[IAUC] CBET 4229: 20151222 : SUPERNOVA 2015bh IN NGC 2770 = PSN J09093496+3307204 = PSN J09093506+3307221

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                                                  Electronic Telegram No. 4229
Central Bureau for Astronomical Telegrams
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SUPERNOVA 2015bh IN NGC 2770 = PSN J09093496+3307204 = PSN J09093506+3307221
     S. Howerton, Arkansas City, KS, USA; A. J. Drake, S. G. Djorgovski, A.
Mahabal, M. J. Graham, and R. Williams, California Institute of Technology; M.
Catelan, Pontificia Universidad Catolica de Chile; and E. Christensen and
S. M. Larson, Lunar and Planetary Laboratory, University of Arizona, report
the Catalina Real-time Transient Survey discovery of a transient source in
public images from the Catalina Sky Survey (CSS).

 SN       2015 UT       R.A. (2000.0) Decl.      Mag.      Offset
 2015bh   Feb. 7.39    9 09 34.96  +33 07 20.4   19.1    15".8 E, 4".4 S

The variable was designated PSN J09093496+3307204 when it was posted at the
Central Bureau's TOCP webpage and is here designated SN 2015bh based on the
spectroscopic confirmation reported below.  An independent discovery was
reported by Z.-j. Xu (Nanjing, Jiangsu, China) and X. Gao (Urumqi, Xinjiang,
China), with the object reported at mag approximately 17.8 on one 40-s survey
image (limiting mag about 18.0) taken by Gao in the course of the Xingming
Sky Survey on Apr. 7.737 UT with a Celestron C14 Schmidt-Cassegrain
telescope; the position end figures were measured to be 35s.03 +/- 0s.03,
21".9 +/- 0".3 (with offset 15".2 east and 2".6 south of the center of NGC
2770, and when they posted the object as a new discovery on the TOCP, it was
automatically assigned the designation PSN J09093496+3307204.  Additional
CCD magnitudes for 2015bh:  1998 Apr. 21, [19.6 (Gao; Digitized Sky Survey);
2014 Dec. 15, [21.0 (CSS); 24.32, 20.3 (CSS); 2015 Jan. 21.35, 19.5 (CSS);
22.38, 19.5 (CSS); Feb. 8.144, 19.9 (Howerton; 0.28-m f/5 reflector; image
posted at URL https://www.flickr.com/photos/watchingthesky/15846899274);
17.825, 19.6 (G. Masi; remotely using a 43-cm telescope at Ceccano, Italy;
position end figures 35s.11, 21".5); Mar. 22, [19.0 (Gao); 24, [18.7 (Gao);
Apr. 9.716, 18.0 (Gao); 9.717, 18.3 (Gao); May 16.928, 16.0 (R. Arbour,
South Wonston, U.K.; position end figures 35s.11, 22".2).  The type-I
supernovae 1999eh (cf. IAUC 7282) and 2007uy (IAUC 8908) also appeared in
NGC 2770.

     A. de Ugarte Postigo, Instituto de Astrofisica de Andalucia, Consejo
Superior de Investigaciones Cientificas (CSIC), Granada, Spain, and Dark
Cosmology Centre, Niels Bohr Institute, Copenhagen; C. C. Thoene, Instituto
de Astrofisica de Andalucia, CSIC; G. Leloudas, Weizmann Institute of Science,
Rehovot, Israel, and Niels Bohr Institute, Copenhagen; and F. Aceituno,
Instituto de Astrofisica de Andalucia, CSIC, report that they found PSN
J09093496+3307204 = PSN J09093506+3307221 = SN 2015bh to be unexpectedly
bright at magnitude R = 16.0 on May 15.865 UT in images obtained with the
0.9-m telescope at Sierra Nevada Observatory (Granada, Spain), the object's
position end figures being measured as 35s.12, 21".3 (offset 16" east, 4"
south of the center of the galaxy NGC 2770); an image taken on May 6 with
the 1.5-m telescope at Sierra Nevada Observatory showed the object fainter
at R = 18.5.  After the object had been first announced by Howerton, Drake et
al. (see above), it was determined to be an eruption of a luminous blue
variable (e.g., see Elias-Rosa et al. 2015, as posted at website URL
http://www.astronomerstelegram.org/?read=7042; and de Ugarte Postigo et al.
2015, website URL http://www.astronomerstelegram.org/?read=7409).  The
absolute magnitude of the object corresponds to -16.6, assuming a distance
of 27 Mpc for NGC 2770; this brightness is unusually luminous for an LBV in
outburst but is fully compatible with supernova explosions.  An optical
spectrogram (resolution 1000; wavelength range 360-1000 nm) was obtained on
May 16.89 UT with the Gran Telescopio Canarias at La Palma.  The spectrum is
typical of type-IIn supernovae, demonstrating prominent Balmer lines with a
narrow component superimposed on a broader asymmetric component.  The full-
width-at-half-maximum of the H-alpha line is about 1800 km/s.  He I lines
(587.6-, 667.8-, and 706.5-nm) are also detected in emission, and the
strongest line (He I 587.6-nm) clearly shows a double component with a
narrow line superimposed on a broader base.  On the blue side of H-beta,
they observe a feature that they identify as He II 468.6-nm, a line that is
often observed in spectra of supernovae, obtained early after explosion, when
the temperature is high.  All features are superimposed on a blue continuum.
It is therefore quite possible from this discovery that the LBV has evolved
into a supernova.


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                         (C) Copyright 2015 CBAT
2015 December 22                 (CBET 4229)              Daniel W. E. Green