Dr Shubham Srivastav

Abstract:

Context. We present the optical follow-up of SNe 2022ywf and 2023zgx, two examples from the Iax subclass of thermonuclear supernova (SN) events. With peak absolute magnitudes of M V = −13.7 and −14.4 mag, respectively, both objects belong to the extremely low-luminosity (EL) population of the class. Aims. The common origin of SNe in the Iax subclass remains under debate, since the distribution of certain observables may indicate that the extremely low-luminosity explosions form a distinct population. We aim to estimate the physical properties of the two EL objects, including mapping the ejecta structure. We compare the results with the predictions of the pure deflagration model with similar luminosity, as well as with the common features of other SNe Iax. Methods. We performed spectral tomography on the spectral series of SNe 2022ywf and 2023zgx around their maxima to map the physical properties of the ejecta. Together with the analysis of BgVriz photometry, we studied a wide range of observables to investigate their distribution against luminosity. We compared the constrained chemical abundances of the ejecta to the predictions of hydrodynamic simulations with similar peak luminosities. Results. Constant abundances provide a good match for the distribution of chemical elements for both SNe 2022ywf and 2023zgx. The discrepancies compared to the least luminous pure deflagration model N5def_hybrid are minor, especially at post-maximum epochs. The two SNe also share similar characteristics in their constrained density structures, as well as in the evolution of the photosphere. Conclusions. The analysis supports the assumption that pure deflagration models can reproduce the main characteristics of SNe Iax, even for the low-luminosity population. The presented indirect observational evidence indicates that these objects show similar intrinsic properties to the well-studied, relatively luminous Iax sample and fit into the velocity distribution of the subclass.

Abstract:

The dynamically driven double-degenerate double-detonation model has emerged as a promising progenitor candidate for Type Ia supernovae. In this scenario, the primary white dwarf ignites due to dynamical interaction with a companion white dwarf, which may also undergo a detonation. Consequently, two scenarios exist: one in which the secondary survives and another in which both white dwarfs detonate. In either case, substantial departures from spherical symmetry are imprinted on the ejecta. Here, we compute full non-local thermodynamic equilibrium nebular-phase spectra in 1D and 3D to probe the innermost asymmetries. Our simulations reveal that the multidimensional structures significantly alter the overall ionization balance, width, and velocity of features, especially when the secondary detonates. In this scenario, some element distributions may produce orientation-dependent line profiles that can be centrally peaked from some viewing angles and somewhat flat-topped from others. Comparison to observations reveals that both scenarios produce most observed features from the optical to mid-infrared. However, the current model realizations do not consistently reproduce all line shapes or relative strengths, and yield prominent optical Ar iii emission which is inconsistent with the data. When the secondary detonates, including 3D effects improves the average agreement with observations, however when compared to observations, particularly weak optical Co iii emission and the presence of optical O i and near-infrared S i challenge its viability for normal Type Ia supernovae. Thus, overall, our comparisons with normal Type Ia’s tentatively favour detonation of only the primary white dwarf but we stress that more model realizations and mid-infrared observations are needed.

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:

We present the discovery of AT 2024wpp (‘Whippet’), a fast and luminous 18cow-like transient. At a redshift of , revealed by Keck Cosmic Web Imager spectroscopy of its faint star-forming host, it is the fourth-nearest example of its class to date. Rapid identification of the source in the Zwicky Transient Facility data stream permitted ultraviolet-through-optical observations to be obtained prior to peak, allowing the first determination of the peak bolometric luminosity ( erg s), maximum photospheric radius ( cm), and total radiated energy ( erg) of an 18cow-like object. We present results from a comprehensive multiwavelength observing campaign, including a far-ultraviolet spectrum from the Cosmic Origins Spectrograph on the Hubble Space Telescope and deep imaging extending 100 d post-explosion from the Very Large Telescope, Hubble Space Telescope, Very Large Array, and Atacama Large Millimetre Array. We interpret the observations under a model in which a rapidly accreting central engine blows a fast ( 0.2 c) wind into the surrounding medium and irradiates it with X-rays. The high Doppler velocities and intense ionization within this wind prevent identifiable spectroscopic features from appearing in the ejecta or in the surrounding circumstellar material. Weak H and He signatures do emerge in the spectra after 35 d in the form of double-peaked narrow lines. Each peak is individually narrow (full width km s) but the two components are separated by km s, indicating stable structures of denser material, possibly representing streams of tidal ejecta or an ablated companion star.