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The Fermi Large Area Telescope has detected over 260 gamma-ray pulsars. Aboutone quarter of these are labeled as radio-quiet. In the population ofnonrecycled gamma-ray pulsars, the fraction of radio-quiet pulsars is higher,about one half. Most radio observations of gamma-ray pulsars have beenperformed at frequencies between 300 MHz and 2 GHz. However, pulsar radiofluxes increase rapidly with decreasing frequency, and their radio beams oftenbroaden at low frequencies. As a consequence, some of these pulsars might bedetectable at low radio frequencies even when no radio flux is detected above300 MHz. Our aim is to test this hypothesis with low-frequency radioobservations. We have observed 27 Fermi-discovered gamma-ray pulsars with theinternational LOw Frequency ARray (LOFAR) station FR606 in single-station mode.We used the LOFAR high band antenna (HBA) band (110-190 MHz). On average, weuse 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. Wedo not detect radio pulsations from any of the 27 sources, and we establishstringent upper limits on their low-frequency radio fluxes. These nondetectionsare compatible with the upper limits derived from radio observations at otherfrequencies. We also determine the pulsars' geometry from the gamma-rayprofiles to see for which pulsars the low-frequency radio beam is expected tocross Earth. This set of observations provides the most constraining upperlimits on the flux density at 150 MHz for 27 radio-quiet gamma-ray pulsars. Inspite of the beam-widening expected at low radio frequencies, most of ournondetections can be explained by an unfavorable viewing geometry; for theremaining observations, especially those of pulsars detected at higherfrequencies, the nondetection is compatible with insufficient sensitivity.

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