Flux density upper limits for 27 radio-quiet gamma-ray pulsars, based on observations with LOFAR/FR606. Empty red triangles (with arrows): Flux density limits from previous observations. Filled green circles: Flux density measurements from previous observations (a typical error bar of 30% is indicated). Empty blue squares (with arrows): Flux density limits from this study. Dark blue line: Equivalent flux density limits for our observation (assuming a spectral index of −1.6). Shaded area between the light blue lines: Equivalent flux density limits for our observation for a spectral index of −1.6 ± 0.54. Dark green line: Same as the dark blue line, but with respect to the most constraining detection. Shaded area between the light green lines: Same as the shaded area between the light blue lines, but with respect to the most constraining detection.