arXiv subjects

Amelia Hankla

Publications and source records attributed to Amelia Hankla.

4 recordsLinked to original sources

arXiv2024

X-ray properties of coronal emission in radio quiet Active Galactic Nuclei

Active galactic nuclei (AGN) are powerful sources of panchromatic radiation. All AGN emit in X-rays, contributing around $\sim 5-10\%$ of the AGN bolometric luminosity. The X-ray emitting region, popularly known as the corona, is geometrically and radiatively compact with a size typically $\lesssim 10 \, R_{\rm G}$ (gravitational radii). The rapid and extreme variability in X-rays also suggest that the corona must be a dynamic structure. Decades of X-ray studies have shed much light on the topic, but the nature and origin of AGN corona are still not clearly understood. This is mostly due to the complexities involved in several physical processes at play in the high-gravity, high-density and high-temperature region in the vicinity of the supermassive black hole (SMBH). It is still not clear how exactly the corona is energetically and physically sustained near a SMBH. The ubiquity of coronal emission in AGN points to their fundamental role in black hole accretion processes. In this review we discuss the X-ray observational properties of corona in radio quiet AGN.

astro-ph.HE↗

arXiv2022

Nonthermal emission from the plunging region: a model for the high-energy tail of black hole X-ray binary soft states

X-ray binaries exhibit a soft spectral state comprising thermal blackbody emission at 1 keV and a power-law tail above 10 keV. Empirical models fit the high-energy power-law tail to radiation from a nonthermal electron distribution, but the physical location of the nonthermal electrons and the reason for their power-law index and high-energy cut-off are still largely unknown. Here, we propose that the nonthermal electrons originate from within the black hole's innermost stable circular orbit (the ''plunging region''). Using an analytic model for the plunging region dynamics and electron distribution function properties from particle-in-cell simulations, we outline a steady-state model that can reproduce the observed spectral features. In particular, our model reproduces photon indices of $Γ\gtrsim2$ and power-law luminosities on the order of a few percent of the disk luminosity for strong magnetic fields, consistent with observations of the soft state. Because the emission originates so close to the black hole, we predict that the power-law luminosity should strongly depend on the system inclination angle and black hole spin. This model could be extended to the power-law tails observed above 400 keV in the hard state of X-ray binaries.

astro-ph.HE↗

arXiv2021

Kinetic Simulations of Imbalanced Turbulence in a Relativistic Plasma: Net Flow and Particle Acceleration

Turbulent high-energy astrophysical systems often feature asymmetric energy injection: for instance, Alfven waves propagating from an accretion disk into its corona. Such systems are "imbalanced": the energy fluxes parallel and anti-parallel to the large-scale magnetic field are unequal. In the past, numerical studies of imbalanced turbulence have focused on the magnetohydrodynamic regime. In the present study, we investigate externally-driven imbalanced turbulence in a collisionless, ultrarelativistically hot, magnetized pair plasma using three-dimensional particle-in-cell (PIC) simulations. We find that the injected electromagnetic momentum efficiently converts into plasma momentum, resulting in net motion along the background magnetic field with speeds up to a significant fraction of lightspeed. This discovery has important implications for the launching of accretion disk winds. We also find that although particle acceleration in imbalanced turbulence operates on a slower timescale than in balanced turbulence, it ultimately produces a power-law energy distribution similar to balanced turbulence. Our results have ramifications for black hole accretion disk coronae, winds, and jets.

astro-ph.HE↗

arXiv2020

Local Simulations of Heating Torques on a Luminous Body in an Accretion Disk

A luminous body embedded in an accretion disk can generate asymmetric density perturbations that lead to a net torque and thus orbital migration of the body. Linear theory has shown that this heating torque gives rise to a migration term linear in the body's mass that can oppose or even reverse that arising from the sum of gravitational Lindblad and co-orbital torques. We use high-resolution local simulations in an unstratified disk to assess the accuracy and domain of applicability of the linear theory. We find agreement between analytic and simulation results to better than 10\% in the appropriate regime (low luminosity, low thermal conductivity), but measure deviations in the non-linear (high luminosity) regime and in the high thermal conductivity regime. In the non-linear regime, linear theory overpredicts the acceleration due to the heating torque, which we find to be due to the neglect of non-linear terms in the heat flux. In the high thermal conductivity regime linear theory underpredicts the acceleration, although here both non-linear and computational constraints play a role. We discuss the impact of the heating torque for the evolution of low-mass planets in protoplanetary disks, and for massive stars or accreting compact objects embedded in AGN disks. For the latter case, we show that the thermal torque is likely to be the dominant physical effect at disk radii where the optical depth drops below a critical value.

astro-ph.EP↗