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Pan-STARRS1 Discovery of the SN IIn PS1-12cht (=LSQ12heq)

ATel #4763; R. Chornock, E. Berger, R. Lunnan, P. Challis, M. Drout, R. J. Foley, R. P. Kirshner, G. Narayan (CfA), A. Rest (STScI), N. E. Sanders, A. M. Soderberg, C. Stubbs (CfA), M. E. Huber, J. Tonry (IfA/Hawaii), A. Riess (JHU), S. Smart (QUB), W. Burgett, K. Chambers, H. Flewelling, J. Heasley, N. Kaiser, E. Magnier, J. Morgan, W. Sweeney, C. Waters (IfA/Hawaii), and P. A. Price (Princeton)
on 25 Jan 2013; 19:20 UT
Credential Certification: Ryan Chornock (rchornock@cfa.harvard.edu)

Subjects: Optical, Black Hole, Supernovae, Transient

We report the independent discovery of the transient PS1-12cht by the Pan-STARRS1 Medium Deep Survey at coordinates:

03:33:04.148 -28:12:49.41 (J2000)

(absolute uncertainty 0.1" in each)

on 2012 Nov. 28.37 UT at z_P1=21.9 mag (AB), before rising to a peak near g_P1=19.3 mag in early December. The light curve has been relatively flat at redder wavelengths, declining by 0.15 mag in z_P1 over the last 35 days. Meanwhile, the g_P1 - r_P1 color has evolved from -0.30 mag on Dec 7.36 to +0.21 mag on Jan 17.25.

This object is the same as the optical transient LSQ12heq, which was discovered by the La Silla-Quest Survey and was initially assumed to be a tidal disruption candidate (ATel #4739, ATel #4758) after the initial spectra showed a smooth blue continuum (ATel #4644).

Prior to the ATels describing this object as a possible tidal disruption candidate, we obtained spectra of PS1-12cht on 2013 Jan. 11 with the Low-Dispersion Survey Spectrograph-3 on the 6.5-m Magellan Clay telescope. These observations show strong nebular emission lines at redshift z=0.148. The continuum is blue, but less so than in the initial observations (ATel #4644, http://www.pessto.org/ ), consistent with the evolving color. Importantly, there are broad, low-amplitude undulations in the continuum similar to those seen in the SN IIn 1998S at about 25-30 days after discovery (Leonard et al. 2000, ApJ, 536, 239; Fassia et al. 2001, MNRAS, 325, 907).

Furthermore, differential astrometry reveals an offset of 0.25 +/- 0.05" (about 1 Pan-STARRS pixel) from PS1-12cht to the centroid of its host galaxy in deep pre-explosion template images.

The combination of the light curve evolution, development of broad spectral features, and spatial offset from the center of the host galaxy are all consistent with a strongly-interacting Type IIn supernova rather than a tidal disruption event.

We thank the staffs at the PS1 and Magellan telescopes for their support. This discovery was enabled using the PS1 System operated by the PS1 Science Consortium (PS1SC) and its member institutions, http://ps1sc.org/PS1_System_ATel.shtml .