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Abstract:

We present the discovery of radio emission from the Be/X-ray binary A0538–66 with the Australian Square Kilometre Array Pathfinder (ASKAP), and results from a subsequent weekly monitoring campaign with the MeerKAT radio telescope. A0538–66, located in the Large Magellanic Cloud, hosts a neutron star with a short spin period (P ≈ 69 ms) in a highly eccentric ≈16.6-day orbit. Its rare episodes of super-Eddington accretion, rapid optical and X-ray flares, and other peculiar properties make it an interesting system among high-mass X-ray binaries. Our MeerKAT data reveal that it is also one of the most radio-luminous neutron star X-ray binaries observed to date, reaching ≈3 × 1022 erg s−1Hz−1, with radio emission that appears to be orbitally modulated. We consider several possible mechanisms for the radio emission, and place A0538–66 in context by comparing it to similar systems.

Abstract:

Context. It is well known that star-forming galaxies (SFGs) exhibit a tight correlation between their radio and infrared emissions, commonly referred to as the infrared-radio correlation (IRRC). Recent empirical studies have reported a dependence of the IRRC on the galaxy stellar mass, in which more massive galaxies tend to show lower infrared-to-radio ratios ( q IR ) with respect to less massive galaxies. One possible, yet unexplored, explanation is a residual contamination of the radio emission from active galactic nuclei (AGNs), not captured through “radio-excess” diagnostics. Aims. To investigate this hypothesis, we aim to statistically quantify the contribution of AGN emission to the radio luminosities of SFGs located within the scatter of the IRRC. Methods. Our Very Large Baseline Array (VLBA) AGN-sCAN program has targeted 500 galaxies that follow the q IR distribution of the IRRC, i.e., with no prior evidence for radio-excess AGN emission based on low-resolution (∼arcsec) VLA radio imaging. Our VLBA 1.4 GHz observations reach a 5 σ sensitivity limit of 25 μJy/beam, corresponding to a radio-brightness temperature of T b  ∼ 10 5 K. This classification serves as a robust AGN diagnostic, regardless of the host galaxy’s star formation rate. Results. We detect four VLBA sources in the deepest regions, which are also the faintest VLBI-detected AGNs in SFGs to date. The effective AGN detection rate is 9%, when considering a control sample matched in mass and sensitivity, which is in good agreement with the extrapolation of previous radio AGN number counts. Despite the non-negligible AGN flux contamination (∼30%) in our individual VLBA detections, we find that the peak of the q IR distribution is completely unaffected by this correction. Although we cannot rule out a high incidence of radio-silent AGNs at (sub)μJy levels among the VLBA non-detections, we derive a conservative upper limit of < 0.1 dex of their cumulative impact on the q IR distribution. We conclude that residual AGN contamination from non-radio-excess AGNs is unlikely to be the primary driver of the M ★ – dependent IRRC.

Abstract:

Context. Microquasars have emerged as promising candidates to explain the cosmic-ray flux at petaelectronvolt energies. LHAASO observations revealed V4641 Sgr as the most extreme example so far. Its gamma-ray spectrum extends up to 800 TeV, which requires particles with multi-PeV energy. The TeV emission is highly extended, which challenges expectations given the reported low-inclination angle of the V4641 Sgr jets. Aims. We spatially and spectrally resolved the gamma-ray emission from V4641 Sgr and investigated the particle acceleration in the system. Methods. Using ≈100 h of H.E.S.S. data, we performed a spectro-morphological study of the gamma-ray emission around V4641 Sgr. We employed HI and dedicated CO observations of the region to infer the target material for cosmic-ray interactions. Results. We detected multi-TeV emission around V4641 Sgr with a high significance. The emission region is elongated, and its major and minor axes are 0.34° ±0.01 syst ±0.04 stat and 0.06° ±0.01 syst ±0.01 stat , respectively. We found a power-law spectrum with an index ≈1.8, and together with results from other gamma-ray instruments, this reveals a spectral energy distribution (SED) that peaks at energies of ≈100 TeV for the first time. We found indications (3 σ ) of a two-component morphology, with indistinguishable spectral properties. The position of V4641 Sgr is inconsistent with the best-fit position of the single-component model and with the dip between the two components. We found no significant evidence of an energy-dependent morphology. No dense gas was found at any distance towards V4641 Sgr, which places an upper limit of n gas ≲ 0.2 cm −3 within the gamma-ray emission region. Conclusions. The peak of the SED at ≈100 TeV identifies V4641 Sgr as a candidate cosmic-ray accelerator beyond the so-called knee. The absence of dense target gas places stringent energetic constraints on hadronic interpretations, however. The H.E.S.S. measurement requires an unusually hard (≈1.5) spectral index for the protons. A leptonic scenario faces fewer obstacles if the particle transport is fast enough to avoid losses and to reproduce the observed energy-independent morphology. The absence of bright X-ray emission across the gamma-ray emission region requires a magnetic field strength ≲3 μG, however. Our findings favour a leptonic origin of the gamma-ray emission. This conclusion does not exclude hadron acceleration in the V4641 Sgr system.

Abstract:

Context. PKS 0346-27 is a low synchrotron peaked blazar at redshift 0.991. The very high energy (VHE; E > 100 GeV) spectra of blazars are always affected by γγ absorption by the extragalactic background light (EBL), and subsequently no blazars have been detected in VHE γ -rays at redshifts exceeding 1. Aims. This is the goal of a target-of-opportunity (ToO) programme by H.E.S.S.: to observe flaring high-redshift ( z ≳ 1) blazars. Importantly, extending the redshift range of VHE-detected blazars to z ≳ 1 will yield insights into the cosmological evolution of both the VHE blazar population and the EBL. Methods. We report H.E.S.S. ToO and multi-wavelength observations of the blazar PKS 0346−27. We analysed and modelled the H.E.S.S. data together with simultaneous data from Fermi -LAT, Swift (XRT and UVOT), using single-zone leptonic and hadronic models. Results. PKS 0346-27 was detected by H.E.S.S. at a significance of 6.3 σ during one night on 3 November 2021, while for other nights before and after this day, upper limits on the VHE flux have been determined. No evidence for intra-night γ -ray variability has been found. A flare in high-energy ( E > 100 MeV) γ -rays detected by Fermi -LAT preceded the H.E.S.S. detection by 2 days. A fit with a single-zone emission model to the contemporaneous spectral energy distribution during the detection night was possible with a proton-synchrotron-dominated hadronic model, requiring a proton-kinetic-energy-dominated jet power temporarily exceeding the source’s Eddington limit, although alternative (e.g. multi-zone) models cannot be ruled out. A one-zone leptonic model is, in principle, also able to fit the flare-state spectral energy distribution. However, it requires implausible parameter choices, in particular, extreme Doppler and bulk Lorentz factors of ≳80.