Decade long RXTE monitoring observations of Be/X-ray binary pulsar EXO 2030+375

TL;DR

This study analyzes over 15 years of RXTE data on the Be/X-ray binary pulsar EXO 2030+375, revealing how its pulse profiles and spectra depend on luminosity, and identifying spectral state transitions and a critical luminosity regime.

Contribution

It provides the first comprehensive timing and spectral analysis of EXO 2030+375 over a decade, linking pulse profile features and spectral changes to accretion regimes.

Findings

Pulse profiles vary with luminosity, showing phase shifts and energy dependence.

No cyclotron resonance scattering feature detected in the spectra.

Spectral parameters indicate transitions between sub-critical and super-critical accretion regimes.

Abstract

We present a comprehensive timing and spectral studies of Be/X-ray binary pulsar EXO 2030+375 using extensive Rossi X-ray Timing Explorer observations from 1995 till 2011, covering numerous Type I and 2006 Type II outbursts. Pulse profiles of the pulsar were found to be strongly luminosity dependent. At low luminosity, the pulse profile consisted of a main peak and a minor peak that evolved into a broad structure at high luminosity with a significant phase shift. A narrow and sharp absorption dip, also dependent on energy and luminosity, was detected in the pulse profile. Comparison of pulse profiles showed that the features at a particular luminosity are independent of type of X-ray outbursts. This indicates that the emission geometry is solely a function of mass accretion rate. The broadband energy spectrum was described with a partial covering high energy cutoff model as well as a physical model based on thermal and bulk Comptonization in accretion column. We did not find any signature of cyclotron resonance scattering feature in the spectra obtained from all the observations. A detailed analysis of spectral parameters showed that, depending on source luminosity, the power-law photon index was distributed in three distinct regions. It suggests the phases of spectral transition from sub-critical to super-critical regimes in the pulsar as proposed theoretically. A region with constant photon index was also observed in ~(2-4) x 10^37 erg/s range, indicating critical luminosity regime in EXO 2030+375.