702 research outputs found
Hydrodynamic Simulation of the Cosmological X-ray Background
(Abridged) We use a hydrodynamic simulation of a LambdaCDM model to predict
the extragalactic X-ray background (XRB), focussing on emission from the
intergalactic medium (IGM). We also include X-rays from point sources
associated with galaxies in the simulation, and make maps of the angular
distribution of the emission. We find that filaments in the maps are not
evident, being diluted by projection. In the soft (0.5-2 keV) band, the mean
intensity of radiation from intergalactic and cluster gas is 2.3*10^-12
ergdeg^-2cm^-2s^-1, 35% of the total soft band emission. This is compatible at
the ~1 sigma level with estimates of the unresolved soft background from ROSAT
and {\it Chandra}. Only 4% of the hard (2-10 keV) emission is associated with
the IGM. Relative to AGN flux, the IGM component peaks at a lower redshift
(median z~0.45) so its clustering makes an important contribution to that of
the total XRB. The angular correlations on 0.1-10 arcmin scales are
significant, with an amplitude roughly consistent with an extrapolation of
recent ROSAT results to small scales. A cross-correlation of the XRB against
nearby galaxies taken from a simulated redshift survey also yields a strong
signal from the IGM. Although some recent papers have argued that the expected
soft band intensity from gas in galaxy, group, and cluster halos would exceed
XRB limits unless much of the gas is expelled by supernova feedback, we obtain
reasonable compatibility with current observations in a simulation that
incorporates cooling, star formation, and only modest feedback. A prediction of
our model is that the unresolved portion of the soft XRB will remain mostly
unresolved.Comment: Improved referencing of related papers. Submitted to ApJ, 19 pages,
17 postscript figures, most reduced in resolution, emulateapj.sty, for full
resolution version, see http://cfa-www.harvard.edu/~rcroft/xray.ps.g
Enhancement of flux-line pinning in all-oxide superconductor/ferromagnet heterostructures
We have studied the local critical current density, jc, in the superconductor
thin film of bilayer structures consisting of YBa2Cu3O7 and the ferromagnets
La2/3Ca1/3MnO3 and SrRuO3, respectively, by means of quantitative
magneto-optics. A pronounced hysteresis of jc was observed which is ascribed to
the magnetization state of the ferromagnetic layer. The results are discussed
within the frame of magnetic vortex - wall interactions.Comment: 9 page
Ultracold quantum gases in triangular optical lattices
Over the last years the exciting developments in the field of ultracold atoms
confined in optical lattices have led to numerous theoretical proposals devoted
to the quantum simulation of problems e.g. known from condensed matter physics.
Many of those ideas demand for experimental environments with non-cubic lattice
geometries. In this paper we report on the implementation of a versatile
three-beam lattice allowing for the generation of triangular as well as
hexagonal optical lattices. As an important step the superfluid-Mott insulator
(SF-MI) quantum phase transition has been observed and investigated in detail
in this lattice geometry for the first time. In addition to this we study the
physics of spinor Bose-Einstein condensates (BEC) in the presence of the
triangular optical lattice potential, especially spin changing dynamics across
the SF-MI transition. Our results suggest that below the SF-MI phase
transition, a well-established mean-field model describes the observed data
when renormalizing the spin-dependent interaction. Interestingly this opens new
perspectives for a lattice driven tuning of a spin dynamics resonance occurring
through the interplay of quadratic Zeeman effect and spin-dependent
interaction. We finally discuss further lattice configurations which can be
realized with our setup.Comment: 19 pages, 7 figure
Maximum black-hole spin from quasi-circular binary mergers
Black holes of mass M must have a spin angular momentum S below the Kerr
limit chi = S/M^2 < 1, but whether astrophysical black holes can attain this
limiting spin depends on their accretion history. Gas accretion from a thin
disk limits the black-hole spin to chi_gas < 0.9980 +- 0.0002, as
electromagnetic radiation from this disk with retrograde angular momentum is
preferentially absorbed by the black hole. Extrapolation of
numerical-relativity simulations of equal-mass binary black-hole mergers to
maximum initial spins suggests these mergers yield a maximum spin chi_eq <
0.95. Here we show that for smaller mass ratios q = m/M << 1, the superradiant
extraction of angular momentum from the larger black hole imposes a fundamental
limit chi_lim < 0.9979 +- 0.0001 on the final black-hole spin even in the
test-particle limit q -> 0 of binary black-hole mergers. The nearly equal
values of chi_gas and chi_lim imply that measurement of supermassive black-hole
spins cannot distinguish a black hole built by gas accretion from one assembled
by the gravitational inspiral of a disk of compact stellar remnants. We also
show how superradiant scattering alters the mass and spin predicted by models
derived from extrapolating test-particle mergers to finite mass ratios.Comment: final version accepted in PRD, new Fig.4 and discussio
Scaling Laws for Advection Dominated Flows: Applications to Low Luminosity Galactic Nuclei
We present analytical scaling laws for self-similar advection dominated
flows. The spectra from these systems range from 10 - 10 Hz, and
are determined by considering cooling of electrons through synchrotron,
bremsstrahlung, and Compton processes. We show that the spectra can be quite
accurately reproduced without detailed numerical calculations, and that there
is a strong testable correlation between the radio and X-ray fluxes from these
systems. We describe how different regions of the spectrum scale with the mass
of the accreting black hole, , the accretion rate of the gas, , and
the equilibrium temperature of the electrons, . We show that the universal
radio spectral index of 1/3 observed in most elliptical galaxies (Slee et al.
1994) is a natural consequence of self-absorbed synchrotron radiation from
these flows. We also give expressions for the total luminosity of these flows,
and the critical accretion rate, , above which the advection
solutions cease to exist. We find that for most cases of interest the
equilibrium electron temperature is fairly insensitive to , , and
parameters in the model. We apply these results to low luminosity black holes
in galactic nuclei. We show that the problem posed by Fabian & Canizares (1988)
of whether bright elliptical galaxies host dead quasars is resolved, as pointed
out recently by Fabian & Rees (1995), by considering advection-dominated flows.Comment: 30 pages, 5 postscript files. Accepted to ApJ. Also available
http://cfa-www.harvard.edu/~rohan/publications.htm
Origin of Intense Magnetic Fields Near Black Holes Due to Non-Minimal Gravitational-Electromagnetic Coupling
The origin of magnetic fields in astrophysical objects is a challenging
problem in astrophysics. Throughout the years, many scientists have suggested
that non-minimal gravitational-electromagnetic coupling (NMGEC) could be the
origin of the ubiquitous astrophysical magnetic fields. We investigate the
possible origin of intense magnetic fields by NMGEC near
rotating neutron stars and black holes, connected with magnetars, quasars, and
gamma-ray bursts. Whereas these intense magnetic fields are difficult to
explain astrophysically, we find that they are easily explained by NMGEC.Comment: 7 pages, accepted for publication in Phys. Lett.
A Possible Correlation Between the Luminosities and Lifetimes of Active Galactic Nuclei
We use the clustering of galaxies around distant AGN to show with ~90%
confidence that fainter AGN are longer lived. Our argument is simple: since the
measured galaxy-AGN cross-correlation length r_0 ~ 5h^-1 Mpc does not vary
significantly over a 10 magnitude range in AGN optical luminosity, faint and
bright AGN must reside in dark matter halos with similar masses. The halos that
host bright and faint AGN therefore have similar intrinsic abundances, and the
large observed variation in AGN number density with luminosity reflects a
change in duty cycle.Comment: 9 emulateapj pages, accepted for publication in the Ap
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