Dr Shubham Srivastav

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, in a dwarf galaxy at z = 0.407. This is one of the most luminous SLSNe discovered to date with a peak absolute magnitude of Mg,peak = –22.75 ± 0.03 and a lower limit for energy radiated of E = 2.9 × 1051 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 ∼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 color 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.

Abstract:

The Vera C. Rubin Observatory’s 10-yr Legacy Survey of Space and Time (LSST) is expected to produce a hundredfold increase in the number of transients we observe. However, there are insufficient spectroscopic resources to follow up on all of the wealth of targets that LSST will provide. As such it is necessary to be able to prioritize objects for follow-up observations or inclusion in sample studies based purely on their LSST photometry. We are particularly keen to identify tidal disruption events (TDEs) with LSST. TDEs are immensely useful for determining black hole parameters and probing our understanding of accretion physics. To assist in these efforts, we present the Many Artificial LSST Light curves based on the Observations of Real Nuclear transients (MALLORN) data set and the corresponding classifier challenge for identifying TDEs. MALLORN comprises 10 178 simulated LSST light curves, constructed from real Zwicky Transient Facility (ZTF) observations of 64 TDEs, 727 nuclear supernovae and 1407 AGN with spectroscopic labels using Gaussian process fitting, empirically motivated spectral energy distributions from SNCosmo and the baseline from the Rubin Survey Simulator. Our novel approach can be easily adapted to simulate transients for any photometric survey using observations from another, requiring only the limiting magnitudes and an estimate of the cadence of observations. The MALLORN Astronomical Classification Challenge, launched on Kaggle on 2025 October 15, will allow competitors to test their photometric classifiers on simulated LSST data to find TDEs and improve upon their capabilities prior to the start of LSST.

Abstract:

We present a photometric and spectroscopic analyses of the Type Ibn supernova (SN) 2024acyl. It rises to an absolute magnitude peak of M o = −17.58 ± 0.15 mag in 10.6 days, and displays a rapid linear post-peak light-curve decline in all bands (e.g. γ 0 − 60 ( V ) = 0.097 ± 0.002 mag day −1 ), similar to most SNe Ibn. The optical pseudobolometric light curve peaks at (3.5 ± 0.8)×10 42 erg s −1 , with a total radiated energy of (5.0 ± 0.4)×10 48 erg. The spectra are dominated by a blue continuum at early stages, with narrow P-Cygni He  I lines and flash-ionisation emission lines of C  III , N  III , and He  II . The P-Cygni He  I features gradually evolve and become emission-dominated in late-time spectra. The H α line is detected throughout the entire spectral evolution, which indicates that the circumstellar material (CSM) is helium-rich with some residual amount of hydrogen. Our multi-band light-curve modelling yields estimates of the ejecta mass of M ej = 0.49 +0.11 −0.09 M ⊙ with a kinetic energy of E k = 0.06 +0.01 −0.01 × 10 51 erg, and a 56 Ni mass of M Ni = 0.018 M ⊙ . The inferred CSM properties are characterised by a mass of M CSM = 0.51 +0.05 −0.04 M ⊙ , an inner radius of R 0 =17.8 +3.6 −3.0 AU, and a density of ρ CSM = (8.3 +2.7 −1.2 ) × 10 −12 g cn −3 . The multi-epoch spectra are well reproduced by the CMFGEN/ he4p0 model, corresponding to a He-ZAMS mass of 4 M ⊙ (H-ZAMS mass 18.11 M ⊙ , pre-SN mass 3.16 M ⊙ ). These findings are consistent with a scenario of an SN powered by ejecta-CSM interaction originating from a low-mass helium star that evolved within an interacting binary system where the CSM with some residual hydrogen may originate from the mass-transfer process. We also discuss an extreme scenario involving the possible merger of a helium white dwarf. In addition, a channel of core-collapse explosion of a late-type Wolf-Rayet (WR) star with hydrogen, or a transitional star between an Of and a WR type (e.g. an Ofpe/WN9 star) with fallback accretion cannot be entirely ruled out.

Abstract:

Supernovae (SNe) IIn are terminal explosions of massive stars that are surrounded by a dense circumstellar medium (CSM). Among SNe IIn, a notable subset is the SN 2009ip-like, which exhibits an initial, fainter peak attributed to stellar variability in the late evolutionary stages, followed by a brighter peak, interpreted as the SN explosion itself. In this context, we analysed the spectrophotometric evolution of SN 2024hpj, an object with a triple-peaked light curve and spectra typical of a SN IIn but with a complex line profile composed of broad P-Cygni features topped by narrow emissions. Comparing it with other SN 2009ip-like events in the literature, as well as with other unpublished objects (SNe 2019mry, 2022ytx, 2024uzf, and 2025csc), we identify star-forming regions as their preferred formation environment. On the other hand, the diversity of spectrophotometric features within the sample suggests that variations in CSM mass and distribution may influence the observed characteristics. We identify four sub-classes based on the luminosity and rapidity of the light curve evolution, which provides insights into possible differences in the progenitors, while a statistical analysis of their observed rate indicates progenitor masses around 25 − 31 M ⊙ or lower.

Abstract:

Context. Type IIb supernovae (SNe IIb) are stellar explosions whose spectra reveal transitional features between hydrogen-rich (Type II) and helium-rich (Type Ib) SNe. Their progenitors are massive stars that were mostly stripped of their hydrogen envelope, likely through binary interaction and/or strong stellar winds. This makes such stars key tools in studies of the late stages of the evolution of massive stars. Aims. We present an extensive photometric and spectroscopic follow-up campaign of the Type IIb SN 2022ngb. Through the detailed modeling of this dataset, we aim to constrain the key physical parameters of the explosion, infer the nature of the progenitor star and its environment, and probe the dynamical properties of the ejecta. Methods. We analyzed photometric and spectroscopic data of SN 2022ngb. By constructing and modeling the bolometric light curve with semi-analytic models, we were able to estimate the primary explosion parameters. The spectroscopic data were compared with those of well-studied SNe IIb and NLTE models to constrain the properties of the progenitor and the structure of the resulting ejecta. Results. SN 2022ngb is a low-luminosity SN IIb with a peak bolometric luminosity of L Bol = 7.76 +1.15 −1.00 × 10 41 erg s −1 and a V -band rising time of 24.32 ± 0.50 days. The light curve modeling indicates an ejecta mass of ∼2.9 − 3.2 M ⊙ , an explosion energy of ∼1.4 × 10 51 erg, and a low synthesized 56 Ni mass of ∼0.045 M ⊙ . The nebular phase spectra exhibit asymmetric line profiles, pointing to a nonspherical explosion and an anisotropic distribution of radioactive material. Our analysis reveals a relatively compact stripped-envelope progenitor with a pre-SN mass of approximately 4.7 M ⊙ (corresponding to a 15–16 M ⊙ ZAMS star). Conclusions. Our analysis suggests that SN 2022ngb originated from the explosion of a moderate-mass relatively compact, stripped-envelope star in a binary system. The asymmetries inferred from the nebular phase spectral line features indicate the occurrence of a nonspherical explosion.