Geminga

Geminga is a neutron star approximately 250 parsecs (around 800 light years) from the Sun in the constellation Gemini. Its name is both a contraction of "Gemini gamma-ray source", and a transcription of the words gh'è minga "it's not there" in the Lombard dialect of Milan (pronounced [ɡɛˈmiŋɡa]).

The nature of Geminga was quite unknown for 20 years after its discovery by NASA's Second Small Astronomy Satellite (SAS-2). Finally, in March 1991 the ROSAT satellite detected a periodicity of 0.237 seconds in soft x-ray emission. Thus, it is supposed that Geminga is a sort of neutron star: the decaying core of a massive star that exploded as a supernova about 300,000 years ago.

It was once thought that this nearby explosion was responsible for the low density of the interstellar medium in the immediate vicinity of the Solar System. This low-density area is known as the Local Bubble. Possible evidence for this includes findings by the Arecibo Observatory that local micrometre-sized interstellar meteor particles appear to originate from its direction. More recently, however, it has been suggested that multiple supernovae in subgroup B1 of the Pleiades moving group were more likely responsible, becoming a remnant supershell.

Geminga was the first example of an unidentified gamma-ray source, a source which could not be associated with any objects known at other wavelengths. It was first detected as a significant excess of gamma-rays over the expected background of diffuse Galactic emission, by the SAS-2 satellite (Fictel et al. 1975) and subsequently by the COS-B satellite. The SAS-2 group reported a pulsation in the gamma-ray signal, with period approximately 59 s, although the limited number of detected gamma-rays (121 over a period of four months) led them to conclude that the pulsation was not statistically compelling. Due to the limited angular resolution of the instrument (approximately 2.5° at 100MeV) and the small number of gamma-rays detected, the exact location of the source was uncertain, constrained only to be within a relatively large "error region". At the time of detection, four weak radio sources were known within this region, two supernova remnants bordered it and a known satellite galaxy to the Milky Way lay nearby. None of these known sources were convincing associations to the gamma-ray source, and the SAS-2 team suggested that an undiscovered radio-pulsar was the most likely progenitor.