arXiv subjects

Marco Regis

Publications and source records attributed to Marco Regis.

At least 19 recordsLinked to original sources

arXiv2026

Probing the Fundamental Nature of Particle Dark Matter

Understanding the fundamental nature of dark matter (DM) is one of the most significant scientific challenges of our time. A compelling hypothesis is that DM consists of a new, yet-to-be-discovered particle. Among the leading candidates are weakly interacting massive particles (WIMPs) and axion-like particles (ALPs), both of which can be investigated using observations with the SKA telescopes. In this chapter, we review the search for particle DM through radio observations, summarizing the current state-of-the-art and presenting forecasts for the SKA-Low and SKA-Mid telescopes in the AA4 baseline design. Radio searches for WIMPs focus on detecting synchrotron radiation originating from the products of DM annihilation using continuum observations. Competitive constraints on sub-TeV WIMPs have already been derived using SKA precursors looking at dwarf galaxies, galaxy clusters, and the Large Magellanic Cloud. We discuss how the superior continuum sensitivity of the SKA telescopes will allow us to progressively close in on the WIMP parameter space. The ALP signal arises from its decay or conversion into photon(s), which typically consists of a nearly monochromatic signature, and from rotation of polarization angles of photons interacting with ALPs. We demonstrate how the spectral resolution, line sensitivity, and polarimetry of the SKA AA4 telescopes can be leveraged to constrain the ALP-photon coupling.

astro-ph.CO↗

arXiv2026

On the potential of pseudo-scalar dark energy

A cosmological pseudo-scalar field provides a compelling realization of dynamical dark energy (DE). If its coupling to photons is non-negligible, the cosmic microwave background acquires a rotation of its polarization plane, known as cosmic birefringence (CB). We present an extended analysis of several pseudo-scalar DE models and derive constraints on the parameters of their potentials by combining observations of the background expansion history with measurements of CB. We find that the axion-like potential constitutes a viable model only for large values of the anomaly coefficient. Scenarios in which the pseudo-scalar field rolls down a potential with quadratic, linear, or Ratra-Peebles forms can successfully explain DE and CB, with a symmetry-breaking scale close to the GUT scale.

astro-ph.CO↗

arXiv2026

NuSTAR as an Axion Helioscope: probing axion-nucleon and axion-electron couplings

We investigate solar X-ray observations as a probe of axions and axion-like particles. These particles can be produced in the interior of the Sun via the conversion of thermal photons, as well as through processes involving axion-electron and axion-nucleon interactions. The resulting axions can then reconvert into photons in the Sun's atmospheric magnetic field, generating a signal in the X-ray energy range. In this work, we derive new limits on axions using X-ray observations with the Nuclear Spectroscopic Telescope Array (NuSTAR) during the 2020 solar minimum. In the regime where ALP production is dominated by couplings to electrons or nucleons, we obtain bounds on the product of couplings $g_{ae}\cdot g_{a\gamma}\lesssim 1.1\times10^{-24}\,\rm GeV^{-1}$ and $g_{aN}^{\rm eff}\cdot g_{a\gamma}\lesssim 2.3\times 10^{-19}\,\rm GeV^{-1}$ at 95% CL, for axion masses $m_a\lesssim10^{-6}\,\rm eV$. These constraints strongly improve current ground-based experimental limits, establishing solar X-ray observations as a powerful and robust method for axion searches.

astro-ph.CO↗

arXiv2026

Searching for dark matter X-ray lines from the Large Magellanic Cloud with eROSITA

We perform a search for an X-ray monochromatic line arising from dark matter (DM) decay in the halo of the Large Magellanic Cloud. An emission line can be expected from two well-motivated DM candidates: sterile neturinos and axion-like particles (ALPs). We analyze the eROSITA-DE DR1 datasets in the energy range between 1 and 9 keV. No evidence for a DM line is found, and we set lower limits on the DM lifetime. We then recast these bounds into upper limits on the active-sterile neutrino mixing angle $\sin^2(2\theta)$ and on the ALP to photon coupling $g_{a\gamma}$, for DM masses between 2 and 18 keV. These results set new strong constraints for masses below 5 keV.

hep-ph↗

arXiv2026

WIMP Dark Matter Searches in Reticulum II Using MeerKAT

In the last decade radio astronomy has emerged as a powerful technique for detecting signatures of Weakly Interacting Massive Particles (WIMPs). Dwarf spheroidal galaxies (dSphs) are particularly promising targets for these searches due to their substantial dark matter (DM) dominance and minimal baryonic background emission. In this study, we utilize the exceptional sensitivity of the MeerKAT radio telescope to search for synchrotron emission from WIMP annihilation/decay in the nearby Reticulum II dSph. Through rigorous data reduction and self-calibration, we establish constraints on WIMP properties that improve upon previous radio studies, demonstrating the potential of MeerKAT and next-generation radio telescopes in exploring increasing swathes of the WIMP parameter space.

astro-ph.CO↗

arXiv2026

The COSMIC WISPers White Paper: The physics case for Weakly Interacting Slim Particles

Axions and other very weakly interacting slim particles (WISPs), with masses below 1 GeV, arise naturally in many extensions of the Standard Model of particle physics. In particular, they could offer a new framework to explain the nature of dark matter and may help address a range of puzzling observations in astrophysics and particle physics. This review provides an overview of ongoing WISP searches and outlines the prospects for the next decade, spanning their theoretical motivation, indirect signatures in astrophysical observations, and dedicated laboratory experiments. It is based on the work carried on by the EU-funded COST Action ``Cosmic WISPers in the Dark Universe: Theory, astrophysics, and experiments'' (CA21106, https://www.cost.eu/actions/CA21106). This network plays a key role in coordinating and supporting WISP searches across Europe, while also contributing to the development of a roadmap aimed at securing European leadership in this research area. It is emphasized that Europe is currently pursuing a rich, diverse, and cost-effective experimental program, with the potential to deliver one or more transformative discoveries.

hep-ph↗

arXiv2025

The diffusion coefficient in the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is the largest satellite galaxy of the Milky Way and provides a unique laboratory for high-energy astrophysics and dark matter studies. In this work, we develop an end-to-end numerical description of cosmic-ray transport and the associated non-thermal emission in the LMC, extending the public DRAGON and HERMES codes. Within this framework, we compute the diffuse synchrotron radiation produced by cosmic-ray electrons in the LMC and compare our predictions with observed low-frequency radio maps. Because electron diffusion imprints a characteristic morphology on the radio emission, this comparison allows us to infer the effective average diffusion coefficient in the LMC. We find a diffusion coefficient D0 = (3-6) $\times 10^{28} \; \rm{cm}^2 \; \rm{s}^{-1}$ at 1 GeV, comparable to but slightly larger than values typically inferred for the Milky Way. More generally, this work provides a scalable tool for interpreting non-thermal signals in nearby galaxies and constraining their cosmic-ray transport properties.

astro-ph.GA↗

arXiv2024

Searching for axion-like particles with SPHEREx

We study prospects to detect axion-like particles (ALPs) with the upcoming near-infrared telescope SPHEREx. The signal under investigation is the ALP decay into two photons. Assuming dark matter (DM) to be in the form of ALPs, we analyze the signal from the DM halos of dwarf spheroidal galaxies, the Large Magellanic Cloud and the Milky Way. We find that SPHEREx can significantly improve current limits on the axion-photon coupling in the 0.5-3 eV ALP mass range.

astro-ph.CO↗

arXiv2024

Bounds on Axions-Like Particles Shining in the Ultra-Violet

Axion-like particles (ALPs) can decay into two photons with a rest-frame frequency given by half of the ALP mass. This implies that ultra-violet searches can be used to investigate ALPs in the multi-eV mass range. We use archival data from the Hubble Space Telescope between 110 and 170 nm to constrain ALPs with mass between 14.4-22.2 eV. We consider observations of a set of dwarf spheroidal galaxies and galaxy clusters and assume the ALP density in these objects to follow their dark matter density. The derived limit on the ALP-photon coupling $g_{a\gamma}$ excludes values above $10^{-12}~{\rm GeV}^{-1}$ over the whole mass range and surpasses previous limits by over one order of magnitude.

hep-ph↗

arXiv2023

The Sun as a target for axion dark matter detection

The exploration of the parameter space of axion and axion-like particle dark matter is a major aim of the future program of astroparticle physics investigations. In this context, we present a possible strategy that focuses on detecting radio emissions arising from the conversion of dark matter axions in the Sun's magnetic field, including conversion in sunspots. We demonstrate that near-future low-frequency radio telescopes, such as the SKA Low, may access regions of unexplored parameter space for masses $m_a\lesssim 10^{-6}$ eV.

hep-ph↗

arXiv2023

Constraints on the origin of the radio synchrotron background via angular correlations

The origin of the radio synchrotron background (RSB) is currently unknown. Its understanding might have profound implications in fundamental physics or might reveal a new class of radio emitters. In this work, we consider the scenario in which the RSB is due to extragalactic radio sources and measure the angular cross-correlation of LOFAR images of the diffuse radio sky with matter tracers at different redshifts, provided by galaxy catalogs and CMB lensing. We compare these measured cross-correlations to those expected for models of RSB sources. We find that low-redshift populations of discrete sources are excluded by the data, while higher redshift explanations are compatible with available observations. We also conclude that at least 20\% of the RSB surface brightness level must originate from populations tracing the large-scale distribution of matter in the universe, indicating that at least this fraction of the RSB is of extragalactic origin. Future measurements of the correlation between the RSB and tracers of high-redshift sources will be crucial to constraining the source population of the RSB.

astro-ph.CO↗

arXiv2023

Anatomy of astrophysical echoes from axion dark matter

If the dark matter in the Universe is made of $\mu$eV axion-like particles (ALPs), then a rich phenomenology can emerge in connection to their stimulated decay into two photons. We discuss the ALP stimulated decay induced by electromagnetic radiation from Galactic radio sources. Three signatures, made by two echoes and one collinear emission, are associated with the decay, and can be simultaneously detected, offering a unique opportunity for a clear ALP identification. We derive the formalism associated with such signatures starting from first principles, and providing the relevant equations to be applied to study the ALP phenomenology. We then focus on the case of Galactic pulsars as stimulating sources and derive forecasts for future observations, which will be complementary to helioscopes and haloscopes results

hep-ph↗

arXiv2023

Robust bounds on ALP dark matter from dwarf spheroidal galaxies in the optical MUSE-Faint survey

Nearby dwarf spheroidal galaxies are ideal targets in the search for indirect dark matter (DM) signals. In this work, we analyze MUSE spectroscopic observations of a sample of five galaxies, composed of both classical and ultra-faint dwarf spheroidals. The goal is to search for radiative decays of axion-like particles (ALPs) in the mass range of 2.7-5.3 eV. After taking into account the uncertainties associated with the DM spatial distribution in the galaxies, we derive robust bounds on the effective ALP-two-photon coupling. They lie well below the QCD axion band and are significantly more constraining than limits from other probes, in the relevant mass range. We also test the possible presence of a positive signal, concluding that none of the channels selected for this analysis, i.e., not affected by large background contamination, is exhibiting such evidence.

astro-ph.CO↗

arXiv2023

Constraining Below-threshold Radio Source Counts With Machine Learning

We propose a machine-learning-based technique to determine the number density of radio sources as a function of their flux density, for use in next-generation radio surveys. The method uses a convolutional neural network trained on simulations of the radio sky to predict the number of sources in several flux bins. To train the network, we adopt a supervised approach wherein we simulate training data stemming from a large domain of possible number count models going down to fluxes a factor of 100 below the threshold for source detection. We test the model reconstruction capabilities as well as benchmark the expected uncertainties in the model predictions, observing good performance for fluxes down to a factor of ten below the threshold. This work demonstrates that the capabilities of simple deep learning models for radio astronomy can be useful tools for future surveys.

astro-ph.IM↗

arXiv2023

The self-confinement of electrons and positrons from dark matter

Radiative emissions from electrons and positrons generated by dark matter (DM) annihilation or decay are one of the most investigated signals in indirect searches of WIMPs. Ideal targets must have large ratio of DM to baryonic matter. However, such ``dark'' systems have a poorly known level of magnetic turbulence, which determines the residence time of the electrons and positrons and therefore also the strength of the expected signal. This typically leads to significant uncertainties in the derived DM bounds. In a novel approach, we compute the self-confinement of the DM-induced electrons and positrons. Indeed, they themselves generate irregularities in the magnetic field, thus setting a lower limit on the presence of the magnetic turbulence. We specifically apply this approach to dwarf spheroidal galaxies. Finally, by comparing the expected synchrotron emission with radio data from the direction of the Draco galaxy collected at the Giant Metre Radio Telescope, we show that the proposed approach can be used to set robust and competitive bounds on WIMP DM.

astro-ph.HE↗

arXiv2022

Dark Matter prospects with COSI: ALPs, PBHs and sub-GeV Dark Matter

We study the prospects in the search of dark matter offered by the newly selected NASA MeV mission COSI (Compton Spectrometer and Imager). This instrument is designed and optimized to detect spectral lines, and we show it offers an exquisite possibility to detect dark matter directly decaying or annihilating into monochromatic gamma-rays. This is the case, for example, for axion-like particles (ALPs) which undergo decay into two photons. Furthermore, we show that COSI can lead to important progress in the quest for primordial black holes (PBHs) dark matter, through measurements of the 511 keV line from the positrons produced via Hawking evaporation. We also outline opportunities for the search of continuum signals, such as those expected from sub-GeV dark matter annihilation/decay into leptons and PBH evaporation into photons. We find that also in this case COSI can lead to improvements of current bounds.

hep-ph↗

arXiv2022

Upper limits on the dark matter content in globular clusters

We present a systematic analysis on the possible presence of dark mass components inside globular clusters (GCs). A spherical Jeans analysis is applied to the stellar kinematics of 9 nearby GCs. On top of the mass distribution provided by the luminous stellar component, we add either dark matter (DM), described by an NFW mass profile, or an intermediate mass black-hole (IMBH), described by a point-like mass. Their existence would have important implications in the context of indirect DM searches. After profiling over the stellar parameters, we find no evidence neither for DM nor for IMBH. Upper limits on the two components are reported.

astro-ph.GA↗

arXiv2022

Snowmass2021 Cosmic Frontier: Synergies between dark matter searches and multiwavelength/multimessenger astrophysics

This whitepaper focuses on the astrophysical systematics which are encountered in dark matter searches. Oftentimes in indirect and also in direct dark matter searches, astrophysical systematics are a major limiting factor to sensitivity to dark matter. Just as there are many forms of dark matter searches, there are many forms of backgrounds. We attempt to cover the major systematics arising in dark matter searches using photons -- radio and gamma rays -- to cosmic rays, neutrinos and gravitational waves. Examples include astrophysical sources of cosmic messengers and their interactions which can mimic dark matter signatures. In turn, these depend on commensurate studies in understanding the cosmic environment -- gas distributions, magnetic field configurations -- as well as relevant nuclear astrophysics. We also cover the astrophysics governing celestial bodies and galaxies used to probe dark matter, from black holes to dwarf galaxies. Finally, we cover astrophysical backgrounds related to probing the dark matter distribution and kinematics, which impact a wide range of dark matter studies. In the future, the rise of multi-messenger astronomy, and novel analysis methods to exploit it for dark matter, will offer various strategic ways to continue to enhance our understanding of astrophysical backgrounds to deliver improved sensitivity to dark matter.

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