Celestial gamma-ray source

A celestial gamma-ray source differs from an astronomical gamma-ray source in that the terms astronomical objects and astronomical bodies only differ from "celestial objects" and "celestial bodies" where the latter terms do not include the Earth. Of the 9,785 gamma-ray sources and gamma-ray bursts contained within the SIMBAD database, 2112 are gamma-ray sources and 7687 are gamma-ray bursts, while fourteen are both.
Celestial sphere
In astronomy and navigation, the 'celestial sphere' is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and geographical poles are the celestial equator and the celestial poles. The celestial sphere is a very practical tool for positional astronomy.
The celestial sphere is divided by projecting the equator into space. This divides the sphere into the north celestial hemisphere and the south celestial hemisphere. Likewise, one can locate the Celestial Tropic of Cancer, Celestial Tropic of Capricorn, North Celestial Pole, and South Celestial Pole. The directions toward various objects in the sky can be quantified by constructing a celestial coordinate system.
There are different celestial coordinate systems each using a system of spherical coordinates projected on the celestial sphere, in analogy to the geographic coordinate system used on the surface of the Earth. The coordinate systems differ only in their choice of the fundamental plane, which divides the sky into two equal hemispheres along a great circle. For example, the fundamental plane of the geographic system is the Earth's equator. Each coordinate system is named for its choice of fundamental plane.
Epoch
In astronomy, an epoch is a moment in time used as a reference point for some time-varying astronomical quantity, such as celestial coordinates, or elliptical orbital elements of a celestial body, where these are (as usual) subject to perturbations and vary with time. The time-varying astronomical quantities might include, for example, the mean longitude or mean anomaly of a body, or of the node of its orbit relative to a reference-plane, or of the direction of the apogee or aphelion of its orbit, or the size of the major axis of its orbit.
The International Astronomical Union (IAU) decided at their General Assembly of 1976 that the new standard equinox of J2000.0 should be used starting in 1984. (Before that, the equinox of B1950.0 seems to have been the standard.) There have been some indications that the epoch J2000.0 is (at least currently) intended to have a longer period of usage than to mid-century.
To indicate the magnitude of change from one epoch to another, for example, according to SIMBAD, gamma-ray source TeV J2203+4217 is located in equatorial coordinates for the epoch 1950 B1950.0 at right ascension (RA) and declination (Dec) and in J2000.0 at RA Dec .
Constellation
The celestial sphere has been divided into 88 constellations. The International Astronomical Union (IAU) modern constellations are areas of the sky. The original boundaries of the IAU modern constellations are along vertical and horizontal lines of right ascension and declination for the epoch B1875.0, which means that due to precession of the equinoxes, the borders on a modern star map (e.g., for epoch J2000.0) are already somewhat skewed and no longer perfectly vertical or horizontal. This skew increases over the years and centuries. However, this does not change the area of any constellation.
The total area of all constellations is the total area of the sky, which is 4π steradians, or 4π x (180/π) , or approximately 41252.961 square degrees.
Although there are only 88 IAU constellations, the sky is actually divided into 89 irregularly shaped boxes as the constellation Serpens is split into two separate sections, Serpens Caput (the snake's head) to the west and Serpens Cauda (the snake's tail) to the east:
: Serpens Caput (428.484 sq. deg.; midpoint: RA: 15 46.49, Dec: +10 58.20), and
: Serpens Cauda (208.444 sq. deg.; midpoint: RA: 18 07.60, Dec: -04 51.73).
The two constellational areas of Serpens are separated by the constellation Ophiuchus.
Each of these eighty-nine irregularly shaped boxes contains remarkable gamma-ray sources. Some of them are galaxies or black holes at the centers of galaxies. Some are pulsars. As with the astronomical gamma-ray sources, striving to understand the generation of gamma rays by the apparent source helps to understand the Sun, the universe as a whole, and how these affect us on Earth.
The only major celestial gamma-ray source that does not belong to a constellation is the Sun. The Sun travels through the 13 constellations along the ecliptic, the 12 of the Zodiac and Ophiuchus. Minor celestial X-ray sources such as the Moon also travel through several constellations.
Andromeda
Andromeda is in the northern sky. Of all the constellations it is 19th in celestial area at 722 sq. deg. A relatively uniform distribution of 1,000 astronomical gamma-ray sources would yield at least seventeen.
The quasar 4C 45.51 is a multifrequency blazar that is a gamma-ray source in Andromeda.
Aquila
If some 1,000 gamma-ray sources are distributed uniformly over the celestial sphere, Aquila at 652.473 sq. deg. should have at least fifteen.
In the image at right is the W44 supernova remnant (SNR). The composite image is made up in part by gamma-rays (magenta) detected with the NASA Fermi Gamma-ray Space Telescope, specifically the Large Area Telescope (LAT). The gamma-ray portion of the image is in the billion-electron-volt (GeV) energy range. Additionally in the image are X-rays (blue) from ROSAT, infrared (red) from the Spitzer Space Telescope, and radio (orange) from the Very Large Array near Socorro, New Mexico. This SNR has never before been resolved in the GeV range. But, it does not appear to radiate in the TeV range.
Cassiopeia
Cassiopeia is 598.407 sq. deg. of the celestial sphere. Of 1,000 uniformly distributed gamma-ray sources it would have at least fourteen.
Cassiopeia A is an extensively studied supernova remnant, having been observed in X-rays by the X-ray astronomy satellites: Uhuru, Ariel 5, HEAO 1, the Einstein Observatory, ROSAT, and the Chandra X-ray Observatory. In the composite image at right, NASA's Fermi Gamma-ray Space Telescope reveals the primary source of gamma-rays (magenta) from Cassiopeia A. Additional portions of the composite image are in X-rays (blue, green) from NASA's Chandra X-ray Observatory; visible light (yellow) from the Hubble Space Telescope; infrared (red) from NASA's Spitzer Space Telescope; and radio (orange) from the Very Large Array near Socorro, New Mexico.
Centaurus
Centaurus is the ninth largest constellation by area at 1060.422 sq. deg. For a uniform distribution of 1,000 gamma-ray sources, Centaurus is expected to have at least twenty-five.
Centaurus A is a prominent galaxy with an active galactic nucleus. It exhibits a relativistic jet with inner parts moving at about half the speed of light. As the jet collides with surrounding gases, X-rays and gamma-rays are produced. The image at right is a composite of a visual image from the Capella Observatory and a gamma-ray image from the Fermi Gamma-ray Space Telescope.
"This is something we've never seen before in gamma rays," said Teddy Cheung, a Fermi team member at the Naval Research Laboratory in Washington. "Not only do we see the extended radio lobes, but their gamma-ray output is more than ten times greater than their radio output." 3C 279 observed by the Compton Gamma Ray Observatory in 1991. It is one of the brightest gamma ray objects in the sky.

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