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?

¹ Institut für Astronomie und Astrophysik, Universität Tübingen (IAAT), Sand 1, 72076 Tübingen, Germany
e-mail: klochkov@astro.uni-tuebingen.de
² Moscow M.V. Lomonosov State University, Faculty of Physics and Sternberg Astronomical Institute, 199992 Moscow, Russia
³ Dr. Remeis-Sternwarte & ECAP, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁴ Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0424, USA

Received: 26 March 2015
Accepted: 20 April 2015

Abstract

Context. The magnetic field is a crucial ingredient of neutron stars. It governs the physics of accretion and of the resulting high-energy emission in accreting pulsars. Studies of the cyclotron resonant scattering features (CRSFs) seen as absorption lines in the X-ray spectra of the pulsars permit direct measurements of the field strength.

Aims. From an analysis of a number of pointed observations with different instruments, the energy of CRSF, E_cyc, has recently been found to decay in Her X-1 , which is one of the best-studied accreting pulsars. We present our analysis of a homogeneous and almost uninterrupted monitoring of the line energy with Swift/BAT.

Methods. We analyzed the archival Swift/BAT observations of Her X-1 from 2005 to 2014. The data were used to measure the CRSF energy averaged over several months.

Results. The analysis confirms the long-term decay of the line energy. The downward trend is highly significant and consistent with the trend measured with the pointed observations: dE_cyc/ dt ~ −0.3 keV per year.

Conclusions. The decay of E_cyc either indicates a local evolution of the magnetic field structure in the polar regions of the neutron star or a geometrical displacement of the line-forming region due to long-term changes in the structure of the X-ray emitting region. The shortness of the observed timescale of the decay, −E_cyc/Ė_cyc ~ 100 yr, suggests that trend reversals and/or jumps of the line energy might be observed in the future.