Time Evolution Of Cyclotron Line of Her X-1; A Detailed StatisticalAnalysis Including New ASTROSAT Data

TL;DR

This study analyzes over three decades of X-ray observations of Her X-1, revealing a long-term decline and possible stabilization in its cyclotron line energy, with new ASTROSAT data supporting a flattening trend.

Contribution

It provides a comprehensive statistical analysis of the cyclotron line evolution in Her X-1, incorporating recent ASTROSAT data and clarifying the trend as a flattening after decline.

Findings

Line energy shows a secular decline from 1995-2014.

Recent data indicates a flattening of the line energy trend.

No evidence supports an upward reversal in line energy.

Abstract

The cyclotron line feature in the X-ray spectrum of the accretion powered pulsar Her X-1 has been observed and monitored for over three decades. The line energy exhibited a slow secular decline over the period 1995-2014, with a possible (not confirmed) indication of a reversal thereafter. Recent works have shown that the temporal evolution of the line energy may be modelled as a flattening after an earlier decrease until MJD 55400 ($\pm200)$. In this work, we present the results of ASTROSAT observations in the context of earlier data and offer a common interpretation through a detailed study of temporal and flux dependence. We find that the variation of the line energy does not support an upward trend but is consistent with the reported flattening after an earlier decrease until MJD $54487^{+515}_{-469}$.