A high-mass X-ray binary (HMXB or HMXRB) is a binary star system containing a compact object (typically a black hole or neutron star) and a normal star more massive than approximately 8–10 solar masses (). They form when the more massive star in a binary undergoes a supernova explosion to become a neutron star or black hole, and then begins to accrete from the other star, which can be a supergiant, a Be star, or, rarely, a Wolf-Rayet star. The compact object, which is almost always a neutron star, usually accretes from the companion's strong stellar winds, releasing bright X-ray emission that can be observed by astronomers. This period ends when the companion begins to evolve and the binary either spirals in, forming a Thorne-Zytkow object, or becomes a double compact object binary system after the companion star explodes in a supernova.
There are several different types of HMXBs, all with different properties, modes of formation, and likely fates. Supergiant X-ray binaries, Be/X-ray binaries, and Wolf-Rayet X-ray binaries are systems containing a supergiant star, Be star, or Wolf-Rayet star respectively. X-ray pulsars contain strongly magnetized neutron stars that funnel accreted plasma onto their magnetic poles. Ultraluminous X-ray sources are HMXBs containing a compact object, likely a black hole, accreting faster than the Eddington limit. Finally, gamma-ray binaries are HMXBs that emit mostly in gamma-ray wavelengths due to interactions between their host pulsar's fast rotation and strong magnetic field. Recent launches of X-ray telescopes and studies of their data have allowed for the detection, identification, and analysis of many new HMXBs within every class.
Formation HMXBs start as binaries of two massive stars. When the more massive star begins to evolve, mass transfer begins, causing the binary's orbital period to shrink. About 4–40 million years after the binary forms, the more massive star explodes in a supernova and becomes a black hole (BH) or neutron star (NS), and the system becomes an HMXB. A supernova kick is believed to send HMXBs away from the Galactic Center; this is corroborated by observations of a larger scale height for HMXBs compared to other star types.
Formation of HMXBs varies. Some HXMBs undergo an initial phase of Roche lobe overflow when the more massive star in the binary begins to evolve, eventually forming a common envelope around both stars. Due to drag, the two stars will spiral in towards each other and ultimately eject the common envelope, leaving behind a binary with a much shorter period. Eventually, the more massive star will supernova, and the system will become an HMXB. Alternatively, an HMXB may be formed by stable, nonconservative mass transfer that can either increase or decrease the binary's orbital period. SgXBs are believed to generally form via the former method, and BeXBs the latter.
Properties Although the compact object can be either a black hole or a neutron star, in the vast majority of HMXBs with known compact-object classifications, it is a neutron star. When BHs are present in HMXBs, they tend to be more massive than the BHs in low-mass X-ray binaries (LMXBs). Donor objects tend to be O-, B-, or Be-type stars.
HMXBs generally have a positive correlation between orbital period and orbital eccentricity. More eccentric systems tend to have greater ranges of luminosity, since the amount of stellar wind they are receiving is continuously fluctuating throughout their orbit. They are brightest in UV-optical and X-ray luminosities, with luminosities of about – erg/s in both spectral bands. Some HMXBs also produce significant emission in gamma rays or radio waves; emission in these wavelengths is associated with relativistic jets.
HMXBs tend to be located in the arms of spiral galaxies and in regions of young stars and rapid star formation. The number of HMXBs in a galaxy is directly proportional with its star-formation rate. Very few HMXBs are located in older star clusters, as both stars in an HMXB will have evolved into compact objects within a few hundred million years of their formation. However, HMXBs are scarce in clusters with very young stars, as the more massive stars in these binaries will not have had time to evolve into compact objects.
Dynamics Unlike LMXBs and cataclysmic variables, which usually accrete via Roche lobe overflow, HMXBs typically undergo mass transfer via the accretion of stellar wind from the massive star onto the compact object. Because massive stars have powerful stellar winds, the X-ray accretion luminosity can be extremely bright, up to erg/s. Although some HMXBs do accrete via Roche lobe overflow, this process is unstable due to the high mass ratio between the two objects and therefore can only last for short timescales.
HMXBs can either be persistent (continuously luminous) or transient (alternating between high- and low-luminosity states). About 40% of galactic HMXBs are transient, including most BeXBs. HMXBs can also exhibit variability, both periodically through superorbital modulation and aperiodically through quasi-periodic oscillations, off-states, and flares.
Disk-fed accretion
Several HMXBs are known to be experiencing disk-fed accretion, in which an accretion disk forms around the compact object. These disks can exist in HMXBs that accrete via Roche lobe overflow or from stellar wind, although in the case of the latter, disks are usually temporary and occur when the stellar wind is focused towards the compact object. HMXBs known to have accretion disks include Centaurus X-3, Cygnus X-1, and Vela X-1. Accretion disks may be able to persist in some Be/X-ray binaries, even when they are not actively accreting (quiescent). Disk-fed HMXBs have the shortest orbital periods of any type of HMXB.
Wind-fed accretion The vast majority of HMXBs transfer mass via wind-fed accretion, in which the compact object accretes mass shed from the normal star in the form of powerful stellar wind. This wind can have speeds up to 2000 kilometers per second or more. Wind-fed systems usually contain neutron stars, which have magnetic fields that channel the accretion flow towards their poles. Wind-fed HMXBs with closer orbits have higher minimum X-ray luminosities than systems with wider orbits. In the case of NS HMXBs, interactions between the accretion flow and the NS's magnetosphere can cause variability in the system's luminosity.
Evolution
The HMXB phase typically lasts approximately 10,000 years. Estimates of the ages of HMXBs vary, but are typically short on a cosmic scale, on the order of millions to tens of millions of years. Because high-mass stars evolve quickly, HMXBs become impossible a few hundred million years after their formation. The average BeXB is about 40–50 million years old, and in general, HMXB formation is most efficient 30–60 million years after the binary itself forms. About 10% of neutron stars and black holes are thought to be the accretors in HMXBs at some point in their lifetimes.
In NS HMXBs with relatively short orbital periods, the supergiant will eventually fill its Roche lobe, triggering accretion onto the neutron star. Since the neutron star is much less massive, this mass transfer will be unstable, causing the formation of a common envelope. Drag from the common envelope will cause the two stars to spiral together, likely forming a Thorne-Zytkow object—a cool red supergiant with a dense neutron star core. Only NS HMXBs can survive this phase, and will eventually evolve into double NSs with short orbital periods and eccentric orbits. The first known double neutron star, the Hulse–Taylor binary pulsar, is believed to be the remnant of such an HMXB. The double NSs could eventually merge due to orbital decay from gravitational wave emission, becoming a single black hole.
BH HMXBs, on the other hand, are more likely to survive Roche-lobe overflow or common envelope evolution because their higher mass ratios relative to their companion star makes accretion more stable. Instead, these HMXBs probably evolve into Wolf-Rayet X-ray binaries (WRXBs) with short orbital periods. Eventually, these will become NS-BH or BH-BH binaries. Due to their close proximity, these double compact objects may be able to merge due to gravitational wave emission within timescales less than the age of the universe.
Subtypes Since HMXBs are mostly wind-fed, their properties depend largely on the properties of the normal star's stellar wind; thus, HMXBs are typically classified based on the type of normal star they host. However, they can also be classed via different factors, such as the type of compact object in the system. The two primary classes of HMXBs are supergiant X-ray binaries (SgXBs) and Be/X-ray binaries (BeXBs), but other categorizations exist.
Supergiant X-ray binaries
Supergiant X-ray binaries (SgXBs) are HMXBs in which a compact object accretes off of a ~20–50 supergiant (evolved) O- or early-B-type star over an orbital period of days to weeks. These stars are usually filling or close to filling their Roche lobe and lose mass quickly via stellar wind, with rates of – per year, roughly 3 orders of magnitude higher than for main-sequence O- and B-type stars. The acceleration of stellar winds in these systems depend on the wind's chemical composition, excitation, and ionization, the latter of which is in turn based on the surface temperature of the star and the X-ray emission from the accreting compact object.
SgXBs are typically variable. Several factors can affect their luminosities: for one, the density of stellar winds can change over time. Additionally, as the accretion rate increases, X-ray emission from the accreting compact object decelerates the stellar wind through photoionization. This can even produce a bow shock depending on the orbital motion of the compact object.
Supergiant fast X-ray transients
Supergiant fast X-ray transients (SFXTs) are a type of transient SgXB that undergo extreme flares, undergoing rapid increases in brightness by up to 5 orders of magnitude over periods of hours. These flares are relatively rare, with SFXTs spending less than 5% of their time in flaring states. Similar to HMXBs in general, most SFXTs are believed to host neutron stars.
Several theories have been put forth to explain the difference between SFXTs and classical SgXBs. One theory is that SFXTs are an early stage of SgXBs where the donor star is more compact and its winds are less structured; differences in SFXT winds compared to classical SgXB winds have been observed. It has also been suggested that flaring states are explained by transitions between different types of accretion. Finally, SFXTs are believed to have longer orbital periods than classical SgXBs, which may explain the periods of quiescence.
Be/X-ray binaries
Be/X-ray binaries (BeXBs) are X-ray binaries containing a compact object–almost universally a neutron star–and a Be or Oe star. The donor stars are not close to filling their Roche lobes, but they do have decretion disks, and as the compact object approaches, reaching its periastron, it accretes material from the decretion disk. This causes X-ray outbursts in the case of a NS. BeXBs have longer orbital periods than SgXBs, typically longer than 20 days, and neutron star spin periods from –s. The orbital periods and spin periods are positively correlated.
BeXBs can be persistent or transient. Persistent BeXBs typically have longer orbital periods (>200 days) and spin periods (>200s). They also generally have eccentric orbits. Meanwhile, transient BeXBs exhibit X-ray outbursts and have luminosity modulations of three to four orders of magnitude, sometimes reaching the Eddington limit. Transient BeXBs can sometimes form accretion disks around the compact object.
The properties of Be stars, which are still under scientific debate, are important to the dynamics of BeXBs. Thus, the mechanisms behind BeXBs are not fully understood.