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 energy band, only Geminga was radio quiet. Very recently, three groups at the Pushchino Radio Astronomy Observatory reported independently the detection of a weak pulsed radio emission from this source at similar to 100 MHz, bringing Geminga back into the standard group of radio-loud, isolated neutron 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 sensitivity of the Arecibo telescope allowed us to put a stringent upper limit 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 peculiar nature of a radio pulsar with an extremely steep radio spectrum. This would severely constrain the models for the radio emission from isolated rotating neutron stars.