ASAS-SN Discovery of a Probable Nova in M33

ATel #8349; S. Kiyota (Variable Star Observers League in Japan), J. L. Prieto (Diego Portales; MAS), J. S. Brown, T. W.-S. Holoien, K. Z. Stanek, C. S. Kochanek, D. Godoy-Rivera, U. Basu (Ohio State), B. J. Shappee (Hubble Fellow, Carnegie Observatories), D. Bersier (LJMU), Subo Dong, Ping Chen (KIAA-PKU), J. Brimacombe (Coral Towers Observatory), I. Cruz (Cruz Observatory)

on 1 Dec 2015; 14:56 UT

Credential Certification: Jose L. Prieto (jose.prietok@mail.udp.cl)

Subjects: Optical, Nova, Transient

Referred to by ATel #: 8352, 8396, 8416

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During the ongoing All Sky Automated Survey for SuperNovae (ASAS-SN or "Assassin"), using data from the quadruple 14-cm "Brutus" telescope in Haleakala, Hawaii, we discovered a new transient source, most likely a nova, in the nearby galaxy M33.

ASASSN-15th was discovered in images obtained on UT 2015-12-01.40 at V~16.5 mag. We do not detect (V>17.1) the object in images taken on UT 2015-11-30.41 and before. An image obtained by S. Kiyota on UT 2015-12-01.51 with a 25-cm telescope at Kamagaya, Japan, confirms the discovery of the transient. This figure shows the archival SDSS g-band archival image (left) and the S. Kiyota V-band confirmation image (right). The red circle has a radius of 15" and is centered on the position of the transient in the S. Kiyota V-band confirmation image.

The position of ASASSN-15th is approximately 765.6" South and 46.8" West from the center of M33 (d=964 kpc, Bonanos et al. 2006, ApJ, 652, 313), giving an absolute V-band magnitude of approximately -8.5 (m-M=24.92, A_V=0.11 mag). This makes it consistent with a possible nova in M33. 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-15th 01:33:47.27 +30:26:50.7 2015-12-01.40 16.5 -8.5 767.1

Obs. UT Date V mag 2015-11-30.41 >17.1 2015-12-01.40 16.5

Follow-up observations are encouraged.

We thank LCOGT and its staff for their continued support of ASAS-SN. ASAS-SN is supported by NSF grant AST-1515927, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics (CCAPP) at OSU, and the Robert Martin Ayers Sciences Fund. For more information about the ASAS-SN project, see the ASAS-SN Homepage and the list of all ASAS-SN transients.