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

¹ LPC2E – Université d’Orléans/CNRS, France
e-mail: jean-mathias.griessmeier@cnrs-orleans.fr
² Station de Radioastronomie de Nançay, Observatoire de Paris – CNRS/INSU, USR 704 – Univ. Orléans, OSUC, Route de Souesmes, 18330 Nançay, France
³ Centre d’Études Nucléaires de Bordeaux Gradignan, IN2P3/CNRS, Université Bordeaux, 33175 Gradignan, France
⁴ Laboratoire d’Astrophysique de Bordeaux, Université Bordeaux, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁵ Laboratoire Univers et Théories LUTh, Observatoire de Paris, CNRS/INSU, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
⁶ College of Science, George Mason University, Fairfax, VA 22030, USA
⁷ Resident at Naval Research Laboratory, Washington, DC 20375, USA
⁸ Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352, USA
⁹ LESIA, Observatoire de Paris, CNRS, PSL, SU/UP/UO, 92195 Meudon, France
¹⁰ South African Radio Astronomy Observatory, 2 Fir Street, Black River Park, Observatory 7925, South Africa
¹¹ Department of Physics and Astronomy, University of the Western Cape, Bellville, Cape Town 7535, South Africa

Received: 19 March 2021
Accepted: 14 July 2021

Abstract

Context. The Fermi Large Area Telescope has detected over 260 gamma-ray pulsars. About one quarter of these are labeled as radio-quiet, that is they either have radio flux densities < 30 μJy at 1400 MHz, or they are not detected at all in the radio domain. In the population of nonrecycled gamma-ray pulsars, the fraction of radio-quiet pulsars is higher, about one half.

Aims. 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.

Methods. 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 band (110−190 MHz), with an average observing time of 13 h per target. Part of the data had to be discarded due to radio frequency interference. On average, we kept 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.

Results. 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.

Conclusions. 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.