AT2018cow
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SN 2018cow (
ATLAS An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets. Atlases have traditio ...
name: ATLAS18qqn; also known as Supernova 2018cow, AT 2018cow (AT = Astronomical Transient), and "The Cow") was a very powerful astronomical explosion 10–100 times brighter than a normal
supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...
, spatially coincident with
galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...
, approximately distant in the Hercules constellation. It was discovered on 16 June 2018 by the ATLAS-HKO telescope, and had generated significant interest among astronomers throughout the world. Later, on 10 July 2018, and after AT 2018cow had significantly faded, astronomers, based on follow-up studies with the
Nordic Optical Telescope The Nordic Optical Telescope (NOT) is an astronomical telescope located at Roque de los Muchachos Observatory, La Palma in the Canary Islands. The telescope saw first light in 1988, and was officially inaugurated during September 1989. Regular o ...
(NOT), formally described AT 2018cow as SN 2018cow, a type Ib supernova, showing an "unprecedented spectrum for a supernova of this class"; although others, mostly at first but also more recently, have referred to it as a type Ic-BL supernova. An explanation to help better understand the unique features of AT 2018cow has been presented. AT2018cow is one of the few reported Fast Blue Optical Transients (FBOTs) observed in the Universe. In May 2020, however, a much more powerful FBOT than AT 2018cow (namely, CRTS-CSS161010 J045834-081803, or CSS161010 for short) was reportedly observed. On 2 November 2018, two independent teams of astronomers both concluded that the AT 2018cow event was "either a newly formed
black hole A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...
in the process of accreting matter, or the frenetic rotation of a
neutron star A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...
." In January 2019, astronomers proposed that the explosion may have been a
white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...
being pulled apart by a black hole; or a supernova leaving behind a black hole or a neutron star, the creation of a compact body being observed for the first time. On 13 December 2021, astronomers reported that AT 2018cow, an extreme FBOT, "could be a neutron star or black hole with a mass less than 850 solar masses" based on high-time-resolution X-ray observation studies.


History

AT 2018cow was discovered on 16 June 2018 at 10:35:02 UTC by the ATLAS-HKO telescope, a twin system, at the Haleakala Observatory in
Hawaii Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...
. It was a powerful astronomical explosion (discovery magnitude 14.739;
redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and increase in frequency and e ...
0.014145, 0.0136), 10 – 100 times brighter than a normal supernova, spatially coincident with galaxy , approximately distant in the Hercules constellation. By 22 June 2018, this transient astronomical event had generated significant interest among astronomers throughout the world. At least 24 major telescopes were observing the event, the largest number, as of 27 June 2018, of concurrent observations (over 35 posted on 27 June 2018) of any astronomical event ever reported on '' The Astronomer's Telegram''. The event had been tentatively identified as a supernova and given the designation Supernova 2018cow and classification SN Ic-BL. The first
X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
and
ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
(UV) observations of AT 2018cow were obtained on 19 June 2018 with the Swift telescope. These observations revealed that the object was a bright X-ray/UV transient, with an X-ray luminosity of ~ and a UV brightness of about 11.7 ( Vega mag) in the range 1600-3600 Å. On 25 June 2018, astronomers, using the Liverpool Telescope and the telescope at
Palomar Observatory The Palomar Observatory is an astronomical research observatory in the Palomar Mountains of San Diego County, California, United States. It is owned and operated by the California Institute of Technology (Caltech). Research time at the observat ...
, noted on '' The Astronomer's Telegram'': "AT2018cow has faded every night since our first observations . ... observations suggest that although a link to Ic-BL SNe and GRBs remains credible given the smooth spectra and luminous
radio Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3  hertz (Hz) and 300  gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...
and
X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
counterpart, AT2018cow is distinct in other ways and its true identity remains unclear. Observations are continuing." On 29 June 2018, astronomers, using telescopes at the
Beijing Astronomical Observatory Beijing Astronomical Observatory (BAO) is an observatory in Chaoyang District, Beijing, China. It was founded in 1958 and is part of the Chinese Academy of Sciences. The observatory comprises 5 observing stations. The principal observing site for ...
, reported further support for the fading of AT 2018cow. However, using the Swift/XRT telescope on 30 June 2018, an increase in the X-ray luminosity of the transient was reported. That would be the beginning of an unusual X-ray variable behavior. On 2 July 2018, astronomers, using the Fermi Large Area Telescope (LAT), reported that there were no significant >100 MeV gamma-ray emissions between 19–26 June 2018. Further, on 3 July 2018, astronomers reported, using the Cadmium Zinc Telluride Imager (CZTI) detector aboard the AstroSat space observatory, no hard X-ray transients were detected between 13–16 June 2018 (event detection time) and, using the UVIT fitted with
F172M filter
observed an AB magnitude of an estimated 17.6 at the AT 2018cow location on 3 July 2018. Moreover, astronomers on 3 July 2018 reported, using the MAXI GSC detector aboard the ISS, that no significant X-ray emissions were detected between 11–21 June 2018. On 4 July 2018, astronomers, using NuSTAR, reported a lessening of hard X-ray emissions from AT 2018cow. On 12 July 2018, astronomers, using
INTEGRAL In mathematics, an integral is the continuous analog of a Summation, sum, which is used to calculate area, areas, volume, volumes, and their generalizations. Integration, the process of computing an integral, is one of the two fundamental oper ...
, reported no detections of the source from 30 June – 8 July 2018; however, GRB-like bursts may have been observed earlier in the vicinity, on 12 and 15 June 2018, although association of these bursts with AT 2018cow may be "disfavored". Radio emissions, at 5 GHz with a flux density of ~ 170 microJy, were detected from the location of AT 2018cow on 3–4 July 2018 by
e-MERLIN The Multi-Element Radio Linked Interferometer Network (MERLIN) is an interferometer array of radio telescopes spread across England. The array is run from Jodrell Bank Observatory in Cheshire by the University of Manchester on behalf of UK Rese ...
; radio emissions at the AT 2018cow location were detected by ATCA at 5.5 GHz with ~0.4 mJy flux density and at 9 GHz with ~1.0 mJy on 3 July 2018, and at 34 GHz with ~10 mJy on 5 July 2018.
VLBI Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth or in space. T ...
observations at 22 GHz, with the NRAO, using the VLBA and Effelsberg
radio telescope A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...
s, found a total flux density of ~5 mJy around 8 July 2018 at a reportedly more accurate (but consistent within uncertainties)
astrometric Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. History ...
location of AT2018cow (RA=16h 16m 00.2242s, DEC=22d 16' 04.890") than that of e-MERLIN. On 10 July 2018, astronomers, based on follow-up studies with the
Nordic Optical Telescope The Nordic Optical Telescope (NOT) is an astronomical telescope located at Roque de los Muchachos Observatory, La Palma in the Canary Islands. The telescope saw first light in 1988, and was officially inaugurated during September 1989. Regular o ...
(NOT), formally described AT 2018cow as SN 2018cow and as a type Ib supernova, showing an "unprecedented spectrum for a supernova of this class". On 19 July 2018, astronomers, using the Kanata telescope at the Higashi-Hiroshima Observatory, observed further declines in the optical and near-infrared luminosity of the AT 2018cow position in early July 2018, and noted that the large decline rates of the light curves were "quite large" compared to Type Ic (Ic-BL) and Type Ib/c supernovae. On 6 August 2018, ultraviolet observations of the AT 2018cow location, using the
Wide Field Camera 3 The Wide Field Camera 3 (WFC3) is the Hubble Space Telescope's last and most technologically advanced instrument to take images in the visible spectrum. It was installed as a replacement for the Wide Field and Planetary Camera 2 during the first ...
(WFC3) on the
Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...
(HST), detected brightness ( Vega mag) of about 19 on all four bands (F218W, F225W, F275W, F336W) studied. On 12 August 2018, astronomers at the Giant Metrewave Radio Telescope (GMRT) detected a low frequency radio emission (1390 MHz band; 438+/-82 uJy) at the AT 2018cow position. On 15 August 2018, astronomers using the
High Energy Stereoscopic System High Energy Stereoscopic System (H.E.S.S.) is a system of imaging atmospheric Cherenkov telescopes (IACTs) for the investigation of cosmic gamma rays in the photon energy range of 0.03 to 100 TeV. The acronym was chosen in honour of Vic ...
(H.E.S.S.) array of Cherenkov telescopes (CTA) reported no significant gamma-ray source at the AT 2018cow location on 3–5 July 2018, which, as a consequence, resulted in the preliminary determination of upper limits on the integrated flux of the Very-High-Energy (VHE) gamma emission from AT 2018cow as follows: above the energy threshold 220 GeV (±2sd) an upper limit of 5e-12 ph cm^-2 s^-1; above 1 TeV (±2sd) an upper limit of 5e-13 ph cm^-2 s^-1.


Properties

According to astronomers at the time of its discovery, the explosion, with a surface temperature of over and traveling , may have been a
cataclysmic variable star In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (), since those with an outburst brightness visible t ...
(CV),
gamma-ray burst In gamma-ray astronomy, gamma-ray bursts (GRBs) are extremely energetic events occurring in distant Galaxy, galaxies which represent the brightest and most powerful class of explosion in the universe. These extreme Electromagnetic radiation, ele ...
(GRB),
gravitational wave Gravitational waves are oscillations of the gravitational field that Wave propagation, travel through space at the speed of light; they are generated by the relative motion of gravity, gravitating masses. They were proposed by Oliver Heaviside i ...
(GW),
supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...
(SN), or something else. However, the CV scenario was rapidly disfavored given the initial featureless
optical spectrum The visible spectrum is the band of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' (or simply light). The optical spectrum is sometimes consider ...
and the large initial
X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
luminosity of the transient. According to astronomer Kate Maguire of
Queen's University Belfast The Queen's University of Belfast, commonly known as Queen's University Belfast (; abbreviated Queen's or QUB), is a public research university in Belfast, Northern Ireland, United Kingdom. The university received its charter in 1845 as part of ...
: "It really just appeared out of nowhere. There are other objects that have been discovered that are as fast, but the fastness and the brightness, that's quite unusual." The classification of type Ic-BL indicates a spectrum with very unusually broad lines, but with no hydrogen lines and weak or missing helium lines. Such a spectrum is produced by the explosion of a very large star which has lost its outer layers of
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
and
helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
. However, according to astronomer Shubham Srivastav, associated with the Himalayan Chandra Telescope (HCT): "Although spectroscopic features indicate a tentative similarity with broad line Ic supernovae, its true nature remains a puzzle." Also, according to Maguire: "We're not sure yet what it is, but the normal powering mechanism for a supernova is radioactive decay of
nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...
, and this event is too bright and too fast for that." The AT 2018cow explosion could have been accompanied by a
gravitational wave Gravitational waves are oscillations of the gravitational field that Wave propagation, travel through space at the speed of light; they are generated by the relative motion of gravity, gravitating masses. They were proposed by Oliver Heaviside i ...
(GW) emission, but the GW emission could not be detected since the LIGO detectors in the states of Washington and
Louisiana Louisiana ( ; ; ) is a state in the Deep South and South Central regions of the United States. It borders Texas to the west, Arkansas to the north, and Mississippi to the east. Of the 50 U.S. states, it ranks 31st in area and 25 ...
were down at the time of the event due to service upgradings. An explanation to help better understand the unique features of AT 2018cow, particularly as a
white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...
tidal disruption event A tidal disruption event (TDE) is a time-domain astronomy, 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 spaghett ...
, has been presented. As of 29 September 2018, AT 2018cow has been explained in various ways, including as a type Ic supernova, a
gamma-ray burst In gamma-ray astronomy, gamma-ray bursts (GRBs) are extremely energetic events occurring in distant Galaxy, galaxies which represent the brightest and most powerful class of explosion in the universe. These extreme Electromagnetic radiation, ele ...
, an interaction between a
white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...
and
black hole A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...
, and as a
magnetar A magnetar is a type of neutron star with an extremely powerful magnetic field (~109 to 1011 T, ~1013 to 1015 G). The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.Ward; Br ...
. Preliminary studies to better understand the exact physical nature of AT 2018cow, using the
European VLBI Network The European VLBI Network (EVN) is a network of radio telescopes located primarily in Europe and Asia, with additional antennas in South Africa and Puerto Rico, which performs very high angular resolution observations of cosmic radio sources usin ...
(EVN), have been presented. On 2 November 2018, two independent teams of astronomers both concluded that the AT 2018cow event was "either a newly formed
black hole A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...
in the process of accreting matter, or the frenetic rotation of a
neutron star A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...
." In January 2019, Anna Ho of the California Institute of Technology in Pasadena, who conducted observations with the Submillimeter Array on Mauna Kea in Hawaii, noted that an unusually protracted period of continuing activity after the event was noticed, enabled more extensive study than typically afforded during such events, allowing observation of it while it was brightening. Subsequently, astronomers proposed that AT 2018cow may have been a
white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...
being pulled apart by a
black hole A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...
; or, a
supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...
leaving behind a black hole or a
neutron star A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...
, the creation of a compact body being observed for the first time.


See also

* *


References


External links


SN 2018cow webpage
on the Transient Name Server
AT2018cow webpage
at Vanbuitenen.nl

by the Astronomy Section Rochester Academy of Science
CGCG 137-068 webpage
at the
NASA/IPAC Extragalactic Database The NASA/IPAC Extragalactic Database (NED) is an online astronomical database for astronomers that collates and cross-correlates astronomical information on extragalactic objects (galaxies, quasars, radio, x-ray and infrared sources, etc.). NED was ...

AT2018cow images from 21 June 2018
by Cedric Raguenaud * * *
AT2018cow – Originated from supernova in strongly magnetized environment/ALMA
at NAOJ {{DEFAULTSORT:SN 2018cow Supernovae Discoveries by ATLAS Hercules (constellation) June 2018 2018 in science 2018 in outer space