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Classes of High Energy Objects

Active Galactic Nuclei (AGN)

AGN is a catch-all term for most types of galaxies that have extremely bright and concentrated emission from their central regions. This light is believed to come from matter in the form of gas, dust, and even whole stars being sucked into the central black hole. As all of this material spirals in, it gets very hot and very bright, much brighter than the stars and other luminous material in the center of the galaxy.


Inner Structure of an Active Galaxy

An AGN is a compact region at the center of a galaxy that has a much higher than normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. Such excess non-stellar emission has 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.

Active galactic nuclei are the most luminous persistent sources of electromagnetic radiation in the universe, and as such can be used as a means of discovering distant objects; their evolution as a function of cosmic time also puts constraints on models of the cosmos.

The observed characteristics of an AGN depend on several properties such as the mass of the central black hole, the rate of gas accretion onto the black hole, the orientation of the accretion disk, the degree of obscuration of the nucleus by dust, and presence or absence of jets.

Numerous subclasses of AGN have been defined based on their observed characteristics; the most powerful AGN are classified as quasars. A blazar is an AGN with a jet pointed toward the Earth, in which radiation from the jet is enhanced by relativistic beaming.

Blazars

A blazar is an active galactic nucleus (AGN) with a relativistic jet (a jet composed of ionized matter traveling at nearly the speed of light) directed very nearly towards an observer. Relativistic beaming of electromagnetic radiation from the jet makes blazars appear much brighter than they would be if the jet were pointed in a direction away from the Earth. Blazars are powerful sources of emission across the electromagnetic spectrum and are observed to be sources of high-energy gamma ray photons. Blazars are highly variable sources, often undergoing rapid and dramatic fluctuations in brightness on short timescales (hours to days). Some blazar jets exhibit apparent superluminal motion, another  consequence of material in the jet traveling toward the observer at nearly the speed of light. Blazars are thought to be supermassive black holes having millions or billions of times the mass of the Sun, found at the centers of some galaxies.

The way an AGN looks to us is dependent upon our line of sight into the central region of the galaxy -- mainly on whether we can see the accretion disk surrounding the black hole or not, and whether relativistic jets are being created by the black hole and are pointed in our direction. If we don't see the central engine of the AGN, and the jets aren't pointed in our direction (or don't exist at all), then it might not appear to vary at all, and might not even be noticed as an AGN without looking carefully at all wavelengths of light. But the blazars are AGN where we not only see the inner accretion disk close to the black hole's event horizon, but we're also looking straight down the beam of the jet. In blazars, we're seeing the most energetic parts of the AGN. Among AGN, the blazars vary on the shortest timescales, and emit radiation across the entire electromagnetic spectrum from radio waves to the highest energy gamma rays.

Blazars are variable at all wavelengths of light observed, from radio waves to high-energy gamma rays. Blazars also produce some of the highest energy gamma rays observed in the universe, millions of times more energetic than the gamma rays emitted by radioactive elements, and billions of times more energetic than the X-rays you receive in a doctor's or dentist's office. Only objects with incredible power -- strong gravity, strong magnetic fields, strong radiation pressure, or all three -- can make electromagnetic radiation that energetic, and supermassive black holes certainly fit the bill!

Even more fascinating is that blazars vary on very short timescales, as short as hours or days. This is because when we look at a blazar, we're looking at the very heart of the AGN, right at the black hole itself. Objects can vary on a timescale proportional to their physical size, essentially however long it takes light to cross the entire size of the object. So for blazars to change on timescales of a few hours, they must only be a few light-hours across. For comparison, it takes a ray of light about 11 hours to cross the Solar System from one side of Pluto's orbit to the other. So in a blazar that varies on that timescale, you have to pack millions, billions, or even trillions of solar masses into a volume the size of Pluto's orbit!

The blazar category includes BL Lac objects and optically violently variable (OVV) quasars. The generally accepted picture is that BL Lac objects are intrinsically low-power radio galaxies while OVV quasars are intrinsically powerful radio-loud quasars. The name "blazar" was originally coined in 1978 by astronomer Edward Spiegel to denote the combination of these two classes. In visible-wavelength images, most blazars appear compact and pointlike, but high-resolution images reveal that they are located at the centers of elliptical galaxies.

Blazars are important topics of research in astronomy and high-energy astrophysics. Blazar research includes investigation of the properties of accretion disks and jets, the central supermassive black holes and the surrounding host galaxies, and the emission of high-energy photons, cosmic rays, and neutrinos.

In July 2018, the IceCube Neutrino Observatory announced that they have traced a neutrino that hit their Antarctica-based detector in September 2017 back to its point of origin in a blazar 3.7 billion light-years away. This is the first time that a neutrino detector has been used to locate an object in space.

In 2003, the AAVSO partnered with the GLAST Telescope Network (GTN), since renamed the GTN – The Global Telescope Network in an Education and Public Outreach project to monitor blazars prior to the launch of GLAST, now known as the Fermi Gamma-Ray Space Telescope. Long term monitoring of a number of blazars helped to establish a baseline of behavior to use for future comparison. The AAVSO International High Energy Network continues to monitor blazars and other high-energy phenomena for these and other projects.

The AAVSO is currently (as of 2008 December) running another campaign on several blazars being monitored by the VERITAS observatory and the XMM-Newton satellite, by request of Dr. Markus Boettcher (Ohio University). Please see AAVSO Alert Notice #353 for more details.

BL Lacertae (BLLACS)


Markarian 501 (Sloan Digital Sky Survey image)

A BL Lacertae object or BL Lac object is a type of active galactic nucleus (AGN) or a galaxy with such an AGN, named after its prototype, BL Lacertae. In contrast to other types of active galactic nuclei, BL Lacs are characterized by rapid and large-amplitude flux variability and significant optical polarization. Because of these properties, the prototype of the class (BL Lac) was originally thought to be a variable star. When compared to the more luminous active nuclei (quasars) with strong emission lines, BL Lac objects have spectra dominated by a relatively featureless non-thermal emission continuum over the entire electromagnetic range. This lack of spectral lines historically hindered BL Lac's identification of their nature and proved to be a hurdle in the determination of their distance.

In the unified scheme of radio-loud active galactic nuclei, the observed nuclear phenomenology of BL Lacs is interpreted as being due to the effects of the relativistic jet closely aligned to the line of sight of the observer. BL Lacs are thought to be intrinsically identical to low-power radio galaxies. These active nuclei appear to be hosted in massive elliptical galaxies. From the point of AGN classification, BL Lacs are a blazar subtype. All known BL Lacs are associated with core dominated radio sources, many of them exhibiting superluminal motion.

The blazar category encompasses all quasars oriented with the relativistic jet directed at the observer giving a unique radio emission spectrum. This includes both BL Lacs along with optically violent variable (OVV) quasars, however in general practice, "Blazar" and "BL Lac Object" are often used interchangeably. OVV quasars are generally more luminous and have stronger emission lines than BL Lac objects.

Some examples of BL Lac objects are BL Lacertae itself, OJ 287, AP Librae, PKS 2155-304, PKS 0521-365, Markarian 421, 3C 371, W Com, ON 325 and Markarian 501.


AAVSO Light Curve for Markarian 401

Gamma Ray Bursts (GRB)

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.

There are two kinds of gamma-ray burst, known as long-soft and short-hard, referring to their duration, and the nature of their gamma-ray emission. Long-soft bursts last for a few dozens of seconds, and emit less energetic ("soft") gamma rays; short-hard bursts last for a second or less and emit very energetic ("hard") gamma rays.

The long-soft GRBs are the ones which have been detected most often at other wavelengths, and they are believed to be associated with the collapse of supermassive stars, in an event known as a hypernova. When a massive star runs out of the nuclear fuel that makes it shine, the core of the star collapses. If the core collapses into a black hole, the remainder of the star will begin to fall onto it. Black holes sometimes produce jets of material that fly away from the black hole at close to the speed of light, and in a hypernova, the infalling stellar material acts as a source for these jets. These events probably happen dozens of times a day across the entire universe, but we only detect them as a gamma ray burst if, by chance, the jet from the black hole happens to be pointed in our direction. GRBs produce the most intense radiation along the direction of the jet, and so we only detect them when they're pointed right at us.

Although they haven't been studied as well, the short-hard GRBs are also believed to originate from the formation of a black hole. In this case astrophysicists think they come from the merger of two black holes or two neutron stars in orbit around one another. Both black holes and neutron stars are very massive and very, very small in size, and when they orbit one another closely, they move very fast! If they spiral together and merge with one another, their collision may result in a huge explosion that occurs very quickly, producing a very rapid burst of gamma rays at high energies.

Most of the energy emitted by a gamma-ray burst comes out as gamma-rays, but the jets that create them and the resulting hypernova emits light at other wavelengths too, and by studying the afterglow, you can learn more about the object that created the GRB than you can from just studying the gamma ray emission. The light emitted in X-rays, optical light, and radio waves can often persist for hours or days after the gamma ray burst, and because of the nature of radiation at these wavelengths, it is easier to pinpoint where the GRB is from the afterglow than it is from the gamma ray burst itself. You can also figure out what kind of star it was that exploded, how the explosion progressed, or what the environment was like around that star by studying the afterglow.

GRB afterglows are hard to find, but there is now a network of space satellites and ground-based observatories dedicated to their detection and localization. Satellites like Swift are designed to quickly detect and localize GRBs to much higher precision than was previously possible. Satellites can now provide gamma ray localizations to less than 0.5 degrees (sometimes much less), making it easier for ground-based observers to concentrate their search on a particular spot in the sky. The satellite radios the coordinates back to Earth, and these coordinates are then relayed to observatories around the world via the Gamma Ray Burst Coordinates Network or GCN.

The AAVSO International High Energy Network is a part of the GCN, which is updated in real time. Observers can then turn their telescopes toward those coordinates, and search for a transient -- an object not previously observed at those coordinates. If they find one, then it's possible that they're looking at the GRB afterglow. The discoverer of an afterglow usually communicates the exact position and their initial observations to the rest of the GCN community, and other observers around the world begin observing the object, too. If the object fades in brightness over the next few hours or days and doesn't move as a minor planet, comet, or asteroid in our Solar System would, then they've found the afterglow! Continued observation of the source gives information on how the GRB explosion evolves, what its environment is like, and sometimes even what the progenitor object was.

We think we understand the basics of how GRBs happen, but we don't know everything, and sometimes we see some surprising things when we study gamma ray bursts and their afterglows. Sometimes the gamma ray light curve is very complex, with lots of rapid changes, sometimes not; sometimes the GRB afterglow light curve seems to evolve like a supernova, sometimes not; sometimes two GRBs with very similar gamma ray light curves will have totally different light curves in the optical, or perhaps one might not have an optical afterglow at all. We still don't completely understand what happens during a GRB, and the more observational data we have the better our understanding will be. Often, we learn more by encountering something we don't expect to see than by seeing what we expect.

There are many observatories searching for afterglows, including many robotic telescopes that search for them automatically, so your chances of being the first to observe an afterglow is not as great now as it was in the first decade of the 21st Century. But robotic telescopes can't be everywhere at once, and there is still a chance for individuals to make contributions to GRB research, especially by obtaining photometry during the outburst.

Magnetars

A magnetar is a type of neutron star believed to have an extremely powerful magnetic field (∼1013 to 1015 G, ∼109 to 1011 T). The magnetic field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays. The theory regarding these objects was proposed by Robert Duncan and Christopher Thompson in 1992, but the first recorded burst of gamma rays thought to have been from a magnetar had been detected on March 5, 1979. During the following decade, the magnetar hypothesis became widely accepted as a likely explanation for soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs).

Pulsars

A pulsar (from pulse and -ar as in quasar) [1] is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Earth (much like the way a lighthouse can be seen only when the light is pointed in the direction of an observer), and is responsible for the pulsed appearance of emission. Neutron stars are very dense, and have short, regular rotational periods. This produces a very precise interval between pulses that ranges from milliseconds to seconds for an individual pulsar. Pulsars are one of the candidates for the source of ultra-high-energy cosmic rays (see also centrifugal mechanism of acceleration).

The periods of pulsars make them very useful tools for astronomers. Observations of a pulsar in a binary neutron star system were used to indirectly confirm the existence of gravitational radiation. The first extrasolar planets were discovered around a pulsar, PSR B1257+12. Certain types of pulsars rival atomic clocks in their accuracy in keeping time.

Flare Stars

A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to the magnetic energy stored in the stars' atmospheres. The brightness increase is across the spectrum, from X rays to radio waves. The first known flare stars (V1396 Cygni and AT Microscopii) were discovered in 1924. However, the best-known flare star is UV Ceti, first observed to flare in 1948. Today similar flare stars are classified as UV Ceti type variable stars (using the abbreviation UV) in variable star catalogs such as the General Catalogue of Variable Stars.

Most flare stars are dim red dwarfs, although recent research indicates that less massive brown dwarfs might also be capable of flaring. The more massive RS Canum Venaticorum variables (RS CVn) are also known to flare, but it is understood that these flares are induced by a companion star in a binary system which causes the magnetic field to become tangled. Additionally, nine stars similar to the Sun had also been seen to undergo flare events prior to the flood of superflare data from the Kepler observatory. It has been proposed that the mechanism for this is similar to that of the RS CVn variables in that the flares are being induced by a companion, namely an unseen Jupiter-like planet in a close orbit.

X-Ray Binaries

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 (usually a relatively normal star), to the other component, called the accretor, which is very compact: a neutron star or black hole. The infalling matter releases gravitational potential energy, up to several tenths of its rest mass, as X-rays. Hydrogen fusion releases only about 0.7 percent of rest mass. 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. An estimated 1041positrons escape per second from a typical low-mass X-ray binary.

Quasi Stellar Objects

A quasar (also known as a quasi-stellar object abbreviated QSO) is an extremely luminous active galactic nucleus (AGN), in which a supermassive black hole with mass ranging from millions to billions of times the mass of the Sun is surrounded by a gaseous accretion disk. As gas in the disk falls towards the black hole, energy is released in the form of electromagnetic radiation, which can be observed across the electromagnetic spectrum. The power radiated by quasars is enormous: the most powerful quasars have luminosities thousands of times greater than a galaxy such as the Milky Way.


Quasar 3C 273 observed by the
Hubble Space Telescope
(ESA/Hubble & NASA)

The term quasar originated as a contraction of quasi-stellar [star-like] radio source, because quasars were first identified during the 1950s as sources of radio-wave emission of unknown physical origin, and when identified in photographic images at visible wavelengths they resembled faint star-like points of light. High-resolution images of quasars, particularly from the Hubble Space Telescope, have demonstrated that quasars occur in the centers of galaxies, and that some host-galaxies are strongly interacting or merging galaxies. As with other categories of AGN, the observed properties of a quasar depend on many factors including the mass of the black hole, the rate of gas accretion, the orientation of the accretion disk relative to the observer, the presence or absence of a jet, and the degree of obscuration by gas and dust within the host galaxy.

Quasars are found over a very broad range of distances, and quasar discovery surveys have demonstrated that quasar activity was more common in the distant past. The peak epoch of quasar activity was approximately 10 billion years ago. As of 2017, the most distant known quasar is ULAS J1342+0928 at redshift z = 7.54; light observed from this quasar was emitted when the universe was only 690 million years old. The supermassive black hole in this quasar, estimated at 800 million solar masses, is the most distant black hole identified to date.

Polars

A Polar is a highly magnetic type of cataclysmic variable binary star system, originally known as an AM Herculis star after the prototype member AM Herculis. Like other cataclysmic variables (CVs), polars contain two stars: an accreting white dwarf (WD), and a low-mass donor star (usually a red dwarf) which is transferring mass to the WD as a result of the WD's gravitational pull, overflowing its Roche lobe. Polars are distinguished from other CVs by the presence of a very strong magnetic field in the WD. Typical magnetic field strengths of polar systems are 10 million to 80 million gauss (1000–8000 teslas). The WD in the polar AN Ursae Majoris has the that of strongest known magnetic field among cataclysmic variables, with a field strength of 230 million gauss (23 kT).

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