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ASASSN-17bw is a Type II Supernova in SBS 1657+505

ATel #10135; J. Brimacombe (Coral Towers Observatory), S. Kiyota (Variable Star Observers League in Japan), R. S. Post (Post Astronomy), J. Klusmeyer (MDM Observatory), T. W.-S. Holoien (Ohio State), J. L. Prieto (Diego Portales; MAS), K. Z. Stanek, C. S. Kochanek, J. S. Brown, J. Shields, T. A. Thompson (Ohio State), B. J. Shappee (Hubble Fellow, Carnegie Observatories), D. Bersier (LJMU), Subo Dong, S. Bose, Ping Chen (KIAA-PKU), J. M. Fernandez (Observatory Inmaculada del Molino), G. Krannich (Roof Observatory Kaufering), G. Masi (Virtual Telescope Project, Ceccano, Italy)
on 27 Feb 2017; 18:17 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Thomas Holoien (tholoien@astronomy.ohio-state.edu)

Subjects: Optical, Supernovae, Transient

The transient source ASASSN-17bw (AT 2017zu) in the galaxy SBS 1657+505 was discovered by the All Sky Automated Survey for SuperNovae (ASAS-SN, Shappee et al. 2014), using data from the quadruple 14-cm "Brutus" telescope in Haleakala, Hawaii. The transient was subsequently classified with a low-resolution spectrum as an AGN, but a recently obtained spectrum reveals it is a Type II supernova.

ASASSN-17bw was discovered in images obtained on UT 2017-02-01.66 at V~17.0 mag. We also detect the object in images obtained on UT 2017-02-23.59 (V~17.2), UT 2017-02-19.61 (V~17.4), UT 2017-02-15.60 (V~17.0), UT 2017-02-14.60 (V~17.1), and UT 2017-01-25.66 (V~17.6). We do not detect (V>17.5) the object in images taken on UT 2017-01-20.67 and before. An image obtained on 2017-02-02 by S. Kiyota confirms the discovery of the transient. This figure shows the archival Pan-STARRS (Chambers et al. 2016, arXiv:1612.05560) g-band 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-17bw is approximately 0.2" South and 0.2" West from the center of the galaxy SBS 1657+505 (z=0.010200, d=41.9 Mpc, via NED), giving an absolute V-band magnitude at discovery of approximately -16.2 (m-M=33.11, A_V=0.059). 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-17bw  16:58:37.69   +50:29:26.50     2017-02-01.66      17.0          -16.2               0.28 
Obs. UT Date         V mag 
2017-01-20.67        >17.5 
2017-01-25.66         17.6 
2017-02-01.66         17.0 
2017-02-14.60         17.1 
2017-02-15.60         17.0 
2017-02-19.61         17.4 
2017-02-23.59         17.2 

We obtained an optical spectrum of ASASSN-17bw on UT 2017 Feb. 24.47 with OSMOS (range 4000-6850 Angstroms, R ~ 2000) mounted on the MDM 2.4m telescope at KPNO. The spectrum shows strong Balmer and Fe II lines with P-Cygni profiles characteristic of a Type II SN, with strong host galaxy contamination (including nebular emission lines). After correcting the spectrum for the redshift of the host galaxy, we measure an expansion velocity from the H-alpha line of 5800 km/s from the minimum of the P-Cygni trough.

While we are participating in the TNS system to minimize potential confusion, ASAS-SN will continue using ASASSN-17xx 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-17xx (AT 2017xyz) 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 funded in part by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University, 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. For more information about the ASAS-SN project, see the ASAS-SN Homepage and the list of all ASAS-SN transients.