arXiv subjects

J. A. García

Publications and source records attributed to J. A. García.

At least 19 recordsLinked to original sources

arXiv2026

Disk reflection as the origin of the X-ray polarization of NGC 4151 with IXPE

We present an X-ray spectro-polarimetric study of the nearby type-1 active galactic nucleus NGC 4151 using two long IXPE observations obtained in 2022 and 2024, supported by simultaneous XMM-Newton and NuSTAR spectroscopy. IXPE measures a polarization degree of $\sim 6-7\%$ above 4 keV, with a polarization angle parallel to the radio jet, and a distinct low-energy component with a different angle, indicating at least two polarized components in the $2-8$ keV band. Previous work interpreted the hard X-ray polarization as evidence for a radially extended slab-like corona. Here we test an alternative scenario in which the observed polarization is produced predominantly by relativistic reflection from an accretion disk illuminated by a compact, lamp-post-like corona. Using recently developed models, we fit the IXPE Stokes spectra with a lamp-post plus distant-torus geometry, including partial-covering absorption and an additional soft polarized power-law component. We find that the data require a low coronal height ($h<9\,R_{\rm g}$ at $3σ$) and a relatively large torus opening angle ($>45^\circ$ at 3$σ$), while the disk reflection contributes $\sim 20\%$ of the 2-8 keV flux. The soft polarized component carries only $\sim 1-5\%$ of the flux but has a high polarization degree ($>10\%$) and a polarization angle around $20^\circ$. The same configuration provides acceptable fits to the $0.4-79$ keV XMM-Newton and NuSTAR spectra, demonstrating that disk reprocessing by a compact corona can simultaneously account for both the polarization and broadband spectral properties of NGC 4151.

astro-ph.HE↗

arXiv2026

Dust grain chemistry in the diffuse ISM towards the black hole transient GX 339-4

We present results on X-ray absorption and the dust grain chemistry in the diffuse interstellar medium (ISM), based on a new Cycle 25 Chandra High Energy Transmission Grating Spectrometer (HETGS) observational campaign targeting the black hole transient GX 339-4. The X-ray source offers an optimal combination of moderate hydrogen column density and high X-ray flux, enabling the first detailed simultaneous fitting of the photoabsorption edges of Fe, O, Si, and Mg which are key elemental constituents of interstellar dust. We performed a joint spectral analysis of Chandra/HETGS data and archival observations from the Reflection Grating Spectrometer (RGS) on board XMM-Newton. We found that the dust grain chemical composition along this diffuse Galactic line of sight is best described by the silicate Mg-rich amorphous pyroxene (Mg0.75Fe0.25SiO3) and metallic iron. We also discuss the elemental abundances and depletions of Fe, O, Si, and Mg, and the presence of absorption features in the X-ray spectrum of this source associated with highly ionised plasma.

astro-ph.HE↗

arXiv2026

Muon tracking in a LiquidO opaque scintillator detector

LiquidO is an innovative radiation detector concept. The core idea is to exploit stochastic light confinement in a highly scattering medium to self-segment the detector volume. In this paper, we demonstrate event-by-event muon tracking in a LiquidO opaque scintillator detector prototype. The detector consists of a 30 mm cubic scintillator volume instrumented with 64 wavelength-shifting fibres arranged in an 8$\times$8 grid with a 3.2 mm pitch and read out by silicon photomultipliers. A wax-based opaque scintillator with a scattering length of approximately 0.5 mm is used. The tracking performance of this LiquidO detector is characterised with cosmic-ray muons and the position resolution is demonstrated to be 450 $μ$m per row of fibres. These results highlight the potential of LiquidO opaque scintillator detectors to achieve fine spatial resolution, enabling precise particle tracking and imaging.

physics.ins-det↗

arXiv2025

Low-energy threshold demonstration for dark matter searches in TREX-DM with an $^{37}$Ar source produced at CNA HiSPANoS

We report on the successful implementation of an $^{37}$Ar calibration source in the TREX-DM detector, a high-pressure time projection chamber designed for low-mass dark matter searches. The $^{37}$Ar source was produced through fast neutron activation of CaO powder at the HiSPANoS facility of Centro Nacional de Aceleradores (CNA) in Spain, yielding $O(1)$ kBq of activity. Using a novel combined GEM-Micromegas readout system, we successfully detected both characteristic emissions from $^{37}$Ar decay (2.82 keV and 270 eV) and achieved unprecedented energy threshold performance in TREX-DM, approaching the single-electron ionization energy of argon.

physics.ins-det↗

arXiv2025

A XRISM/Resolve view of the dynamics in the hot gaseous atmosphere of M87

The XRISM/Resolve microcalorimeter directly measured the gas velocities in the core of the Virgo Cluster, the closest example of AGN feedback in a cluster. This proximity allows us to resolve the kinematic impact of feedback on scales down to 5 kpc. Our spectral analysis reveals a high velocity dispersion of $σ_v$=262 (+45 / -38) km/s near the AGN, which steeply declines to ~60 km/s between 5 and 25 kpc in the northwest direction. The observed line-of-sight bulk velocity in all regions is broadly consistent with the central galaxy, M87, with a mild trend toward blueshifted motions at larger radii. Systematic uncertainties have been carefully assessed and do not affect the measurements. The central velocities, if attributed entirely to isotropic turbulence, correspond to a transonic ICM at sub-6 kpc scales with three-dimensional Mach number 0.69 (+0.14 / -0.11) and a non-thermal pressure fraction of 21 (+7 / -5)%. Simple models of weak shocks and sound waves and calculations assuming isotropic turbulence both support the hypothesis that the velocity field reflects a mix of shock-driven expansion and turbulence. Compared to other clusters observed by XRISM to date, M87's central region stands out as the most kinematically disturbed, exhibiting both the highest velocity dispersion and the largest 3D Mach number, concentrated at the smallest physical scales.

astro-ph.GA↗

arXiv2025

XRISM/Resolve reveals the complex iron structure of NGC 7213: Evidence for radial stratification between inner disk and broad-line region

We present the first high-resolution X-ray spectrum of NGC 7213 obtained with XRISM/Resolve, supported by simultaneous XMM-Newton, NuSTAR, and SOAR optical data. The XRISM spectrum resolves the neutral Fe\,K$α$ into two components: a narrow core ($\rm FWHM = 650_{-220}^{+240}\,\rm km\,s^{-1}$) consistent with emission at the dust sublimation radius, and a broader, asymmetric line best described by disk-like emission from $\sim 100\,\rm R_{g}$. This disk component mirrors the profile of the double-peaked H$α$ line observed in the optical. In addition, we detect broadened Fe XXV and Fe XXVI emission lines whose inferred locations bridge the gap between the inner disk and the optical broad-line region. The weak narrow Fe K$α$ equivalent width ($\rm EW = 32 \pm 6\,eV$) and absence of a Compton hump imply a low-covering-fraction, Compton-thin torus. Together, these results reveal a radially stratified structure in NGC 7213, spanning nearly four orders of magnitude in radius, and place the source in an intermediate accretion state ($\rm λ_{Edd} = 0.001-0.01$) where the inner disk and BLR remain, while the torus shows signs of dissipation.

astro-ph.HE↗

arXiv2025

Spatial resolution studies with the BabyIAXO Micromegas prototype

The spatial resolution of the Micromegas prototype developed for the BabyIAXO experiment was evaluated using a low-energy X-ray beam at the SOLEIL synchrotron facility. BabyIAXO, currently under construction, aims to search for hypothetical solar axions. A key component of the experiment is a low-background X-ray detector with high efficiency in the 1-10 keV energy range and stringent background rejection capabilities. Achieving a spatial resolution on the order of, or better than, 1 mm is critical for accurately reconstructing signal shapes and positions, and for effectively discriminating between signal and background events. Therefore, a precise characterization of the detector's spatial resolution is essential to validate its suitability for the experiment. This study involved scanning the IAXO-D1 Micromegas detector under various beam energies, positions, and drift field configurations to evaluate their influence on spatial resolution. A resolution of approximately 100 $μ$m at 6 keV was achieved, confirming the strong potential of this technology for application in the final BabyIAXO setup.

physics.ins-det↗

arXiv2025

Recovery of the X-ray polarisation of Swift J1727.8$-$1613 after the soft-to-hard spectral transition

We report on the detection of X-ray polarisation in the black-hole X-ray binary Swift J1727.8$-$1613 during its dim hard spectral state by the Imaging X-ray Polarimetry Explorer (IXPE). This is the first detection of X-ray polarisation at the transition from the soft to the hard state in an X-ray binary. We find an averaged 2$-$8 keV polarisation degree of (3.3 ${\pm}$ 0.4) % and a corresponding polarisation angle of 3° ${\pm}$ 4°, which matches the polarisation detected during the rising stage of the outburst, in September$-$October 2023, within 1$σ$ uncertainty. The observational campaign complements previous studies of this source and enables comparison of the X-ray polarisation properties of a single transient across the X-ray hardness-intensity diagram. The complete recovery of the X-ray polarisation properties, including the energy dependence, came after a dramatic drop in the X-ray polarisation during the soft state. The new IXPE observations in the dim hard state at the reverse transition indicate that the accretion properties, including the geometry of the corona, appear to be strikingly similar to the bright hard state during the outburst rise despite the X-ray luminosities differing by two orders of magnitude.

astro-ph.HE↗

arXiv2025

Search for solar axions produced through the axion-electron coupling $g_{ae}$ using a new GridPix detector at CAST

We present a search for solar axions produced through the axion-electron coupling $(g_{ae})$ using data from a novel 7-GridPix detector installed at the CERN Axion Solar Telescope (CAST). The detector, featuring ultra-thin silicon nitride windows and multiple veto systems, collected approximately 160 hours of solar tracking data between 2017-2018. Using machine learning techniques and the veto systems, we achieved a background rate of $1.06\times 10^{-5}\,\text{keV}^{-1}\text{cm}^{-2}\text{s}^{-1}$ at a signal efficiency of about $80\,\%$ in the $0.2$-$8\,\text{keV}$ range. Analysis of the data yielded no significant excess above background, allowing us to set a new upper limit on the product of the axion-electron and axion-photon couplings of $g_{ae}\cdot g_{aγ} < 7.35\times 10^{-23}\,\text{GeV}^{-1}$ at $95\,\%$ confidence level. This result improves upon the previous best helioscope limit and demonstrates the potential of GridPix technology for rare event searches. Additionally, we derived a limit on the axion-photon coupling of $g_{aγ} < 9.0\times 10^{-11}\,\text{GeV}^{-1}$ at $95\,\%$ CL, which, while not surpassing CAST's best limit, provides complementary constraints on axion models.

hep-ex↗

arXiv2025

The Structure of the Relativistic Fe Line in GX 340+0 as Viewed with XRISM/Resolve, NICER, and NuSTAR

We present a 152 ks XRISM/Resolve observation of the persistently accreting Z source GX 340+0. Simultaneous observations also occurred with NuSTAR and NICER for 22.47 ks and 2.7 ks, respectively. The source covered the normal branch to the flaring branching during the observations. The data from all three missions were modeled concurrently for each spectral branch. The superior energy resolution of XRISM/Resolve reveals structure within the iron emission line complex regardless of spectral state. We model the reprocessed Fe K line with a reflection model tailored for thermal illumination of the accretion disk by a neutron star. The currently available model encompasses the broad components, but narrow emission features remain at the ~5% level. These remaining features may be described by the presence of an ionized plasma in the system as has been observed in the Z source Cygnus X-2, but subsequent updates to the reflection model code may be able to explain these features.

astro-ph.HE↗

arXiv2025

On the nature of the X-ray binary transient MAXI J1834-021: clues from its first observed outburst

MAXI J1834-021 is a new X-ray transient that was discovered in February 2023. We analysed the spectral and timing properties of MAXI J1834-021 using NICER, NuStar and Swift data collected between March and October 2023. The light curve showed a main peak followed by a second activity phase. The majority of the spectra extracted from the individual NICER observations could be adequately fitted with a Comptonisation component alone, while a few of them required an additional thermal component. The spectral evolution is consistent with a softening trend as the source gets brighter in X-rays. We also analysed the broadband spectrum combining data from simultaneous NICER and NuStar observations on 2023 March 10. This spectrum can be fitted with a disc component with a temperature at the inner radius of $kT_{\rm in} \sim 0.4$ keV and a Comptonisation component with a power-law photon index of $Γ\sim 1.8$. By including a reflection component in the modelling, we obtained a 3$σ$ upper limit for the inner disc radius of 11.4 gravitational radii. We also detected a quasi-periodic oscillation (QPO), whose central frequency varies with time (from 2 Hz to $\sim$0.9 Hz) and anti-correlates with the hardness ratio. Based on the observed spectral-timing properties, MAXI J1834-021, can be classified as a low-mass X-ray binary in outburst. However, we are not able to draw a definitive conclusion on the nature of the accreting compact object, which at the moment could as well be a black hole or a neutron star.

astro-ph.HE↗

arXiv2025

Micromegas with GEM preamplification for enhanced energy threshold in low-background gaseous time projection chambers

Background: we develop the concept of a Micromegas (MICRO-MEsh GAseous Structure) readout plane with an additional GEM (Gas Electron Multiplier) preamplification stage placed a few mm above it, to increase the maximum effective gain of the combined readout. We implement it and test it in realistic conditions for its application to low-background dark matter searches like the TREX-DM experiment. Methods: for this, we use a Micromegas of microbulk type, built with radiopure materials. A small test chamber allowing for systematic scanning of voltages and pressures is used. In addition, a TREX-DM full-scale set-up has also been built and tested, featuring a replica of the fully-patterned TREX-DM microbulk readout. Results: we report on GEM effective extra gain factors of about 90, 50 and 20 in 1, 4 and 10 bar of Ar-1%iC$_{4}$H$_{10}$. Conclusions: the results here obtained show promise to lower the threshold of the experiment down to 50 eV$_{ee}$, corresponding to substantially enhanced sensitivity to low-mass WIMPs (Weakly Interacting Massive Particles).

physics.ins-det↗

arXiv2025

COCOA: a compact Compton camera for astrophysical observation of MeV-scale gamma rays

COCOA (COmpact COmpton cAmera) is a next-generation gamma-ray telescope designed for astrophysical observations in the MeV energy range. The detector comprises a scatterer volume employing the LiquidO detection technology and an array of scintillating crystals acting as absorber. Surrounding plastic scintillator panels serve as a veto system for charged particles. The detector's compact, scalable design enables flexible deployment on microsatellites or high-altitude balloons. Gamma rays at MeV energies have not been well explored historically (the so-called "MeV gap") and COCOA has the potential to improve the sensitivity in this energy band.

astro-ph.IM↗

arXiv2025

GX 340+0: In and Out of Focus

The Nuclear Spectroscopic Telescope Array (NuSTAR) enables detailed high-energy X-ray observations from 3--79 keV, but its performance can be constrained by telemetry saturation when observing bright sources, leading to reduced effective exposure times. In this study, we investigate the use of serendipitous stray light (SL) observations to infer properties of an X-ray bright source in comparison to focused data. Our case study is performed on the neutron star (NS) low-mass X-ray binary (LMXB) GX 340+0, a prominent Z source, where we execute a spectral analysis comparing 25 SL and 7 focused NuSTAR observations. Our findings demonstrate that SL observations can significantly enhance long-term temporal coverage; detecting variations in the thermal components of the system across the baseline of the mission, which could not be inferred from focused observations alone.

astro-ph.HE↗

arXiv2025

The Stochastic Light Confinement of LiquidO

Light-based detectors have been widely used in fundamental research and industry since their inception in the 1930s. The energy particles deposit in these detectors is converted to optical signals via the Cherenkov and scintillation mechanisms that are then propagated through transparent media to photosensors placed typically on the detector's periphery, sometimes up to tens of metres away. LiquidO is a new technique pioneering the use of opaque media to stochastically confine light around each energy deposition while collecting it with an array of fibres that thread the medium. This approach preserves topological event information otherwise lost in the conventional approach, enabling real-time imaging down to the MeV scale. Our article demonstrates LiquidO's imaging principle with a ten-litre prototype, revealing successful light confinement of 90% of the detected light within a 5 cm radius sphere, using a custom opaque scintillator with a scattering length on the order of a few millimetres. These high-resolution imaging capabilities unlock opportunities in fundamental physics research and applications beyond. The absolute amount of light detected is also studied, including possible data-driven extrapolations to LiquidO-based detectors beyond prototyping limitations. Additionally, LiquidO's timing capabilities are explored through its ability to distinguish Cherenkov light from a slow scintillator.

physics.ins-det↗

arXiv2025

Simultaneous NICER and NuSTAR Observations of the Neutron Star Low-mass X-ray Binary Serpens X-1

We present the first contemporaneous NICER and NuSTAR analysis of the low-mass X-ray binary Serpens X-1 obtained in June 2023, performing broadband X-ray spectral analysis modeling of the reprocessed emission with RELXILLNS from $0.4-30$ keV. We test various continuum and background estimation models to ensure that our results do not hinge on the choice of model used and found that the detection of reflection features is independent of the choice of both continuum and background model. The position of the inner accretion disk is consistent with the last stable circular orbit ($R_{\rm in} \leq 1.2$~$R_{ISCO}$) and a low inclination of $i\leq 8.3 ^{\circ}$. Additionally, we investigate the presence of the low energy ($\sim$ 1 keV) Fe L complex in the data from NICER and the Reflection Grating Spectrometer (RGS) on XMM-Newton that was previously reported in the literature. We find that the line is at most a 2% feature relative to the reprocessed continuum and are unable to claim a definitive detection for the current dataset. However, we discuss plausible conditions and systems that would increase the likelihood of detecting this feature in the future.

astro-ph.HE↗

arXiv2024

A new upper limit on the axion-photon coupling with an extended CAST run with a Xe-based Micromegas detector

Hypothetical axions provide a compelling explanation for dark matter and could be emitted from the hot solar interior. The CERN Axion Solar Telescope (CAST) has been searching for solar axions via their back conversion to X-ray photons in a 9-T 10-m long magnet directed towards the Sun. We report on an extended run with the IAXO (International Axion Observatory) pathfinder detector, doubling the previous exposure time. The detector was operated with a xenon-based gas mixture for part of the new run, providing technical insights for future detector configurations in IAXO. No counts are detected in the 95% signal-encircling region during the new run, while 0.75 are expected. The new data improve the axion-photon coupling limit to 5.8$\times 10^{-11}\,$GeV$^{-1}$ at 95% C.L. (for $m_a \lesssim 0.02$ eV), the most restrictive experimental limit to date.

hep-ex↗

arXiv2024

Argon X-ray absorption in the local ISM

We present the first comprehensive analysis of the argon K-edge absorption region (3.1-4.2 Å) using high-resolution HETGS {\it Chandra} spectra of 33 low-mas X-ray binaries. Utilizing R-matrix theory, we computed new K photoabsorption cross-sections for {\rm Ar}~{\sc i}--{\rm Ar}~{\sc xvi} species. For each X-ray source, we estimated column densities for the {\rm Ar}~{\sc i}, {\rm Ar}~{\sc ii}, {\rm Ar}~{\sc iii}, {\rm Ar}~{\sc xvi}, {\rm Ar}~{\sc xvii} and {\rm Ar}~{\sc xviii} ions, which trace the neutral, warm and hot components of the gaseous Galactic interstellar medium. We examined their distribution as a function of Galactic latitude, longitude, and distances to the sources. However, no significant correlations were discerned among distances, Galactic latitude, or longitude. Future X-ray observatories will allow us to benchmark the atomic data as the main resonance lines will be resolved.

astro-ph.HE↗