SN 2018cow

SN 2018cow (ATLAS 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, spatially coincident with galaxy CGCG 137-068, approximately 200 million ly (60 million pc) 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 (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 in the process of accreting matter, or the frenetic rotation of a neutron star." In January 2019, astronomers proposed that the explosion may have been a white dwarf 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 0.5 m (20 in) system, at the Haleakala Observatory in Hawaii. It was a powerful astronomical explosion (discovery magnitude 14.739; redshift 0.014145, 0.0136), 10 – 100 times brighter than a normal supernova, spatially coincident with galaxy CGCG 137-068, approximately 200 million ly (60 million pc) 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 and ultraviolet (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 ~1×1043 erg/s and a UV brightness of about 11.7 (Vega mag) in the range 1600-3600 Å. On 25 June 2018, astronomers, using the 2.0 m (79 in) Liverpool Telescope and the 1.5 m (59 in) telescope at Palomar Observatory, noted on The Astronomer's Telegram: "AT2018cow has faded every night since our first observations [19 June 2018]. ... [The reported] observations suggest that although a link to Ic-BL SNe and GRBs remains credible given the smooth spectra and luminous radio and X-ray counterpart[s], AT2018cow is distinct in other ways and its true identity remains unclear. Observations are continuing." On 29 June 2018, astronomers, using 0.6 m (24 in) telescopes at the Beijing Astronomical Observatory, 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.

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