In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic events occurring in distant galaxies which represent the brightest and "most powerful class of explosions [sic] in the universe." These extreme electromagnetic events are second only to the Big Bang as the most energetic and luminous phenomenon ever known. Gamma-ray bursts can last from ten milliseconds to several hours. After the initial flash of gamma rays, a longer-lived § Afterglow is emitted, usually in the longer wavelengths of X-ray, ultraviolet, optical, infrared, microwave or radio frequencies.
Cygnus X-1 (abbreviated Cyg X-1) is a galactic X-ray source in the constellation Cygnus and was the first such source widely accepted to be a black hole. It was discovered in 1964 during a rocket flight and is one of the strongest X-ray sources detectable from Earth, producing a peak X-ray flux density of 2.3×10−23 W/(m2⋅Hz) (2.3×103 jansky). It remains among the most studied astronomical objects in its class. The compact object is now estimated to have a mass about 21.2 times the mass of the Sun and has been shown to be too small to be any known kind of normal star or other likely object besides a black hole. If so, the radius of its event horizon has 300 km "as upper bound to the linear dimension of the source region" of occasional X-ray bursts lasting only for about 1 ms.
An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that this luminosity is not produced by the stars. Such excess, non-stellar emissions have been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.
X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the donor, to the other component, called the accretor, which is either a neutron star or black hole. The infalling matter releases gravitational potential energy, up to 30 percent of its rest mass, as X-rays. The lifetime and the mass-transfer rate in an X-ray binary depends on the evolutionary status of the donor star, the mass ratio between the stellar components, and their orbital separation.
3C 273 is a quasar located at the center of a giant elliptical galaxy in the constellation of Virgo. It was the first quasar ever to be identified and is the visually brightest quasar in the sky as seen from Earth, with an apparent visual magnitude of 12.9. The derived distance to this object is 749 megaparsecs. The mass of its central supermassive black hole is approximately 886 million times the mass of the Sun.
Messier 87 is a supergiant elliptical galaxy in the constellation Virgo that contains several trillion stars. One of the largest and most massive galaxies in the local universe, it has a large population of globular clusters—about 15,000 compared with the 150–200 orbiting the Milky Way—and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs, traveling at a relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.
47 Tucanae or 47 Tuc is a globular cluster located in the constellation Tucana. It is about 4.45 ± 0.01 kpc (15,000 ± 33 ly) from Earth, and 120 light years in diameter. 47 Tuc can be seen with the naked eye, with an apparent magnitude of 4.1. It appears about 44 arcminutes across including its far outreaches. Due to its far southern location, 18° from the south celestial pole, it was not catalogued by European astronomers until the 1750s, when the cluster was first identified by Nicolas-Louis de Lacaille from South Africa.
An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as extended beams along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets become relativistic jets as they show effects from special relativity.
NGC 4395 is a nearby low surface brightness spiral galaxy located about 14 million light-years from Earth in the constellation Canes Venatici. The nucleus of NGC 4395 is active and the galaxy is classified as a Seyfert Type I known for its very low-mass supermassive black hole.
The Cloverleaf quasar is a bright, gravitationally lensed quasar. It receives its name because of gravitational lensing spitting the single quasar into four images.
V404 Cygni is a microquasar and a triple star system in the constellation of Cygnus. It contains a black hole with a mass of about 9 M☉ and an early K giant star companion with a mass slightly smaller than the Sun, and an evolved tertiary component. The inner star and the black hole orbit each other every 6.47129 days at fairly close range, while the outer tertiary takes 70000 years to orbit the inner binary system. Due to their proximity and the intense gravity of the black hole, the secondary star loses mass to an accretion disk around the black hole and ultimately to the black hole itself.
Markarian 501 is a galaxy with a spectrum extending to the highest energy gamma rays. It is a blazar or BL Lac object, which is an active galactic nucleus with a jet that is shooting towards the Earth. The object has a redshift of z = 0.034.
A tidal disruption event (TDE) is a transient astronomical source produced when a star passes so close to a supermassive black hole (SMBH) that it is pulled apart by the black hole's tidal force. The star undergoes spaghettification, producing a tidal stream of material that loops around the black hole. Some portion of the stellar material is captured into orbit, forming an accretion disk around the black hole, which emits electromagnetic radiation. In a small fraction of TDEs, a relativistic jet is also produced. As the material in the disk is gradually consumed by the black hole, the TDE fades over several months or years.
An accretion disk is a structure formed by diffuse material in orbital motion around a massive central body. The central body is most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other forces induce instabilities causing orbiting material in the disk to spiral inward toward the central body. Gravitational and frictional forces compress and raise the temperature of the material, causing the emission of electromagnetic radiation. The frequency range of that radiation depends on the central object's mass. Accretion disks of young stars and protostars radiate in the infrared; those around neutron stars and black holes in the X-ray part of the spectrum. The study of oscillation modes in accretion disks is referred to as diskoseismology.
NGC 3862 is an elliptical galaxy located 300 million light-years away in the constellation Leo. Discovered by astronomer William Herschel on April 27, 1785, NGC 3862 is an outlying member of the Leo Cluster.
3C 120, also known as Markarian 1506, is an active galaxy located in the constellation of Taurus, at a distance of about 420 million light years. It has been categorised as a type I Seyfert galaxy and a broad-line radio galaxy. 3C 120 has been found to be a variable source in all wavelengths and hosts a superluminal jet.
4C +71.07 known as S5 0836+71, is a quasar located in the constellation Ursa Major. Based on its high redshift, the object is located 10.7 billion light-years away from Earth and such, classified as a blazar with a flat-spectrum radio source and features a radio jet.
1H 0323+342 known as 2MASX J032441.19+341045.9, is a galaxy located in the constellation of Perseus. It is located 831 million light years from Earth. It is classified a gamma-ray emitting narrow-line Seyfert galaxy, the nearest known example of this subtype.
PKS 0208-512 is a blazar located in the southern constellation of Eridanus. It has a redshift of 1.003 and was first discovered in 1975 by astronomers conducting the Parkes 2700 MHz survey in Australia as a bright astronomical radio source. This object is also classified highly polarized with the radio spectrum appearing to be flat, thus making it a flat-spectrum radio quasar.
PKS 0420-014 is a blazar located in the constellation of Eridanus. This is a high polarized quasar with a redshift of (z) 0.915, first discovered as an astronomical radio source by astronomers in 1975. The radio spectrum of this source appears to be flat, making it a flat-spectrum radio quasar (FRSQ).
