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ASAS-SN Discovery of A Probable Supernova in GALEXASC J205408.07-551628.8

ATel #7941; S. Kiyota (Variable Star Observers League in Japan), T. W.-S. Holoien, K. Z. Stanek, C. S. Kochanek, J. S. Brown, 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), Subo Dong (KIAA-PKU), J. Brimacombe (Coral Towers Observatory), E. Falco (CfA), P. R. Wozniak (LANL), G. Pojmanski (Warsaw University Observatory), I. Cruz (Cruz Observatory), G. Masi (Virtual Telescope Project, Ceccano, Italy)
on 22 Aug 2015; 17:21 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 #: 7997

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, most likely a supernova, in the galaxy GALEXASC J205408.07-551628.8.

ASASSN-15os was discovered in images obtained on UT 2015-08-22.12 at V~17.1 mag. We also detect the object in images obtained on UT 2015-08-18.15 (V~17.2). We do not detect (V>17.4) the object in images taken on UT 2015-08-14.11 and before. An image obtained by S. Kiyota on UT 2015-08-22.55 using a 0.5-m CDK + FLI PL-9000 at the ITelescope.NET site at Siding Springs Observatory confirms the discovery of the transient. This figure shows the archival DSS image of the host (left) and the S. Kiyota confirmation image (right). The red circle has a radius of 5" and is centered on the position of the transient in the S. Kiyota image.

The position of ASASSN-15os is approximately 0.3" South and 0.2" West from the center of the galaxy GALEXASC J205408.07-551628.8, which has no available redshift in NED. 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-15os  20:54:08.04    -55:16:28.91      2015-08-22.12      17.1          N/A               0.36 
Obs. UT Date         V mag 
2015-08-14.11        >17.4 
2015-08-18.15         17.2 
2015-08-22.12         17.1 

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 NSF grant AST-1515927. 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.