The Cow: discovery of a luminous, hot and rapidly evolving transient

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 ($\sim$1.7 $\times$ 10$^{44}$ erg s$^{-1}$), rapidly evolving light curves ($>$5 mag rise to peak in $\sim$3.5 days), and hot blackbody spectra, peaking at $\sim$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 $0.1-0.4$ \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 lines appear after about 20 days, and we tentatively identify them as He I although they would be redshifted from their rest wavelengths. We rule out that there are any features in the spectra due to intermediate mass elements up to and including the Fe-group. The presence of r-process elements cannot be ruled out. If these lines are due to He, then we suggest a low-mass star with residual He as a potential progenitor. Alternatively, models of magnetars formed in neutron-star mergers give plausible matches to the data.