Follow-up observations of ASASSN-15lh establish it as the most luminous supernova ever discovered
ATel #7774; Subo Dong (KIAA-PKU), B. J. Shappee (Hubble Fellow, Carnegie Observatories), J. L. Prieto (Diego Portales; MAS), S. W. Jha (Rutgers), K. Z. Stanek, T. W.-S. Holoien, C. S. Kochanek, T. A. Thompson (Ohio State), N. Morell, I. B. Thompson (Carnegie Observatories), F. Olivares, G. Pignata (Andres Bello; MAS)
on 8 Jul 2015; 14:01 UT
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ASASSN-15lh was reported in ATel #7642 as a probable supernova in an
unknown redshift galaxy APMUKS(BJ) B215839.70-615403.9. Detailed
analysis of ASAS-SN data reveal that it peaked around June 5, 2015.
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).
We 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, our 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.
Details of the observations will be presented in an upcoming paper
(Dong et al. 2015, in prep.).
Further multi-wavelength observations of this unique event are encouraged.
We thank the SALT staff, in particular, Brent Miszalski, Paul Kotze,
and Eric Depagne, for assistance with the observations.
We thank LCOGT and its staff for their continued support of ASAS-SN.
ASAS-SN is supported in part by Mt. Cuba Astronomical Foundation. For
more information about the ASAS-SN project, see the ASAS-SN Homepage
and the list of all ASAS-SN transients.