with strong beaming.…”

“…The first one is a large observed PF, which is expected to be reduced in systems with strong beaming. Recently, however, it was proposed that strong pulsations in ULXs appear in the case of a lucky combination between geometrical parameters of the accreting system (King & Lasota 2020). Another problematic point is related to bright transient X-ray pulsars, which do not show any evidence of beaming at high luminosity states (Tsygankov et al 2017;Doroshenko et al 2020).…”

“…In this paper, we test numerically the possibility of a simultaneous presence of strong beaming and large PF in pulsating ULX using a simplified geometry of accretion flow similar to that proposed by King & Lasota (2020).…”

Models of ultraluminous X-ray transient sources

“…This certainly must be the case for ULXs containing neutron stars, since luminosities exceeding up to one hundred times the Eddington luminosity cannot be attained if L x increases as lnṁ. Other options have been suggested, such as the presence of a magnetar-type magnetic field, but they are not all viable (see King & Lasota 2020). Beaming implies that a large number of sources exist that would be seen off axis, and therefore would not appear as ULXs, although they would be surrounded by a nebula; an example of such a source is SS 433, which is most probably an ULX seen from the side.…”

“…A&A 643, A171 (2020) sources, which are defined by L X > 10 41 erg s −1 ; one of them, NGC 5907 ULX1, with L X 10 41 erg s −1 is a pulsating ultraluminous X-ray source (PULXs, see King & Lasota 2020, for their properties and references). The luminosity of HLX-1 is variable and can exceed 10 42 erg s −1 at maximum.…”