Pulsars (4) ...

The nearest known pulsar is called Geminga; it lies in the constellation Gemini at a distance of about 160 parsecs (about 520 light years).
The Geminga Pulsar
Geminga is one of the three pulsars known to emit gamma rays. It also emits X-rays and very weak visible light (apparent visual magnitude 25.5), but no radio pulses have been detected from it. The adjacent figure shows Geminga and the Crab Pulsar in gamma rays (and a distant galaxy that is a gamma ray source). Geminga was known as a gamma ray source for nearly twenty years before ROSAT soft X-ray observations in 1991 showed that it was a pulsar with a period of 237 milliseconds. The name "Geminga" has a double meaning. It stands for "Gemini gamma ray source" and also means "is not there" in Milanese dialect, presumably a reference to the original mystery surrounding its nature.

Geminga and the "Local Bubble"

There is a local region of the galaxy about 300 light years across and containing the Sun where the density is lower than the surrounding region. This region is called the "local bubble". One cause of such a bubble could be a supernova. It has been proposed that the supernova that produced Geminga might also have been responsible for clearing out the local bubble.

A Nearby Supernova?
If Geminga is indeed a spinning neutron star, it must have formed in a supernova. From the slowing of Geminga's rotation rate, we can estimate its age to be about 340,000 years. An extrapolation of the present spatial motion of Geminga back for that length of time suggests that the supernova that produced the pulsar could have occurred as close as 200 light years to Earth. Such a nearby supernova would have been at least as bright as the full Moon and gamma rays and X-rays from it could have had a damaging effect on Earth's upper atmosphere. Our distant ancestors may have witnessed an amazing sight and experienced a close escape!

However, speculation concerning Geminga should be tempered with some caution. Although the weight of evidence indicates that it is a pulsar, it is the only pulsar known that emits gamma rays and X-rays but no detectable radio waves. This might indicate only that the radio pulses are not beamed toward Earth, but it might instead indicate that Geminga is not a normal pulsar.