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SkyMapper Discovery of a Nearby SN IIn

ATel #7711; M. Childress, R. Scalzo, F. Yuan, B. Tucker, B. Zhang, A. Ruiter, I. Seitenzahl, B. Schmidt (Australian National University), S. Ryder (Australian Astronomical Observatory)
on 25 Jun 2015; 08:05 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Michael Childress (mjc@mso.anu.edu.au)

Subjects: Optical, Supernovae

Referred to by ATel #: 7762

We report the discovery of a nearby Type IIn SN as part of the SkyMapper Transient (SMT) survey conducted with the 268-megapixel camera on the SkyMapper 1.3-m telescope at Siding Spring Observatory near Coonabarabran, NSW, Australia (Keller et al., 2007, PASA, 24, 1). SMTJ07350518-6907531 was discovered at RA = 07:35:05.18, DEC = -69:07:53.1 at magnitude g = 19.3, r = 19.3 in SkyMapper images taken 2015 Apr 27.9 UT; no source was detected at this position on Apr 26 to a limiting magnitude of r = 20.2. The SN light curve demonstrated an initial peak of r = 18.2 on May 08.9 before beginning to decline, then rose again to a second peak at r = 16.8 on Jun 09.9 before undergoing a brief decline then rising again. Currently we estimate the SN to be about r=16.0 and rising.

A 20 minute spectrum was obtained with the Wide Field Spectrograph (WiFeS; Dopita et al. 2007, ApSS, 310, 255) on 2015 Jun 24 UT. The spectrum is consistent with a Type IIn SN, and shows broad structured emission in the Balmer series (particularly Halpha and Hbeta) with an additional strong broad emission feature consistent with rest-frame He 4686 AA. We determine the peak emission in Halpha to be at 6598 AA, which corresponds to a very nearby redshift of z=0.0054. There are no obvious host galaxies near the SN, though we note NGC 2434 (which has a similar redshift z=0.004637 via NED) is a projected 62 kpc distant and NGC 2442 (z=0.004890 via NED, also host to SN 2015F) is a projected 166 kpc away; the SN may be associated with this group. It also sits in the region between NGC 2442 and its associated 10^9 Msun gas cloud HIPASS J0731-69 (Ryder et al. 2001, ApJ, 555, 232) for which no optical counterpart has been detected.

Given the observed redshift (distance modulus of 31.83 for H0=70) we estimate the initial peak on 2015 May 08.9 to be M_r = -13.60, and the second peak on 2015 Jun 09.9 to be M_r = -15.01. We have posted images of the preliminary light curve and spectrum. Further observations at all wavelengths are strongly encouraged.