An upper limit on the power-law index of the radio spectrum of Geminga

Among the similar to 20 isolated neutron stars that emit in more than one e nergy band, only Geminga was radio quiet. Very recently, three groups at th e Pushchino Radio Astronomy Observatory reported independently the detectio n of a weak pulsed radio emission from this source at similar to 100 MHz, b ringing Geminga back into the standard group of radio-loud, isolated neutro n stars. Here we report a series of observations of Geminga performed with the Arecibo radio telescope at various frequencies between 318 and 1400 MHz , performed from 1982 to 1984. No pulsed emission was detected. The high se nsitivity of the Arecibo telescope allowed us to put a stringent upper limi t to the spectral index in the radio band: alpha less than or equal to -3.6 . If the 100 MHz pulsed emission is confirmed, Geminga will retain its pecu liar nature of a radio pulsar with an extremely steep radio spectrum. This would severely constrain the models for the radio emission from isolated ro tating neutron stars.