Luminosity-dependent changes of the cyclotron line energy and spectral hardness in Cepheus X-4

Abstract

Context. X-ray spectra of accreting pulsars are generally observed to vary with their X-ray luminosity. In particular, the hardness of the X-ray continuum is found to depend on luminosity. In a few sources, the correlation between the energy of the cyclotron resonance scattering feature (CRSF) and the luminosity is clear. Different types (signs) of the correlation are believed to reflect different accretion modes. Aims. We analyse two NuSTAR observations of the transient accreting pulsar Cep X-4 during its 2014 outburst. Our analysis is focused on a detailed investigation of the dependence of the CRSF energy and of the spectral hardness on X-ray luminosity, especially on short timescales. Methods. To investigate the spectral changes as a function of luminosity within each of the two observations, we used the intrinsic variability of the source on the timescale of individual pulse cycles (tens of seconds) , the so-called pulse-to-pulse variability. Results. We find that the NuSTAR spectrum of Cep X-4 contains two CRSFs: the fundamental line at~30 keV and its harmonic at~55 keV. We find for the first time that the energy of the fundamental CRSF increases and the continuum becomes harder with increasing X-ray luminosity not only between the two observations, that is, on the long timescale, but also within an individual observation, on the timescale of a few tens of seconds. We investigate these dependencies in detail including their non-linearity. We discuss a possible physical interpretation of the observed behaviour in the frame of a simple one-dimensional model of the polar emitting region with a collisionless shock formed in the infalling plasma near the neutron star surface. With this model, we are able to reproduce the observed variations of the continuum hardness ratio and of the CRSF energy with luminosity.

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“…The 20-year decay of E cyc has ended with a strong indication of an inversion. (Klochkov et al 2011;Sartore et al 2015), Vela X-1 (Fürst et al 2014), GX 304−1 , and Cep X-4 (Vybornov et al (2017). A possible additional source is V0332+53 at low luminosity towards the end of an outburst (Doroshenko et al 2016).…”

On the geometry of the X-ray emission from pulsars: the changing aspect of the Be/X-ray pulsar SXP348

“…Cyclotron line studies also support this interpretation (e.g. Rothschild et al (2017); Vybornov et al (2017)). The behavior of pulsars on the HB indicates that substantial spectral hardening can occur without the formation of an extended column.…”

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“…Nagel 1981; Meszaros & Nagel 1985;Burnard et al 1991;Hickox et al 2004;Becker & Wolff 2007), its hardness as a function of pulse-phase as has been noted in several X-ray pulsars (XRPs) (e.g. Klochkov et al 2011;Malacaria et al 2015;Vybornov et al 2017;Koliopanos & Vasilopoulos 2018) and also the presence of non-pulsating components such as the thermal-like excess below ∼1 keV, which can be attributed to the presence of a truncated accretion disk (e.g. Hickox et al 2004, and references therein).…”