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ASAS-SN Discovery of A Possible Hostless Supernova or High Amplitude CV Outburst

ATel #7944; J. Brimacombe (Coral Towers Observatory), 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), E. Falco (CfA), P. R. Wozniak (LANL), G. Pojmanski (Warsaw University Observatory), I. Cruz (Cruz Observatory), S. Kiyota (Variable Star Observers League in Japan), G. Masi (Virtual Telescope Project, Ceccano, Italy)
on 23 Aug 2015; 20:01 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 #: 8011

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 supernova or cataclysmic variable star outburst, with no apparent host.

ASASSN-15ou was discovered in images obtained on UT 2015-08-23.31 at V~16.9 mag. We also detect the object in images obtained on UT 2015-08-19.27 (V~16.9), UT 2015-08-15.41 (V~16.9), and UT 2015-08-14.29 (V~17.0). We do not detect (V>17.2) the object in images taken on UT 2015-08-13.29 and before. An image obtained by J. Brimacombe on UT 2015-08-23.76 with the CDK 43-cm telescope at Savannah Skies Observatory confirms the discovery of the transient. This figure shows the archival DSS image of the host (left) and the J. Brimacombe confirmation image (right). The red circle has a radius of 5" and is centered on the position of the transient in the J. Brimacombe image.

There are no galaxies listed within an arcminute of ASASSN-15ou position in NED, nor are there any stellar sources near the location in Vizier. The relatively constant photometry would seem to indicate that ASASSN-15ou is a supernova, given the lack of a host a CV outburst is also a possible explanation. 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-15ou  03:27:25.95    -47:49:43.70      2015-08-23.31      16.9          N/A               N/A 
Obs. UT Date         V mag 
2015-08-13.29        >17.2 
2015-08-14.29         17.0 
2015-08-15.41         16.9 
2015-08-19.27         16.9 
2015-08-23.31         16.9 

Follow-up observations, especially spectroscopy, 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 and by the 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.