[IAUC] CBET 4171: 20151115 : SUPERNOVA 2015Z IN NGC 5837 = PSN J15044078+1237436

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                                                  Electronic Telegram No. 4171
Central Bureau for Astronomical Telegrams
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SUPERNOVA 2015Z IN NGC 5837 = PSN J15044078+1237436
     R. Arbour, South Wonston, Hants, UK, reports the discovery of an
apparent supernova (mag 17.5) in the galaxy NGC 5837 on an unfiltered CCD
image taken on June 16.028 UT using a 35-cm f/6 Schmidt-Cassegrain telescope
(+ SXVF H9 camera) in the course of his supernova patrol.  The new object is
located at R.A. = 15h04m40s.78, Decl. = +12d37'43".6 (equinox 2000.0), which
is 3" east and 16" south of the center of NGC 5837.  Nothing is visible at
this position on digitized Palomar Sky Survey plates from 1989 Feb. 11 (red)
and 1993 Mar. 19 (blue).  The variable was designated PSN J15044078+1237436
when it was posted at the Central Bureau's TOCP webpage and is here
designated SN 2015Z based on the spectroscopic confirmation reported below.
The type-Ia supernova 2014ac also appeared in NGC 5837 (cf. CBET 3829).
Additional CCD magnitudes for 2015Z:  June 16.932, 17.7 (G. Masi; remotely
using a 43-cm telescope at Ceccano, Italy; position end figures 40s.76,
43".5); 19.965, 17.4 (Arbour; position end figures 40s.79, 43".3).

     D. C. Leonard, San Diego State University; P. Sheehan and D. McCarthy,
University of Arizona; K. Follette, Stanford University; J. Moustakas, Siena
College; D. Cantillo, A. Cazares-Kelly, S. Cazares-Kelly, Y. Cendes, N. Damm,
A. Donati, E. Douglas, L. Ferrell, H.  Fosbiner-Elkins, C. Fox, M. Greenberg,
K. Hart, H. Hensley, A. Holt, E.  Hooper, C. Juran, J. Keane, K. Key, L.
Korus, T. Lee, K. Leidig, E. Merchak, K. Nessmann, S. Pendyala, S. Pirkl, J.
Reeder, A. Roos, S. Rounseville, E. Ruddy, A. Schlingman, W. Schlingman,
W. M. Schlingman, E. Schwartzman, V. Shanmugam, E. Silver, A. Stein, N.
Stock, B. Svoboda, B. Thomas, N. Thomas, K. Thompson-Taylor, and H. Walton,
2015 Advanced Teen Astronomy Camp, report on optical spectroscopic
observations of PSN J15044078+1237436 = SN 2015Z taken on June 20.3 and 21.3
UT with the 2.3-m Bok telescope (+ Boller & Chivens spectrograph) at Kitt
Peak.  The spectra cover the range 370-680 nm, with a resolution of 0.8 nm.
The spectra are characterized by a blue continuum with Balmer emission lines
(at the approximate redshift of NGC 5837 -- z = 0.0287, from Fisher et al.
1995, Ap.J. Suppl 100, 69, via NED) that are well-fitted with Lorentzian
profiles (FWHM around 1100 km/s; HWZI around 3500 km/s); sharp absorption
dips at velocity about 800 km/s are apparent in the blue wings of the Balmer
emission lines.  He I 587.6- and 501.6-nm emission is also seen.  There does
not appear to be significant evolution of the spectra over the one-day time
interval between them, although the signal-to-noise ratio does not allow a
detailed comparison.  Cross-correlation with a library of spectra using the
comparison tool GELATO (Harutyunyan et al. 2008, A.Ap. 488, 383) finds best
matches with spectra of the peculiar type-IIn event 2009ip taken during the
broad peak of its second optical brightening in 2012 (i.e., the "2012-B
event", which peaked at absolute r magnitude about -18.3 between 2012 Oct. 6
and 15; see Graham et al. 2014, Ap.J. 787, 163), during which the ejecta
were believed to be strongly interacting with a dense circumstellar medium.
The true nature of SN 2009ip remains a subject of considerable debate, with
some contending the 2012 events to have been associated with a true supernova
(e.g., Smith et al. 2014, MNRAS 438, 1191) and others proposing a non-terminal
event (e.g., Moriya et al. 2015, Ap.J. Let. 803, 26, and references therein).
The present authors note that a reasonable spectral match is also found with
the "classical" type-IIn supernova 1995G at two days after its discovery
(Pastorello et al. 2002, MNRAS 333, 27); in particular, the Balmer line
profiles -- including especially the low-velocity, blue-shifted absorption
dip (not seen in the spectra of SN 2009ip) -- are extremely similar, although
PSN J15044078+1237436 = SN 2015Z does not show any evidence of the multiple
Fe II lines seen in the spectrum of SN 1995G.  The present authors add that,
taking the approximate magnitude of the transient to be 17.5 (in R-band, from
the discovery photometry posted at the TOCP webpage), an estimated distance
modulus to the host galaxy of m-M = 35.25 +/- 0.41 (via NED; the mean of five
Tully-Fisher distances), and an R-band Galactic extinction of 0.08 mag
(Schlafly and Finkbeiner 2011, Ap.J. 737, 103) yields an absolute R-band
magnitude of approximately -17.8 for 2015Z.  Given the similarities of 2015Z
to type-IIn events and, especially, to spectra of SN 2009ip during a
particularly interesting phase, further observations at all wavelengths are
encouraged, as are examinations of pre-discovery images to potentially
constrain prior outbursts.


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                         (C) Copyright 2015 CBAT
2015 November 15                 (CBET 4171)              Daniel W. E. Green