Electronic Telegram No. 4106
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
CBAT Director:  Daniel W. E. Green; Hoffman Lab 209; Harvard University;
 20 Oxford St.; Cambridge, MA  02138; U.S.A.
e-mail:  cbatiau@eps.harvard.edu (alternate cbat@iau.org)
URL http://www.cbat.eps.harvard.edu/index.html
Prepared using the Tamkin Foundation Computer Network


SUPERNOVA 2015I IN NGC 2357 = PSN J07174570+2320406
     S. Nakano, Sumoto, Japan, reports the discovery by Toshihide Noguchi
(Katori, Chiba-ken, Japan) of an apparent supernova (mag 15.7) on a 20-s
unfiltered CCD frame (limiting magnitude 17.5) taken on May 2.482 UT using a
0.23-m f/6.3 Schmidt-Cassegrain reflector (+ FLI ML0261E camera).  The new
object is located at R.A. = 7h17m45s.70, Decl. = +23d20'40".6 (equinox
2000.0; UCAC4 reference stars), which is 65".9 east and 42".6 south of the
nucleus of the galaxy NGC 2357.  Nothing is visible at this position on a
red Digitized Sky Survey image from 1997 Jan. 10.  Noguchi has posted his
discovery image at URL http://park8.wakwak.com/~ngc/images/PSNinNGC2357.jpg.
The variable was designated PSN J07174570+2320406 when it was posted at the
Central Bureau's TOCP webpage and is here designated SN 2015I based on the
spectroscopic confirmation reported below.  Additional CCD magnitudes for
2015I:  May 2.804, 15.4 (G. Masi; remotely using a 43-cm telescope at
Ceccano, Italy; position end figures 45s.71, 40".9); 4.171, V = 15.21, I_c =
15.41 (S. Kiyota, Kamagaya, Japan; remotely with an iTelescope 0.61-m f/6.5
CDK astrograph + FLI PL09000 CCD camera at the Sierra Remote Observatory
near Auberry, CA, USA); 5.406, 15.4 (J. Brimacombe, Cairns, Australia;
position end figures 45s.71, 40".5; image posted at website URL
https://www.flickr.com/photos/43846774@N02/17180015357/); 9.519, I = 14.8 (Y.
Suzuki, T. Yusa, N. Wada, S. Tsunoda, and K. Chiba; 0.30-m reflector
+ SBIG ST-1001E camera at the Osaki Lifelong Learning Center; stacked 120-s
frames; limiting mag 16.5); 16.490, V = 14.3, R = 14.3 (Yusa, Suzuki, Wada,
Tsunoda, S. Sasaki, and H. Goto; as on May 9.519; color composite image posted
at URL http://space.geocities.jp/yusastar77/supernova/PSNinN2357_150509.htm);
16.740, 14.8 (P. Corelli, Pagnacco, Italy; 0.4-m telescope; image posted at
https://www.flickr.com/photos/52195265@N06/17599305678/in/dateposted-public/).
The type-IIP supernova 2010bj also appeared in NGC 2357 (cf. CBET 2230).

     T. Nakaoka, K. Takaki, M. Kawabata, and K. S. Kawabata, Hiroshima
University; and M. Yamanaka, Konan University, obtained a low-resolution
optical spectrum (range 450-900 nm) of PSN J07174570+2320406 = SN 2015I
on May 4.5 UT with the 1.5-m Kanata telescope (+ HOWPol) at Higashi-Hiroshima
Observatory.  The spectrum shows absorption lines of Fe II multiplets, Si II
635.5-nm, and the Ca II near-infraredtriplet, suggesting that this is a
type-Ia supernova in its early phase.  The absorption line of Si II 635.5-nm
is blueshifted by around 12000 km/s.  A comparison with a library of supernova
spectra using GELATO (Harutyunyan et al. 2008, A.Ap. 488, 383) reveals that
the spectrum gives a good match to normal type-Ia supernovae such as 2003du
at a few days before their B-band maximun.

     E. Karamehmetoglu, M. Messa, M. Hayes, and J. Sollerman, Department of
Astronomy, Stockholm University, report the spectroscopic confirmation of PSN
J07174570+2320406 = SN 2015I obtained with the Nordic Optical Telescope (NOT)
on May 5.88 UT, using ALFOSC and GRISM 4 (range 320-910 nm, 1.62-nm
resolution).  Cross-correlation against a library of template spectra with
SNID (Blondin and Tonry 2007, Ap.J. 666 1024) gives a best match to many
normal type-Ia supernovae at the redshift of the visible host galaxy (NGC
2357; z = 0.0076, obtained via the NASA/IPAC Extragalactic Database), at
phases a few days before maximum.  These observations were done remotely with
the aide of T. Kuutma.

     K. Ayani, Bisei Astronomical Observatory (BAO), Ibara, Okayama, Japan,
obtained a low-resolution spectrogram (range 400-800 nm; resolution 0.5 nm at
H-alpha) of PSN J07174570+2320406 = SN 2015I with the BAO 1.01-m telescope on
May 13.5 UT.  The spectrum is that of a type-Ia supernova around maximum,
which confirms the report by Karamehmetoglu et al. (above).  Cross-correlation
with a library of supernova spectra using the "SuperNova Identification" code
(SNID; Blondin and Tonry 2007, Ap.J. 666, 1024) shows that 2015I matches
best with SN 2003cg at two days before maximum.  Assuming the heliocentric
velocity for NGC 2357 of 2275 km/s (Giovanelli et al. 1997, A.J. 114, 122; via
NED), the blueshift of the Si II 635.5-nm absorption is about 11000 km/s.


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2015 CBAT
2015 May 25                      (CBET 4106)              Daniel W. E. Green