Volume 590, June 2016
|Number of page(s)||7|
|Section||Stellar structure and evolution|
|Published online||19 May 2016|
Continued decay in the cyclotron line energy in Hercules X-1
1 Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
2 Dr. Remeis-Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
3 Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
Received: 14 December 2015
Accepted: 18 March 2016
The centroid energy Ecyc of the cyclotron line in the spectrum of the binary X-ray pulsar Her X-1 has been found to decrease with time on a time scale of a few tens of years – surprisingly short in astrophysical terms. This was found for the pulse phase-averaged line centroid energy using observational data from various X-ray satellites over the time period 1996 to 2012, establishing a reduction of ~4 keV. Here we report on the result of a new observation by NuSTAR performed in August 2015. The earlier results are confirmed and strengthened with respect to both the dependence of Ecyc on flux (it is still present after 2006) and the dependence on time: the long-term decay continued with the same rate, corresponding to a reduction of ~5 keV in 20 yr.
Key words: magnetic fields / stars: neutron / radiation mechanisms: general / radiation mechanisms: non-thermal / accretion, accretion disks / X-rays: binaries
© ESO, 2016
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