Electronic Telegram No. 3560
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 2013dg
     A. J. Drake, S. G. Djorgovski, M.J. Graham, A. Mahabal, and R. Williams,
California Institute of Technology; J. L. Prieto, Princeton University; 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 an apparent supernova in
unfiltered Mount Lemmon Survey (MLS) images:

    SN       2013 UT       R.A. (2000.0) Decl.      Mag.
    2013dg   May 17.26   13 18 41.35  - 7 04 43.0   19.5

Further CCD magnitudes for 2013dg from the MLS and the Catalina Sky Survey
(CSS):  Apr. 29.27 UT, [21.3 (MLS); May 30.18, 19.0 (CSS); June 1.20, 19.0
(MLS); June 6.24, 19.0 (CSS); June 13.22, 19.1 (MLS).  Archival NOAO Deep
Ecliptic Survey images reaching a 5-sigma VR magnitude limit of 24 reveal
the presence of a faint source at the location of 2013dg.

     S. J. Smartt, M. Nicholl, C. Inserra, D. Wright, and T.-W. Chen,
Queen's University, Belfast; and Andy Lawrence and Alex Mead, University of
Edinburgh, report that a spectrogram (range 330-975 nm; resolution about 1000)
of 2013dg was obtained on June 10.96 UT with the 4.2-m William Herschel
Telescope (+ ISIS).  The spectrum has a blue continuum with several broad
absorption features.  Narrow absorption from the Mg II H and K resonance
doublet (279.55/280.27 nm), arising either in the host-galaxy interstellar
medium or in the intervening intergalactic medium, is detected at redshift
of z = 0.1918 +/- 0.002.  At this redshift of z about 0.2, the blue continuum
and broad absorption resemble that of SN 2010gx (Pastorello et al. 2010,
Ap.J. 724, L16; Quimby et al. 2011, Nature 474, 487) and the object
designated PTF09cnd by Quimby et al. (2011, op.cit.) around peak,  but an
exact redshift of z = 0.26 is required to match the restframe broad
absorption of the photospheric O II and Mg II lines.  This discrepancy could
either be due to the narrow Mg II absorption arising, not in the host galaxy,
but in lower-redshift intervening material or due to 2013dg showing different
ionic species in absorption.  The absolute B-band magnitude of the supernova
would be approximately in the range -21 to -22 in the restframe, depending
on the source redshift.  In conclusion, this appears to be a super-luminous
supernova of type Ic (Gal-Yam 2012, Science 337, 927; Inserra et al. 2013,
Ap.J. 770, 128) at or around peak magnitude.  The discrepancy between the
redshift derived from the interstellar-medium features and the supernova's
photospheric absorption requires further work to resolve.  No host galaxy is
visible in the CSS or MLS deeper reference images, and no narrow host- galaxy
emission lines are observed in the WHT spectrum (e.g., H-alpha, H-beta, or
[O III], [O II]).


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

                         (C) Copyright 2013 CBAT
2013 June 15                     (CBET 3560)              Daniel W. E. Green