X-ray variability of the HMXB Cen X−3: evidence for inhomogeneous accretion flows

Abstract

Cen X−3 is a compact high-mass X-ray binary likely powered by Roche lobe overflow. We present a phase-resolved X-ray spectral and timing analysis of two pointed XMM–Newton observations. The first one took place during a normal state of the source, when it has a luminosity LX ∼ 1036 erg s−1. This observation covered orbital phases ϕ = 0.00–0.37, i.e. the egress from the eclipse. The egress light curve is highly structured, showing distinctive intervals. We argue that different intervals correspond to the emergence of different emitting structures. The light-curve analysis enables us to estimate the size of such structures around the compact star, the most conspicuous of which has a size ∼0.3R*, of the order of the Roche lobe radius. During the egress, the equivalent width of Fe emission lines, from highly ionized species, decreases as the X-ray continuum grows. On the other hand, the equivalent width of the Fe K α line, from near-neutral Fe, strengthens. This line is likely formed due to the X-ray illumination of the accretion stream. The second observation was taken when the source was 10 times X-ray brighter and covered the orbital phases ϕ = 0.36–0.80. The X-ray light curve in the high state shows dips. These dips are not caused by absorption but can be due to instabilities in the accretion stream. The typical dip duration, of about 1000 s, is much longer than the time-scale attributed to the accretion of the clumpy stellar wind of the massive donor star, but is similar to the viscous time-scale at the inner radius of the accretion disc.

Cen x-3 as seen by MAXI during six years

“…For such fluxes, the radius of the blackbody emitting area is found R bb = 0.71 +0.07 −0.06 km (averaged spectra), R bb = 1.6 ± 0.5 km (high states) and R bb = 0.36 +0.12 −0.10 km (low states). All values are on the order of 1 to 2 km which is compatible with a hotspot on the NS surface (Sanjurjo-Ferrín et al 2021).…”

“…Recently, Aftab et al (2019) and Sanjurjo-Ferrín et al (2021) carried out spectral analysis of XMM-Newton data in the eclipse and out-of-eclipse phases in the energy band (0.8-10.0 keV). They obtained best fits by combining blackbody and power-law components and used them to describe both eclipse and out-ofeclipse spectra.…”

“…Previous studies following this direction of analysis have been performed in one or two orbits (Nagase et al 1992;Suchy et al 2008) or in shorter orbital phase (Ebisawa et al 1996;Audley et al 1996;Wojdowski et al 2001;Aftab et al 2019;Sanjurjo-Ferrín et al 2021). We have obtained orbital phase-resolved spectra of the HMXB pulsar Cen X-3, accumulating the 60 s duration scans into ten orbital phase bins covering entirely its orbit (Rodes-Roca et al 2015).…”

Pulsating iron spectral features in the emission of X-ray pulsar V0332+53

“…Such absorption enhancements are observed in different X-ray pulsars (e.g. Hemphill et al 2014;Jaisawal & Naik 2014;Sanjurjo-Ferrín et al 2021;Liu et al 2021;Ji et al 2021) and usually related to the structures like wind or accretion stream through the inner Lagrange point. To our knowledge, such a clear and isolated episode of increased absorption is detected in Be/X-ray binary system for the first time.…”

“…Later, Kohmura et al (2001) proposed for this source that the reprocessing site is the accreting matter flowing along the magnetic field lines at the distance of ≈ 1.7 × 10 8 cm from the neutron star. Sanjurjo-Ferrín et al (2021) propose the accretion stream in Cen X-3 as a source of reprocessed neutral iron. Similarly, in GX 301-2 Liu et al (2018) detected transient pulsations of iron line flux, suggesting that the reprocessing medium might be the accretion stream or the secondary's surface.…”