Deciphering the Accretion Disk Structure of an X-ray Binary System
A schematic diagram of the Her X-1 X-ray binary system. Astronomers have analyzed the variable X-ray emission from Her X-1 and concluded that its accretion disk is warped and precessing, and used this model to refine the physical processes in the system.
The binary star system Her X-1 consists of a 1.5 solar-mass neutron star – the super-dense, collapsed remnant of a massive star – in orbit around the 2.2 solar-mass star, HZ Herculis. Located about twenty-two thousand light-years away, Her X-1 was the first known X-ray emitting binary star, discovered by the Uhuru satellite in 1971, and is the prototype object for its class of X-ray binaries. The X-rays are produced as material from the outer atmosphere of the normal star, pulled away by gravity, falls onto a hot accretion disk around the neutron star. Her X-1 is a pulsar, and its beam rotates with a period of 1.24 seconds. The two stars in the system orbit each other every 1.7 days in a roughly circular orbit that is highly inclined to our line-of-sight, resulting in regular eclipsing of the flux from the neutron star in addition to its pulses. Not least, in addition to these variable features Her X-1 shows regular flux variations over a 35-day timescale, much longer than the 1.7 day orbital period. These so-called "superorbital periods" are seen in other luminous X-ray binaries as well and are thought to be the result of warping in the accretion disk. Astronomers are trying to analyze the behavior of this complex and variable X-ray emission in order to model the detailed structure of the accretion disk around neutron stars and better understand the physical mechanisms at play in these systems.
CfA astronomer Saeqa Vrtilek was on a team that has completed a new study of the variations in the X-ray pulses in Her X-1. Their data come from the XMM-Newton and NuSTAR missions and span a complete 35-day superorbital cycle, and they supplement the observations with archival datasets. Their modeling finds that the accretion disk is warped and precessing, conclusions that agree with earlier findings. While they are not able to constrain the geometry of the ejected pulsar beams, they do conclude that the most energetic of the X-rays are in the pulsar beam while the less energetic X-ray emission comes from regions of the warped disk that are illuminated by the rotating pulsar beam. Their model successfully constrains the geometry of the jet-disk system and the distribution of energy across the X-ray spectrum.
Reference(s): "A Broad-Band X-Ray View of the Precessing Accretion Disk and Pre-eclipse Dip in the Pulsar Her X-1 With NuSTAR and XMM-Newton," McKinley C. Brumback, Ryan C. Hickox, Felix S. Furst, Katja Pottschmidt, John A. Tomsick, Jorn Wilms, Rudiger Staubert, and Saeqa Vrtilek, The Astrophysical Journal 2021 (in press).