Implications of ELT observations of pulsars, anomalous X-ray pulsars and supernova remnants

Abstract

Supernova remnants are the remains of the outer envelope of the supernova progenitor star. Neutron stars are one of the possible remnants of the region where the explosion detonated. Our knowledge, from galactic studies, of SNRs is well established as their morphology can be understood in terms of the initial blast and out-gassing during the last stages of the progenitor. However, observationally the link between neutron stars and SNRs is only poorly established. During a supernova there are a number of possibilities to produce a condensed remnant - no remnant; a neutron star (pulsar?); a magnetar; a black hole or something more exotic. We do not know what fraction of supernovae go down these possible paths. In the ELT era we will have the first real opportunity to sample the pulsar population in external galaxies and get a more comprehensive survey of optical emission from local pulsars. Such a survey would have significantly reduced biases compared to the current state of radio surveys particularly in the area of pulsar-SNR statistics. Furthermore, a 50m telescope will be able to survey galaxies out to at least 20 Mpc for young SNRs using H$\alpha$:[OIII] and H$\alpha$:[SII] ratios. Currently there are over 1500 radio pulsars detected, 14 of which have been observed at optical wavelengths. Although small in number the family of optical pulsars yield much useful information in bridging the gap between the long wavelength radio emission and the high energy gamma-ray emission from pulsars. Specifically optical techniques are currently the only way of detecting polarisation in the high-energy regime. The advent of ELTs will increase the detection rate of local galactic pulsars and provides the possibility of detecting a significant number of extragalactic optical pulsars. Phenomenologically, the Pacini scaling law predicts $\sim$150 galactic pulsars to have pulsed optical emission with an m$_{V} <$32. Using SKA, Crab-like giant radio pulses should be detectable out to 7 Mpc. In contrast Crab-like pulsars would have a normal peak pulsed m$_{V}\,\sim$31 at 10 Mpc making ELT optical observations more sensitive than radio observations and the best method for extragalactic pulsar discovery. To date only five AXPs have observed IR emission and two optical emission. ELTs will be able to sample the AXP population within the Galaxy as well as the local group again providing better statistics for the birth rate of AXPs compared to ‘normal’ pulsars. A combination of an AXP and ‘normal’ pulsar survey will make a significant contribution to the birthrate question - what fraction of supernovae produce pulsars compared to AXPs and other condensed objects?