Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Hercules X-1: variations of the cyclotron line energy with flux, with
  phase and with time   Her X-1 is one of the the most intensively studied Accreting X-ray Binary
Pulsars (AXBPs). This is largely because it is a bright and persistent X-ray
pulsar, showing the largest variety of observable phenomena, partly due to the
favorable geometry in observing the binary nearly edge-on. Her X-1 is the
binary X-ray pulsar in which the first cyclotron line (or Cyclotron Resonant
Scattering Feature - CRSF) was detected in 1975. Since then, we have made an
effort to observe the source and the CRSF around 40 keV as often as possible,
using those X-ray satellites which cover the high energy X-ray range. Extended
contributions are from the Rossi X-ray Timing Explorer (RXTE), {INTEGRAL,
Suzaku and Swift. The most recent contribution with a new degree of quality is
from the high energy imaging telescope on NuStar. We have found that the CRSF
is variable with flux, with phase (both pulsational and precessional phase) and
with time. The historical evolution of the pulse phase averaged CRSF centroid
energy Ecyc since its discovery is characterized by an initial value around 35
keV, an abrupt jump upwards to beyond ~40$ keV between 1990 and 1994 and an
apparent secular 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, amounting to a change of ~7% in energy for a factor of
two in luminosity. In observations since 2007, however, we now find a
statistically significant trend of a true secular decrease of the cyclotron
line energy. In addition, we discuss recent results of pulse phase spectroscopy
and evidence for the suspected variation in Ecyc with phase of the 35 d
precessional period (which is observed both in the modulation of the X-ray flux
and in the systematic variation in shape of the 1.24 s pulse profile.

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The categories of this paper are: plasm-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: acc-phys
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The categories of this paper are: supr-con
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The categories of this paper are: mtrl-th
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The categories of this paper are: adap-org
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The categories of this paper are: quant-ph
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The categories of this paper are: hep-th
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The categories of this paper are: q-alg
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The categories of this paper are: solv-int
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The categories of this paper are: gr-qc
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The categories of this paper are: cs
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The categories of this paper are: nlin
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The categories of this paper are: cmp-lg
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The categories of this paper are: patt-sol