Soft gamma repeater
Encyclopedia
A soft gamma repeater is an astronomical
object which emits large bursts of gamma-rays and X-ray
s at irregular intervals. It is conjectured that they are a type of magnetar
or, alternatively, neutron star
s with fossil disks around them.
On March 5, 1979 a powerful gamma-ray burst was noted. As a number of receivers at different locations in the solar system
saw the burst at slightly different times, its direction could be determined, and it was shown to originate from near a supernova remnant
in the Large Magellanic Cloud
.
Over time it became clear that this was not a normal gamma-ray burst. The photon
s were less energetic in the soft gamma-ray and hard X-ray range, and repeated bursts came from the same region.
Astronomer Chryssa Kouveliotou of the Universities Space Research Association (USRA) at NASA's Marshall Space Flight Center decided to test the theory that soft gamma repeaters were magnetars. According to the theory, the bursts would cause the object to slow down its rotation. In 1998, she made careful comparisons of the periodicity of soft gamma repeater SGR 1806-20
. The period had increased by 0.008 seconds since 1993, and she calculated that this would be explained by a magnetar with a magnetic field
strength of 8×1010 tesla
s (8×1014 gauss
). This was enough to convince the international astronomical community that soft gamma repeaters are indeed magnetars.
An unusually spectacular soft gamma repeater burst was SGR 1900+14
observed on August 27, 1998. Despite the large distance to this SGR, estimated at 20,000 light years, the burst had large effects on the Earth's atmosphere. The atoms in the ionosphere
, which are usually ionized by the Sun's radiation by day and recombine to neutral atoms by night, were ionized at nighttime at levels not much lower than the normal daytime level. The Rossi X-Ray Timing Explorer (RXTE), an X-ray
satellite
, received its strongest signal from this burst at this time, even though it was directed at a different part of the sky, and should normally have been shielded from the radiation.
Known soft gamma repeaters include:
The numbers give the position in the sky, for example, SGR 0526-66 has a right ascension
of 5h26m and a declination
of −66°. The date of discovery sometimes appears in a format such as 1979/1986 to refer to the year the object was discovered, and the year soft gamma repeaters were recognized as a separate class of objects rather than "normal" gamma-ray bursts.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
object which emits large bursts of gamma-rays and X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s at irregular intervals. It is conjectured that they are a type of magnetar
Magnetar
A magnetar is a type of neutron star with an extremely powerful magnetic field, the decay of which powers the emission of copious high-energy electromagnetic radiation, particularly X-rays and gamma rays...
or, alternatively, 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...
s with fossil disks around them.
On March 5, 1979 a powerful gamma-ray burst was noted. As a number of receivers at different locations in 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...
saw the burst at slightly different times, its direction could be determined, and it was shown to originate from near a supernova remnant
Supernova remnant
A supernova remnant is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.There are two...
in the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
.
Over time it became clear that this was not a normal gamma-ray burst. The photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s were less energetic in the soft gamma-ray and hard X-ray range, and repeated bursts came from the same region.
Astronomer Chryssa Kouveliotou of the Universities Space Research Association (USRA) at NASA's Marshall Space Flight Center decided to test the theory that soft gamma repeaters were magnetars. According to the theory, the bursts would cause the object to slow down its rotation. In 1998, she made careful comparisons of the periodicity of soft gamma repeater SGR 1806-20
SGR 1806-20
|- style="vertical-align: top;"| Distance | 50,000 light-years SGR 1806-20 is a magnetar, a particular type of neutron star. It has been identified as a soft gamma repeater. SGR 1806-20 is located about 14.5 kiloparsecs from Earth on the far side of our Milky Way galaxy in the constellation of...
. The period had increased by 0.008 seconds since 1993, and she calculated that this would be explained by a magnetar with a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
strength of 8×1010 tesla
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
s (8×1014 gauss
Gauss (unit)
The gauss, abbreviated as G, is the cgs unit of measurement of a magnetic field B , named after the German mathematician and physicist Carl Friedrich Gauss. One gauss is defined as one maxwell per square centimeter; it equals 1 tesla...
). This was enough to convince the international astronomical community that soft gamma repeaters are indeed magnetars.
An unusually spectacular soft gamma repeater burst was SGR 1900+14
SGR 1900+14
|- style="vertical-align: top;"| Distance | SGR 1900+14, is located in the constellation of Aquila about 45,000 light-years away. It is an example of an intensely magnetic star, known as a magnetar, which is thought to arise from a fairly recent supernova explosion; only four are known to exist for...
observed on August 27, 1998. Despite the large distance to this SGR, estimated at 20,000 light years, the burst had large effects on the Earth's atmosphere. The atoms in the ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
, which are usually ionized by the Sun's radiation by day and recombine to neutral atoms by night, were ionized at nighttime at levels not much lower than the normal daytime level. The Rossi X-Ray Timing Explorer (RXTE), an X-ray
X-ray astronomy
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...
satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
, received its strongest signal from this burst at this time, even though it was directed at a different part of the sky, and should normally have been shielded from the radiation.
Known soft gamma repeaters include:
| Object | Discovery | Notes |
|---|---|---|
| SGR 0525-66 SGR 0525-66 |- style="vertical-align: top;"| Distance | 165.000 LySGR 0525-66 is a star located in the Super-Nova Remnant 0525-66.1, otherwise known as N49, in the Large Magellanic Cloud.-References:* * Soft gamma repeater*... |
1979 | |
| SGR 1806-20 SGR 1806-20 |- style="vertical-align: top;"| Distance | 50,000 light-years SGR 1806-20 is a magnetar, a particular type of neutron star. It has been identified as a soft gamma repeater. SGR 1806-20 is located about 14.5 kiloparsecs from Earth on the far side of our Milky Way galaxy in the constellation of... |
1979/1986 | The most powerful soft gamma repeater burst yet recorded was observed coming from this object on December 27, 2004. |
| SGR 1900+14 SGR 1900+14 |- style="vertical-align: top;"| Distance | SGR 1900+14, is located in the constellation of Aquila about 45,000 light-years away. It is an example of an intensely magnetic star, known as a magnetar, which is thought to arise from a fairly recent supernova explosion; only four are known to exist for... |
1979/1986 | |
| SGR 1627-41 SGR 1627-41 SGR 1627-41, is a soft gamma repeater. It was discovered June 15, 1998 using the Burst and transient Source Experiment and was the first soft gamma repeater to be discovered since 1979. During a period of 6 weeks, the star bursted approximately 100 times, and then went quiet. The measured bursts... |
1998 | |
| SGR J1550-5418 SGR J1550-5418 SGR J1550-5418 is a soft gamma-ray repeater , the sixth to be discovered, located in the constellation Norma.Long known as an X-ray source, it was noticed to have become active on 23 October 2008, and then after a relatively quiescent interval, became much more active on 22 January 2009.It has been... |
2008 | Rotates once every 2.07 seconds, holds the record for the fastest-spinning magnetar. |
| SGR 0501+4516 SGR 0501+4516 SGR 0501+4516 is one of 15 known magnetars, or soft gamma repeaters , and is an ancient stellar remnant. It is located approximately 15,000 light years from earth and has a magnetic field 100 trillion times stronger than the Earth’s.-Discovery:... |
2008 | Distance 15,000 light-years; X-ray outburst detected by Swift satellite 22 August 2008. |
The numbers give the position in the sky, for example, SGR 0526-66 has a right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
of 5h26m and a declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
of −66°. The date of discovery sometimes appears in a format such as 1979/1986 to refer to the year the object was discovered, and the year soft gamma repeaters were recognized as a separate class of objects rather than "normal" gamma-ray bursts.