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

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 E cyc 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 ∼ 40 keV. Results. The historical evolution of the pulse phase averaged CRSF centroid energy E cyc since its discovery in 1976 is characterized by an initial value around 35 keV, an abrupt jump upwards to beyond ∼ 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 E cyc and the X-ray luminosity L x 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 E cyc on X-ray luminosity is still valid with an increase of ∼ 5% in energy for a factor of two increase in luminosity. A decrease in E cyc 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.

Inversion of the decay of the cyclotron line energy in Hercules X-1

“…It seems that this speculation has now come true. It was further speculated that the observed behavior -a rather fast rise followed by a decay over tens of yearsmight possibly be a cyclic behavior that could be understood in the framework of a physical scenario, the basics of which were outlined by Staubert et al (2014), Staubert (2014), and Staubert et al (2016), and are partly repeated here.…”

“…The exposure time of NuSTAR was 37 ks. The details of the data analysis are similar to those described by Staubert et al (2014) and Staubert et al (2016). We used the standard nupipeline and nuproducts utilities and XSPEC 1 v12.9 as part of HEASOFT 2 6.18.…”

Neutron Star Physics in the Square Kilometre Array Era: An Indian Perspective

“…Her X-1 has shown long term evolution of the CRSF independent to other observables both in the form of a sudden jump followed by a gradual decay (Staubert et al 2014;Klochkov et al 2015). 4U 1538-522 also shows signatures of an evolving CRSF energy (Hemphill et al 2016).…”

Swift/BAT measurements of the cyclotron line energy decay in the accreting neutron star Hercules X-1: indication of an evolution of the magnetic field?

“…Among more than a dozen confirmed CRSF sources, Her X-1 possesses one of the most prominent cyclotron lines whose characteristics can be reliably measured with shorter observations than in other XBPs at a similar flux level. A&A 578, A88 (2015) Staubert et al 2014, for details).…”

“…As a result, an unprecedentedly long record of CRSF measurements in a single pulsar has been accumulated. For a detailed description of the observational data base of the cyclotron line measurements in Her X-1, we refer to the recent work by Staubert et al 2014, hereafter, Paper I). The CRSF centroid energy E cyc in Her X-1 does not remain constant.…”