# New Clues to the Power Sources of Hydrogen-poor Superluminous Supernovae

Abstract
slugcomment: To be submitted to the Astrophysical Journal Letter

It is not completely understood what powers the extremely large energy output from a Superluminous Supernova (SLSN). However, with increasing number of newly discovered SLSNe, and in particular, with better coverage of bolometric light curves and spectroscopy from early to late times, new insights have been obtained regarding this unsolved problem. This paper presents the observations of iPTF15esb, which is a hydrogen-poor SLSN discovered by intermediate Palomar Transient Factory at $$z$$ =0.224. iPTF15esb has three unique features. Its multi-band light curves (LC) show several bumps, unlike a typical smooth rise-and-decline SLSN LC. The LC undulation is most pronounced at the shorter wavelength. In addition, its LC also has a plateau lasting $$\sim$$40 days, somewhat similar to that of a SN IIP. The third intriguing feature is the detection of a strong, broad H$$\alpha$$ emission at $$\sim$$70 days from the primary peak, similar to what has been seen in SLSN-I iPTF13ehe. iPTF15esb

After that, it undergoes a rapid decline with $$L_{bol}\sim t^{-2.5}$$

, have two prominent bumps and a plateau with a duration of $$\sim$$40 days, similar to that of a SN IIP. The LC undulation is most pronounced at the shorter wavelength. The third intriguing feature is the detection of a strong, broad H$$\alpha$$ emission at $$\sim$$70 days from the primary LC peak.

reports the observations of iPTF15esb at $$z$$ = 0.224, which reveal three unique characteristics

The multi-band light curves reveal several prominent bumps on top of a general trend of rising and declining. These bumps are most pronounced at shorter wavelength. In addition, late-time spectra display a strong, broad H$$\alpha$$ emission, similar to iPTF13ehe. These new features observed in iPTF15esb could be explained by three different models, (1) shell-shell collisions or ejecta-CSM (H-poor) interaction, (2) changing of opacity, (3) fall-back accretion of a central blackhole with varying fall-back/accretion rates…

Stars: supernova, massive stars
slugcomment: To be submitted to the Astrophysical Journal Letter
slugcomment: To be submitted to the Astrophysical Journal Letter

## Introduction

Superluminous supernovae (SLSNe) are stellar explosions which are about a factor of $$(10-100)$$ more luminous than a normal type I or type II supernova. The enormous radiative energy ($$10^{51-52}$$ erg) produced by very long rise time scales of $$\sim 40-100$$ days and extremely high peak luminosities of $$\sim 10^{44-45}$$ erg/s can not be explained by the standard supernova models. Ten years after the first discovery of SLSNe, although we do not yet have a clear and definitive picture of what powers SLSNe, several possible scenarios have been proposed. This includes (1) electron-positron pair-instability supernova (PISN), or a similar but less energetic one — pulsational pair-instability supernova (PPSN) from extreme metal poor massive stars; (2) central power source due to spin-down of a magnetar or fall-back accretion of a blackhole; (3) ejecta-circumstellar medium (CSM) interaction where CSM can be either H-rich or H-poor.

Similar to a normal SN, a SLSN is classified into a Type I or a Type II by the absence or the presence of hydrogen features in the spectra taken during photospheric phase. The underlying physical difference between these two types of events is whether H-rich material exists or not before the supernova explosion. A more significant implication is that progenitor stars of SLSNe-I and II must have very different progenitor star mass loss histories. For SLSN-II, one major power source is ejecta-CSM (H-rich) interaction where the kinetic energy from fast moving ejecta gets efficiently converted into thermal radiative emission. This is supported by the detections of both narrow and broad H$$\alpha$$ emission lines in early-time SLSN-II spectra. Here the narrow components ($$\sim 100$$ km/s) indicate the hydrogen recombination in the outer layers of ionized medium moving at low velocities, similar to a mass loss wind speed from massive stars.

For SLSN-I, the situation seems much less clear. Amongst all of the observational evidence, there is no smoking gun which clearly identifies one physical mechanism — either PISN, or magnetar or interaction model — as the power source for all SLSNe-I. It is not clear if all SLSNe-I have uniform observed characteristics which could be explained by a single physical model. Theoretically, this does not have to be the case, and there may be various types of power sources. Observationally, a better characterization of a large sample of SLSNe-I would identify generic features versus unique properties. With the increasing number of newly discovered SLSNe-I, more efforts are now focused on getting observations at both very early and late times, as well as complete phase coverage. These efforts have led to discoveries of new features for SLSNe-I, including double peaks at early times (Nicholl et al., 2015; Vreeswijk et al., 2016; Smith et al., 2016) and H$$\alpha$$ emission appearing in late-time spectra of a SLSN classified as an hydrogen-poor event by its photospheric spectra (Yan et al., 2015). SLSN-I SN2015bn is one example with complete photometric and spectroscopic observations from early to late times (Nicholl et al., 2016). Its well sampled light curve shows a clear deviation from the smooth rise and fall, with excess emission at several places near the peak. A similar case was also found in the light curve of iPTF13dcc (Vreeswijk et al., 2016). The interpretation of both SN2015bn and iPTF13dcc seems to favor ejecta interacting with a H-poor CSM.

It is not clear if undulation in light curves is a feature for a large fraction of SLSNe-I or just for a small number of events. It is also not clear at all what the physical explanation is. In this paper, we report the observations for iPTF15esb, a SLSN-I at $$z=0.224$$. Our light curve revealed several prominent bumps on top of a smooth curve. The unique feature of our dataset is its extensive spectroscopy, which covers the specific times when the excess emission occurs in the light curve. In addition, the late-time spectra uncovered the second case of a hydrogen-poor SLSN with broad H$$\alpha$$ emission at late times, like iPTF13ehe (Yan et al., 2015). § \ref{data} presents the observational data, § \ref{sec_results} reports the results of our analyses of the light curves and the spectra covering both early and late times. § \ref{sec_interp} discusses the interpretation and implications for various models and § \ref{sec_summ} summarizes the key points of this paper.

Throughout the paper, we adopt a $$\Lambda$$CDM cosmological model with $$\Omega_{\rm{M}}$$ = 0.286, $$\Omega_{\Lambda}$$ = 0.714, and $$H_{0}$$ = 69.6 $$\rm{km}\rm{s}^{-1}\rm{Mpc}^{-1}$$.