2014/12/27 by Rüdiger Staubert, Staubert, Rüdiger
Physics and Astronomy · #FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE) #astro-ph.HE
paper · pdf · doi:10.48550/arxiv.1412.8067
10 pages, 3 figures, conference: INTEGRAL 2014, Annapolis, ML, USA
arxiv created 2014/12/27 · arxiv updated 2014/12/30
Her~X-1 is one of the most remarkable members of the class of binary X-ray pulsars. It does not only show a large number of observable features, but has repeatedly been the first object for which fundamental discoveries were made: it was the first to show a super-orbital modulation, the first to reveal a cyclotron line in its spectrum and the first in which systematic variations of the cyclotron line energy were detected, namely variations with pulse phase (by ∼ 25%) and a positive correlation with X-ray luminosity (∼ 5% increase for a factor of two increase in luminosity). Now we have found another 'first': a long-term decrease of the pulse phase averaged cyclotron line energy E\rm cyc by ∼ 5 keV in 20 years. At the time of the discovery of the cyclotron line in 1976, its energy was ∼ 35 keV, remeasured around a similar energy by various instruments until 1990. Between 1990 and 1994 a jump upwards beyond 41 keV occurred, we are now back at ∼ 37 keV With respect to the physical cause of the discovered effect, we suggest it to be connected to a geometric displacement of the cyclotron resonant scattering region in the polar magnetic field or due to to a true physical change in the field configuration at the accretion mound or column by the continued accretion. The variation with time might be due to a non-perfect equilibrium between the rate at which material is accreted and the rate at which material is lost at the base of the accretion mound, allowing for a variation of the configuration of the accretion mound (height, total mass, field distribution). We do believe that we see the signature of a local change in the field configuration, rather than a decay of the global magnetic field, since the observed timescale of a few decades is very short.