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ASAS-SN Discovery of A Possible Superluminous Supernova in ESO 384-G055

ATel #7583; T. W.-S. Holoien (Ohio State), Subo Dong (KIAA-PKU), K. Z. Stanek, C. S. Kochanek, J. S. Brown, A. B. Danilet, G. Simonian, U. Basu, J. F. Beacom, T. A. Thompson (Ohio State), B. J. Shappee (Hubble Fellow, Carnegie Observatories), J. L. Prieto (Diego Portales; MAS), D. Bersier (LJMU), J. Brimacombe (Coral Towers Observatory), E. Falco (CfA), P. R. Wozniak (LANL), D. Szczygiel, G. Pojmanski (Warsaw University Observatory)
on 2 Jun 2015; 17:38 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Thomas Holoien (tholoien@astronomy.ohio-state.edu)

Subjects: Optical, Supernovae, Transient

Referred to by ATel #: 7600

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, we discovered a new transient source, possibly a superluminous supernova, in the galaxy ESO 384-G055.

ASASSN-15ki was discovered in images obtained on UT 2015-06-02.19 at V~15.2 mag. We do not detect (V>16.5) the object in images taken on UT 2015-05-31.17 and before. An image obtained on UT 2015-06-02.42 with the LCOGT 1-m robotic telescope at Siding Spring Observatory confirms the discovery of the transient. This figure shows the archival DSS image of the host (left) and the LCOGT g-band confirmation image (right). The red circle has a radius of 3" and is centered on the position of the transient in the LCOGT image.

The position of ASASSN-15ki is approximately 2.7" North and 3.7" West from the center of the galaxy ESO 384-G055 (z=0.038070, d=164 Mpc, via NED), giving an absolute V-band magnitude of approximately -21.1 (m-M=36.08, A_V=0.211). PSF photometry on the LCOGT follow-up image gives a g-band magnitude of 15.6, corresponding to an absolute g-band magnitude of approximately -20.7 (A_g=0.254). We note that the transient is approximately co-incident with a bright source in the host galaxy, but given the flux ratio between the host nucleus and the location of the transient in the follow-up image, we believe a new source is present. Properties of the new source and photometry are summarized in the tables below:

Object       RA (J2000)     DEC (J2000)      Disc. UT Date   Disc. V mag  Approx. Abs. Mag   Offset from Host (") 
ASASSN-15ki  14:07:40.28   -37:17:06.82      2015-06-02.19      15.2          -21.1               4.58 
Obs. UT Date          Mag        Filter 
2015-05-31.17        >16.5	ASAS-SN V 
2015-06-02.19         15.2      ASAS-SN V 
2015-06-02.42         15.6       LCOGT g 

Follow-up observations are encouraged.

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.