Spectrum formation in superluminous supernovae (Type I)

Abstract

The near-maximum spectra of most superluminous supernovae that are not dominated by interaction with a H-rich CSM (SLSN-I) are characterised by a blue spectral peak and a series of absorption lines which have been identified as O II. SN 2011kl, associated with the ultra-long gamma-ray burst GRB111209A, also had a blue peak but a featureless optical/UV spectrum. Radiation transport methods are used to show that the spectra (not including SN 2007bi, which has a redder spectrum at peak, like ordinary SNe Ic) can be explained by a rather steep density distribution of the ejecta, whose composition appears to be typical of carbon-oxygen cores of massive stars which can have low metal content. If the photospheric velocity is ∼ 10000 − 15000 km s −1 several lines form in the UV. O II lines, however, arise from very highly excited lower levels, which require significant departures from Local Thermodynamic Equilibrium to be populated. These SLSNe are not thought to be powered primarily by 56 Ni decay. An appealing scenario is that they are energised by X-rays from the shock driven by a magnetar wind into the SN ejecta. The apparent lack of evolution of line velocity with time that characterises SLSNe up to about maximum is another argument in favour of the magnetar scenario. The smooth UV continuum of SN 2011kl requires higher ejecta velocities (∼ 20000 km s −1 ): line blanketing leads to an almost featureless spectrum. Helium is observed in some SLSNe after maximum. The high ionization near maximum implies that both He and H may be present but not observed at early times. The spectroscopic classification of SLSNe should probably reflect that of SNe Ib/c. Extensive time coverage is required for an accurate classification.

Light-curve and spectral properties of ultrastripped core-collapse supernovae leading to binary neutron stars

“…dominance of He ii) than in a large occupation number within He i excited states possibly generating lines. This is a situation somewhat similar to that of SLSNe (Mazzali et al 2016). Figure 11 shows a comparison between our synthetic LCs and observational data of rapidly-evolving SNe.…”

The evolution of superluminous supernova LSQ14mo and its interacting host galaxy system

“…In the case of SLSNe, Mazzali et al (2016) find that a steep density gradient yields good fits to the absorption-line spectra of different SLSNe. They find that thermal conditions in SLSNe explain some of the lines, but O ii lines near maximum and He i lines after maximum require nonthermal excitation/ionisation, which they claim may be the result of X-rays leaking from a magnetar-interaction.…”

“…In Sect. 4, we examine the evolution of the lines and continuum in more detail using spectral synthesis models (Mazzali et al 2016). (2013); Nicholl et al (2013Nicholl et al ( , 2015bNicholl et al ( , 2016b; Vreeswijk et al (2014).…”

“…The spectra of LSQ14mo were modelled following the approach described in Mazzali et al (2016). These models use a Monte Carlo supernova radiative transfer code (Mazzali & Lucy 1993;Mazzali et al 2000;Lucy 2000), and have been used for SNe Ia (e.g.…”

“…These models use a Monte Carlo supernova radiative transfer code (Mazzali & Lucy 1993;Mazzali et al 2000;Lucy 2000), and have been used for SNe Ia (e.g. Mazzali et al 2014) as well as SNe Ib/c (Mazzali et al 2013), and have recently been applied to SLSNe (Mazzali et al 2016) 8 The code assumes that the SN luminosity is emitted at a sharp inner boundary and follows the propagation of energy 8 The luminosity of PTF09cnd is one order of magnitude smaller than what indicated in their Table 3. This error only affect the table, not the actual calculation.…”