INTEGRAL observations of the variability of OAO 1657-415

TL;DR

This study analyzes INTEGRAL satellite data to examine the spectral and timing variability of the pulsar OAO 1657-415, revealing complex spin period evolution, spectral changes with pulse phase, and no evidence of a cyclotron line.

Contribution

It provides a detailed analysis of the pulsar's spectral and timing properties, including long-term period evolution and phase-resolved spectra, with improved orbital parameters.

Findings

Long-term spin-up with episodes of spin-down/spin-up repeating every 4.8 years.

Spectral changes correlated with pulse phase, especially during the main peak.

No detection of cyclotron line in any spectrum.

Abstract

The Galactic Plane Scan (GPS) was one of the core observation programmes of the INTEGRAL satellite. The highly variable accreting pulsar OAO 1657-415 was frequently observed within the GPS. We investigate the spectral and timing properties of OAO 1657-415 and their variability on short and long time scales in the energy range 6-160 keV. During the time covered by the INTEGRAL observations, the pulse period evolution shows an initial spin-down, which is followed by an equally strong spin-up. In combining our results with historical pulse period measurements (correcting them for orbital variation) and with stretches of continuous observations by BATSE, we find that the long-term period evolution is characterised by a long-term spin-up overlayed by sets of relative spin-down/spin-up episodes, which appear to repeat quasi-periodically on a 4.8 yr time scale. We measure an updated local ephemeris and confirm the previously determined orbital period with an improved accuracy. The spectra clearly change with pulse phase. The spectrum measured during the main peak of the pulse profile is particularly hard. We do not find any evidence of a cyclotron line, wether in the phase-averaged spectrum or in phase-resolved spectra.