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Context. Pulsars have a very stable rotation. However, sudden increases in their rotation frequency known as glitches, perturb their evolution. While large glitches are commonly detected, small glitches are harder to detect because of the lack of daily-cadence observations over long periods of time. Aims. We aim to explore the timing behaviour of young pulsars at daily timescales looking for small glitches and other irregularities. This will further our comprehension of the distribution of glitch sizes, which has also consequences for the theoretical modeling of the glitch mechanism. Methods. We observed six pulsars with up to daily cadence during 5 years with the antennas of the Argentine Institute of Radio Astronomy (IAR). We used standard pulsar timing tools to characterise the rotation of pulsars and developed an algorithm to look for small timing events in the data and calculate the changes in $\nu$ and $\dot\nu$ at those epochs. Results. We found that the rotation of pulsars in this dataset is affected by small step changes in $\nu$ and $\dot\nu$. We found three glitches that had not been reported before: two in PSR J1048-5832 with relative sizes $\Delta\nu / \nu= 9.1(4) \times 10^{-10}$ and $\Delta\nu / \nu = 4.5(1) \times 10^{-10}$, and one in the Vela pulsar with a size $\Delta\nu / \nu = 2.0(2) \times 10^{-10}$. We also report new decay terms on the 2021 Vela giant glitch, and on the 2022 giant glitches in PSR J0742-2822 and PSR J1740-3015 respectively. Besides, we found that the red noise contribution significantly diminished in PSR J0742-2822 after its giant glitch in 2022. Conclusions. Our results highlight the importance of high-cadence monitoring with an exhaustive analysis of the residuals to better characterize the distribution of glitch sizes and to deepen our understanding of the mechanisms behind glitches, red noise and timing irregularities.

The aim of this study is to probe the sub-mJy polarized source population with LOFAR. We present the method used to stack LOFAR polarization datasets, the resulting catalog of polarized sources, and the derived polarized source counts. The ELAIS-N1 field was selected for a polarimetric study at 114.9-177.4 MHz. A total area of 25 deg2 was imaged at 6"- resolution in the Stokes Q and U parameters. Alignment of polarization angles was done both in frequency and in Faraday space before stacking datasets from 19 eight-hour-long epochs. A search for polarized sources was carried out in the final, stacked dataset, and the properties of the detected sources were examined. The depolarization level of sources known to be polarized at 1.4 GHz was quantified. A one-sigma noise level of 19 {\mu}Jy/beam was reached in the central part of the field after stacking. Twenty-five polarized sources were detected above 8\sigma, five of which had not been detected in polarization at any other radio frequencies before. Seven additional polarized components were found by lowering the threshold to 6\sigma at positions corresponding to sources known to be polarized at 1.4 GHz. In two radio galaxies, polarization was detected from both radio lobes, so the final number of associated radio continuum sources is 31. The detected sources are weakly polarized, with a median degree of polarization of 1.75% for the sample of sources detected in polarized emission. The sources previously detected in polarization at 1.4 GHz are significantly depolarized at 150 MHz. The catalog is used to derive the polarized source counts at 150 MHz. This is the deepest and highest-resolution polarization study at 150 MHz to date., Comment: 18 pages, 10 figures, accepted for publication in A&A

The origin of a compact millimeter (mm, 100-250 GHz) emission in radio-quiet active galactic nuclei (RQ AGN) remains debated. Recent studies propose a connection with self-absorbed synchrotron emission from the accretion disk X-ray corona. We present the first joint ALMA ($\sim$100 GHz) and X-ray (NICER/XMM-Newton/Swift; 2-10 keV) observations of the unobscured RQ AGN, IC 4329A ($z = 0.016$). The time-averaged mm-to-X-ray flux ratio aligns with recently established trends for larger samples (Kawamuro et al. 2022, Ricci et al. 2023), but with a tighter scatter ($\sim$0.1 dex) compared to previous studies. However, there is no significant correlation on timescales of less than 20 days. The compact mm emission exhibits a spectral index of $-0.23 \pm 0.18$, remains unresolved with a 13 pc upper limit, and shows no jet signatures. Notably, the mm flux density varies significantly (factor of 3) within 4 days, exceeding the contemporaneous X-ray variability (37% vs. 18%) and showing the largest mm variations ever detected in RQ AGN over daily timescales. The high amplitude variability rules out scenarios of heated dust and thermal free-free emission, pointing toward a synchrotron origin for the mm radiation in a source of $\sim$1 light day size. While the exact source is not yet certain, an X-ray corona scenario emerges as the most plausible compared to a scaled-down jet or outflow-driven shocks.}, Comment: Submitted to A&A

The Pulsar Monitoring in Argentina (PuMA) is a collaboration between the Argentine Institute for Radioastronomy (IAR) and the Rochester Institute of Technology (RIT) that since 2017 has been observing southern sky pulsars with high cadence using the two restored IAR antennas in the L-Band (1400MHz). We briefly review the first set of results of this program to study transient phenomena, such as magnetars and glitching pulsars, as well as to perform precise timing of millisecond pulsars. Access to lower frequency bands, where most of the pulsars are brighter, would allow us to reach additional pulsars, currently buried into the background noise. We identify two dozen additional glitching pulsars that could be observable in the 400MHz band by the IAR's projected Multipurpose Interferometer Array (MIA). We also discuss the relevance and challenges of single-pulse pulsar timing at low frequencies and the search for Fast Radio Burst (FRB) in the collected data since 2017 using machine learning techniques., Comment: 11 pages, 10 figures, 5 tables. arXiv admin note: text overlap with arXiv:2108.13462

Accretion disks around compact objects are expected to enter an unstable phase at high luminosity. One instability may occur when the radiation pressure generated by accretion modifies the disk viscosity, resulting in the cyclic depletion and refilling of the inner disk on short timescales. Such a scenario, however, has only been quantitatively verified for a single stellar-mass black hole. Although there are hints of these cycles in a few isolated cases, their apparent absence in the variable emission of most bright accreting neutron stars and black holes has been a lingering puzzle. Here we report the presence of the same multiwavelength instability around an accreting neutron star. Moreover, we show that the variability across the electromagnetic spectrum-from radio to X-ray-of both black holes and neutron stars at high accretion rates can be explained consistently if the accretion disks are unstable, producing relativistic ejections during transitions that deplete or refill the inner disk. Such new association allows us to identify the main physical components responsible for the fast multiwavelength variability of highly accreting compact objects., Comment: Published in Nature. 26 pages, 10 figures. DOI: 10.1038/s41586-022-05648-3

Context: Massive colliding-wind binaries (CWBs) can be non-thermal sources. The emission produced in their wind-collision region (WCR) encodes information of both the shocks properties and the relativistic electrons accelerated in them. The recently discovered system Apep, a unique massive system hosting two Wolf-Rayet stars, is the most powerful synchrotron radio emitter among the known CWBs, being an exciting candidate to investigate the non-thermal processes associated with stellar wind shocks. Aims: We intend to break the degeneracy between the relativistic particle population and the magnetic field strength in the WCR of Apep by probing its hard X-ray spectrum, where inverse-Compton (IC) emission is expected to dominate. Methods: We observe Apep with NuSTAR for 60 ks and combine this with a re-analysis of a deep archival XMM-Newton observation to better constrain the X-ray spectrum. We use a non-thermal emission model to derive physical parameters from the results. Results: We detect hard X-ray emission consistent with a power-law component. This is compatible with IC emission produced in the WCR for a magnetic field of 100-160 mG and a fraction of ~1.5e-4 of the total wind kinetic power being converted into relativistic electron acceleration. Conclusions: This is the first time that the non-thermal emission from a CWB is detected both in radio and high energies. This allows us to derive the most robust constraints of the particle acceleration efficiency and magnetic field intensity in a CWB so far, reducing the typical uncertainty of a few orders of magnitude to just within a factor of two. This constitutes an important step forward in our characterisation of the physical properties of CWBs., Comment: 12 pages, 6 figures, accepted for publication in A&A

Context. There is a population of runaway stars that move at extremely high speeds with respect to their surroundings. The fast motion and the stellar wind of these stars, plus the wind-medium interaction, can lead to particle acceleration and non-thermal radiation. Aims. We characterise the interaction between the winds of fast runaway stars and their environment, in particular to establish their potential as cosmic-ray accelerators and non-thermal emitters. Methods. We model the hydrodynamics of the interaction between the stellar wind and the surrounding material. We self-consistently calculate the injection and transport of relativistic particles in the bow shock using a multi-zone code, and compute their broadband emission from radio to $\gamma$-rays. Results. Both the forward and reverse shocks are favourable sites for particle acceleration, although the radiative efficiency of particles is low and therefore the expected fluxes are in general rather faint. Conclusions. We show that high-sensitivity observations in the radio band can be used to detect the non-thermal radiation associated with bow shocks from hypervelocity and semi-relativistic stars. Hypervelocity stars are expected to be modest sources of sub-TeV cosmic rays, accounting perhaps for a $\sim 0.1$% of that of galactic cosmic rays., Comment: 11 pages, 8 figures, accepted for publication section 2. Astrophysical processes of Astronomy and Astrophysics

All disc-accreting astrophysical objects produce powerful outflows. In binaries containing neutron stars (NS) or black holes, accretion often takes place during violent outbursts. The main disc wind signatures during these eruptions are blue-shifted X-ray absorption lines, which are preferentially seen in disc-dominated "soft states". By contrast,optical wind-formed lines have recently been detected in "hard states", when a hot corona dominates the luminosity. The relationship between these signatures is unknown, and no erupting system has revealed wind-formed lines between the X-ray and optical bands yet, despite the many strong resonance transitions in this ultraviolet (UV) region. Here, we show that the transient NS binary Swift J1858.6-0814 exhibits wind-formed, blue-shifted absorption associated with C IV, N V and He II in time-resolved UV spectroscopy during a luminous hard state. This represents the first evidence for a warm, moderately ionized outflow component in this state. Simultaneously observed optical lines also display transient blue-shifted absorption. Decomposing the UV data into constant and variable components, the blue-shifted absorption is associated with the former. This implies that the outflow is not connect to the luminous flares in the data. The joint presence of UV and optical wind features reveals a multi-phase and/or stratified outflow from the outer disc. This type of persistent mass loss across all accretion states has been predicted by radiation-hydrodynamic simulations and helps to explain the shorter-than-expected outbursts duration., Comment: Published in Nature. Submitted: 9 July 2021

The recently discovered massive binary system Apep is the most powerful synchrotron emitter among the known Galactic colliding-wind binaries. This makes this particular system of great interest to investigate stellar winds and the non-thermal processes associated with their shocks. This source was detected at various radio bands, and in addition the wind-collision region was resolved by means of very-long baseline interferometric observations. We use a non-thermal emission model for colliding-wind binaries to derive physical properties of this system. The observed morphology in the resolved maps allows us to estimate the system projection angle on the sky to be $\psi \approx 85^\circ$. The observed radio flux densities also allow us to characterise both the intrinsic synchrotron spectrum of the source and its modifications due to free--free absorption in the stellar winds at low frequencies; from this we derive mass-loss rates of the stars of $\dot{M}_\mathrm{WN} \approx 4\times10^{-5}$ $\mathrm{M}_\odot$ yr$^{-1}$ and $\dot{M}_\mathrm{WC} \approx 2.9\times10^{-5}$ $\mathrm{M}_\odot$ yr$^{-1}$. Finally, the broadband spectral energy distribution is calculated for different combinations of the remaining free parameters, namely the intensity of the magnetic field and the injected power in non-thermal particles. We show that the degeneracy of these two parameters can be solved with observations in the high-energy domain, most likely in the hard X-rays but also possibly in $\gamma$-rays under favourable conditions., Comment: 10 pages, 5 figures, 2 tables. This version has been accepted for publication in PASA after improvements and updates in the discussion of the results

Star-forming galaxies (SFGs) emit non-thermal radiation from radio to gamma-rays. We aim to investigate the main mechanisms of global CR transport and cooling in SFGs. The way they contribute in shaping the relations between non-thermal luminosities and SFR could shed light onto their nature. We develop a model to compute the CR populations of SFGs, taking into account their production, transport, and cooling. The model is parameterised only through global galaxy properties, and describes the non-thermal emission in both radio and gamma-rays. We focus on the role of diffusive and advective transport by galactic winds, either driven by turbulent or thermal instabilities. We compare model predictions to observations, for which we compile a homogeneous set of luminosities in these radio bands, and update those available in gamma-rays. Our model reproduces reasonably well the observed relations between the gamma-ray or 1.4 GHz radio luminosities and the SFR, assuming a single power-law scaling of the magnetic field with the latter with index beta=0.3, and winds blowing either at Alfvenic speeds or typical starburst wind velocities. Escape of CR is negligible for > 30 Mo/yr. A constant ionisation fraction of the interstellar medium fails to reproduce the 150 MHz radio luminosity throughout the whole SFR range. Our results reinforce the idea that galaxies with high SFR are CR calorimeters, and that the main mechanism driving proton escape is diffusion, whereas electron escape also proceeds via wind advection. They also suggest that these winds should be CR or thermally-driven at low and intermediate SFR, respectively. Our results globally support that magnetohydrodynamic turbulence is responsible for the dependence of the magnetic field strength on the SFR and that the ionisation fraction is strongly disfavoured to be constant throughout the whole SFR range., Comment: Accepted for publication in Astronomy & Astrophysics (on 18/09/2021)

The Argentine Institute of Radio astronomy (IAR) is equipped with two single-dish 30-m radio antennas capable of performing daily observations of pulsars and radio transients in the southern hemisphere at 1.4 GHz. We aim to contribute to pulsar timing studies related to short time-scale interstellar scintillation and searches for sources of continuous gravitational waves. We performed high-cadence (almost daily) and long-duration observations of the bright millisecond pulsar J0437$-$4715 for over a year, gathering more than 700 hours of good-quality data with timing precision better than 1~$\mu$s. We characterize the white and red timing noise in IAR's observations of J0437$-$4715. We quantify the effects of scintillation in this data set and perform single pulsar searches of continuous gravitational waves, setting constraints in the nHz--$\mu$Hz frequency range. We demonstrate IAR's potential for performing pulsar monitoring in the 1.4 GHz radio band for long periods of time with a daily cadence. In particular, we conclude that the ongoing observational campaign of the millisecond pulsar J0437$-$4715 can contribute to increase the sensitivity of the existing pulsar timing arrays., Comment: 16 pages, 13 figures, 5 tables. version to appear in The Astrophysical Journal

We present an analysis of multi-wavelength observations of an area of 0.27 deg x 0.27 deg around the Galactic HII region G18.88-0.49, which is powered by an O-type star (age ~10^5 years). The Herschel column density map reveals a shell-like feature of extension ~12 pc x 7 pc and mass ~2.9 x 10^4 Msun around the HII region; its existence is further confirmed by the distribution of molecular (12CO, 13CO, C18O, and NH3) gas at [60, 70] km/s. Four subregions are studied toward this shell-like feature, and show a mass range of ~0.8-10.5 x 10^3 Msun. These subregions associated with dense gas are dominated by non-thermal pressure and supersonic non-thermal motions. The shell-like feature is associated with the HII region, Class I protostars, and a massive protostar candidate, illustrating the ongoing early phases of star formation (including massive stars). The massive protostar is found toward the position of the 6.7 GHz methanol maser, and is associated with outflow activity. Five parsec-scale filaments are identified in the column density and molecular maps, and appear to be radially directed to the dense parts of the shell-like feature. This configuration is referred to as a "hub-filament" system. Significant velocity gradients (0.8-1.8 km/s/pc) are observed along each filament, suggesting that the molecular gas flows towards the central hub along the filaments. Overall, our observational findings favor a global non-isotropic collapse scenario as discussed in Motte et al. (2018), which can explain the observed morphology and star formation in and around G18.88-0.49., Comment: 20 pages, 10 figures, 3 Tables, Accepted for publication in The Astrophysical Journal

V341 Ara was recently recognised as one of the closest (d ~ 150 pc) and brightest (V~ 10) nova-like cataclysmic variables. This unique system is surrounded by a bright emission nebula, likely to be the remnant of a recent nova eruption. Embedded within this nebula is a prominent bow-shock, where the system's accretion disc wind runs into its own nova shell. In order to establish its fundamental properties, we present the first comprehensive multi-wavelength study of the system. Long-term photometry reveals quasi-periodic, super-orbital variations with a characteristic time-scale of 10-16 days and typical amplitude of ~1 mag. High-cadence photometry from TESS reveals for the first time both the orbital period and a "negative superhump" period. The latter is usually interpreted as the signature of a tilted accretion disc. We propose a recently developed disc instability model as a plausible explanation for the photometric behaviour. In our spectroscopic data, we clearly detect anti-phased absorption and emission line components. Their radial velocities suggest a high mass ratio, which in turn implies an unusually low white dwarf mass. We also constrain the wind mass-loss rate of the system from the spatially resolved [O iii] emission produced in the bow-shock; this can be used to test and calibrate accretion disc wind models. We suggest a possible association between V341 Ara and a "guest star" mentioned in Chinese historical records in AD1240. If this marks the date of the system's nova eruption, V341 Ara would be the oldest recovered nova of its class and an excellent laboratory for testing nova theory., Comment: Accepted in MNRAS

Context. Near a dozen star-forming galaxies have been detected in gamma-rays by the Fermi observatory in the last decade. A remarkable property of this sample is the quasi-linear relation between the gamma-ray luminosity and the star formation rate, obtained assuming that the latter is well traced by the infra-red luminosity of the galaxies. The non-linearity of this relation has not been fully explained yet. Aims. We aim at determining the biases derived from the use of the infrared luminosity as a proxy for the star formation rate, and shed light onto the more fundamental relation between the latter and the gamma-ray luminosity. We expect to quantify and explain some trends observed in this relation. Methods. We compile from the literature a near-homogeneous set of distances, ultraviolet, optical, infra-red, and gamma-ray fluxes for all known gamma-ray emitting star-forming galaxies. From these data we compute the infra-red and gamma-ray luminosities, and star formation rates. We determine the best-fitting relation between the latter two, and describe the trend using simple, population-oriented models for cosmic-ray transport and cooling. Results. We find that the gamma-ray luminosity-star formation rate relation obtained from infra-red luminosities is biased to shallower slopes. The actual relation is steeper than previous estimates, having a power-law index of 1.35 +- 0.05, in contrast to 1.23 +- 0.06. Conclusions. The unbiased gamma-ray luminosity-star formation rate relation can be explained at high star formation rates by assuming that the cosmic ray cooling region is kiloparsec-sized, and pervaded by mild to fast winds. Combined with previous results about the scaling of wind velocity with star formation rate, our work provides support to advection as the dominant cosmic ray escape mechanism in low-star formation rate galaxies.

We present multi-scale and multi-wavelength observations of the Galactic HII region S305, which is excited by massive O8.5V and O9.5V stars. Infrared images reveal an extended sphere-like shell (extension ~7.5 pc; at T_d = 17.5-27 K) enclosing the S305 HII region (size ~5.5 pc; age ~1.7 Myr). The extended structure observed in the Herschel temperature map indicates that the molecular environment of S305 is heated by the massive O-type stars. Regularly spaced molecular condensations and dust clumps are investigated toward the edges of the infrared shell, where the PAH and H_2 emission is also observed. The molecular line data show a signature of an expanding shell of molecular gas in S305. GMRT 610 and 1280 MHz continuum maps reveal overdensities of the ionized emission distributed around two O-type stars, which are surrounded by the horseshoe envelope (extension ~2.3 pc). A molecular gas deficient region/cavity is identified toward the center of the horseshoe envelope, which is well traced with PAH, H_2, molecular, and dust emission. The edges of the infrared shell are found to be located in the front of the horseshoe envelope. All these outcomes provide the observational evidence of the feedback of O-type stars in S305. Moreover, non-thermal radio emission is detected in S305 with an average spectral index alpha ~-0.45. The variations in alpha, ranging from -1.1 to 1.3, are explained due to soft synchrotron emission and either optically-thicker thermal emission at high frequencies or a suppression of the low-frequency emission by the Razin-Tsytovich effect., Comment: 21 pages, 13 figures, Accepted for publication in The Astrophysical Journal

We conducted an observational campaign towards one of the most massive and luminous colliding wind binaries in the Galaxy, HD~93129A, close to its periastron passage in 2018. During this time the source was predicted to be in its maximum of high-energy emission. Here we present our data analysis from the X-ray satellites \textit{Chandra} and \textit{NuSTAR} and the $\gamma$-ray satellite \textit{AGILE}. High-energy emission coincident with HD~93129A was detected in the X-ray band up to $\sim$18~keV, whereas in the $\gamma$-ray band only upper limits were obtained. We interpret the derived fluxes using a non-thermal radiative model for the wind-collision region. We establish a conservative upper limit for the fraction of the wind kinetic power that is converted into relativistic electron acceleration, $f_\mathrm{NT,e} < 0.02$. In addition, we set a lower limit for the magnetic field in the wind-collision region as $B_\mathrm{WCR} > 0.3$~G. We also argue a putative interpretation of the emission from which we estimate $f_\mathrm{NT,e} \approx 0.006$ and $B_\mathrm{WCR} \approx 0.5$~G. We conclude that multi-wavelength, dedicated observing campaigns during carefully selected epochs are a powerful tool for characterising the relativistic particle content and magnetic field intensity in colliding wind binaries., Comment: 11 pages, 10 figures, accepted for publication in MNRAS

The Argentine Institute of Radio astronomy (IAR) is equipped with two single-dish 30mts radio antennas capable of performing daily observations of pulsars and radio transients in the southern hemisphere at 1.4 GHz. We aim to introduce to the international community the upgrades performed and to show that IAR observatory has become suitable for investigations in numerous areas of pulsar radio astronomy, such as pulsar timing arrays, targeted searches of continuous gravitational waves sources, monitoring of magnetars and glitching pulsars, and studies of short time scale interstellar scintillation. We refurbished the two antennas at IAR to achieve high-quality timing observations. We gathered more than $1\,000$ hours of observations with both antennas to study the timing precision and sensitivity they can achieve. We introduce the new developments for both radio telescopes at IAR. We present observations of the millisecond pulsar J0437$-$4715 with timing precision better than 1~$\mu$s. We also present a follow-up of the reactivation of the magnetar XTE J1810--197 and the measurement and monitoring of the latest (Feb. 1st. 2019) glitch of the Vela pulsar (J0835--4510). We show that IAR is capable of performing pulsar monitoring in the 1.4 GHz radio band for long periods of time with a daily cadence. This opens the possibility of pursuing several goals in pulsar science, including coordinated multi-wavelength observations with other observatories. In particular, observations of the millisecond pulsar J0437$-$4715 will increase the gravitational wave sensitivity of the NANOGrav array in their current blind spot. We also show IAR's great potential for studying targets of opportunity and transient phenomena such as magnetars, glitches, and fast-radio-burst sources., Comment: 13 pages, 12 figures, some important revisions included

We present observations of the Cygnus OB2 region obtained with the Giant Metrewave Radio Telescope (GMRT) at the frequencies of 325 MHz and 610 MHz. In this contribution we focus on the study of proplyd-like objects (also known as free-floating Evaporating Gas Globules or frEGGs) that typically show an extended cometary morphology. We identify eight objects previously studied at other wavelengths and derive their physical properties by obtaining their optical depth at radio-wavelengths. Using their geometry and the photoionization rate needed to produce their radio-continuum emission, we find that these sources are possibly ionized by a contribution of the stars Cyg OB2 #9 and Cyg OB2 #22. Spectral index maps of the eight frEGGs were constructed, showing a flat spectrum in radio frequencies in general. We interpret these as produced by optically-thin ionized gas, although it is possible that a combination of thermal emission, not necessarily optically thin, produced by a diffuse gas component and the instrument response (which detects more diffuse emission at low frequencies) can artificially generate negative spectral indices. In particular, for the case of the Tadpole we suggest that the observed emission is not of non-thermal origin despite the presence of regions with negative spectral indices in our maps., Comment: Acepted Astronomy & Astrophysics, 9 pages, 4 figures

The innermost parts of powerful jets in active galactic nuclei are surrounded by dense, high-velocity clouds from the broad-line region, which may penetrate into the jet and lead to the formation of a strong shock. Such jet-cloud interactions are expected to have measurable effects on the $\gamma$-ray emission from blazars. We characterise the dynamics of a typical cloud-jet interaction scenario, and the evolution of its radiative output in the 0.1-30~GeV energy range, to assess to what extent these interactions can contribute to the $\gamma$-ray emission in blazars. We use semi-analytical descriptions of the jet-cloud dynamics, taking into account the expansion of the cloud inside the jet and its acceleration. Assuming that electrons are accelerated in the interaction and making use of the hydrodynamical information, we then compute the high-energy radiation from the cloud, including the absorption of $\gamma$-rays in the ambient photon field through pair creation. Jet-cloud interactions can lead to significant $\gamma$-ray fluxes in blazars with a broad-line region, in particular when the cloud expansion and acceleration inside the jet are taken into account. This is caused by 1) the increased shocked area in the jet, which leads to an increase in the energy budget for the non-thermal emission; 2) a more efficient inverse Compton cooling with the boosted photon field of the broad-line region; and 3) an increased observer luminosity due to Doppler boosting effects. For typical broad-line region parameters, either (i) jet-cloud interactions contribute significantly to the persistent $\gamma$-ray emission from blazars or (ii) the broad-line region is far from spherical or the fraction of energy deposited in non-thermal electrons is small., Comment: 9 pages, 9 figures. Accepted for publication in A&A

Context: Runaway stars produce bowshocks that are usually observed at infrared (IR) wavelengths. Non-thermal radio emission has been detected so far only from the bowshock of BD+43{\deg}3654, whereas the detection of non-thermal radiation from these bowshocks at high energies remains elusive. Aims: We aim at characterising in detail the radio, X-ray, and gamma-ray emission from stellar bowshocks accounting for the structure of the region of interaction between the stellar wind and its environment. Methods: We develop a broadband-radiative, multi-zone model for stellar bowshocks that takes into account the spatial structure of the emitting region and the observational constraints. The model predicts the evolution and the emission of the relativistic particles accelerated and streaming together with the shocked flow. Results: We present broadband non-thermal spectral energy distributions for different scenarios, synthetic radio-cm synchrotron maps that reproduce the morphology of BD+43{\deg}3654, and updated predictions in X-ray and gamma-ray energy ranges. We also compare the results of the multi-zone model applied in this work with those of a refined one-zone model. Conclusions: A multi-zone model provides better constraints than a one-zone model on the relevant parameters, namely the magnetic field intensity and the amount of energy deposited in non-thermal particles. However, one-zone models can be improved by carefully characterising the intensity of the IR dust photon field and the escape rate of the plasma from the shocked region. Finally, comparing observed radio maps with those obtained from a multi-zone model enables constraints to be obtained on the direction of stellar motion with respect to the observer., Comment: 12 pages, 10 figures. Accepted for publication in A&A

The stellar winds of the massive stars in high-mass microquasars are thought to be inhomogeneous. The interaction of these inhomogeneities, or clumps, with the jets of these objects may be a major factor in gamma-ray production. Our goal is to characterize a typical scenario of clump-jet interaction, and calculate the contribution of these interactions to the gamma-ray emission from these systems. We use axisymmetric, RHD simulations to model the emitting flow in a typical clump-jet interaction. Using the simulation results we perform a numerical calculation of the high-energy emission from one of these interactions. The radiative calculations are done for relativistic electrons locally accelerated at the jet shock, and the synchrotron and IC radiation spectra are computed for different stages of the shocked clump evolution. We also explore different parameter values, such as viewing angle and magnetic field strength. We generalize phenomenologically the results derived from one clump-jet interaction to multiple interactions under different wind models, estimating the clump-jet interaction rates, and the resulting luminosities in the GeV range. If particles are efficiently accelerated in clump-jet interactions, the apparent gamma-ray luminosity through IC scattering with the stellar photons can be significant even for rather strong magnetic fields and thus efficient synchrotron cooling. Moreover, despite the standing nature or slow motion of the jet shocks for most of the interaction stage, Doppler boosting in the postshock flow is relevant even for mildly relativistic jets. For clump-to-average wind density contrasts $\gtrsim 10$, clump-jet interactions could be bright enough to match the observed GeV luminosity in Cyg~X-1 and Cyg~X-3 when a jet is present in these sources, with required non-thermal-to-total available power fractions $\sim 0.01$ and $0.1$, respectively., Comment: 9 pages, 7 figures, accepted for publication in A&A

Several massive early-type binaries exhibit non-thermal emission which has been attributed to synchrotron radiation from particles accelerated by diffusive shock acceleration (DSA) in the wind-collision region (WCR). If the magnetic field in the strong shocks is ordered, its component parallel to the shock front should be enhanced, and the resultant synchrotron radiation would be polarized. However, such polarization has never been measured. We performed spatially-unresolved radio continuum observations of WR 146, a well known a WC6+O8 system, at 5 cm and 20 cm with the Karl G. Jansky Very Large Array. No linearly polarized radio emission was detected. The data constrain the fractional linear polarization to less than 0.6% between 1 to 8 GHz. This is compatible with a high level of turbulence and a dominant random component in the magnetic field. In this case the relativistic particles could be produced by turbulent magnetic reconnection (MR). In order for this scenario to satisfy the required non-thermal energy budget, we find through numerical modeling that the strength of the magnetic field in the WCR must be as high as ~150 mG. However, if the magnetic field is ordered and DSA is ongoing, then a combination of internal and external Faraday rotation could equally account for the depolarization of the emission. It is not clear whether it is possible to develop the high level of turbulence and strong magnetic fields required for efficient MR in a long-period binary such as WR 146. This scenario might also have trouble explaining the low-frequency cutoff in the spectrum. We therefore favor a scenario where particles are accelerated through DSA and the depolarization is produced by mechanisms other than a large ratio between random to regular magnetic fields., Comment: 7 pages, 2 figures, A&A in press

We present coordinated multiwavelength observations of the bright, nearby BL Lac object Mrk 421 taken in 2013 January-March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT, MAGIC, VERITAS, and other collaborations and instruments, providing data from radio to very-high-energy (VHE) gamma-ray bands. NuSTAR yielded previously unattainable sensitivity in the 3-79 keV range, revealing that the spectrum softens when the source is dimmer until the X-ray spectral shape saturates into a steep power law with a photon index of approximately 3, with no evidence for an exponential cutoff or additional hard components up to about 80 keV. For the first time, we observed both the synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to frequencies below the typical quiescent state by an order of magnitude. The fractional variability as a function of photon energy shows a double-bump structure which relates to the two bumps of the broadband SED. In each bump, the variability increases with energy which, in the framework of the synchrotron self-Compton model, implies that the electrons with higher energies are more variable. The measured multi-band variability, the significant X-ray-to-VHE correlation down to some of the lowest fluxes ever observed in both bands, the lack of correlation between optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short estimated electron cooling time, and the significantly longer variability timescale observed in the NuSTAR light curves point toward in-situ electron acceleration, and suggest that there are multiple compact regions contributing to the broadband emission of Mrk 421 during low-activity states., Comment: 32 pages, 14 figures; accepted for publication in ApJ