arXiv subjects

Ilya Mandel

Publications and source records attributed to Ilya Mandel.

At least 37 records · Page 2Linked to original sources

arXiv2025

Discovery of a new transitional type of evolved massive stars with hard ionizing flux

Wolf-Rayet (WR) stars are the evolved descendants of the most massive stars and show emission-line dominated spectra formed in their powerful stellar winds. Marking the final evolution stage before core collapse, the standard picture of WR stars has been that they evolve through three well-defined spectral subtypes known as WN, WC, and WO. Here, we present a detailed analysis of five objects that defy this scheme, demonstrating that WR stars can also evolve directly from the WN to the WO stage. Our study reveals that this direct transition is connected to low metallicity and weaker winds. The WN/WO stars and their immediate WN precursors are hot and emit a high flux of photons capable of fully ionizing helium. The existence of these stages unveil that high mass stars which manage to shed off their outer hydrogen layers in a low-metallicity environment can spend a considerable fraction of their lifetime in a stage that is difficult to detect in integrated stellar populations, but at the same time yields hard ionizing flux. The identification of the WN to WO evolution path for massive stars has significant implications for understanding the chemical enrichment and ionizing feedback in star-forming galaxies, in particular at earlier cosmic times.

astro-ph.SR↗

arXiv2025

Rapid stellar and binary population synthesis with COMPAS: methods paper II

The COMPAS public rapid binary population synthesis code has undergone a number of key improvements since the original COMPAS methods paper (Team COMPAS: Riley et al., 2022) was published. These include more sophisticated and robust treatments of binary interactions: mass transfer physics, common-envelope events, tides and gravitational-wave radiation reaction; and updated prescriptions for stellar evolution, winds and supernovae. The code structure and outputs have also been updated, with a focus on improving resolution without sacrificing computational speed. This paper describes the substantive changes in the code between the previous methods paper and COMPAS v03.22.01.

astro-ph.SR↗

arXiv2025

The Kick Velocity Distribution of Isolated Neutron Stars

Neutron stars (NSs) are thought to receive natal kicks at their formation in supernovae. In order to investigate the magnitude of these kicks, we analyze the proper motions and distance estimates -- either through parallax or dispersion measures -- of young isolated pulsars and infer their three-dimensional velocities relative to their local standard of rest. We find that the velocities based on parallax distances of pulsars younger than 10 Myr follow a log-normal distribution with $μ=5.60\pm0.12$ and $σ=0.68\pm0.10$, peaking at ${\sim}$150--200 km/s, which we adopt as our fiducial kick distribution. Using a previously established method that infers kick magnitudes through the eccentricity of Galactic trajectories, we also estimate the kick velocities of older pulsars, which we find to be consistent with our fiducial kick distribution. A log-normal fit to all pulsars with ages below 40 Myr yields a more constraining (but possibly more prone to systematic errors) fit with $μ=5.67\pm0.10$ and $σ=0.59\pm0.08$, respectively. Moreover, we (1) resolve the tension between our results and the Maxwellian distribution found by Hobbs et al. (2005), which has a ${\sim}50\%$ higher median velocity, by showing that their analysis is missing a Jacobian needed to correct for its logarithmic histogram bin sizes, and (2) argue that the bimodality found by others is not statistically significant and that previous results are consistent with our inferred kick distribution, effectively reconciling the literature on observed NS kicks.

astro-ph.HE↗

arXiv2025

Common envelopes in massive stars III. The obstructive role of radiation transport in envelope ejection

We present 3D radiation hydrodynamics simulations of common-envelope (CE) evolution involving a 12 solar mass red supergiant donor and a 3 solar mass companion. Existing 3D simulations are predominantly adiabatic, focusing strongly on low-mass donors on the red giant and asymptotic giant branches. However, the adiabatic assumption breaks down once the perturbed CE material becomes optically thin or when entering a longer-timescale evolutionary phase after the dynamical plunge-in. This is especially important for high-mass red supergiant donors, which have short thermal timescales, adding significant uncertainty to our understanding of how massive binary stars evolve into gravitational-wave sources, X-ray binaries, stripped-envelope supernovae, and more. We compare our radiation hydrodynamics simulations with an adiabatic simulation from Paper I that is otherwise identical, finding that radiative diffusion strongly inhibits CE ejection. The fraction of ejected mass is roughly half that of the adiabatic case without accounting for recombination energy release. Almost no material is ejected during the dynamical plunge-in, and longer-timescale ejection during the slow spiral-in is suppressed. However, the orbital separation reached at the end of the dynamical plunge-in does not differ significantly. The large amount of remaining bound mass tentatively supports the emerging view that the dynamical plunge-in is followed by a non-adiabatic phase, during which a substantial fraction of the envelope is ejected and the binary orbit may continue to evolve.

astro-ph.SR↗

arXiv2025

Radio emission from tidal disruption events produced by the collision between super-Eddington outflows and the circumnuclear medium

In this Letter, we simulate the collision between outflows from the tidal disruption of a 1M$_\odot$ main sequence star around a $10^6$M$_\odot$ black hole and an initially spherically symmetric circumnuclear cloud. We launch super-Eddington outflows self-consistently by simulating the disruption of stars on both bound and unbound initial orbits using general relativistic smoothed particle hydrodynamics. We find shocks formed as early as $\sim 10~$days after the initial stellar disruption produce prompt radio emission. The shock radius ($\approx~10^{17}$~cm), velocity ($\sim 0.15$c) and total energy ($\sim 10^{51}$ erg) in our simulations match those inferred from radio observations of tidal disruption events (TDEs). We ray-trace to produce synthetic radio images and spectra to compare with the observations. While the TDE outflow is quasi-spherical, the synchrotron emitting region is aspherical but with reflection symmetry above and below the initial orbital plane. Our synthetic spectra show continuous decay in peak frequency, matching prompt radio TDE observations. Our model supports the hypothesis that synchrotron radio flares from TDEs result from the collision between outflows and the circumnuclear material.

astro-ph.HE↗

arXiv2025

The kangaroo's first hop: the early fast cooling phase of EP250108a/SN 2025kg

Fast X-ray transients (FXTs) are a rare and poorly understood population of events. Previously difficult to detect in real time, the launch of the Einstein Probe with its wide field X-ray telescope has led to a rapid expansion in the sample and allowed the exploration of these transients across the electromagnetic spectrum. EP250108a is a recently detected example linked to an optical counterpart, SN 2025kg, or 'the kangaroo'. Together with a companion paper (Rastinejad et al. 2025), we present our observing campaign and analysis of this event. In this letter, we focus on the early evolution of the optical counterpart over the first six days, including our measurement of the redshift of $z=0.17641$. We find that the source is well-modelled by a rapidly expanding cooling blackbody. We show the observed X-ray and radio properties are consistent with a collapsar-powered jet that is low energy ($\lesssim10^{51}$ erg) and/or fails to break out of the dense material surrounding it. While we examine the possibility that the optical emission emerges from the shock produced as the supernova ejecta expand into a dense shell of circumstellar material, due to our X-ray and radio inferences, we favour a model where it arises from a shocked cocoon resulting from the trapped jet. This makes SN 2025kg one of the few examples of this currently observationally rare event.

astro-ph.HE↗

arXiv2025

Interpretation of the binary black hole mass spectrum

This is a summary of an invited talk given at the Moriond Gravitation meeting on March 31, 2025. I touch on some of the practical challenges of measuring the mass spectrum of merging binary black holes through their gravitational-wave signatures. I then describe my take on the current state of interpreting the observed binary black hole mass spectrum from the perspective of models for the formation of these sources. I conclude that meaningful progress must rely on the combination of gravitational-wave observations and a broad range of electromagnetic observations of massive stellar binaries at earlier stages of their evolution. This is my very personal and necessarily brief take on the current state of the field and does not aspire to the balance or completeness of a review.

astro-ph.HE↗

arXiv2025

Prompt Periodicity in the GRB 211211A Precursor: Black-hole or magnetar engine?

The merger origin long GRB 211211A was a class (re-)defining event. A precursor was identified with a $\sim 1$ s separation from the main burst, as well as a claimed candidate quasi-periodic oscillation (QPO) with a frequency $\sim20$ Hz. Here, we explore the implications of the precursor, assuming the quasi-periodicity is real. The precursor variability timescale requires relativistic motion with a Lorentz factor $Γ\gtrsim80$, and implies an engine driven jetted outflow. The declining amplitude of the consecutive pulses requires an episodic engine with an `on/off' cycle consistent with the QPO. For a black-hole central engine, the QPO can have its origin in Lense-Thirring precession of the inner disk at $\sim6-9$ $r_g$ (gravitational radii) for a mass $M_\bullet\leq4.5$ $M_{\odot}$, and $\lesssim 7$ $r_g$ for $M_\bullet>4.5$ $M_{\odot}$ and dimensionless spin $χ\sim 0.3 - 0.9$. Alternatively, at a disk density of $\sim10^{8 - 12}$ g cm$^{-3}$, the required magnetic field strength for a QPO via magnetohydrodynamic effects will be on the order $B\sim10^{12 - 14}$ G. If the central engine is a short lived magnetar or hypermassive neutron star, then a low-frequency QPO can be produced via instabilities within the disk at a radius of $\sim20 - 70$ km, for a disk density $\sim10^{9 - 12}$ g cm$^{-3}$ and magnetic field $\gtrsim10^{13 - 14}$ G. The QPO cannot be coupled to the neutron star spin, as the co-rotation radius is beyond the scale of the disk. Neither engine can be ruled out -- however, we favour an origin for the precursor candidate QPO as early jet-disk coupling for a neutron star -- black hole merger remnant with mass $M_\bullet>4.5$ $M_{\odot}$.

astro-ph.HE↗

arXiv2025

Constraints on LIGO/Virgo Compact Object Mergers from Late-time Radio Observations

We present results from a search for radio afterglows of compact object mergers conducted with the Australian SKA Pathfinder. We used data from four epochs of the Rapid ASKAP Continuum Survey to search compact binary merger localization regions observed during the LIGO/Virgo O2, and O3 observing runs. Our investigation focused on eleven events (published in the GWTC-1, GWTC-2, and GWTC-3 catalogues of gravitational-wave events) with 90\% posterior localisations smaller than $150\,°^2$ and $\ge$99\% probabilities of being of astrophysical origin, to identify potential radio afterglow-like transients up to $\lesssim$1500 days post-merger. We identified candidate afterglow-type variable sources in the 90\% localisation for events -- GW190503, GW200202 and GW200208, which were ruled out as unlikely to be related to the corresponding GW event on further analysis. Since we find no likely candidate counterparts, we constrain the inclination angle and the circum-merger density at isotropic equivalent energies ranging from $2\times10^{51} -1\times10^{54}\rm \:erg$. These constraints are based on the assumption that the electron energy distribution in the associated jets follows a power-law index of $ p = 2.2$, with 1% of the shock energy in the magnetic field ($ ε_B = 0.01$) and 10% in the electrons ($ε_e = 0.1$). We discuss the detectability of late-time afterglows as a function of merger distance and inclination angles with millijansky surveys.

astro-ph.HE↗

arXiv2025

Rarity of precession and higher-order multipoles in gravitational waves from merging binary black holes

The latest binary black hole population estimates argue for a subpopulation of unequal component mass binaries with spins that are likely small but isotropically distributed. This implies a non-zero probability of detecting spin-induced orbital precession and higher order multipole moments in the observed gravitational-wave signals. In this work we directly calculate the probability for precession and higher-order multipoles in each significant gravitational-wave candidate observed by the LIGO--Virgo--KAGRA collaborations (LVK). We find that only one event shows substantial evidence for precession: GW200129_065458, and two events show substantial evidence for higher-order multipoles: GW190412 and GW190814; any evidence for precession and higher-order multipole moments in other gravitational-wave signals is consistent with random fluctuations caused by noise. We then compare our observations with expectations from population models, and confirm that current population estimates from the LVK accurately predict the number of observed events with significant evidence for precession and higher-order multipoles. In particular, we find that this population model predicts that a binary with significant evidence for precession will occur once in every $\sim 50$ detections, and a binary with significant evidence for higher-order multipoles will occur once in every $\sim 70$ observations. However, we emphasise that since substantial evidence for precession and higher-order multipoles have only been observed in three events, any population model that includes a subpopulation of binaries yielding $\sim 2\%$ of events with detectable precession and higher-order multipole moments will likely be consistent with the data.

gr-qc↗

arXiv2025

Hot Jupiter engulfment by a red giant in 3D hydrodynamics

Hot Jupiters are gas giant planets with orbital periods of a few days and are found in 0.1-1% of Sun-like stars. They are expected to be engulfed during their host star's radial expansion on the red giant branch, which may account for observed rapidly rotating and chemically enriched giant stars. We performed 3D hydrodynamical simulations of hot Jupiter engulfment by a 1 solar mass, 4 solar radii early red giant. Our 'global' simulations simultaneously resolve the stellar envelope and planetary structure, modelling the hot Jupiter as a polytropic gas sphere. The hot Jupiter spirals in due to ram-pressure drag. A substantial fraction of its mass is continuously ablated in this process, although the mass-loss rate is resolution dependent. We estimate that this could enhance the surface lithium abundance by up to 0.1 dex. The hot Jupiter is disrupted by a combination of ram pressure and tidal forces near the base of the convective envelope, with the deepest material penetrating to the radiative zone. The star experiences modest spin-up (~ 1 km/s), and engulfing a more massive companion may be required to produce a rapidly rotating giant. Drag heating near the surface and hydrogen recombination in the small amount of unbound ejecta recorded in the simulation could power an optical transient, although this needs to be confirmed by a calculation that has adequate resolution at the stellar surface.

astro-ph.SR↗

arXiv2024

Common envelopes at StanFest

We describe some of our group's recent work on common envelopes. Our goal is to understand the onset and outcomes of dynamically unstable mass transfer, including the properties of the binaries left behind and the outflows during the common envelope stage. We have also started thinking about light curves of common envelope events. During a talk at StanFest, a meeting in honour of Stan Owocki's retirement held in Leuven in July, 2024, the first author reported on some of the results we recently obtained and briefly outlined future prospects.

astro-ph.SR↗

arXiv2024

How to Escape from a Trap: Outcomes of Repeated Black Hole Mergers in AGN

Stellar-mass black holes (BHs) embedded in active galactic nuclei (AGN) may be major sources of astrophysical gravitational waves (GWs), contributing both to the observed LIGO-Virgo-KAGRA population of binary BH mergers and to future populations of LISA-band extreme mass ratio inspirals (EMRIs). The ability of these BHs to pair up into binaries, inspiral, and produce GWs will be shaped by the existence of migration traps, regions in the AGN where hydrodynamic torques vanish. Previous works have studied the existence and location of migration traps in AGN disks. Here, we investigate how individual BHs may escape such traps as an outcome of mergers, potentially suppressing hierarchical growth. We find that while GW recoil kicks are strong enough to kick merged BHs onto inclined orbits, gas drag quickly realigns them into the AGN disk. A more robust escape mechanism is gap opening: once a BH grows above a critical mass, its gravity disturbs the AGN gas sufficiently to eliminate the trap. In low-mass AGN relevant for LISA, gaps open easily and the resulting ``wet EMRI'' masses are unlikely to reflect protracted hierarchical mergers. In combination with our previous work, we find that migration traps only exist in a relatively narrow range of AGN luminosities between [10^{43.5},10^{45.5}] erg/s. We identify an even narrower AGN luminosity range for which stellar mass BHs can grow into the pair instability mass gap and beyond. This characteristic luminosity scale may assist in indirect tests of the ``AGN channel'' for binary BH mergers.

astro-ph.HE↗

arXiv2024

Theoretical expectations for high-mass X-ray binaries, supernova remnants, and their evolutionary paths: An overview talk for LIAC41: the eventful life of stellar multiples

In this invited talk at the 41st Liège International Astrophysical Colloquium on "The eventful life of massive star multiples", I reviewed some aspects of our current understanding of neutron stars and black holes as end products of stellar evolution as well as the evolutionary paths leading to the formation of high-mass X-ray binaries.

astro-ph.HE↗

arXiv2024

Evolution of the Convective Core Mass during the Main Sequence

We construct a semi-analytical model that describes the convective core mass evolution of massive stars experiencing mass loss during the main-sequence stage. We first conduct a suite of 1D stellar evolution calculations to build insight into how convective core masses behave under idealized mass loss. Based on these simulations, we find several universal relations between global properties of the star that hold regardless of the mass loss history. By combining these relations, we construct a semi-analytic framework that can predict the convective core mass evolution for arbitrary mass loss histories and hence the helium core mass at the end of the main sequence. Our formulae improve upon existing methods for predicting the core mass in rapid population synthesis codes.

astro-ph.SR↗

arXiv2024

No evidence for a dip in the binary black hole mass spectrum

Stellar models indicate that the core compactness of a star, which is a common proxy for its explodability in a supernova, does not increase monotonically with the star's mass. Rather, the core compactness dips sharply over a range of carbon-oxygen core masses; this range may be somewhat sensitive to the star's metallicity and evolutionary history. Stars in this compactness dip are expected to experience supernovae leaving behind neutron stars, whereas stars on either side of this range are expected to form black holes. This results in a hypothetical mass range in which black holes should seldom form. Quantitatively, when applied to binary stripped stars, these models predict a dearth of binary black holes with component masses $\approx 10 M_\odot - 15 M_\odot$. The population of gravitational-wave signals indicates potential evidence for a dip in the distribution of chirp masses of merging binary black holes near $\approx 10 M_\odot - 12 M_\odot$. This feature could be linked to the hypothetical component mass gap described above, but this interpretation depends on what assumptions are made of the binaries' mass ratios. Here, we directly probe the distribution of binary black hole component masses to look for evidence of a gap. We find no evidence for this feature using data from the third gravitational-wave transient catalogue (GWTC-3). If this gap does exist in nature, we find that it is unlikely to be resolvable by the end of the current (fourth) LIGO-Virgo-KAGRA (LVK) observing run.

astro-ph.HE↗

arXiv2024

Bumpy Superluminous Supernovae Powered by a Magnetar-star Binary Engine

Wolf-Rayet stars in close binary systems can be tidally spun up by their companions, potentially leaving behind fast-spinning highly-magnetized neutron stars, known as ``magnetars", after core collapse. These newborn magnetars can transfer rotational energy into heating and accelerating the ejecta, producing hydrogen-poor superluminous supernovae (SLSNe). In this {\em{Letter}}, we propose that the magnetar wind of the newborn magnetar could significantly evaporate its companion star, typically a main-sequence or helium star, if the binary system is not disrupted by the {abrupt mass loss and} SN kick. The subsequent heating and acceleration of the evaporated star material along with the SN ejecta by the magnetar wind can produce a post-peak bump in the SLSN lightcurve. Our model can reproduce the primary peaks and post-peak bumps of four example observed multiband SLSN lightcurves, revealing that the mass of the evaporated material could be $\sim0.4-0.6\,M_\odot$ if the material is hydrogen-rich. {We propose that the magnetar could induce strongly enhanced evaporation from its companion star near the pericenter if the orbit of the post-SN binary is highly eccentric, ultimately generating multiple post-peak bumps in the SLSN lightcurves. This ``magnetar-star binary engine" model may offer a possible explanation for the evolution of polarization, along with the origin and velocity broadening of late-time hydrogen or helium broad spectral features observed in some bumpy SLSNe.} The diversity in the lightcurves and spectra of SLSNe may be attributed to the wide variety of companion stars and post-SN binary systems.

astro-ph.HE↗

arXiv2024

Eddington envelopes: The fate of stars on parabolic orbits tidally disrupted by supermassive black holes

Stars falling too close to massive black holes in the centres of galaxies can be torn apart by the strong tidal forces. Simulating the subsequent feeding of the black hole with disrupted material has proved challenging because of the range of timescales involved. Here we report a set of simulations that capture the relativistic disruption of the star, followed by one year of evolution of the returning debris stream. These reveal the formation of an expanding asymmetric bubble of material extending to hundreds of astronomical units -- an outflowing Eddington envelope with an optically thick inner region. Such envelopes have been hypothesised as the reprocessing layer needed to explain optical/UV emission in tidal disruption events, but never produced self-consistently in a simulation. Our model broadly matches the observed light curves with low temperatures, faint luminosities, and line widths of 10,000--20,000 km/s.

astro-ph.HE↗