Statistical analysis of archival Vela X-1 data

TL;DR

This paper provides a comprehensive statistical analysis of Vela X-1's archival INTEGRAL data, revealing its variable activity, spectral differences between states, and a log-normal distribution of its flaring behavior.

Contribution

It offers the first detailed statistical characterization of Vela X-1's flaring activity and spectral states based on extensive archival data.

Findings

Vela X-1 exhibits both active and quiet states with distinct spectral properties.

The source's intensity distribution follows a log-normal pattern.

Flares can reach flux levels exceeding 2.0 Crab.

Abstract

We present a detailed analysis of all archival INTEGRAL data of the accreting X-ray pulsar Vela X-1. We extracted lightcurves in several energy bands from 20 keV up to 60 keV. The lightcurves show that the source was found in very active as well as quiet states. During the active states several giant flares were detected. For these states spectra between 5 keV and 120 keV were obtained. The spectra of the active states were found to be significantly softer than those from the quiet states. We performed a statistical analysis of the flaring behavior. The resulting log-normal distribution of the intensity of Vela X-1 shows that the source spends most of the time at an average flux level of 300 mCrab but also that the distribution extends well up to more than 2.0 Crab.