[IAUC] CBET 876: 20070312 : SUPERNOVA 1996cr IN ESO 97-G13

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                                                  Electronic Telegram No. 876
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
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SUPERNOVA 1996cr IN ESO 97-G13
     F. Bauer, Columbia Astrophysics Laboratory, reports the discovery of
an apparent type-IIn supernova in ESO 97-G13 (the "Circinus galaxy"), the
position of 1996cr given as R.A. = 14h13m10s.01, Decl. = -65o20'44".4
(equinox 2000.0), which is about 23" south of the center of the galaxy.
SN 1996cr was originally identified as a variable x-ray source but later
discovered to be a young supernova candidate via archival optical and
radio data.  The x-ray object that led to the discovery of the optical
images of 1996cr was designated CG X-2 in a paper by Sambruna et al. (2001,
Ap.J. 546, L13) and later identified as a potential young supernova by
Bauer et al. (2001, A.J. 122, 182); both of these papers rely on Chandra
HETGS data taken on 2000 June 15.  Smith and Wilson (2001, Ap.J. 557, 180)
later identified the same source in earlier Chandra ACIS-S observations
taken on 2000 Mar. 14.  CG X-2 was not detected in any of the five
previous ROSAT HRI observations (thus, it was at least 30 times fainter in
the 0.5- to 2.0-keV band prior to 2000).  Notably, the x-ray spectrum of
CG X-2 was unlike most ultraluminous x-ray sources, showing a thermal
spectrum of about 9-10 keV and a broad emission complex with a high
equivalent width of about 6.6-7.0 keV; such spectral features are more
typical of type-II x-ray-emitting supernovae.
     The archival discovery CCD images were taken with the Anglo-Australian
Telescope (+ TAURUS); the raw data were obtained through the AAT archive
website.  The observations that best capture the explosion date are
several narrow-band-filtered images centered on typical emission lines of
He II, [O III], and [S II] lines in the rest-frame of the Circinus galaxy
taken in March 1996; photometric calibrations were derived by matching each
AAT image against photometry measured by Bauer et al. 2001 (Ap.J. 122, 182)
with band conversions calculated using SYNPHOT.  Measured narrow-band and
broadband magnitudes -- corresponding to the He II, [O III], H-alpha + [N
II], and S[II] emission lines -- for 1996cr (including 2-sigma limiting
magnitudes from earlier AAT images), are as follows:

DATE      UT       FILTER (BANDWIDTH)     MAGNITUDE
1995 Feb. 21.64      501 nm (3 nm)         [22.0
     Feb. 22.65      659 nm (4)            [18.3
     Feb. 28.8       662 nm (5)            [20.3
     Feb. 28.73       R                    [19.6
     Feb. 28.74       B                    [22.1
     Feb. 28.8       659 nm (4)            [18.1
1996 Mar. 16.710     469 nm (3)             19.5 +/- 0.6
     Mar. 16.728     673 nm (3)             15.8 +/- 0.5
     Mar. 17.8       469 nm (3)             19.8 +/- 0.5
     Mar. 19.73      501 nm (2)             18.7 +/- 0.4
     Mar. 19.738     673 nm (3)             15.4 +/- 0.3
     Mar. 19.755     469 nm (3)             19.7 +/- 0.3

The position of 1996cr coincides with a faint H II region.
     A flux-calibrated reduced spectrogram (range 300-850 nm; four 1200-s
exposures), obtained of the region around 1996cr with the Very Large
Telescope (+ FORS1) in 0".7 seeing on 2006 Jan. 26.30, shows narrow
emission lines (most notably strong H_alpha) superimposed on classic broad,
asymmetric [O III], [O I], and [Ca II] blended emission profiles --
characteristic features of late-time type-IIn supernovae.  A deblended
fit to the narrow H_alpha-and-[N II] emission blend gives a line width
of about 700 km/s, with a small broader H_alpha base remaining unfitted;
the various forbidden lines in the spectrum have similar line widths.
     Radio data published by Elmouttie et al. (1998, MNRAS 297, 1202), and
additional radio images at 3 and 6 cm (comprised of data taken between
1994 and 1996; Elmouttie, private communication), confirm that 1996cr has
a faint (about 1- to 2-mJy) radio counterpart.  From a radio light curve
constructed by Bauer from ATCA-archive data, 1996cr begins to peak up above
the noise by 1996 Dec. 5 (about 1-2 mJy).  From the available data, it is
unclear whether the peak of the radio emission has been reached yet; as of
2004 Apr. 2, 1996cr had the following radio-flux densities:  8.6 GHz, 162
mJy; 4.8 GHz, 240 mJy; 2.4 GHz, 364 mJy; 1.4 GHz, 481 mJy.
     The explosion date for 1996cr is constrained by these data to lie
between 1995 Feb. 28 and 1996 Mar. 16.


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2007 March 12                    (CBET 876)               Daniel W. E. Green