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SN 2013dh in NGC 5936 is probably a type Iax supernova

ATel #5143; Saurabh W. Jha, Curtis McCully (Rutgers), Ryan J. Foley (Harvard-Smithsonian CfA), S. Bradley Cenko (NASA/GSFC), WeiKang Zheng, Kelsey I. Clubb, Isaac Shivvers, Alexei V. Filippenko (UC Berkeley), Brad E. Tucker (Australian National University), and Peter M. Garnavich (Notre Dame)
on 18 Jun 2013; 14:38 UT
Credential Certification: Saurabh Jha (saurabh@physics.rutgers.edu)

Subjects: Request for Observations, Supernovae

Further to CBET 3561 (Kumar et al. 2013; Cenko et al. 2013), we have analyzed spectra of SN 2013dh obtained on June 14.3 and June 17.3 UT with the Shane 3-m reflector (+ Kast spectrograph) at Lick Observatory. The observations continue to show that SN 2013dh has a blue continuum with relatively narrow (FWHM approximately 3000 km/s) Fe III absorption.

The host-galaxy redshift z = 0.0133 for NGC 5936 (Ahn et al. 2012, ApJS, 203, 21; http://dr9.sdss3.org/spectrumDetail?plateid=2754&mjd=54240&fiber=372 ) is consistent with the observed nebular emission lines superimposed on our supernova spectrum. Adopting this redshift, cross-correlation of the SN 2013dh spectra with a library of supernova spectra gives good matches with SN 2005hk (Phillips et al. 2007, PASP, 119, 360), although SN 2013dh has lower velocity features than SN 2005hk. The June 17.3 UT spectrum best matches SN 2005hk at an epoch of approximately 9 days before maximum light.

Cenko et al. (2013, CBET 3561) noted that SN 2013dh spectroscopically resembles SN 1991T. However, the relative faintness of SN 2013dh (M_R ~ -17 mag, with no sign of dust extinction), the low expansion velocities, and the spectral similarity to SN 2005hk (along with SNe 2009J, 2009ku, and 2012Z) suggests that it is actually a member of the type Iax class of supernovae (Foley et al. 2013, ApJ, 767, 57), also known as SN 2002cx-like supernovae (Li et al. 2003, PASP, 115, 453; Jha et al. 2006, AJ, 132, 189).

We have triggered HST UV spectroscopy (program GO-12973, PI: McCully) and Swift UV photometry of SN 2013dh. The HST observations are scheduled to begin on June 21, and we encourage additional follow-up observations of this supernova.

We thank Peter Brown (TAMU) and Boris Sbarufatti (INAF-OAB/PSU) for advocacy and assistance in scheduling the Swift observations, and Patricia Royle and Stephen Holland (STScI) for assistance in planning the HST observations.