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ASASSN-16oh: An Unusual Transient in the Vicinity of the SMC

ATel #9859; S. W. Jha (Rutgers), E. Romero Colmenero (SALT/SAAO), K. Z. Stanek, J. S. Brown, T. W.-S. Holoien, C. S. Kochanek, J. Shields, T. A. Thompson (Ohio State), B. J. Shappee (Hubble Fellow, Carnegie Observatories), J. L. Prieto (Diego Portales; MAS), D. Bersier (LJMU), Subo Dong, S. Bose, Ping Chen (KIAA-PKU), J. Brimacombe (Coral Towers Observatory)
on 14 Dec 2016; 17:11 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Krzysztof Stanek (stanek.32@osu.edu)

Subjects: Optical, Transient

Referred to by ATel #: 9866, 9867

During the ongoing All Sky Automated Survey for SuperNovae (ASAS-SN or "Assassin"), using data from the quadruple 14-cm "Cassius" telescope in Cerro Tololo, Chile, we discovered a new transient source, of unknown nature, in the vicinity of the Small Magellanic Cloud.

ASASSN-16oh (AT 2016irh) was discovered in images obtained on UT 2016-12-02.12 at V~15.7 mag, and detected in several epochs before and after (see the Table below). We do not detect (V>17.8) this object in images taken on UT 2016-11-20.07 and before (ASAS-SN has been observing this region of the sky since 2014-05-31). At the position of the transient Vizier reports a blue match, 0.9" away, with photographic USNO magnitudes B=20.4, R=20.1, and also a Gaia DR1 match with G=21.0.

Two spectra were obtained with SALT (+RSS) on UT 2016-Dec-07.8 (range 350-920 nm, spectral resolution R ~ 400) and UT 2016-Dec-13.8 (350-940 nm, R ~ 1000). These show a blue continuum with narrow (unresolved) emission lines of He II 4686, H-alpha, and weak He I 6678. Weak H-gamma, H-delta, and H-epsilon absorption lines are also present. These lines are consistently redshifted by +223 +/- 15 km/s in the 2016-Dec-13.8 spectrum. The lines show a higher redshift in the previous spectrum, but this may be an artifact of the lower spectral resolution. The spectra have been uploaded to the Transient Name Server AT 2016irh page.

The position of ASASSN-16oh is approximately 4.74 degrees from the center of the Small Magellanic Cloud, towards the LMC. If ASASSN-16oh is indeed associated with the SMC, its absolute V-band magnitude is approximately M_V=-2.1 (m-M=18.9, A_V=0.13). Properties of the new source and photometry are summarized in the tables below:

 
Object       RA (J2000)     DEC (J2000)      Disc. UT Date   Disc. Vmag 
ASASSN-16oh  01:57:43.64   -73:37:32.5       2016-12-02.15     16.9 
 
 
Obs. UT Date       V mag sig_V 
2016-11-20.07     >17.9 
2016-11-24.16      17.2  0.12 
2016-11-28.17      17.0  0.11 
2016-11-29.02      17.1  0.18 
2016-12-02.15      16.9  0.11 
2016-12-07.09      16.9  0.12 
Follow-up observations are encouraged.

While we are participating in the TNS system to minimize potential confusion, ASAS-SN will continue using ASASSN-16xx transient names as our primary nomenclature (including supernovae, but also other classes of transients), and we encourage others to do the same. We prefer merging the names as ASASSN-16xx (AT2016xyz) to preserve, rather than anonymize, the origin of the transient.

We thank Las Cumbres Observatory and its staff for their continued support of ASAS-SN. ASAS-SN is supported by NSF grant AST-1515927, the Center for Cosmology and AstroParticle Physics (CCAPP) at OSU, the Chinese Academy of Sciences South America Center for Astronomy (CASSACA), and the Mt. Cuba Astronomical Foundation.

TNS page for ASASSN-16oh/AT 2016irh