Long-term change in the cyclotron line energy in Hercules X-1

TL;DR

This study analyzes the long-term evolution of the cyclotron line energy in Her X-1, revealing a significant decrease from 1996 to 2012, likely due to changes in the magnetic field or scattering region.

Contribution

It provides the first clear evidence of a true long-term decrease in the cyclotron line energy in Her X-1 after accounting for luminosity correlations.

Findings

A significant decrease of 4.2 keV in Ecyc from 1996 to 2012.

Ecyc correlates with X-ray luminosity, increasing by 5% for a doubling of luminosity.

The long-term decay can be modeled as linear or with a two-phase decay pattern.

Abstract

Aims. We investigate the long-term evolution of the Cyclotron Resonance Scattering Feature (CRSF) in the spectrum of the binary X-ray pulsar Her X-1 and present evidence of a true long-term decrease in the centroid energy Ecyc of the cyclotron line in the pulse phase averaged spectra from 1996 to 2012. Methods. Our results are based on repeated observations of Her X-1 by those X-ray observatories capable of measuring clearly beyond the cyclotron line energy of about 40 keV. Results. The historical evolution of the pulse phase averaged CRSF centroid energy Ecyc since its discovery in 1976 is characterized by an initial value around 35 keV, an abrupt jump upwards to beyond about 40 keV between 1990 and 1994, and an apparent decay thereafter. Much of this decay, however, was found to be due to an artifact, namely a correlation between Ecyc and the X-ray luminosity Lx discovered in 2007. In observations after 2006, however, we now find a statistically significant true decrease in the cyclotron line energy. At the same time, the dependence of Ecyc on X-ray luminosity is still valid with an increase of about 5% in energy for a factor of two increase in luminosity. A decrease in Ecyc by 4.2 keV over the 16 years from 1996 to 2012 can either be modeled by a linear decay, or by a slow decay until 2006 followed by a more abrupt decrease thereafter. Conclusions. We speculate that the physical reason could be connected to a geometric displacement of the cyclotron resonant scattering region in the polar field or to a true physical change in the magnetic field configuration at the polar cap by the continued accretion.