arXiv subjects

Bradley J. Kavanagh

Publications and source records attributed to Bradley J. Kavanagh.

At least 19 recordsLinked to original sources

arXiv2026

Gamma Rays from ALP-Photon Conversion and Inverse Compton Reprocessing in Neutron Star Magnetospheres

Exploring axion-like particle (ALP) signatures from neutron stars (NSs) in the \emph{Fermi}-LAT energy range remains largely unexplored. Neutron stars with exceptionally strong magnetic fields, such as magnetars and pulsars with magnetar-like magnetic fields, provide particularly promising environments for ALP--photon conversion. Magnetars are characterized by surface magnetic fields as large as $B_0\sim(10^{14}$--$10^{15})\,\mathrm{G}$; however, despite their extreme magnetic fields, no steady magnetar emission has been firmly detected in the \emph{Fermi}-LAT energy range, with high-energy activity generally associated with rare flaring episodes. In this work, we investigate ALP production in the interiors of different classes of NSs and the subsequent conversion of ALPs into photons in their magnetospheres. The ALP emissivity is determined by the stellar density and temperature $T$, while the conversion probability is enhanced by the strong magnetic fields surrounding the star. We further account for photon propagation through the Galactic magnetic field, which can provide an additional contribution to the observable photon flux. We investigate the resulting gamma-ray signatures and assess whether ALP-induced emission from NS magnetospheres could be detectable at energies $E\gtrsim100,\mathrm{MeV}$ in the \emph{Fermi}-LAT band. In addition, we consider if the reprocessing of the magnetospheric photons through inverse Compton scattering can shift part of the emission to higher energies and provide an additional observational signature. We use the resulting fluxes to derive constraints from existing gamma-ray observations and to estimate the sensitivity of future MeV--GeV observations, taking COSI as a representative example.

astro-ph.HE↗

arXiv2026

Axions at the meV Crossroads: Theory, Cosmology, Astrophysics, and Experiments

The meV mass range has emerged as a focal point in axion physics, where advances in theory, cosmology, astrophysics, and experimental techniques converge. Axions in this mass range are theoretically well motivated, can arise in ultraviolet-complete models, and can have significant cosmological impacts as dark matter or dark radiation. In parallel, their efficient production in stellar and supernova environments provides powerful astrophysical probes. Here, we provide a comprehensive overview of meV axions across these domains, highlighting both established results and open questions. We discuss the theoretical underpinnings of meV axions, their cosmological and astrophysical signatures, and the diverse experimental strategies -- ranging from helioscopes and haloscopes to quasiparticle systems and large-volume Cherenkov detectors -- that aim to explore this regime. The convergence of these approaches emphasizes the pivotal role of the meV mass range for axion discovery in the coming years, identifying meV axions as a key probe for testing beyond-Standard-Model physics. This review document is the direct outcome of the discussions at the dedicated workshop "The meV Mass Axion Frontier: Challenges and Opportunities", held at Laboratori Nazionali di Frascati (IT) on 27--28 October 2025, and organized by the EU funded COST Action "Cosmic WISPers in the Dark Universe: Theory, astrophysics, and experiments" (CA21106, https://www.cost.eu/actions/CA21106). Its aim is to provide an overview of current efforts in meV axion research, their motivations, and the research goals that animate the community involved in this search.

hep-ph↗

arXiv2026

The swallowed spike: the formation of light primordial black hole structures around heavy seeds

Spikes are steep enhancements in the dark matter (DM) distribution around a heavy compact object. If the compact object is primordial, and the bulk of the DM is also composed of (lighter) primordial compact objects, for instance asteroid-mass primordial black holes (PBHs), the phenomenology of spike formation is highly non-trivial. In fact, lighter PBHs have negligible angular momentum at formation with respect to the massive central object and would therefore be captured unless enough torque is exerted from either small-scale or large-scale matter fluctuations. In this paper, we present the first comprehensive assessment of this scenario. We define the mechanisms and the initial conditions that allow light PBHs to avoid capture. We then quantify the different types of torque and follow the corresponding angular momentum evolution with a combination of analytical prescriptions and numerical simulations. We find that in the innermost region no mechanism studied here is capable of providing enough torque; the resulting inner core is expected to be significantly less dense than in particle scenarios, potentially leading to interesting phenomenology.

astro-ph.CO↗

arXiv2026

On the survival of dark matter spikes: Stellar and compact-object perturbations

Establishing realistic expectations for the dark matter (DM) distribution near a supermassive black hole (BH) is essential for assessing environmental imprints on gravitational wave (GW) signals. Using the Galactic Center as an observationally constrained case study, we investigate the evolution of DM density spikes under gravitational perturbations from the nuclear stellar and black hole populations surrounding the central BH. We find that scattering of DM particles by the nuclear star cluster depletes the DM distribution at radii $r \sim 10^{-1}~\mathrm{pc}$, far outside the region where the relevant GW signals are produced. At smaller radii, at $r \sim 10^{-3}~\mathrm{pc}$, the closest known stellar perturbers, S2 and S38, induce only negligible changes to the density profile. At still smaller radii, where no stellar perturbers are currently known, we assess the cumulative impact of past EMRIs by modelling successive mergers with stellar-mass BHs of mass $\sim10~\mathrm{M}_\odot$, in numbers consistent with the expected rate over $10~\mathrm{Gyr}$. We find that these events do not erase the central overdensity, reducing the density only to approximately $82 \%$ of its initial value at $r \lesssim 10^{-5}~\mathrm{pc}$. Our results indicate that, at least at the Galactic Center, DM overdensities around the central BH are expected to remain largely intact under stellar perturbations and plausible stellar-BH merger histories.

astro-ph.GA↗

arXiv2026

Dark matter mounds from the collapse of supermassive stars: a general-relativistic analysis

Recent work has highlighted the importance of a fully relativistic treatment of the dephasing of gravitational waves induced by dark-matter overdensities in extreme mass-ratio inspirals (EMRIs). However, a general-relativistic description of the dark matter phase-space distribution is currently available only for the case of a dark matter "spike" arising from adiabatic black hole growth. Here we develop a fully general-relativistic formalism for the more realistic scenario in which a supermassive stellar progenitor collapses to a black hole and produces a shallower dark matter overdensity, or "mound". We follow self-consistently the evolution of the supermassive star, its collapse, and the subsequent growth of the resulting black hole, together with the collisionless dark matter orbits. We find that in the regime where the collapse becomes non-adiabatic, the dark matter distribution function is significantly reshaped, with a clear depletion in the low-binding-energy region of phase space. Our results provide a more realistic prediction for the dark matter phase-space distribution around supermassive black holes, which is an essential step in our programme to use future EMRI observations to extract information about both the nature of dark matter and the formation history of the black hole.

gr-qc↗

arXiv2026

Caustic crossings in giant arcs with extended dark matter objects

Caustic-crossing stars observed in giant arcs behind galaxy clusters provide a powerful probe of dark matter substructure. While previous work has focused on point-like lenses such as primordial black holes, we extend this framework to extended dark objects (EDOs), including ultracompact minihalos formed from the collapse of primordial overdensities. We develop an analytic model of microlensing by EDOs embedded in a macrolensing cluster potential and derive the resulting caustics and light curves. Depending on the EDO size relative to the effective Einstein radius, we show that they may generate additional narrow caustics, leading to novel features in the light curve. Applying our framework to the MACS J1149 LS1 ``Icarus'' event, we constrain EDOs with radii up to $10^7 R_\odot$. Our results demonstrate that caustic-crossing events complement galactic microlensing searches, as they can probe EDOs with larger physical size. We discuss the implications for current and future observations, which promise to deliver a statistical sample of caustic transients and correspondingly sharper constraints on dark objects.

astro-ph.CO↗

arXiv2026

Axion search with telescope for radio astronomy (ASTRA): forecast for observations between 0.5 and 4~GHz

Axion dark matter (DM) is predicted to convert into radio waves in neutron star magnetospheres. We assess the detectability of this signal using a 5 m radio telescope to be installed at the Fan Mountain Observatory, operating in the UHF, L- and S-bands from 0.5 to 4~GHz. We demonstrate that such a telescope can search new parameter space for axion-like particles over a broad range from $2\,μ\text{eV}<m_a<17\,μ\text{eV}$ for axion-photon couplings $g_{aγγ} \gtrsim 2\times 10^{-12}\text{ GeV}^{-1}$ with a three year observing period assuming the standard halo model -- improving neutron star observations by more than an order of magnitude. The search is broadband and is thus complementary to other techniques in the same frequency range. We describe in detail our neutron star population model, noise model, and proposed observing strategy. Most constraining power comes from neutron stars at the Galactic centre, where the smooth DM halo is densest. If a DM spike exists at the Galactic centre, the search is sensitive in the QCD axion model band. UHF and L-band observations (0.5 to 2~GHz) represent the pathfinder phase of a wider program we call ``Axion Search with Telescope for Radio Astronomy'' (ASTRA). Future higher mass searches aimed at discovery potential for the post-inflation axion require further hardware development to cover S, C, X and Ku bands (2 to 18~GHz).

hep-ph↗

arXiv2026

Hints of Dark Matter Spikes in Low-mass X-ray Binaries: a critical assessment

Three black-hole low-mass X-ray binaries (LMXBs) in the Milky Way show rates of period decay which cannot be easily explained by standard mechanisms. Recently, it has been claimed that the anomalous period decays in two of these systems may be explained by dynamical friction due to very high dark matter (DM) densities around the black holes. We critically assess these claims by performing $N$-body simulations of binaries embedded in dense DM ``spikes". We simulate the previously-studied systems XTE J1118+480 and A0620--00, as well as studying the third binary Nova Muscae 1991 for the first time in this context. These simulations show that feedback on the DM distribution plays a crucial role and we rule out previously-claimed shallow DM spikes. We set lower limits on the steepness $γ$ of DM density profiles required to explain the period decay in these LMXBs, requiring $γ\gtrsim 2.15-2.20$ in XTE J1118+480 and A0620--00 and $γ\gtrsim 2.3$ in Nova Muscae 1991. Improved modeling of the long-term evolution of binaries embedded in DM spikes may allow us to exclude even larger densities in future.

hep-ph↗

arXiv2025

A Fast Earth-scattering Formalism for Light Dark Matter with Dark Photon Mediators

While Dark Matter (DM) is typically assumed to interact only very weakly with the particles of the Standard Model, many direct detection experiments are currently exploring regions of parameter space where DM can have a large scattering cross section. In this scenario, DM may scatter in the atmosphere and Earth before reaching the detector, leading to a distortion of the DM flux and a daily modulation of the signal rate as the detector is shielded by more or less of the Earth at different times of day. This modulation is a distinctive signature of strongly-interacting DM and provides a powerful method of discriminating against time-independent backgrounds. However, the calculation of these Earth-scattering effects by Monte Carlo methods is computationally intensive, inhibiting a systematic exploration of the DM parameter space. Here, we present a semi-analytic formalism for calculating Earth-scattering effects, for models of MeV-mass DM which interacts via a dark photon mediator, and release the associated code Verne2. This formalism assumes that DM travels along straight-line trajectories until it scatters and is reflected back along its incoming path, along us to taking into account the affects of both attenuation and reflection in the Earth. We compare this formalism with the results of full Monte Carlo simulations for cross sections within reach of current and future DM-electron scattering searches. We find that Verne2 is accurate to better than 10-30%, making it suitable for performing signal modeling in the search for daily modulation, while reducing the computational cost by a factor of $\sim10^4$ compared to full Monte Carlo simulations.

hep-ph↗

arXiv2025

Sharpening the dark matter signature in gravitational waveforms I: Accretion and eccentricity evolution

Dark matter overdensities around black holes can alter the dynamical evolution of a companion object orbiting around it, and cause a dephasing of the gravitational waveform. Here, we present a refined calculation of the co-evolution of the binary and the dark matter distribution, taking into account the accretion of dark matter particles on the companion black hole, and generalizing previous quasi-circular calculations to the general case of eccentric orbits. These calculations are validated by dedicated N-body simulations. We show that accretion can lead to a large dephasing, and therefore cannot be neglected in general. We also demonstrate that dark matter spikes tend to circularize eccentric orbits faster than previously thought.

gr-qc↗

arXiv2025

Dark Matter Mounds: towards a realistic description of dark matter overdensities around black holes

Dark matter overdensities around black holes can be searched for by looking at the characteristic imprint they leave on the gravitational waveform of binary black hole mergers. Current theoretical predictions of the density profile of dark matter overdensities are based on highly idealised formation scenarios, in which black holes are assumed to grow adiabatically from an infinitesimal seed to their final mass, compressing dark matter cusps at the center of galactic halos into very dense `spikes'. These scenarios were suitable for dark matter indirect detection studies, since annihilating dark matter cannot reach very high densities, but they fail to capture the dark matter distribution in the innermost regions where the gravitational wave signal is produced. We present here a more realistic formation scenario where black holes form from the collapse of supermassive stars, and follow the evolution of the dark matter density as the supermassive star grows and collapses to a black hole. We show that in this case dark matter forms shallower `mounds', instead of `spikes', on scales comparable with the size of the supermassive stars originating them. We discuss the implications for the detectability of these systems.

astro-ph.CO↗

arXiv2024

Indirect detection of the QCD axion

The QCD axion, originally proposed to solve the strong CP problem in QCD, is a prominent candidate for dark matter (DM). In the presence of strong magnetic fields, such as those around neutron stars, axions can theoretically convert into photons, producing detectable electromagnetic signals. This axion-photon coupling provides a unique experimental pathway to probe axions within a specific mass range. We investigate a novel observational approach using the Green Bank Telescope (GBT) to search for radio transients that could arise from interactions between neutron stars and dense DM clumps known as axion miniclusters. By observing the core of Andromeda with the VErsatile GBT Astronomical Spectrometer (VEGAS) and the X-band receiver (8 to 10 GHz), we achieve sensitivity to axions with masses in the range of (33 - 42)$\,μ$eV, with a mass resolution of $3.8 \times 10^{-4}\,μ$eV. We detail our observational and analytical strategies developed to capture transient signals from axion-photon conversion, achieving an instrumental sensitivity of $2\,$mJy per spectral channel. Despite our sensitivity threshold, no candidate signals exceeding the 5$σ$ level were identified. Future implementations will extend this search across additional spectral bands and refine the modeling used for the processes involved, strengthening the constraints on axion DM models.

astro-ph.CO↗

arXiv2024

The cosmic history of Primordial Black Hole accretion and its uncertainties

Primordial Black Holes (PBHs) have not been experimentally detected so far, but their existence would provide important insights about the early Universe and serve as one of the possible candidates of dark matter (DM). In this work, we explore the accretion of radiation and matter by PBHs, with relevance for the growth of PBH seeds to form early Supermassive Black Holes; the emission from accreting PBHs; and constraints from gravitational wave observations, among others. We study the growth of PBH masses in the early Universe due to the accretion of radiation, highlighting uncertainties which arise from estimates of the PBH formation time. For baryonic accretion, we review the traditional Bondi-Hoyle-Lyttleton (BHL) and its refined version known as the Park-Ricotti (PR) model, which also includes radiative feedback. We find that in the BHL model, PBHs heavier than $\sim 100 \,\mathrm{M_{\odot}}$ can grow in mass by several orders of magnitude by $z \lesssim 10$, though only when surrounded by DM halos and only when the accretion efficiency is large. By contrast, the inclusion of radiation feedback in the PR model can drastically suppress the baryonic accretion rate of PBHs, leading to a negligible change in PBH mass over cosmic time. Furthermore our calculations show that the accretion rate depends sensitively on the modelling of various parameters such as the speed of sound in the baryonic gas and the velocity of PBHs. These findings highlight the uncertainties associated with accretion onto PBHs, and we find that a large increase in the PBH mass due to accretion is by no means guaranteed.

astro-ph.CO↗

arXiv2024

The Lunar Gravitational-wave Antenna: Mission Studies and Science Case

The Lunar Gravitational-wave Antenna (LGWA) is a proposed array of next-generation inertial sensors to monitor the response of the Moon to gravitational waves (GWs). Given the size of the Moon and the expected noise produced by the lunar seismic background, the LGWA would be able to observe GWs from about 1 mHz to 1 Hz. This would make the LGWA the missing link between space-borne detectors like LISA with peak sensitivities around a few millihertz and proposed future terrestrial detectors like Einstein Telescope or Cosmic Explorer. In this article, we provide a first comprehensive analysis of the LGWA science case including its multi-messenger aspects and lunar science with LGWA data. We also describe the scientific analyses of the Moon required to plan the LGWA mission.

gr-qc↗

arXiv2024

Axions in Andromeda: Searching for Minicluster -- Neutron Star Encounters with the Green Bank Telescope

The QCD axion and axion-like particles are compelling candidates for galactic dark matter. Theoretically, axions can convert into photons in the presence of a strong external magnetic field, which means it is possible to search for them experimentally. One approach is to use radio telescopes with high-resolution spectrometers to look for axion-photon conversion in the magnetospheres of neutron stars. In this paper, we describe the results obtained using a novel approach where we used the Green Bank Telescope (GBT) to search for radio transients produced by collisions between neutron stars and dark matter clumps known as axion miniclusters. We used the VErsatile GBT Astronomical Spectrometer (VEGAS) and the X-band receiver (8 to 10 GHz) to observe the core of Andromeda. Our measurements are sensitive to axions with masses between 33 and 42 $μ$eV with $Δ$$m_a$ = 3.8$\times10^{-4}$ $μ$eV. This paper gives a description of the search method we developed, including observation and analysis strategies. Given our analysis algorithm choices and the instrument sensitivity ($\sim$2 mJy in each spectral channel), we did not find any candidate signals greater than 5$σ$. We are currently implementing this search method in other spectral bands.

astro-ph.CO↗

arXiv2024

Sharpening the dark matter signature in gravitational waveforms II: Numerical simulations with the NbodyIMRI code

Future gravitational wave observatories can probe dark matter by detecting the dephasing in the waveform of binary black hole mergers induced by dark matter overdensities. Such a detection hinges on the accurate modelling of the dynamical friction, induced by dark matter on the secondary compact object in intermediate and extreme mass ratio inspirals. In this paper, we introduce NbodyIMRI, a new publicly available code designed for simulating binary systems within cold dark matter `spikes'. Leveraging higher particle counts and finer timesteps, we validate the applicability of the standard dynamical friction formalism and provide an accurate determination of the maximum impact parameter of particles which can effectively scatter with a compact object, across various mass ratios. We also show that in addition to feedback due to dynamical friction, the dark matter also evolves through a `stirring' effect driven by the time-dependent potential of the binary. We introduce a simple semi-analytical scheme to account for this effect and demonstrate that including stirring tends to slow the rate of dark matter depletion and therefore enhances the impact of dark matter on the dynamics of the binary.

gr-qc↗

arXiv2023

Impact of dark matter spikes on the merger rates of Primordial Black Holes

Mergers of Primordial Black Holes (PBHs) may contribute to the gravitational wave mergers detected by the LIGO-Virgo-KAGRA (LVK) Collaboration. We study the dynamics of PBH binaries dressed with dark matter (DM) spikes, for PBHs with extended mass functions. We analyze the impact of DM spikes on the orbital parameters of the PBH binaries formed in the early Universe and calculate their merger rates at the age of the Universe today. We consider two possible scenarios for the dynamics of the dressed binaries: assuming that either the DM spikes are completely evaporated from the binaries before merger or they remain static until the merger. Contrary to previous studies, we find that the presence of spikes may increase or decrease the present-day PBH merger rates, in some cases dramatically. Comparing with merger rates reported by the LVK Collaboration in the third Gravitational Wave Transient Catalog (GWTC-3), we derive approximate constraints on the fraction of Solar-mass PBHs in cold dark matter as $f_\mathrm{pbh}\leq \mathcal{O}(10^{-5} - 10^{-3})$, depending on the mass function. Our calculations are valid only for the idealized scenarios in which the DM spikes are either evaporated or static. However, they suggest that the impact of DM spikes on PBH merger rates may be more complicated than previously thought and motivate the development of a more general description of the merger dynamics, including feedback of the DM spikes in highly eccentric PBH binaries.

astro-ph.CO↗

arXiv2023

Phonon dynamics for light dark matter detection

The search for low-mass dark matter (DM) goes in parallel with the identification of new detection channels and the development of suitable detectors. Detection of the resulting small energy depositions is challenging: it requires extremely high sensitivity, only achievable by cryogenic thermal detectors, which might be put to the limit. Understanding the processes which can limit performances of these detectors can be thus crucial for evaluating the feasibility of the proposed new detection schemes and to design the detectors and tune their performance. In this paper we focus on a promising detection scheme, the excitation of single optical phonons in polar materials, to evaluate one of the possible limiting factors of cryogenic thermal detectors, i.e. the phonon dynamics in the target/absorber. We present a detailed theoretical analysis, within an entirely ab initio scheme, of the downconversion and propagation processes undergone by optical phonons, created by the interaction of a low-mass DM particle in an Al2O3 target, until they reach the interface with a phonon Al collector. After a preliminary methodological survey that reveals the limitations of any Relaxation Time Approximation based method, we developed a 3D beyond-RTA phonon Monte Carlo that allowed us to introduce the spatial dimension of the device and address questions about impact of target size and scattering position. We analyse also the effect of the phonon energy and wavevector and show that isotopes can, perhaps counterintuitively, result in a larger heat flux by providing transport channels of higher velocities, thus favoring detection. Our results suggest that, though challenging, the direct detection of light DM via athermal phonon generation appears feasible, and that the phonon downconversion followed by quasi-ballistic propagation does not appear to be a major bottleneck in terms of reducing the signal.

cond-mat.mtrl-sci↗