Gamma-ray astronomy

Abstract:

Most population studies of pulsars have hitherto focused on the disc of the Galaxy, the Galactic centre, globular clusters, and nearby galaxies. It is expected that pulsars, by virtue of their natal kicks, are also to be found in the Galactic halo. We investigate the possible population of canonical (i.e. non-recycled) radio pulsars in the halo, estimating the number of such pulsars, and the fraction that is detectable via single pulse and periodicity searches. Additionally, we explore the distributions of flux densities and dispersion measures (DMs) of this population. We also consider the effects of different velocity models and the evolution of inclination angle and magnetic field on our results. We show that ∼33  % of all pulsars beaming towards the Earth are in the halo but the fraction reduces to ∼1.5  % if we let the inclination angle and the magnetic field evolve as a falling exponential. Moreover, the fraction that is detectable is significantly limited by the sensitivity of surveys. This population would be most effectively probed by surveys using time-domain periodicity search algorithms. The current non-detections of pulsars in the halo can be explained if we assume that the inclination angle and magnetic field of pulsars evolve with time. We also highlight a possible confusion between bright pulses from halo pulsars and fast radio bursts with low DMs where further follow-up is warranted.

Abstract:

Here, we present initial results from the ALFABURST radio transient survey, which is currently running in a commensal mode with the ALFA receiver at the Arecibo telescope. We observed for a total of 1400 hours and have detected single pulses from known pulsars but did not detect any FRBs. The non-detection of FRBs is consistent with the current FRB sky rates.

Abstract:

Accreting stellar-mass black holes appear to populate two branches in a radio:X-ray luminosity plane. We have investigated the X-ray variability properties of a large number of black hole low-mass X-ray binaries, with the aim of unveiling the physical reasons underlying the radio-loud/radio-quiet nature of these sources, in the context of the known accretion–ejection connection. A reconsideration of the available radio and X-ray data from a sample of black hole X-ray binaries confirms that being radio-quiet is the more normal mode of behaviour for black hole binaries. In the light of this we chose to test, once more, the hypothesis that radio-loudness could be a consequence of the inclination of the X-ray binary. We compared the slope of the ‘hard-line’ (an approximately linear correlation between X-ray count rate and rms variability, visible in the hard states of active black holes), the orbital inclination, and the radio-nature of the sources of our sample. We found that high-inclination objects show steeper hard-lines than low-inclination objects, and tend to display a radio-quiet nature (with the only exception of V404 Cyg), as opposed to low-inclination objects, which appear to be radio-loud(er). While in need of further confirmation, our results suggest that – contrary to what has been believed for years – the radio-loud/quiet nature of black-hole low-mass X-ray binaries might be an inclination effect, rather than an intrinsic source property. This would solve an important issue in the context of the inflow–outflow connection, thus providing significant constraints to the models for the launch of hard-state compact jets.