Geminga

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Geminga
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Gemini
Right ascension 06h 33m 54.15s
Declination +17° 46′ 12.9″
Apparent magnitude (V) 25.5
Distance 815 ly
(250 pc)
Spectral type Pulsar
Other designations
SN 437, PSR B0633+17, PSR J0633+1746

Geminga is a neutron star approximately 250 parsecs[1] away from the Sun in the constellation Gemini. Its name is both a contraction of "Gemini gamma-ray source", and gh'è minga "it's not there" in the Lombard dialect of Milan (pronounced [ɡɛˈmiŋɡa]).[2]

Pulsar[edit]

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. 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.[3]

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.[4] Possible evidence for this includes findings by the Arecibo Observatory that local micrometre-sized interstellar meteor particles appear to originate from its direction.[5]

Discovery and identification[edit]

Position of Geminga in the Milky Way. Credit : NASA/DOE/International LAT Team.

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.[6]

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.[2] 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,[7] which was also seen in gamma-rays by the EGRET instrument[8] and retrospectively in the COS-B and SAS-2 data.[9][10]

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).

Proper motion[edit]

The proper motion of Geminga is 178.2 mas/year which corresponds to a projected velocity of 205 kilometers per second.[1] This is very fast for a star, comparable to Barnard's star.

Timing measurements[edit]

Geminga underwent a minor glitch in the late part of 1996, with a fractional change in frequency of 6.2 × 10−10.[11] A 1998 study of the pre-glitch ephemeris suggested that the timings were being affected by reflex motion due to the presence of a low-mass planet in a 5.1-year orbit,[12] however this was later shown to be an artifact of noise that affects the pulse times from Geminga rather than a genuine orbital effect.[11]

References[edit]

  1. ^ a b Faherty, J.; Walter, F. M.; Anderson, J. (2007). "The trigonometric parallax of the neutron star Geminga". Astrophysics and Space Science 308: 225. Bibcode:2007Ap&SS.308..225F. doi:10.1007/s10509-007-9368-0.  edit
  2. ^ a b 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
  3. ^ Geminga, Internet Encyclopedia of Science
  4. ^ Gehrels, N.; Chen, W. (1993). "The Geminga supernova as a possible cause of the local interstellar bubble". Nature 361 (6414): 706. Bibcode:1993Natur.361..706G. doi:10.1038/361706a0.  edit
  5. ^ "The Sun's Exotic Neighborhood". Centauri Dreams. 2008-02-28. 
  6. ^ D. J. Thompson et al. Final SAS-2 gamma-ray results on sources in the galactic anticenter region. ApJ, 213:252–262, April 1977.
  7. ^ 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.
  8. ^ D. L. Bertsch et al. Pulsed high-energy gamma-radiation from Geminga (1E0630 + 178). Nature, 357:306, May 1992.
  9. ^ G. F. Bignami and P. A. Caraveo. Geminga - New Period Old Gamma-Rays. Nature, 357:287, May 1992.
  10. ^ J. R. Mattox et al. SAS 2 observation of pulsed high-energy gamma radiation from Geminga. ApJ, 401:L23–L26, December 1992.
  11. ^ a b Jackson, M. S.; Halpern, J. P.; Gotthelf, E. V.; Mattox, J. R. (2002). "A High-Energy Study of the Geminga Pulsar". The Astrophysical Journal 578 (2): 935–942. arXiv:astro-ph/0207001. Bibcode:2002ApJ...578..935J. doi:10.1086/342662. 
  12. ^ Mattox, J. R.; Halpern, J. P.; Caraveo, P. A. (1998). "Timing the Geminga Pulsar with Gamma-Ray Observations". The Astrophysical Journal 493 (2): 891–897. Bibcode:1998ApJ...493..891M. doi:10.1086/305144. 
  • C. E. Fichtel et al. High-energy gamma-ray results from the second small astronomy satellite. ApJ, 198:163–182, May 1975.

External links[edit]