arXiv subjects

Christopher Eckner

Publications and source records attributed to Christopher Eckner.

At least 19 recordsLinked to original sources

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

Regge trajectories for UV completions of graviton scattering from polynomial boundedness

We study graviton scattering amplitudes. Assuming they are UV completed by a theory of weakly coupled massive higher spins, we demonstrate that the UV completion must possess infinitely many Regge trajectories, and thus they are forced to have a stringy spectrum. We extend and simplify a previous proof by some of us for open-string like states to the case of external gravitons. In the present new proof, we trace the need for infinitely many trajectories to the constraint of polynomial boundedness, ultimately tied to causality. We further present numerical results based on the stringy ansatz of H\"aring-Zhiboedov, which illustrates how single-trajectory-like solutions actually emerge as extremal solutions of numerical bootstrap. In our numerics, these trajectories curiously show up as numerically very large \textit{sister} trajectories. We provide solid evidence that the solutions are spurious as they appear to admit a divergent limit for infinite ansatz size.

hep-th↗

arXiv2025

Prospects on detection of the Fermi Bubbles with CTAO

In 2010, the Fermi Gamma-ray Space Telescope observed two gamma-ray emitting structures, the Fermi Bubbles (FBs), that extend up to 55{\deg} above and below the Galactic plane and that seem to emanate from the Galactic center region. Although the spectrum at latitudes |b| > 10{\deg} has a softening or a cutoff around 100 GeV, the one at the base of the FBs, |b| <10{\deg}, extends up to about 1 TeV without a significant cutoff in the Fermi LAT data. The mechanism behind the FBs production is currently under debate. More observations of the FBs at different energies are required to improve our understanding of their origin. Recently, H.E.S.S. and HAWC observatory have set upper limits on the FBs. In this work, we assess the sensitivity of the Cherenkov Telescope Array Observatory (CTAO) using the "alpha configuration" in the South site to detect the FBs and investigate the optimal strategies for their detection at low latitudes. We simulate the observations using the official CTAO science tool gammapy, considering several benchmark models for the FBs and the interstellar emission and test different observational strategies taking advantage of the proposed CTAO consortium surveys. We use these simulations to estimate the CTAO sensitivity to the FBs.

astro-ph.HE↗

arXiv2025

Stellar-like Galactic center excess challenges particle dark matter

The Galactic Center (GC) is potentially hosting the largest indirect signal from particle dark matter (DM), which in many well-motivated models would produce gamma rays as their final states. However, this region has often been dismissed for DM studies because of the evidence for an unexpected gamma-ray component over astrophysical backgrounds at GeV energies, firstly discovered in the data of the \textit{Fermi} Large Area Telescope (LAT), the so-called Galactic Center Excess (GCE). While this was initially considered to hint at GeV thermal relics, recent work supports a GCE interpretation in terms of a stellar population of millisecond pulsar-like sources in the Galactic bulge. Building on this preference, we re-evaluate the GC as a powerful target for indirect DM searches via gamma rays. This is achieved by combining adaptive template fitting and photon-count statistical methods to assess the role of sub-threshold point sources in the observed \textit{Fermi}-LAT gamma-ray counts, while minimizing the mismodeling of Galactic diffuse emission backgrounds. In a fully self-consistent way, the gamma-ray data are fitted with a mixed model comprising a DM signal and a stellar bulge, both potentially contributing to the GCE. The space left for signals from weak-scale DM particle annihilations is quantified by extracting 95\% C.L. upper limits on the annihilation cross section, which, depending on the DM density profile, result in stringent limits for masses $\lesssim 300$ GeV. The robustness of our results is supported by tests on simulated data.

hep-ph↗

arXiv2025

Probing the Parameter Space of Axion-Like Particles Using Simulation-Based Inference

Axion-like particles (ALPs), hypothetical pseudoscalar particles that couple to photons, are among the most actively investigated candidates for new physics beyond the Standard Model. Their interaction with gamma rays in the presence of astrophysical magnetic fields can leave characteristic, energy-dependent modulations in observed spectra. Capturing such subtle features requires precise statistical inference, but standard likelihood-based methods often fall short when faced with complex models, large number of nuisance parameters and limited analytical tractability. In this work, we investigate the application of simulation-based inference (SBI), specifically Truncated Marginal Neural Ratio Estimation (TMNRE), to constrain ALP parameters using simulated observations from the upcoming Cherenkov Telescope Array Observatory (CTAO). We model the gamma-ray emission from the active galactic nucleus NGC 1275, accounting for photon-ALP mixing, extragalactic background light (EBL) absorption, and the full CTAO instrument response. Leveraging the Swyft framework, we infer posteriors for the ALP mass and coupling strength and demonstrate its potential to extract meaningful constraints on ALPs from future real gamma-ray data with CTAO.

astro-ph.HE↗

arXiv2025

Lecture Notes: WISPs in gamma-ray astrophysics

These lecture notes provide an overview of high-energy astrophysical processes involving axions, axion-like particles (ALPs), and other weakly interacting slim particles (WISPs) focusing on their potential observational signatures in astrophysical environments. After introducing key concepts in high-energy astrophysics, we present the fundamental properties of WISPs, emphasizing their phenomenological implications. Particular attention is given to ALP-photon conversion in strong magnetic fields and the possible decay signatures of ALPs in sources such as active galactic nuclei, galaxy clusters, and cosmic-ray accelerators. These effects can lead to distinctive modifications in astrophysical spectra, spatial distributions, and polarization patterns, providing unique probes of physics beyond the Standard Model. We discuss their role in dark matter scenarios and their potential impact on high-energy observations. The lecture series is supplemented by hands-on tutorials, including exercises on axion electrodynamics and an analysis of gamma-ray data from NGC 1275 to search for ALP-photon conversion signatures.

astro-ph.HE↗

arXiv2025

A robust neural determination of the source-count distribution of the Fermi-LAT sky at high latitudes

Over the past 16 years, the Fermi Large Area Telescope (LAT) has significantly advanced our view of the GeV gamma-ray sky, yet several key questions remain - such as the composition of the isotropic gamma-ray background, the origin of the Fermi Bubbles or the potential presence of signatures from exotic physics like dark matter. Addressing these challenges requires sophisticated astrophysical modeling and robust statistical methods capable of handling high-dimensional parameter spaces. In this work, we analyze 14 years of high-latitude ($|b|\geq30^{\circ}$) Fermi-LAT data in the range from 1 to 10 GeV using simulation-based inference (SBI) via neural ratio estimation. This approach allows us to detect individual gamma-ray sources and derive a list of significant gamma-ray emitters containing more than 98\% of all sources listed in the Fermi-LAT Fourth Source Catalog (4FGL) with a flux $S>3\times10^{-10}\;\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$ (about a factor of three larger than the flux above which 4FGL is nearly complete), without any non-4FGL source detected in that flux range. Additionally, we reconstruct the source-count distribution in both parametric and non-parametric forms, achieving large agreement with previous literature results as well as those sources detected by our SBI pipeline. We also quantitatively validate our gamma-ray emission simulator via an anomaly detection technique, demonstrating that the synthetic data closely reproduces the complexity of the real observations.

astro-ph.HE↗

arXiv2025

Detecting dark matter sub-halos in the Galactic plane with the Cherenkov Telescope Array Observatory

Numerous observations confirm the existence of dark matter (DM) at astrophysical and cosmological scales. Theory and simulations of galaxy formation predict that DM should cluster on small scales in bound structures called sub-halos or DM clumps. While the most massive DM sub-halos host baryonic matter, less massive, unpopulated sub-halos could be abundant in the Milky Way (MW), as well and yield high-energy gamma rays as final products of DM annihilation. Recently, it has been highlighted that the brightest halos should also have a sizeable extension in the sky. In this study, we examine the prospects offered by the Cherenkov Telescope Array Observatory (CTAO), a next-generation gamma-ray instrument, for detecting and characterizing such objects. Previous studies have primarily focused on high-latitude observations; here, we assess the potential impact of the CTAO's Galactic Plane Survey, which will provide unprecedentedly deep survey data for the inner five degrees of the Galactic plane. Our modeling accounts for tidal effects on the sub-halo population, examining the conditions under which DM sub-halos can be detected and distinguished from conventional astrophysical sources. We find that regions a few degrees above or below the Galactic plane offer the highest likelihood for DM sub-halo detection. For an individual sub-halo -- the brightest from among various realizations of the MW subhalo population -- we find that detection at the 5$\sigma$ level is achievable for an annihilation cross section of $\langle \sigma v \rangle \sim 3\times10^{-25}$ cm$^3$/s for TeV-scale DM annihilating into $b\bar{b}$. For a full population study, depending on the distribution and luminosity model of Galactic sub-halos, yet unconstrained cross sections in the range $\langle \sigma v \rangle \sim 10^{-23}-10^{-22}$ cm$^3$/s for TeV DM candidates are necessary for the brightest sub-halos to be detected.

astro-ph.HE↗

arXiv2024

No evidence for gamma-ray emission from the Sagittarius dwarf spheroidal galaxy

More than a decade ago, the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope unveiled the existence of two gigantic gamma-ray lobes known as the Fermi bubbles. While their origin is still unknown, various studies identified intricate spectral and morphological structures within the bubbles. One peculiar region, the cocoon, has recently been associated with gamma-ray emissions from the Sagittarius dwarf spheroidal (Sgr) galaxy. We assess the validity of this claim through adaptive-template fitting and pixel-count statistical methods. Our approach introduces a substantial advancement in data interpretation by enabling a data-driven optimisation of astrophysical background models, thereby reducing the impact of background mis-modelling. We do not find evidence for gamma-ray emission from the Sgr region at the level obtained in previous work. We demonstrate that there is no pronounced difference between the source population located within the cocoon region and a reference region at similar latitudes. We examine the hypothesis that a millisecond pulsar population in Sgr causes the putative signal via dedicated simulations finding that it is unlikely that we may have detected their presence in Sgr.

astro-ph.HE↗

arXiv2024

On the number of Regge trajectories for dual amplitudes

Regge poles connect the analytic structure of scattering amplitudes, analytically continued in spin, to the high-energy limit in momentum space. Dual models are expected to have only Regge poles, and string theory suggests there should be an infinite number of them. In this study, we investigate the number of Regge trajectories these models may have. We prove, based solely on crossing symmetry and unitarity, that meromorphic amplitudes, with or without subtractions, cannot produce a reggeizing amplitude if they contain any finite number of Regge trajectories. We argue that this should exclude the existence of such amplitudes altogether. Additionally, we develop and apply a linear programming dual bootstrap method to exclude these amplitudes directly in momentum space.

hep-th↗

arXiv2024

Simulation-based inference of radio millisecond pulsars in globular clusters

Millisecond pulsars (MSPs) are abundant in globular clusters (GCs), which offer favorable environments for their creation. While the advent of recent, powerful facilities led to a rapid increase in MSP discoveries in GCs through pulsation searches, detection biases persist. In this work, we investigate the ability of current and future detections in GCs to constrain the parameters of the MSP population in GCs through a careful study of their luminosity function. Parameters of interest are the number of MSPs hosted by a GC, as well as the mean and the width of their luminosity function, which are typically affected by large uncertainties. While, as we show, likelihood-based studies can lead to ill-behaved posterior on the size of the MSP population, we introduce a novel, likelihood-free analysis, based on Marginal Neural Ratio Estimation, which consistently produces well-behaved posteriors. We focus on the GC Terzan 5, which currently counts 48 detected MSPs. We find that about 158 MSPs should be hosted in this GC, but the uncertainty on this number remains large. We explore the performance of our new method on simulated Terzan 5-like datasets mimicking possible future observational outcomes. We find that significant improvement on the posteriors can be obtained by adding a reliable measurement of the diffuse radio emission of the GC to the analysis or by improving the detection threshold of current radio pulsation surveys by at least a factor two.

astro-ph.HE↗

arXiv2024

Probing protoneutron stars with gamma-ray axionscopes

Axion-like particles (ALPs) coupled to nucleons can be efficiently produced in the interior of protoneutron stars (PNS) during supernova (SN) explosions. If these ALPs are also coupled to photons they can convert into gamma rays in the Galactic magnetic field. This SN-induced gamma-ray burst can be observable by gamma-ray telescopes like ${\textit Fermi}$-LAT if the SN is in the field of view of the detector. We show that the observable gamma-ray spectrum is sensitive to the production processes in the SN core. In particular, if the nucleon-nucleon bremsstrahlung is the dominant axion production channel, one expects a thermal spectrum with average energy $E_a \simeq 50$ MeV. In this case the gamma-ray spectrum observation allows for the reconstruction of the PNS temperature. In case of a sizable pion abundance in the SN core, one expects a second spectral component peaked at $E_a\simeq 200$ MeV due to axion pionic processes. We demonstrate that, through a dedicated LAT analysis, we can detect the presence of this pionic contribution, showing that the detection of the spectral shape of the gamma-ray signal represents a unique probe of the pion abundance in the PNS.

hep-ph↗

arXiv2024

Uncovering axion-like particles in supernova gamma-ray spectra

A future Galactic Supernova (SN) explosion can lead to a gamma-ray signal induced by ultralight Axion-Like Particles (ALPs) thermally produced in the SN core and converted into high-energy photons in the Galactic magnetic field. The detection of such a signal is in the reach of the Large Area Telescope aboard the \emph{Fermi} Gamma-Ray Space Telescope. The observation of gamma-ray emission from a future SN has a sensitivity to $g_{aγ}\gtrsim 4\times 10^{-13}$ GeV$^{-1}$ for a SN at fiducial distance of $10$ kpc and would allow us to reconstruct the ALP-photon coupling within a factor of $\sim2$, mainly due to the uncertainties on the modeling of the Galactic magnetic field.

astro-ph.HE↗

arXiv2024

Robust inference of the Galactic centre gamma-ray excess spatial properties

The gamma-ray Fermi-LAT Galactic centre excess (GCE) has puzzled scientists for over 15 years. Despite ongoing debates about its properties, and especially its spatial distribution, its nature remains elusive. We scrutinize how the estimated spatial morphology of this excess depends on models for the Galactic diffuse emission, focusing particularly on the extent to which the Galactic plane and point sources are masked. Our main aim is to compare a spherically symmetric morphology - potentially arising from the annihilation of dark matter (DM) particles - with a boxy morphology - expected if faint unresolved sources in the Galactic bulge dominate the excess emission. Recent claims favouring a DM-motivated template for the GCE are shown to rely on a specific Galactic bulge template, which performs worse than other templates for the Galactic bulge. We find that a non-parametric model of the Galactic bulge derived from the VVV survey results in a significantly better fit for the GCE than DM-motivated templates. This result is independent of whether a GALPROP-based model or a more non-parametric ring-based model is used to describe the diffuse Galactic emission. This conclusion remains true even when additional freedom is added in the background models, allowing for non-parametric modulation of the model components and substantially improving the fit quality. When adopted, optimized background models provide robust results in terms of preference for a boxy bulge morphology of the GCE, regardless of the mask applied to the Galactic plane.

astro-ph.GA↗

arXiv2024

The Regge bootstrap, from linear to non-linear trajectories

We present a numerical linear programming bootstrap to construct dual model scattering amplitudes. Dual models describe tree-level exchanges of higher spin resonances in theories like string theory and large $N$ gauge theories. Despite being very simple objects, their numerical bootstrap has proven challenging due to slow convergence of the infinite sums over resonances. Our bootstrap succeeds thanks to an efficient parametrization of the amplitude in terms of Mandelstam-Regge poles and the use of combined regions that make crossing symmetry constraining. Along the way, we discover and conjecture a property of "super-unitarity" of the Veneziano amplitude, which we use to keep a linear problem. As results, we present first the study of a class of string-like amplitudes with linear trajectories, for which we observe that the Veneziano amplitude lies at a preferred location, at the bottom of a pit, which minimizes crossing. Then, we introduce a toy-model deformation to non-linear trajectories, mimicking some features of QCD, for which our algorithm also detects a clear pit. This gives compelling evidence that our bootstrap is able to produce amplitudes that can exhibit non-trivial phenomenological features.

hep-th↗

arXiv2024

Constraining MeV-scale axion-like particles with Fermi-LAT observations of SN 2023ixf

The Fermi-LAT observations of SN 2023ixf, a Type II supernova in the nearby Pinwheel Galaxy, Messier 101 (M101), presents us with an excellent opportunity to constrain MeV-scale Axion-Like Particles (ALPs). By examining the photon decay signature from heavy ALPs that could be produced in the explosion, the existing constraints on the ALP-photon coupling can be improved, under optimistic assumptions, by up to a factor of $ \sim 2 $ for masses $ m_a \lesssim 3 $ MeV. Under very conservative assumptions, we find a bound that is slightly weaker than the existing ones for $ m_a \lesssim 0.5$ MeV. The exact reach of these searches depends mostly on properties of the SN progenitor. This study demonstrates the relevance of core-collapse supernovae, also beyond the Magellanic Clouds, as probes of fundamental physics.

astro-ph.HE↗

arXiv2023

Detecting and characterizing pulsar halos with the Cherenkov Telescope Array Observatory

The recently identified source class of pulsar halos may be populated and bright enough at TeV energies to constitute a large fraction of the sources that will be observed with the Cherenkov Telescope Array (CTA), especially in the context of the planned Galactic Plane Survey (GPS). In this study, we examine the prospects offered by CTA for the detection and characterization of such objects. CTA will cover energies from 20 GeV to 300 TeV, bridging the ranges already probed with the Fermi Large Area Telescope and High Altitude Water Cherenkov Observatory, and will also have a better angular resolution than the latter instruments, thus providing a complementary look at the phenomenon. From simple models for individual pulsar halos and their population in the Milky Way, we examine under which conditions such sources can be detected and studied from the GPS observations. In the framework of a full spatial-spectral likelihood analysis, using the most recent estimates for the instrument response function and prototypes for the science tools, we derive the spectral and morphological sensitivity of the CTA GPS to the specific intensity distribution of pulsar halos. From these, we quantify the physical parameters for which pulsar halos can be detected, identified, and characterized, and what fraction of the Galactic population could be accessible. We also discuss the effect of interstellar emission and data analysis systematics on these prospects.

astro-ph.HE↗

arXiv2023

Investigating the gamma-ray burst from decaying MeV-scale axion-like particles produced in supernova explosions

We investigate the characteristics of the gamma-ray signal following the decay of MeV-scale Axion-Like Particles (ALPs) coupled to photons which are produced in a Supernova (SN) explosion. This analysis is the first to include the production of heavier ALPs through the photon coalescence process, enlarging the mass range of ALPs that could be observed in this way and giving a stronger bound from the observation of SN 1987A. Furthermore, we present a new analytical method for calculating the predicted gamma-ray signal from ALP decays. With this method we can rigorously prove the validity of an approximation that has been used in some of the previous literature, which we show here to be valid only if all gamma rays arrive under extremely small observation angles (i.e. very close to the line of sight to the SN). However, it also shows where the approximation is not valid, and offers an efficient alternative to calculate the ALP-induced gamma-ray flux in a general setting when the observation angles are not guaranteed to be small. We also estimate the sensitivity of the Fermi Large Area Telescope (Fermi-LAT) to this gamma-ray signal from a future nearby SN and show that in the case of a non-observation the current bounds on the ALP-photon coupling $ g_{aγ} $ are strengthened by about an order of magnitude. In the case of an observation, we show that it may be possible to reconstruct the product $ g_{aγ}^2 m_a $, with $ m_a $ the mass of the ALP.

astro-ph.HE↗