AT2018cow: Continued optical fading and weakening of spectral features
ATel #11776; D. A. Perley (LJMU), N. Blagorodnova, J. D. Neill, R. Walters (Caltech)
on 25 Jun 2018; 10:41 UT
Credential Certification: Daniel Perley (d.a.perley@ljmu.ac.uk)
Subjects: Optical, Gamma-Ray Burst, Supernovae, Transient
We have continued to monitor the evolution of the optical transient AT2018cow/ATLAS18qqn (Smartt et al., ATEL#11727). Low-resolution spectra have been taken each night, using both SPRAT on the Liverpool Telescope (wavelength coverage from 400-800 nm; resolution of 2 nm) and the SED Machine on the Palomar 60-inch telescope (wavelength coverage from 380-920 nm; resolution of 5 nm). We are also monitoring the transient in ugrizH filters nightly at LT.
AT2018cow has faded every night since our first observations (ATEL#11732). The spectrum and broadband optical colors have become slowly redder. Beginning on 2018-06-19 UT a broad spectral feature (with a peak of around 550 nm, or alternatively a dip around 450 nm) emerges, as noted by several other groups (ATEL#11740, #11753, #11759, #11766). Although this feature broadly resembles the dominant spectral feature in optical spectra of Ic-BL SNe, its identity is not definitive. A possible spectral bump may be present close to 850 nm in all subsequent SEDM spectra, although this is outside the coverage of the LT spectrum and we have not confirmed its reality.
This 450/550 nm feature is strongest on 2018-06-21 UT, about five days after the ATLAS discovery. However, from then on it appears to weaken. In our latest spectra, acquired with LT at 21:22 on 2018-06-24, it has almost disappeared. Photometrically, the SN has faded below the luminosity of SN1998bw at a comparable epoch in all visible bands, although it remains more luminous in the u-band. The u-band is not within our nightly spectroscopic coverage, but the u-g color excess suggests that there may be a separate feature in the u-band that is persisting.
These observations suggest that although a link to Ic-BL SNe and GRBs remains credible given the smooth spectra and luminous radio and X-ray counterpart (ATEL#11737, #11749, #11773, #11774, #11775), AT2018cow is distinct in other ways and its true identity remains unclear. Observations are continuing.