Pulsars, transients and relativistic astrophysics

Abstract:

We investigate the relation between the radio (Fr) and optical (Fo) flux densities of a variety of classes of radio transients and variables, with the aim of analysing whether this information can be used, in the future, to classify such events. Using flux density values between 1 and 10 GHz and the optical bands V and R, we build a sample with a total of 12 441 Fr and Fo measurements. The sample contains both Galactic objects, such as stellar sources and X-ray binaries, and extragalactic objects, such as gamma-ray bursts and quasars. By directly comparing the two parameters, it is already possible to distinguish between the Galactic and extragalactic populations. Although individual classes are harder to separate from the Fr − Fo parameter space to a high accuracy, and can only provide approximations, the basic approach provides an already useful foundation to develop a more accurate classification technique. In addition, we illustrate how example objects from different classes move in the parameter space as they evolve over time, offering a feature that could be used to reduce the confusion between classes. A small, blind test of the classification performance is also undertaken using a catalogue of VLA FIRST transient and variable sources, to demonstrate the advantages and current limitations of the approach. With more multiwavelength data becoming available in the future, we discuss other classification techniques which the Fr − Fo method could be combined with and potentially become an important part of an automatic radio transient classification system.

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.