242 research outputs found
Extragalactic Zeeman Detections in OH Megamasers
We have measured the Zeeman splitting of OH megamaser emission at 1667 MHz
from five (ultra)luminous infrared galaxies ([U]LIRGs) using the 305 m Arecibo
telescope and the 100 m Green Bank Telescope. Five of eight targeted galaxies
show significant Zeeman-splitting detections, with 14 individual masing
components detected and line-of-sight magnetic field strengths ranging from
~0.5-18 mG. The detected field strengths are similar to those measured in
Galactic OH masers, suggesting that the local process of massive star formation
occurs under similar conditions in (U)LIRGs and the Galaxy, in spite of the
vastly different large-scale environments. Our measured field strengths are
also similar to magnetic field strengths in (U)LIRGs inferred from synchrotron
observations, implying that milligauss magnetic fields likely pervade most
phases of the interstellar medium in (U)LIRGs. These results provide a
promising new tool for probing the astrophysics of distant galaxies.Comment: 32 pages, 14 figures, 8 tables. Accepted for publication in The
Astrophysical Journal v680n2, June 20, 2008; corrected 2 typo
Pressure anisotropy and viscous heating in weakly collisional plasma turbulence
Pressure anisotropy can strongly influence the dynamics of weakly collisional, high-beta plasmas, but its effects are missed by standard magnetohydrodynamics (MHD). Small changes to the magnetic-field strength generate large pressure-anisotropy forces, heating the plasma, driving instabilities and rearranging flows, even on scales far above the particles’ gyroscales where kinetic effects are traditionally considered most important. Here, we study the influence of pressure anisotropy on turbulent plasmas threaded by a mean magnetic field (Alfvénic turbulence). Extending previous results that were concerned with Braginskii MHD, we consider a wide range of regimes and parameters using a simplified fluid model based on drift kinetics with heat fluxes calculated using a Landau-fluid closure. We show that viscous (pressure-anisotropy) heating dissipates between a quarter (in collisionless regimes) and half (in collisional regimes) of the turbulent-cascade power injected at large scales; this does not depend strongly on either plasma beta or the ion-to-electron temperature ratio. This will in turn influence the plasma's thermodynamics by regulating energy partition between different dissipation channels (e.g. electron and ion heat). Due to the pressure anisotropy's rapid dynamic feedback onto the flows that create it – an effect we term ‘magneto-immutability’ – the viscous heating is confined to a narrow range of scales near the forcing scale, supporting a nearly conservative, MHD-like inertial-range cascade, via which the rest of the energy is transferred to small scales. Despite the simplified model, our results – including the viscous heating rate, distributions and turbulent spectra – compare favourably with recent hybrid-kinetic simulations. This is promising for the more general use of extended-fluid (or even MHD) approaches to model weakly collisional plasmas such as the intracluster medium, hot accretion flows and the solar wind
Testing Radiatively-Inefficient Accretion Flow Theory: an XMM-Newton Observation of NGC 3998
We present the results of a 10 ks XMM-Newton observation of NGC 3998, a
``type-I'' LINER galaxy. Our goal is to test the extent to which
radiatively-inefficient accretion flow (RIAF) models and/or scaled-down AGN
models are consistent with the observed properties of NGC 3998. The upper-limit
for narrow Fe-K emission derived from a combined fit of the XMM-Newton and
BeppoSAX spectra is 25 eV, which is one of the strictest limits to date for any
AGN. This significantly rules out Fe-K emission as is expected to be observed
in typical Seyfert 1 galaxies. The lack of any reflection features suggests
that any optically-thick, geometrically-thin accretion disk must be truncated,
probably at a radius of order 100-300 R_s. RIAF models fit the UV to X-ray
spectral energy distribution of NGC 3998 reasonably well. In these models the
mid-IR flux also constrains the emission from any outer thin disk component
that might be present. The UV to X-ray SED is also consistent with a
Comptonized thin disk with a very low accretion rate, in which case the lack of
Fe-K emission may be due to an ionized accretion disk. Accretion models in
general do not account for the observed radio flux of NGC 3998, and the radio
flux may be due to a jet. Recent jet models may also be consistent with the
nuclear fluxes of NGC 3998 in general, including the X-ray, optical/UV and
mid-IR bands. We also derive nuclear fluxes using archival HST WFPC2 data to
constrain the SED of NGC 3998. We discuss a possible OM U band and USNO-B
detection of the NGC 3998 ULX.Comment: Accepted for publication in ApJ. 13 pages and 5 figures formatted
with emulateapj. Version with black-and-white only plots available at
http://www.pha.jhu.edu/~ptak/paper
Extreme Plasma Astrophysics
This is a science white paper submitted to the Astro-2020 and Plasma-2020
Decadal Surveys. The paper describes the present status and emerging
opportunities in Extreme Plasma Astrophysics -- a study of
astrophysically-relevant plasma processes taking place under extreme conditions
that necessitate taking into account relativistic, radiation, and QED effects.Comment: A science white paper submitted to the Astro-2020 and Plasma-2020
Decadal Surveys. 7 pages including cover page and references. Paper updated
in late March 2019 to include a several additional co-authors and references,
and a few small change
Low-Luminosity Accretion in Black Hole X-ray Binaries and Active Galactic Nuclei
At luminosities below a few percent of Eddington, accreting black holes
switch to a hard spectral state which is very different from the soft
blackbody-like spectral state that is found at higher luminosities. The hard
state is well-described by a two-temperature, optically thin, geometrically
thick, advection-dominated accretion flow (ADAF) in which the ions are
extremely hot (up to K near the black hole), the electrons are also
hot ( K), and thermal Comptonization dominates the X-ray
emission. The radiative efficiency of an ADAF decreases rapidly with decreasing
mass accretion rate, becoming extremely low when a source reaches quiescence.
ADAFs are expected to have strong outflows, which may explain why relativistic
jets are often inferred from the radio emission of these sources. It has been
suggested that most of the X-ray emission also comes from a jet, but this is
less well established.Comment: To appear in "From X-ray Binaries to Quasars: Black Hole Accretion on
All Mass Scales" edited by T. Maccarone, R. Fender, L. Ho, to be published as
a special edition of "Astrophysics and Space Science" by Kluwe
Radiative Models of Sagittarius A* and M87 from Relativistic MHD Simulations
Ongoing millimeter VLBI observations with the Event Horizon Telescope allow
unprecedented study of the innermost portion of black hole accretion flows.
Interpreting the observations requires relativistic, time-dependent physical
modeling. We discuss the comparison of radiative transfer calculations from
general relativistic MHD simulations of Sagittarius A* and M87 with current and
future mm-VLBI observations. This comparison allows estimates of the viewing
geometry and physical conditions of the Sgr A* accretion flow. The viewing
geometry for M87 is already constrained from observations of its large-scale
jet, but, unlike Sgr A*, there is no consensus for its millimeter emission
geometry or electron population. Despite this uncertainty, as long as the
emission region is compact, robust predictions for the size of its jet
launching region can be made. For both sources, the black hole shadow may be
detected with future observations including ALMA and/or the LMT, which would
constitute the first direct evidence for a black hole event horizon.Comment: 8 pages, 2 figures, submitted to the proceedings of AHAR 2011: The
Central Kiloparse
Hybrid viscosity and the magnetoviscous instability in hot, collisionless accretion disks
We aim to illustrate the role of hot protons in enhancing the
magnetorotational instability (MRI) via the ``hybrid'' viscosity, which is due
to the redirection of protons interacting with static magnetic field
perturbations, and to establish that it is the only relevant mechanism in this
situation. It has recently been shown by Balbus \cite{PBM1} and Islam & Balbus
\cite{PBM11} using a fluid approach that viscous momentum transport is key to
the development of the MRI in accretion disks for a wide range of parameters.
However, their results do not apply in hot, advection-dominated disks, which
are collisionless. We develop a fluid picture using the hybrid viscosity
mechanism, that applies in the collisionless limit. We demonstrate that viscous
effects arising from this mechanism can significantly enhance the growth of the
MRI as long as the plasma \beta \gapprox 80. Our results facilitate for the
first time a direct comparison between the MHD and quasi-kinetic treatments of
the magnetoviscous instability in hot, collisionless disks.Comment: To appear in the proceedings of the first Kodai-Trieste workshop on
Plasma Astrophysics (Aug 27-Sept 07 2007), Springer Astrophysics and Space
Science Proceedings serie
Properties of Accretion Flows Around Coalescing Supermassive Black Holes
What are the properties of accretion flows in the vicinity of coalescing
supermassive black holes (SBHs)? The answer to this question has direct
implications for the feasibility of coincident detections of electromagnetic
(EM) and gravitational wave (GW) signals from coalescences. Such detections are
considered to be the next observational grand challenge that will enable
testing general relativity in the strong, nonlinear regime and improve our
understanding of evolution and growth of these massive compact objects. In this
paper we review the properties of the environment of coalescing binaries in the
context of the circumbinary disk and hot, radiatively inefficient accretion
flow models and use them to mark the extent of the parameter space spanned by
this problem. We report the results from an ongoing, general relativistic,
hydrodynamical study of the inspiral and merger of black holes, motivated by
the latter scenario. We find that correlated EM+GW oscillations can arise
during the inspiral phase followed by the gradual rise and subsequent drop-off
in the light curve at the time of coalescence. While there are indications that
the latter EM signature is a more robust one, a detection of either signal
coincidentally with GWs would be a convincing evidence for an impending SBH
binary coalescence. The observability of an EM counterpart in the hot accretion
flow scenario depends on the details of a model. In the case of the most
massive binaries observable by the Laser Interferometer Space Antenna, upper
limits on luminosity imply that they may be identified by EM searches out to
z~0.1-1. However, given the radiatively inefficient nature of the gas flow, we
speculate that a majority of massive binaries may appear as low luminosity AGN
in the local universe.Comment: Revised version accepted to Class. Quantum Grav. for proceedings of
8th LISA Symposium. 15 pages, 3 figures, includes changes suggested in
referee report
Relativistic MHD Winds from Rotating Neutron Stars
We solve for the time-dependent dynamics of axisymmetric, general
relativistic magnetohydrodynamic winds from rotating neutron stars. The mass
loss rate is obtained self-consistently as a solution to the MHD equations,
subject to a finite thermal pressure at the stellar surface. We consider both
monopole and dipole magnetic field geometries and we explore the parameter
regime extending from low magnetization (low-sigma_o), almost thermally-driven
winds to high magnetization (high-sigma_o), relativistic Poynting-flux
dominated outflows. We compute the angular momentum and rotational energy loss
rates as a function of sigma_o and compare with analytic expectations from the
classical theory of pulsars and magnetized stellar winds. In the case of the
monopole, our high-sigma_o calculations asymptotically approach the analytic
force-free limit. If we define the spindown rate in terms of the open magnetic
flux, we similarly reproduce the spindown rate from recent force-free
calculations of the aligned dipole. However, even for sigma_o as high as ~20,
we find that the location of the Y-type point (r_Y), which specifies the radius
of the last closed field line in the equatorial plane, is not the radius of the
light cylinder R_L = c/omega (R = cylindrical radius), as has previously been
assumed in most estimates and force-free calculations. Instead, although the
Alfven radius at intermediate latitudes quickly approaches R_L as sigma_o
exceeds unity, r_Y remains significantly less than R_L. Because r_Y < R_L, our
calculated spindown rates thus exceed the classic ``vacuum dipole'' rate. We
discussthe implications of our results for models of rotation-powered pulsars
and magnetars, both in their observed states and in their hypothesized rapidly
rotating initial state.Comment: Accepted for publication in MNRAS. 19 pages, 21 figure
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