Xray Burster

X-ray bursters are a specialised class of X-ray binaries characterised by sudden, intense flashes of X-ray radiation known as X-ray bursts. These bursts appear as rapid and periodic increases in luminosity, often rising by a factor of ten or more above the persistent emission, and peaking in the X-ray region of the electromagnetic spectrum. X-ray bursters are among the most important astrophysical laboratories for studying thermonuclear processes under extreme conditions and for probing the physical properties of neutron stars.

Basic Structure and System Composition

An X-ray burster consists of a neutron star accreting matter from a companion donor star, which is typically a main-sequence star. The two stars form a close binary system, in which the donor star loses material to the neutron star through gravitational interaction. Unlike the broader class of X-ray binaries, which may host either neutron stars or black holes, the detection of X-ray bursts immediately identifies the compact object as a neutron star, since black holes lack a solid surface on which matter can accumulate and ignite.
X-ray bursters are most commonly found in low-mass X-ray binaries (LMXBs), though neutron-star systems may also be classified according to the mass of the donor star as either low-mass or high-mass X-ray binaries.

Types of X-ray Bursts

X-ray bursts are divided into two principal categories based on their physical origin:

• Type I X-ray bursts, which are caused by thermonuclear runaway reactions on the surface of a neutron star.
• Type II X-ray bursts, which arise from the sudden release of gravitational potential energy associated with unstable accretion processes.

Type I bursts are by far the most common and are the defining feature of classical X-ray bursters. Type II bursts have been observed from only a very small number of sources and are linked to peculiar accretion instabilities rather than nuclear burning.

Observational Characteristics of X-ray Bursts

X-ray bursts typically display a sharp rise time of about 1–10 seconds, followed by a slower decay phase lasting from tens of seconds to several minutes. During the decay, the emitted spectrum softens, a behaviour consistent with the cooling of a black-body radiator, interpreted as the neutron-star surface.
Individual bursts release an integrated energy of approximately 10³²–10³³ joules, compared with the persistent luminosity from steady accretion, which is of the order of 10³⁰ watts. As a result, the ratio of burst flux to persistent flux usually lies between 10 and 1000, with a typical value close to 100.
Burst recurrence times range from hours to days in most systems, though some sources exhibit much longer intervals between bursts. Weak bursts recurring on timescales of 5–20 minutes have also been observed, though their origin remains poorly understood.

Thermonuclear Burst Astrophysics

The physical mechanism behind Type I X-ray bursts begins with mass transfer from the donor star. When the donor fills its Roche lobe, matter flows through the first Lagrange point towards the neutron star. Additional mass loss may occur through stellar winds or when the donor exceeds its Eddington luminosity. In close systems with massive companions, more than one transfer mechanism may operate simultaneously.
Because the infalling material possesses angular momentum, it forms an accretion disc around the neutron star. Viscous interactions within the disc cause the material to lose energy and spiral inward, eventually settling onto the neutron-star surface. The accreted matter, rich in hydrogen and helium, forms a dense outer layer.
After several hours of accumulation, gravitational compression raises the temperature and pressure sufficiently to initiate nuclear fusion. Burning begins as stable hydrogen fusion via the hot CNO cycle, but continued accretion leads to the formation of degenerate matter. Under these conditions, rising temperatures above 10⁹ K do not relieve pressure, allowing runaway reactions to occur.
The triple-alpha process rapidly ignites, producing a helium flash. This energy input drives the system into thermonuclear runaway, powering the X-ray burst. Early stages are dominated by alpha reactions, followed by the rp-process, which involves rapid proton captures and beta decays. Nucleosynthesis may proceed to nuclei with mass numbers approaching 100, terminating near tellurium isotopes that undergo alpha decay.
Within seconds, most of the accumulated fuel is consumed, producing an intense and observable X-ray flash.

Burning Regimes and Superbursts

Theoretical models predict several nuclear burning regimes, leading to variations in burst duration, energy release, and recurrence time. These regimes depend strongly on the composition of the accreted material and the ashes left behind by previous bursts, particularly the relative proportions of hydrogen, helium, and carbon.
In rare cases, ignition of deeply buried carbon layers may produce superbursts, which are far more energetic and longer-lasting than ordinary Type I bursts. These events are extremely uncommon and remain an active area of research.

Observation and Detection

X-ray bursts are detected using space-based X-ray observatories, as Earth’s atmosphere is opaque to X-rays. The enormous energy released over a short period produces a dramatic increase in observed luminosity, making bursts readily distinguishable from persistent emission.
Most X-ray bursters exhibit recurrent behaviour, since the bursts do not disrupt the binary system or halt accretion. However, burst intervals are irregular and depend on system parameters such as stellar masses, orbital separation, accretion rate, and fuel composition.
From an observational standpoint, Type I bursts show a fast-rise, slow-decay light curve, whereas Type II bursts appear as rapid pulses, sometimes occurring in clusters separated by only minutes.

Spectroscopy and Neutron Star Properties

High-resolution X-ray spectroscopy has revealed absorption features during bursts in certain systems, including lines associated with hydrogen-like and helium-like iron. In some cases, these features have been interpreted as gravitationally redshifted spectral lines, offering potential constraints on the mass–radius relationship of neutron stars.
Such measurements are of fundamental importance, as they provide rare observational access to the equation of state of ultra-dense matter. However, alternative explanations, such as line formation in the accretion disc, remain under consideration.

Applications to Astronomy

X-ray bursts have significant applications in observational astronomy. Because the peak luminosity of a burst is largely determined by the neutron-star mass and the Eddington limit, luminous bursts can be used as standard candles for distance estimation. Comparing observed fluxes with theoretical luminosities allows relatively accurate distance measurements to X-ray bursters.