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From hydrated silica to quartz: Potential hydrothermal precipitates found in Jezero crater, Mars
No full textInternational audienceOn Earth, silica-rich phases from opal to quartz are important indicators and tracers of geological processes. Hydrated silica, such as opal, is a particularly good matrix for the preservation of molecular and macroscopic biosignatures. Cherts, a type of silica-dominated rocks, provide a unique archive of ancient terrestrial life while quartz is the emblematic mineral of the Earth's continental crust. On Mars, hydrated silica has been detected in several locations based on remote sensing and rover-based studies. In the present article we report on the detection of cobbles made of hydrated silica (opal or chalcedony), as well as well-crystallized quartz. These detections were made with the SuperCam instrument onboard Perseverance (Mars 2020 mission), using a combination of LIBS, infrared and Raman spectroscopy. Quartz-dominated stones are detected unambiguously for the first time on the Martian surface, and based on grain size and crystallinity are proposed to be of hydrothermal origin. Although these rocks were all found as float, we propose that these detections are part of a common hydrothermal system, and represent different depths / temperatures of precipitation. This attests that hydrothermal processes were active in and around Jezero crater, possibly triggered by the Jezero crater-forming impact. These silica-rich rocks, in particular opaline silica, are very promising targets for sampling and return to Earth given their high biosignature preservation potential.</div
Cosmic rays cannot explain the high ionisation rates in the Galactic centre
No full textInternational audienceContext. The H2 ionisation rate in the central molecular zone, located in the Galactic centre, is estimated to be ζ ∼ 2 × 10−14 s−1, based on observations of H3+ lines. This value is two to three orders of magnitude larger than that measured anywhere else in the Galaxy.Aims. Due to the high density of the gas in the central molecular zone, UV and X-ray photons do not penetrate this region. Hence, cosmic rays are expected to be the exclusive agents of ionisation. A high cosmic-ray density has been invoked to explain the unusually high ionisation rate. However, this excess is not seen in the γ-ray emission from this region, which is produced by high-energy cosmic rays. Therefore, an excess is expected only in the low-energy cosmic-ray spectrum. Here, we derive constraints on this hypothetical low-energy component in the cosmic-ray spectra, and we question its plausibility.Methods. To do so, we numerically solved the cosmic-ray transport equation in the central molecular zone, considering spatial diffusion, advection in the Galactic wind, re-acceleration in the ambient turbulence, and energy losses due to interactions with matter and radiation in the interstellar medium. We derived stationary solutions under the assumption that cosmic rays are continuously injected by a source located in the Galactic centre. The high-energy component in the cosmic-ray spectrum was then fitted to available γ-ray and radio data, and a steep low-energy component was added to the cosmic-ray spectrum to explain the large ionisation rates.Results. We find that injection spectra of p−7 for protons below penh, pc ≃ 780 MeV and p−5.2 for electrons below penh, ec = 1.5 GeV are needed to reach the observed ionisation rates. This corresponds to a cosmic-ray power of the order of ∼1040 − 41 erg s−1 injected at the Galactic centre. Not only is this unrealistic, but it is also impossible to reproduce a constant ionisation rate across the region, as observations suggest.Conclusions. We conclude that cosmic rays alone cannot explain the high ionisation rates in the Galactic centre
Constraints on the state of the IGM at using redshifted 21-cm observations with LOFAR
No full textInternational audienceThe power spectra of the redshifted 21-cm signal from the Epoch of Reionization (EoR) contain information about the ionization and thermal states of the intergalactic medium (IGM), and depend on the properties of the EoR sources. Recently, Mertens et al 2025 has analysed 10 nights of LOFAR high-band data and estimated upper limits on the 21-cm power spectrum at redshifts 8.3, 9.1 and 10.1. Here we use these upper limit results to constrain the properties of the IGM at those redshifts. We focus on the properties of the ionized and heated regions where the temperature is larger than that of the CMB. We model the 21-cm power spectrum with the code GRIZZLY, and use a Bayesian inference framework to explore the source parameters for uniform priors on their ranges. The framework also provides information about the IGM properties in the form of derived parameters. In a model which includes a radio background in excess of the CMB, the 95 (68) per cent credible intervals of disfavoured models at redshift 9.1 for the chosen priors correspond to IGM states with averaged ionization and heated fraction below 0.46 (), an average gas temperature below 44 K (4 K), and a characteristic size of the heated region (). The 68 per cent credible interval suggests an excess radio background which is more than 100 per cent of the CMB at 1.42 GHz, while the 95 per cent credible interval of the radio background efficiency parameter spans the entire prior range. The behaviour of the credible intervals is similar at all redshifts. The models disfavoured by the LOFAR upper limits are extreme ones, as they are mainly driven by rare and large ionized or heated regions
Transient Gamma Rays from the 2021 Outburst of the Recurrent Nova RS Ophiuchi: The Effect of Gamma-Ray Absorption
No full textInternational audienceIn 2021, RS Ophiuchi was the first nova to be detected in the very-high-energy (TeV) gamma-ray domain, directly testifying to efficient acceleration of charged particles up to at least the TeV range at the nova shock. Surprisingly, the TeV gamma-ray signal peaks ∼2 days after the GeV signal, and the origin of this delay has still not been clearly understood. We investigate the possibility that this delay is due to the effect of gamma-ray absorption resulting from interactions between gamma rays and optical photons copiously emitted during the outburst. We model particle acceleration at a nova shock to obtain the gamma-ray emission produced in interactions between the accelerated particles and the shocked gas. The effect of gamma-ray absorption is then included in detail using the radiative transfer equation. We find that this can naturally account for the delay between the peaks of GeV and TeV gamma-ray lightcurves. This result emphasizes the importance of gamma-ray absorption for interpreting gamma-ray observations of novae in the TeV range, which, in turn, demonstrates the necessity of a multiwavelength view for unraveling the underlying physics of particle acceleration in these systems
Unsupervised Classification of Gamma-ray Bursts from Blazars (GRBBLs) with Machine Learning
No full textInternational audienceBlazars dominate the extragalactic -ray sky and show pronounced flares. Using public Fermi-LAT light curves for 732 blazars with secure redshifts, I implement an automated pipeline to identify and characterize -ray bursts from blazars (GRBBLs). Each event is modeled with an exponential rise/decay profile, and spectral variability is quantified via a constant fit. From 679 high-quality GRBBLs, I apply extreme deconvolution for unsupervised classification. The GRBBL population is remarkably homogeneous; the most robust split is in achromatic vs. chromatic events, with significant overlap. Removing spectral information yields a luminosity-driven classification in type-1 and type-2 GRBBLs, although this classification is not identified in all tests. This study establishes GRBBL population studies as a tool to study blazars. As a by-product of this project I identify a correlation between peak luminosity and timescales in GRBBLs
A double dipole geometry for PSR J0740+6620
No full textInternational audienceContext. Millisecond pulsars are known to show complex radio pulse profiles and polarisation position angle evolution with rotational phase. Small-scale surface magnetic fields and multipolar components are believed to be responsible for this complexity, due to the radiation mechanisms occurring close to the stellar surface but within the relatively small light cylinder compared to the stellar radius.Aims. In this work, we use the latest NICER phase aligned thermal X-ray pulse profile of PSR J0740+6620 combined with radio and γ-ray pulse profiles and radio polarisation to deduce the best magnetic field configuration that can simultaneously reproduce the light curves in these respective bands.Methods. We assumed a polar cap model for the radio emission and used the rotating vector model for the associated polarisation and a striped wind model for the γ-ray light curves, whereas we relied on the NICER collaboration results for the hot spot geometry.Results. We demonstrate that an almost centred dipole can account for the hot spot location with a magnetic obliquity of α ≈ 51° and a line of sight inclination angle of ζ ≈ 82°. However, with this geometry, the hot spot areas are three times too large. We found a better solution consisting of two dipoles located just below the surface in approximately antipodal positions.Conclusions. Our double dipole model is able to reproduce all the salient radio and γ-ray characteristics of PSR J0740+6620 including radio polarisation data. A double dipole solution is more flexible than an off-centred dipole because it has two independent magnetic axes and could hint at a magnetic field mostly concentrated within the crust and not in the core
Mitigating incoherent excess variance in high-redshift 21-cm observations with multi-output cross Gaussian process regression
No full textInternational audienceSystematic effects that limit the achievable sensitivity of current low-frequency radio telescopes to the 21-cm signal are among the foremost challenges in observational 21-cm cosmology. The standard approach to retrieving the 21-cm signal from radio interferometric data separates it from bright astrophysical foregrounds by exploiting their spectrally smooth nature, in contrast to the finer spectral structure of the 21-cm signal. Contaminants exhibiting rapid frequency fluctuations, on the other hand, are difficult to separate from the 21-cm signal using standard techniques, and the power from these contaminants contributes to low-level systematics that can limit our ability to detect the 21-cm signal. Many of these low-level systematics are incoherent across multiple nights of observation, resulting in an incoherent excess variance above the thermal noise sensitivity of the instrument. In this paper, we develop a method called cross-GPR (cross covariance Gaussian process regression) that exploits the incoherence of these systematics to separate them from the 21-cm signal, which remains coherent across multiple nights of observation. We first develop and demonstrate the technique on synthetic signals in a general setting, and then apply it to gridded interferometric visibility cubes. We perform realistic simulations of visibility cubes containing foregrounds, 21-cm signal, noise, and incoherent systematics. The simulations show that the method can successfully separate and subtract incoherent contributions to the excess variance, and its advantages over standard techniques become more evident when the spectral behavior of the contaminants resembles that of the 21-cm signal. Simulations performed on a variety of 21-cm signal shapes also reveal that the cross-GPR approach can subtract incoherent contributions to the excess variance, without suppressing the 21-cm signal
Effect of water saturation on the diffusive transport of water and solutes in compacted kaolinite, illite and vermiculite
No full textInternational audienceThrough-diffusion experiments were carried out under partially water saturated conditions and using uncharged tracers (HTO and HDO; water molecule with one tritium and deuterium atom, respectively), negatively charged tracers (125I− and 36Cl−) and positively charged tracer (22Na+) in two reference clayey materials: (i) illite, a negatively-charged clay mineral with only interparticle porosity and (ii) vermiculite, a negatively-charged clay mineral with both inter-particle and interlayer porosities. For both types of porous media, a sharp decrease in diffusive flux was observed for all three types of tracer when water saturation (Sw) was reduced from Sw = 0.95 to 0.88. The data were compared to those obtained previously with porous media made of kaolinite, a weakly-charged clay mineral having only inter-particle porosity, to quantify the relative influences of surface charge and pore geometry/pore size distribution on diffusion properties as a function of water saturation. The latter parameter had a prominent role on the diffusional properties in the investigated range of Sw, regardless of the type of clayey material. Then, a relationship predicting diffusivities of cation under partially water saturated conditions as a function of diffusivities of anion at the same level of water saturation is proposed for the reference clayey materials investigated
Explaining JWST Counts with Galaxy Formation Models
No full textInternational audienceA distinct power-law break is apparent at m ∼ 21 in the deep near-infrared PEARLS-JWST galaxy counts. The break becomes more pronounced at longer wavelengths, with the slope flattening smoothly with apparent magnitude in the shortest band used at 0.9 μm, and trending toward an increasingly broken slope by the longest wavelength passband of JWST’s Near Infrared Camera, 4.4 μm. This behaviour is remarkably well predicted by the GALFORM semi-analytical model of galaxy formation. We use the model to diagnose the origin of this behaviour. The features that are responsible for the break are (1) the inherent break in the luminosity function (LF); (2) the change in the volume element with redshift; (3) the redshift-dependent nature of the k-correction (with 1 contributing to the existence of the break and 2–3 contributing to its shape). We study the contribution to these effects by their morphology using the bulge-to-total stellar mass ratio. The way in which bulge-dominated galaxies populate the bright end of the LF while disk-dominated galaxies dominate the faint end is preserved in the galaxy number counts, with a characteristic stellar mass at a break of ∼10M. The shape of the number counts is mainly driven by galaxies with relatively low redshift (z ≲ 2) for a limit of m ≲ 28. We give a comprehensive description of why the galaxy number counts in the near-infrared PEARLS-JWST observation look the way they do and which population of galaxies is dominant at each apparent magnitude
Diffuse gamma-ray and neutrino emission from the Milky Way and the local knee in the cosmic ray spectrum
No full textInternational audienceThe LHAASO observatory has recently measured details of the cosmic-ray (CR) spectrum in the knee region (1 -- 10 PeV) with unprecedented precision, including its average CR mass composition and the spectrum of the proton component. We use these precision measurements, combined with direct measurements of CRs by space-based detectors, to derive predictions for the spectrum of diffuse gamma-ray and neutrino emission from the interstellar medium under the assumption that the CR spectrum is universal throughout the Milky Way. We compare these predictions with the Fermi-LAT and LHAASO measurements of the diffuse gamma-ray flux from inner and outer Galactic Plane regions and with estimates of the neutrino flux based on the IceCube data for the same Galactic Plane regions. We notice that the model predictions exceed LHAASO gamma-ray measurements at energies above 100 TeV. This excess can be interpreted within a CR knee model assuming a "local PeV CR bubble''. Within this model, we infer the extension of the local PeV CR bubble of 1.5 +/- 0.3 kpc in the direction of the inner Galaxy and of 0.7+/-0.3 kpc toward the outer Galaxy