Radiation Backgrounds at Cosmic Dawn: X-Rays from Compact Binaries

We compute the expected X-ray diffuse background and radiative feedback on the intergalactic medium (IGM) from X-ray binaries prior and during the epoch of reionization. The cosmic evolution of compact binaries is followed using a population synthesis technique that treats separately neutron stars and black hole binaries in different spectral states and is calibrated to reproduce the observed X-ray properties of galaxies at z<4. Together with an updated empirical determination of the cosmic history of star formation, recent modeling of the stellar mass-metallicity relation, and a scheme for absorption by the IGM that accounts for the presence of ionized HII bubbles during the epoch of reionization, our detailed calculations provide refined predictions of the X-ray volume emissivity and filtered radiation background from "normal" galaxies at z>6. Radiative transfer effects modulate the background spectrum, which shows a characteristic peak between 1 and 2 keV. While the filtering of X-ray radiation through the IGM slightly increases the mean excess energy per photoionization, it also weakens the radiation intensity below 1 keV, lowering the mean photoionization and heating rates. Numerical integration of the rate and energy equations shows that the contribution of X-ray binaries to the ionization of the bulk IGM is negligible, with the electron fraction never exceeding 1%. Direct HeI photoionizations are the main source of IGM heating, and the temperature of the largely neutral medium in between HII cavities increases above the temperature of the cosmic microwave background (CMB) only at z0.1. Therefore, in this scenario, it is only at relatively late epochs that the bulk of neutral intergalactic hydrogen may be observable in 21-cm emission against the CMB.Comment: 14 pages, 8 figures, accepted for publication in The Astrophysical Journa

Lagrangian modelling of nonlinear waves in optical fibres

The Lagrangian perturbation method for the NLS is revisited in the form of an equivalent direct problem. The analogy can be extended to arbitrarily perturbed systems. It is then possible to provide first order perturbation expansions for the fundamental soliton. The case of the damped NLS is considered and shown to fully comply with IST predictions. Subsequently the problem of NLS initial condition not corresponding to an exact soliton is examined. There are two issues that need to be considered: the location of the soliton solution and the modelling of the continuum. The location of the soliton solution is handled by considering the integrals of motion of the NLS. The improvement arises by the inclusion of the contributions due to the continuum. The results are compared with numerical calculations and are proved to be satisfactory provided that the initial pulse shape does not depart greatly from the Asech(z) functional form. The propagation problem is handled by considering the evolution of the soliton and the continuum separately and recombining them at the required time. Two cases are considered: the far field pattern and the position where the peak of the soliton lies. For the former the recombination of continuum with the soliton is achieved with the help of the inverse part of the IST. For the peak position a Bäcklund transform is considered. Results from both regimes are compared with numerical results and shown to agree satisfactorily

On the origin of the metallicity dependence in dynamically formed extragalactic low-mass X-ray binaries

Globular clusters (GCs) effectively produce dynamically-formed low-mass X-ray binaries (LMXBs). Observers detect ~100 times more LMXBs per stellar mass in GCs compared to stars in the fields of galaxies. It has also been observationally established that metal-rich GCs are about 3 times more likely to contain an X-ray source than their metal-poor counterparts. Recent observations have shown that this ratio holds in extragalactic GCs for all bright X-ray sources with Lx between 2X10^{37} and 5X10^{38} erg/s. In this Letter, we propose that the observed metallicity dependence of LMXBs in extragalactic GCs can be explained by the differences in the number densities and average masses of red giants in populations of different metallicities. Red giants serve as seeds for the dynamical production of bright LMXBs via two channels - binary exchanges and physical collisions - and the increase of the number densities and masses of red giants boost LMXB production, leading to the observed difference. We also discuss a possible effect of the age difference in stellar populations of different metallicities.Comment: 5 pages, 2 figures, submitted to ApJ

Constraining population synthesis models via the binary neutron star population

The observed sample of double neutron-star (NS-NS) binaries presents a challenge to population-synthesis models of compact object formation: the parameters entering into these models must be carefully chosen so as to match (i) the observed star formation rate and (ii) the formation rate of NS-NS binaries, which can be estimated from the observed sample and the selection effects related to the discoveries with radio-pulsar surveys. In this paper, we select from an extremely broad family of possible population synthesis models those few (2%) which are consistent with the observed sample of NS-NS binaries. To further sharpen the constraints the observed NS-NS population places upon our understanding of compact-object formation processes, we separate the observed NS-NS population into two channels: (i) merging NS-NS binaries, which will inspiral and merge through the action of gravitational waves within $10$ Gyr, and (ii) wide NS-NS binaries, consisting of all the rest. With the subset of astrophysically consistent models, we explore the implications for the rates at which double black hole (BH-BH), black hole-neutron star (BH-NS), and NS-NS binaries will merge through the emission of gravitational waves.Comment: (v1) Submitted to ApJ. Uses emulateapj.cls. 8 pages, 7 figures. (v2) Minor textual changes in response to referee queries. Substantial additions in appendicies, including a detailed discussion of sample multidimensional population synthesis fit