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.
Highlights
The Large Area Telescope (LAT) on the Fermi satellite has strongly increased the number of known gamma-ray pulsars
The large differences in the upper limits for different targets result from four effects: (a) The number of 1 h observations was different for different targets; (b) during some nights, the radio frequency interference (RFI) conditions were worse than during others, forcing us to remove a larger number of observations from processing; (c) the low elevation of some sources led to a low effective oanbdse(rdv)indgeptiemndeitnoegffbs despite a large number of observations Nobs; on the direction, the background sky temperature Tsky varied by more than one order of magnitude
This is related to their proximity to the Galactic plane (Galactic coordinates are given in Table 1, Cols. 2 and 3), which causes a high sky temperature (Tsky, Table 1 Col. 11), and at the same time this reduces the effective area of the telescope because of their low obtained declination
Summary
The Large Area Telescope (LAT) on the Fermi satellite has strongly increased the number of known gamma-ray pulsars (see e.g., the “Fermi 2nd Pulsar Catalog” Abdo et al 2013, hereafter 2PC). A major update, “3PC”, which is in preparation, will characterize at least 260 gamma-ray pulsars. As radio pulsars (Clark et al 2018) Some of these are very faint: In 2PC, a pulsar is designated as radio-quiet (RQ) if its flux density at 1400 MHz, S 1400, is
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