Geminga

Starbox short
name = Geminga
epoch = J2000.0
constell = Gemini
ra = RA|06|33|54.15
dec = DEC|+17|46|12.9
appmag_v = ?
dist_ly = 552
dist_pc = 169
spectral = Pulsar
names = SN 437, PSR B0633+17

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.G. F. Bignami "et al." An identification for ’Geminga’ (2CG 195+04) 1E 0630+178 - A unique object in the error box of the high-energy gamma-ray source. ApJ, 72:L9–L13, September 1983]

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. [ [http://www.daviddarling.info/encyclopedia/G/Geminga.html Geminga] , Internet Encyclopedia of Science]

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. [cite web| url=http://www.centauri-dreams.org/?p=1741| title=The Sun’s Exotic Neighborhood| publisher=Centauri Dreams| date=2008-02-28]

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 expectedbackground 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, theexact 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 wereconvincing associations to the gamma-ray source, and the SAS-2 team suggested that an undiscovered radio-pulsar was the most likely progenitor.D. J. Thompson "et al." Final SAS-2 gamma-ray results on sources in the galactic anticenter region. ApJ, 213:252–262, April 1977.]

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 associationbetween the gamma-ray and x-ray sources was not conclusively made until the
ROSAT x-ray imager detected a 237 ms pulsation,J. P. Halpern and S. S. Holt. Discovery of soft X-ray pulsations from the gamma-ray source Geminga. Nature, 357:222–224, May 1992.] which was also seen in gamma-rays by the EGRET instrumentD. L. Bertsch "et al." Pulsed high-energy gamma-radiation from Geminga (1E0630 + 178). Nature, 357:306, May 1992.] and retrospectively in the COS-B and SAS-2 data.G. F. Bignami and P. A. Caraveo. Geminga - New Period Old Gamma-Rays. Nature, 357:287, May 1992.] J. R. Mattox "et al." SAS 2 observation of pulsed high-energy gamma radiation from Geminga. ApJ, 401:L23–L26, December 1992.]

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, mustbe identified in one of the prospective candidates (or vice-versa as in the case ofGeminga).

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.

PlanetboxOrbit speculation
exoplanet = b
mass_earth = 1.7
period = 5.1 y
semimajor = 3.3
eccentricity = 0.00

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

* [http://spaceflightnow.com/news/n0309/06pulsar/ Spaceflight Now: 'Cannonball pulsar' seen flying across space]
* [http://www.xs4all.nl/~carlkop/gimga.html ESA: Hipparcos pinpoints an amazing gamma-ray clock]
* [http://www.extrasolar.net/startour.asp?StarCatId=disproven&StarId=25 Extrasolar Visions: Geminga] + [http://www.extrasolar.net/planettour.asp?StarCatID=disproven&PlanetID=33 Geminga b]