Geminga
Encyclopedia
Geminga is a neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...

 approximately 250 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

s away from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

 Gemini
Gemini (constellation)
Gemini is one of the constellations of the zodiac. It was one of the 48 constellations described by the 2nd century astronomer Ptolemy and it remains one of the 88 modern constellations today. Its name is Latin for "twins", and it is associated with the twins Castor and Pollux in Greek mythology...

. 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
Milanese
Milanese is the central variety of the Western Lombard language spoken in the city and province of Milan....

 .

Pulsar

The nature of Geminga was quite unknown for 20 years after its discovery by NASA's Second Small Astronomy Satellite
Second Small Astronomy Satellite
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 low Earth orbit with a periapsis of 443 km and an apoapsis of 632 km...

 (SAS-2). Finally, in March 1991 the ROSAT
ROSAT
ROSAT was a German Aerospace Center-led satellite X-ray telescope, with instruments built by Germany, the UK and the US...

 satellite detected a periodicity
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 of 0.237 seconds in soft x-ray emission
Soft x-ray emission spectroscopy
Soft X-ray emission spectroscopy is an experimental technique for determining the electronic structure of materials. It is a form of X-ray spectroscopy.-Uses:...

. Thus, it is supposed that Geminga is a sort of neutron star: the decaying core of a massive star that exploded as a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

 about 300,000 years ago.

This nearby explosion may be responsible for the low density of the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

 in the immediate vicinity of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. This low-density area is known as the Local Bubble
Local Bubble
The Local Bubble is a cavity in the interstellar medium of the Orion Arm of the Milky Way. It is at least 300 light years across and has a neutral hydrogen density of about 0.05 atoms per cubic centimetre, or approximately one tenth of the average for the ISM in the Milky Way , and half that for...

. Possible evidence for this includes findings by the Arecibo Observatory
Arecibo Observatory
The Arecibo Observatory is a radio telescope near the city of Arecibo in Puerto Rico. It is operated by SRI International under cooperative agreement with the National Science Foundation...

 that local micrometre-sized interstellar meteor particles appear to originate from its direction.

Discovery and identification

Geminga was the first example of an unidentified gamma-ray source
Gamma-ray astronomy
Gamma-ray astronomy is the astronomical study of the cosmos with gamma rays. Gamma-rays are the most energetic form of "light" that travel across the universe, and gamma-rays thus have the smallest wavelength of any wave in the electromagnetic spectrum.Gamma-rays are created by celestial events...

, 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
Cos-B
Cos-B was the first European Space Research Organisation mission to study gamma-ray sources. COS-B was first put forward by the European scientific community in the mid 1960s and approved by the ESRO council in 1969. The mission consisted of a satellite containing gamma-ray detectors, which was...

 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
Einstein Observatory
Einstein Observatory was the first fully imaging X-ray telescope put into space and the second of NASA's three High Energy Astrophysical Observatories...

, 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
VLA may refer to:Transport* RUM-139 VL-ASROC, a vertically-launched anti-submarine rocket* Very Light Aircraft, a category of aircraft, per the European EASA CS-VLA specificationsGovernment Agencies...

 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
ROSAT was a German Aerospace Center-led satellite X-ray telescope, with instruments built by Germany, the UK and the US...

 x-ray imager detected a 237 ms pulsation, which
was also seen in gamma-rays by the EGRET
Egret
An egret is any of several herons, most of which are white or buff, and several of which develop fine plumes during the breeding season. Many egrets are members of the genera Egretta or Ardea which contain other species named as herons rather than egrets...

 instrument
and retrospectively in the COS-B and SAS-2 data.

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

The proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

 of Geminga is 178.2 mas/year which corresponds to a projected velocity of 205 kilometers per second. This is very fast for a star, comparable to Barnard's star
Barnard's star
Barnard's Star, also known occasionally as Barnard's "Runaway" Star, is a very low-mass red dwarf star approximately six light-years away from Earth in the constellation of Ophiuchus . In 1916, the American astronomer E.E...

.

Possible planetary system

In 1997, John Mattox et al. claimed to have discovered a planet orbiting Geminga by gamma-ray timing of Geminga. This hypothesized planet, Geminga b, was thought to orbit about 3.3 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 from Geminga in a 5.1 year orbit. With a mass of 1.7 Earths, Geminga b would be a terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

. However, this discovery is now doubtful because recent analysis of the data indicates that the detected timing changes were due to signal noise, not a planet.

External links

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