Abstract:
We present an extensive, panchromatic photometric (UV, optical, and near-IR) and low-resolution optical
spectroscopic coverage of a Type IIP supernova SN 2018gj that occurred on the outskirts of the host galaxy
NGC 6217. From the V-band light curve, we estimate the plateau length to be ∼ 70 ± 2 days, placing it among the
very few well-sampled short plateau supernovae (SNe). With V-band peak absolute magnitude
MV −17.0 ± 0.1 mag, it falls in the middle of the luminosity distribution of the Type II SNe. The color
evolution is typical to other Type II SNe except for an early elbow-like feature in the evolution of V − R color
owing to its early transition from the plateau to the nebular phase. Using the expanding photospheric method, we
present an independent estimate of the distance to SN 2018gj. We report the spectral evolution to be typical of a
Type II SNe. However, we see a persistent blueshift in emission lines until the late nebular phase, not ordinarily
observed in Type II SNe. The amount of radioactive nickel (
56Ni) yield in the explosion was estimated to be
0.026 ± 0.007 Me. We infer from semianalytical modeling, nebular spectrum, and 1D hydrodynamical modeling
that the probable progenitor was a red supergiant with a zero-age-main-sequence mass 13 Me. In the simulated
hydrodynamical model light curves, reproducing the early optical bolometric light curve required an additional
radiation source, which could be the interaction with the proximal circumstellar matter.
