View source for Geminga

{{short description|X-ray pulsar in the constellation Gemini}}
{{Starbox begin
| name = Geminga
}}
{{Starbox image
 | image= [[File:Geminga by Chandra and Spitzer.jpg|250px]]
 | caption= Geminga as seen by [[Chandra X-ray Observatory|Chandra]] and [[Spitzer Space Telescope|Spitzer]]
 | credit= X-ray: [[NASA]]/CXC/PSU/B. Posselt et al; Infrared: NASA/[[Jet Propulsion Laboratory|JPL]]–[[California Institute of Technology|Caltech]]
}}
{{Starbox observe
| epoch = [[J2000.0]] 
| constell = [[Gemini (constellation)|Gemini]]<ref name=constellation/>
| ra = {{RA|06|33|54.15}}<ref name="SIMBAD"/>
| dec = {{DEC|+17|46|12.9}}<ref name="SIMBAD"/>
| appmag_v = 25.5<ref name="Shearer2018"/>
}}
{{Starbox character
| type = [[Pulsar]]
}}
{{Starbox astrometry
| parallax = 4.0
| p_error = 1.3
| parallax_footnote = <ref name="Faherty 2007"/>
| dist_ly = ~815
| dist_pc = {{val|250|120|62}}<ref name="Faherty 2007"/>
}}
{{Starbox detail
| rotation = {{val|237|ul=ms}}<ref name="Jackson2005"/>
| age = 339,000<ref name="Mignani2011"/>
}}
{{Starbox catalog
| names = SN 437, PSR B0633+17, PSR J0633+1746<ref name="SIMBAD"/>
}}
{{Starbox reference
| Simbad = Geminga
}}
{{Starbox end}}

'''Geminga''' {{IPAc-en|g|@|'|m|I|N|g|@}} is a gamma ray and x-ray pulsar source thought to be a [[neutron star]] approximately 250 [[parsec]]s<ref name="Faherty 2007"/> (around 800 [[light-year]]s) from the [[Sun]] in the [[constellation]] [[Gemini (constellation)|Gemini]]. 

Its name, attributed by its discoverer [[Giovanni Bignami]], is both a contraction of '''''Gemin'''i '''ga'''mma-ray source'', and a transcription of the words '''''g'''h'''è minga''''' ({{IPA|lmo|ɡɛ ˈmĩːɡa|pron}}), meaning "it's not there" in the [[Milanese dialect]] of [[Lombard language|Lombard]].<ref name="Bignami 1983">{{cite journal |first=G. F. |last=Bignami |display-authors=etal |title=An identification for 'Geminga' (2CG 195+04) 1E 0630+178 – A unique object in the error box of the high-energy gamma-ray source |journal=Astrophysical Journal |volume=272 |pages=L9–L13 |date=September 1983| doi = 10.1086/184107 |bibcode = 1983ApJ...272L...9B }}</ref> The name was approved by the [[International Astronomical Union]] on 4 April 2022.<ref>{{Cite web |title=Naming Stars |url=https://www.iau.org/public/themes/naming_stars/}}</ref>

== Pulsar ==
[[File:Halo around pulsar Geminga.jpg|thumb|left|left: Geminga, [[IC 443]] and the [[Crab Nebula]]. right: The halo around the pulsar Geminga seen by [[Fermi Gamma-ray Space Telescope|Fermi]] after removing bright sources]]
The nature of Geminga was quite unknown for 20&nbsp;years after its discovery by NASA's [[Second Small Astronomy Satellite]] (SAS-2). Finally, in March 1991 the [[ROSAT]] satellite detected a [[Frequency|periodicity]] of 0.237&nbsp;seconds in [[X-ray emission spectroscopy|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&nbsp;years ago.<ref>{{cite web |url=http://www.daviddarling.info/encyclopedia/G/Geminga.html |title=Geminga |website=[[Internet Encyclopedia of Science]]}}</ref>

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]].<ref>{{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}}</ref> Possible evidence for this includes findings by the [[Arecibo Observatory]] that local micrometre-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 |df=dmy-all}}</ref> More recently, however, it has been suggested that multiple supernovae in subgroup B1 of the [[Pleiades]] moving group were more likely responsible,<ref name="T. W. Berghoefer, D. Breitschwerdt 2002">{{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 }}</ref> becoming a remnant [[supershell]].<ref>{{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 }}</ref>

A study from 2019, using data from NASA's [[Fermi Gamma-ray Space Telescope]] discovered a large [[Gamma ray|gamma-ray]] halo around Geminga. Accelerated [[electron]]s and [[positron]]s 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 Experiment|High-Altitude Water Cherenkov Gamma-ray Observatory]] found only a small gamma-ray halo around Geminga at higher energies.<ref>{{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}}</ref><ref>{{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}}</ref>

== Discovery and identification ==
[[Image:267641main allsky labeled HI.jpg|right|thumb|350px|Position of Geminga in the Milky Way gamma-ray sky. '''Credit:''' ''NASA/DOE/International LAT Team'']]
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 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)<ref name="Fichtel1975"/> 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">{{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 }}</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 [[Very Large Array|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">{{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 }}</ref> which was also seen in gamma rays by the [[EGRET]] instrument<ref name="Bertsch 1992">{{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 }}</ref> and retrospectively in the COS-B and SAS-2 data.<ref name="Bignami 1992">{{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 }}</ref><ref name="Mattox 1992">{{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 }}</ref> 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&nbsp;MHz and below.<ref>{{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}}</ref>

== Proper motion ==
The [[proper motion]] of Geminga is 178.2 ± 1.8 [[Minute and second of arc#Astronomy|mas]]/year which corresponds (at a distance of 250 pc) to a transverse velocity of 211 kilometers per second.<ref name="Faherty 2007">{{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 }}</ref><ref>{{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 }}</ref> This velocity is very fast for a star, comparable to [[Barnard's Star]].

== Timing measurements ==

Geminga underwent a minor [[glitch (astronomy)|glitch]] in the late part of 1996, with a fractional change in frequency of 6.2&nbsp;×&nbsp;10<sup>−10</sup>.<ref name="Jackson2002">{{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 }}</ref> 

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;<ref>{{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 }}</ref> however, this was later shown to be an artifact of [[Noise (electronics)|noise]] that affects the pulse times from Geminga rather than a genuine orbital effect.<ref name="Jackson2002"/>

== References ==
{{Reflist|refs=

<ref name="SIMBAD">{{cite simbad |title=NAME Geminga}}</ref>

<ref name=constellation>{{cite constellation|Geminga}}</ref>

<ref name="Fichtel1975">{{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 }}</ref>

<ref name="Jackson2005">{{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}}</ref>

<ref name="Mignani2011">{{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}}</ref>

<ref name="Shearer2018">{{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}}</ref>

}}

== External links ==
* [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]

{{Stars of Gemini}}
{{Portal bar|Astronomy|Stars|Outer space}}

[[Category:Gemini (constellation)]]
[[Category:Pulsars]]
[[Category:Optical pulsars]]
[[Category:Stars with proper names|Geminga]]