Slowly fading super-luminous supernovae that are not pair-instability explosions

Abstract

The high luminosity and slow decline of their light curves ( Fig PTF12dam is not detected in z P1 images on 1 January 2012, 132 days before the peak.Although their light curves match the declining phases of SN 2007bi and the PISN models quite well, PTF12dam and PS1-11ap rise to maximum light a factor of ~2 faster than these models.The spectra of PTF12dam and PS1-11ap show them to be similar supernovae. After 50 days from the respective light curve peaks, these spectra are almost identical to that of SN 2007bi at the same epoch ( Particularly around and after maximum light, PISN colours are expected to evolve to the red owing to increasing blanketing by iron group elements 7,8 abundant in their ejecta. We see no evidence of line blanketing in our spectra, even down to 2,000 Å (rest frame) in PS1-11ap, which suggests lower iron group abundances and a higher degree of ionization than in PISN models. Such conditions are fulfilled in models of ejecta reheated by magnetars-highly magnetic, rapidly rotating nascent pulsars 13,16,17 . The pressure of the magnetar wind on the inner ejecta can form a dense shell 13,14,17 at near-constant photospheric velocity. ForPTF12dam, the velocities of spectral lines are close to 10,000 km s −1 at all times. Intriguingly, Page 4 of 26 the early spectra of our objects are very similar to those of superluminous supernovae of type I (refs 2, 11, 12) and evolve in the same way, but on longer timescales and with lower line velocities (Fig. 2).Nebular modelling of SN 2007bi spectra has been used to argue 1 for large ejected oxygen and magnesium masses of 8-15M ! and 0.07-0.13M ! , respectively (where M ! is the solar mass). Such masses are actually closer to values in massive core-collapse models 18 than in PISN models, which eject ~40M ! oxygen and ~4M ! magnesium 1,8,9 . In the work reported in ref.1, an additional 37M ! in total of Ne, Si, S, and Ar were added to the model, providing a total ejecta mass consistent with a PISN. However, this was not directly measured 1 , because these elements lack any identified lines. These constraints are important, so we investigated line formation in this phase using our own non-local thermodynamic equilibrium code We suggest here one model that can consistently explain the data. A magnetarpowered supernova can produce a light curve with the observed rise and decline rates as the neutron star spins down and reheats the ejecta 13,14,16,17 . It has been suggested that ~10% of core-collapses may form magnetars 14 . Although their initial-spin distribution is unknown, periods ≳1 ms are physically plausible. This mechanism has already been proposed for SN (Fig. 4), and found a good fit for magnetic field B ≈ 10 14 G and spin period P ≈ 2.6 ms, with an ejecta mass of ~10-16M ! . At peak, the r-band luminosities of PTF12dam and PS1-11ap are ~1.5 times that of SN 2007bi. Scaling our light curve by this factor, our model implies a similar ejected mass for SN 2007bi, with a slower-spinning magnetar (P ≈ 3.3 ms), comparable to previous models 14 . If the mag...

Spectrum formation in superluminous supernovae (Type I)

“…This is consistent with a 56 Ni-driven light curve, and suggestive of the explosion of a very massive star with a zero-age main sequence mass close to, or in excess of 100 M⊙. This scenario has been criticized (e.g., Nicholl et al 2013), but in our opinion the presence of forbidden emission lines of Fe in an amount compatible with the 56 Ni that is necessary to drive the light curve (Gal-Yam et al 2009) makes the Ni-driven scenario uncontroversial. The massive collapse scenario (Moriya et al 2010) agrees on this despite uncertainties.…”

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

“…The peak luminosity, M g ≈ −21, is typical of these fast-declining events and fainter than those slowlyfading objects (for a larger sample comparison, we refer to Nicholl et al 2015a). The rise time and overall lightcurve width are very similar to SN 2011ke (Inserra et al 2013), SN 2006oz (Leloudas et al 2012, and SN 2010gx (Pastorello et al 2010) and much narrower than other events, such as PTF12dam (Nicholl et al 2013;Chen et al 2015) and LSQ14bdq (Nicholl et al 2015b). LSQ14mo appears to transition to a shallower de- 4000 4500 5000 5500 6000 6500…”

“…In contrast, the PISN model can only explain slowly-fading lightcurve events, such as SN 2007bi, which was initially suggested to be a PISN (Gal-Yam et al 2009) based on a 56 Co decay-like tail. However, the relatively rapid rise time of similar SLSNe PTF12dam and PS1-11ap (Nicholl et al 2013;McCrum et al 2014) argues against this interpretation. Recently, Kozyreva et al (2016) demonstrated that radiative-transfer simulations have essential scatter depending on the input ingredients, a relatively fast rising time was found for the helium PISN model (Kasen et al 2011).…”

“…The first is that of SLSNe, which do not generally exhibit hydrogen or helium lines in spectra and show no spectral signatures of interaction between fast moving ejecta and circumstellar shells, and thus are classified as SLSNe type I or type Ic (e.g. Pastorello et al 2010;Quimby et al 2011;Chomiuk et al 2011;Inserra et al 2013;Howell et al 2013;Nicholl et al 2013). The lightcurves of SLSNe type I span a wide range of rise (∼ 15-40 d) and decline timescales (∼ 30-100 d).…”

“…Firstly, the millisecond magnetar model can reproduce a wide range of lightcurves, fitting both fast and slow decliners (e.g. Inserra et al 2013;Nicholl et al 2013); Chen et al (2015) found that the slowly-fading late-time lightcurve also fits a magnetar model well if the escape of high-energy gamma rays is taken into account (for time-varying leakage see Wang et al 2015). In contrast, the PISN model can only explain slowly-fading lightcurve events, such as SN 2007bi, which was initially suggested to be a PISN (Gal-Yam et al 2009) based on a 56 Co decay-like tail.…”