377 research outputs found
Powering Anomalous X-ray Pulsars by Neutron Star Cooling
Using recently calculated analytic models for the thermal structure of
ultramagnetized neutron stars, we estimate the thermal fluxes from young
( yr) ultramagnetized ( G) cooling neutron stars.
We find that the pulsed X-ray emission from objects such as 1E 1841-045 and 1E
2259+586 as well as many soft-gamma repeaters can be explained by photon
cooling if the neutron star possesses a thin insulating envelope of matter of
low atomic weight at densities g/cm. The total mass
of this insulating layer is .Comment: 8 pages, 1 figure, to appear in Ap.J. Letters (one reference entry
corrected, no other changes
Quantifying and Controlling Prethermal Nonergodicity in Interacting Floquet Matter
The use of periodic driving for synthesizing many-body quantum states depends crucially on the existence of a prethermal regime, which exhibits drive-tunable properties while forestalling the effects of heating. This dependence motivates the search for direct experimental probes of the underlying localized nonergodic nature of the wave function in this metastable regime. We report experiments on a many-body Floquet system consisting of atoms in an optical lattice subjected to ultrastrong sign-changing amplitude modulation. Using a double-quench protocol, we measure an inverse participation ratio quantifying the degree of prethermal localization as a function of tunable drive parameters and interactions. We obtain a complete prethermal map of the drive-dependent properties of Floquet matter spanning four square decades of parameter space. Following the full time evolution, we observe sequential formation of two prethermal plateaux, interaction-driven ergodicity, and strongly frequency-dependent dynamics of long-time thermalization. The quantitative characterization of the prethermal Floquet matter realized in these experiments, along with the demonstration of control of its properties by variation of drive parameters and interactions, opens a new frontier for probing far-from-equilibrium quantum statistical mechanics and new possibilities for dynamical quantum engineering
Characterizing Entanglement Entropy Produced by Non-Linear Scalar Interactions During Inflation
The density fluctuations that we observe in the universe today are thought to
originate from quantum fluctuations produced during a phase of the early
universe called inflation. By evolving a wavefunction describing two coupled
Fourier modes of a scalar field forward through an inflationary epoch, we
demonstrate that non-linear effects can result in a generation of entanglement
entropy between modes with different momenta in a scalar field during the
inflationary period when just one of the modes is observed. Through this
mechanism, the field would experience decoherence and appear more like a
classical distribution today; however the mechanism is not sufficiently
efficient to explain classicality. We find that the amount of entanglement
entropy generated scales roughly as a power law S \propto \lambda^{1.75}, where
\lambda is the coupling coefficient of the non-linear potential term. We also
investigate how the entanglement entropy scales with the duration of inflation
and compare various entanglement measures from the literature with the von
Neumann entropy. This demonstration explicitly follows particle creation and
interactions between modes; consequently, the mechanism contributing to the
generation of the von Neumann entropy can be easily seen.Comment: 11 pages, 9 figures, version accepted by Phys. Rev.
Anomalous X-ray Pulsars and Soft gamma-ray Repeaters: Spectral Fits and the Magnetar Model
The energy source powering the X-ray emission from anomalous X-ray pulsars
(AXPs) and soft gamma-ray repeaters (SGRs) is still uncertain. In one scenario,
the presence of an ultramagnetized neutron star, or ``magnetar'', with B on the
order of 10^{14} - 10^{15} G is invoked. To investigate this hypothesis, we
have analyzed archival ASCA data for several known AXPs and SGRs, and fitted
them with a model in which all or part of the X-ray flux originates as thermal
emission from a magnetar. Our magnetar spectral model includes the effects of
the anisotropy of the heat flow through an ultramagnetized neutron star
envelope, reprocessing by a light element atmosphere, and general relativistic
corrections to the observed spectrum. We obtain good fits to the data with
radii for the emitting areas which are generally consistent with those expected
for neutron stars, in contrast to blackbody (BB) fits, which imply much smaller
radii. Furthermore, the inclusion of atmospheric effects results in inferred
temperatures which are lower than those implied by BB fits, but however still
too high to be accounted by thermal cooling alone. An extra source of heating
(possibly due to magnetic field decay) is needed. Despite the harder tail in
the spectrum produced by reprocessing of the outgoing flux through the
atmosphere, spectral fits still require a considerable fraction of the flux to
be in a power-law component.Comment: 14 pages, 2 tables, 1 figure, ApJ in press; note added to Table
1.65mic (H-band) surface photometry of galaxies. VI: The history of star formation in normal late-type galaxies
We have collected a large body of NIR (H band), UV (2000 A) and Halpha
measurements of late-type galaxies. These are used, jointly with spectral
evolutionary synthesis models, to study the initial mass function (IMF) in the
mass range m > 2 Mo. For spirals (Sa-Sd), Magellanic irregulars (Im) and blue
compact dwarfs (BCD), our determination is consistent with a Salpeter IMF with
an upper mass cutoff M_up = 80 Mo. The history of star formation and the amount
of total gas (per unit mass) of galaxies are found to depend primarily on their
total masses (as traced by the H band luminosities) and only secondarily on
morphological type. The present star formation activity of massive spirals is
up to 100 times smaller than that average over their lifetime, while in low
mass galaxies it is comparable to or higher than that at earlier epochs. Dwarf
galaxies have presently larger gas reservoirs per unit mass than massive
spirals. The efficiency in transforming gas into stars and the time scale for
gas depletion (10 Gyrs) are independent of the luminosity and/or of the
morphological type. These evidences are consistent with the idea that galaxies
are coeval systems,that they evolved as closed-boxes forming stars following a
simple, universal star formation law whose characteristic time scale is small
(1 Gyr) in massive spirals and large (10 Gyr) in low mass galaxies. A similar
conclusion was drawn by Gavazzi and Scodeggio (1996) to explain the
colour-magnitude relation of late-type galaxies. The consequences of this
interpretation on the evolution of the star formation rate and of the gas
density per comoving volume of the Universe with look-back time are discussed.Comment: LaTex, 24 pages, 12 figures, accepted for publication on Astronomical
Journa
Black hole mergers in the universe
Mergers of black-hole binaries are expected to release large amounts of
energy in the form of gravitational radiation. However, binary evolution models
predict merger rates too low to be of observational interest. In this paper we
explore the possibility that black holes become members of close binaries via
dynamical interactions with other stars in dense stellar systems. In star
clusters, black holes become the most massive objects within a few tens of
millions of years; dynamical relaxation then causes them to sink to the cluster
core, where they form binaries. These black-hole binaries become more tightly
bound by superelastic encounters with other cluster members, and are ultimately
ejected from the cluster. The majority of escaping black-hole binaries have
orbital periods short enough and eccentricities high enough that the emission
of gravitational radiation causes them to coalesce within a few billion years.
We predict a black-hole merger rate of about per year per
cubic megaparsec, implying gravity wave detection rates substantially greater
than the corresponding rates from neutron star mergers. For the first
generation Laser Interferometer Gravitational-Wave Observatory (LIGO-I), we
expect about one detection during the first two years of operation. For its
successor LIGO-II, the rate rises to roughly one detection per day. The
uncertainties in these numbers are large. Event rates may drop by about an
order of magnitude if the most massive clusters eject their black hole binaries
early in their evolution.Comment: 12 pages, ApJL in pres
Strange Star Heating Events as a Model for Giant Flares of Soft Gamma-ray Repeaters
Two giant flares were observed on 5 March 1979 and 27 August 1998 from the
soft gamma-ray repeaters SGR 0526-66 and SGR 1900+14, respectively. The
striking similarity between these remarkable bursts strongly implies a common
nature. We show that the light curves of the giant bursts may be easily
explained in the model where the burst radiation is produced by the bare quark
surface of a strange star heated, for example, by impact of a massive
comet-like object.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Letter
Weak Lensing Reconstruction and Power Spectrum Estimation: Minimum Variance Methods
Large-scale structure distorts the images of background galaxies, which
allows one to measure directly the projected distribution of dark matter in the
universe and determine its power spectrum. Here we address the question of how
to extract this information from the observations. We derive minimum variance
estimators for projected density reconstruction and its power spectrum and
apply them to simulated data sets, showing that they give a good agreement with
the theoretical minimum variance expectations. The same estimator can also be
applied to the cluster reconstruction, where it remains a useful reconstruction
technique, although it is no longer optimal for every application. The method
can be generalized to include nonlinear cluster reconstruction and photometric
information on redshifts of background galaxies in the analysis. We also
address the question of how to obtain directly the 3-d power spectrum from the
weak lensing data. We derive a minimum variance quadratic estimator, which
maximizes the likelihood function for the 3-d power spectrum and can be
computed either from the measurements directly or from the 2-d power spectrum.
The estimator correctly propagates the errors and provides a full correlation
matrix of the estimates. It can be generalized to the case where redshift
distribution depends on the galaxy photometric properties, which allows one to
measure both the 3-d power spectrum and its time evolution.Comment: revised version, 36 pages, AAS LateX, submitted to Ap
Discovery of 5.16s pulsations from the isolated neutron star RBS 1223
The isolated neutron star candidate RBS 1223 was observed with the Advanced
CCD Imaging Spectrometer aboard the Chandra X-ray observatory on 2000 June 24.
A timing analysis of the data yielded a periodic modulation with a period
P=5.1571696^(+1.57*10^(-4) -1.36*10^(-4)s. Using ROSAT HRI archived
observations we detected a period P=5.1561274 \pm 4.4*10^(-4)s and determined
period derivative dP/dt=(0.7 - 2.0)*10^(-11) s*s^(-1). The detection of this
period and dP/dt indicates that RBS 12223 has a ``characteristic'' age of
6000-12000 years and huge magnetic field at the surface (B(dipole)~(1.7-
3.2)*10^(+14) G) typical for anomalous X-ray pulsars (AXPs).Comment: 7 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysic
First IXPE Observations of the Accreting X-ray Pulsar Her X-1
Theoretical models for the X-ray emission of accretion-powered pulsars predict a high degree and a strong spin-phase dependence of the X-ray polarization. Using observations of the Imaging X-ray Polarimetry Explorer of the accreting pulsar Her X-1, we were able to test these predictions for the first time ever
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