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THE ASTRONOMER Electronic Circular No 3089     2015 Jly 09 20.05UT
Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise,  Basingstoke,
Hants, RG22 4PP,England.Telephone/FAX(01256)471074Int:+441256471074
INTERNET: GUY@TAHQ.DEMON.CO.UK      Backup: gmh@wdcc1.bnsc.rl.ac.uk
WORLD WIDE WEB                         http://www.theastronomer.org
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ASASSN-15lh: MOST LUMINOUS SUPERNOVA EVER DISCOVERED
B. Nicholls, Mt Vernon Obs, New Zealand et. al. relay on ATEL 7642
that during the ongoing All Sky Automated Survey for SuperNovae (ASAS-SN or
"Assassin"), using data from the double 14-cm "Cassius" telescope in Cerro
Tololo, Chile, they discovered a new transient source, most likely a
supernova, in the galaxy APMUKS(BJ) B215839.70-615403.9. 

ASASSN-15lh was discovered in images obtained on UT 2015 June 14.25UT at
V~17.0 mag. They did not detect (V>17.3) the object in images obtained on UT
2015 May 15.33 and before. The position of ASASSN-15lh is approximately
0.04" South and 0.41" East from the centre of the galaxy APMUKS(BJ)
B215839.70-615403.9, which has no redshift available in NED. Properties of
the new source and photometry are summarized in the tables below: 

Object       RA (J2000)     DEC (J2000)      Disc. UT Date   Disc. 
ASASSN-15lh  22:02:15.45   -61:39:34.64      2015-06-14.25   17.0  


Subo Dong (KIAA-PKU) et. al. added on ATEL #7774 that spectroscopic
follow-up of ASASSN-15lh was obtained on June 21 UT with the du Pont 2.5-m
telescope (+ WFCCD) at Las Campanas Observatory, on June 30 UT and July 7 UT
with the SALT 10-m telescope (+RSS), on June 24th with SOAR 4-m telescope
(+Goodman) at Cerro Pachon, and on July 6 UT with the Clay 6.5-m telescope
(+ MagE) at Las Campanas Observatory. All spectra reveal very blue, mostly
featureless continuum, except for broad (OII) absorption features
characteristic of hydrogen-poor super-luminous supernovae -- SLSN-I
(prototype SN 2005ap, Quimby et al., 2007, ApJ 668, L99). 

They also detect narrow Mg II (rest 279.5, 280.2 nm) absorption in the June
30 SALT spectrum with spectral resolution R = 800, subsequently confirmed by
higher spectral resolution Clay (+MagE) spectroscopy on July 6 and R=2500
SALT (+RSS) spectroscopy on July 7.0 UT. They all yield a consistent
redshift for the supernova host galaxy of z = 0.2326 (luminosity distance of
1170.1 Mpc). Presence of a very blue, slowly evolving transient is also
confirmed by our ongoing photometric follow-up in BVRI from LCOGT 1-m
robotic telescopes and in optical/UV with SWIFT UVOT/XRT. 

At redshift z = 0.2326, their follow-up observations give an absolute
magnitude in u(AB) of -23.5 and bolometric luminosity ~2.2x10^45 erg/s,
making ASASSN-15lh the most luminous supernova ever discovered.

Guy M Hurst