Nebular-phase Spectra of Superluminous Supernovae: Physical Insights from Observational and Statistical Properties

Abstract

We study the spectroscopic evolution of superluminous supernovae (SLSNe) later than 100 days after maximum light. We present new data for Gaia16apd and SN 2017egm, and analyse these with a larger sample comprising 41 spectra of 12 events. The spectra become nebular within 2-4 e-folding times after light curve peak, with the rate of spectroscopic evolution correlated to the light curve timescale. Emission lines are identified with well-known transitions of oxygen, calcium, magnesium, sodium and iron. SLSNe are differentiated from other Type Ic SNe by a prominent O I λ7774 line and higher-ionisation states of oxygen. The iron-dominated region around 5000Å is more similar to broad-lined SNe Ic than to normal SNe Ic. Principal Component Analysis shows that 5 'eigenspectra' capture 70% of the variance, while a clustering analysis shows no clear evidence for multiple SLSN sub-classes. Line velocities are 5000-8000 km s −1 , and show stratification of the ejecta. O I λ7774 likely arises in a dense inner region that also produces calcium emission, while [O I] λ6300 comes from further out until 300-400 days. The luminosities of O I λ7774 and Ca II suggest significant clumping, in agreement with previous studies. Ratios of [Ca II] λ7300/[O I] λ6300 favour progenitors with relatively massive helium cores, likely 6 M , though more modelling is required here. SLSNe with broad light curves show the strongest [O I] λ6300, suggesting larger ejecta masses. We show how the inferred velocity, density and ionisation structure point to a central power source.

SN 2010kd: Photometric and Spectroscopic Analysis of a Slow-decaying Superluminous Supernova

“…In the case of SN 2010kd, the luminosity of the strongest lines is ∼ 10 42 erg s −1 at +96 days, dropping to ∼ 4 × 10 40 erg s −1 at +194 days, consistent with the result of Nicholl et al (2019) for different SLSNe I. For SN 2010kd at early epochs, Mg I] has the highest luminosity, followed by [Ca II], the Ca II NIR triplet, and O I λ7774; the rest of the lines have even lower luminosities.…”

“…In comparison to SN 2010kd, fast-decaying SLSNe as well as PTF12dam appear to have velocities with steeper decay rates, whereas SN 2015bn seems to have lower velocity values with similar decay rates. and evolve much later, which indicate a central engine power source heating the inner ejecta as suggested by Nicholl et al (2019).…”

“…First, the continuum was subtracted by a linear fit, and the values of luminosities are estimated by direct numerical integration only for the lines that are isolated and do not have highly blended features, except [O I] λ5577. For the [O I] λ5577 line, we fitted a double Gaussian, because it is blended with [Fe II] λ5250, the same method adopted by Nicholl et al (2019). We also attempted subtracting the contribution of the host-galaxy flux from the nebular-phase spectra of SN 2010kd using the host spectrum published by Leloudas et al (2015) as described by Nicholl et al (2019); however, the host-galaxy continuum flux of SN 2010kd is found to be negligible.…”

“…We compare luminosities of various lines of SN 2010kd with a set of well-studied SLSNe I (see Figure 14). All of the published comparison data were taken from Nicholl et al (2019) and references therein; Nicholl et al (2013Nicholl et al ( , 2016aNicholl et al ( ,b, 2017 In the present study, for comparison we mainly discuss the line-luminosity evolution of PTF12dam and SN 2015bn, because these SLSNe I have a significant number of spectroscopic data points and are also at comparable redshifts. Lineluminosity values of these objects are fitted with straight lines.…”

“…We as-sume that [O I] λ5577 and the [O I] doublet are from the same zone, so their ratio will depend only on optical depth and temperature. Thus, the ratio of these lines can indicate the single-zone local thermodynamic equilibrium (LTE) temperature, as discussed by Jerkstrand et al (2017), Nicholl et al (2019). To calculate the temperature, we used Equation 2 from Jerkstrand et al (2014) and assume β ratio = 1.5.…”