X-ray burster

X-ray bursters are one class of X-ray binary stars exhibiting periodic and rapid increases in luminosity (typically a factor of 10 or greater) that peak in the X-ray regime of the electromagnetic spectrum. These astrophysical systems are composed of an accreting compact object, and a main sequence companion 'donor' star. A compact object in an X-ray binary system consists of either a neutron star or a black hole; however, with the emission of an X-ray burst, the companion star can immediately be classified as a neutron star, since black holes do not have a surface and all of the accreting material disappears past the event horizon. The donor star's mass falls to the surface of the neutron star where the hydrogen fuses to helium which accumulates until it fuses in a burst, producing X-rays.

The mass of the donor star is used to categorize the system as either a high mass (above 10 solar masses (M)) or low mass (less than 1 M) X-ray binary, abbreviated as HMXB and LMXB, respectively. X-ray bursters differ observationally from other X-ray transient sources (such as X-ray pulsars and soft X-ray transients), showing a sharp rise time (1 – 10 seconds) followed by spectral softening (a property of cooling black bodies). Individual burst energetics are characterized by an integrated flux of 1032–33 joules,[1] compared to the steady luminosity which is of the order 1032 joules for steady accretion onto a neutron star.[2] As such the ratio α, of the burst flux to the persistent flux, ranges from 10 to 103 but is typically on the order of 100.[1] The X-ray bursts emitted from most of these systems recur on timescales ranging from hours to days, although more extended recurrence times are exhibited in some systems, and weak bursts with recurrence times between 5–20 minutes have yet to be explained but are observed in some less usual cases.[3] The abbreviation XRB can refer either the object (X-ray burster) or the associated emission (X-ray burst). There are two types of XRB's, designated I and II. Type I are far more common than type II, and have a distinctly different cause. Type I are caused by thermonuclear runaway, while type II are caused by gravitational energy release.

Thermonuclear burst astrophysics

When a star in a binary fills its Roche lobe (either due to being very close to its companion or having a relatively large radius), it begins to lose matter, which streams towards its neutron star companion. The star may also undergo mass loss by exceeding its Eddington luminosity, or through strong stellar winds, and some of this material may become gravitationally attracted to the neutron star. In the circumstance of a short orbital period and a massive partner star, both of these processes may contribute to the transfer of material from the companion to the neutron star. In both cases, the falling material originates from the surface layers of the partner star and is rich in hydrogen and helium. The matter streams from the donor into the accretor at the intersection of the two Roche Lobes, which is also the location of the first LaGrange point, or L1. Because of the rotation of the two stars around a common center of gravity, the material then forms a jet travelling towards the accretor. Because compact stars have high gravitational fields, the material falls with a high velocity and angular momentum towards the neutron star. However, the angular momentum prevents it from immediately joining the surface of the accreting star. It continues to orbit the accretor in the plane of the orbital axis, colliding with other accreting material en route, thereby losing energy, and in so doing forming an accretion disk, which also lies on the plane of the orbital axis. In an X-ray burster, this material accretes onto the surface of the neutron star, where it forms a dense layer. After mere hours of accumulation and gravitational compression, nuclear fusion starts in this matter. This begins as a stable process, the hot CNO cycle, however, continued accretion causes a degenerate shell of matter, in which the temperature rises (greater than 1 × 109 kelvin) but this does not alleviate thermodynamic conditions. This causes the triple-α cycle to quickly become favored, resulting in a He flash. The additional energy provided by this flash allows the CNO burning to breakout into thermonuclear runaway. In the early phase of the burst is the alpha-p process, which quickly yields to the rp-process. Nucleosynthesis can proceed as high as A=100, but was shown to end definitively with Te107.[4] Within seconds most of the accreted material is burned, powering a bright X-ray flash that is observable with X-ray (or Gamma ray) telescopes. Theory suggests that there are several burning regimes which cause variations in the burst, such as ignition condition, energy released, and recurrence, with the regimes caused by the nuclear composition, both of the accreted material and the burst ashes. This is mostly dependent on either Hydrogen, Helium, or Carbon content. Carbon ignition may also be the cause of the extremely rare "superbursts".

The behavior of X-ray bursters is similar to the behavior of recurrent novae. In that case the compact object is a white dwarf that accretes hydrogen that finally undergoes explosive burning.

Observation of bursts

Because an enormous amount of energy is released in a short period of time, much of the energy is released as high energy photons in accordance with the theory of black-body radiation, in this case X-rays. This release of energy may be observed as in increase in the star's luminosity with a space telescope, and is called an X-ray burst. These bursts cannot be observed on Earth's surface because our atmosphere is opaque to X-rays. Most X-ray bursting stars exhibit recurrent bursts because the bursts are not powerful enough to disrupt the stability or orbit of either star, and the whole process may begin again. Most X-ray bursters have irregular periods, which can be on the order of a few hours to many months, depending on factors such as the masses of the stars, the distance between the two stars, the rate of accretion, and the exact composition of the accreted material. Observationally, the X-ray burst categories exhibit different features. A Type I X-ray burst has a sharp rise followed by a slow and gradual decline of the luminosity profile. A Type II X-ray burst exhibits a quick pulse shape and may have many fast bursts separated by minutes. However, only from two sources have Type II X-ray bursts been observed, and most X-ray bursts are of Type I.

More finley detailed variations in burst observation have been recorded as the X-ray imaging telescopes improve. Within the familiar burst lightcurve shape, anomalies such as oscillations (called quasi-periodic oscillations) and dips have been observed, with various nuclear and physical explanations being offered, though none yet has been proven.[5] Spectroscopy reveals a 4 keV absorption feature and H and He-like absorption lines in Fe, but these are thought to derive from the accretion disc. The subsequent derivation of redshift of Z=35 for EXO 0748-676 has provided an important constraint for the mass-radius equation of the neutron star, a relationship which is still a mystery but is a major priority for the astrophysics community.[6]

Applications to astronomy

Luminous X-ray bursts can be considered standard candles, since the mass of neutron star determines the luminosity of the burst. Therefore, comparing the observed X-ray flux to the predicted value yields relatively accurate distances. Observations of X-ray bursts allow also the determination of the radius of the neutron star.

See also

References

  1. ^ a b Lewin, Walter H. G.; van Paradijs, Jan; Taam, R. E (1993). "X-Ray Bursts". Space Science Reviews. 62 (3–4): 223–389. Bibcode:1993SSRv...62..223L. doi:10.1007/BF00196124.
  2. ^ Ayasli, S.; Joss, P. C. (1982). "Thermonuclear processes on accreting neutron stars - A systematic study". Astrophysical Journal. 256: 637–665. Bibcode:1982ApJ...256..637A. doi:10.1086/159940.
  3. ^ Iliadis, Christian; Endt, Pieter M.; Prantzos, Nikos; Thompson, William J. (1999). "Explosive Hydrogen Burning of 27Si, 31S, 35Ar, and 39Ca in Novae and X-Ray Bursts". Astrophysical Journal. 524 (1): 434–453. Bibcode:1999ApJ...524..434I. doi:10.1086/307778.
  4. ^ Schatz, H.; Rehm, K.E. (October 2006). "X-ray binaries". Nuclear Physics A. 777: 601–622. arXiv:astro-ph/0607624. Bibcode:2006NuPhA.777..601S. doi:10.1016/j.nuclphysa.2005.05.200.
  5. ^ Watts, Anna L. (2012-09-22). "Thermonuclear Burst Oscillations". Annual Review of Astronomy and Astrophysics. 50 (1): 609–640. arXiv:1203.2065. doi:10.1146/annurev-astro-040312-132617. ISSN 0066-4146.
  6. ^ Schatz, H.; Rehm, K.E. (October 2006). "X-ray binaries". Nuclear Physics A. 777: 601–622. arXiv:astro-ph/0607624. doi:10.1016/j.nuclphysa.2005.05.200.
Astronomical object

An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms object and body are often used interchangeably. However, an astronomical body or celestial body is a single, tightly bound, contiguous entity, while an astronomical or celestial object is a complex, less cohesively bound structure, which may consist of multiple bodies or even other objects with substructures.

Examples of astronomical objects include planetary systems, star clusters, nebulae, and galaxies, while asteroids, moons, planets, and stars are astronomical bodies. A comet may be identified as both body and object: It is a body when referring to the frozen nucleus of ice and dust, and an object when describing the entire comet with its diffuse coma and tail.

Bursting Pulsar

The Bursting Pulsar (GRO J1744-28) is a low-mass x-ray binary with a period of 11.8 days. It was discovered in December 1995 by the Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory, the second of the NASA Great Observatories. The pulsar is unique in that it has a "bursting phase" where it emits gamma rays and X-rays peaking at approximately 20 bursts per hour after which the frequency of bursts drops off and the pulsar enters a quiescent phase. After a few months, the bursts reappear, though not yet with predictable regularity.The Bursting Pulsar is the only known X-ray pulsar that is also a Type II X-ray burster.

Circinus X-1

Not to be confused with Cygnus X-1

Circinus X-1 is an X-ray binary star system that includes a neutron star. Observation of Circinus X-1 in July 2007 revealed the presence of X-ray jets normally found in black hole systems; it is the first of the sort to be discovered that displays this similarity to black holes. Circinus X-1 may be among the youngest X-ray binaries observed.

Index of physics articles (X)

The index of physics articles is split into multiple pages due to its size.

To navigate by individual letter use the table of contents below.

List of stars in Ara

This is the list of notable stars in the constellation Ara, sorted by decreasing brightness.

Notes

List of stars in Norma

This is the list of notable stars in the constellation Norma, sorted by decreasing brightness.

List of stars in Scorpius

This is the list of notable stars in the constellation Scorpius, sorted by decreasing brightness.

List of stars in Volans

This is the list of notable stars in the constellation Volans, sorted by decreasing brightness.

NGC 6441

NGC 6441 is a globular cluster in the southern constellation of Scorpius. It was discovered by the Scottish astronomer James Dunlop on May 13, 1826, who described it as "a small, well-defined rather bright nebula, about 20″ in diameter". The cluster is located 5 arc minutes east-northeast of the star G Scorpii, and is some 44,000 light years from the Sun.This is one of the most massive and luminous globular clusters in the Milky Way, with an estimated 1.6 million solar masses of stars. It is located in the bulge of the galaxy at a distance of 13 kilolight-years (3.9 kpc) from the core, and is considered metal "rich". That is, it has a relatively high abundance of elements with higher mass than helium. The core region of the cluster subtends an angle of 0.11 arc minutes, compared to the half-mass radius of 0.64 arc minutes. The density of stars in the core region is indicated by the luminosity density: 5.25 L⊙ pc−3. The cluster has a half-light radius of 7.1 ly (2.18 pc).This cluster has an abnormally large number of RR Lyrae variables—68 candidates as of 2006, and their periods are longer than is typical for their respective metallicities. (The mean period for the cluster's RRab stars is 0.759 day.) There are also several type II Cepheid stars, which is unusual given the high metallicity of this cluster. Examination of the red giant branch section of the color-magnitude diagram suggests that there are at least two and possibly three distinct populations in the cluster. The brightest and higher temperature members of the red clump stars are more concentrated toward the center of the cluster. This group may be a helium-enriched second generation of stars.The cluster contains at least four millisecond pulsars, of which two are in binary systems. One of these binaries, PSR J1750−37A, is in a highly eccentric orbit with an eccentricity of 0.71. The cluster has an X-ray burster, X1746-370, which has the longest period known in any globular cluster and is consistent with the galaxy as a whole. Finally, there is a planetary nebula, JaFu 2, one of only four planetary nebulas known to inhabit globular clusters in the Milky Way.

Neutron star

A neutron star is the collapsed core of a giant star which before collapse had a total mass of between 10 and 29 solar masses. Neutron stars are the smallest and densest stars, not counting black holes, hypothetical white holes, quark stars and strange stars. Neutron stars have a radius on the order of 10 kilometres (6.2 mi) and a mass lower than 2.16 solar masses. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.

Once formed, they no longer actively generate heat, and cool over time; however, they may still evolve further through collision or accretion. Most of the basic models for these objects imply that neutron stars are composed almost entirely of neutrons (subatomic particles with no net electrical charge and with slightly larger mass than protons); the electrons and protons present in normal matter combine to produce neutrons at the conditions in a neutron star. Neutron stars are partially supported against further collapse by neutron degeneracy pressure, a phenomenon described by the Pauli exclusion principle, just as white dwarfs are supported against collapse by electron degeneracy pressure. However neutron degeneracy pressure is not sufficient to hold up an object beyond 0.7M☉ and repulsive nuclear forces play a larger role in supporting more massive neutron stars. If the remnant star has a mass exceeding the Tolman–Oppenheimer–Volkoff limit, it continues collapsing to form a black hole.

Neutron stars that can be observed are very hot and typically have a surface temperature of around 600000 K. They are so dense that a normal-sized matchbox containing neutron-star material would have a weight of approximately 3 billion metric tons, the same weight as a 0.5 cubic kilometre chunk of the Earth (a cube with edges of about 800 metres). Their magnetic fields are between 108 and 1015 (100 million to 1 quadrillion) times stronger than Earth's magnetic field. The gravitational field at the neutron star's surface is about 2×1011 (200 billion) times that of Earth's gravitational field.

As the star's core collapses, its rotation rate increases as a result of conservation of angular momentum, hence newly formed neutron stars rotate at up to several hundred times per second. Some neutron stars emit beams of electromagnetic radiation that make them detectable as pulsars. Indeed, the discovery of pulsars by Jocelyn Bell Burnell in 1967 was the first observational suggestion that neutron stars exist. The radiation from pulsars is thought to be primarily emitted from regions near their magnetic poles. If the magnetic poles do not coincide with the rotational axis of the neutron star, the emission beam will sweep the sky, and when seen from a distance, if the observer is somewhere in the path of the beam, it will appear as pulses of radiation coming from a fixed point in space (the so-called "lighthouse effect"). The fastest-spinning neutron star known is PSR J1748-2446ad, rotating at a rate of 716 times a second or 43,000 revolutions per minute, giving a linear speed at the surface on the order of 0.24 c (i.e. nearly a quarter the speed of light).

There are thought to be around 100 million neutron stars in the Milky Way, a figure obtained by estimating the number of stars that have undergone supernova explosions. However, most are old and cold, and neutron stars can only be easily detected in certain instances, such as if they are a pulsar or part of a binary system. Slow-rotating and non-accreting neutron stars are almost undetectable; however, since the Hubble Space Telescope detection of RX J185635−3754, a few nearby neutron stars that appear to emit only thermal radiation have been detected. Soft gamma repeaters are conjectured to be a type of neutron star with very strong magnetic fields, known as magnetars, or alternatively, neutron stars with fossil disks around them.Neutron stars in binary systems can undergo accretion which typically makes the system bright in X-rays while the material falling onto the neutron star can form hotspots that rotate in and out of view in identified X-ray pulsar systems. Additionally, such accretion can "recycle" old pulsars and potentially cause them to gain mass and spin-up to very fast rotation rates, forming the so-called millisecond pulsars. These binary systems will continue to evolve, and eventually the companions can become compact objects such as white dwarfs or neutron stars themselves, though other possibilities include a complete destruction of the companion through ablation or merger. The merger of binary neutron stars may be the source of short-duration gamma-ray bursts and are likely strong sources of gravitational waves. In 2017, a direct detection (GW170817) of the gravitational waves from such an event was made, and gravitational waves have also been indirectly detected in a system where two neutron stars orbit each other.

In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be directly related to the historic GW170817 and associated with the merger of two neutron stars. The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, are "striking", suggesting the two separate events may both be the result of the merger of neutron stars, and both may be a kilonova, which may be more common in the universe than previously understood, according to the researchers.

Nova

A nova (plural novae or novas) or classical nova (CN, plural CNe, or Q) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star, that slowly fades over several weeks or many months.

Causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars. All observed novae involve a white dwarf in a close binary system. The main sub-classes of novae are classical novae, recurrent novae (RNe), and dwarf novae. They are all considered to be cataclysmic variable stars.

Classical nova eruptions are the most common type of nova. They are likely created in a close binary star system consisting of a white dwarf and either a main sequence, subgiant, or red giant star. When the orbital period falls in the range of several days to one day, the white dwarf is close enough to its companion star to start drawing accreted matter onto the surface of the white dwarf, which creates a dense but shallow atmosphere. This atmosphere is mostly hydrogen and is thermally heated by the hot white dwarf, which eventually reaches a critical temperature causing rapid runaway ignition by fusion.

From the dramatic and sudden energies created, the now hydrogen-burnt atmosphere is then dramatically expelled into interstellar space, and its brightened envelope is seen as the visible light created from the nova event, and previously was mistaken as a "new" star. A few novae produce short-lived nova remnants, lasting for perhaps several centuries. Recurrent nova processes are the same as the classical nova, except that the fusion ignition may be repetitive because the companion star can again feed the dense atmosphere of the white dwarf.

Novae most often occur in the sky along the path of the Milky Way, especially near the observed galactic centre in Sagittarius; however, they can appear anywhere in the sky. They occur far more frequently than galactic supernovae, averaging about ten per year. Most are found telescopically, perhaps only one every year to eighteen months reaching naked-eye visibility. Novae reaching first or second magnitude occur only several times per century. The last bright nova was V1369 Centauri reaching 3.3 magnitude on 14 December 2013.

Small Astronomy Satellite 3

The Small Astronomy Satellite 3 (SAS 3, also known as SAS-C before launch) was a NASA X-ray astronomy space telescope. It functioned from May 7, 1975 to April 1979. It covered the X-ray range with four experiments on board. The satellite, built by the Johns Hopkins University Applied Physics Laboratory (APL), was proposed and operated by MIT's Center for Space Research (CSR). It was launched on a Scout vehicle from the Italian San Marco launch platform near Mombasa, Kenya, into a low-Earth, nearly equatorial orbit. It was also known as Explorer 53, as part of NASA's Explorer program.The spacecraft was 3-axis stabilized with a momentum wheel that was used to establish stability about the nominal rotation, or z-axis. The orientation of the z-axis could be altered over a period of hours using magnetic torque coils that interacted with the Earth's magnetic field. Solar panels charged batteries during the daylight portion of each orbit, so that SAS 3 had essentially no expendables to limit its lifetime beyond the life of the tape recorders, batteries, and orbital drag. The spacecraft typically operated in a rotating mode, spinning at one revolution per 95-min orbit, so that the LEDs, tube and slat collimator experiments, which looked out along the y-axis, could view and scan the sky almost continuously. The rotation could also be stopped, allowing extended (up to 30 min) pointed observations of selected sources by the y-axis instruments. Data were recorded on board by magnetic tape recorders, and played back during station passes every orbit.SAS 3 was commanded from the NASA Goddard Space Flight Center (GSFC) in Greenbelt MD, but data were transmitted by modem to MIT for scientific analysis, where scientific and technical staff were on-duty 24 hours a day. The data from each orbit were subjected to quick-look scientific analysis at MIT before the next orbital station pass, so the science operational plan could be altered by telephoned instruction from MIT to GSFC in order to study targets in near real-time.

Star

A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the estimated 300 sextillion (3×1023) stars in the observable universe are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.

For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.

A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole.

Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.

Terzan 5

Terzan 5 is a heavily obscured globular cluster belonging to the bulge (the central star concentration) of the Milky Way galaxy. It was one of six globulars discovered by French astronomer Agop Terzan in 1968 and was initially labeled Terzan 11. The cluster was cataloged by the Two-Micron Sky Survey as IRC–20385. It is situated in the Sagittarius constellation in the direction of the Milky Way's center. Terzan 5 probably follows an unknown complicated orbit around the center of the galaxy, but currently it is moving towards the Sun with a speed of around 90 km/s.

Transient astronomical event

A transient astronomical event, often shortened by astronomers to a transient, is an astronomical object or phenomenon whose duration may be from seconds to days, weeks, or even several years. This is in contrast to the timescale of the millions or billions of years during which the galaxies and their component stars in our universe have evolved. Singularly, the term is used for violent deep-sky events, such as supernovae, novae, dwarf nova outbursts, gamma-ray bursts, and tidal disruption events, as well as gravitational microlensing, transits and eclipses. These events are part of the broader topic of time domain astronomy.

X-ray star

X-ray star may refer to:

Be/X-ray binary, a class of high-mass X-ray binaries that consist of a Be star and a neutron star

X-ray binary, a class of binary stars that are luminous in X-rays

X-ray burster, a class of X-ray binary stars exhibiting periodic and rapid increases in luminosity that peak in the X-ray regime of the electromagnetic spectrum

X-ray pulsar, a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity

X-ray transient

X-ray emission occurs from many celestial objects. These emissions can have a pattern, occur intermittently, or as a transient astronomical event. In X-ray astronomy many sources have been discovered by placing an X-ray detector above the Earth's atmosphere. Often, the first X-ray source discovered in many constellations is an X-ray transient. These objects show changing levels of X-ray emission. NRL astronomer Dr. Joseph Lazio stated: " ... the sky is known to be full of transient objects emitting at X- and gamma-ray wavelengths, ...". There are a growing number of recurrent X-ray transients. In the sense of traveling as a transient, the only stellar X-ray source that does not belong to a constellation is the Sun. As seen from Earth, the Sun moves from west to east along the ecliptic, passing over the course of one year through the twelve constellations of the Zodiac, and Ophiuchus.

Types
Single pulsars
Binary pulsars
Properties
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