Pulse-amplitude-resolved spectroscopy of bright accreting pulsars: indication of two accretion regimes

TL;DR

This study analyzes high-resolution X-ray data of bright accreting pulsars to reveal two distinct accretion regimes based on spectral changes correlated with pulse amplitude, advancing understanding of neutron star accretion physics.

Contribution

It introduces a novel pulse-to-pulse spectral analysis method to identify two different accretion regimes in pulsars, based on spectral variability with pulse amplitude.

Findings

Spectral changes depend on pulse amplitude in all studied pulsars.

Two groups of pulsars exhibit different spectral dependencies on pulse height.

Evidence suggests the existence of two distinct accretion regimes.

Abstract

Context: In addition to coherent pulsation, many accreting neutron stars exhibit flaring activity and strong aperiodic variability on time scales comparable to or shorter than their pulsation period. Such a behavior shows that the accretion flow in the vicinity of the accretor must be highly non-stationary. Observational study of this phenomenon is often problematic as it requires very high statistics of X-ray data and a specific analysis technique. Aims: In our research we used high-resolution data taken with RXTE and INTEGRAL on a sample of bright transient and persistent pulsars, to perform an in-depth study of their variability on time scales comparable to the pulsation period - "pulse-to-pulse variability". Methods: The high-quality data allowed us to collect individual pulses of different amplitude and explore their X-ray spectrum as a function of pulse amplitude. The described approach allowed us for the first time to study the luminosity-dependence of pulsars' X-ray spectra in observations where the averaged (over many pulse cycles) luminosity of the source remains constant. Results: In all studied pulsars we revealed significant spectral changes as a function of the pulse height both in the continuum and in the cyclotron absorption features. The sources appear to form two groups showing different dependencies of the spectrum on pulse height. We interpret such a division as a manifestation of two distinct accretion regimes that are at work in different pulsars.