arXiv:1807.05965 -- The Cow: discovery of a luminous, hot and rapidly evolving transient

Paper: The Cow: discovery of a luminous, hot and rapidly evolving transient
Authors: S. J. Prentice, et al.

Abstract: We present the ATLAS discovery and initial analysis of the first 18 days of the unusual transient event, ATLAS18qqn/AT2018cow. It is characeterized by a high peak luminosity (∼1.7×1044 erg s−1), rapidly evolving light curves (>5 mag rise in ∼3 days), 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 0.1−0.4 M⊙, which lies between that of low-energy core-collapse events and the kilonova, AT2017gfo. The spectra of AT2018cow showed a number of shallow features overlying a blackbody continuum. The spectra cooled rapidly from 27000 to 15000 K in just over 2 weeks but the positions of shallow bumps in the spectra did not evolve, suggesting that they are produced in a shell or are potentially emission features. Using spectral modelling, we tentatively identify some features as being due to He I and He II and rule out that the features in the spectra are due to most 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. 

My Comment: Weird things that go bump in the night are always neat. This object seems to offer some unique challenges (spectra) in terms of matching what it is. Best guess being a magnetar-like object. The combination of Cow and magnetar is honestly why this paper was chosen. Moooooo.

My Scrawling Notes: