305 research outputs found
Double Compton and Cyclo-Synchrotron in Super-Eddington Disks, Magnetized Coronae, and Jets
We present an extension to the general relativistic radiation
magnetohydrodynamic code HARMRAD to account for emission and absorption by
thermal cyclo-synchrotron, double Compton, bremsstrahlung, low-temperature OPAL
opacities as well as Thomson and Compton scattering. We approximate the
radiation field as a Bose-Einstein distribution and evolve it using the
radiation number-energy-momentum conservation equations in order to track
photon hardening. We perform various simulations to study how these extensions
affect the radiative properties of magnetically-arrested disks accreting at
Eddington to super-Eddington rates. We find that double Compton dominates
bremsstrahlung in the disk within a radius of (gravitational
radii) at a hundred times the Eddington accretion rate, and within smaller
radii at lower accretion rates. Double Compton and cyclo-synchrotron regulate
radiation and gas temperatures in the corona, while cyclo-synchrotron regulates
temperatures in the jet. Interestingly, as the accretion rate drops to
Eddington, an optically thin corona develops whose gas temperature of K is times higher than the disk's black body temperature. Our
results show the importance of double Compton and synchrotron in
super-Eddington disks, magnetized coronae, and jets.Comment: 25 pages, 14 figures, 3 tables, submitted to MNRA
A Hydrodynamical Approach to CMB mu-distortions
Spectral distortion of the cosmic microwave background provides a unique
opportunity to probe primordial perturbations on very small scales by
performing large-scale measurements. We discuss in a systematic and pedagogic
way all the relevant physical phenomena involved in the production and
evolution of the mu-type spectral distortion. Our main results agree with
previous estimates (in particular we show that a recently found factor of 3/4
arises from relativistic corrections to the wave energy). We also discuss
several subleading corrections such as adiabatic cooling and the effects of
bulk viscosity, baryon loading and photon heat conduction. Finally we calculate
the transfer function for mu-distortions between the end of the mu-era and now.Comment: 45 page
Is a Classical Language Adequate in Assessing the Detectability of the Redshifted 21cm Signal from the Early Universe?
The classical radiometer equation is commonly used to calculate the
detectability of the 21cm emission by diffuse cosmic hydrogen at high
redshifts. However, the classical description is only valid in the regime where
the occupation number of the photons in phase space is much larger than unity
and they collectively behave as a classical electromagnetic field. At redshifts
z<20, the spin temperature of the intergalactic gas is dictated by the
radiation from galaxies and the brightness temperature of the emitting gas is
in the range of mK, independently from the existence of the cosmic microwave
background. In regions where the observed brightness temperature of the 21cm
signal is smaller than the observed photon energy, of 68/(1+z) mK, the
occupation number of the signal photons is smaller than unity. Neverethless,
the radiometer equation can still be used in this regime because the weak
signal is accompanied by a flood of foreground photons with a high occupation
number (involving the synchrotron Galactic emission and the cosmic microwave
background). As the signal photons are not individually distinguishable, the
combined signal+foreground population of photons has a high occupation number,
thus justifying the use of the radiometer equation.Comment: 4 pages, Accepted for publication in JCA
TiNi-based films for elastocaloric microcooling. Fatigue life and device performance
The global trend of miniaturization and concomitant increase of functionality in microelectronics, microoptics, and various other fields in microtechnology leads to an emerging demand for temperature control at small scales. In this realm, elastocaloric cooling is an interesting alternative to thermoelectrics due to the large latent heat and good down-scaling behavior. Here, we investigate the elastocaloric effect due to a stress-induced phase transformation in binary TiNi and quaternary TiNiCuCo films of 20 μm thickness produced by DC magnetron sputtering. The mesoscale mechanical and thermal performance, as well as the fatigue behavior are studied by uniaxial tensile tests combined with infrared thermography and digital image correlation measurements. Binary films exhibit strong features of fatigue, involving a transition from Lüders-like to homogeneous transformation behavior within three superelastic cycles. Quaternary films, in contrast, show stable Lüders-like transformation without any signs of degradation. The elastocaloric temperature change under adiabatic conditions is −15 K and −12 K for TiNi and TiNiCuCo films, respectively. First-of-its-kind heat pump demonstrators are developed that make use of out-of-plane deflection of film bridges. Owing to their large surface-to-volume ratio, the demonstrators reveal rapid heat transfer. The TiNiCuCo-based devices, for instance, generate a temperature difference of 3.5 K within 13 s. The coefficients of performance of the demonstrators are about 3
Cosmological parameters constraints from galaxy cluster mass function measurements in combination with other cosmological data
We present the cosmological parameters constraints obtained from the
combination of galaxy cluster mass function measurements (Vikhlinin et al.,
2009a,b) with new cosmological data obtained during last three years: updated
measurements of cosmic microwave background anisotropy with Wilkinson Microwave
Anisotropy Probe (WMAP) observatory, and at smaller angular scales with South
Pole Telescope (SPT), new Hubble constant measurements, baryon acoustic
oscillations and supernovae Type Ia observations.
New constraints on total neutrino mass and effective number of neutrino
species are obtained. In models with free number of massive neutrinos the
constraints on these parameters are notably less strong, and all considered
cosmological data are consistent with non-zero total neutrino mass \Sigma m_\nu
\approx 0.4 eV and larger than standard effective number of neutrino species,
N_eff \approx 4. These constraints are compared to the results of neutrino
oscillations searches at short baselines.
The updated dark energy equation of state parameters constraints are
presented. We show that taking in account systematic uncertainties, current
cluster mass function data provide similarly powerful constraints on dark
energy equation of state, as compared to the constraints from supernovae Type
Ia observations.Comment: Accepted for publication in Astronomy Letter
Creation of the CMB spectrum: precise analytic solutions for the blackbody photosphere
The blackbody spectrum of CMB was created in the blackbody photosphere at
redshifts z>2x10^6. At these early times, the Universe was dense and hot enough
that complete thermal equilibrium between baryonic matter (electrons and ions)
and photons could be established. Any perturbation away from the blackbody
spectrum was suppressed exponentially. New physics, for example annihilation
and decay of dark matter, can add energy and photons to CMB at redshifts z>10^5
and result in a Bose-Einstein spectrum with a non-zero chemical potential
(). Precise evolution of the CMB spectrum around the critical redshift of
z~2x10^6 is required in order to calculate the -type spectral distortion
and constrain the underlying new physics. Although numerical calculation of
important processes involved (double Compton process, comptonization and
bremsstrahlung) is not difficult, analytic solutions are much faster and easier
to calculate and provide valuable physical insights. We provide precise (better
than 1%) analytic solutions for the decay of , created at an earlier
epoch, including all three processes, double Compton, Compton scattering on
thermal electrons and bremsstrahlung in the limit of small distortions. This is
a significant improvement over the existing solutions with accuracy ~10% or
worse. We also give a census of important sources of energy injection into CMB
in standard cosmology. In particular, calculations of distortions from
electron-positron annihilation and primordial nucleosynthesis illustrate in a
dramatic way the strength of the equilibrium restoring processes in the early
Universe. Finally, we point out the triple degeneracy in standard cosmology,
i.e., the and distortions from adiabatic cooling of baryons and
electrons, Silk damping and annihilation of thermally produced WIMP dark matter
are of similar order of magnitude (~ 10^{-8}-10^{-10})
The first spectral line surveys searching for signals from the Dark Ages
Our aim is to observationally investigate the cosmic Dark Ages in order to
constrain star and structure formation models, as well as the chemical
evolution in the early Universe. Spectral lines from atoms and molecules in
primordial perturbations at high redshifts can give information about the
conditions in the early universe before and during the formation of the first
stars in addition to the epoch of reionisation. The lines may arise from moving
primordial perturbations before the formation of the first stars (resonant
scattering lines), or could be thermal absorption or emission lines at lower
redshifts. The difficulties in these searches are that the source redshift and
evolutionary state, as well as molecular species and transition are unknown,
which implies that an observed line can fall within a wide range of
frequencies. The lines are also expected to be very weak. Observations from
space have the advantages of stability and the lack of atmospheric features
which is important in such observations. We have therefore, as a first step in
our searches, used the Odin satellite to perform two sets of spectral line
surveys towards several positions. The first survey covered the band 547-578
GHz towards two positions, and the second one covered the bands 542.0-547.5 GHz
and 486.5-492.0 GHz towards six positions selected to test different sizes of
the primordial clouds. Two deep searches centred at 543.250 and 543.100 GHz
with 1 GHz bandwidth were also performed towards one position. The two lowest
rotational transitions of H2 will be redshifted to these frequencies from
z~20-30, which is the predicted epoch of the first star formation. No lines are
detected at an rms level of 14-90 and 5-35 mK for the two surveys,
respectively, and 2-7 mK in the deep searches with a channel spacing of 1-16
MHz. The broad bandwidth covered allows a wide range of redshifts to be
explored for a number of atomic and molecular species and transitions. From the
theoretical side, our sensitivity analysis show that the largest possible
amplitudes of the resonant lines are about 1 mK at frequencies <200 GHz, and a
few micro K around 500-600 GHz, assuming optically thick lines and no
beam-dilution. However, if existing, thermal absorption lines have the
potential to be orders of magnitude stronger than the resonant lines. We make a
simple estimation of the sizes and masses of the primordial perturbations at
their turn-around epochs, which previously has been identified as the most
favourable epoch for a detection. This work may be considered as an important
pilot study for our forthcoming observations with the Herschel Space
Observatory.Comment: 15 pages, 9 figures, 3 on-line pages. Accepted for publication in
Astronomy & Astrophysics 8 March 2010
HeII->HeI Recombination of Primordial Helium Plasma Including the Effect of Neutral Hydrogen
The HeII->HeI recombination of primordial helium plasma (z = 1500 - 3000) is
considered in terms of the standard cosmological model. This process affects
the formation of cosmic microwave background anisotropy and spectral
distortions. We investigate the effect of neutral hydrogen on the HeII->HeI
recombination kinetics with partial and complete redistributions of radiation
in frequency in the HeI resonance lines. It is shown that to properly compute
the HeII->HeI recombination kinetics, one should take into account not only the
wings in the absorption and emission profiles of the HeI resonance lines, but
also the mechanism of the redistribution of resonance photons in frequency.
Thus, for example, the relative difference in the numbers of free electrons for
the model using Doppler absorption and emission profiles and the model using a
partial redistribution in frequency is 1 - 1.3% for the epoch z = 1770 - 1920.
The relative difference in the numbers of free electrons for the model using a
partial redistribution in frequency and the model using a complete
redistribution in frequency is 1 - 3.8% for the epoch z = 1750 - 2350.Comment: 28 pages, 9 figures, 2 tabel
Probing the last scattering surface through the recent and future CMB observations
We have constrained the extended (delayed and accelerated) models of hydrogen
recombination, by investigating associated changes of the position and the
width of the last scattering surface. Using the recent CMB and SDSS data, we
find that the recent data constraints favor the accelerated recombination
model, though the other models (standard, delayed recombination) are not ruled
out at 1- confidence level. If the accelerated recombination had
actually occurred in our early Universe, baryonic clustering on small-scales is
likely to be the cause of it. By comparing the ionization history of baryonic
cloud models with that of the best-fit accelerated recombination model, we find
that some portion of our early Universe has baryonic underdensity. We have made
the forecast on the PLANCK data constraint, which shows that we will be able to
rule out the standard or delayed recombination models, if the recombination in
our early Universe had proceeded with or lower, and
residual foregrounds and systematic effects are negligible.Comment: v2: matched with the accepted version (conclusions unchanged
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