The Intermediate Palomar Transient Factory reported the discovery of an unusual type II-P supernova iPTF14hls. Instead of a ∼ 100-day plateau as observed for ordinary type II-P supernovae, the light curve of iPTF14hls has at least five distinct peaks, followed by a steep decline at ∼ 1000 days since discovery. Until 500 days since discovery, the effective temperature of iPTF14hls is roughly constant at 5000-6000 K. In this paper we propose that iPTF14hls is likely powered by intermittent fallback accretion. It is found that the light curve of iPTF14hls can be well fit by the usual t −5/3 accretion law until ∼ 1000 days post discovery when the light curve transitions to a steep decline. To account for this steep decline, we suggest a power-law density profile for the late accreted material, rather than the constant profile as appropriated for the t −5/3 accretion law. Detailed modeling indicates that the total fallback mass is ∼ 0.2M , with an ejecta mass M ej 21M . We find the third peak of the light curve cannot be well fit by the fallback model, indicating that there could be some extra rapid energy injection. We suggest that this extra energy injection may be a result of a magnetic outburst if the central object is a neutron star. These results indicate that the progenitor of iPTF14hls could be a massive red supergiant.

We present extensive ultraviolet (UV) and optical photometry, as well as dense optical spectroscopy for type II Plateau (IIP) supernova SN 2016X that exploded in the nearby (∼ 15 Mpc) spiral galaxy UGC 08041. The observations span the period from 2 to 180 days after the explosion; in particular, the Swift UV data probably captured the signature of shock breakout associated with the explosion of SN 2016X. It shows very strong UV emission during the first week after explosion, with contribution of ∼ 20 -30% to the bolometric luminosity (versus 15% for normal SNe IIP). Moreover, we found that this supernova has an unusually long rise time of about 12.6 ± 0.5 days in the R band (versus ∼ 7.0 days for typical SNe IIP). The optical light curves and spectral evolution are quite similar to the fast-declining type IIP object SN 2013ej, except that SN 2016X has a relatively brighter tail. Based on the evolution of photospheric temperature as inferred from the Swif t data in the early phase, we derive that the progenitor of SN 2016X has a radius of about 930 ± 70 R ⊙ . This large-size star is expected to be a red supergiant star with an initial mass of 19 -20 M ⊙ based on the mass −− radius relation of the Galactic red supergiants, and it represents one of the most largest and massive progenitors found for SNe IIP.

It is well-known that ordinary supernovae (SNe) are powered by 56 Ni cascade decay. Broad-lined type Ic SNe (SNe Ic-BL) are a subclass of SNe that are not all exclusively powered by 56 Ni decay. It was suggested that some SNe Ic-BL are powered by magnetar spin-down. iPTF16asu is a peculiar broad-lined type Ic supernova discovered by the intermediate Palomar Transient Factory. With a rest-frame rise time of only 4 days, iPTF16asu challenges the existing popular models, for example, the radioactive heating ( 56 Ni-only) and the magnetar+ 56 Ni models. Here we show that this rapid rise could be attributed to interaction between the SN ejecta and a preexisting circumstellar medium ejected by the progenitor during its final stages of evolution, while the late-time light curve can be better explained by energy input from a rapidly spinning magnetar. This model is a natural extension to the previous magnetar model. The mass-loss rate of the progenitor and ejecta mass are consistent with a progenitor that experienced a common envelope evolution in a binary. An alternative model for the early rapid rise of the light curve is the cooling of a shock propagating into an extended envelope of the progenitor. It is difficult at this stage to tell which model (interaction+magnetar+ 56 Ni or cooling+magnetar+ 56 Ni) is better for iPTF16asu. However, it is worth noting that the inferred envelope mass in the cooling+magnetar+ 56 Ni is very high.

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