11,696 research outputs found
The Non-Linear Dependence of Flux on Black Hole Mass and Accretion Rate in Core Dominated Jets
We derive the non-linear relation between the core flux F_{nu} of accretion
powered jets at a given frequency and the mass M of the central compact object.
For scale invariant jet models, the mathematical structure of the equations
describing the synchrotron emission from jets enables us to cancel out the
model dependent complications of jet dynamics, retaining only a simple, model
independent algebraic relation between F_{nu} and M. This approach allows us to
derive the F_{nu}-M relation for any accretion disk scenario that provides a
set of input boundary conditions for the magnetic field and the relativistic
particle pressure in the jet, such as standard and advection dominated
accretion flow (ADAF) disk solutions. Surprisingly, the mass dependence of
F_{nu} is very similar in different accretion scenarios. For typical
flat-spectrum core dominated radio jets and standard accretion scenarios we
find F_{nu}~M^{17/12}. The 7-9 orders of magnitude difference in black hole
mass between microquasars and AGN jets imply that AGN jets must be about 3-4
orders of magnitude more radio loud than microquasars, i.e., the ratio of radio
to bolometric luminosity is much smaller in microquasars than in AGN jets.
Because of the generality of these results, measurements of this F_{nu}-M
dependence are a powerful probe of jet and accretion physics. We show how our
analysis can be extended to derive a similar scaling relation between the
accretion rate mdot and F_{nu} for different accretion disk models. For
radiatively inefficient accretion modes we find that the flat spectrum emission
follows F_{nu}~(mdot*M)^{17/12}.Comment: Added key words and acknowledgements, minor editorial corrections. 6
pages, to appear in MNRAS 343, L59-L6
Fast and dense magneto-optical traps for Strontium
We improve the efficiency of sawtooth-wave-adiabatic-passage (SWAP) cooling
for strontium atoms in three dimensions and combine it with standard
narrow-line laser cooling. With this technique, we create strontium
magneto-optical traps with bosonic Sr (
fermionic Sr) atoms at phase-space densities of
(). Our method is simple to implement and is faster and more
robust than traditional cooling methods.Comment: 9 pages, 6 figure
A deep Chandra observation of Abell 4059: a new face to radio-mode AGN feedback?
A deep Chandra observation of the cooling core cluster Abell 4059 (A4059) is
presented. Previous studies have found two X-ray cavities in the central
regions of A4059 together with a ridge of X-ray emission 20kpc south-west of
the cluster center. These features are clearly related to the radio galaxy
PKS2354-35 which resides in the cD galaxy. Our new data confirm these previous
findings and strengthen previous suggestions that the south-western ridge is
colder and denser than, but in approximate pressure equilibrium with, the
surrounding ICM atmosphere. In addition, we find evidence for a weak shock that
wraps around the north and east sides of the cavity structure. Our data allow
us to map the 2-dimensional distribution of metals in the ICM of A4059 for the
first time. We find that the SW ridge possesses an anomalously high
(super-solar) metalicity. The unusual morphology, temperature structure and
metal distribution all point to significant asymmetry in the ICM atmosphere
prior to the onset of radio-galaxy activity. Motivated by the very high
metalicity of the SW ridge, we hypothesize that the ICM asymmetry was caused by
the extremely rapid stripping of metal enriched gas from a starburst galaxy
that plunged through the core of A4059. Furthermore, we suggest that the onset
of powerful radio-galaxy activity in the cD galaxy may have been initiated by
this starburst/stripping event, either via the tidal-shocking of cold gas
native to the cD galaxy, or the accretion of cold gas that had been stripped
from the starburst galaxy.Comment: Accepted for publication in the Astrophysical Journal. 12 pages, 11
figures. A version of this paper including full resolution figures can be
found at http://www.astro.umd.edu/~chris/publications/papers/a4059_2008.pd
An RXTE study of M87 and the core of the Virgo cluster
We present hard X-ray observations of the nearby radio galaxy M87 and the
core of the Virgo cluster using the Rossi X-ray Timing Explorer. These are the
first hard X-ray observations of M87 not affected by contamination from the
nearby Seyfert 2 galaxy NGC4388. Thermal emission from Virgo's intracluster
medium is clearly detected and has a spectrum indicative of kT=2.5keV plasma
with approximately 25% cosmic abundances. No non-thermal (power-law) emission
from M87 is detected in the hard X-ray band, with fluctuations in the Cosmic
X-ray Background being the limiting factor. Combining with ROSAT data, we infer
that the X-ray spectrum of the M87 core and jet must be steep (Gamma_core>1.90$
and Gamma_jet>1.75), and we discuss the implications of this result. In
particular, these results are consistent with M87 being a mis-aligned BL-Lac
object.Comment: 8 pages, 2 postscript figures. Accepted for publication in Ap
Symmetry constraints for the emission angle dependence of Hanbury Brown--Twiss radii
We discuss symmetry constraints on the azimuthal oscillations of two-particle
correlation (Hanbury Brown--Twiss interferometry) radii for non-central
collisions between equal spherical nuclei. We also propose a new method for
correcting in a model-independent way the emission angle dependent correlation
function for finite event plane resolution and angular binning effects.Comment: 8 pages revtex4, 2 tables, no figures. Short Section VI added and
correction algorithm in Section VII made more explicit. Submitted to Physical
Review
A fundamental plane of black hole activity
We examine the disc--jet connection in stellar mass and supermassive black
holes by investigating the properties of their compact emission in the X-ray
and radio bands. We compile a sample of ~100 active galactic nuclei with
measured mass, 5 GHz core emission, and 2-10 keV luminosity, together with 8
galactic black holes with a total of ~50 simultaneous observations in the radio
and X-ray bands. Using this sample, we study the correlations between the radio
(L_{R}) and the X-ray (L_{X}) luminosity and the black hole mass (M). We find
that the radio luminosity is correlated with {\em both} M and L_{X}, at a
highly significant level. In particular, we show that the sources define a
``fundamental plane'' in the three-dimensional (log L_{R},log L_{X},log M)
space, given by log L_{R}=(0.60^{+0.11}_{-0.11}) log L_{X}
+(0.78^{+0.11}_{-0.09}) log M + 7.33^{+4.05}_{-4.07}, with a substantial
scatter of \sigma_{R}=0.88. We compare our results to the theoretical relations
between radio flux, black hole mass, and accretion rate derived by Heinz and
Sunyaev (2003). Such relations depend only on the assumed accretion model and
on the observed radio spectral index. Therefore, we are able to show that the
X-ray emission from black holes accreting at less than a few per cent of the
Eddington rate is unlikely to be produced by radiatively efficient accretion,
and is marginally consistent with optically thin synchrotron emission from the
jet. On the other hand, models for radiatively inefficient accretion flows seem
to agree well with the data.Comment: 21 pages, 8 figures (2 in colour). Revised version accepted for
publication by MNRAS. Improved and extended discussio
Shock heating by FR I radio sources in galaxy clusters
Feedback by active galactic nuclei (AGN) is frequently invoked to explain the
cut-off of the galaxy luminosity function at the bright end and the absence of
cooling flows in galaxy clusters. Meanwhile, there are recent observations of
shock fronts around radio-loud AGN. Using realistic 3D simulations of jets in a
galaxy cluster, we address the question what fraction of the energy of active
galactic nuclei is dissipated in shocks. We find that weak shocks that
encompass the AGN have Mach numbers of 1.1-1.2 and dissipate at least 2% of the
mechanical luminosity of the AGN. In a realistic cluster medium, even a
continuous jet can lead to multiple shock structures, which may lead to an
overestimate of the AGN duty cycles inferred from the spatial distribution of
waves.Comment: accepted by MNRAS Letter
Thermalization vs. Isotropization & Azimuthal Fluctuations
Hydrodynamic description requires a local thermodynamic equilibrium of the
system under study but an approximate hydrodynamic behaviour is already
manifested when a momentum distribution of liquid components is not of
equilibrium form but merely isotropic. While the process of equilibration is
relatively slow, the parton system becomes isotropic rather fast due to the
plasma instabilities. Azimuthal fluctuations observed in relativistic heavy-ion
collisions are argued to distinguish between a fully equilibrated and only
isotropic parton system produced in the collision early stage.Comment: 12 pages, presented at `Correlations and Fluctuations in Relativistic
Nuclear Collisions', MIT, April 05, minor correction
State-Dependent Optical Lattices for the Strontium Optical Qubit
We demonstrate state-dependent optical lattices for the Sr optical qubit at
the tune-out wavelength for its ground state. We tightly trap excited state
atoms while suppressing the effect of the lattice on ground state atoms by more
than four orders of magnitude. This highly independent control over the qubit
states removes inelastic excited state collisions as the main obstacle for
quantum simulation and computation schemes based on the Sr optical qubit. Our
results also reveal large discrepancies in the atomic data used to calibrate
the largest systematic effect of Sr optical lattice clocks.Comment: 6 pages, 4 figures + 6 pages supplemental materia
Impact of tangled magnetic fields on AGN-blown bubbles
There is growing consensus that feedback from AGN is the main mechanism
responsible for stopping cooling flows in clusters of galaxies. AGN are known
to inflate buoyant bubbles that supply mechanical power to the intracluster gas
(ICM). High Reynolds number hydrodynamical simulations show that such bubbles
get entirely disrupted within 100 Myr, as they rise in cluster atmospheres,
which is contrary to observations. This artificial mixing has consequences for
models trying to quantify the amount of heating and star formation in cool core
clusters of galaxies. It has been suggested that magnetic fields can stabilize
bubbles against disruption. We perform MHD simulations of fossil bubbles in the
presence of tangled magnetic fields using the high order PENCIL code. We focus
on the physically-motivated case where thermal pressure dominates over magnetic
pressure and consider randomly oriented fields with and without maximum
helicity and a case where large scale external fields drape the bubble.We find
that helicity has some stabilizing effect. However, unless the coherence length
of magnetic fields exceeds the bubble size, the bubbles are quickly shredded.
As observations of Hydra A suggest that lengthscale of magnetic fields may be
smaller then typical bubble size, this may suggest that other mechanisms, such
as viscosity, may be responsible for stabilizing the bubbles. However, since
Faraday rotation observations of radio lobes do not constrain large scale ICM
fields well if they are aligned with the bubble surface, the draping case may
be a viable alternative solution to the problem. A generic feature found in our
simulations is the formation of magnetic wakes where fields are ordered and
amplified. We suggest that this effect could prevent evaporation by thermal
conduction of cold Halpha filaments observed in the Perseus cluster.Comment: accepted for publication in MNRAS, (downgraded resolution figures,
color printing recommended
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