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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
Distributed as an Instant Email Notice Supernovae
Credential Certification: Subo Dong (dongsubo@pku.edu.cn)

Subjects: Supernovae

Referred to by ATel #: 7776, 7843, 8216, 8388

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.