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Neutron stars that show X-ray and $\gamma$-ray pulsed emission must,somewhere in the magnetosphere, generate electron-positron pairs. Such pairsare also required for radio emission, but then why do a number of these sourcesappear radio quiet? Here, we carried out a deep radio search towards four suchneutron stars that are isolated X-ray/$\gamma$-ray pulsars but for which noradio pulsations have been detected yet. These sources are 1RXSJ141256.0+792204 (Calvera), PSR J1958+2846, PSR J1932+1916 and SGR J1907+0919.Searching at lower radio frequencies, where the radio beam is thought to bewider, increases the chances of detecting these sources, compared to theearlier higher-frequency searches. We thus carried a search for periodic andsingle-pulse radio emission with the LOFAR radio telescope at 150 MHz. We usedthe known periods, and searched a wide range of dispersion measures, as thedistances are not well constrained. We did not detect pulsed emission from anyof the four sources. However, we put very constraining upper limits on theradio flux density at 150 MHz, of $\lesssim$1.4 mJy.

New upper limits on low-frequency radio emission from isolated neutron stars with LOFAR