arXiv subjects

J. Chluba

Publications and source records attributed to J. Chluba.

64 records · Page 4Linked to original sources

arXiv2007

Cosmological hydrogen recombination: Lyn line feedback and continuum escape

We compute the corrections to the cosmological hydrogen recombination history due to delayed feedback of Lyman-series photons and the escape in the Lyman-continuum. The former process is expected to slightly delay recombination, while the latter should allow the medium to recombine a bit faster. It is shown that the subsequent feedback of released Lyman-n photons on the lower lying Lyman-(n-1) transitions yields a maximal correction of DN_e/N_e 0.22% at z~ 1050. Including only Lyman-βfeedback onto the Lyman-αtransition, accounts for most of the effect. We find corrections to the cosmic microwave background TT and EE power spectra \change{with typical peak to peak amplitude |DC^{TT}_l/C^{TT}_l|~0.15% and |ΔC^{EE}_l/C^{EE}_l|~0.36% at l<~3000. The escape in the Lyman-continuum and feedback of Lyman-αphotons on the photoionization rate of the second shell lead to modifications of the ionization history which are very small (less than |DN_e/N_e|~few x 10^{-6}).

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arXiv2007

The double Compton emissivity in a mildly relativistic thermal plasma within the soft photon limit

We provide simple and accurate analytic approximations for the low frequency double Compton emission coefficient that are applicable in a broad range of physical situations up to mildly relativistic temperatures. These approximations may be useful for checking in which circumstances the double Compton process is important. We perform series expansions of the double Compton emission integral for low energies of the incident photon and electron and compare the derived analytic expressions with the results obtained by numerical integrations of the full double Compton cross section. We explicitly derived analytic approximations to the low frequency double Compton emission coefficient for initial monochromatic photons and Wien spectra. We show that combining the analytic approximations given in this paper, an accuracy of better than 5% over a very broad range of temperatures and under various physical conditions can be achieved. The double Compton emissivity strongly depends on the ratio of the incoming photon and electron energies: for hard photons and cold electrons the emission is strongly suppressed compared to the case of similar photon and electron energy, whereas in the opposite situation, i.e. hot electrons and soft initial photons, the emission is enhanced. For photons and electrons close to thermodynamic equilibrium the double Compton emissivity increases less rapidly with temperature than in the Lightman-Thorne approximation and the corrections exceed ~10% for temperatures above 4 keV.

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arXiv2006

Cosmological hydrogen recombination: populations of the high level sub-states

We present results for the spectral distortions of the Cosmic Microwave Background (CMB) arising due to bound-bound transitions during the epoch of cosmological hydrogen recombination at frequencies down to nu~100MHz. We extend our previous treatment of the recombination problem now including the main collisional processes and following the evolution of all the hydrogen angular momentum sub-states for up to 100 shells. We show that, due to the low baryon density of the Universe, even within the highest considered shell full statistical equilibrium (SE) is not reached and that at low frequencies the recombination spectrum is significantly different when assuming full SE for n>2. We also directly compare our results for the ionization history to the output of the Recfast code, showing that especially at low redshifts rather big differences arise. In the vicinity of the Thomson visibility function the electron fraction differs by roughly -0.6% which affects the temperature and polarization power spectra by <~1%. Furthermore we shortly discuss the influence of free-free absorption and line broadening due to electron scattering on the bound-bound recombination spectrum and the generation of CMB angular fluctuations due to scattering of photons within the high shells.

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arXiv2006

Free-bound emission from cosmological hydrogen recombination

In this paper we compute the emission coming from the direct recombination of free electrons to a given shell (n>=2) during the epoch of cosmological hydrogen recombination. This contribution leads to a total of one photon per recombined hydrogen atom and therefore a ~30-88% increase of the recombination spectrum within the frequency range 1 GHz<=nu<=100 GHz. In particular the Balmer-continuum emission increases the distortion at nu~ 690 GHz by ~92%. With our 100 shell calculations for the hydrogen atom we find that a total of ~5 photons per hydrogen atom are emitted when including all the bound-bound transitions, the 2s two-photon decay channel and the optically thin free-bound transitions. Since the direct recombination continuum at high n is very broad only a few n-series continuua are distinguishable and most of this additional emission below nu<~30 GHz is completely featureless.

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arXiv2006

Lines in the Cosmic Microwave Background Spectrum from the Epoch of Cosmological Hydrogen Recombination

We compute the spectral distortions of the Cosmic Microwave Background (CMB) arising during the epoch of cosmological hydrogen recombination within the standard cosmological (concordance) model for frequencies in the range 1 GHz-3500 GHz. We follow the evolution of the populations of the hydrogen levels including states up to principle quantum number $n=30$ in the redshift range $500\leq z\leq 3500$. All angular momentum sub-states are treated individually, resulting in a total number of 465 hydrogen levels. The evolution of the matter temperature and the fraction of electrons coming from HeII are also included. We present a detailed discussion of the distortions arising from the main dipolar transitions, e.g. Lyman and Balmer series, as well as the emission due to the two-photon decay of the hydrogen 2s level. Furthermore, we investigate the robusteness of the results against changes in the number of shells considered. The resulting spectral distortions have a characteristic oscillatory behaviour, which might allow experimentalists to separate them from other backgrounds. The relative distortion of the spectrum exceeds a value of $10^{-7}$ at wavelengths longer than 21cm. Our results also show the importance of detailed follow-up of the angular momentum sub-states, and their effect on the amplitude of the lines. The effect on the residual electron fraction is only moderate, and mainly occurs at low redshifts. The CMB angular power spectrum is changed by less than 1%. Finally, our computations show that if the primordial radiation field is described by a pure blackbody, then there is no significant emission from any hydrogen transition at redshifts greater than $z \sim 2000$. This is in contrast to some earlier works, where the existence of a `pre-recombination' peak was claimed.

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arXiv2006

Accretion of helium and metal-rich gas onto neutron stars and black holes at high luminosities

Ultraluminous X-ray sources fed by Wolf-Rayet star winds and X-ray bursters in ultracompact binaries with He or C white dwarfs have accretion disks, whose properties may significantly differ from those of pure H alpha-accretion disks. We have therefore included the dependence on charge number Z and mean molecular weights mu_{e/I} into the Shakura and Sunyaev (1973) scaling relations for the key parameters of the disk. Furthermore, we also consider the case of the pseudo-Newtonian potential of Paczynsky and Wiita (1980). These scaling relations might become useful, e.g., when estimating the illumination efficiency of the external parts of the disk. We also address the changes in the structure of the boundary (spreading) layer on the surface of neutron stars, occurring in the case of H depleted accretion disks.

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arXiv2005

Induced two-photon decay of the 2s level and the rate of cosmological hydrogen recombination

Induced emission due to the presence of soft CMB photons slightly increases the two-photon decay rate of the 2s level of hydrogen defining the rate of cosmological recombination. This correspondingly changes the degree of ionization, the visibility function and the resulting primordial temperature anisotropies and polarization of the CMB on the percent level. These changes exceed the precision of the widely used CMBFAST and CAMB codes by more than one order of magnitude and can be easily taken into account.

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arXiv2005

Clusters of galaxies in the microwave band: influence of the motion of the Solar System

In this work we consider the changes of the SZ cluster brightness, flux and number counts induced by the motion of the Solar System with respect to the frame defined by the cosmic microwave background (CMB). These changes are connected with the Doppler effect and aberration and exhibit a strong spectral and spatial dependence. The correction to the SZ cluster brightness and flux has an amplitude and spectral dependence, which is similar to the first order cluster peculiar velocity correction to the thermal SZ effect. Due to the change in the received cluster CMB flux the motion of the Solar System induces a dipolar asymmetry in the observed number of clusters above a given flux level. Similar effects were discussed for $γ$-ray bursts and radio galaxies, but here, due to the very peculiar frequency-dependence of the thermal SZ effect, the number of observed clusters in one direction of the sky can be both, decreased or increased depending on the frequency band. A detection of this asymmetry should be possible using future full sky CMB experiments with mJy sensitivities.

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arXiv2004

Superposition of blackbodies and the dipole anisotropy: A possibility to calibrate CMB experiments

The CMB angular temperature fluctuations observed by COBE and WMAP enable us to place a lower limit on the spectral distortions of the CMB at any angular scale. These distortions are connected with the simple fact, that the superposition of blackbodies with different temperatures in general is not a blackbody. We show, that in the limit of small temperature fluctuations, the superposition of blackbodies leads to a y-type spectral distortion. It is known, that the CMB dipole induces a y-type spectral distortion with quadrupole and monopole angular distribution leading to a corresponding whole sky y-parameter of y~2.6\cdot 10^{-7}.We show here that taking the difference of the CMB signal in the direction of the maximum and minimum of the CMB dipole due to the superposition of two blackbodies leads to a spectral distortion with y~3.1\cdot 10^{-6}. The amplitude of this distortion can be calculated to the same precision as the CMB dipole, i.e. 0.3% today. Therefore it may be used as a source with brightness of several or tens of \muK to cross calibrate and calibrate different frequency channels of CMB surveys with precision of a few tens or hundreds of nK. Furthermore, we show in this work that primordial anisotropies for multipoles 2\leq l \leq 1000 also lead to spectral distortions but with much smaller y-parameter, i.e. y\sim 10^{-11}-10^{-9}.

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arXiv2002

Kinetic Sunyaev-Zeldovich effect from galaxy cluster rotation

We show how the temperature and the polarisation of the cosmic microwave background are affected by bulk rotation of clusters of galaxies owing to the kinetic Sunyaev-Zeldovich effect. The main effects of rotation are (i) a shift of the position of the peak of the temperature fluctuation relative to the center of the cluster by a few percent of the core radius and (ii) a tilt of the direction of the plane of linear polarisation by several degrees.

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