452 research outputs found
Pulse phase and precession phase resolved spectroscopy of Her X-1: studying a representative Main-On with RXTE
We performed a detailed pulse phase resolved spectroscopy of the accreting
binary X-ray pulsar Her X-1 in the energy range 3.5-75 keV and have established
pulse phase profiles for all spectral parameters. For the centroid of the
cyclotron line, the photon index and the flux of the 6.4 keV iron line, we have
studied the variation as a function of 35 d phase. We analyzed RXTE
observations of the Main-On of November 2002. Four different time intervals of
about 1 d duration were selected to provide a good coverage of a complete
Main-On. The intervals are centered at 35 d phase 0.03, 0.10, 0.15, and 0.20,
respectively. All spectral parameters show a strong modulation with pulse
phase. While the centroid energy of the cyclotron line follows roughly the
shape of the pulse profile, both the photon index and the iron line intensity
exhibit distinct minima around the peak of the X-ray pulse. With respect to
variations of the observed profiles with 35 d phase, we find that there is a
clear evolution of the shape of the pulse profiles (flux versus pulse phase), a
moderate increase of the maximum cyclotron line energy (found around pulse
phase 0.7), but no significant evolution of the shape of the pulse phase
profiles of the cyclotron line energy, the spectral power law index or the iron
line intensity. The variation of spectral parameters as a function of the pulse
phase provides important information about the system: 1. the disappearance of
the Fe line flux near the highest continuum flux may be an indication of a
hollow cone geometry of the accretion structure; ii. the apparent
non-dependence of the cyclotron line energy profiles on 35 d phase provides a
new possibility to test the model of free precession of the neutron star,
proposed to be responsible for the systematic variations in the pulse profiles.Comment: 10 pages, 11 figures, Accepted by A&A on the 22/12/201
Variable pulse profiles of Her X-1 repeating with the same irregular 35d clock as the turn-ons
The accreting X-ray pulsar Her X-1 shows two types of long-term variations,
both with periods of ~35 days: 1) Turn-on cycles, a modulation of the flux},
with a ten-day long Main-On and a five-day long Short-On, separated by two
Off-states, and 2) a systematic variation in the shape of the 1.24 s pulse
profile. While there is general consensus that the flux modulation is due to
variable shading of the X-ray emitting regions on the surface of the neutron
star by the precessing accretion disk, the physical reason for the variation in
the pulse profiles has remained controversial. Following the suggestion that
free precession of the neutron star may be responsible for the variation in the
pulse profiles, we developed a physical model of strong feedback interaction
between the neutron star and the accretion disk in order to explain the
seemingly identical values for the periods of the two types of variations,
which were found to be in basic synchronization. In a deep analysis of pulse
profiles observed by several different satellites over the last three decades
we now find that the clock behind the pulse profile variations shows exactly
the same erratic behavior as the turn-on clock, even on short time scales (a
few 35 d cycles), suggesting that there may in fact be only one 35 d clock in
the system. If this is true, it raises serious questions with respect to the
idea of free precession of the neutron star, namely how the neutron star can
change its precessional period every few years by up to 2.5% and how the
feedback can be so strong, such that these changes can be transmitted to the
accretion disk on rather short time scales.Comment: 9 pages, 13 figures, accepted by Astronomy & Astrophysics. arXiv
admin note: substantial text overlap with arXiv:1110.671
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