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AT2018cow : further updates to lightcurve and spectra

ATel #11742; S. J. Smartt, S. Prentice, K. Maguire, P. Clark K. W. Smith, O. McBrien, (Queen's University Belfast), J. Tonry, L. Denneau, B. Stalder, A. Heinze, H. Weiland, H. Flewelling (IfA, University of Hawaii), A. Rest (STScI), K. Z. Stanek, C. S. Kochanek, T. A. Thompson (Ohio State), B. J. Shappee (IfA-Hawaii), T. W.-S. Holoien (Carnegie Observatories), J. L. Prieto (Diego Portales; MAS), Subo Dong (KIAA-PKU), M. Stritzinger (Aarhus)
on 20 Jun 2018; 20:37 UT
Credential Certification: Stephen Smartt (s.smartt@qub.ac.uk)

Subjects: Optical, Gamma-Ray Burst, Supernovae, Transient

Referred to by ATel #: 11748, 11749, 11753, 11757, 11758, 11760, 11772, 11796

We report further observations and basic analysis of ATLAS18qqn/AT2018cow with ATLAS, ASASSN and the Liverpool Telescope. Two spectra were taken with the LT + SPRAT on 2016-06-18 and and 2016-06-19. Similar to Perley (ATel #11732) and Jones et al. (ATel #11736) we see a very blue, mostly featureless continuum. After correcting for foreground extinction (E(B-V)=0.08), the continuum is well matched to a thermal black-body of T=25000K.

The latest ATLAS point (Smartt et al. ATel #11727) is cyan c=13.60+/- 0.02 on MJD=58287.45 (similar to Chen and Schady ATel #11734). Making it rather luminous at M_c = -20.5 if it is in CGCG 137-068.

Correcting the spectrum to the dereddened c-band magnitude and assuming the object's SED follows a thermal black body, provides an estimate of total luminosity at this epoch of L = 4.1e44 erg/s, at a distance of 60 Mpc. At Teff=25,000 K, this implies a radius of 1.2e15cm (or 1.75e4 R_sol).

There are recent non-detections by ATLAS (Smartt et al. ATel #11727) and ZTF (Fremling et al. ATel #11738) and ASASSN on MJD=58284. This implies a rise time (to c=13.6) of t_rise < 3.3 days. This is remarkably short for a supernova type transient to reach this luminosity.

The rise time would imply an expansion velocity (to get to 1.2e15cm) of v = 42,000 km/s (or shock breakout from a very extended star).

The detection of host galaxy absorption features of CGCG 137-068 (Jones et al. ATel #11736, Selsing et al. Atel #11741) at z=0.014, the Swift x-ray detection by Rivera Sandoval et al. ATel #11737, #11739), and the GROND rapid decline (Chen and Schady ATel #11734) suggest either a very high velocity (mildy relativistic) explosion (v>0.14c) or shock breakout from an extraordinary progenitor.