New upper limits on low-frequency radio emission from isolated neutron stars with LOFAR

TL;DR

This study used LOFAR at 150 MHz to search for radio emissions from four isolated neutron stars that are X-ray and gamma-ray pulsars but lack detected radio signals, setting strict upper limits on their radio flux.

Contribution

It provides the first deep low-frequency radio limits on these neutron stars, exploring the possibility of radio emission at frequencies where beams may be wider.

Findings

No pulsed radio emission detected from the four sources.

Established upper limits of approximately 1.4 mJy on radio flux density at 150 MHz.

Results suggest these neutron stars are radio quiet at low frequencies within the sensitivity limits.

Abstract

Neutron stars that show X-ray and $\gamma$-ray pulsed emission must, somewhere in the magnetosphere, generate electron-positron pairs. Such pairs are also required for radio emission, but then why do a number of these sources appear radio quiet? Here, we carried out a deep radio search towards four such neutron stars that are isolated X-ray/$\gamma$-ray pulsars but for which no radio pulsations have been detected yet. These sources are 1RXS J141256.0+792204 (Calvera), PSR J1958+2846, PSR J1932+1916 and SGR J1907+0919. Searching at lower radio frequencies, where the radio beam is thought to be wider, increases the chances of detecting these sources, compared to the earlier higher-frequency searches. We thus carried a search for periodic and single-pulse radio emission with the LOFAR radio telescope at 150 MHz. We used the known periods, and searched a wide range of dispersion measures, as the distances are not well constrained. We did not detect pulsed emission from any of the four sources. However, we put very constraining upper limits on the radio flux density at 150 MHz, of $\lesssim$1.4 mJy.