AT2018cow: VLT X-shooter observations
ATel #11741; J. Selsing (DARK/DAWN, NBI), L. Izzo (HETH/IAA-CSIC), A. J Levan (Warwick), D. B. Malesani (DARK/DAWN, NBI), E. R. Stanway (Warwick), D. A. Perley (LJMU), D. A. Kann (HETH/IAA-CSIC), S. Campana, S. Covino (Brera), A. de Ugarte Postigo (HETH/IAA-CSIC, DARK), M. Arabsalmani (Paris-Saclay), K. Wiersema (Warwick), D. Hartmann (Clemson), C. Kouveliotou (GWU), N. R. Tanvir (Leicester), G. Pugliese (API, Amsterdam), J Fynbo (DAWN/NBI), J. Hjorth (DARK/NBI)
on 20 Jun 2018; 18:55 UT
Credential Certification: Andrew Levan (A.J.Levan@warwick.ac.uk)
Subjects: Infra-Red, Optical, Gamma-Ray Burst, Supernovae, Transient
We observed AT2018cow/ATLAS18qqn (Smartt et al. ATEL #11727) with VLT X-shooter on 20 June 2018, beginning at 02:18UT. A total of 600s of exposure time was obtained with spectra spanning the range 3000-25000A.
The source is well detected across the wavelength range with very high signal to noise. We identify Ca H&K and Na ID absorption from the putative host galaxy CGCG 137-068. This confirms other spectroscopy (Jones et al. ATEL #11736) that AT2018cow is not Galactic and lies at (or beyond) z=0.0140. We also identify a range of narrow emission lines from OII (3727), Hbeta, OIII (4959,5007), H-alpha and NII that are likely from the host galaxy.
The source is very blue, and can be adequately, but not perfectly, described by a power-law with beta=-3 (F_nu ~ nu^{-beta}) Alternatively a thermal model with a temperature of ~18,000K can be used. Neither scenario provides a good fit across the entire wavelength range. In particular, the presence of a very broad feature in the 5000-6000A range is noted in both cases, somewhat redder than reported by Xu et al. (ATEL #11740). There is also some evidence from deviation away from power-law of thermal models around 3750A.
We thank the staff of Paranal for their excellent assistance with these observations.