Photometric and Spectroscopic Observations of Gaia16cfr/AT 2016jbu: A Probable Supernova Impostor in NGC 2442

ATel #9937; S. Bose (KIAA-PKU), L. A. G. Monard (Klein Karoo Observatory), M. K. Seidel (Carnegie Observatories), S. Dong (KIAA-PKU), E. Hsiao (FSU), B. J. Shappee (Hubble Fellow, Carnegie Observatories), H. E. Bond (PSU), J. L. Prieto (UDP), N. Morrell, M. Phillips (Las Campanas Observatory)
on 4 Jan 2017; 18:38 UT
Credential Certification: Subhash Bose (bose@pku.edu.cn)

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Gaia16cfr/AT 2016jbu was discovered by Gaia Alerts at 19.6 mag on UT 2016-12-01 (http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia16cfr/). This transient in NGC 2442 was also found by Berto Monard at 17.8 +/- 0.2 mag (clear filter) on UT 2016-12-22.85, and the photometric observations by Monard from 2016-12-22 to 2017-01-01 are listed below:

 
Time (UT)            Mag (clear filter) 
--------------------------------------- 
2016-12-22.85        17.8 +/- 0.2 
2016-12-30.88        17.7 +/- 0.1 
2016-12-31.96        17.7 +/- 0.1 
2017-01-01.87        17.4 +/- 0.2 

The distance NGC 2442 (z = 0.0049) is estimated as 23.9 +/- 0.4 Mpc using SN Ia 2015F (Im et al., 2015), and the absolute magnitude of Gaia16cfr/AT 2016jbu on 2017-01-01.87 is thus about -14.5 mag without taking into account of extinction (the Galactic extinction is about A_R ~ 0.4 mag according to NED). HST archival image shows that the transient is spatially coincident with a young association in the host galaxy.

We report optical spectroscopic observation of Gaia16cfr/AT 2016jbu using the du Pont 2.5-m telescope (+ WFCCD) at Las Campanas Observatory on UT 2016-12-31.32. The transient's spectrum shows almost flat continuum with prominent narrow P-Cygni Balmer lines and relatively weaker FeII triplet (4924, 5018, 5169 \AA; rest wavelength). The peaks of the emission lines are consistent with host redshift, z=0.0049. The expansion velocities estimated from absorption troughs of these P-Cygni profiles are ~700 km/s. An absorption line is visible near 5909 \AA which is most likely absorption from NaID P-Cygni profile at similar velocity but with significantly weaker emission counterpart.

We also report infrared spectrum observed on UT 2017-01-02.29 using FIRE spectrometer (0.8-2.5 micron) mounted on 6.5-m Magellan telescope. Paschen beta, gamma and delta emission lines are visible in the spectrum, where P-Cyni profile is clearly detectable for Paschen beta and gamma lines.

The spectral features, expansion velocity and absolute magnitude of Gaia16cfr/AT 2016jbu is consistent with a supernova impostor (see, e.g., Smith et al., 2011).