29 research outputs found
Particle acceleration in rotating and shearing jets from AGN
We model the acceleration of energetic particles due to shear and centrifugal
effects in rotating astrophysical jets. The appropriate equation describing the
diffusive transport of energetic particles in a collisionless, rotating
background flow is derived and analytical steady state solutions are discussed.
In particular, by considering velocity profiles from rigid, over flat to
Keplerian rotation, the effects of centrifugal and shear acceleration of
particles scattered by magnetic inhomogeneities are distinguished. In the case
where shear acceleration dominates, it is confirmed that power law particle
momentum solutions exist, if the mean scattering
time is an increasing function of momentum. We show
that for a more complex interplay between shear and centrifugal acceleration,
the recovered power law momentum spectra might be significantly steeper but
flatten with increasing azimuthal velocity due to the increasing centrifugal
effects. The possible relevance of shear and centrifugal acceleration for the
observed extended emission in AGN is demonstrated for the case of the jet in
the quasar 3C273.Comment: 15 pages (including 8 pages Appendix), 4 figures; accepted for
publication in A&
Thin Disk Theory with a Non-Zero Torque Boundary Condition and Comparisons with Simulations
We present an analytical solution for thin disk accretion onto a Kerr black
hole that extends the standard Novikov-Thorne alpha-disk in three ways: (i) it
incorporates nonzero stresses at the inner edge of the disk, (ii) it extends
into the plunging region, and (iii) it uses a corrected vertical gravity
formula. The free parameters of the model are unchanged. Nonzero boundary
stresses are included by replacing the Novikov-Thorne no torque boundary
condition with the less strict requirement that the fluid velocity at the
innermost stable circular orbit is the sound speed, which numerical models show
to be the correct behavior for luminosities below ~30% Eddington. We assume the
disk is thin so we can ignore advection. Boundary stresses scale as alpha*h and
advection terms scale as h^2 (where h is the disk opening angle (h=H/r)), so
the model is self-consistent when h < alpha. We compare our solution with slim
disk models and general relativistic magnetohydrodynamic disk simulations. The
model may improve the accuracy of black hole spin measurements.Comment: 11 pages, 8 figures, MNRAS accepte
4U 1909+07: a well-hidden pearl
We present the first detailed spectral and timing analysis of the High Mass
X-ray Binary (HMXB) 4U 1909+07 with INTEGRAL and RXTE. 4U 1909+07 is detected
in the ISGRI 20-40 keV energy band with an average countrate of 2.6 cps. The
pulse period of ~604 sec is not stable, but changing erratically on timescales
of years. The pulse profile is strongly energy dependent: it shows a double
peaked structure at low energies, the secondary pulse decreases rapidly with
increasing energy and above 20 keV only the primary pulse is visible. This
evolution is consistent between PCA, HEXTE, and ISGRI. The phase averaged
spectrum can be well described by the sum of a photoabsorbed power law with a
cutoff at high energies and a blackbody component. To investigate the pulse
profile, we performed phase resolved spectral analysis. We find that the
changing spectrum can be best described with a variation of the folding energy.
We rule out a correlation between the black body component and the continuum
variation and discuss possible accretion geometries.Comment: 9 pages, 11 figures, accepted for publication in A&A Sect.
Jet Precession Driven by Neutrino-Cooled Disc for Gamma-Ray Bursts
A model of jet precession driven by a neutrino-cooled disc around a spinning
black hole is present in order to explain the temporal structure and spectral
evolution of gamma-ray bursts (GRBs). The differential rotation of the outer
part of a neutrino dominated accretion disc may result in precession of the
inner part of the disc and the central black hole, hence drives a precessed jet
via neutrino annihilation around the inner part of the disc. Both analytic and
numeric results for our model are present. Our calculations show that a black
hole-accretion disk system with black hole mass ,
accretion rate , spin parameter
and viscosity parameter may drive a precessed jet with period P=1
s and luminosity erg s, corresponding to the scenario for
long GRBs. A precessed jet with s and erg s may be
powered by a system with , , , and , possibly being responsible for the
short GRBs. Both the temporal and spectral evolution in GRB pulse may explained
with our model. GRB central engines likely power a precessed jet driven by a
neutrino-cooled disc. The global GRB lightcurves thus could be modulated by the
jet precession during the accretion timescale of the GRB central engine. Both
the temporal and spectral evolution in GRB pulse may be due to an viewing
effect due to the jet precession.Comment: 5 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
Modeling of non-stationary accretion disks in X-ray novae A 0620-00 and GRS 1124-68 during outburst
We address the task of modeling soft X-ray and optical light curves of X-ray
novae in the high/soft state. The analytic model of viscous evolution of an
externally truncated accretion \alpha-disk is used. Relativistic effects near a
Kerr black hole and self-irradiation of an accretion disk are taken into
account. The model is applied to the outbursts of X-ray nova Monocerotis 1975
(A 0620-00) and X-ray nova Muscae 1991 (GRS 1124-68). Comparison of
observational data with the model yields constraints on the angular momentum
(the Kerr parameter) of the black holes in A 0620-00 and GRS 1124-68: 0.3-0.6
and \leq 0.4, and on the viscosity parameter \alpha of the disks: 0.7-0.95 and
0.55-0.75. We also conclude that the accretion disks should have an effective
geometrical thickness 1.5-2 times greater than the theoretical value of the
distance between the photometric layers.Comment: 12 pages, 11 figures, 1 table, accepted for publication in A&A (minor
changens following the referee's comments, five references added
A Toy Model for Magnetized Neutrino-Dominated Accretion Flows
In this paper, we present a simplified model for magnetized
neutrino-dominated accretion flow (NDAF) in which effect of black hole (BH)
spin is taken into account by adopting a set of relativistic correction factor,
and the magnetic field is parameterized as \beta, the ratio of the magnetic
pressure to the total pressure. It is found that the disc properties are
sensitive to the values of the BH spin and \beta, and more energy can be
extracted from NDAF for the faster spin and lower \beta.Comment: 4 pages, 3 figures, accepted for publication in Science in China
Series
Black Hole Spin via Continuum Fitting and the Role of Spin in Powering Transient Jets
The spins of ten stellar black holes have been measured using the
continuum-fitting method. These black holes are located in two distinct classes
of X-ray binary systems, one that is persistently X-ray bright and another that
is transient. Both the persistent and transient black holes remain for long
periods in a state where their spectra are dominated by a thermal accretion
disk component. The spin of a black hole of known mass and distance can be
measured by fitting this thermal continuum spectrum to the thin-disk model of
Novikov and Thorne; the key fit parameter is the radius of the inner edge of
the black hole's accretion disk. Strong observational and theoretical evidence
links the inner-disk radius to the radius of the innermost stable circular
orbit, which is trivially related to the dimensionless spin parameter a_* of
the black hole (|a_*| < 1). The ten spins that have so far been measured by
this continuum-fitting method range widely from a_* \approx 0 to a_* > 0.95.
The robustness of the method is demonstrated by the dozens or hundreds of
independent and consistent measurements of spin that have been obtained for
several black holes, and through careful consideration of many sources of
systematic error. Among the results discussed is a dichotomy between the
transient and persistent black holes; the latter have higher spins and larger
masses. Also discussed is recently discovered evidence in the transient sources
for a correlation between the power of ballistic jets and black hole spin.Comment: 30 pages. Accepted for publication in Space Science Reviews. Also to
appear in hard cover in the Space Sciences Series of ISSI "The Physics of
Accretion onto Black Holes" (Springer Publisher). Changes to Sections 5.2,
6.1 and 7.4. Section 7.4 responds to Russell et al. 2013 (MNRAS, 431, 405)
who find no evidence for a correlation between the power of ballistic jets
and black hole spi
Foundations of Black Hole Accretion Disk Theory
This review covers the main aspects of black hole accretion disk theory. We
begin with the view that one of the main goals of the theory is to better
understand the nature of black holes themselves. In this light we discuss how
accretion disks might reveal some of the unique signatures of strong gravity:
the event horizon, the innermost stable circular orbit, and the ergosphere. We
then review, from a first-principles perspective, the physical processes at
play in accretion disks. This leads us to the four primary accretion disk
models that we review: Polish doughnuts (thick disks), Shakura-Sunyaev (thin)
disks, slim disks, and advection-dominated accretion flows (ADAFs). After
presenting the models we discuss issues of stability, oscillations, and jets.
Following our review of the analytic work, we take a parallel approach in
reviewing numerical studies of black hole accretion disks. We finish with a few
select applications that highlight particular astrophysical applications:
measurements of black hole mass and spin, black hole vs. neutron star accretion
disks, black hole accretion disk spectral states, and quasi-periodic
oscillations (QPOs).Comment: 91 pages, 23 figures, final published version available at
http://www.livingreviews.org/lrr-2013-
The Relational Power of Education: The immeasurability of knowledge, value and meaning
Recognizing the challenge of adequate evaluation in
higher education, this essay introduces some of the critical,
alternative-seeking conversation about educational measurement.
The thesis is that knowledge, value, and meaning emerge in the
relational dynamics of education, thus requiring complex
approaches to evaluation, utilizing relational criteria. The method
of the essay is to analyse two educational case studies à à à ¢ a travel
seminar and a classroom course à à à ¢ in dialogue with educational
literature and a process-relational philosophy of education.
Building from this analysis, the essay concludes with proposals for
relational criteria of evaluation: relations with self, community and
culture, difference, earth, and social structures