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

Neutron stars that show X-ray and ?-ray pulsed emission must generate electron-positron pairs somewhere in the magnetosphere. Pairs like this are also required for radio emission, which poses the question why a number of these sources appear to be radio quiet. We carried out a deep radio search toward four such neutron stars that are isolated X-ray or ?-ray pulsars, but for which no radio pulsations have been detected so far. These sources are 1RXS J141256.0+792204 (Calvera), PSR J1958+2846, PSR J1932+1916, and SGR J1907+0919. A search at lower radio frequencies, where the radio beam is thought to be wider, increases the chances of detecting these sources compared to the earlier higher-frequency searches. We thus carried out a search for periodic and single-pulse radio emission with the LOFAR radio telescope at 150 MHz. We used the known periods and searched a wide range of dispersion measures because the distances are only poorly constrained. We did not detect pulsed emission from any of the four sources. However, we place highly constraining upper limits on the radio flux density at 150 MHz, of ?1.4 mJy.