AT2018cow

AT2018cow was a luminous, rapidly evolving optical transient discovered in June 2018. It exhibited an unusually rapid rise and decline, blue continuum emission, and bright high-energy counterparts, provoking comparisons to supernovae, tidal disruption events, and exotic transients observed by facilities such as Pan-STARRS, GALEX, and Fermi Gamma-ray Space Telescope. The object catalyzed coordinated campaigns by observatories including Hubble Space Telescope, Chandra X-ray Observatory, Swift, and ground-based arrays such as Very Large Array and Atacama Large Millimeter/submillimeter Array.

Discovery and classification

The object was first reported by the Asteroid Terrestrial-impact Last Alert System on 2018 June 16 in the northern constellation Hercules and rapidly attracted follow-up from teams at Las Cumbres Observatory, Palomar Observatory, and Keck Observatory. Early photometry and spectroscopy showed a rise to peak in days, prompting classification efforts comparing it to fast transients cataloged by Zwicky Transient Facility, Pan-STARRS1 Medium Deep Survey, and historical events like the SN 2006jc and the sample compiled by the Open Supernova Catalog. Debates on taxonomy invoked classes such as Type I supernova, superluminous supernova, and tidal disruption event; collaboration papers from groups at Harvard–Smithsonian Center for Astrophysics, University of Cambridge, and Max Planck Institute for Astrophysics proposed the designation as a member of emergent fast blue optical transients (FBOTs), contrasted with classical samples from Sloan Digital Sky Survey supernova surveys.

Observational characteristics

Photometry revealed an unprecedentedly fast rise (<3 days) to an absolute magnitude rivaling bright Type IIn supernovae and a blue spectral energy distribution with temperatures inferred from early spectra exceeding 15,000 K. Spectroscopic campaigns with Keck II, Gemini Observatory, and Very Large Telescope produced featureless continua initially, later developing broad and narrow features compared with lines in Type Ic supernova spectra and emission seen in some active galactic nucleus flares. High-energy detections included luminous X-rays observed by Chandra X-ray Observatory and XMM-Newton, and radio emission mapped by Very Large Array and Atacama Large Millimeter/submillimeter Array, displaying temporal and spectral evolution consistent with synchrotron emission similar to models developed for GRB afterglows and particular cases like SN 1998bw. Polarization constraints from instruments at William Herschel Telescope and spectro-polarimetric data from the European Southern Observatory provided limits on asymmetry compared with aspherical explosions such as those associated with Gamma-Ray Burst (GRB) 980425.

Progenitor and explosion models

Interpretations of the progenitor landscape considered compact objects and stellar endpoints studied by groups at Caltech, Massachusetts Institute of Technology, and University of California, Berkeley. Proposed models included a central engine scenario invoking a nascent neutron star or rapidly accreting black hole analogous to central engines in long-duration gamma-ray burst models, magnetar spin-down models developed in the context of SLSN theory, and shock-interaction models where dense circumstellar material shaped emission similar to mechanisms explored for Type IIn supernovae and luminous blue variable eruptions studied in the Large Magellanic Cloud. Alternative hypotheses invoked a tidal disruption of a stellar object by an intermediate-mass black hole in a low-mass galaxy, drawing on event comparisons from ASASSN-14li and theoretical frameworks advanced at institutions like Princeton University and Ohio State University.

Multiwavelength follow-up and evolution

Rapid multiwavelength campaigns coordinated by teams at NASA, European Space Agency, and national observatories tracked the transient from radio through X-ray bands. Radio light curves from Very Large Array and MeerKAT evolved over months, constraining ejecta velocities and circumstellar densities using formalisms from relativistic blast wave theory developed in connection with GRB afterglows. X-ray luminosity and variability patterns captured by Chandra X-ray Observatory and XMM-Newton implied a compact, high-energy source persisting beyond optical decline, leading to comparisons with persistent engines in ultraluminous X-ray source studies. Optical/UV decline monitored by Swift and Hubble Space Telescope showed cooling and spectral development that informed radiative-transfer models from teams at Argonne National Laboratory and Los Alamos National Laboratory. Long-term follow-up placed limits on late-time nebular emission and searched for radio remnants analogous to those of historical nearby explosions like SN 1987A.

Host galaxy and environment

The transient occurred in a star-forming dwarf host identified in surveys by Sloan Digital Sky Survey and targeted spectroscopy with Keck Observatory and Gemini Observatory, yielding a redshift corresponding to a distance of roughly 60 megaparsecs. Host properties—low stellar mass, modest metallicity, and localized star-formation—echo environments noted for some long-duration gamma-ray burst hosts and certain superluminous supernova hosts cataloged by the Pan-STARRS and Dark Energy Survey teams. Integral-field observations with instruments on Very Large Telescope and photometric analyses from Hubble Space Telescope constrained the transient’s offset from host star-forming regions, informing progenitor population studies carried out by researchers at University of Oxford and University of Tokyo.

Category:Fast blue optical transients