The Swift-BAT monitoring reveals a long term decay of the cyclotron line energy in Vela X-1

TL;DR

This study uses 11 years of Swift BAT data to analyze the cyclotron line in Vela X-1, revealing a long-term decay in its energy that suggests magnetic field evolution possibly due to accreted matter effects.

Contribution

First detection of secular variation in the cyclotron line energy in Vela X-1 over a long-term monitoring period.

Findings

Second harmonic CRSF energy varies between 53 and 58 keV, correlated with luminosity.

CRSF energy decreases by approximately 0.36 keV/year over 11 years.

Decay in magnetic field strength estimated at about 3×10^{10} G/year.

Abstract

We study the behaviour of the cyclotron resonant scattering feature (CRSF) of the high mass X-ray binary Vela X-1 using the long-term hard X-ray monitoring performed by the Burst Alert Telescope (BAT) on board Swift. High statistics, intensity selected spectra were built along 11 years of BAT survey. While the fundamental line is not revealed, the second harmonic of the CRSF can be clearly detected in all the spectra, at an energy varying between $\sim 53$ keV and $\sim 58$ keV, directly correlated with the luminosity. We have further investigated the evolution of the CRSF in time, by studying the intensity selected spectra built along four 33-month time intervals along the survey. For the first time we find in this source a secular variation in the CRSF energy: independent of the source luminosity, the CRSF second harmonic energy decreases by $\sim 0.36$ keV/year between the first and the third time interval, corresponding to an apparent decay of the magnetic field of $\sim 3\times 10^{10}$ G/year. The intensity-cyclotron energy pattern is consistent between the third and the last time intervals. A possible interpretation for this decay could be the settling of an accreted mound that produces either a distortion of the poloidal magnetic field on the polar cap or a geometrical displacement of the line forming region. This hypothesis seems supported by the correspondance between the rate of the line shift per unit accreted mass and the mass accreted on the polar cap per unit area in Vela X-1 and Her X-1, respectively.