Pulsars are remarkable objects that emit across the entire electromagnetic spectrum, providing a powerful probe of the interstellar medium. In this study, we investigate the relation between dispersion measure (DM) and X-ray absorption column density N H using 68 radio pulsars detected at X-ray energies with the Chandra X-ray Observatory or XMM-Newton. We find a best-fit empirical linear relation of N H (10 20 cm −2 ) = 0.30 +0.13 −0.09 DM (pc cm −3 ), which corresponds to an average ionization of 10 +4 −3 %, confirming the ratio of one free electron per ten neutral hydrogen atoms commonly assumed in the literature. We also compare different N H estimates and note that some N H values obtained from X-ray observations are higher than the total Galactic Hi column density along the same line of sight, while the optical extinction generally gives the best N H predictions.

Emission from neutron stars and accretion disks in low-mass X-ray binaries is anisotropic. The non-spherical shape of the disk as well as blocking of the neutron star by the disk make the observed flux dependent on the inclination angle of the disk with respect to the line of sight. This is of importance for the interpretation of thermonuclear X-ray bursts from neutron stars. Because part of the X-ray burst is reflected off the disk, the observed burst flux depends on the anisotropies for both direct emission from the neutron star and reflection off the disk. This influences measurements of source distance, mass accretion rate, and constraints on the neutron star's equation of state. Previous predictions of the anisotropy factors assumed a geometrically flat disk. Detailed observations of two so-called superbursts allowed for the direct and the reflected burst fluxes to each be measured separately. The reflection fraction was much higher than what the anisotropies of a flat disk can account for. We create numerical models to calculate the anisotropy factors for different disk shapes, including concave disks. We present the anisotropy factors of the direct and reflected burst fluxes separately, as well as the anisotropy of the persistent flux. Reflection fractions substantially larger than unity are produced in the case where the inner accretion disk increases steeply in height, such that part of the star is blocked from view. Such a geometry could possibly be induced by the X-ray burst if X-ray heating causes the inner disk to puff up.

We present radiation-magneto-hydrodynamic simulations of star formation in selfgravitating, turbulent molecular clouds, modeling the formation of individual massive stars, including their UV radiation feedback. The set of simulations have cloud masses between m gas = 10 3 M to 3 × 10 5 M and gas densities typical of clouds in the local universe (n gas ∼ 1.8 × 10 2 cm −3 ) and 10× and 100× denser, expected to exist in high-redshift galaxies. The main results are: i) The observed Salpeter power-law slope and normalisation of the stellar initial mass function at the high-mass end can be reproduced if we assume that each star-forming gas clump (sink particle) fragments into stars producing on average a maximum stellar mass about 40% of the mass of the sink particle, while the remaining 60% is distributed into smaller mass stars. Assuming that the sinks fragment according to a power-law mass function flatter than Salpeter, with log-slope 0.8, satisfy this empirical prescription. ii) The star formation law that best describes our set of simulation is dρ * /dt ∝ ρ 1.5 gas if n gas < n cri ≈ 10 3 cm −3 , and dρ * /dt ∝ ρ 2.5 gas otherwise. The duration of the star formation episode is roughly 6 cloud's sound crossing times (with c s = 10 km/s). iii) The total star formation efficiency in the cloud is f * = 2%(m gas /10 4 M ) 0.4 (1 + n gas /n cri ) 0.91 , for gas at solar metallicity, while for metallicity Z < 0.1 Z , based on our limited sample, f * is reduced by a factor of ∼ 5. iv) The most compact and massive clouds appear to form globular cluster progenitors, in the sense that star clusters remain gravitationally bound after the gas has been expelled.

We calculate the hydrogen and helium-ionizing radiation escaping star forming molecular clouds, as a function of the star cluster mass and compactness, using a set of high-resolution radiation-magneto-hydrodynamic simulations of star formation in selfgravitating, turbulent molecular clouds. In these simulations, presented in He, Ricotti and Geen (2019), the formation of individual massive stars is well resolved, and their UV radiation feedback and lifetime on the main sequence are modelled selfconsistently. We find that the escape fraction of ionizing radiation from molecular clouds, f MC esc , decreases with increasing mass of the star cluster and with decreasing compactness. Molecular clouds with densities typically found in the local Universe have negligible f MC esc , ranging between 0.5% to 5%. Ten times denser molecular clouds have f MC esc ≈ 10% − 20%, while 100× denser clouds, which produce globular cluster progenitors, have f MC esc ≈ 20% − 60%. We find that f MC esc increases with decreasing gas metallicity, even when ignoring dust extinction, due to stronger radiation feedback. However, the total number of escaping ionizing photons decreases with decreasing metallicity because the star formation efficiency is reduced. We conclude that the sources of reionization at z > 6 must have been very compact star clusters forming in molecular clouds about 100× denser than in today's Universe, which lead to a significant production of old globular clusters progenitors.Recently, a handful of galaxies at high redshifts have been confirmed to have large Lyman continuum (LyC) escape fractions. Ion2 and Q1549-C25 are the only two z ∼ 3 galaxies with a direct spectroscopic detection of uncontaminated LyC emission (Vanzella et al. 2016;Shapley et al. 2016). Escape fractions of 50% is inferred for both of them. Vanzella et al. (2018) reported the highest redshift individually-confirmed LyC-leaky galaxy, Ion3, at z = 4. As a proxy for high-z galaxies, Izotov et al. (2018) selected local compact star-forming galaxies in the redshifts range z = 0.2993 − 0.4317, using the Cosmic Origins Spectrograph on HST. They found LyC emission with f esc in a range of 2-72 per cent. We should note that f gal esc in models of reion-