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Register a new userModeling of 35-day superorbital cycle of $B$ and $V$ light curves of IMXB HZ Her/Her X-1
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The X-ray binary Her X-1 consists of an accreting neutron star and the optical component HZ Her. The 35-day X-ray superorbital variability of this system is known since its discovery in 1972 by the Uhuru satellite and is believed to be caused by forced precession of a warped accretion disk tilted to the orbital plane. We argue that the observed features of the 35-day optical variability of HZ Her can be explained by free precession of the neutron star with a period close to that of the forced disk. The model parameters include a) the X-ray luminosity of the neutron star; b) the optical flux from the accretion disk; c) the tilt of the inner and outer edges of the accretion disk. A possible synchronization mechanism based on the coupling between the neutron star free precession and the dynamical action of non-stationary gas streams is discussed.
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Regular variations of the pulse period of Her X-1 with X-ray flux observed by Fermi-GBM are examined. We argue that these regular variations result from the free precession of the neutron star in Her X-1.
We report on the timing analysis of the observation of the X-ray binary pulsar Her X-1 performed during the BeppoSAX Science Verification Phase. The observation covered more that two full orbital cycles near the maximum of the main-on in the 35 day cycle of Her X-1. We present the pulse profiles from 0.1 to 100 keV. Major changes are present below 1 keV, where the appearance of a broad peak is interpreted as re-processing from the inner part of the accretion disk, and above 10 keV, where the pulse profile is less structured and the main peak is appreciably harder. The hardness ratios show complex changes with pulse phase at different energies.
We present an analysis of several high-resolution Chandra grating observations of the X-ray binary pulsar Her X-1. With a total exposure of 170 ks, the observations are separated by years and cover three combinations of orbital and super-orbital phases. Our goal is to determine distinct properties of the photoionized emission and its dependence on phase-dependent variations of the continuum. We find that the continua can be described by a partial covering model which above 2 keV is consistent with recent results from rxte studies and at low energies is consistent with recent xmm and sax studies. Besides a powerlaw with fixed index, an additional thermal blackbody of 114 eV is required to fit wavelengths above 12 AA ($sim$ 1 keV). We find that likely all the variability is caused by highly variable absorption columns in the range (1 -- 3)$times 10^{23}$ cm$^{-2}$. Strong Fe K line fluorescence in almost all observations reveals that dense, cool material is present not only in the outer regions of the disk but interspersed throughout the disk. Most spectra show strong line emission stemming from a photoionized accretion disk corona. We model the line emission with generic thermal plasma models as well as with the photoionization code XSTAR and investigate changes of the ionization balance with orbital and superorbital phases. Most accretion disk coronal properties such as disk radii, temperatures, and plasma densities are consistent with previous findings for the low state. We find that these properties change negligibly with respect to orbital and super-orbital phases. A couple of the higher energy lines exhibit emissivities that are significantly in excess of expectations from a static accretion disk corona.
We report high-cadence photometry of the ultra-fast ($t_2sim1.2$ d) nova V1674 Her during its rise to maximum light ($Vsim6.3$) and the beginning of its subsequent decline. These observations from Evryscope and the Mount Laguna Observatory All-Sky Camera reveal a plateau in the pre-maximum light curve at $gsim14$ ($sim$8 mag below peak) that lasted for at least three hours. Similar features (so-called pre-maximum halts) have been observed in some novae near maximum light, but to our knowledge the detection of a plateau in the light curve $sim$8 mag below peak is unprecedented.
We present spin-resolved X-ray data of the neutron star binary Her X-1. We find evidence that the Iron line at 6.4 keV originates from the same location as the blackbody X-ray component. The line width and energy varies over both the spin period and the 35 day precession period. We also find that the correlation between the soft and hard X-ray light curves varies over the 35 day period.
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