Gamma-ray astronomy

Abstract:

We report on Atacama Large Millimeter Array (ALMA) continuum observations of the black hole X-ray binary A0620–00 at an X-ray luminosity nine orders of magnitude sub-Eddington. The system was significantly detected at 98 GHz (at 44 ± 7 μJy) and only marginally at 233 GHz (20 ± 8 μJy), about 40 d later. These results suggest either an optically thin sub-mm synchrotron spectrum, or highly variable sub-mm jet emission on month time-scales. Although the latter appears more likely, we note that, at the time of the ALMA observations, A0620–00 was in a somewhat less active optical-IR state than during all published multiwavelength campaigns when a flat-spectrum, partially self-absorbed jet has been suggested to extend from the radio to the mid-IR regime. Either interpretation is viable in the context of an internal shock model, where the jet’s spectral shape and variability are set by the power density spectrum of the shells’ Lorentz factor fluctuations. While strictly simultaneous radio–mm-IR observations are necessary to draw definitive conclusions for A0620–00, the data presented here, in combination with recent radio and sub-mm results from higher luminosity systems, demonstrate that jets from black hole X-ray binaries exhibit a high level of variability – either in flux density or intrinsic spectral shape, or both – across a wide spectrum of Eddington ratios. This is not in contrast with expectations from an internal shock model, where lower jet power systems can be expected to exhibit larger fractional variability owing to an overall decrease in synchrotron absorption.

Abstract:

The cosmic-ray (CR) accelerator at the Galactic center (GC) is not yet established by current observations. Here we investigate the radiative-inefficient accretion flow (RIAF) of Sagittarius A∗; (SgrA∗;) as a CR accelerator assuming acceleration by turbulent magnetic reconnection, and derive possible emission fluxes of CRs interacting within the RIAF (the central ∼1013 cm). The target environment of the RIAF is modeled with numerical, general relativistic magnetohydrodynamics together with leptonic radiative transfer simulations. The acceleration of the CRs is not computed here. Instead, we inject CRs constrained by the magnetic reconnection power of the accretion flow and compute the emission/absorption of γ-rays due to these CRs interacting with the RIAF, through Monte Carlo simulations employing the CRPropa 3 code. The resulting very-high-energy (VHE) fluxes are not expected to reproduce the point source HESS J1745-290 as the emission of this source is most likely produced at parsec scales. The emission profiles derived here intend to trace the VHE signatures of the RIAF as a CR accelerator and provide predictions for observations of the GC with improved angular resolution and differential flux sensitivity as those of the forthcoming Cerenkov Telescope Array (CTA). Within the scenario presented here, we find that for mass accretion rates 10-7 M oyr-1, the RIAF of SgrA∗; produces VHE fluxes that are consistent with the High Energy Stereoscopic System (H.E.S.S.) upper limits for the GC and potentially observable by the future CTA. The associated neutrino fluxes are negligible compared with the diffuse neutrino emission measured by the IceCube.

Abstract:

Reverberation mapping (RM) is a powerful approach for determining the nature of the broad-line region (BLR) in active galactic nuclei. However, inferring physical BLR properties from an observed spectroscopic time series is a difficult inverse problem. Here, we present a blind test of two widely used RM methods: MEMECHO (developed by Horne) and CARAMEL (developed by Pancoast and collaborators). The test data are simulated spectroscopic time series that track the Hα emission line response to an empirical continuum light curve. The underlying BLR model is a rotating, biconical accretion disc wind, and the synthetic spectra are generated via self-consistent ionization and radiative transfer simulations. We generate two mock data sets, representing Seyfert galaxies and QSOs. The Seyfert model produces a largely negative response, which neither method can recover. However, both fail ‘gracefully', neither generating spurious results. For the QSO model both CARAMEL and expert interpretation of MEMECHOś output both capture the broadly annular, rotation-dominated nature of the line-forming region, though MEMECHO analysis overestimates its size by 50 per cent, but CARAMEL is unable to distinguish between additional inflow and outflow components. Despite fitting individual spectra well, the CARAMEL velocity-delay maps and RMS line profiles are strongly inconsistent with the input data. Finally, since the Hα line-forming region is rotation dominated, neither method recovers the disc wind nature of the underlying BLR model. Thus considerable care is required when interpreting the results of RM analyses in terms of physical models.

Abstract:

We find that hydromagnetic flux tubes in back-flows in the lobes of radio galaxies offer a suitable environment for the acceleration of cosmic rays (CR) to ultra-high energies. We show that CR can reach the Hillas (1984) energy even if the magnetised turbulence in the flux tube is not sufficiently strong for Bohm diffusion to apply. First-order Fermi acceleration by successive weak shocks in a hydromagnetic flux tube is shown to be equivalent to second-order Fermi acceleration by strong turbulence.