Valenti Bosch-Ramon

Context. Thermonuclear and core-collapse supernovae are statistically expected to happen within extragalactic jets, as these jets cross the inner regions of their host galaxies, and supernova ejecta–jet interactions can potentially lead to strong non-thermal activity. Aims. We model the non-thermal emission from a supernova interacting with a jet on kiloparsec scales, using detailed numerical simulations of the interaction. Methods. Using a post-processing method, we applied a non-thermal model of the acceleration and transport of electron and their synchrotron and inverse Compton emissions, simplified and tuned for flexibility and computational speed, to the results of a three-dimensional relativistic hydrodynamics simulation of the ejecta of a supernova interacting with an extragalactic jet. Results. For typical magnetic field strengths, a jet power 10 ⁴⁴ erg s ⁻¹ , and an electron energy distribution characteristic of diffusive shock acceleration, we predict: synchrotron emission that peaks in X-rays but reaches up to 100 MeV, with an overall luminosity of a few times 10 ⁴¹ erg s ⁻¹ ; and emission from inverse Compton scattering mostly off galactic infrared photons, peaking at ∼1 TeV and with a luminosity up to ∼10 ⁴⁰ erg s ⁻¹ . We also computed the evolution of the emission, and the distribution of the flux in the sky at its maximum, for five different energy bands: radio, optical, X-rays, high-energy and very high-energy γ -rays. The emission is roughly steady on a few 1000 yr timescales, with an extension on arcsec scales for a source in the local universe. Conclusions. For intermediate jet powers, radio, optical and X-ray fluxes from supernova ejecta-jet interactions may be detectable for distances ≲100 Mpc. The non-thermal luminosity budget and local conditions associated with these interactions could be enough to explain knots seen in kiloparsec scale jets at different energies. These conditions might also be suitable for the production of heavy ultra high-energy cosmic rays, but photon and neutrino generation from hadronic processes would be minor on kiloparsec scales. The leptonic γ -ray fluxes are modest but might non-negligibly contribute in TeV for the nearest or more powerful jet sources.

Context. Low-power extragalactic jets are known to be decelerated and dissipate large amounts of energy within their host galaxies. However, the exact process by which this occurs is still elusive. Aims. The aim of this work is to probe the role of stars as triggers of jet mass-loading, deceleration and dissipation in Fanaroff–Riley type I radio galaxies. This is motivated by a theoretical model that proposes that stars interacting with the jet boundaries could facilitate entrainment of interstellar-medium (ISM) gas into jets, favouring mixing and dissipation. Methods. We performed a numerical experiment of stars entering a relativistic flow using a relativistic hydrodynamics code. Our setup is limited to the interaction of three stars with the jet boundary in order to assess the results in a limited, controlled, environment, although this number of stars may be plausible in the inner kiloparsec region of a massive galaxy. Results. Our results allow us to estimate the amount of entrained ISM gas as the stars enter the jet. We show that the entrainment temporally induced on scales of tens of parsecs and thousands of years by evolved stars is comparable to the initial jet mass rate. The way in which this entrainment happens is by the creation of a low-pressure region behind the stellar objects, which drags ambient gas into the jet flow. Conclusions. Our results confirm that stars interacting with the jet boundaries, and acting as catalysts of gas entrainment from both the ISM and the shear layer, can significantly contribute to jet mass load and deceleration.

Microquasar remnants (MQRs), the long-lived cocoons inflated by extinct microquasar jets, have recently been proposed as hidden Galactic PeVatrons capable of producing ultra-high-energy gamma rays without an active central engine. While hadronic interactions can account for bright gamma-ray emission from nearby clouds, the direct detection of MQRs remains challenging because their intrinsic emission is expected to be extended and of low surface brightness. In this work, we explore the broadband emission of MQRs by focusing on the leptonic component confined within the cocoon and on particle interactions in the shocked shell surrounding it. We modeled the injection and time-dependent transport of relativistic particles, including stochastic reacceleration driven by internal turbulence, which we treated as a second-order Fermi process. We considered both sub-Eddington and super-Eddington microquasar systems and computed the resulting nonthermal emission from radio to gamma-ray energies, together with the thermal soft X-ray emission produced in the shocked shell. In the fiducial super-Eddington case, the intrinsic emission reaches peak values of ν L _ν ∼ 10 ³⁵ − 10 ³⁶ erg s ⁻¹ , whereas sub-Eddington remnants are typically several orders of magnitude fainter. At 1.3 GHz, the modeled cocoon surface brightness is of order Σ _ν ∼ 10 ⁻¹⁹ W m ⁻² Hz ⁻¹ sr ⁻¹ for young powerful remnants and decreases rapidly as the remnant evolves. We find that the direct detectability of MQRs is therefore controlled mainly by surface brightness rather than by integrated luminosity. Powerful remnants may be detectable as extended synchrotron radio cocoons and shell-dominated soft X-ray structures under favorable conditions, whereas sub-Eddington remnants are expected to be much harder to identify directly. Our results suggest that MQRs constitute a hidden population of extended Galactic nonthermal sources, whose identification requires multiwavelength searches that combine morphology, surface brightness, and environmental contexts.

Context. LHAASO J2108+5157 is one of the most intriguing Galactic ultra-high-energy γ -ray sources. It is detected up to hundreds of TeV but lacks a clear counterpart at lower frequencies. Fermi -LAT observations have revealed a compact GeV source spatially coincident with the PeV emission, while VERITAS and LST-1 impose stringent upper limits in the intermediate TeV band, producing a pronounced spectral suppression between the GeV and TeV domains. Aims. We investigate a time-dependent hadronic scenario in which a past episode of microquasar jet activity injected relativistic protons that are now observed in a relic phase. Methods. Energy-dependent diffusion naturally segregates the particle population: Lower-energy protons remain confined near dense molecular gas and account for the GeV emission, whereas higher-energy particles reach more distant material, producing the hard TeV–PeV component. Results. For representative interstellar conditions, the model is simultaneously consistent with the broadband spectral energy distribution, the spatial compact–extended dichotomy, and the lack of detectable radio or X-ray counterparts. Conclusions. The observed bimodal γ -ray phenomenology thus emerges as a transport effect of a single injected proton population, supporting the interpretation of LHAASO J2108+5157 as a fossil microquasar remnant observed at an intermediate evolutionary stage and highlighting relic jet systems as a viable class of Galactic PeVatrons.

Context. Gamma-ray binaries hosting a non-accreting neutron star and a massive star exhibit multi-wavelength emission on different spatial scales. The interaction between the neutron star and stellar winds mixes them and produces an outflow whose interaction with the medium on large scales can inflate a bubble or form a bow shock in systems that are old enough. The well-studied but still mysterious high-energy emitting binary LS 5039 shows extended (1 pc scale) X-rays that might arise from one of these two interaction regimes. Aims. We explain and predict the large-scale emission from LS 5039. Methods. We modelled the LS 5039 large-scale emission, including the thermal and non-thermal contributions. For the latter, we assumed particle acceleration at the termination shock of the mixed-wind outflow. We considered five scenarios: a very young and an intermediate-age bubble interacting with the interstellar medium, and a bubble evolving inside a supernova remnant, studied using a one-zone model; and two older bow shocks with a moderate and a high neutron star power, respectively, for which we used a multi-zone approach. We also investigated the particles escaping from these structures and their radiation. Results. The large-scale X-rays surrounding LS 5039 are best explained as synchrotron radiation, as previously proposed. In some cases, very extended radio emission might be detectable, while particles escaping the region may be a minor but non-negligible steady contribution to the source gamma rays in the powerful bow-shock scenario. Additionally, escaping protons and nuclei with 0.1–1 PeV might inject up to ∼10 ³⁶ erg s ⁻¹ into the Galactic cosmic rays for optimistic injection luminosities. Conclusions. Gamma-ray binaries can be efficient accelerators at large scales and produce non-negligible amounts of broadband emission and 0.1–1 PeV cosmic rays. Our study can help future multi-wavelength observational campaigns to better characterise the large-scale medium interaction, age, and birthplace of LS 5039.

We present IRAM 30 m observations aimed at identifying potential outflow–interstellar medium interaction sites in the vicinity of the black hole X-ray binary Cygnus X-1, which displays persistent relativistic jets and a prominent stellar wind. Using this data set, we construct molecular line emission maps, identifying a never-before-seen molecular structure potentially linked to X-ray-binary-driven feedback. This structure, surrounding the source, exhibits properties consistent with an interaction powered primarily by the stellar wind of the massive stellar companion and further sculpted by the relativistic jets. Our results indicate that multiple outflow mechanisms (stellar winds and relativistic jets) may simultaneously be shaping the interstellar medium around Cygnus X-1 and that molecular line imaging can help to disentangle complex feedback processes in environments where multiple outflows take place.

The Large High Altitude Air Shower Observatory (LHAASO) has revealed numerous ultrahigh-energy gamma-ray sources of unknown origin. We propose that a fraction of them can be explained by microquasar remnants, i.e., binary systems where mass transfer has ceased and the central engine is quenched. Cosmic rays injected during the active phase of a microquasar may remain confined within its cocoon and subsequently interact with nearby molecular clouds, producing bright gamma-ray emission through pp collisions. Remnants of former super-Eddington systems can act as dark PeVatrons, releasing particles up to ∼10 PeV that illuminate surrounding clouds producing gamma rays reaching hundreds of tera-electron volts. This scenario provides a natural explanation for several unidentified Galactic LHAASO sources.

Context. Jets in active galactic nuclei (AGN) have to cross significant distances within their host galaxies, meeting large numbers of stars of different masses and evolution stages on their paths. Given enough time, supernova explosions within the jet will eventually happen, and they may have a strong impact on its dynamics, potentially triggering powerful non-thermal activity. Aims. We aim to carry out a detailed numerical study to explore the dynamics of the interaction between the ejecta of a supernova explosion and a relativistic extragalactic jet. Methods. By means of relativistic hydrodynamics simulations using the code RATPENAT , we simulated the jet-ejecta interaction in two different geometries or scenarios: a two-dimensional, axisymmetric simulation, and a three-dimensional one, which includes the orbital velocity of the exploding star. In both scenarios, the supernova ejecta is located within the jet at a distance of ∼1 kpc from the central black hole, which is the spatial scale on which these events are most likely. Results. Although initially filling a region much smaller than the jet radius, the ejecta expands and eventually covers most of the jet’s cross-section. The expansion is enhanced as more energy from the jet is converted into kinetic and internal energy of the ejecta, which also favours the ejecta disruption, all of this occurs on timescales of ∼10 ⁴ yr. Although a complete numerical convergence of the results is unattainable given the subsonic, turbulent nature of the interaction region, the simulations are consistent in their description of the gross morphological and dynamical properties of the interaction process. Conclusions. At the end of the simulations, the supernova ejecta already partially mixed with the relativistic jet. The results also suggest that the jet-ejecta interaction may be a non-negligible non-thermal emitter. Moreover, due to efficient mixing, the interaction region can be a potential source of ultra-high-energy cosmic rays of heavy composition.