Geminga

Geminga, is a neutron star approximately 552 light-years away from the Sun in the constellation Gemini. Its name is a contraction of "Gemini gamma-ray source", and coincidentally means "it's not there" in the Italian Milanese dialect.

Pulsar

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 behemoth star that went supernova about 300,000 years ago.

This nearby explosion may be 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.

Discovery and identification

Geminga was the first example of an unidentified gamma-ray source, a source which could not be associated with any objects known at other wavelength. 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.

Despite the investment of a significant amount of observation time, the source remained unidentified through the COS-B era; their data did, however, rule out the claimed 59 s pulsation. Many claims were made about the source during this time, but its nature remained a mystery until the identification of a candidate source by the Einstein x-ray satellite, 1E 0630+178. The characteristics of the x-ray source were unique: large x-ray to optical luminosity, no radio emission detected by the sensitive VLA instrument, point-like emission in the Einstein imager and an estimated distance of approximately 100 pc, placing it within the Galaxy. An association between the gamma-ray and x-ray sources was not conclusively made until the ROSAT x-ray imager detected a 237 ms pulsation, which was also seen in gamma-rays by the EGRET instrument and retrospectively in the COS-B and SAS-2 data.

Geminga is the first example of a radio-quiet pulsar, and serves as an illustration of the difficulty of associating gamma-ray emission with objects known at other wavelengths: either no credible object is detected in the error region of the gamma-ray source, or a number are present and some characteristic of the gamma-ray source, such as periodicity or variability, must be identified in one of the prospective candidates (or vice-versa as in the case of Geminga).

Possible planetary system

In 1997, John Mattox et al. claimed to have discovered a planet orbiting Geminga by gamma-ray timing of Geminga. This hypothesized planet, Geminga b, was thought to orbit about 3.3 AU from Geminga in a 5.1 year orbit. With a mass of 1.7 earths, Geminga b would be a terrestrial planet. However, this discovery is now doubtful because recent analysis of the data indicates that the detected timing changes were due to signal noise, not a planet.

References

• C. E. Fichtel et al. High-energy gamma-ray results from the second small astronomy satellite. ApJ, 198:163–182, May 1975.

External links

• Spaceflight Now: 'Cannonball pulsar' seen flying across space
• ESA: Hipparcos pinpoints an amazing gamma-ray clock
• Extrasolar Visions: Geminga + Geminga b