67 research outputs found
Striped Jets in Post Neutron Star Merger Systems
Models invoking magnetic reconnection as the particle acceleration mechanism
within relativistic jets often adopt a gradual energy dissipation profile
within the jet. However, such a profile has yet to be reproduced in
first-principles simulations. Here, we perform a suite of 3D general
relativistic magnetohydrodynamic simulations of post-neutron star merger disks
with an initially purely toroidal magnetic field. We explore the variations in
both the microphysics (e.g., nuclear recombination, neutrino emission) and
system parameters (e.g., disk mass). In all our simulations, we find the
formation of magnetically striped jets. The stripes result from the reversals
in the poloidal magnetic flux polarity generated in the accretion disk. The
simulations display large variations in the distributions of stripe duration,
, and power, . We find that more massive disks
produce more powerful stripes, the most powerful of which reaches ~erg~s at ~ms. The power and
variability that result from the magnetic reconnection of the stripes agree
with those inferred in short duration gamma-ray bursts. We find that the
dissipation profile of the cumulative energy is roughly a power-law in both
radial distance, , and , with the slope in the range, ;
more massive disks display larger slopes
A jet model for Galactic black-hole X-ray sources: Some constraining correlations
Some recent observational results impose significant constraints on all the
models that have been proposed to explain the Galactic black-hole X-ray sources
in the hard state. In particular, it has been found that during the hard state
of Cyg X-1 the power-law photon number spectral index is correlated with the
average time lag between hard and soft X-rays. Furthermore, the peak
frequencies of the four Lorentzians that fit the observed power spectra are
correlated with both the photon index and the time lag. We performed Monte
Carlo simulations of Compton upscattering of soft, accretion-disk photons in
the jet and computed the time lag between hard and soft photons and the
power-law index of the resulting photon number spectra. We demonstrate that our
jet model naturally explains the above correlations, with no additional
requirements and no additional parameters
Magnetic Reconnection in Extreme Astrophysical Environments
Magnetic reconnection is a basic plasma process of dramatic rearrangement of
magnetic topology, often leading to a violent release of magnetic energy. It is
important in magnetic fusion and in space and solar physics --- areas that have
so far provided the context for most of reconnection research. Importantly,
these environments consist just of electrons and ions and the dissipated energy
always stays with the plasma. In contrast, in this paper I introduce a new
direction of research, motivated by several important problems in high-energy
astrophysics --- reconnection in high energy density (HED) radiative plasmas,
where radiation pressure and radiative cooling become dominant factors in the
pressure and energy balance. I identify the key processes distinguishing HED
reconnection: special-relativistic effects; radiative effects (radiative
cooling, radiation pressure, and Compton resistivity); and, at the most extreme
end, QED effects, including pair creation. I then discuss the main
astrophysical applications --- situations with magnetar-strength fields
(exceeding the quantum critical field of about 4 x 10^13 G): giant SGR flares
and magnetically-powered central engines and jets of GRBs. Here, magnetic
energy density is so high that its dissipation heats the plasma to MeV
temperatures. Electron-positron pairs are then copiously produced, making the
reconnection layer highly collisional and dressing it in a thick pair coat that
traps radiation. The pressure is dominated by radiation and pairs. Yet,
radiation diffusion across the layer may be faster than the global Alfv\'en
transit time; then, radiative cooling governs the thermodynamics and
reconnection becomes a radiative transfer problem, greatly affected by the
ultra-strong magnetic field. This overall picture is very different from our
traditional picture of reconnection and thus represents a new frontier in
reconnection research.Comment: Accepted to Space Science Reviews (special issue on magnetic
reconnection). Article is based on an invited review talk at the
Yosemite-2010 Workshop on Magnetic Reconnection (Yosemite NP, CA, USA;
February 8-12, 2010). 30 pages, no figure
Accretion and ejection in black-hole X-ray transients
Aims: We summarize the current observational picture of the outbursts of
black-hole X-ray transients (BHTs), based on the evolution traced in a
hardness-luminosity diagram (HLD), and we offer a physical interpretation.
Methods: The basic ingredient in our interpretation is the Poynting-Robertson
Cosmic Battery (PRCB, Contopoulos & Kazanas 1998), which provides locally the
poloidal magnetic field needed for the ejection of the jet. In addition, we
make two assumptions, easily justifiable. The first is that the mass-accretion
rate to the black hole in a BHT outburst has a generic bell-shaped form. This
is guaranteed by the observational fact that all BHTs start their outburst and
end it at the quiescent state. The second assumption is that at low accretion
rates the accretion flow is geometrically thick, ADAF-like, while at high
accretion rates it is geometrically thin.
Results: Both, at the beginning and the end of an outburst, the PRCB
establishes a strong poloidal magnetic field in the ADAF-like part of the
accretion flow, and this explains naturally why a jet is always present in the
right part of the HLD. In the left part of the HLD, the accretion flow is in
the form of a thin disk, and such a disk cannot sustain a strong poloidal
magnetic filed. Thus, no jet is expected in this part of the HLD. The
counterclockwise traversal of the HLD is explained as follows: the poloidal
magnetic field in the ADAF forces the flow to remain ADAF and the source to
move upwards in the HLD rather than to turn left. Thus, the history of the
system determines the counterclockwise traversal of the HLD. As a result, no
BHT is expected to ever traverse the entire HLD curve in the clockwise
direction.
Conclusions: We offer a physical interpretation of accretion and ejection in
BHTs with only one parameter, the mass transfer rate.Comment: Accepted for publication in A&
The characterization of the distant blazar GB6 J1239+0443 from flaring and low activity periods
In 2008 AGILE and Fermi detected gamma-ray flaring activity from the
unidentified EGRET source 3EG J1236+0457, recently associated with a flat
spectrum radio quasar GB6 J1239+0443 at z=1.762. The optical counterpart of the
gamma-ray source underwent a flux enhancement of a factor 15-30 in 6 years, and
of ~10 in six months. We interpret this flare-up in terms of a transition from
an accretion-disk dominated emission to a synchrotron-jet dominated one. We
analysed a Sloan Digital Sky Survey (SDSS) archival optical spectrum taken
during a period of low radio and optical activity of the source. We estimated
the mass of the central black hole using the width of the CIV emission line. In
our work, we have also investigated SDSS archival optical photometric data and
UV GALEX observations to estimate the thermal-disk emission contribution of GB6
J1239+0443. Our analysis of the gamma-ray data taken during the flaring
episodes indicates a flat gamma-ray spectrum, with an extension of up to 15
GeV, with no statistically-relevant sign of absorption from the broad line
region, suggesting that the blazar-zone is located beyond the broad line
region. This result is confirmed by the modeling of the broad-band spectral
energy distribution (well constrained by the available multiwavelength data) of
the flaring activity periods and by the accretion disk luminosity and black
hole mass estimated by us using archival data.Comment: 30 pages, 7 figures, 4 tables MNRAS Accepted on 2012 June 1
AGILE detection of extreme gamma-ray activity from the blazar PKS 1510-089 during March 2009. Multifrequency analysis
We report on the extreme gamma-ray activity from the FSRQ PKS 1510-089
observed by AGILE in March 2009. In the same period a radio-to-optical
monitoring of the source was provided by the GASP-WEBT and REM. Moreover,
several Swift ToO observations were triggered, adding important information on
the source behaviour from optical/UV to hard X-rays. We paid particular
attention to the calibration of the Swift/UVOT data to make it suitable to the
blazars spectra. Simultaneous observations from radio to gamma rays allowed us
to study in detail the correlation among the emission variability at different
frequencies and to investigate the mechanisms at work. In the period 9-30 March
2009, AGILE detected an average gamma-ray flux of (311+/-21)x10^-8 ph cm^-2
s^-1 for E>100 MeV, and a peak level of (702+/-131)x10^-8 ph cm^-2 s^-1 on
daily integration. The gamma-ray activity occurred during a period of
increasing activity from near-IR to UV, with a flaring episode detected on
26-27 March 2009, suggesting that a single mechanism is responsible for the
flux enhancement observed from near-IR to UV. By contrast, Swift/XRT
observations seem to show no clear correlation of the X-ray fluxes with the
optical and gamma-ray ones. However, the X-ray observations show a harder
photon index (1.3-1.6) with respect to most FSRQs and a hint of
harder-when-brighter behaviour, indicating the possible presence of a second
emission component at soft X-ray energies. Moreover, the broad band spectrum
from radio-to-UV confirmed the evidence of thermal features in the optical/UV
spectrum of PKS 1510-089 also during high gamma-ray state. On the other hand,
during 25-26 March 2009 a flat spectrum in the optical/UV energy band was
observed, suggesting an important contribution of the synchrotron emission in
this part of the spectrum during the brightest gamma-ray flare, therefore a
significant shift of the synchrotron peak.Comment: 13 pages, 7 figures, 3 tables. Accepted for publication in Astronomy
and Astrophysic
High-Energy Polarimetry - a new window to probe extreme physics in AGN jets
The constantly improving sensitivity of ground-based and space-borne
observatories has made possible the detection of high-energy emission (X-rays
and gamma-rays) from several thousands of extragalactic sources. Enormous
progress has been made in measuring the continuum flux enabling us to perform
imaging, spectral and timing studies. An important remaining challenge for
high-energy astronomy is measuring polarization. The capability to measure
polarization is being realized currently at X-ray energies (e.g. with IXPE),
and sensitive gamma-ray telescopes capable of measuring polarization, such as
AMEGO, AdEPT, e-ASTROGAM, etc., are being developed. These future gamma-ray
telescopes will probe the radiation mechanisms and magnetic fields of
relativistic jets from active galactic nuclei at spatial scales much smaller
than the angular resolution achieved with continuum observations of the
instrument. In this white paper, we discuss the scientific potentials of
high-energy polarimetry, especially gamma-ray polarimetry, including the
theoretical implications, and observational technology advances being made. In
particular, we will explore the primary scientific opportunities and wealth of
information expected from synergy of multi-wavelength polarimetry that will be
brought to multi-messenger astronomy.Comment: submitted to Astro2020 (Astronomy and Astrophysics Decadal Survey
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