Anna Wolter

Context. Ultraluminous X-ray sources (ULXs) are thought to be powered, in many cases, by super-Eddington accretion onto compact objects. While soft X-ray lags, commonly interpreted as reverberation in optically thick outflows, have been detected in several ULXs, hard lags remain rare and poorly understood. Aims. We investigate the temporal and energy dependence of X-ray lags in NGC 7456 ULX-1 to constrain their physical origin and probe the structure of the super-Eddington accretion flow. Methods. We analysed the two deepest XMM-Newton observations of NGC 7456 ULX-1, taken in 2018 and 2023. Hard (1–10 keV) light curves were cross correlated with soft (0.3–1 keV) light curves using adaptive-binning techniques optimized for Poissonian low-count data. Time-resolved lag measurements were performed over consecutive 10 ks intervals, and the energy dependence of the delays was investigated. Broadband spectra were modelled using multi-component thermal and Comptonization models. Results. We detect significant hard X-ray lags in both observations, with the 1–10 keV emission delayed by ∼10 ³ s relative to the soft band during phases of rapid flux variability. The energy-resolved analysis shows that the delays are primarily driven by the lowest-energy photons. Spectral modelling indicates a Comptonization-dominated super-Eddington flow comprising a cooler, extended outer region and a hotter, compact inner flow embedded in an optically thick wind. We interpret the observed delays as the combined effect of inward propagation of accretion-rate fluctuations and photon diffusion within the dense outflow. In this scenario, fluctuations first enhance the soft-emitting outer regions and subsequently propagate towards the hotter inner flow, where photons undergo stronger Comptonization before escaping with a kilosecond delay. The relatively small inferred inner emitting radius disfavours an intermediate-mass black hole accretor, while the observed delays imply diffusion through an extended optically thick wind. Conclusions. The sign, amplitude, and energy dependence of the observed delays disfavour a standard reverberation scenario. Instead, propagation-driven variability coupled with radiative transfer in optically thick winds appears to play a major role in shaping the timing properties of super-Eddington accretion flows.

Context. Analyzing environmental effects in galaxy groups is pivotal for understanding how galaxies evolve in these moderate-density settings. The degree to which ram pressure versus tidal interactions drive the unusual morphologies and kinematics seen in group galaxies remains a subject of active debate. Aims. This study focuses on the nearby galaxy NGC 2276, which has the longest radio continuum tail in galactic groups. It is a member of the NGC 2300 group that possibly experiences both processes, and we aim to determine which has the greatest impact on its overall structure. Methods. We combined broadband images with synthetic narrow band filters around emission line maps from integral field spectrograph to construct spatially resolved spectral energy distributions (SEDs), which we modeled using the BAGPIPES software package to reconstruct kiloparsec-scale star formation histories. These are compared with those derived from adaptive mesh refinement wind tunnel simulations of an NGC 2276-like system. Results. We show that the spatial distribution of the oldest stellar populations (>1.1 Gyr) is highly symmetric compared to the youngest ones. This is the most compelling evidence so far pointing toward ram pressure stripping (RPS) being the only morphological disturber of this system, as it primarily affects the gaseous component. Simulated data show similar results, with older stellar populations being more symmetric. Our RPS-only model showed no significant differences in the morphology of the stellar populations compared to the RPS+tidal model with an initial separation of 50 kpc.

The ASTRI-Horn telescope is a prototype Imaging Atmospheric Cherenkov Telescope (IACT) developed within the Astrofisica con Specchi a Tecnologia Replicante Italiana (ASTRI) project. It represents the first implementation of a dual-mirror Schwarzschild–Couder (SC) optical configuration for ground-based gamma-ray IACT observations. It also served as a testbed for the Small-Sized Telescopes (SSTs) of the Cherenkov Telescope Array Observatory (CTAO) that will host at least 37 SSTs at the Paranal site in Chile, and for the forthcoming ASTRI Mini-Array. Installed in 2014 at the INAF “M.C. Fracastoro” observing station on Mount Etna, the telescope is equipped with a SiPM-based Cherenkov camera and a complete control and data acquisition system. Between 2020 and 2022, after the demonstration of the validity of the SC configuration in observing gamma-ray sources, ASTRI-Horn underwent refurbishment activities that included mirror replacement, new optical coatings, camera upgrades, and software improvements. The telescope has since been re-validated through calibration campaigns and observational tests, including the detection and spectral characterization of the Crab Nebula at TeV energies. The results confirm the correct operation of the optical system, camera electronics, and data processing chain, providing a consolidated basis for the design and development of the ASTRI Mini-Array and future CTAO-SST telescopes.

The detection of gravitational waves (GWs) from a binary neutron star (BNS) merger by Advanced LIGO and Advanced Virgo (GW170817), together with its electromagnetic counterpart, the short gamma-ray burst GRB 170817A, heralded the birth of multimessenger astronomy. The detection of TeV emission from GRBs motivates follow-up observations with the Cherenkov Telescope Array Observatory (CTAO), which is ideal for detecting such signals due to its unprecedented sensitivity, rapid response, and wide-field survey capabilities. The aim of this work is to evaluate GeV–TeV GW follow-up strategies for CTAO using a multistep simulation pipeline and to estimate the expected rate of joint GW–GRB detections during observing run O5. Using a simulated sample of BNS systems with corresponding GW detections, gamma-ray emission is simulated through phenomenological prescriptions based on the observed population of short GRBs, including off-axis jet scenarios. CTAO observations are simulated to account for instrument response, sky tiling strategies, integration times, and varying observing conditions. Strategies with variable and constant integration times are investigated. We find that, via an optimized follow-up strategy, about 5% of simulated GW-associated short GRBs produce GeV–TeV radiation detectable by CTAO. Detectability is strongly influenced by the jet opening angle and viewing angle, suggesting that even rough estimates of the viewing angle in GW alerts could enhance targeting. This framework motivates future follow-ups of GW-detectable events, including neutron star–black hole mergers, and further supports the development of advanced strategies incorporating galaxy distributions and synergies with future detectors such as the Einstein Telescope.

Context. Ultraluminous X-ray sources (ULXs) are non-nuclear X-ray binary systems that exceed the Eddington luminosity for a 10 M _⊙ black hole. The majority of these sources are thought to be stellar-mass compact objects accreting at super-Eddington rates, exhibiting powerful relativistic winds. These winds have been identified through the detection of absorption lines with a blueshift as high as 0.3 c and emission lines typically found at their laboratory wavelengths. Aims. In this work, we analysed the XMM-Newton data of the ULX NGC 5204 X-1, which has been observed to exhibit emission lines with a blueshift of about 0.3 c . The aim of this study is to examine the geometry and physical properties of the accretion disc and the relativistic outflows. In addition, we aim to explore the factors that influence the ULX spectral transitions. Methods. We undertook an observing campaign with XMM-Newton to explore the source behaviour at different luminosities. In this first paper of the series, we performed high-resolution X-ray spectroscopy, including archival data, with the Reflection Grating Spectrometer (RGS) instrument, which allowed us to resolve both emission and absorption lines. The outflows features were characterised using physical models of plasma in collisional ionisation and photoionisation equilibrium. Results. We identify collisionally ionised blueshifted and redshifted components at about 0.3 c . These findings have a high statistical significance and suggest a biconical structure for the outflow. Additionally, the analysis of the O VII line triplet observed in the spectrum enables us to infer physical properties of the low-velocity line-emitting plasma; for example, electron density ( n _e ∼ 10 ¹⁰ cm ⁻³ ) and temperature ( T _e ≥ 1.5 × 10 ⁵ K). A hybrid plasma whose ionisation balance is affected by both collisions and radiation is favoured.

Context. Several studies have reported a nearly linear correlation between the molecular gas and star formation rate surface density, the so-called Kennicutt-Schmidt (KS) law. Aims. We aim to retrieve the KS relation for a sample of four star-forming galaxies located in nearby clusters, disturbed by the effects of the ram pressure stripping, as testing this law in galaxies subject to different environmental conditions can provide key information on the physics of star formation. Methods. To perform our analysis, we used ALMA band 6 and band 3 data coupled with MUSE data at spatial resolution of ∼1 kpc. Moreover, we analyzed data of star-forming complexes detected through their H α ionized gas emission. We also derived the star formation efficiencies of the star-forming regions nested in these big complexes using the star formation rates derived from spatially resolved HST images and various recipes for the corresponding cold gas phase. Results. We find that ram-pressure-stripped galaxies show normal-to-low star formation efficiencies, depending on the position within the galaxy and on the local gas density: the inner dense regions in the disk show higher efficiencies with respect to the outer regions, including the gaseous tails. The global relation between the star formation rate density and the molecular gas surface density is superlinear, likely suggesting the shortening of the depletion times at high gas mass densities caused by the ram pressure. Within the star-forming complexes, the star formation efficiency is very similar to the one observed at 1 kpc scale in undisturbed star-forming disks. Interestingly, this result holds also for the star-forming complexes located in the stripped gas tails. The analysis of HST resolved clumps suggests that the molecular gas is not uniformly distributed within the star-forming complexes, but its density distribution follows a steeper profile.

We present the full sample of 76 galaxies in 39 galaxy cluster fields at z = 0.04 to 0.07 observed with VLT/MUSE by the GAs Stripping Phenomena in galaxies (GASP) survey. Most of them (64) were observed as possible ram pressure stripped galaxies (stripping candidates) based on optical B-band images, while the remaining 12 were a control sample of both star-forming and passive galaxies. Based on spatially resolved ionized gas and stellar kinematics, we assessed the physical origin of the gas asymmetries and find that 89% of the stripping candidates are confirmed by the VLT/MUSE data. In addition, three of the four star-forming galaxies in the control sample also show signs of ram pressure. These control galaxies display a ring of unusual emission line ratios, which we observe in field galaxies as well, possibly originating from the interaction with a hotter surrounding medium. The stripped galaxies are classified into various classes corresponding to different degrees of stripping, from the weakest to the strongest and most extreme (jellyfish galaxies) stripping, as well as truncated gas disks with gas remaining only in the galaxy center. Our results show that selecting cluster stripping candidates based on optical imaging yields a sample that is indeed largely dominated by galaxies affected by ram pressure at different stages and stripping strength, though some contamination is present mostly due to tidal processes. Strong ram pressure cases are found in galaxies over the whole range of stellar masses studied (10⁹−10^11.5 M_⊙), both in low-mass and high-mass clusters (cluster velocity dispersions σ = 500−1100 km s⁻¹). We examine the possible connection between the progressive stages of stripping, up to the phase of a truncated gas disk, and the subsequent complete stripping of gas. We discuss the incompleteness intrinsic to this and other methods of selection to obtain a complete census of ram pressure stripping in clusters.

Context. The mass function (MF) of young stellar clumps (age ≲ 200 Myr) is an indicator of the mechanism driving the collapse of the interstellar medium (ISM) into the giant molecular clouds. Typically, the MF of clumps in main-sequence galaxies is described by a power law (dN/dM_* ∝ M_*^−α) with slope α = 2, hinting that the collapse is driven by turbulence. Aims. To understand whether the local environment affects star formation, we modelled the clump MF of six cluster galaxies, from the GASP survey, undergoing strong ram-pressure stripping (RPS). This phenomenon, exerted by the hot and high-pressure intracluster medium (ICM), has produced long tails of stripped ISM, where clumps form far away from the galactic disk and are surrounded by the ICM itself. Methods. Clumps were selected from HST-UVIS/WFC3 images, covering from near-UV to red-optical bands and including Hα emission-line maps. The catalogue comprises 398 Hα clumps (188 in tails, 210 in the disk outskirts known as the extraplanar region) and 1270 UV clumps (593 in tails, 677 extraplanar). Using mock images, we quantified the mass completeness and bias of our sample. Taking these two effects into account, we adopted a Bayesian approach to fit the clump mass catalogue to a suitable MF. Results. The resulting MFs are steeper than the expected value of 2. In the tails the Hα clumps have slope α = 2.31 ± 0.12, while the UV slope is larger (2.60 ± 0.09), in agreement with ageing effects. Similar results are found in the extraplanar region, with Hα slope α = 2.45_−0.16^+0.20 and UV slope α = 2.63_−0.18^+0.20, even though in this case they are consistent within the uncertainties. Conclusions. We suggest that the steepening results from the higher-than-usual turbulent environment, arising from the interaction between the ISM and the ICM. As shown by recent works, this process can favour the fragmentation of the largest ISM clouds, inhibiting the formation of very massive clumps.

Context. Ultraluminous X-ray sources (ULX) are extragalactic objects with observed X-ray luminosities largely above the Eddington limit for a 10 M_⊙ black hole. Currently, it is believed that ULXs host super-Eddington accreting neutron stars or stellar mass black holes. However, the exact proportion of the two populations of compact objects is not yet known. Aims. We investigate the properties of the ULX NGC 4559 X7 (hereafter X7), which shows flux variability up to a factor of five on both long (months to years) and short (hours to days) timescales. A flaring activity was also observed during the highest flux epochs of the source. Flares are unpredictable. They have different durations (but similar rising and decay times) and are all flat topped in flux. The latter suggests that at the flare peaks, there is likely a common switch-off mechanism for the accretion onto the compact object. Methods. We analysed all available XMM-Newton and Swift/XRT observations in order to fully investigate the spectral and temporal evolution of X7, looking for short- and long-term variability. We applied a Lomb-Scargle search to look for long-term periodicities. We also looked for coherent signals through accelerated searches that included orbital corrections. We described the X7 spectral properties with two thermal components plus a cut-off power-law model. Results. We found three well-defined spectral states where the spectral variability is mainly driven by the two harder components, with the thermal one clearly following a correlation between its temperature and luminosity. In addition, a pulsed signal at 2.6 s–2.7 s was detected in two XMM-Newton observations. The significance of these coherent signals is relatively weak, but they are found in two different observations with the same parameter space for the orbital properties. If confirmed, the pulsation would imply a high spin-down of 10⁻⁹ s s⁻¹, which could be extreme amongst the known pulsating ULXs, and X7 would become a new extragalactic ULX pulsar. Conclusions. We discuss the spectral and temporal results of X7 in the context of super-Eddington accretion onto a stellar-mass compact object. In particular, we suggest that the source might likely host a neutron star.