Geminga

File:Halo around pulsar Geminga.jpg and the Crab Nebula. right: The halo around the pulsar Geminga seen by Fermi after removing bright sources]]

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 degenerate core of a massive star that exploded as a supernova about 300,000 years ago.{{cite web |url=http://www.daviddarling.info/encyclopedia/G/Geminga.html |title=Geminga |website=Internet Encyclopedia of Science}}

It was once thought that this nearby explosion was 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.{{cite journal |last1=Gehrels |first1=N. |last2=Chen |first2=W. |title=The Geminga supernova as a possible cause of the local interstellar bubble |journal=Nature |volume=361 |pages=706 |year=1993 |doi=10.1038/361706a0 |bibcode=1993Natur.361..706G |issue=6414 |s2cid=4338940 |url=https://zenodo.org/record/1233141}} 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 |df=dmy-all}} More recently, however, it has been suggested that multiple supernovae in subgroup B1 of the Pleiades moving group were more likely responsible,{{cite journal |author1=Berghoefer, T. W. |author2=Breitschwerdt, D. |year=2002 |title=The origin of the young stellar population in the solar neighborhood – a link to the formation of the Local Bubble? |journal=Astronomy and Astrophysics |volume=390 |issue=1 |pages=299–306 |arxiv=astro-ph/0205128v2 |doi=10.1051/0004-6361:20020627 |bibcode=2002A&A...390..299B|s2cid=6002327 }} becoming a remnant supershell.{{cite web |url=http://aas.org/archives/BAAS/v29n5/aas191/abs/S051009.html |id=51.09 |title=Model of an Expanding Supershell Structure in the LISM |author1=Gabel, J. R. |author2=Bruhweiler, F. C. |date=8 January 1998 |publisher=American Astronomical Society |access-date=2014-03-14 |df=dmy-all |archive-url=https://web.archive.org/web/20140315102240/http://aas.org/archives/BAAS/v29n5/aas191/abs/S051009.html |archive-date=15 March 2014 |url-status=dead }}

A study from 2019, using data from NASA's Fermi Gamma-ray Space Telescope discovered a large gamma-ray halo around Geminga. Accelerated electrons and positrons collide with nearby starlight. The collision boosts the light up to much higher energies. Geminga alone could be responsible for as much as 20% of the high-energy positrons seen by the AMS-02 experiment. Previous studies using data from the High-Altitude Water Cherenkov Gamma-ray Observatory found only a small gamma-ray halo around Geminga at higher energies.{{Cite web|url=http://www.nasa.gov/feature/goddard/2019/nasa-s-fermi-mission-links-nearby-pulsar-s-gamma-ray-halo-to-antimatter-puzzle|title=Fermi Links Nearby Pulsar's Gamma-ray 'Halo' to Antimatter Puzzle|last=Garner|first=Rob|date=2019-12-19|website=NASA|access-date=2020-01-26}}{{Cite journal|last1=Di Mauro|first1=Mattia|last2=Manconi|first2=Silvia|last3=Donato|first3=Fiorenza|author3-link=Fiorenza Donato|date=December 2019|title=Detection of a γ-ray halo around Geminga with the Fermi-LAT data and implications for the positron flux|journal= Physical Review D|language=en|volume=100|issue=12|pages=123015|doi=10.1103/PhysRevD.100.123015|issn=1550-7998|bibcode=2019PhRvD.100l3015D|arxiv=1903.05647|s2cid=119218479}}

Image:267641main allsky labeled HI.jpg

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 (Fichtel 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.{{cite journal |first=D. J. |last=Thompson |display-authors=etal |title=Final SAS-2 gamma-ray results on sources in the galactic anticenter region |journal=Astrophysical Journal |volume=213 |pages=252–262 |date=April 1977|bibcode = 1977ApJ...213..252T |doi = 10.1086/155152 |hdl=2060/19760025006 |s2cid=121094983 |hdl-access=free }}

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,{{cite journal |first1=J. P. |last1=Halpern |first2=S. S. |last2=Holt |title=Discovery of soft X-ray pulsations from the gamma-ray source Geminga |journal=Nature |volume=357 |issue=6375 |pages=222–224 |date=May 1992|bibcode = 1992Natur.357..222H |doi = 10.1038/357222a0 |s2cid=4281635 |url=https://zenodo.org/record/1233123 |doi-access=free }} which was also seen in gamma rays by the EGRET instrument{{cite journal |first=D. L. |last=Bertsch |s2cid=4304133 |display-authors=etal |title=Pulsed high-energy gamma-radiation from Geminga (1E0630 + 178) |journal=Nature |volume=357 |issue=6376 |pages=306–307 |date=May 1992|bibcode = 1992Natur.357..306B |doi = 10.1038/357306a0 }} and retrospectively in the COS-B and SAS-2 data.{{cite journal |first1=G. F. |last1=Bignami |first2=P. A. |last2=Caraveo |s2cid=36168064 |author2-link=Patrizia A. Caraveo |title=Geminga – New Period Old Gamma-Rays |journal=Nature |volume=357 |issue=6376 |pages=287 |date=May 1992|bibcode = 1992Natur.357..287B |doi = 10.1038/357287a0 |doi-access=free }}{{cite journal |first=J. R. |last=Mattox |display-authors=etal |title=SAS 2 observation of pulsed high-energy gamma radiation from Geminga |journal=Astrophysical Journal |volume=401 |pages=L23–L26 |date=December 1992|bibcode = 1992ApJ...401L..23M |doi = 10.1086/186661 }} Geminga thus appeared to be the first example of a radio-quiet pulsar, and served as an illustration of the difficulty of associating gamma-ray emission with objects known at other wavelengths: some characteristic of the gamma-ray source, such as periodicity or variability, must be identified in candidate counterparts at other wavelengths in order to make the connection of their identity.

Finally, this principle held true when radio emissions of matching 237 ms periodicity were found at previously unsurveyed frequencies of 100 MHz and below.{{cite journal|last1 = Gil, J. A.|last2 = Khechinashvili, D. G.|last3 = Melikidze, G. I.|title = Why is the Geminga pulsar radio quiet at frequencies higher than about 100 MHz?|journal = ASP Conference Series|year = 1998|volume = 138|pages = 119 |bibcode = 1998ASPC..138..119G}}

The proper motion of Geminga is 178.2 ± 1.8 mas/year which corresponds (at a distance of 250 pc) to a transverse velocity of 211 kilometers per second.{{Cite journal| last1 = Faherty | first1 = J.|author1-link=Jackie Faherty| last2 = Walter | first2 = F. M.| last3 = Anderson | first3 = J.| s2cid = 122256682| title = The trigonometric parallax of the neutron star Geminga| journal = Astrophysics and Space Science| volume = 308| issue = 1–4| pages = 225–230| year = 2007| doi = 10.1007/s10509-007-9368-0|bibcode = 2007Ap&SS.308..225F }}{{Cite journal |last=Donato |first=Fiorenza |last2=Manconi |first2=Silvia |last3=Mauro |first3=Mattia Di |date=2020-02-01 |title=Detection of a γ-ray halo around Geminga with the Fermi-LAT and implications for the positron flux |url=https://iopscience.iop.org/article/10.1088/1742-6596/1468/1/012084 |journal=Journal of Physics: Conference Series |volume=1468 |issue=1 |pages=012084 |doi=10.1088/1742-6596/1468/1/012084 |issn=1742-6588|doi-access=free }} This velocity is very fast for a star, comparable to Barnard's Star.

Geminga underwent a minor glitch in the late part of 1996, with a fractional change in frequency of 6.2 × 10⁻¹⁰.{{cite journal|title=A High-Energy Study of the Geminga Pulsar|author1=Jackson, M. S. |author2=Halpern, J. P. |author3=Gotthelf, E. V. |author4=Mattox, J. R. |date=2002|journal=The Astrophysical Journal|volume=578|issue=2|pages=935–942|doi=10.1086/342662|bibcode=2002ApJ...578..935J|arxiv=astro-ph/0207001|s2cid=119067655 }}

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;{{cite journal |title=Timing the Geminga Pulsar with Gamma-Ray Observations |author=Mattox, J. R. |author2=Halpern, J. P. |author3-link=Patrizia A. Caraveo |author3=Caraveo, P. A. |date=1998 |journal=The Astrophysical Journal |volume=493 |issue=2 |pages=891–897 |doi=10.1086/305144 |bibcode=1998ApJ...493..891M|doi-access=free }} however, this was later shown to be an artifact of noise that affects the pulse times from Geminga rather than a genuine orbital effect.

{{Reflist|refs=

{{cite simbad |title=NAME Geminga}}

{{cite constellation|Geminga}}

{{cite journal |first=C. E. |last=Fichtel |display-authors=etal |title=High-energy gamma-ray results from the second small astronomy satellite. |journal=Astrophysical Journal |volume=198 |pages=163–182 |date=May 1975|bibcode = 1975ApJ...198..163F |doi = 10.1086/153590 |hdl=2060/19740027105 |hdl-access=free }}

{{cite journal |last1=Jackson |first1=M. S. |last2=Halpern |first2=J. P. |date=November 2005 |title=A Refined Ephemeris and Phase-resolved X-Ray Spectroscopy of the Geminga Pulsar |journal=The Astrophysical Journal |volume=633 |issue=2 |pages=1114-1125 |doi=10.1086/491615 |arxiv=astro-ph/0509038 |bibcode=2005ApJ...633.1114J}}

{{cite journal |last1=Mignani |first1=Roberto P. |date=April 2011 |title=Optical, ultraviolet, and infrared observations of isolated neutron stars |journal=Advances in Space Research |volume=47 |issue=8 |pages=1281-1293 |doi=10.1016/j.asr.2009.12.011 |arxiv=0912.2931 |bibcode=2011AdSpR..47.1281M}}

{{cite journal |last1=Shearer |first1=Andrew |last2=O’ Connor |first2=Eoin |date=June 2018 |title=Optical pulsars and polarimetry |journal=Proceedings of the International Astronomical Union |language=en |volume=13 |issue=S337 |pages=191-194 |doi=10.1017/S174392131700998X |doi-access=free |bibcode=2018IAUS..337..191S |issn=1743-9213}}

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• [https://spaceflightnow.com/news/n0309/06pulsar Spaceflight Now: 'Cannonball pulsar' seen flying across space]
• [https://carlkop.home.xs4all.nl/gimga.html ESA: Hipparcos pinpoints an amazing gamma-ray clock]

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Geminga