Query : 2014A&A...572A.119S

2014A&A...572A.119S - Astronomy and Astrophysics, volume 572A, 119-119 (2014/12-1)

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


Abstract (from CDS):

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 Ecyc of the cyclotron line in the pulse phase averaged spectra from 1996 to 2012. 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 ∼40keV; these are RXTE, INTEGRAL, Suzaku, and NuSTAR. We consider results based on our own successful observing proposals as well as results from the literature. The historical evolution of the pulse phase averaged CRSF centroid energy Ecyc since its discovery in 1976 is characterized by an initial value around 35keV, an abrupt jump upwards to beyond ∼40keV 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 Ecyc and the X-ray luminosity Lx 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 Ecyc on X-ray luminosity is still valid with an increase of ∼5% in energy for a factor of two increase in luminosity. We also report on the first evidence of a weak dependence of Ecyc on phase of the 35d precessional period, which manifests itself not only in the modulation of the X-ray flux, but also in the systematic variation in the shape of the 1.24s pulse profile. One of our motivations for repeatedly observing Her X-1, namely the suspicion that the cyclotron line energy may be gradually decreasing after its strong upward jump in the early 1990s, is finally confirmed. A decrease in Ecyc by 4.2keV 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. The observed timescale for the decrease in Ecyc of a few decades is too short for a decay of the global magnetic field. 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. In the second scenario, the upward jump in Ecyc observed around 1991 may have been due to a relatively fast event in which the polar magnetic field rearranged itself after releasing part of the accumulated material to larger areas of the neutron star surface.

Abstract Copyright:

Journal keyword(s): radiation mechanisms: non-thermal - binaries: eclipsing - pulsars: individual: Her X-1 - accretion, accretion disks - magnetic fields - X-rays: binaries

Simbad objects: 8

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Number of rows : 8
N Identifier Otype ICRS (J2000)
ICRS (J2000)
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2024
1 V* V635 Cas HXB 01 18 31.9704955824 +63 44 33.072603144 17.05 16.64 15.19 14.34 13.22 B0.2Ve 730 0
2 V* BQ Cam HXB 03 34 59.9116110096 +53 10 23.296879776   17.16 15.42 14.26 13.04 O8.5Ve 463 1
3 M 1 SNR 05 34 30.9 +22 00 53           ~ 6120 1
4 HD 245770 HXB 05 38 54.5748918624 +26 18 56.836952784 9.30 9.84 9.39 8.77 8.30 O9/B0III/Ve 964 0
5 V* GP Vel HXB 09 02 06.8608812864 -40 33 16.899168060 6.85 7.37 6.87 6.31 6.05 B0.5Ia 1487 0
6 V* V850 Cen HXB 13 01 17.0968941216 -61 36 06.637549788   15.6 14.40     B2Vne 323 0
7 V* HZ Her LXB 16 57 49.8110126616 +35 20 32.486555472 11.90 14.89 13.63 13.88   B3/6ep 2061 0
8 V* V490 Cep HXB 21 39 30.6903047304 +56 59 10.421969280   15.2       B1.5Ve 162 1

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