Pulse-to-pulse variability of bright accreting pulsars

TL;DR

This study investigates the rapid variability of bright accreting pulsars on the timescale of individual pulses using high-resolution X-ray data, revealing spectral differences and luminosity-dependent behaviors near neutron stars.

Contribution

First detailed pulse-to-pulse spectral analysis of accreting pulsars, linking variability to accretion flow models near neutron stars.

Findings

Spectral differences observed between pulses of varying amplitudes.

Luminosity-dependent spectral changes identified at constant average luminosity.

Pulse-to-pulse spectroscopy provides new insights into accretion physics.

Abstract

In addition to coherent pulsation, many accreting neutron stars exhibit flaring activity and strong aperiodic variability on time scales similar 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. Although from the theoretical point of view the problem of non-stationary accretion has been addressed by many authors, observational study of this phenomenon is often problematic as it requires very high statistics of X-ray data and a specific analysis technique. 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". The high-quality data allowed us to collect individual pulses of different amplitude and reveal differences in their spectra (such an analysis we refer to as "pulse-to-pulse spectroscopy"). The described approach allowed us for the first time to study luminosity-dependence of pulsars' X-ray spectra in observations where the averaged (over many pulse cycles) luminosity of the source remained constant and discuss them in the frame of the current physical models of the accretion flow close to the neutron star surface.