Swift/BAT and RXTE/ASM Observations of the 35-day X-ray Cycle of Hercules X-1. (arXiv:2009.07246v1 [astro-ph.HE])

Swift/BAT and RXTE/ASM Observations of the 35-day X-ray Cycle of Hercules X-1. (arXiv:2009.07246v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Leahy_D/0/1/0/all/0/1">Denis Leahy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_Y/0/1/0/all/0/1">Yuyang Wang</a>

Swift/BAT and RXTE/ASM observations have monitored the X-ray binary system
Her X-1 for approximately 14.5 years each, and both were monitoring Her X-1 for
a period of ~5.5 years. Here we study the 35-day cycle using these
observations. Using a cross-correlation method we find the times of peaks of
the 35-day cycles for ~150 cycles observed by Swift/BAT and ~150 cycles
observed by RXTE/ASM. These cycles include ~60 observed with both instruments.
The noise level of the RXTE/ASM measurements is larger than that of Swift/BAT,
resulting in larger uncertainty in peak times. The distribution of 35-day cycle
lengths can be fit with a Gaussian with mean 34.79 d and $sigma$ of 1.1 d. The
distribution of orbital phases of 35-day cycle peaks is well fit by a uniform
distribution, with 76 percent of the cycles, plus a Gaussian distribution
peaked at orbital phase ~0.5, with 24 percent of the cycles. We construct the
long-term average 35-day lightcurve in the 15-50 keV band from Swift/BAT, and
in the 2-12 keV band from RXTE/ASM. The high energy band shows more variability
in the Short High state and the low energy band shows more variability in the
Main High state. This is consistent with a precessing accretion disk model as
cause of the 35-day cycle.

Swift/BAT and RXTE/ASM observations have monitored the X-ray binary system
Her X-1 for approximately 14.5 years each, and both were monitoring Her X-1 for
a period of ~5.5 years. Here we study the 35-day cycle using these
observations. Using a cross-correlation method we find the times of peaks of
the 35-day cycles for ~150 cycles observed by Swift/BAT and ~150 cycles
observed by RXTE/ASM. These cycles include ~60 observed with both instruments.
The noise level of the RXTE/ASM measurements is larger than that of Swift/BAT,
resulting in larger uncertainty in peak times. The distribution of 35-day cycle
lengths can be fit with a Gaussian with mean 34.79 d and $sigma$ of 1.1 d. The
distribution of orbital phases of 35-day cycle peaks is well fit by a uniform
distribution, with 76 percent of the cycles, plus a Gaussian distribution
peaked at orbital phase ~0.5, with 24 percent of the cycles. We construct the
long-term average 35-day lightcurve in the 15-50 keV band from Swift/BAT, and
in the 2-12 keV band from RXTE/ASM. The high energy band shows more variability
in the Short High state and the low energy band shows more variability in the
Main High state. This is consistent with a precessing accretion disk model as
cause of the 35-day cycle.

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