SN 2023taz: Implications for the UV Diversity of Superluminous Supernovae

TL;DR

SN 2023taz represents one of the most luminous hydrogen-poor SLSNe yet observed and reveals a pronounced deficit in rest-frame UV flux that cannot be explained by extinction or cooler photospheric temperatures. Through comprehensive multi-wavelength data, host-galaxy analysis, SED-based blackbody fits, and magnetar-based light-curve modeling (MOSFiT slsnni), the work shows that a rapidly spinning, low-field magnetar in conjunction with a low ejecta mass can power the peak luminosity, while the UV deficit is best attributed to enhanced UV line blanketing from intermediate-mass elements, particularly Mg. The study combines spectral evolution with quantitative Mg II absorption measurements to argue for deeper Mg-rich zones or enhanced mixing as the likely cause, rather than Fe-group line blanketing. These results underscore the substantial UV diversity among SLSNe and highlight the necessity of UV coverage for robust classification and physical interpretation, especially for high-redshift transients where rest-frame UV shifts into the optical/NIR bands.

Abstract

Superluminous supernovae (SLSNe) are some of the brightest explosions in the Universe representing the extremes of stellar deaths. At the upper end of their distribution is SN\,2023taz, one of the most luminous SLSNe discovered to date with a peak absolute magnitude of $M_{g,\rm{peak}}=-22.75 \pm 0.03$ and a lower limit for energy radiated of $E=2.9 \times 10^{51}$\,erg. Magnetar model fits reveal individual parameter values typical of the SLSN population, but the combination of a low $B$-field and ejecta mass with a short spin period places SN\,2023taz in a unusual region of parameter space, accounting for its extreme luminosity. The optical data around peak are consistent with a temperature of $\sim$17\,000\,K but SN\,2023taz shows a surprising deficit in the UV compared to other events in this temperature range. We find no indication of dust extinction that could plausibly explain the UV deficit. The lower level of UV flux is reminiscent of the absorption seen in lower-luminosity events like SN\,2017dwh, where Fe-group elements are responsible for the effect. However, in the case of SN\,2023taz, there is no evidence for a larger amount of Fe-group elements which could contribute to line blanketing. Comparing to SLSNe with well-observed UV spectra, an underlying temperature of $8000-9000$\,K would match the UV spectral slope, but is not consistent with the optical colour temperatures of these events. The most likely explanation is enhanced absorption by intermediate-mass elements, challenging previous findings that SLSNe exhibit similar UV absorption line equivalent widths. This highlights the need for expanded UV spectroscopic coverage of SLSNe, especially at early times, to build a framework for interpreting their diversity and to enable classification at higher redshifts where optical observations will exclusively probe rest-frame UV emission.

Figures (9)

• Figure 1: Spectrum of SN 2023taz obtained with X-Shooter in the visible arm at a phase of +310.6 days post peak. Host galaxy emission lines from the Balmer series are highlighted with grey dashed lines.
• Figure 2: The light curve of SN 2023taz, with all magnitudes in the AB system and uncorrected for Milky Way extinction. Phases are given in rest-frame days relative to the $w-$band maximum. The figure compiles photometric data from ATLAS, Pan-STARRS, ZTF, Swift, LCOgt, and NTT. MJD is in the observer frame, with 3$\sigma$ upper limits marked as inverted triangles. For clarity, ATLAS $c$ and $o$-band points are shown without upper limits and have been binned to a daily cadence. Vertical tick marks on the top axis indicate the epochs of the spectra displayed in Figure \ref{['fig:spectra']}.
• Figure 3: Photometric colours for SN 2023taz in blue, pink, purple, and indigo compared to the colours from the sample in Gomez2024 shown in black. Measurements given in AB mag with lower limits denoted by triangles. Photometric colours for Gaia16apd Kangas2017Yan2017bNicholl2017d and SN 2017dwh Blanchard2019 are also plotted in green squares and orange diamonds respectively. Phases are given relative to peak.
• Figure 4: Parameters derived from SED fits of the light curve. Unfilled stars indicate where less than three bands were available for the fit. The colours of each line correspond to the peak blackbody temperature calculated for each event from the sample in Gomez2024, and the parameters for Gaia16apd are plotted in green squares. Top: Bolometric luminosity. Middle: Blackbody temperature. The temperature from blackbody fits to the spectra are also plotted in purple. Bottom: Blackbody radius.
• Figure 5: MOSFiT fits to the light curve of SN 2023taz using the slsnni model. This model combines the magneter engine with a flexible level of contribution from the radioactive decay of $^{56}$Ni. Upper limits are indicated via inverted triangles.