Probing circumstellar interaction with spectroscopy of ZTF hydrogen-rich superluminous supernovae

abstract / block 1 · abstract

Hydrogen-rich superluminous supernovae (H-rich SLSNe) are rare, exceptionally luminous transients. Most exhibit narrow Balmer emission and can be further classified as SLSNe-IIn, indicating interaction with dense circumstellar material (CSM). We present and analyse 304 optical spectra of 106 H-rich SLSNe observed by the Zwicky Transient Facility (ZTF). We examine their spectral evolution and Balmer-line properties, and model the H$α$ profiles using Monte Carlo electron-scattering calculations. Of the 106 objects, 105 exhibit narrow Balmer emission consistent with SLSN-IIn classifications, including six with additional Ia-CSM-like characteristics and three with possible flash-ionisation features; the remaining one object exhibits only broad Balmer emission and is classified as a SLSN-II. Post-peak Balmer profiles range from approximately symmetric, exponential profiles to asymmetric profiles with prominent blue excesses or shoulders. Objects with Balmer line blue excess favour higher fitted photon-emitting shell velocities and tend to be fainter than those with persistently symmetric profiles. The H$α$ pseudo-equivalent width (pEW) and FWHM of SLSNe-IIn overlap with those of lower-luminosity SNe IIn, while the pEW evolution is more diverse than in the broad-lined SLSN-II comparison sample. Our SLSNe-IIn sample's median Balmer decrement increases from 2.85 before $+100$ d to 4.40 afterwards, remaining consistently below that of the comparison Ia-CSM sample. We identify no Balmer-line morphology unique to SLSNe-IIn in the available comparison with lower-luminosity SNe IIn, although some SLSNe-IIn exhibit prolonged high-ionisation emission. Our results also support interaction with dense CSM formed through enhanced pre-explosion mass loss. We also highlight a possible hidden Ia-CSM population among luminous H-rich interacting SNe.

S4.SS4: IV.4 SED modeling / block 1 · html

The resulting best-fit SEDs for some of the epochs are shown in Figure 5 . The blackbody temperature corresponding to the photosphere drops drastically from > > 15000 K at t ∼ \sim 16.5 days to ∼ \sim 5500 K. This is also accompanied by a rise in the blackbody radius from ∼ \sim 2 × 10 15 2\times 10^{15} cm to 7.5 × 10 15 7.5\times 10^{15} cm. This high-temperature blackbody traces the photosphere of the SN, which expanded outward until t ∼ \sim 130 days and then started receding inwards. The temperature of the receding photosphere is constant throughout ( ∼ \sim 5000 K). The temperature and radius of the NIR emitting region, including Wise W1, W2 data, are tabulated in Table 1 . The dust temperature estimated is ∼ \sim 1600 K. The radius of the NIR emitting region is ∼ \sim 4 × 10 16 4\times 10^{16} cm, which is the lower limit since we are assuming the dust to be optically thick.

S7.SS1: VII.1 The SN environment: the properties of the CSM / block 1 · html

Figure 13: Comparison of SN 2019vxm (red) with results of radiative transfer models given by [ 16 ] , details of which are mentioned in the text.

S2.SS5: 2.5 JWST /NIRSpec spectroscopy / block 1 · html

Table 1: Summary of available photometric data of GNz7q used in this study

A1: Appendix A Testing Photometric Systematics Using Nearby Sources / block 1 · html

Figure 9 shows the resulting normalized light curves for these comparison sources, including only measurements with S / N > 3 {\rm S/N}>3 . None of the control objects exhibits statistically significant long-term or wavelength-dependent variability comparable to that observed in GNz7q. The absence of correlated variability in these sources demonstrates that the observed behavior of GNz7q cannot be attributed to instrumental differences, filter-response mismatches, PSF variations, or details of the photometric extraction, and instead reflects genuine intrinsic variability of the source.

S6: 6 Summary / block 1 · html

Stage II – The LRD Phase: The central engine drives fast winds into the pre-existing envelope. Kinetic energy from the resulting shocks is converted highly efficiently into radiation ( ≈ 50 % \approx 50\% efficiency vs. < 1 % <1\% in typical SNe). This explains the LRDs’ immense luminosity with physics similar to interaction transients, which rank among the most luminous transients. Accretion radiation may complement or act in lieu of CSM interaction luminosity.

S6: 6 Summary / block 1 · html

We estimate a mass-loss rate of ≈ 0.3 ​ M ⊙ \approx 0.3M_{\rm{\odot}} yr -1 and a fast wind velocity of ≈ 8000 \approx 8000 km s -1 that crashes into a slow wind of ≈ 500 \approx 500 km s -1 . The wind velocities are comparable to IIn SNe and the GE, while the mass-loss rates are ≈ 10 − 100 × \approx 10-100\times higher, consistent with a more massive central engine. [§ 3.3 ]

S8: VIII Conclusions / block 1 · html

The early phase spectra display broad H α \alpha and He I P-Cygni profiles with initial velocities of ∼ 15 500 \sim 15\,500 km s -1 and ∼ 14 000 \sim 14\,000 km s -1 that decline to ∼ 10 000 \sim 10\,000 km s -1 and ∼ 7 000 \sim 7\,000 km s -1 by t ≈ 30 ​ days t\approx 30\,\text{days} , reflecting a hydrogen envelope overlying the He rich ejecta.

S7: VII Results and Discussion / block 1 · html

All shock-cooling models applied in this work reproduce the early observations of SN 2024uwq reasonably well, with a few notable distinctions. As seen in Figure 9 , each model predicts an early UV/optical peak followed by a decline, which is steeper in the UV bands (e.g., W2 , M2 , W1 , U ) compared to the subtler decline in optical passbands ( B , g , V , r , o , i ). The models diverge in their ability to match the earliest UV detections and the inferred explosion epochs. The 85 model yields an explosion epoch closest to the first ATLAS o -band detection, whereas the 99 ( n = 1.5 n=1.5 [convective] and n = 3 n=3 [radiative]) and 74 models predict earlier onset of shock-cooling signatures compared to the ATLAS o -band detection. The 74 model provides the most consistent fit to the earliest ATLAS- o detections in comparison with the other models. Both the 99 models over predict fluxes in the earliest Swift UV bands, whereas the 85 model underestimates UV fluxes at t > 1.5 t>1.5 days. The 74 model better matches the UV light-curve morphology at all phases, owing to its updated treatment of radiative transfer as well as line blanketing effects. The steep density gradients as assumed in 85 favor compact progenitors (with R ≈ 14.6 ​ R ⊙ R\approx 14.6\,R_{\odot} ), while models with gradual density profiles ( 74 ) align with more extended progenitors (up to R ≈ 57 ​ R ⊙ R\approx 57\,R_{\odot} ). The tighter constraints in 85 arise from non-exclusion of intrinsic scatter parameter that reflect the systematic uncertainties from unmodeled envelope inhomogeneities or density gradients, whereas the other models adopts broader priors for σ \sigma , reflecting more conservative error estimates. All of the models consistently fit the optical data well, underscoring the importance of early UV observations in disentangling the progenitor structures by breaking any known degeneracies.

S5: V Conclusions / block 1 · html

With ZTF public light curves that span a rest-frame timescale of ∼ 5 \sim 5 yrs, we were able to derive stringent constraints on the intrinsic optical continuum variability of the three local LRDs, which vary by less than 3 − 4 % 3-4\% over this time period. Additionally, using multi-epoch spectra of J1025 and J1047 that span ∼ 15 \sim 15 years in their rest-frame, we place constraints on the fractional broad H α \alpha variability to be less than a few percent. These are by far the strongest constraints on the variability properties of local LRD analogs on multi-year to decade timescales, and are consistent with the majority of variability constraints on much shorter timescales [ 37 , 72 , 61 , ≲ 1 \lesssim 1 yr in restframe, e.g.,] .

S3: III Results / block 1 · html

Table 1: Photometric and H α \alpha spectral variability measurements. The mean photometric uncertainty σ i ¯ \overline{\sigma_{i}} is shown for each filter. The total H α \alpha flux measurements may suffer from aperture effects for the narrow-line emission.

S6: 6 Conclusions / block 1 · html

We thank Koutarou Kyutoku, Kohta Murase, and Xuejian Shen for motivating this work. We also thank Takashi Hosokawa and Kunihito Ioka for fruitful discussions and comments.

S2.SS1: 2.1 Structure of Envelopes / block 1 · html

where σ SB \sigma_{\rm SB} denotes the Stefan-Boltzmann constant, and L ph = 10 10 ​ L ⊙ ​ L ph , 10 L_{\rm ph}=10^{10}~L_{\odot}~L_{\rm ph,10} and T ph = 6000 ​ K ​ T ph , 0 T_{\rm ph}=6000~{\rm K}~T_{\rm ph,0} are fiducial values in the following discussion ( Inayoshi and Ho, 2025 , e.g.,) .

abstract / block 1 · abstract

The recent discovery of a large number of massive black holes within the first two billion years after the Big Bang, as well as their peculiar properties, have been largely unexpected based on the extrapolation of the properties of luminous quasars. These findings have prompted the development of several theoretical models for the early formation and growth of black holes, which are, however, difficult to differentiate. We report the metallicity measurement around a gravitationally lensed massive black hole at redshift 7.04 (classified as a Little Red Dot), hosted in a galaxy with very low dynamical mass. The weakness of the [OIII]5007 emission line relative to the narrow H$β$ emission indicates extremely low metallicity, about $4\times 10^{-3}$ solar, and even more metal poor in the surrounding few 100 pc. We argue that such properties cannot be uncommon among accreting black holes around this early cosmic epoch. Explaining such a low chemical enrichment in a system that has developed a massive black hole is challenging for most theories. Models assuming heavy black hole seeds (such as Direct Collapse Black Holes) or super-Eddington accretion scenarios struggle to explain the observations, although they can potentially reproduce the observed properties in some cases. Models invoking "primordial black holes" (i.e. putative black holes formed shortly after the Big Bang) may potentially explain the low chemical enrichment associated with this black hole, although this class of models also requires further developments for proper testing.

S8: VIII Summary / block 1 · html

For sources with available multi-epoch spectroscopy, we performed a variability analysis for our sample. We found that 11 out of 13 sources ( 85 − 15 + 5 % 85^{+5}_{-15}\% ) exhibit variability either in flux (10/13, 77 − 15 + 8 % 77^{+8}_{-15}\% ) or N H N_{\rm H} (4/12, 33 − 10 + 15 % 33^{+15}_{-10}\% ). These results are in excellent agreement with recent findings on lower-luminosity ( log ⁡ L X / erg ​ s − 1 ∼ 43 \log L_{\rm X}/\mathrm{erg\,s^{-1}}\sim 43 ), Compton-thin AGNs.

S8: VIII Summary / block 1 · html

For 20/21 sources with adequate optical spectroscopy, we measured the [ Ne v ] λ ​ 3426 \lambda 3426 emission line and found a detection rate of 85 − 11 + 5 % 85^{+5}_{-11}\% (17/20). This confirms that the [ Ne v ] λ ​ 3426 \lambda 3426 emission line is a strong tracer of AGN activity even in cases of heavy obscuration, in agreement with previous studies;