Follow-up of 27 radio-quiet gamma-ray pulsars at 110-190 MHz using the international LOFAR station FR606

TL;DR

This study used LOFAR to observe 27 gamma-ray pulsars at low radio frequencies, setting upper limits on their fluxes and exploring whether beam widening at low frequencies could reveal previously undetected radio emissions.

Contribution

First low-frequency radio observations of 27 gamma-ray pulsars were conducted, providing the most stringent flux upper limits and analyzing beam geometry effects on detectability.

Findings

No radio pulsations detected at 150 MHz for any pulsar.

Most nondetections are due to unfavorable viewing geometry.

Some nondetections are consistent with sensitivity limits at higher frequencies.

Abstract

The Fermi Large Area Telescope has detected over 260 gamma-ray pulsars. About one quarter of these are labeled as radio-quiet. In the population of nonrecycled gamma-ray pulsars, the fraction of radio-quiet pulsars is higher, about one half. Most radio observations of gamma-ray pulsars have been performed at frequencies between 300 MHz and 2 GHz. However, pulsar radio fluxes increase rapidly with decreasing frequency, and their radio beams often broaden at low frequencies. As a consequence, some of these pulsars might be detectable at low radio frequencies even when no radio flux is detected above 300 MHz. Our aim is to test this hypothesis with low-frequency radio observations. We have observed 27 Fermi-discovered gamma-ray pulsars with the international LOw Frequency ARray (LOFAR) station FR606 in single-station mode. We used the LOFAR high band antenna (HBA) band (110-190 MHz). On average, we use 9 h of observation per target after the removal of affected datasets, resulting in a sensitivity for pulse-averaged flux on the order of 1-10 mJy. We do not detect radio pulsations from any of the 27 sources, and we establish stringent upper limits on their low-frequency radio fluxes. These nondetections are compatible with the upper limits derived from radio observations at other frequencies. We also determine the pulsars' geometry from the gamma-ray profiles to see for which pulsars the low-frequency radio beam is expected to cross Earth. This set of observations provides the most constraining upper limits on the flux density at 150 MHz for 27 radio-quiet gamma-ray pulsars. In spite of the beam-widening expected at low radio frequencies, most of our nondetections can be explained by an unfavorable viewing geometry; for the remaining observations, especially those of pulsars detected at higher frequencies, the nondetection is compatible with insufficient sensitivity.