Volume 578, June 2015
|Number of page(s)||5|
|Section||Stellar structure and evolution|
|Published online||11 June 2015|
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
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
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, Ecyc, 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: dEcyc/ dt ~ −0.3 keV per year.
Conclusions. The decay of Ecyc 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, −Ecyc/Ėcyc ~ 100 yr, suggests that trend reversals and/or jumps of the line energy might be observed in the future.
Key words: accretion, accretion disks / stars: neutron / stars: magnetic field
© ESO, 2015
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