arXiv subjects

Quentin Pognan

Publications and source records attributed to Quentin Pognan.

10 recordsLinked to original sources

arXiv2026

A first-principles binary neutron star merger model of GW170817, GRB170817A, and AT2017gfo

The multimessenger observation of the binary neutron star merger event GW170817, associated with its electromagnetic counterparts GRB170817A and AT2017gfo, marked a milestone in astrophysics, yet its unified physical explanation remains elusive. We conduct an end-to-end simulation based on a first-principles general-relativistic magnetohydrodynamics neutrino-radiation transfer merger simulation, followed by nucleosynthesis calculations and photon radiative transfer to generate kilonova light curves. We show that the large-scale dynamo simultaneously produces a relativistic jet with an isotropic-equivalent luminosity of $\sim 10^{51}~{\rm erg~s^{-1}}$ and $\approx 0.08M_\odot$ of neutron-rich ejecta, reproducing the GRB170817A afterglow and the AT2017gfo kilonova light curves. Our results establish a unified first-principles framework for interpreting binary neutron star mergers across gravitational wave, gamma-ray burst, and kilonova observations.

astro-ph.HE↗

arXiv2025

Infrared spectral signatures of light r-process elements in kilonovae

A central question regarding neutron star mergers is whether they are able to produce all the r-process elements, from first to third peak. The high abundances of first-peak elements (atomic number $Z \sim 31-40$) in the solar composition means they may dominate the ejecta mass in kilonovae. We here study theoretical infrared signatures of such light 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 ($\sim$10d), the product of ion mass and electron density in late phases ($\gtrsim$75d), and in some cases direct ionic masses at intermediate phases. Emission by [Se I] 5.03 \mum\ + [Se III] 4.55 \mum\ can produce satisfactory fits to 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 \mum\ 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 data, in particular covering the so far unobserved $>5$ \mum\ region, 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 outflow. We identify specific needs for further atomic data on recombination rates and collision strengths for $Z=31-40$ elements.

astro-ph.SR↗

arXiv2025

Lanthanide Impact on the Infra-Red Spectra of Nebular Phase Kilonovae

Nebular phase kilonovae (KNe) have significant infra-red (IR) emission thought to be mostly forbidden emission lines from rapid neutron capture (r-process) species in neutron star merger ejecta. Lanthanide elements in particular have complex, open f-shell atomic structures with many IR transitions. Using non-local thermodynamic equilibrium (NLTE) radiative transfer simulations, we explore the impact of lanthanides on the IR spectra of KNe in the nebular phase, exploring a parameter space of ejecta mass and lanthanide fraction. We find that lanthanide impact is greater at higher densities, corresponding to earlier epochs and greater ejecta masses. The wavelengths most affected are found to be $\lambda \lesssim 4~\mu$m, with the species Ce\,\textsc{iii} and Nd \textsc{ii} being the most important contributors to spectral formation. We also find significant emission from species proposed in observations, notably Te\,\textsc{iii} at 2.1 $\mu$m, and Se\,\textsc{iii} at 4.5 and 5.7 $\mu$m, while W\,\textsc{iii} is subdominant at 4.5 $\mu$m. The Te\,\textsc{iii} feature at 2.1 $\mu$m is always blended, particularly with Zr\,\textsc{ii}, Ce\,\textsc{iii}, and Nd\,\textsc{ii}. We do not reproduce the smooth blackbody-like continua observed in AT2023vfi. Based on our results, we argue that line opacity alone is likely insufficient to produce optically thick continua in the nebular phase, even in the case of lanthanide/actinide-rich ejecta, as our models are optically thin in the IR at these epochs. Given that lanthanide contributions are dominant below 4 $\mu$m, we suggest that NIR observations best probe these elements, while MIR spectroscopy with \textit{JWST} can reliably probe non-lanthanide emission even in relatively lanthanide-rich cases.

astro-ph.HE↗

arXiv2025

Nebular spectra of kilonovae with detailed recombination rates -- I. Light r-process composition

To investigate spectra of kilonovae in the NLTE phase (t>= 1 week), we perform atomic calculations for dielectronic recombination (DR) rates for the light r-process elements Se (Z = 34), Rb (Z = 37), Sr (Z = 38), Y (Z = 39), and Zr (Z = 40) using the HULLAC code. For the different elements, our results for the DR rate coefficients for recombining from the ionization states of II to I, III to II, and IV to III vary between 2x10^{-12} - 5x10^{-11} cm^3/s, 10^{-13} - 5x10^{-11} cm^3/s and 2x10^{-15} - 10^{-11} cm^3/s, respectively, at a temperature of T = 10,000 K. Using this new atomic data (DR), we study the impact on kilonova model spectra at phases of t = 10 days and t = 25 days after the merger using the spectral synthesis code SUMO. Compared to models using the previous treatment of recombination as a constant rate, the new models show significant changes in ionization and temperature, and correspondingly, in emergent spectra. With the new rates, we find that Zr (Z = 40) plays a yet more dominant role in kilonova spectra for light r-process compositions. Further, we show that previously predicted mid-infrared (e.g. [Se III] 4.55 mum) and optical (e.g. Rb I 7802, 7949 {\AA}) lines weaken in the new model. Instead [Se I] 5.03 mum emerges as a signature. These results demonstrate the importance of considering the detailed microphysics for modelling and interpreting the late-time kilonova spectra.

astro-ph.HE↗

arXiv2024

Actinide signatures in low electron fraction kilonova ejecta

Neutron star (NS) mergers are known to produce heavy elements through rapid neutron capture (r-process) nucleosynthesis. Actinides are expected to be created solely by the r-process in the most neutron rich environments. Confirming if NS mergers provide the requisite conditions for actinide creation is therefore central to determining their origin in the Universe. Actinide signatures in kilonova (KN) spectra may yield an answer, provided adequate models are available in order to interpret observational data. In this study, we investigate actinide signatures in neutron rich merger ejecta. We use three ejecta models with different compositions and radioactive power, generated by nucleosynthesis calculations using the same initial electron fraction ($Y_e = 0.15$) but with different nuclear physics inputs and thermodynamic expansion history. These are evolved from 10 - 100 days after merger using the SUMO non-local thermodynamic equilibrium (NLTE) radiative transfer code. We highlight how uncertainties in nuclear properties, as well as choices in thermodynamic trajectory, may yield entirely different outputs for equal values of $Y_e$. We consider an actinide-free model and two actinide-rich models, and find that the emergent spectra and lightcurve evolution are significantly different depending on the amount of actinides present, and the overall decay properties of the models. We also present potential key actinide spectral signatures, of which doubly ionized $_{89}$Ac and $_{90}$Th may be particularly interesting as spectral indicators of actinide presence in KN ejecta.

astro-ph.HE↗

arXiv2023

NLTE Spectra of Kilonovae

The electromagnetic transient following a binary neutron star merger is known as a kilonova (KN). Owing to rapid expansion velocities and small ejecta masses, KNe rapidly transition into the Non-Local Thermodynamic Equilibrium (NLTE) regime. In this study, we present synthetic NLTE spectra of KNe from 5 to 20 days after merger using the \texttt{SUMO} spectral synthesis code. We study three homogeneous composition, 1D multi-zone models with characteristic electron fractions of $Y_e \sim 0.35, 0.25$ and $0.15$. We find that emission features in the spectra tend to emerge in windows of reduced line blocking, as the ejecta are still only partially transparent even at 20 days. For the $Y_e \sim 0.35$ (lanthanide-free) ejecta, we find that the neutral and singly ionised species of Rb, Sr, Y and Zr dominate the spectra, all with good potential for identification. We directly test and confirm an impact of Sr on the 10000 angstrom spectral region in lanthanide-free ejecta, but also see that its signatures may be complex. We suggest the Rb I $\rm{5p^{1}}$- $\rm{5s^{1}}$ 7900 angstrom transition as a candidate for the $\lambda_0 \sim$ 7500--7900 angstrom P-Cygni feature in AT2017gfo. For the $Y_e \sim 0.25$ and $0.15$ compositions, lanthanides are dominant in the spectral formation, in particular Nd, Sm, and Dy. We identify key processes in KN spectral formation, notably that scattering and fluorescence play important roles even up to 20 days after merger, implying that the KN ejecta are not yet optically thin at this time.

astro-ph.HE↗

arXiv2022

NLTE Effects on Kilonova Expansion Opacities

A binary neutron star merger produces a rapidly evolving transient known as a kilonova (KN), which peaks a few days after merger. Modelling of KNe has often been approached assuming local thermodynamic equilibrium (LTE) conditions in the ejecta. We present the first analysis of non-local thermodynamic equilibrium (NLTE) level populations, using the spectral synthesis code SUMO, and compare these to LTE values. We investigate the importance of the radiation field by conducting NLTE excitation calculations with and without radiative transfer. Level populations, in particular higher lying ones, start to show deviations from LTE a few days after merger. Excitation is lower in NLTE for the majority of ions and states, and this tends to give lower expansion opacities. While the difference is small for the first few days, it grows to factors 2-10 after this. Our results are important both for demonstrating validity of LTE expansion opacities for an initial phase (few days), while highlighting the need for NLTE modelling during later phases. Considering also NLTE ionisation, our results indicate that NLTE can give both higher or lower opacities, depending on composition and wavelength, sometimes by orders of magnitudes.

astro-ph.HE↗

arXiv2021

On the Validity of Steady-State for Nebular Phase Kilonovae

The radioactively powered transient following a binary neutron star (BNS) merger, known as a kilonova (KN), is expected to enter the steady-state nebular phase a few days after merger. Steady-state holds until thermal reprocessing time-scales become long, at which point the temperature and ionisation states need to be evolved time-dependently. We study the onset and significance of time-dependent effects using the non-local thermodynamic equilibrium (NLTE) spectral synthesis code SUMO. We employ a simple single-zone model with an elemental composition of Te, Ce, Pt and Th, scaled to their respective solar abundances. The atomic data are generated using the Flexible Atomic Code (FAC), and consist of energy levels and radiative transitions, including highly forbidden lines. We explore the KN evolution from 5 to 100 days after merger, varying ejecta mass and velocity. We also consider variations in the degree of electron magnetic field trapping, as well as radioactive power generation for alpha and beta decay (but omitting fission products). We find that the transition time, and magnitude of steady-state deviations are highly sensitive to these parameters. For typical KN ejecta, the deviations are minor within the time-frame studied. However, low density ejecta with low energy deposition show significant differences from $\sim 10$ days. Important deviation of the ionisation structure solution impacts the temperature by altering the overall line cooling. Adiabatic cooling becomes important at $t \geq 60$ days which, in addition to the temperature and ionisation effects, lead to the bolometric light curve deviating from the instantaneous radioactive power deposited.

astro-ph.HE↗

arXiv2020

Searching for Super-Eddington Quasars using a Photon Trapping Accretion Disc Model

Accretion onto black holes at rates above the Eddington limit has long been discussed in the context of supermassive black hole (SMBH) formation and evolution, providing a possible explanation for the presence of massive quasars at high redshifts (z$\gtrsim$7), as well as having implications for SMBH growth at later epochs. However, it is currently unclear whether such `super-Eddington' accretion occurs in SMBHs at all, how common it is, or whether every SMBH may experience it. In this work, we investigate the observational consequences of a simplistic model for super-Eddington accretion flows -- an optically thick, geometrically thin accretion disc (AD) where the inner-most parts experience severe photon-trapping, which is enhanced with increased accretion rate. The resulting spectral energy distributions (SEDs) show a dramatic lack of rest-frame UV, or even optical, photons. Using a grid of model SEDs spanning a wide range in parameter space (including SMBH mass and accretion rate), we find that large optical quasar surveys (such as SDSS) may be missing most of these luminous systems. We then propose a set of colour selection criteria across optical and infra-red colour spaces designed to select super-Eddington SEDs in both wide-field surveys (e.g., using SDSS, 2MASS and WISE) and deep & narrow-field surveys (e.g., COSMOS). The proposed selection criteria are a necessary first step in establishing the relevance of advection-affected super-Eddington accretion onto SMBHs at early cosmic epochs.

astro-ph.GA↗

arXiv2018

The Solar Wind Environment in Time

We use magnetograms of 8 solar analogues of ages 30~Myr to 3.6~Gyr obtained from Zeeman Doppler Imaging (ZDI) and taken from the literature, together with two solar magnetograms, to drive magnetohydrodynamical (MHD) wind simulations and construct an evolutionary scenario of the solar wind environment and its angular momentum loss rate. With observed magnetograms of the radial field strength as the only variant in the wind model, we find that power law model fitted to the derived angular momentum loss rate against time, $t$, results in a spin down relation $Ω\propto t^{-0.51}$, for angular speed $Ω$, which is remarkably consistent with the well-established Skumanich law $Ω\propto t^{-0.5}$. We use the model wind conditions to estimate the magnetospheric standoff distances for an Earth-like test planet situated at 1~AU for each of the stellar cases, and to obtain trends of minimum and maximum wind ram pressure and average ram pressure in the solar system through time. The wind ram pressure declines with time as $\overline{P_{ram}}\propto t^{2/3}$, amounting to a factor of 50 or so over the present lifetime of the solar system.

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