High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: Stellar mass black holes in X-ray binaries (XRBs) are known to display different states characterized by different spectral and timing properties, understood in the framework of a hot corona coexisting with a thin accretion disk whose inner edge is truncated. There are several open questions related to the nature and properties of the corona, the thin disk, and dynamics behind the hard state. This motivated us to perform two-dimensional hydrodynamical simulations of accretion flows onto a 10 solar masses black hole. We consider a two-temperature plasma, incorporate radiative cooling with bremmstrahlung, synchrotron and comptonization losses and approximate the Schwarzschild spacetime via a pseudo-Newtonian potential. We varied the mass accretion rate in the range 0.02 <= Mdot/Mdot_Edd <= 0.35. Our simulations show the natural emergence of a colder truncated thin disk embedded in a hot corona, as required to explain the hard state of XRBs. We found that as Mdot increases, the corona contracts and the inner edge of the thin disk gets closer to the event horizon. At a critical accretion rate 0.02 <= Mdot_crit\Mdot_Edd <= 0.06, the thin disk disappears entirely. We discuss how our simulations compare with XRB observations in the hard state.

Abstract: Temporal broadening is a commonly observed property of fast radio bursts (FRBs), associated with turbulent media which cause radiowave scattering. Similarly to dispersion, scattering is an important probe of the media along the line of sight to an FRB source, such as the circum-burst or circum-galactic mediums (CGM). Measurements of characteristic scattering times alone are insufficient to constrain the position of the dominant scattering media along the line of sight. However, where more than one scattering screen exists, Galactic scintillation can be leveraged to form strong constraints. We quantify the scattering and scintillation in 10 FRBs with 1) known host galaxies and redshifts and 2) captured voltage data enabling high time resolution analysis, obtained from the Commensal Real-time ASKAP (Australian Square Kilometre Array Pathfinder) Fast Transient survey science project (CRAFT). We find strong evidence for two screens in three cases. For FRBs 20190608B and 20210320C, we find evidence for scattering screens less than approximately 16.7 and 3000 kpc respectively, from their sources. For FRB 20201124A we find evidence for a scattering screen at $\approx$26 kpc. Each of these measures is consistent with the scattering occurring in the host ISM (inter-stellar medium) or CGM. If pulse broadening is assumed to be contributed by the host galaxy ISM or circum-burst environment, the definitive lack of observed scintillation in four FRBs in our sample suggests that existing models may be over-estimating scattering times associated with the Milky Way's ISM, similar to the anomalously low scattering observed for FRB 20201124A.

Abstract: We have studied the effect of changing the density and magnetic field strength in the coherent pulses that are emitted as energetic showers develop in the atmosphere. For this purpose we have developed an extension of ZHS, a program to calculate coherent radio pulses from electromagnetic showers in homogeneous media, to account for the Lorentz force due to a magnetic field. This makes it possible to perform quite realistic simulations of radio pulses from air showers in a medium similar to the atmosphere but without variations of density with altitude. The effects of independently changing the density, the refractive index and the magnetic field strength are studied in the frequency domain for observers in the Cherenkov direction at far distances from the shower. This approach is particularly enlightening providing an explanation of the spectral behavior of the induced electric field in terms of shower development parameters. More importantly, it clearly displays the complex scaling properties of the pulses as density and magnetic field intensity are varied. The usually assumed linear behavior of electric field amplitude with magnetic field intensity is shown to hold up to a given magnetic field strength at which the extra time delays due to the deflection in the magnetic field break it. Scaling properties of the pulses are obtained as the density of air decreases relative to sea level. A remarkably accurate scaling law is obtained that relates the spectra of pulses obtained when reducing the density and increasing the magnetic field.