An ionized accretion disc wind in Hercules X-1

Abstract

Hercules X-1 is one of the best studied highly magnetised neutron star X-ray binaries with a wealth of archival data. We present the discovery of an ionised wind in its X-ray spectrum when the source is in the high state. The wind detection is statistically significant in most of the XMM-Newton observations, with velocities ranging from 200 to 1000 km/s. Observed features in the iron K band can be explained by both wind absorption or by a forest of iron emission lines. However, we also detect nitrogen, oxygen and neon absorption lines at the same systematic velocity in the high-resolution RGS grating spectra. The wind must be launched from the accretion disc, and could be the progenitor of the UV absorption features observed at comparable velocities, but the latter likely originate at significantly larger distances from the compact object. We find strong correlations between the ionisation level of the outflowing material and the ionising luminosity as well as the super-orbital phase. If the luminosity is driving the correlation, the wind could be launched by a combination of Compton heating and radiation pressure. If instead the super-orbital phase is the driver for the variations, the observations are likely scanning the wind at different heights above the warped accretion disc. If this is the case, we can estimate the wind mass outflow rate, corrected for the limited launching solid angle, to be roughly 70% of the mass accretion rate.

Multi-frequency long-term observations of Her X-1 - The 35-d cycle

“…Resent XMM-Newton satellite observations showed-the presence of the modulation of disk wind mass outflow rate with 35 d superorbital period [28]. This result is in according to the model of free precession of the neutron star.…”

Observations of Her X-1 in low states during SRG/eROSITA all-sky survey

“…The observed short-time variability could be due to thermal instability in the photoionized plasma (see, e.g., Mehdipour et al (2016)). Indeed, variable ionized mass outflows from Her X-1 are observed originating from the nonstationary hot corona above the accretion disk and optical star (Kosec et al 2020).…”

Do the periodic activities of repeating fast radio bursts represent the spins of neutron stars?

“…"Disk winds" are thought to be observed in four NS-systems, which are Her X-1 (Kosec et al 2020;Nixon & Pringle 2020), GX 13+1 (Ueda et al 2004), IGR J17480-2446 (Miller et al 2011) and IGR J17591-2342 (Nowak et al 2019). In the first three systems, outflows are probably from the inner disk in super-Eddington accretion phase, which may be explained by the mass loss at the spherization radius because the Eddington-limited accretion is enabled (Shakura & Sunyaev 1973;Lipunova 1999;Grebenev 2017).…”