Dr Shubham Srivastav

Abstract:

ABSTRACT We present photometric and spectroscopic studies of two core-collapse supernovae (SNe) 2008aq and 2019gaf in the optical wavelengths. Light curve and spectral sequence of both the SNe are similar to those of other Type IIb SNe. The pre-maximum spectrum of SN 2008aq showed prominent H $\alpha$ lines, the He lines started appearing in the near maximum spectrum. The near maximum spectrum of SN 2019gaf shows shallow H $\alpha$ absorption and He lines with almost similar strength. Both the SNe show transition from hydrogen-dominated spectra to helium-dominated spectra within a month after maximum brightness. The velocity evolution of SN 2008aq matches well with those of other well-studied Type IIb SNe, while SN 2019gaf shows higher velocities. Close to maximum light, the H $\alpha$ and He i line velocities of SN 2019gaf are $\sim$ 2000 and $\sim$ 4000 km s$^{-1}$ higher than other well-studied Type IIb SNe. Semi-analytical modelling indicates SN 2019gaf to be a more energetic explosion with a smaller ejecta mass than SN 2008aq. The zero-age main-sequence (ZAMS) mass of the progenitor estimated using the nebular spectra of SN 2008aq ranges between 13 and 20 M$_\odot$, while for SN 2019gaf, the inferred ZAMS mass is between 13 and 25 M$_\odot$. The [O i] to [Ca ii] lines flux ratio favours a less massive progenitor star in a binary system for both the SNe.

Abstract:

Public data release accompanying the ATLAS100 sample definition paper by Srivastav et al. (2026). The data release includes the cleaned and binned ATLAS light curves of 1729 transients in the sample. Also included is a catalog csv file with additional useful metadata for the transients in the sample, including host galaxy associations, any updated classifications, etc.

Abstract:

Type Ia supernovae (SNe Ia) are among the most precise cosmological distance indicators used to study the expansion history of the Universe. The vast increase in SN Ia data due to large-scale astrophysical surveys has led to the discovery of a wide variety of SN Ia sub-classes, such as transitional and fast-declining SNe Ia. However, their distinct photometric and spectroscopic properties differentiate them from the population of normal SNe Ia such that their use as cosmological tools remains challenged. Here, we present a high-cadenced photometric and spectroscopic dataset of two SNe Ia, SNe 2020ue and 2020nlb, which were discovered in the nearby Virgo cluster of galaxies. Our study shows that SN 2020nlb is a normal SN Ia whose unusually red colour is intrinsic, arising from a lower photospheric temperature rather than interstellar reddening, providing clear evidence that colour diversity among normal SNe Ia can have a physical origin. In contrast, SN 2020ue has photometric properties, such as colour evolution and light curve decay rate, similar to those of transitional SNe. It is hence more spectroscopically aligned with normal SNe Ia. This is evident from spectroscopic indicators such as the pseudo-equivalent width of lines. Thus, such SNe Ia, which lie photometrically at the edge of the standard normal SNe Ia range, may be missed in cosmological SNe Ia samples. Our results highlight that a spectroscopic analysis of SNe Ia around peak brightness is crucial for identifying intrinsic colour variations and constructing a more complete and physically homogeneous SN Ia sample for precision cosmology. Si II

Abstract:

Kilonovae are the scientifically rich—but observationally elusive—optical transient phenomena associated with compact binary mergers. Only a handful of events have been discovered to date, all through multiwavelength (gamma-ray) and multimessenger (gravitational-wave) signals. Given their scarcity, it is important to maximise the discovery possibility of new kilonova events. To this end, we present our follow-up observations of the gravitational-wave signal S250818k—a plausible binary neutron star merger at a distance of 237 ± 62 Mpc. Pan-STARRS tiled 286 and 318 deg2 (32% and 34% of the 90% sky localisation region) within 3 and 7 days of the GW signal, respectively. ATLAS covered 65% of the sky map within 3 days, but with lower sensitivity. These observations uncovered 47 new transients; however, none were deemed to be linked to S250818k. We undertook an expansive follow-up campaign of AT2025ulz, the purported counterpart to S250818k. The griz-band light curve, combined with our redshift measurement (z = 0.0849 ± 0.0003), all indicate that SN2025ulz is a type IIb supernova and thus not the counterpart to S250818k. We rule out the presence of an AT2017gfo-like kilonova within ≈27% of the distance posterior sampled by our Pan-STARRS pointings (≈9.1% across the total 90% 3D sky localisation). We demonstrate that early observations are optimal for probing the distance posterior of the 3D gravitational-wave sky map, and that SN2025ulz was a plausible kilonova candidate for ≲5 days, before ultimately being ruled out.

Abstract:

We present optical observations of the type IIb supernova (SN) 2017ckj, covering approximately 180 days after the explosion. Its early-time multi-band light curves display no clear evidence of a shock-cooling tail, resembling the behaviour of SN 2008ax. The V -band light curve exhibits a short rise time of about 5 days and reaches an absolute fitted peak magnitude of M V  = −18.49 ± 0.18 mag. The late-time multi-band light curves reveal a linear decline. We modelled the bolometric light curve of SN 2017ckj to constrain the progenitor and the explosion parameters. We estimated a total mass of 56 Ni synthesised by SN 2017ckj of M Ni = 0.21 +0.05 −0.03 M ⊙ , with a massive H-rich envelope of M env = 0.4 +0.1 −0.1 M ⊙ . Both the 56 Ni mass and the envelope mass of SN 2017ckj are higher than those of typical SNe IIb, in agreement with its peculiar light curve evolution. The early-time spectra of SN 2017ckj are dominated by a blue continuum, accompanied by narrow H α and He  II emission lines. The earliest spectrum exhibits flash ionisation features, from which we estimated a progenitor mass-loss rate of ∼3 × 10 −4  M ⊙  yr −1 . At later epochs, the spectra develop broad P-Cygni profiles and become increasingly similar to those of SNe IIb, especially SN 2018gk. The late-time spectrum at around 139 days does not show a distinct decline in the strength of its H α emission profile, also indicating a relatively massive envelope of its progenitor. Aside from the H α feature, the nebular spectrum exhibits prominent emission lines of O  I , Ca  II , [Ca  II ], and Mg  I ], which are consistent with the prototypical SN 1993J.