X-ray Bursts

Bursts of X-ray energy that occur in low-mass X-ray binaryClose binary system where a neutron star (or rarely a black hole) accretes matter from what is usually a main sequence star (left). X-ray binaries are some of the most luminous X-ray sources in the sky. X-rays are produced as material from the companion star is drawn to the compact systems in which a neutron starDense ball of neutrons that remains at the core of a star after a supernova explosion has destroyed the rest of a star with mass 8-18 (?) Msun. A neutron star has mass ~2-3 Msun, density ~1014 g/cm3, and is supported by neutron degeneracy pressure. Typical neutron stars are 10-20 and low-mass main sequence starSelf-luminous object held together by its own self-gravity. Often refers to those objects which generate energy from nuclear reactions occurring at their cores, but may also be applied to stellar remnants such as neutron stars. are in orbitThe elliptical path of one body around another, typically the path of a small body around a much larger body. However, depending on the mass distribution of the objects, they may rotate around an empty spot in space • The Moon orbits around the Earth. • The Earth orbits around around one another. Due to their close proximity and the extreme gravityAttractive force between all matter - one of the four fundamental forces. of the neutronCharge-neutral hadron with a mass of 1.6748 x 10-27 kg, equivalent to 939.573 MeV, and an intrinsic angular momentum, or spin, of ½ (in units of h/2π). The neutron is a nucleon, one of the two basic constituents of all atomic nuclei (apart from 1H, which consists of a single star, the companion star overflows its Roche lobe and H is drawn into an accretion diskDisk-shaped cloud of gas and solids in orbital motion around a central protostar or some other massive central body. around the neutron star. H is eventually accreted to the surface of the neutron star, where it is immediately converted into He by the extreme temperatures and pressures that exist there. A thin surface layer of He builds up, and once a critical mass is reached, the He ignites explosively, heating the entire surface of the neutron star and releasing a sudden burst of X-raysHigh-energy electromagnetic radiation, with short wavelength (~10-0.01 nm) and high frequency (greater than ~1016 Hertz). Although the boundaries are somewhat arbitrary, wavelengths shorter than 0.01 nm are called gamma-rays and those longer than 10 nm extreme ultraviolet (EUV). X-rays would be produced by blackbody radiation at temperatures in excess of. After the burst, the systemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with returns to a quiescent state and the neutron star begins to re-accumulate the He surface layer. The process repeats resulting in recurrent X-ray bursts. The mechanisms that produce X-ray bursts and recurrent novae are similar. Recurrent novae form when a white dwarfRemnant of a star with mass <8 Msun. White dwarfs have masses <1.4 Msun (the Chandrasekhar mass) and are supported by electron degeneracy pressure. White dwarfs have radii ~Rearth (<0.02 Rsun) and densities ~105-6 g/cm3. No nuclear fusion or gravitational contraction occurs in white dwarfs, they shine by residual heat. accretes a surface layer of H that undergoes explosive burning.

X-ray bursts generally occur at regular intervals separated by several hours or days. They last from a few seconds to a few minutes, with the burst profile showing a rapid rise (0.3-10 s) followed by a slower decline (5-100 seconds). The rapid rise reflects the sudden increase in temperature brought about by explosive He ignition, while the longer decline reflects the slower cooling of the surface of the star.