Gamma-ray burst progenitors

Gamma-ray burst progenitors are the types of celestial objects that can emit gamma-ray bursts (GRBs). GRBs show an extraordinary degree of diversity. They can last anywhere from a fraction of a second to many minutes. Bursts could have a single profile or oscillate wildly up and down in intensity, and their spectra are highly variable unlike other objects in space. The near complete lack of observational constraint led to a profusion of theories, including evaporating black holes, magnetic flares on white dwarfs, accretion of matter onto neutron stars, antimatter accretion, supernovae, hypernovae, and rapid extraction of rotational energy from supermassive black holes, among others.[1][2]

There are at least two different types of progenitors (sources) of GRBs: one responsible for the long-duration, soft-spectrum bursts and one (or possibly more) responsible for short-duration, hard-spectrum bursts. The progenitors of long GRBs are believed to be massive, low-metallicity stars exploding due to the collapse of their cores. The progenitors of short GRBs are thought to arise from mergers of compact binary systems like neutron stars, which was confirmed by the GW170817 observation of a neutron star merger and a kilonova.

EtaCarinae
Eta Carinae, in the constellation of Carina, one of the nearer candidates for a hypernova

Long GRBs: massive stars

Collapsar model

As of 2007, there is almost universal agreement in the astrophysics community that the long-duration bursts are associated with the deaths of massive stars in a specific kind of supernova-like event commonly referred to as a collapsar or hypernova.[2][3] Very massive stars are able to fuse material in their centers all the way to iron, at which point a star cannot continue to generate energy by fusion and collapses, in this case, immediately forming a black hole. Matter from the star around the core rains down towards the center and (for rapidly rotating stars) swirls into a high-density accretion disk. The infall of this material into the black hole drives a pair of jets out along the rotational axis, where the matter density is much lower than in the accretion disk, towards the poles of the star at velocities approaching the speed of light, creating a relativistic shock wave[4] at the front. If the star is not surrounded by a thick, diffuse hydrogen envelope, the jets' material can pummel all the way to the stellar surface. The leading shock actually accelerates as the density of the stellar matter it travels through decreases, and by the time it reaches the surface of the star it may be traveling with a Lorentz factor of 100 or higher (that is, a velocity of 0.9999 times the speed of light). Once it reaches the surface, the shock wave breaks out into space, with much of its energy released in the form of gamma-rays.

Three very special conditions are required for a star to evolve all the way to a gamma-ray burst under this theory: the star must be very massive (probably at least 40 Solar masses on the main sequence) to form a central black hole in the first place, the star must be rapidly rotating to develop an accretion torus capable of launching jets, and the star must have low metallicity in order to strip off its hydrogen envelope so the jets can reach the surface. As a result, gamma-ray bursts are far rarer than ordinary core-collapse supernovae, which only require that the star be massive enough to fuse all the way to iron.

Evidence for the collapsar view

This consensus is based largely on two lines of evidence. First, long gamma-ray bursts are found without exception in systems with abundant recent star formation, such as in irregular galaxies and in the arms of spiral galaxies.[5] This is strong evidence of a link to massive stars, which evolve and die within a few hundred million years and are never found in regions where star formation has long ceased. This does not necessarily prove the collapsar model (other models also predict an association with star formation) but does provide significant support.

Second, there are now several observed cases where a supernova has immediately followed a gamma-ray burst. While most GRBs occur too far away for current instruments to have any chance of detecting the relatively faint emission from a supernova at that distance, for lower-redshift systems there are several well-documented cases where a GRB was followed within a few days by the appearance of a supernova. These supernovae that have been successfully classified are type Ib/c, a rare class of supernova caused by core collapse. Type Ib and Ic supernovae lack hydrogen absorption lines, consistent with the theoretical prediction of stars that have lost their hydrogen envelope. The GRBs with the most obvious supernova signatures include GRB 060218 (SN 2006aj),[6] GRB 030329 (SN 2003dh),[7] and GRB 980425 (SN 1998bw),[8] and a handful of more distant GRBs show supernova "bumps" in their afterglow light curves at late times.

Possible challenges to this theory emerged recently, with the discovery[9][10] of two nearby long gamma-ray bursts that lacked the signature of any type of supernova: both GRB060614 and GRB 060505 defied predictions that a supernova would emerge despite intense scrutiny from ground-based telescopes. Both events were, however, associated with actively star-forming stellar populations. One possible explanation is that during the core collapse of a very massive star a black hole can form, which then 'swallows' the entire star before the supernova blast can reach the surface.

Short GRBs: degenerate binary systems

Short gamma-ray bursts appear to be an exception. Until 2007, only a handful of these events have been localized to a definite galactic host. However, those that have been localized appear to show significant differences from the long-burst population. While at least one short burst has been found in the star-forming central region of a galaxy, several others have been associated with the outer regions and even the outer halo of large elliptical galaxies in which star formation has nearly ceased. All the hosts identified so far have also been at low redshift.[11] Furthermore, despite the relatively nearby distances and detailed follow-up study for these events, no supernova has been associated with any short GRB.

Neutron star and neutron star/black hole mergers

While the astrophysical community has yet to settle on a single, universally favored model for the progenitors of short GRBs, the generally preferred model is the merger of two compact objects as a result of gravitational inspiral: two neutron stars,[12][13] or a neutron star and a black hole.[14] While thought to be rare in the Universe, a small number of cases of close neutron star - neutron star binaries are known in our Galaxy, and neutron star - black hole binaries are believed to exist as well. According to Einstein's theory of general relativity, systems of this nature will slowly lose energy due to gravitational radiation and the two degenerate objects will spiral closer and closer together, until in the last few moments, tidal forces rip the neutron star (or stars) apart and an immense amount of energy is liberated before the matter plunges into a single black hole. The whole process is believed to occur extremely quickly and be completely over within a few seconds, accounting for the short nature of these bursts. Unlike long-duration bursts, there is no conventional star to explode and therefore no supernova.

This model has been well-supported so far by the distribution of short GRB host galaxies, which have been observed in old galaxies with no star formation (for example, GRB050509B, the first short burst to be localized to a probable host) as well as in galaxies with star formation still occurring (such as GRB050709, the second), as even younger-looking galaxies can have significant populations of old stars. However, the picture is clouded somewhat by the observation of X-ray flaring[15] in short GRBs out to very late times (up to many days), long after the merger should have been completed, and the failure to find nearby hosts of any sort for some short GRBs.

Magnetar giant flares

One final possible model that may describe a small subset of short GRBs are the so-called magnetar giant flares (also called megaflares or hyperflares). Early high-energy satellites discovered a small population of objects in the Galactic plane that frequently produced repeated bursts of soft gamma-rays and hard X-rays. Because these sources repeat and because the explosions have very soft (generally thermal) high-energy spectra, they were quickly realized to be a separate class of object from normal gamma-ray bursts and excluded from subsequent GRB studies. However, on rare occasions these objects, now believed to be extremely magnetized neutron stars and sometimes termed magnetars, are capable of producing extremely luminous outbursts. The most powerful such event observed to date, the giant flare of 27 December 2004, originated from the magnetar SGR 1806-20 and was bright enough to saturate the detectors of every gamma-ray satellite in orbit and significantly disrupted Earth's ionosphere.[16] While still significantly less luminous than "normal" gamma-ray bursts (short or long), such an event would be detectable to current spacecraft from galaxies as far as the Virgo cluster and, at this distance, would be difficult to distinguish from other types of short gamma-ray burst on the basis of the light curve alone. To date, three gamma-ray bursts have been associated with SGR flares in galaxies beyond the Milky Way: GRB 790503b in the Large Magellanic Cloud, GRB 051103 from M81 and GRB 070201 from M31.[17]

Diversity in the origin of long GRBs

HETE II and Swift observations reveal that long gamma-ray bursts come with and without supernovae, and with and without pronounced X-ray afterglows. It gives a clue to a diversity in the origin of long GRBs, possibly in- and outside of star-forming regions, with otherwise a common inner engine. The timescale of tens of seconds of long GRBs hereby appears to be intrinsic to their inner engine, for example, associated with a viscous or a dissipative process.

The most powerful stellar mass transient sources are the above-mentioned progenitors (collapsars and mergers of compact objects), all producing rotating black holes surrounded by debris in the form of an accretion disk or torus. A rotating black hole carries spin-energy in angular momentum [18] as does a spinning top:

where and denote the moment of inertia and the angular velocity of the black hole in the trigonometric expression [19] for the specific angular momentum of a Kerr black hole of mass . With no small parameter present, it has been well-recognized that the spin energy of a Kerr black hole can reach a substantial fraction (29%) of its total mass-energy , thus holding promise to power the most remarkable transient sources in the sky. Of particular interest are mechanisms for producing non-thermal radiation by the gravitational field of rotating black holes, in the process of spin-down against their surroundings in aforementioned scenarios.

By Mach's principle, spacetime is dragged along with mass-energy, with the distant stars on cosmological scales or with a black hole in close proximity. Thus, matter tends to spin-up around rotating black holes, for the same reason that pulsars spin down by shedding angular momentum in radiation to infinity. A major amount of spin-energy of rapidly spinning black holes can hereby be released in a process of viscous spin-down against an inner disk or torus—into various emission channels.

Spin-down of rapidly spinning stellar mass black holes in their lowest energy state takes tens of seconds against an inner disk, representing the remnant debris of the merger of two neutron stars, the break-up of a neutron star around a companion black hole or formed in core-collapse of a massive star. Forced turbulence in the inner disk stimulates the creation of magnetic fields and multipole mass-moments, thereby opening radiation channels in radio, neutrinos and, mostly, in gravitational waves with distinctive chirps shown in the diagram [20] with the creation of astronomical amounts of Bekenstein-Hawking entropy.[21][22][23]

Chirp diagram vanputten
Diagram of van Putten (2009) showing the gravitational radiation produced in binary coalescence of neutron stars with another neutron star or black hole and, post-coalescence or following core-collapse of a massive star, the expected radiation by high-density turbulent matter around stellar mass Kerr black holes. As the ISCO (ellipse) relaxes to that around a slowly rotating, nearly Schwarzschild black hole, the late-time frequency of gravitational radiation provides accurate metrology of the black hole mass.

Transparency of matter to gravitational waves offers a new probe to the inner-most workings of supernovae and GRBs. The gravitational-wave observatories LIGO and Virgo are designed to probe stellar mass transients in a frequency range of tens to about fifteen hundred Hz. The above-mentioned gravitational-wave emissions fall well within the LIGO-Virgo bandwidth of sensitivity; for long GRBs powered by "naked inner engines" produced in the binary merger of a neutron star with another neutron star or companion black hole, the above-mentioned magnetic disk winds dissipate into long-duration radio-bursts, that may be observed by the novel Low Frequency Array (LOFAR).

See also

References

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  8. ^ Kulkarni, S.R.; et al. (1998). "Radio emission from the unusual supernova 1998bw and its association with the gamma-ray burst of 25 April 1998". Nature. 395 (6703): 663. Bibcode:1998Natur.395..663K. doi:10.1038/27139.
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  17. ^ Frederiks 2008
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  19. ^ van Putten, M.H.P.M., 1999, Science, 284, 115
  20. ^ Maurice H.P.M. van Putten (2009). "On the origin of long gamma-ray bursts". MNRAS Letters. 396 (1): L81–L84. Bibcode:2009MNRAS.396L..81V. doi:10.1111/j.1745-3933.2009.00666.x.
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Ergosphere

The ergosphere is a region located outside a rotating black hole's outer event horizon. Its name was proposed by Remo Ruffini and John Archibald Wheeler during the Les Houches lectures in 1971 and is derived from the Greek word ἔργον (ergon), which means "work". It received this name because it is theoretically possible to extract energy and mass from this region. The ergosphere touches the event horizon at the poles of a rotating black hole and extends to a greater radius at the equator. With a low spin of the central mass the shape of the ergosphere can be approximated by an oblate spheroid, while with higher spins it resembles a pumpkin shape. The equatorial (maximal) radius of an ergosphere corresponds to the Schwarzschild radius of a non-rotating black hole; the polar (minimal) radius can be as little as half the Schwarzschild radius (the radius of a non-rotating black hole) in the case that the black hole is rotating maximally (at higher rotation rates the black hole could not have formed).

GRB 060614

GRB 060614 was a gamma-ray burst detected by the Neil Gehrels Swift Observatory on June 14, 2006, with peculiar properties. It challenged a previously-held scientific consensus on gamma-ray burst progenitors and black holes.

Prior to this detection, it was believed that a long gamma-ray burst, like GRB 060614, was probably caused by gravitational collapse of a large star into a black hole, and would be accompanied by detectable supernova, whilst short gamma-ray bursts were thought to be the merger of two neutron stars. However, the lack of any supernova and the vanishing spectral lags during GRB 060614 are typical of short GRBs, at odds with the long (102s) duration of this event and its origin in a galaxy 1.6 billion light years away in the constellation Indus.In December 2006, an article on the burst was published in the journal Nature, with the editors describing a hunt by scientists to define a new GRB classification system to account for this burst. GRB 060614 was subsequently classified as a "hybrid gamma-ray burst", defined as a long burst without accompanying supernova, and was hypothesized to have been an observation of a new type of black hole formation.

Gamma-ray burst

In gamma-ray astronomy, gamma-ray bursts (GRBs) are extremely energetic explosions that have been observed in distant galaxies. They are the brightest electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several hours. After an initial flash of gamma rays, a longer-lived "afterglow" is usually emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, microwave and radio).The intense radiation of most observed GRBs is thought to be released during a supernova or superluminous supernova as a high-mass star implodes to form a neutron star or a black hole.

A subclass of GRBs (the "short" bursts) appear to originate from the merger of binary neutron stars. The cause of the precursor burst observed in some of these short events may be the development of a resonance between the crust and core of such stars as a result of the massive tidal forces experienced in the seconds leading up to their collision, causing the entire crust of the star to shatter.The sources of most GRBs are billions of light years away from Earth, implying that the explosions are both extremely energetic (a typical burst releases as much energy in a few seconds as the Sun will in its entire 10-billion-year lifetime) and extremely rare (a few per galaxy per million years). All observed GRBs have originated from outside the Milky Way galaxy, although a related class of phenomena, soft gamma repeater flares, are associated with magnetars within the Milky Way. It has been hypothesized that a gamma-ray burst in the Milky Way, pointing directly towards the Earth, could cause a mass extinction event.GRBs were first detected in 1967 by the Vela satellites, which had been designed to detect covert nuclear weapons tests; this was declassified and published in 1973. Following their discovery, hundreds of theoretical models were proposed to explain these bursts, such as collisions between comets and neutron stars. Little information was available to verify these models until the 1997 detection of the first X-ray and optical afterglows and direct measurement of their redshifts using optical spectroscopy, and thus their distances and energy outputs. These discoveries, and subsequent studies of the galaxies and supernovae associated with the bursts, clarified the distance and luminosity of GRBs, definitively placing them in distant galaxies.

Index of physics articles (G)

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.

Lists of black holes

This is a list of lists of black holes.

List of black holes.

List of most massive black holes

List of nearest black holes

SMSS J215728.21-360215.1

SMSS J215728.21-360215.1, commonly known as J2157-3602, is one of the fastest growing black holes and one of the most powerful quasars known to exist as of 2018. The quasar is located at redshift 4.75, corresponding to a comoving distance of 25×109 ly from Earth and to a light-travel distance of 12.5×109 ly. It was discovered with the SkyMapper telescope at Australian National University's Siding Spring Observatory, announced in May 2018. It has an intrinsic bolometric luminosity of 6.95×1014 L☉ (2.66×1041 W).

Superluminous supernova

A super-luminous supernova (SLSN, plural super luminous supernovae or SLSNe), also known as a hypernova, is a type of stellar explosion with a luminosity 10 or more times higher than that of standard supernovae. Like supernovae, SLSNe seem to be produced by several mechanisms, which is readily revealed by their light-curves and spectra. There are multiple models for what conditions may produce an SLSN, including core collapse in particularly massive stars, millisecond magnetars, interaction with circumstellar material (CSM model), or pair-instability supernovae.

Hypernovae produce long gamma ray bursts (GRBs), which range from 2 seconds to over a minute in duration.

GRBs were initially detected on July 2, 1967 by US military satellites in high orbit, which were meant to detect gamma radiation. The US had suspected the USSR of conducting secret nuclear tests despite signing the Nuclear Test Ban Treaty of 1963, and the Vela satellites were capable of detecting explosions behind the moon. The satellites detected a signal, but it was unlike that of a nuclear weapon signature, nor could it be correlated to solar flares.Over the next few decades, the GRBs posed a compelling mystery. Gamma rays require highly energetic events to be produced, yet GRBs could not be correlated to supernovae, solar flares, or any other activity in the sky. Their brevity made them difficult to trace. Once their direction could be determined, it was found that they were evenly spread across the sky. Thus they were not originating in the Milky Way or nearby galaxies, but from deep space.

In February 1997, Dutch-Italian satellite BeppoSAX was able to trace GRB 970508 to a faint galaxy roughly 6 billion light years away. From analyzing the spectroscopic data for both the burst and the galaxy, Bloom et al. concluded that a hypernova was the likely cause. That same year, hypernovae were hypothesized in greater detail by Polish astronomer Bohdan Paczyński.The first hypernova observed was SN 1998bw, with a luminosity 100 times higher than a standard Type Ib.The first confirmed superluminous supernova connected to a gamma ray burst was not found until 2003, when GRB 030329 illuminated the Leo constellation. SN 2003dh represented the death of a star 25 times more massive than the sun, with material being blasted out at over a tenth the speed of light.In June 2018, AT2018cow was detected and found to be a very powerful astronomical explosion, 10 – 100 times brighter than a normal supernova.Today, it is believed that stars with M ≥ 40 M☉ produce superluminous supernovae.

Thermodynamic temperature

Thermodynamic temperature is the absolute measure of temperature and is one of the principal parameters of thermodynamics.

Thermodynamic temperature is defined by the third law of thermodynamics in which the theoretically lowest temperature is the null or zero point. At this point, absolute zero, the particle constituents of matter have minimal motion and can become no colder. In the quantum-mechanical description, matter at absolute zero is in its ground state, which is its state of lowest energy. Thermodynamic temperature is often also called absolute temperature, for two reasons: one, proposed by Kelvin, that it does not depend on the properties of a particular material; two that it refers to an absolute zero according to the properties of the ideal gas.

The International System of Units specifies a particular scale for thermodynamic temperature. It uses the kelvin scale for measurement and selects the triple point of water at 273.16 K as the fundamental fixing point. Other scales have been in use historically. The Rankine scale, using the degree Fahrenheit as its unit interval, is still in use as part of the English Engineering Units in the United States in some engineering fields. ITS-90 gives a practical means of estimating the thermodynamic temperature to a very high degree of accuracy.

Roughly, the temperature of a body at rest is a measure of the mean of the energy of the translational, vibrational and rotational motions of matter's particle constituents, such as molecules, atoms, and subatomic particles. The full variety of these kinetic motions, along with potential energies of particles, and also occasionally certain other types of particle energy in equilibrium with these, make up the total internal energy of a substance. Internal energy is loosely called the heat energy or thermal energy in conditions when no work is done upon the substance by its surroundings, or by the substance upon the surroundings. Internal energy may be stored in a number of ways within a substance, each way constituting a "degree of freedom". At equilibrium, each degree of freedom will have on average the same energy: where is the Boltzmann constant, unless that degree of freedom is in the quantum regime. The internal degrees of freedom (rotation, vibration, etc.) may be in the quantum regime at room temperature, but the translational degrees of freedom will be in the classical regime except at extremely low temperatures (fractions of kelvins) and it may be said that, for most situations, the thermodynamic temperature is specified by the average translational kinetic energy of the particles.

WR 104

WR 104 is a triple star system located about 7,500 light-years (2,300 pc) from Earth. The primary star is a Wolf-Rayet star, abbreviated as WR, with a B0.5 main sequence star in close orbit and another more distant fainter companion.

The WR star is surrounded by a distinctive spiral Wolf–Rayet nebula, often referred to as a pinwheel nebula. The rotational axis of the binary system, and likely of the two closest stars, is directed approximately towards Earth. Within the next few hundred thousand years, the Wolf-Rayet star is predicted to probably become a core-collapse-supernova with a small chance of producing a long duration gamma-ray burst.

The possibility of a supernova explosion from WR 104 having destructive consequence for life on Earth stirred interest in the mass media and several popular science articles have been issued in the press since 2008. Some articles decide to reject the catastrophic scenario, while others leave it as an open question. Scientists currently believe the odds of WR 104 posing a risk to be small.

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