X-ray Binary Hercules Research Articles

Constraining the Number Density of the Accretion Disk Wind in Hercules X-1 Using Its Ionization Response to X-Ray Pulsations

X-ray binaries are known to launch powerful accretion disk winds that can have a significant impact on the binary systems and their surroundings. To quantify the impact and determine the launching mechanisms of these outflows, we need to measure the wind plasma number density, an important ingredient in the theoretical disk wind models. While X-ray spectroscopy is a crucial tool for understanding the wind properties, such as their velocity and ionization, in nearly all cases, we lack the signal-to-noise ratio to constrain the plasma number density, weakening the constraints on the outflow location and mass outflow rate. We present a new approach to determining this number density in the X-ray binary Hercules X-1, by measuring the speed of the wind ionization response to the time-variable illuminating continuum. Hercules X-1 is powered by a highly magnetized neutron star, pulsating with a period of 1.24 s. We show that the wind number density in Hercules X-1 is sufficiently high to respond to these pulsations by modeling the ionization response with the time-dependent photoionization model tpho. We then perform a pulse-resolved analysis of the best-quality XMM-Newton observation of Hercules X-1 and directly detect the wind response, confirming that the wind density is at least 1012 cm−3. Finally, we simulate XRISM observations of Hercules X-1 and show that they will allow us to accurately measure the number density at different locations within the outflow. With XRISM, we will rule out ∼3 orders of magnitude in density parameter space, constraining the wind mass outflow rate, energetics, and its launching mechanism.

Accretion disc viscosity: a limit on the anisotropy

Observations of warped discs can give insight into the nature of angular momentum transport in accretion discs. Only a few objects are known to show strong periodicity on long timescales, but when such periodicity is present it is often attributed to precession of the accretion disc. The X-ray binary Hercules X-1/HZ Herculis (Her X-1) is one of the best examples of such periodicity and has been linked to disc precession since it was first observed. By using the current best-fitting models to Her X-1, which invoke precession driven by radiation warping, I place a constraint on the effective viscosities that act in a warped disc. These effective viscosities almost certainly arise due to turbulence induced by the magneto-rotational instability. The constraints derived here are in agreement with analytical and numerical investigations into the nature of magneto-hydrodynamic disc turbulence, but at odds with some recent global simulations.

FUSEObservations of a Full Orbit of Hercules X‐1: Signatures of Disk, Star, and Wind

We observed an entire 1.7 day orbit of the X-ray binary Hercules X-1 with the Far Ultraviolet Spectroscopic Explorer (FUSE). Changes in the O VI 1032,1037 line profiles through eclipse ingress and egress indicate a Keplerian accretion disk spinning prograde with the orbit. These observations may show the first double-peaked accretion disk line profile to be seen in the Hercules X-1 system. Doppler tomograms of the emission lines show a bright spot offset from the Roche lobe of the companion star HZ Her, but no obvious signs of the accretion disk. Simulations show that the bright spot is too far offset from the Roche lobe to result from uneven X-ray heating of its surface. The absence of disk signatures in the tomogram can be reproduced in simulations which include absorption from a stellar wind. We attempt to diagnose the state of the emitting gas from the C III 977, C III 1175, and N III 991 emission lines. The latter may be enhanced through Bowen fluorescence.

Atmospheric Reflection during an Anomalous Low State of Hercules X‐1

We present RXTE observations of the eclipsing X-ray binary Hercules X-1 during an anomalous low state. Data reduction reveals a light curve over 2.7 orbital cycles remarkably similar to optical and UV light curves dominated by the companion star. Count rates are modulated close to the orbital period, attaining a maximum when the inner face of the companion star, irradiated by X-rays from the compact source, is most visible. Cold reflection provides an acceptable fit to the energy spectrum. Employing binary geometry to scale the model and assuming companion-star reflection, we are able to reconstruct the incident X-rays that are removed from our direct line of sight (presumably by the accretion disk). We find the flux of the hidden source to be identical to the observed flux of Her X-1 at the peak of its main high state. Consequently, Her X-1 is emitting a reflected spectrum, largely uncontaminated by direct X-rays in the anomalous low state. The spectral energy distribution, period, amplitude, and phasing of the modulation are all consistent with a companion-star origin. Since this source occurs in a well-understood binary environment, it provides an excellent case study for more sensitive experiments in the future.

RXTEObservations of Hercules X‐1 during the 1998 July Short High State

We present RXTE monitoring of the eclipsing X-ray binary Hercules X-1 conducted over the short high state of 1998 July. This was one of the last major short high states before the source entered an anomalous low state of activity. A comparison with previous epochs finds no evidence for special behavior during these observations. We determine orbital and pulsar spin periods to facilitate measurements of spin and orb during the subsequent anomalous low state and the next epoch of high-state activity. Spectrally, the decay of the short high state and concurrent pre-eclipse dips are consistent with obscuration of a central X-ray source by a cloud of nonuniform column density. The standard model of a warped accretion disk of finite vertical scale height fits the characteristics of this absorber well. Pre-eclipse dips have durations a factor of a few longer than the characteristic durations of dips during main high states. Pulse profile structure increases in complexity toward the tail of the short high state, suggesting changes in accretion curtain geometry.

Eclipsing Broad Emission Lines in Hercules X‐1: Evidence for a Disk Wind?

We present disk wind model calculations for the broad emission lines seen in the ultraviolet spectra of the X-ray binary Hercules X-1. Recent HST/STIS observations of these lines suggest that they are kinematically linked to the orbital motion of the neutron star and exhibit a red-shifted to blue-shifted evolution of the line shape during the progression of the eclipse from ingress to egress which is indicative of disk emission. Furthermore, these lines are single-peaked which implies that they may be formed in a disk wind similar to those we have proposed as producing the broad emission lines seen in the UV spectra of active galactic nuclei. We compute line profiles as a function of eclipse phase and compare them to the observed line profiles. Various effects may modify the appearance of the lines including resonant scattering in the wind itself, self-shadowing of the warped disk from the central continuum, and self-obscuration of parts of the disk along the observer's line-of-sight. These latter two effects can cause orbital and precessional phase dependent variations in the emission lines. Hence, examination of the line profiles as a function of these phases can, in principle, provide additional information on the characteristics of the disk warp.

Hopkins Ultraviolet Telescope Observations of Hercules X‐1

We have obtained a far-ultraviolet spectrum of the X-ray binary Hercules X-1/HZ Herculis using the Hopkins Ultraviolet Telescope aboard the Astro-1 Space Shuttle mission in 1990 December. This is the first spectrum of Her X-1 that extends down to the Lyman limit at 912 Å. We observed emission lines of O VI λλ1031.9, 1037.6, N V λλ1238.8, 1242.8, and C IV λλ1548.2, 1550.8 and the far-UV continuum extending to the Lyman limit. We examine the conditions of the emitting gas through line strengths, line ratios, and doublet ratios. The UV flux is lower by about a factor of 2 than expected at the orbital phase of the observation. We model the UV continuum with a simple power law and with a detailed model of an X-ray-illuminated accretion disk and companion star. The power law provides a superior fit, as the detailed model predicts too little flux below 1200 Å. We note, however, that there are uncertainties in the interstellar reddening, in the background airglow spectrum, and in the long-term phase of the accretion disk. We have searched the data for UV line and continuum pulsations near the neutron star's spin period but found none at a detectable level.

Gamma-ray light curves from statistically limited data; Hercules X-l

The problem of finding the light curve for noisy or sparse data is a familiar one. We develop a model-independent method which gives the light curve as a linear transformation of the data. Our approach emphasizes the fundamental choices between resolution and statistical error that is inherent in any data analysis. The analysis is treated as a special case of the general problem of data analysis based upon Fredholm equations of the first kind. The results, with certain approximations, are formally equivalent to the kernel density estimators but the interpretation is much different. We apply our technique to a TeV gamma-ray observation of the X-ray binary Hercules X-l.

A model of pulsed gamma radiation from the X-ray binary Hercules X-1/HZ Herculis

A model of pulsed gamma radiation from the X-ray binary Hercules X-1/HZ Herculis

The 35 day cycle of the X-ray binary Hercules X-1

The 35 day cycle of the X-ray binary Hercules X-1

The 35 day cycle of Hercules X-1

Current models for the eclipsing X-ray binary Hercules X-1 appear inadequate to account properly for the 35-day cycle of X-ray on and off periods, particularly its phase lock with orbital cycle. The observations can be explained by a model in which cessation of detectable X-ray emission is caused by occultation by circumstellar gas streams leaving the binary system through the outer Lagrange points. The specific values of the turn-on phases appear to be an accident of orbital inclination.

Optical study of the X-ray binary Hercules X-1

The 1.7 day B-V light curve of HZ Herculis shows the existence of a small hot component which is present only during the X-ray active portion of the 35-day cycle. This hot component is associated with the X-ray source since it is absent during X-ray eclipse.