Geminga is a gamma ray and x-ray pulsar source thought to be a
neutron star
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white ...
approximately 250
parsec
The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...
s
(around 800
light-year
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
s) from the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
in the
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The origins of the e ...
Gemini
Gemini may refer to:
Space
* Gemini (constellation), one of the constellations of the zodiac
** Gemini in Chinese astronomy
* Project Gemini, the second U.S. crewed spaceflight program
* Gemini Observatory, consisting of telescopes in the Northern ...
.
Its name, attributed by its discoverer
Giovanni Bignami
Giovanni Fabrizio Bignami (10 April 1944 – 24 May 2017) was an Italian physicist. From March 2007 until August 2008, he was Chairman of the Italian Space Agency. Between 2010 and 2014, he was the first Italian to chair the Committee on Space ...
, is both a contraction of ''Gemini gamma-ray source'', and a transcription of the words ''ghè minga'' (), meaning "it's not there" in the
Milanese dialect
Milanese (endonym in traditional orthography , ') is the central variety of the Western dialect of the Lombard language spoken in Milan, the rest of its metropolitan city, and the northernmost part of the province of Pavia. Milanese, due to t ...
of
Lombard.
The name was approved by the
International Astronomic Union on 4 April 2022.
Pulsar
![Halo around pulsar Geminga](https://upload.wikimedia.org/wikipedia/commons/7/7d/Halo_around_pulsar_Geminga.jpg)
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
ROSAT (short for Röntgensatellit; in German X-rays are called Röntgenstrahlen, in honour of Wilhelm Röntgen) was a German Aerospace Center-led satellite X-ray telescope, with instruments built by West Germany, the United Kingdom and the Uni ...
satellite detected a
periodicity
Periodicity or periodic may refer to:
Mathematics
* Bott periodicity theorem, addresses Bott periodicity: a modulo-8 recurrence relation in the homotopy groups of classical groups
* Periodic function, a function whose output contains values tha ...
of 0.237 seconds in
soft x-ray emission. Thus, it is supposed that Geminga is a sort of
neutron star
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white ...
: the decaying core of a massive star that exploded as a
supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
about 300,000 years ago.
It was once thought that this nearby explosion was responsible for the low density of the
interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
in the immediate vicinity of the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
. This low-density area is known as the
Local Bubble
The Local Bubble, or Local Cavity, is a relative cavity in the interstellar medium (ISM) of the Orion Arm in the Milky Way. It contains the closest of celestial neighbours and among others, the Local Interstellar Cloud (which contains the Sola ...
. Possible evidence for this includes findings by the
Arecibo Observatory
The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
that local micrometre-sized interstellar meteor particles appear to originate from its direction. More recently, however, it has been suggested that multiple supernovae in subgroup B1 of the
Pleiades
The Pleiades (), also known as The Seven Sisters, Messier 45 and other names by different cultures, is an asterism and an open star cluster containing middle-aged, hot B-type stars in the north-west of the constellation Taurus. At a distance of ...
moving group were more likely responsible,
becoming a remnant
supershell
A superbubble or supershell is a cavity which is hundreds of light years across and is populated with hot (106 K) gas atoms, less dense than the surrounding interstellar medium, blown against that medium and carved out by multiple supernov ...
.
A study from 2019, using data from NASA's
Fermi Gamma-ray Space Telescope
The Fermi Gamma-ray Space Telescope (FGST, also FGRST), formerly called the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is ...
discovered a large
gamma-ray
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nucleus, atomic nuclei. It consists of the shortest wavelength electromagnetic wav ...
halo around Geminga. Accelerated
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
s and
positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...
s collide with nearby
starlight
Starlight is the light emitted by stars. It typically refers to visible electromagnetic radiation from stars other than the Sun, observable from Earth at night, although a component of starlight is observable from Earth during daytime.
Sunligh ...
. 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.
Discovery and identification
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
The Small Astronomy Satellite 2, also known also as SAS-2, SAS B or Explorer 48, was a NASA gamma ray telescope. It was launched on 15 November 1972 into the low Earth orbit with a periapsis of 443 km and an apoapsis of 632 km. It compl ...
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.
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 VLA or vla may refer to:
Organizations
* Vermont Library Association, professional organization for librarians from Vermont
* Veterinary Laboratories Agency, a UK government agency for researching animal and public health
* Victoria Legal Aid, an ...
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
ROSAT (short for Röntgensatellit; in German X-rays are called Röntgenstrahlen, in honour of Wilhelm Röntgen) was a German Aerospace Center-led satellite X-ray telescope, with instruments built by West Germany, the United Kingdom and the Uni ...
x-ray imager detected a 237 ms pulsation,
which was also seen in gamma rays by the
EGRET
Egrets ( ) are herons, generally long-legged wading birds, that have white or buff plumage, developing fine plumes (usually milky white) during the breeding season. Egrets are not a biologically distinct group from herons and have the same build ...
instrument
and retrospectively in the COS-B and SAS-2 data.
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.
Proper motion
The
proper motion
Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more dista ...
of Geminga is 178.2
mas/year which corresponds (at a distance of 250 pc) to a projected velocity of 205 kilometres per second.
This velocity is very fast for a star, comparable to
Barnard's Star
Barnard's Star is a red dwarf about six light-years from Earth in the constellation of Ophiuchus. It is the fourth-nearest-known individual star to the Sun after the three components of the Alpha Centauri system, and the closest star in the ...
.
Timing measurements
Geminga underwent a minor
glitch
A glitch is a short-lived fault in a system, such as a transient fault that corrects itself, making it difficult to troubleshoot. The term is particularly common in the computing and electronics industries, in circuit bending, as well as among ...
in the late part of 1996, with a fractional change in frequency of 6.2 × 10
−10.
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;
however, this was later shown to be an artifact of
noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...
that affects the pulse times from Geminga rather than a genuine orbital effect.
See also
*
Gemini (Chinese astronomy) The modern constellation Gemini lies across two of the quadrants, symbolized by the White Tiger of the West (西方白虎, ''Xī Fāng Bái Hǔ'') and the Vermilion Bird of the South (南方朱雀, ''Nán Fāng Zhū Què''), that divide the sky in ...
*
List of stars in Gemini
This is the list of notable stars in the constellation Gemini, sorted by decreasing brightness.
See also
*List of stars by constellation
References
*
*
*
*
{{DEFAULTSORT:List of stars in Gemini
*List
Gemini
Gemini may refer to:
Space
* ...
References
*
External links
Spaceflight Now: 'Cannonball pulsar' seen flying across space
{{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System
Gemini (constellation)
Pulsars