Eruptive mass loss less than a year before the explosion of superluminous supernovae. II. A systematic search for pre-explosion eruptions with VLT/X-shooter

TL;DR

This work presents a targeted near-UV spectroscopic search for pre-explosion CSM shells around hydrogen-poor SLSNe-I using VLT/X-shooter, combining 21 high-quality spectra from triggered programs and literature. A Monte-Carlo radiative-transfer-like framework models MgII shell absorption, spanning a wide parameter grid in $v_{ m max}$, $R_{ m in}$, $\Delta r$, and $\tau$, with a Bayesian statistic to distinguish shell signatures from a continuum. Five objects show robust $>5\sigma$ CSM detections, including two new shells around DES15S2nr and DES16C3ggu with $v_{ m max}\sim 4700$–$4800$ km s$^{-1}$ and ejection times of roughly $2$–$3$ months before explosion; combined with previous detections this suggests a sub-population of SLSN-I progenitors undergoing late-stage mass loss. The study finds no strong correlations between CSM properties and SN photometric behavior, though a tentative link between faster CSM speeds and shorter decline times is noted, and it emphasizes that detectable CSM features require high-quality data (S/N $\gtrsim 8$). Collectively, these results imply that eruptive mass loss shortly before core collapse is real for at least a subset of SLSNe-I and can inform progenitor evolution scenarios, including LBV-like eruptions and pulsational pair-instability, while highlighting the need for larger samples and follow-up at late times to understand geometry, mass, and fueling mechanisms.

Abstract

We present X-shooter observations of a sample of 21 hydrogen-poor superluminous supernovae (SLSNe-I), spanning a redshift range of z=0.13-0.95, aimed at searching for shells of circumstellar material (CSM). Specifically, we focus on identifying broad Mg II absorption features that are blueshifted by several thousand kilometers per second and have previously been interpreted as arising from resonance line scattering of the SLSN continuum by rapidly expanding CSM ejected shortly before explosion. Utilizing high-quality spectra, we model the region around 2800A to characterize the Mg II line profiles, enabling us to either confirm their presence or place constraints on undetected CSM shells. We identify five objects in our sample that show broad Mg II absorption features consistent with the presence of CSM. While SN2018ibb, SN2020xga and SN2022xgc have been previously reported, we identify previously undiscovered CSM shells in DES15S2nr and DES16C3ggu. These shells were likely expelled approximately two and three months, respectively, before the explosion of their associated SNe, timescales consistent with late-stage mass-loss episodes. We do not find any correlations between the shell properties and the SN properties, except for a marginal correlation between the light curve decline time scale and the shell velocities. We further demonstrate that CSM configurations similar to the majority of the detected shells would have been observable in spectra with signal-to-noise >8 per resolution element, and that the lines from a shell are in general detectable except in the cases where the shell is either very geometrically and/or optically thin. Therefore, we conclude that the detection of CSM shells is not a selection effect, but may instead point to the existence of a subclass of SLSNe-I undergoing late-stage shell ejections shortly before explosion.

Figures (11)

• Figure 1: Near-UV spectroscopic observations of the 21 SLSNe-I in the final X-shooter sample analyzed in this study. Each panel shows the observed spectrum of a single object around the 2800 Å region, with the corresponding $1\sigma$ uncertainty shaded in grey. All spectra have been corrected for Milky Way extinction. The objects are presented in order of descending S/N.
• Figure 2: Synthetic spectra generated using the Monte Carlo scattering code, where three of the four parameters are held fixed while the fourth is varied: optical depth $\tau$ in panel (a), maximum velocity $v_{\rm max}$ in panel (b), shell width $\Delta r$ in panel (c), and inner radius $R_{\rm in}$ in panel (d). The colorbar indicates the value of the parameter being varied in each case, while the values of the fixed parameters are noted in the title of each panel. The $\Delta r$ and $R_{\rm in}$ are in units of photospheric radius $R_{\rm ph}$ and the colorbars are in logarithmic scale for better visualization. For clarity, the host galaxy MgII lines (black dashed lines) have been excluded from the model spectra. The grey curve shows the observed spectrum of SN 2020rmv, with the light blue shaded region indicating the 1$\sigma$ uncertainty.
• Figure 3: Corner plots for four of the 16 events in our sample, showing the percentage of CSM models ruled out as a function of increasing S/N ratio. The colorbar indicates the fraction of excluded models. The parameters correspond to the maximum velocity $v_{\rm max}$, inner radius $R_{\rm in}$, shell thickness $\Delta r$ and optical depth $\tau$.
• Figure 4: Column density of the CSM shells as a function of the spectrum’s S/N. Triangular markers denote the lower limits for objects with no detected shell, representing the column density above which a shell would have been detectable given the noise level. The dashed line shows the best-fit relation, and the shaded region marks the parameter space where a CSM shell would remain undetectable at the given S/N. Star symbols indicate the measured column densities (or lower limits) for objects with detected CSM shells.
• Figure 5: Boxplots for DES15S2nr, DES16C3ggu, SN 2022xgc, SN 2018ibb and SN 2020xga. The colored boxes indicate the parameter space where models with CSM are statistically favored over continuum-only models at a confidence level exceeding $>5\sigma$. The white horizontal lines within each box mark the best-fit values. The parameters explored are the maximum velocity ($v_{\rm max}$), the inner radius ($R_{\rm in}$), the shell thickness ($\Delta r$), and the optical depth ($\tau$).