248 research outputs found
Kinetic simulations of mildly relativistic shocks I: particle acceleration in high Mach number shocks
We use fully kinetic particle-in-cell simulations with unprecedentedly large
transverse box sizes to study particle acceleration in weakly-magnetized mildly
relativistic shocks traveling at a velocity and a Mach number
of 15. We examine both subluminal (quasi-parallel) and superluminal
(quasi-perpendicular) magnetic field orientations. We find that quasi-parallel
shocks are mediated by a filamentary non-resonant (Bell) instability driven by
non-thermal ions, producing magnetic fluctuations on scales comparable to the
ion gyro-radius. In quasi-parallel shocks, both electrons and ions are
accelerated into non-thermal power-laws whose maximum energy grows linearly
with time. The upstream heating of electrons is small, and the two species
enter the shock front in rough thermal equilibrium. The shock's structure is
complex; the current of reflected non-thermal ions evacuates cavities in the
upstream which form filaments of amplified magnetic fields once advected
downstream. At late times, of the shock's energy goes into non-thermal
protons and into magnetic fields. We find that properly capturing
the magnetic turbulence driven by the non-thermal ions is important for
properly measuring the energy fraction of non-thermal electrons, .
We find for quasi-parallel shocks with
, slightly larger than what was measured in simulations of
non-relativistic shocks. In quasi-perpendicular shocks, no non-thermal
power-law develops in ions or electrons. The ion acceleration efficiency in
quasi-parallel shocks suggests that astrophysical objects that could host
mildly relativistic quasi-parallel shocks -- for example, the jets of active
galactic nuclei or microquasars -- may be important sources of cosmic rays and
their secondaries, such as gamma-rays and neutrinos.Comment: Submitted to MNRAS. 14 pages, 18 figure
The rise and fall of the compact jet in GRO J1655-40
In this work, we present some preliminary results on a multi-wavelength (radio/infrared/optical/X-ray) study of GRO J1655-40 during its 2005 outburst. We focus on the broadband spectral energy distribution during the different stages of the outburst. In particular, using this unprecedented coverage, and especially thanks to the new constraints given in the mid-IR by Spitzer, we can test the physical self-consistent disk-jet model during the hard state, where the source shows radio emission from a compact jet. The hard state broadband spectra of the observations during the decay of the outburst, are fairly well fit using the jet model with parameters overall similar to those found for Cyg X-1 and GX 339-4 in a previous work. However, we find that, compared to the other two BHs, GRO J1655-40 has a much higher jet power (at least a factor of 3), and that, most notably, the model seems to underestimate the radio emissio
Transient jet formation and state transitions from large-scale magnetic reconnection in black hole accretion discs
Magnetically arrested accretion discs (MADs), where the magnetic pressure in
the inner disc is dynamically important, provide an alternative mechanism for
regulating accretion to what is commonly assumed in black hole systems. We show
that a global magnetic field inversion in the MAD state can destroy the jet,
significantly increase the accretion rate, and move the effective inner disc
edge in to the marginally stable orbit. Reconnection of the MAD field in the
inner radii launches a new type of transient outflow containing hot plasma
generated by magnetic dissipation. This transient outflow can be as powerful as
the steady magnetically-dominated Blandford-Znajek jet in the MAD state. The
field inversion qualitatively describes many of the observational features
associated with the high luminosity hard to soft state transition in black hole
X-ray binaries: the jet line, the transient ballistic jet, and the drop in rms
variability. These results demonstrate that the magnetic field configuration
can influence the accretion state directly, and hence the magnetic field
structure is an important second parameter in explaining observations of
accreting black holes across the mass and luminosity scales.Comment: 5 pages, 3 figures, submitted to MNRAS Letter
Accelerating AGN jets to parsec scales using general relativistic MHD simulations
Accreting black holes produce collimated outflows, or jets, that traverse
many orders of magnitude in distance, accelerate to relativistic velocities,
and collimate into tight opening angles. Of these, perhaps the least understood
is jet collimation due to the interaction with the ambient medium. In order to
investigate this interaction, we carried out axisymmetric general relativistic
magnetohydrodynamic simulations of jets produced by a large accretion disc,
spanning over 5 orders of magnitude in time and distance, at an unprecedented
resolution. Supported by such a disc, the jet attains a parabolic shape,
similar to the M87 galaxy jet, and the product of the Lorentz factor and the
jet half-opening angle, , similar to values found from very
long baseline interferometry (VLBI) observations of active galactic nuclei
(AGN) jets; this suggests extended discs in AGN. We find that the interaction
between the jet and the ambient medium leads to the development of pinch
instabilities, which produce significant radial and lateral variability across
the jet by converting magnetic and kinetic energy into heat. Thus pinched
regions in the jet can be detectable as radiating hotspots and may provide an
ideal site for particle acceleration. Pinching also causes gas from the ambient
medium to become squeezed between magnetic field lines in the jet, leading to
enhanced mass-loading of the jet and potentially contributing to the
spine-sheath structure observed in AGN outflows.Comment: 18 pages, 24 figures, submitted to MNRAS, revised version. See our
Youtube channel for accompanying animations:
https://www.youtube.com/playlist?list=PLjldVlE2vDFzHMGn75tgc2Lod0kcTWZd
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