arXiv subjects

Q. Remy

Publications and source records attributed to Q. Remy.

15 recordsLinked to original sources

arXiv2026

Evidence for a spectral steepening of the gamma-ray emission from the Galactic Center ridge

Very-high-energy (VHE) $\gamma$-ray emission detected from the central molecular zone (CMZ) hints at diffusive propagation of cosmic rays (CRs) injected continuously near the Galactic center (GC). Using H.E.S.S. VHE $\gamma$-ray observations, we aim to construct a multi-component description of the region in order to derive the spatial and spectral distributions of CRs and discuss their potential origin. We rely on a spectro-morphological analysis, in which spectral and spatial parameters of different components are fit simultaneously, and on a three-dimensional description of the CMZ and CR distributions. The GC ridge emission is well reproduced with the assumed gas distribution model weighted by a $1/r^{\alpha}$ CR density profile with $\alpha = 1.10 \pm 0.05_{\rm{stat}} \pm 0.10_{\rm{syst}}$. We detect no significant spectral variations across the CMZ, which supports a continuous injection from the GC. We report a significant (> 3 $\sigma$) curvature in the GC ridge $\gamma$-ray spectrum, implying a parent proton break (or cutoff energy) of $E_b \sim 15-50$ TeV ($E_{\rm{cut}} \sim 60-160$ TeV). We also derive the best-fit spectral parameters of the other sources in the region and report a significant cutoff at $E \sim 5$ TeV in the spectrum of the pulsar wind nebula G0.9+0.1. Finally we constrain the position of the injection site to be close to the GC (ruling out the Arches and Quintuplet clusters as major contributors to the emission). We discuss possible origins for the GC ridge emission, potential mechanisms for causing the observed curvature, a possible contribution of the gas content to the emission of HESS J1745-290 and derive qualitative constraints on the line-of-sight positions of the clouds shaping the CMZ.

astro-ph.HE↗

arXiv2026

H.E.S.S. detection of the PSR J0855-4644 nebula

HESS J0852-463 is a TeV {\gamma}-ray source located in the Galactic plane. The region consists of a supernova remnant (SNR, RX J0852.0-4622) with a shell-like morphology, commonly referred to as Vela Junior, and a pulsar denoted PSR J0855-4644. Pulsars are among the most efficient leptonic accelerators in our Galaxy, making this region particularly interesting to study. We utilise the most recent data taken by the High Energy Stereoscopic System (H.E.S.S.), to investigate any {\gamma}-ray emission associated with the pulsar in this region, PSR J0855-4644. We applied a full forward folding method on the H.E.S.S. data. Utilising 3D modelling techniques, we evaluated the TeV {\gamma}-ray emission towards the various components of this complex system. The distinct energy-dependent morphology observed in our data motivates further investigation of this source. We resolved the emission in the Vela Junior region into various components, several of which correspond to the SNR itself. In particular, we find a new extended component which is coincident with the position of PSR J0855-4644. The spectrum follows a power-law distribution with a best-fit index of {\Gamma}E = 1.81 \pm 0.07stat which differs from the properties of the surrounding {\gamma}-ray emission of the Vela Junior SNR. A one-zone leptonic joint fit between the X-rays (from XMM-Newton) and {\gamma}-rays (from H.E.S.S.) leads to a lower limit on the magnetic field of 1.6{\mu}G and a spectral index of {\alpha} = 1.88 \pm 0.01, in line with expectations of pulsar wind nebulae (PWNe). In this paper, we report the first detection of the PWN of PSR J0855-4644 at TeV energies with the H.E.S.S. experiment at a significance of 12.2{\sigma}. This is attributed to the advanced techniques of the 3D analysis. Based on the pulsar's characteristics, its PWN is consistent with the known TeV PWNe population in the Galaxy.

astro-ph.HE↗

arXiv2026

The Fourth HAWC Catalog of Very-High-Energy Gamma-Ray Sources

We present an updated catalog of TeV gamma-ray sources based on the fifth pass of data from the High-Altitude Water Cherenkov (HAWC) Observatory. This release benefits from improved event reconstruction and nearly three additional years of observations. It also incorporates a systematic multi-source fitting framework, enabling more flexible and accurate modeling of the gamma-ray sky. This fitting procedure was modeled after the manual approach used in HAWC analyses of individual sources and regions, as well as other gamma-ray catalogs, like the 4FGL. In addition to more varied modeling of source morphology and spectral parameters compared to previous HAWC catalogs, this catalog uses a robust modeling of Galactic diffuse TeV emission. The fitting procedure uses both point-like and symmetric Gaussian spatial templates to model the source morphology. The spectral shape of the emission is modeled with either a simple power-law or log-parabola to explore curvature in the spectral energy distribution. We report 85 sources at the 4{\sigma} level, including 11 sources not associated with any TeVCat source using a distance-based association criterion. Distance-based association with the 1LHAASO catalog results in 22 4HWC sources without a counterpart. Additionally, there are 12 sources not associated with any physical counterpart in the Low- or High-Mass X-Ray Binary, the ATNF, or Fermi Pulsar, or SNR catalogs of sources. Five of the aforementioned sources have no counterpart in any of the catalogs searched and represent an opportunity for follow-up observations.

astro-ph.HE↗

arXiv2025

Constraining the nature of the most extreme Galactic particle accelerator. H.E.S.S. observations of the microquasar V4641 Sgr

Microquasars have emerged as promising candidates to explain the cosmic-ray flux at petaelectronvolt energies. LHAASO observations revealed V4641~Sgr as the most extreme example so far. Using $\approx$100~h of H.E.S.S. data, we performed a spectro-morphological study of the gamma-ray emission around V4641~Sgr. We employed HI and dedicated CO observations of the region to infer the target material for cosmic-ray interactions. We detected multi-TeV emission around V4641~Sgr with a high significance. The emission region is elongated. We found a power-law spectrum with an index $\approx$1.8, and together with results from other gamma-ray instruments, this reveals a spectral energy distribution that peaks at energies of $\approx$100~TeV for the first time. We found indications (3$\sigma$) of a two-component morphology, with indistinguishable spectral properties. The position of V4641~Sgr is inconsistent with the best-fit position of the single-component model and with the dip between the two components. We found no significant evidence of an energy-dependent morphology. No dense gas was found at any distance towards V4641~Sgr. The peak of the SED at $\approx$100~TeV identifies V4641~Sgr as a candidate cosmic-ray accelerator beyond the so-called knee. The absence of dense target gas places stringent energetic constraints on hadronic interpretations, however. The H.E.S.S. measurement requires an unusually hard ($\approx 1.5$) spectral index for the protons. A leptonic scenario faces fewer obstacles if the particle transport is fast enough to avoid losses and to reproduce the observed energy-independent morphology. The absence of bright \xray emission across the gamma-ray emission region requires a magnetic field strength $\lesssim3$~$\mu$G, however. Our findings favour a leptonic origin of the gamma-ray emission. This conclusion does not exclude hadron acceleration in the V4641~Sgr system.

astro-ph.HE↗

arXiv2025

Energy-dependent gamma-ray morphology estimation tool in Gammapy

An understanding of the energy dependence of gamma-ray sources can yield important information on the underlying emission mechanisms. However, despite the detection of energy-dependent morphologies in many TeV sources, we lack a proper quantification of such measurements. We introduce an estimation tool within the Gammapy landscape, an open-source Python package for the analysis of gamma-ray data, for quantifying the energy-dependent morphology of a gamma-ray source. The proposed method fits the spatial morphology in a global fit across all energy slices (null hypothesis) and compares this to separate fits for each energy slice (alternative hypothesis). These are modelled using forward-folding methods, and the significance of the variability is quantified by comparing the test statistics of the two hypotheses. We present a general tool for probing changes in the spatial morphology with energy, employing a full forward-folding approach with a 3D likelihood. We present its usage on a real dataset from H.E.S.S. and on a simulated dataset to quantify the significance of the energy dependence for sources of different sizes. In the first example, which utilises a subset of data from HESSJ1825-137, we observe extended emission at lower energies that becomes more compact at higher energies. The tool indicates a very significant variability (9.8{\sigma}) in the case of the largely extended emission. In the second example, a source with a smaller extent (~0.1{\deg}), simulated using the CTAO response, shows the tool can still provide a statistically significant variation (9.7{\sigma}) on small scales.

astro-ph.HE↗

arXiv2025

Efficient broadband terahertz generation by above band-gap excitation of the pyroelectric ZnSnN2

Terahertz (THz) radiation is a powerful probe of low-energy excitations in all phases of matter. However, it remains a challenge to find materials that efficiently generate THz radiation in a broad range of frequencies following optical excitation. Here, we investigate a pyroelectric material, ZnSnN2, and find that above-band-gap excitation results in the efficient formation of an ultrafast photocurrent generating THz radiation. The resulting THz electric field spans a frequency range from below 1 to above 30 THz. Our results suggest that the photocurrent is primarily driven by an ultrafast pyroelectric effect where the photo-excited carriers screen the spontaneous electric polarization of ZnSnN2. Strong structural disorder reduces the photocarrier lifetime significantly and, thus, enables broadband operation. ZnSnN2 shows similar THz-emitter performance as the best spintronic THz emitters regarding bandwidth and amplitude. Our study unveils the large potential of pyroelectric materials as efficient and broadband THz emitters with built-in bias fields.

cond-mat.mtrl-sci↗

arXiv2025

Cosmic rays, gas and dust in the Central Molecular Zone I -- $X_{CO}$ factors, cosmic-ray densities and dust opacities

Our goal is to estimate the total gas mass in the direction of the Central Molecular Zone (CMZ), quantify the various uncertainties associated, and discuss the implications for the estimates of CR energy densities and dust opacities. The $H_{\rm{I}}$ 21 cm line and the carbon monoxide isotopes ($^{12}\rm{CO}$, $^{13}\rm{CO}$ and $\rm{C}^{18}\rm{O}$) line emission maps are used to derive the total gas column density. The gas in the CMZ is separated from the disk contribution in position and velocity thanks to its different properties in term of velocity dispersion and brightness ratio of CO isotopes. The variations of the $X_{\rm{CO}}$ factors are modelled relying on both theoretical trends from simulations and empirical corrections. We use the new gas column density estimated together with gamma-ray and dust emission measurements to derive the CR energy density and dust opacities, respectively. The $X_{\rm{CO}}$ values in the CMZ range from $(0.32 - 1.37) \ \times$ $10^{20}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s, with a distribution that is highly asymmetric and skewed. The median value is $ \rm{\overline{X}_{CO}^{CMZ}} = 0.39 \ \times$ $10^{20}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s. The total gas mass in the CMZ is estimated to be $2.3_{-0.3}^{+0.3}\times10^{7} \; \rm{M_{\odot}}$ with $\sim 10 \%$ contribution from the atomic phase. Without removing the disk contamination the total mass is about twice higher, and the atomic gas fraction increases to $\sim30\%$. The cosmic-ray (CR) energy density in the CMZ, assuming a 1/r profile, is higher by a factor of two compared to the previous calculations at TeV energies. Using molecular gas tracers which probes only the densest molecular cores leads to an overestimation of the CR energy density, while ignoring the foreground/background contribution leads to an underestimation of the CR energy density in the CMZ.

astro-ph.GA↗

arXiv2023

Gammapy: present status and future roadmap

Since its start in 2014, the lightweight open source Python library Gammapy has come a long way to become a popular data analysis package for high-energy astrophysics. Selected as the official CTAO Science Analysis tool, it is also an approved analysis software within the H.E.S.S. and MAGIC collaborations. The first long-term version, Gammapy v1.0 was released on late 2022. It is compliant with several well-established data conventions in high-energy astrophysics, and provides serialised data products that are interoperable with other software. Event lists and instrument response functions curated within the same format from various instruments can be reduced to data binned in energy, time or spatial coordinates. Thereafter, the flux and morphology of one or more gamma-ray sources can be estimated using Poisson maximum likelihood fitting and assuming a variety of spectral, temporal and spatial models. Flux points, likelihood profiles and light curves extractions are supported. Complex user defined likelihoods and models can also be implemented. In this contribution, we will highlight the main features of Gammapy v1.0, including data reduction and analysis examples from different space and ground-based instruments, applications of various background rejection techniques, and a simultaneous fitting across multiple instruments with astrophysical models. We will also present our plans for the future, showcasing new features such as the support of different event types, unbinned likelihood analysis, spectral unfolding and transient source detections. In addition to an improved API with distributed computing for scalable analysis, enhanced support for all-sky instruments like Fermi-LAT and HAWC is foreseen.

astro-ph.IM↗

arXiv2023

Towards the second H.E.S.S. Galactic plane catalogue

The H.E.S.S. Galactic Plane Survey (HGPS), carried out between 2004 and 2013, is the most extensive survey of our Galaxy at very-high energies that covers the southern sky. Since the first HPGS catalogue release, the new observations accumulated provide a deeper scan of many Galactic sources, and a number of improvements have been made at various stages of the data processing chain, notably on events reconstruction and background modeling techniques. In parallel a new catalog production workflow has been tested and optimized on simulations done in preparation for the future Galactic Plane survey to be conducted by the Cherenkov Telescope Array (CTA). The development of a common data format and open scientific tools for gamma-ray astronomy allowed a smooth transition from the exploratory work done on CTA simulations to its application for H.E.S.S. data analysis. These elements offered a solid ground to build the second H.E.S.S. Galactic Plane Survey catalogue (2HGPS). In the following we will focus on the description of the catalogue workflow and show few results along the way.

astro-ph.HE↗

arXiv2023

Multiple emission components in the Cygnus cocoon detected from Fermi-LAT observations

Star-forming regions may play an important role in the life cycle of Galactic cosmic rays. Gamma-ray observations of Cygnus X have revealed the presence of an excess of hard-spectrum gamma-ray emission, possibly related to a cocoon of freshly accelerated particles. Based on ~13 years of observations with the Fermi-Large Area Telescope (LAT), we performed an improved spectro-morphological characterisation of the residual emission including the cocoon. The best-fit model for the cocoon includes two main emission components: an extended component FCES G78.74+1.56, described by a 2D Gaussian of extension $r_{68} = 4.4^\circ \pm 0.1^\circ\,^{+0.1^\circ}_{-0.1^\circ}$, and a central component FCES G80.00+0.50, traced by the distribution of ionised gas within the borders of the photo-dissociation regions. The two have significantly different spectra. An additional extended emission component FCES G78.83+3.57, located on the edge of the central cavities in Cygnus X and with a spectrum compatible with that of FCES G80.00+0.50, is likely related to the cocoon. For the two main components, spectra and radial-azimuthal profiles of the emission can be accounted for in a diffusion-loss framework involving one single population of non-thermal particles. Particles span the full extent of FCES G78.74+1.56 as a result of diffusion from a central source, and give rise to source FCES G80.00+0.50 by interacting with ionised gas in the innermost region. For this simple diffusion-loss model, viable setups can be very different in terms of energetics, transport conditions, and timescales involved, and both hadronic and leptonic scenarios are possible. The solutions range from long-lasting particle acceleration, possibly in prominent star clusters such as Cyg OB2 and NGC 6910, to a more recent and short-lived release of particles within the last 10-100 kyr, likely from a supernova remnant. (Abridged)

astro-ph.HE↗

arXiv2021

Survey of the Galactic Plane with the CherenkovTelescope Array

Observations with the current generation of very-high-energy gamma-ray telescopes have revealed an astonishing variety of particle accelerators in the Milky Way, such as supernova remnants, pulsar wind nebulae, and binary systems. The upcoming Cherenkov Telescope Array (CTA) will be the first instrument to enable a survey of the entire Galactic plane in the energy range from a few tens of GeV to 300 TeV with unprecedented sensitivity and improved angular resolution. In this contribution we will revisit the scientific motivations for the survey, proposed as a Key ScienceProject for CTA. We will highlight recent progress, including improved physically-motivated models for Galactic source populations and interstellar emission, advance on the optimization of the survey strategy, and the development of pipelines to derive source catalogues tested on simulated data. Based on this, we will provide a new forecast on the properties of the sources thatCTA will detect and discuss the expected scientific return from the study of gamma-ray source populations.

astro-ph.IM↗

arXiv2020

Prospects on high-energy source searches based on pattern recognition: Object detection in the H.E.S.S. Galactic Plane Survey and catalogue cross-matches

The H.E.S.S. Galactic Plane Survey (HGPS) represents one of the most sensitive surveys of the Galactic Plane at very high energies (VHE, 0.1-100 TeV). However the source detection algorithm of the HGPS pipeline is not well-suited for complex regions, including sources with shell-like morphologies. As an alternative and complementary approach, we have investigated blind search methods for VHE gamma-ray source detection based on well-known and widely used image processing and pattern recognition techniques. Our goal is to build in a short amount of computational time a list of potentially valuable objects without prior case-specific morphological assumptions. We aim to classify and rank the detected objects in order to identify only the most promising source candidates for further multi-wavelength-association searches, dedicated analyses, or deeper observations. In the approach proposed, we extract sparse and pertinent structural information from the significance maps using a edge detection operator. We then apply a Hough circle transform and detect a collection of objects as local maxima in the Hough space. On the basis of morphological parameters we can characterize different object classes. Classification can be used to identify valuable source candidates sharing the characteristics of well-known sources. We show that using these pattern recognition techniques we can detect objects with partial circular symmetry irrespective of a morphological template (e.g. point-like, Gaussian-like, or shell-like). All the shell-type supernova remnants (SNRs) catalogued in the HGPS are associated with at least one detected object. Catalogue cross-matches indicate that several detected objects not catalogued in the HGPS are spatially coincident with multi-wavelength counterparts. Further investigation on the most promising candidates will be conducted in dedicated follow-up analyses.

astro-ph.HE↗

arXiv2018

Cosmic-rays, gas, and dust in nearby anti-centre clouds : III -- Dust extinction, emission, and grain properties

We have explored the capabilities of dust extinction and $γ$ rays to probe the properties of the interstellar medium in the nearby anti-centre region. We have jointly modelled the $γ$-ray intensity and the stellar reddening, E(B-V) as a combination of H$_{\rm I}$-bright, CO-bright, and ionised gas components. The complementary information from dust reddening and $γ$ rays is used to reveal the dark gas not seen, or poorly traced, by H$_{\rm I}$, free-free, and $^{12}$CO emissions. We compare the total gas column densities, $N_{\rm{H}}$, derived from the $γ$ rays and stellar reddening with those inferred from a similar analysis (Remy et al. 2017) of $γ$ rays and of the optical depth of the thermal dust emission, $τ_{353}$, at 353 GHz. We can therefore compare environmental variations in specific dust reddening, E(B-V)/$N_{\rm H}$, and in dust emission opacity (dust optical depth per gas nucleon), $τ_{353}/N_{\rm{H}}$. Over the whole anti-centre region, we find an average E(B-V)/$N_{\rm H}$ ratio of $(2.02\pm0.48)\times$ $10^{-22}$~mag~cm$^2$, with maximum local variations of about $\pm30\%$ at variance with the two to six fold coincident increase seen in emission opacity as the gas column density increases. In the diffuse medium, the small variations in specific reddening, E(B-V)/$N_{\rm H}$ implies a rather uniform dust-to-gas mass ratio in the diffuse parts of the anti-centre clouds. The small amplitude of the E(B-V)/$N_{\rm H}$ variations with increasing $N_{\rm{H}}$ column density confirms that the large opacity $τ_{353}/N_{\rm{H}}$ rise seen toward dense CO clouds is primarily due to changes in dust emissivity. The environmental changes are qualitatively compatible with model predictions based on mantle accretion on the grains and the formation of grain aggregates.

astro-ph.HE↗

arXiv2017

Cosmic-rays, gas, and dust in nearby anticentre clouds : II -- Interstellar phase transitions and the Dark Neutral Medium

H$_{\rm I}$ 21-cm and $^{12}$CO 2.6-mm line emissions respectively trace the atomic and molecular gas phases, but they miss most of the opaque H$_{\rm I}$ and diffuse H$_2$ present in the Dark Neutral Medium (DNM) at the transition between the H$_{\rm I}$-bright and CO-bright regions. Jointly probing H$_{\rm I}$, CO, and DNM gas, we aim to constrain the threshold of the H$_{\rm I}$-H$_2$ transition. We also aim to measure gas mass fractions in the different phases and to test their relation to cloud properties. We have used dust optical depth measurements at 353 GHz, $γ$-ray maps at GeV energies, and H$_{\rm I}$ and CO line data to trace the gas column densities and map the DNM in nearby clouds toward the Galactic anticentre and Chamaeleon regions. The onset of detectable CO intensities varies by only a factor of 4 from cloud to cloud, between 0.6$ \times 10^{21}$ cm$^{-2}$ and 2.5$ \times 10^{21}$ cm$^{-2}$ in total gas column density. We observe larger H$_2$ column densities than linearly inferred from the CO intensities at $A_{\rm{V}}> 3$ magnitudes because of the large CO optical thickness: the additional H$_2$ mass in this regime represents on average 20$\%$ of the CO-inferred molecular mass. In the DNM envelopes, we find that the fraction of diffuse CO-dark H$_2$ in the molecular column densities decreases with increasing $A_{\rm{V}}$ in a cloud. For a half molecular DNM, the fraction decreases from more than 80$\%$ at 0.4 magnitude to less than 20$\%$ beyond 2 magnitudes. In mass, the DNM fraction varies with the cloud properties. The mass present in the DNM envelopes appears to scale with the molecular mass seen in CO as $M_{\rm H}^{\rm DNM} = (62 \pm 7) {M_{\rm H_2}^{\rm CO}}^{\,0.51 \pm 0.02}$ across two decades in mass. The phase transitions in these clouds show both common trends and environmental differences.

astro-ph.HE↗

arXiv2017

Cosmic rays, gas and dust in nearby anticentre clouds : I -- CO-to-H2 conversion factors and dust opacities

We aim to explore the capabilities of dust emission and rays for probing the properties of the interstellar medium in the nearby anti-centre region, using gamma-ray observations with the Fermi Large Area Telescope (LAT), and the thermal dust optical depth inferred from Planck and IRAS observations. In particular, we aim at quantifying potential variations in cosmic-ray density and dust properties per gas nucleon across the different gas phases and different clouds, and at measuring the CO-to-H2 conversion factor, X$_{CO}$ , in different environments. We have separated six nearby anti-centre clouds that are coherent in velocities and distances, from the Galactic-disc background in HI 21-cm and $^{12}$CO 2.6-mm line emission. We have jointly modelled the gamma-ray intensity recorded between 0.4 and 100 GeV, and the dust optical depth at 353 GHz as a combination of HI-bright, CO-bright, and ionised gas components. The complementary information from dust emission and gamma rays was used to reveal the gas not seen, or poorly traced, by HI , free-free, and $^{12}$CO emissions, namely (i) the opaque HI and diffuse H$_2$ present in the Dark Neutral Medium at the atomic-molecular transition, and (ii) the dense H$_2$ to be added where $^{12}$CO lines saturate. The measured interstellar gamma-ray spectra support a uniform penetration of the cosmic rays with energies above a few GeV through the clouds. We find a gradual increase in grain opacity as the gas becomes more dense. The increase reaches a factor of four to six in the cold molecular regions that are well shielded from stellar radiation. Consequently, the X$_{CO}$ factor derived from dust is systematically larger by 30% to 130% than the gamma-ray estimate. We also evaluate the average gamma-ray X$_{CO}$ factorfor each cloud, and find that X$_{CO}$ tends to decrease from diffuse to more compact molecular clouds, as expected from theory.

astro-ph.HE↗