DETECTION OF GEMINGA AS A RADIO PULSAR

Geminga was discovered as a strong gamma-ray source in the constellati on Gemini over two decades ago(1,2), and was later detected at X-ray(3 ) and optical(4) wavelengths. X-ray pulsations(5) with a period of 237 ms established that it is a rotating neutron star. Although gamma-ray pulses were subsequently discovered (once the period was known) in ar chived data(6), no evidence for radio emission (either continuum or pu lsed) was found; in this respect, Geminga is different from every othe r neutron star with pulsed emission. Here we report the detection of p ulsed 102.5-MHz radio emission from Geminga, with a period of 237 ms. The nux density varies within the range 5-500 mJy and the pulse width varies between 10 and 80 ms. The small dispersion measure (2.9 +/- 0.5 pc cm(-3)) confirms Geminga's proximity to the Sun and establishes it as the weakest known radio pulsar. This observation poses a considera ble challenge for pulsar emission models, which must now be able to ex plain the exceptional contrast between the strength of the gamma-ray a nd radio emission from this object.