# Long-term evolutions of the cyclotron line energies in Her X-1, Vela X-1 and Cen X-3 as observed with Swift/BAT

## TL;DR

This study analyzes the long-term evolution of cyclotron line energies in three X-ray pulsars using Swift/BAT data, revealing significant decreases in some sources and stability in others, informing magnetic field models.

## Contribution

It provides the first long-term observational analysis of CRSF energy evolution in Her X-1, Vela X-1, and Cen X-3 using Swift/BAT survey data.

## Key findings

- Her X-1 shows a significant decrease in CRSF energy since 2004.
- Vela X-1's first harmonic line energy decreased until 2012.
- Cen X-3's CRSF energy remains stable over 14 years.

## Abstract

We study the long-term evolution of the centroid energy of cyclotron lines - often referered to as Cyclotron Resonance Scattering Features (CRSF) - in Her X-1, Vela X-1 and Cen X-3, using survey observations of the Burst Alert Telescope (BAT) onboard Swift. We find a significant decrease of the fundamental CRSF energy in Her X-1 and the first harmonic line energy in Vela X-1, since the launch of Swift in 2004 and until 2010 and 2012, respectively.In both sources the decreases stopped at some time, with a quite stable centroid energy thereafter. Unlike in Her X-1 and Vela X-1, the CRSF energy in Cen X-3 does not show a long-term decrease. It is observed not to change for at least the past 14 years. The long-term variation of the line energy is a direct way to investigate the magnetic field structure in the polar regions of pulsars. Our results may stimulate the development of theoretical models, especially regarding to how the accreted mass accumulates in the accretion mound or how the magnetic field distorts around the polar cap.