Volume 654, October 2021
|Number of page(s)||14|
|Section||Galactic structure, stellar clusters and populations|
|Published online||08 October 2021|
Follow-up of 27 radio-quiet gamma-ray pulsars at 110–190 MHz using the international LOFAR station FR606
LPC2E – Université d’Orléans/CNRS, France
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
Accepted: 14 July 2021
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.
Key words: pulsars: general / telescopes / ISM: general
© J.-M. Grießmeier et al. 2021
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