The Cow: discovery of a luminous, hot and rapidly evolving transient
S. J. Prentice, K. Maguire, S. J. Smartt, M. R. Magee, P. Schady, S., Sim, T.-W. Chen, P. Clark, C. Colin, M. Fulton, O. McBrien, D. O`Neill, K. W., Smith, C. Ashall, K. C. Chambers, L. Denneau, H. A. Flewelling, A. Heinze, T., W.-S. Holoien, M. E. Huber, C. S. Kochanek

TL;DR
This paper reports the discovery and analysis of ATLAS18qqn/AT2018cow, a luminous, rapidly evolving transient with unique spectral features, suggesting a central engine and possible progenitors involving low-mass stars or neutron-star mergers.
Contribution
It provides the first detailed analysis of this unusual transient, proposing a magnetar-powered model and exploring potential progenitors, which advances understanding of such fast, luminous events.
Findings
High peak luminosity (~1.7 x 10^{44} erg s^{-1})
Rapid light curve evolution (>5 mag in 3.5 days)
Featureless spectra with late emergence of broad He I lines
Abstract
We present the ATLAS discovery and initial analysis of the first 18 days of the unusual transient event, ATLAS18qqn/AT2018cow. It is characterized by a high peak luminosity (1.7 10 erg s), rapidly evolving light curves (5 mag rise to peak in 3.5 days), and hot blackbody spectra, peaking at 27000 K that are relatively featureless and unchanging over the first two weeks. The bolometric light curve cannot be powered by radioactive decay under realistic assumptions. The detection of high-energy emission may suggest a central engine as the powering source. Using a magnetar model, we estimated an ejected mass of \msol, which lies between that of low-energy core-collapse events and the kilonova, AT2017gfo. The spectra cooled rapidly from 27000 to 15000 K in just over 2 weeks but remained smooth and featureless. Broad and shallow emission…
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