Geminga
| Pulsar Geminga | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
.jpg) | |||||||||||||
| Aufnahme der Infrarot- und Röntgenstrahlung von Geminga, drunter Illustration über das Zustandekommen durch Jets und Magnetfeld | |||||||||||||
  | |||||||||||||
| AladinLite | |||||||||||||
| Beobachtungsdaten Äquinoktium: J2000.0, Epoche: J2000.0 | |||||||||||||
| Sternbild | Zwillinge | ||||||||||||
| Rektaszension | 06h 33m 54,15s [1] | ||||||||||||
| Deklination | +17° 46′ 12,9″ [1] | ||||||||||||
| Astrometrie | |||||||||||||
| Trigonometrische Parallaxe | (4,0 ± 1,3) mas[2] | ||||||||||||
| Entfernung [2] | 815 Lj 250 pc | ||||||||||||
| Dispersionsmaß | (2,9 ± 0,5) pc cm−3 | ||||||||||||
| Eigenbewegung: | |||||||||||||
| in Rektaszension | 142,2 ± 1,2 mas/a | ||||||||||||
| in Deklination | 107,4 ± 1,2 mas/a | ||||||||||||
| Physikalische Eigenschaften | |||||||||||||
| Helligkeit | V-Band: ca. 25,5 mag | ||||||||||||
| Rotationsperiode | 237 ms | ||||||||||||
| Alter | 300 000 a [3] | ||||||||||||
| Geschichte | |||||||||||||
| Entdeckung | als Gammastrahlungsquelle: Fichtel et al. mit SAS-2, 1972 Bestätigung als Pulsar: 1992 | ||||||||||||
| Andere Bezeichnungen und Katalogeinträge | |||||||||||||
| |||||||||||||
| AladinLite | |||||||||||||
Geminga ist ein Pulsar im Sternbild Zwillinge (lateinisch Gemini). Die Entfernung beträgt etwa 800 Lichtjahre,[2] ist jedoch mit großer Ungenauigkeit behaftet. Geminga und der etwa gleich weit entfernte Vela-Pulsar sind die erdnächsten bekannten Pulsare.
Der Name leitet sich von GEMINi GAmma ray source (Gemini-Gammastrahlenquelle) ab. Geminga wurde 1972 mit Hilfe des Satelliten SAS-2 entdeckt und ist die zweithellste bekannte Quelle für Gammastrahlung von über 100 MeV Energie (die hellste ist der Vela-Pulsar und die dritthellste der Pulsar im Krebsnebel). 1992 konnte der Röntgensatellit ROSAT eine Periodizität der Strahlung von 0,237 Sekunden nachweisen, womit Geminga einen Pulsar darstellt. Im Gegensatz zu anderen bekannten Pulsaren emittiert Geminga jedoch nur schwach im Radiobereich.[4]
Position von Geminga in der Milchstraße (NASA/DOE/International LAT Team)
Geminga entstand vor circa 300.000 Jahren bei einer Supernovaexplosion. Diese Explosion ist einigen Theorien zufolge die Ursache für die relativ geringe Dichte an interstellarer Materie in der Umgebung des Sonnensystems.[5] Dieses Phänomen wird als Lokale Blase bezeichnet.
Messungen von Variationen der Periode der Gammapulse von Geminga legten 1998 die mögliche Existenz eines Begleiters in einer Umlaufbahn um Geminga nahe.[6] Spätere Beobachtungen sahen dann aber im timing noise die wahrscheinlichere Erklärung;[7] die Planetenhypothese wird nicht mehr weiterverfolgt.[8]
Weblinks
- Was ist Geminga? aus der Fernseh-Sendereihe alpha-Centauri (ca. 15 Minuten). Erstmals ausgestrahlt am 17. Jan. 2007.
- Paul Heeren: Pulsare – Ein toter Stern als Staubsauger vom 6. Juli 2017 bei Spektrum.de
Einzelnachweise
- ↑ SIMBAD-Datenbank
- ↑ a b c Faherty J., Walter F.M., Anderson J.: The trigonometric parallax of the neutron star Geminga. In: Astrophys. Space Sci. Band 308, 2007, S. 225–230, doi:10.1007/s10509-007-9368-0.
- ↑ Bignami G.F., Caraveo P.A.: GEMINGA: Its Phenomenology, Its Fraternity, and Its Physics. In: Annu. Rev. Astron. Astrophys. Band 34, 1996, S. 331, doi:10.1146/annurev.astro.34.1.331.
- ↑ V. M. Malofeev und O. I. Malov: Detection of Geminga as a radio pulsar. In: Nature. Band 389, 16. Oktober 1997, S. 697–699, doi:10.1038/39530 (englisch).
- ↑ Gehrels N., Chen W.: The Geminga supernova as a possible cause of the local interstellar bubble. In: Nature. Band 361, 1993, S. 706–707, doi:10.1038/361706a0.
- ↑ J. R. Mattox, J. P. Halpern, and P. A. Caraveo: Timing the Geminga pulsar with gamma-ray observations. In: Astrophys. J. Band 493. The American Astronomical Society, 1. Februar 1998, S. 891–897, doi:10.1086/305144/meta (englisch, iop.org [abgerufen am 2. Februar 2017]).
- ↑ J. R. Mattox, J. P. Halpern, P. A. Caraveo: An Update on Timing the Geminga Pulsar with the EGRET Gamma-Ray Telescope. In: Bulletin of the American Astronomical Society. Band 31, 1999, S. 904 (aas.org).
- ↑ M. S. Jackson, J. P. Halpern, E. V. Gotthelf und J. R. Mattox: A High-Energy Study of the Geminga Pulsar. In: Astrophys. J. Band 578, Nr. 2, S. 935, doi:10.1086/342662.
Auf dieser Seite verwendete Medien
This all-sky view from GLAST (now Fermi) reveals bright gamma-ray emission in the plane of the Milky Way (center), bright pulsars and super-massive black holes
Opaque red circle
NASA'S Chandra X-ray Observatory has taken deep exposures of two nearby energetic pulsars flying through the Milky Way galaxy. The shape of their X-ray emission suggests there is a geometrical explanation for puzzling differences in behavior shown by some pulsars.
Pulsars − rapidly rotating, highly magnetized, neutron stars born in supernova explosions triggered by the collapse of massive stars − were discovered 50 years ago via their pulsed, highly regular, radio emission. Pulsars produce a lighthouse-like beam of radiation that astronomers detect as pulses as the pulsar's rotation sweeps the beam across the sky.
Since their discovery, thousands of pulsars have been discovered, many of which produce beams of radio waves and gamma rays. Some pulsars show only radio pulses and others show only gamma-ray pulses. Chandra observations have revealed steadier X-ray emission from extensive clouds of high-energy particles, called pulsar wind nebulas, associated with both types of pulsars. New Chandra data on pulsar wind nebulas may explain the presence or absence of radio and gamma-ray pulses.
This four-panel graphic shows the two pulsars observed by Chandra. Geminga is in the upper left and B0355+54 is in the upper right. In both of these images, Chandra’s X-rays, colored blue and purple, are combined with infrared data from NASA’s Spitzer Space Telescope that shows stars in the field of view. Below each data image, an artist’s illustration depicts more details of what astronomers think the structure of each pulsar wind nebula looks like.
For Geminga, a deep Chandra observation totaling nearly eight days over several years was analyzed to show sweeping, arced trails spanning half a light year and a narrow structure directly behind the pulsar. A five-day Chandra observation of the second pulsar, B0355+54, showed a cap of emission followed by a narrow double trail extending almost five light years.
The underlying pulsars are quite similar, both rotating about five times per second and both aged about half a million years. However, Geminga shows gamma-ray pulses with no bright radio emission, while B0355+54 is one of the brightest radio pulsars known yet not seen in gamma rays.
A likely interpretation of the Chandra images is that the long narrow trails to the side of Geminga and the double tail of B0355+54 represent narrow jets emanating from the pulsar’s spin poles. Both pulsars also contain a torus of emission spreading from the pulsar’s spin equator. These disk-shaped structures and the jets are crushed and swept back as the pulsars fly through the Galaxy at supersonic speeds
In the case of Geminga, the view of the torus is close to edge-on, while the jets point out to the sides. B0355+54 has a similar structure, but with the torus viewed nearly face-on and the jets pointing nearly directly towards and away from Earth. In B0355+54, the swept-back jets appear to lie almost on top of each other, giving a doubled tail.
Both pulsars have magnetic poles quite close to their spin poles, as is the case for the Earth’s magnetic field. These magnetic poles are the site of pulsar radio emission so astronomers expect the radio beams to point in a similar direction as the jets. By contrast the gamma-ray emission is mainly produced along the spin equator and so aligns with the torus.
For Geminga, astronomers view the bright gamma-ray pulses along the edge of the torus, but the radio beams near the jets point off to the sides and remain unseen. For B0355+54, a jet points almost along our line of sight towards the pulsar. This means astronomers see the bright radio pulses, while the torus and its associated gamma-ray emission are directed in a perpendicular direction to our line of sight, missing the Earth.
These two deep Chandra images have, therefore, exposed the spin orientation of these pulsars, helping to explain the presence, and absence, of the radio and gamma-ray pulses.
The Chandra observations of Geminga and B0355+54 are part of a large campaign, led by Roger Romani of Stanford University, to study six pulsars that have been seen to emit gamma-rays. The survey sample covers a range of ages, spin-down properties and expected inclinations, making it a powerful test of pulsar emission models.
A paper on Geminga led by Bettina Posselt of Penn State University was accepted for publication in The Astrophysical Journal and is available online. A paper on B0355+54 led by Noel Klingler of the George Washington University was published in the December 20th, 2016 issue of The Astrophysical Journal and is available online. NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra’s science and flight operations.Autor/Urheber: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg), Lizenz: CC BY 3.0
IAU Gemini chart