Geminga
1639651
224576632
2008-07-09T14:19:54Z
85.227.159.145
{{Starbox short
| name = Geminga
| epoch = [[J2000.0]]
| constell = [[Gemini (constellation)|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-year]]s away from the [[Sun]] in the [[constellation]] [[Gemini (constellation)|Gemini]]. Its name is a contraction of "Gemini gamma-ray source", and coincidentally means "it's not there" in the [[Italy|Italian]] [[Milan]]ese dialect.<ref name="Bignami 1983">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</ref>
== Pulsar ==
[[Image:Geminga.jpg|thumb|left|Geminga.]]
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 spectroscopy|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.<ref>[http://www.daviddarling.info/encyclopedia/G/Geminga.html Geminga], [[Internet Encyclopedia of Science]]</ref>
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 micron-sized interstellar meteor particles appear to originate from its direction.<ref>{{cite web| url=http://www.centauri-dreams.org/?p=1741| title=The Sun’s Exotic Neighborhood| publisher=Centauri Dreams| date=2008-02-28}}</ref>
== Discovery and identification ==
Geminga was the first example of an unidentified [[Gamma-ray astronomy|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.<ref name="Thompson 1977">D. J. Thompson ''et al.'' Final SAS-2 gamma-ray results on sources in the galactic anticenter region. ApJ, 213:252–262, April 1977.</ref>
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 Observatory|Einstein x-ray satellite]], 1E 0630+178.<ref name="Bignami 1983"/> 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,<ref name="Halpern 1992">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.</ref> which
was also seen in gamma-rays by the [[EGRET]] instrument<ref name="Bertsch 1992">D. L. Bertsch ''et al.'' Pulsed high-energy gamma-radiation from Geminga (1E0630 + 178). Nature, 357:306, May 1992.</ref>
and retrospectively in the COS-B and SAS-2 data.<ref name="Bignami 1992">G. F. Bignami and P. A. Caraveo. Geminga - New Period Old Gamma-Rays. Nature, 357:287, May 1992.</ref><ref name="Mattox 1992">J. R. Mattox ''et al.'' SAS 2 observation of pulsed high-energy gamma radiation from Geminga. ApJ, 401:L23–L26, December 1992.</ref>
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 [[astronomical unit|AU]] from Geminga in a 5.1 year orbit. With a mass of 1.7 [[earth]]s, 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 begin}}
{{PlanetboxOrbit speculation
| exoplanet = b
| mass = 1.7 [[Earth mass|M<sub>⊕</sub>]]
| period = 5.1 [[year|y]]
| semimajor = 3.3
| eccentricity = 0.00
}}
{{PlanetboxOrbit end}}
== References ==
{{Reflist}}
*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]
[[Category:Gemini constellation]]
[[Category:Neutron stars]]
[[Category:Pulsars]]
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[[fr:PSR J0633+1746]]
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[[ru:Геминга]]
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