Dr. James Gillanders

Abstract:

Abstract Spectroscopic observations of the kilonova AT 2017gfo provide a unique opportunity to identify signatures from individual heavy elements freshly synthesised via the r-process, the nucleosynthetic channel responsible for producing ∼ half of all trans-iron-group elements. Limitations in the available atomic data have historically hampered comprehensive line identification studies; however, renewed interest has led to the generation of improved (more complete and accurately calibrated) line lists for r-process species. Here we demonstrate the utility of such data, by exploiting newly generated line lists for the lanthanides to model the photospheric-phase 3.4 d X-shooter spectrum of AT 2017gfo with the radiative transfer tool tardis. We find the data can only be reproduced by invoking a substantially diminished lanthanide mass fraction ($X_{\rm {\small IN}}$) than that proposed by previous studies. Specifically, our model necessitates $X_{\rm {\small IN}} \approx 2.5 \times 10^{-3}$ in the line-forming region, a value 20 × lower than previously claimed. This substantial reduction in $X_{\rm {\small IN}}$ is driven by our inclusion of much more complete lanthanide line information that enables better estimation of their total contribution to the observations. We encourage future modelling works to exploit all atomic data advances, and also encourage continued efforts to generate the necessary data for the remaining r-process species of interest.

Abstract:

Abstract A central question regarding neutron star mergers is whether they are able to produce all the r-process elements, from first to third peak. We here study theoretical infrared signatures of first-peak elements with spectral synthesis modelling. By combining state-of-the-art NLTE physics with new radiative and collisional data for these elements, we identify several promising diagnostic lines from Ge, As, Se, Br, Kr and Zr. The models give self-consistent line luminosities and indicate specific features that probe emission volumes at early phases (∼10d), the product of ion mass and electron density in late phases (≳75d), and in some cases direct ionic masses at intermediate phases. Emission by [Se I] 5.03 μm + [Se III] 4.55 μm is the only one from the first r-process peak that could explain the Spitzer photometry of AT2017gfo. However, the models show consistently that with a Kr/Te and Se/Te ratio following the solar r-process pattern, Kr + Se emission is dominant over Te for the blend at 2.1 μm observed in both AT2017gfo and AT2023vfi. The somewhat better line profile fit with [Te III] may suggest that both AT2017gfo and AT2023vfi had a strongly sub-solar production of the light r-process elements. An alternative scenario could be that Kr + Se in an asymmetric morphological distribution generates the feature. Further JWST spectral observations holds promise to determine the light r-process production of kilonovae, and in particular whether the light elements are made in a slow disk wind or in a fast proto-NS wind. We identify specific needs for further atomic data for Z = 31 − 40 elements.

Abstract:

The extremely rapid evolution of kilonovae results in spectra that change on an hourly basis. These spectra are key to understanding the processes occurring within the event, but this rapid evolution is an unfamiliar domain compared to other explosive transient events, such as supernovae. In particular, the most obvious P Cygni feature in the spectra of AT2017gfo – commonly attributed to strontium – possesses an emission component that emerges after, and ultimately outlives, its associated absorption dip. This delay is theorized to arise from reverberation effects, wherein photons emitted earlier in the kilonova’s evolution are scattered before reaching the observer, causing them to be detected at later times. We aim to examine how the finite speed of light – and therefore the light travel time to an observer – contributes to the shape and evolution of spectral features in kilonovae. Using a simple model, and tracking the length of the journey photons undertake to an observer, we are able to test the necessity of accounting for this time delay effect when modelling kilonovae. In periods where the photospheric temperature is rapidly evolving, we show spectra synthesized using a time-independent approach are visually distinct from those where these time delay effects are accounted for. Therefore, in rapidly evolving events such as kilonovae, time dependence must be taken into account.

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.