New Limits on Radio Emission from X-ray Dim Isolated Neutron Stars

TL;DR

This study conducted the most sensitive radio observations to date of six X-ray dim isolated neutron stars, finding no radio emission and setting new flux limits, which constrains their possible radio luminosities and beaming properties.

Contribution

It provides the first sensitive radio flux limits for X-ray dim isolated neutron stars and discusses implications for their radio emission and beaming characteristics.

Findings

No radio emission detected from the six neutron stars.

Flux limits are about 0.01 mJy for pulsed emission.

Results suggest these objects have lower radio luminosity or unfavorable beaming.

Abstract

We have carried out a search for radio emission at 820 MHz from six X-ray dim isolated neutron stars with the Robert C. Byrd Green Bank Radio Telescope. No transient or pulsed emission was found using fast folding, fast Fourier transform, and single-pulse searches. The corresponding flux limits are about 0.01 mJy for pulsed emission, depending on the integration time for the particular source and assuming a duty cycle of 2%, and 20 mJy for single dispersed pulses. These are the most sensitive limits to date on radio emission from X-ray dim isolated neutron stars. There is no evidence for isolated radio pulses, as seen in a class of neutron stars known as rotating radio transients. Our results imply that either the radio luminosities of these objects are lower than those of any known radio pulsars, or they could simply be long-period nearby radio pulsars with high magnetic fields beaming away from the Earth. To test the latter possibility, we would need around 40 similar sources to provide a 1 sigma probability of at least one of them beaming toward us. We also give a detailed description of our implementation of the Fast Folding Algorithm.