Spectroscopic observations of ASASSN-14dc

ATel #6284; T.-W. Chen, A. Jerkstrand, S. J. Smartt, C. Inserra (Queen's University Belfast), A. Bruce, A. Lawrence (University of Edinburgh)
on 30 Jun 2014; 21:47 UT
Credential Certification: Stephen Smartt (s.smartt@qub.ac.uk)

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We report that a spectrogram of ASASSN-14dc (Holoien et al. ATel#6267) was obtained on Jun. 27.20 UT with the 4.2-m William Herschel Telescope + ISIS double armed spectograph with the R300B (0.86Angs per pix) and R158R (1.8 Angs per pix) gratings.The combined spectrum covers 3300 to 9990 Ang at a resolution (for 1 arcsec slit) about 1000 to 1300.

We detected narrow unresolved (V < 380 km/s) emission lines from the host galaxy of H-alpha, H-beta, H-gamma and [OIII]5007 implying a host redshift of z=0.044. With a discovery magnitude of V = 15.8 mag (ATel#6267), and correcting for Milky Way extinction of Av=0.2 (Schlafly & Finkbeiner 2011, ApJ,737,103). The intrinsic brightness is V = -20.6, suggesting the transient is a luminous supernova. A synthetic V-band magnitude from our flux-calibrated spectrum gives a similar result to that from ATel#6267 (V = 16 mag).

The spectrum has several broad absorption features and the H-alpha (and resolved [NII]) lines extended spatially over 5-6 arseconds. H-alpha is particularly strong but it is not clear if all the H-alpha flux is galactic or some is SN related.

We employed a library of supernova spectra via the "Supernova Identification" code (SNID; Blondin and Tonry 2007, ApJ, 666, 1024) and also checked with GELATO (Harutyunyan et al., 2008, A&A, 488, 383).

Fixing the galaxy redshift z~0.04, there are reasonable matches with SN2007if (Ia-pec) at 7days after maximum. However ASASSN-14dc lacks strong Si absorption lines around 6100 Ang. We find a better spectral match with broad-line Ic SNe, e.g. SN1997ef and SN2004aw around maximum light. However, the redshift required to match (in SNID) is z~0.056. Significantly higher than that derived from the galaxy emission lines.

Finally, if the H-alpha flux (or part of it) is related to the SN, then ASASSN-14dc can be matched with SN2005gj (supported by both GELATO and SNID). Suggesting either a Ia+CSM, or Ic+CSM type object.

In summary, this appears to be an unusually bright SN (around M_V ~ -20) but further data required to determine if the H-Balmer lines are CSM or completely caused by galaxy emission. Higher resolution spectra to resolve the H-alpha width and distinguish between multiple H-alpha components and host [NII] are required.