Manami Sasaki

Context. Diffuse X-ray emission in galaxies arises from both stellar-feedback-heated gas and unresolved compact sources. Previous studies mainly focused on the integrated X-ray luminosity, leaving the thermal component, especially in low star formation rate (SFR) galaxies, largely unexplored. Aims. We aim to characterize the diffuse X-ray emission of nearby galaxies over a wide range of SFRs and to test how the X-ray luminosities L _X scale with star formation in a blind volume-limited sample. Methods. We analyzed data of the four eROSITA/ Spectrum-Roentgen-Gamma all-sky surveys (eRASS:4) for 140 galaxies within 25 Mpc drawn from the Updated Nearby Galaxy Catalog and cross-matched with HECATEv2. Early-type galaxies were treated separately, while the star-forming galaxies were divided by specific SFR (sSFR) into low-, medium-, and high-sSFR subsamples. We extracted spectra of the integrated emission and modeled them with thermal plasma and power law components. For each subsample, we stacked the spectra and derived total and thermal-component X-ray luminosities. We then fitted relations for the star-forming galaxies. Results. The stacked spectra show a systematic change with sSFR: High-sSFR galaxies exhibit a pronounced soft thermal component, whereas low-sSFR and early-type galaxies are increasingly dominated by the harder nonthermal continuum. For the total 0.5–8 keV luminosity, we find log L _X ∼ 39.5 + 0.9 logSFR, with an intrinsic scatter of ∼0.9 dex. For the 0.5–2 keV luminosity of the thermal component, we find log L _X ∼ 38.7 + 1.1 logSFR, with a smaller intrinsic scatter of ∼0.7 dex. Conclusions. The X-ray luminosity of the thermal component remains well correlated with the SFR down to ∼10 ⁻⁵ M _⊙ yr ⁻¹ and is consistent with previous relations for star-forming galaxies, while the total L _X − SFR relation is flatter and more scattered. This result suggests that at low SFR, the total X-ray output is likely dominated by a small number of unresolved X-ray binaries, whereas the thermal X-ray luminosity remains a robust tracer of stellar-feedback-related X-ray emission.

We present ALMA observations of DEML 241/LMC P3, the most luminous γ -ray binary consisting of a compact object and an O star, in CO emission. We have found a one-sided jet-like CO feature of 8 pc length and 1 pc width, which accompanies another weaker CO jet candidate with slightly different orientation. The one-sided CO jet exhibits striking alignment with LMC P3, suggesting that the jet was driven by LMC P3. We have determined the kinetic temperature of the CO jet to be significantly high at 33−60 K as compared with ∼15 K in the nearby nonjet CO cloud, where no radiative heat source is found. We interpret that the high temperatures are due to shock heating of a microquasar jet driven by the γ -ray binary, where the compact object has an accretion disk fed by the O star winds. The CO jet matches existing predictions from magnetohydrodynamical simulations, which show that CO jet can form from the interaction of the microquasar jet and an ambient interstellar medium (ISM) cloud. These results provide strong evidence that CO jets are a signature sculpted by microquasar jets, lending support for mass accretion in LMC P3 as the γ -ray origin. The results suggest a second case of CO jets potentially driven by a microquasar along with the CO jets in the microquasar candidate HESS J1023-575 recently identified in the Milky Way. Furthermore, our results suggest the use of submillimeter observations for identifying microquasars, opening a new possible window for their discovery and study.

Context . Very high-energy (VHE) γ -ray emission detected from the central molecular zone (CMZ) hints at diffusive propagation of cosmic rays (CRs) injected continuously near the Galactic center (GC). Aims . Using H.E.S.S. VHE γ -ray observations, we aim to construct a multicomponent description of the region in order to derive the spatial and spectral distributions of CRs and discuss their potential origin. Methods . 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. Results . The GC ridge emission is well reproduced with the assumed gas distribution model weighted by a 1 /r ^α CR density profile with α = 1.10 ± 0.05 _stat ± 0.10 _syst . We detect no significant spectral variations across the CMZ, which supports a continuous injection from the GC. We report a significant (>3 σ ) curvature in the GC ridge γ -ray spectrum, implying a parent proton break (or cutoff energy) of E _b ∼ 15–50 TeV ( E _cut ∼ 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 ∼ 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, and 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.

Aims. We report on the first X-ray detection of the symbiotic star CN Cha with the extended Roentgen Survey with an Imaging Telescope Array (eROSITA) on board Spektrum-Roentgen Gamma (SRG). Methods. We analysed the eROSITA observations from eROSITA All Sky Surveys eRASS2 to eRASS5 individually, as well as the combined data of all five surveys (eRASS:5) to investigate the X-ray spectral and timing properties of CN Cha. Results. Although CN Cha was not detected in eRASS1; it was detected for the first time in eRASS2. Our analysis shows that the X-ray emission of CN Cha gradually increased from eRASS2 to eRASS5. The X-ray spectrum of the source, with the main emission at < 2.4 keV, allowed us to classify it as a β -type symbiotic star. However, a more detailed analysis suggests that there is a very soft component, which may originate either from thermonuclear burning on the white dwarf or from wind collisions. Additional X-ray observations are required to better understand the origin and emission mechanisms of the source.

Context . The eROSITA bubbles (eRObub) were discovered in 2020 in the first SRG/eROSITA All-Sky Survey. They are some of the most extended structures in the X-ray sky. In projection, the eRObub encloses the γ -ray Fermi bubbles, and the northern eRObub appears to be bounded by the Loop I/North Polar Spur. Aims . Using eROSITA all-sky maps and spatially resolved spectra, we infer the three-dimensional structure and measure the hot gas properties of the eRObub. Methods . We fitted spectra binned to a constant signal-to-noise ratio and spectra with a high signal-to-noise ratio from custom regions to examine the gas properties in more detail. We fitted the morphology of eRObub using a parametrised geometrical model that describes a blast wave propagating from the Galactic Centre into an idealised Galactic halo. Results . The interior of the western eRObub is best characterised by two emission components with relatively uniform temperatures: a hotter component at kT = 0.60 ± 0.02keV, and a colder component at kT =0.21 _−0.01 ^+0.03 keV, where the emission measure of the colder component is higher by about five times on average. Our spectra suggest sub-solar abundances (Z = 0.2 ± 0.1 Z _⊙ ), consistent with expectations for the Galactic halo. We found no conclusive evidence of α -element enhancement. In contrast, the abundances of the North Polar Spur are higher (Z > 0.5 Z _⊙ ). At face value, this disfavours a common origin. We spectrally confirmed an apparent cool shell at kT ~ 0.18−0.2keV surrounding the northern eRObub, assuming collisional ionisation equilibrium. We found no noticeable difference in X-ray emission in regions overlapping with the Fermi bubbles. Our geometrical model suggests that the horizontal size of the two eRObub is well constrained (semi-minor axis of ~6 kpc), but their vertical extent is uncertain because the observed X-ray emission is almost insensitive to the existence and location of a bubble cap. Additionally, a tilt (~30°) towards l ~ 220° is needed to reproduce the projected image of the northern eRObub, whereas the southern bubble requires a slight tilt.

The propagation of very-high-energy (VHE; E _γ ≥ 100 GeV) gamma rays from extragalactic sources is affected by interactions with photons of the extragalactic background light (EBL), resulting in pair production that attenuates the intrinsic gamma-ray flux. This interaction renders the Universe increasingly opaque to VHE photons at high energies and redshifts. New physics scenarios involving axion-like particles (ALPs) could modify this expected optical depth. In particular, ALPs with masses m _a ∼ 10 eV can decay into two photons over cosmological timescales, thereby contributing to the diffuse EBL. If such ALPs constitute a significant fraction of the dark matter density, their decay would enhance the EBL intensity and consequently increase the gamma-ray optical depth. In this study, we investigate this scenario using a large sample of gamma-ray spectra observed with the High Energy Stereoscopic System (H.E.S.S.) and the Fermi Large Area Telescope. We model the contribution of decaying ALPs to the EBL and assess their impact on the spectra of blazars across redshifts. By comparing these observations with standard EBL models, we place constraints on the properties of heavy ALPs — specifically their mass and photon coupling — and evaluate their viability as a dark matter candidate capable of modifying the gamma-ray transparency of the Universe. From the combined analysis, and under the assumption that ALPs constitute the entire dark matter density, we derive 95% confidence exclusion limits on the photon-ALP coupling down to m _aγ ∼ 7×10 ^-12 GeV ^-1 for masses m _a ∼ 15 eV. These constraints are competitive with existing astrophysical bounds and provide complementary sensitivity to other techniques, closing a previously unconstrained region of parameter space in the m _a ∼ 2.5–20 eV range.

The eROSITA telescope array on board the Spektrum-Roentgen-Gamma (SRG) mission began its all-sky survey program in December 2019, scanning the sky at an approximately six-month cadence. Here, we present new catalogues of point-like and extended X-ray sources derived from the first three completed eROSITA all-sky surveys (eRASS:3), covering the western Galactic hemisphere, which Germany’s eROSITA consortium holds proprietary rights for. We describe the observational strategy, data processing, and analysis pipelines. We also characterise the resulting X-ray source populations. The eRASS:3 main catalogue contains nearly two million sources, including 1 911 744 point-like and 63 796 extended sources, detected in the 0.2–2.3 keV energy band, eROSITA’s most sensitive energy window. This volume has doubled the X-ray source content of eRASS1 and provides a comprehensive census of X-ray-emitting objects across diverse astrophysical classes. We also released a secondary hard catalogue of nearly 15 000 sources detected in the harder 2.3–5.0 keV energy band. In addition, we released six catalogues, three for the main sample and three for the hard sample, in which we identify and classify the optical and infrared counterparts of eRASS:3 point-like sources. This approach delivers a homogeneous, high-quality identification of mostly extragalactic X-ray emitters; for example, we estimate that ~88% of the 1.4 million counterparts identified within the footprints of the Legacy Survey Imaging for DESI are extragalactic sources. This work has enabled the generation of samples optimised for completeness and purity, while expanding the discovery space for rare populations. The second data release (DR2) of the SRG/eROSITA all-sky survey is a catalogue-only release, comprising the catalogues presented in this work together with an updated version of the eROSITA upper flux limit server.

Ultraluminous X-ray sources (ULXs) are luminous non-nuclear X-ray point sources embedded in galaxies. Their luminosities exceeding 10^ 39 , , ^ erg s -1 potentially require sub-Eddington accretion by objects with masses from ∼10^2,M_⊙ to ∼10^4,M_⊙ or supercritical accretion beyond the Eddington limit by stellar mass black holes or neutron stars. As such, ULXs are excellent laboratories for extreme accretion flows in X-ray binaries and, in some cases, potential candidates for intermediate-mass black holes. The first catalog by the eROSITA instrument on board the Spectrum-Roentgen-Gamma mission provides the most complete X-ray view of the whole of the western Galactic hemisphere and is therefore an excellent resource in which to find new ULXs. We identified ULXs by searching for X-ray counterparts in the ∼10^5 local galaxies of the catalog in the footprint of the first eROSITA all-sky survey. The catalog is unaffected by X-ray selection biases. We characterized the completeness of the sample and its contamination by unidentified active galactic nuclei (AGN) and determined the influence of source confusion. The catalog provides the basis for ULX population studies and follow-up observations that can be used to perform a more detailed investigation of individual sources and variability. HECATE We searched for X-ray point sources exceeding the ULX threshold in HECATE galaxies while excluding the nuclear regions of the galaxies to avoid the selection of AGN. We removed known contaminants such as supernovae, stars, and AGN, and we manually vetted all candidates to further remove potential contaminants. We derived the X-ray selection function and fraction of unidentified AGN interlopers using a simulation with subsequent source detection. By comparing our sample to other ULX catalogs, we identified previously unknown candidates and those not detected although expected from their flux, indicating source intrinsic variability. We present two samples of ULX candidates. The main sample consists of 90 sources with highly confident X-ray detections, 53 of which are identified for the first time. This sample is complete to a distance of ∼7, contains at most 29% of unknown background AGN, and is mostly unaffected by source confusion. Several candidates are newly identified even though their locations have been observed by other instruments before, and some previously reported candidates we expected to identify are absent from our sample, demonstrating the transient behavior of ULXs. Based on our list of less confident detections, we also provide an extended catalog of 260 sources identified as potential candidates for further ULX identifications. Of these, 245 are identified as potential candidates for the first time.