Magnetar

A magnetar is a neutron star whose magnetic field has 1,000 times the usual neutron stars value. It is estimated that about 10 % of all neutron stars belong to this class of stars. The theory of magnetars was developed by Robert Duncan and Christopher Thompson. They were discovered in 1998 by Chryssa Kouveliotou.

Formation and properties

Neutron stars are formed according to the established theories in the collapse of stars with a core mass of about 1.4 to 3 solar masses within the framework of a supernova. They have a typical diameter of only about 10 to 20 miles and an extremely strong magnetic field with a flux density of the order 108 Tesla (T). These results based on the laws of electrodynamics, and the product of star cross section and magnetic field remains constant during the collapse of the progenitor star.

Due to the pirouette effect ( angular momentum conservation ) rotating neutron stars immediately after the collapse with rotation periods in the millisecond range. A Magnetar occurs only when the rotation period is less than 10 ms, and the precursor star had a relatively strong magnetic field. Otherwise, there is an ordinary neutron star or pulsar. This is caused by convection in the ultra dense neutron matter, which rotate immediately after the collapse with rotation periods of 10 ms. Rotating the total rating faster so employs a dynamo effect that converts the kinetic energy of the massive convection within about 10 seconds in the magnetic field energy.

This creates a magnetic field that a thousand times as strong with 1011 T as an ordinary neutron star. The mass density that can be associated with such a magnetic field over its energy density in combination with the equivalence of mass and energy according to, is in the range of a few dozen kilograms per cubic millimeter ( kg/mm3 ). Such a magnetic field is so strong that it changes the structure of the quantum vacuum, so that the matter-free space is birefringent.

The axis of the magnetic field to the axis of rotation is inclined so a periodic radio wave having a typical range of the power is radiated in the 108 -fold of the total radiant power of the Sun. The energy required for this is taken from the rotational energy, which is thus largely absorbed within 10,000 years. The rotation period is then several seconds. Ordinary pulsars are slowed considerably less and therefore rotate much faster.

A Magnetar may also result from the merger of two neutron stars in a close binary system. The magnetar formed thereby generates its strong magnetic field due to the rapid differential rotation, which is a consequence of the merger process.

Isolated neutron stars, which do not have a companion in a binary star system are counted as magnetars, if at least three of the following properties are observed:

• The period of rotation is in the range of 1 to 12 seconds
• The rate of deceleration of the rotation exceeds 10-12 s s -1
• A high and variable continuous X-ray brightness of the order of 1032 - 1036 s-1 erg
• Emission of short peaks having a duration of 0.1 to 10 seconds in the field of X-ray and gamma radiation from 1034 to 1047 ergs s-1

Magnetars also emit their rest periods outside outbursts of radiation from X-rays with a luminosity 1027-1029 W. This is to heat radiation from the surface of the neutron star below 1 keV, and a second component in the range of 10 to 100 keV, but could not be detected in all magnetars. The higher-energy component is pulsed due to the rotation of the neutron star. We have developed two hypotheses for the hard component of the X-rays:

• Relativistic particles move along magnetic field lines and strike at the magnetic poles of the neutron star on. The observed X-ray Bremsstrahlung would in this case.
• Positron / electron pairs scatter in the magnetosphere of photons and transfer their energy to it. In this case, most of the X-rays would have to arise above the magnetic pole at a distance of some star radii.

Ray bursts

Man knows more than a dozen X-ray sources in the Milky Way, which are considered as candidates for magnetars. These objects show at irregular intervals gamma and X-ray bursts with a duration of a few tenths of a second. In this short time, typically much high energy radiation energy is released as the sun emits in about 10,000 years across the spectrum. This brief and extreme radiation pulse is followed by a relaxation period of several minutes in which the radiation decreases while having periodic variations in the range of several seconds, the rotation period of the magnetar.

This major outbreaks follow in the hours to years after most other smaller. One calls these radiation sources are therefore also soft gamma repeater ( SGR). A statistical analysis of these outbreaks shows an obvious affinity with the earthquakes. In fact, it is believed that these are breaks in the outer crust of the magnetar consisting of a plasma of electrons and crystalline arranged iron and other nuclei, as in all neutron stars. The cause of this strength of the magnetic field are considered acting on these solid crust.

The larger outbreaks thought to be due to large-scale rearrangement of the magnetic field become unstable as qualitatively similar happen on the sun's surface and produce the so-called flares. Then the observed high-energy radiation from a fireball of hot plasma would be sent to the surface of the magnetar, is bound locally for a few tenths of a second by the strong magnetic field, which requires field strengths over 1010 T. The intensity of the emitted radiation is also so associated that the radiation can pass through this freely fireball, since the strong magnetic field prevents the free electrons from swinging with the electromagnetic wave.

Soft gamma repeaters and anomalous Röntgenpulsare (English anomalous X -ray pulsar, AXP ) show a constant X-ray radiation from 1026 to 1029 W at a rotation period of 2-12 s your rotation is slowing down at a rate of 10-13 to 10-10. Sporadically they show outbursts from fractions of seconds to minutes with energies 1031-1040 J. Following the outbreaks the constant X-ray brightness is usually for years above the resting level.

It is assumed that magnetars only during the first 10,000 years after their formation show such outbursts and have subsequently stabilized their magnetic fields. The still hot neutron star is still shining few thousand years as anomalous Röntgenpulsar continue until its temperature it is no longer sufficient. Perhaps the Milky Way is home to several million such inconspicuous magnetars.

On December 27, 2004 at 22:30:26 CET spectacular radiation burst ( " Superflare ") of the Soft Gamma Repeater SGR 1806-20 was observed, which is in the direction of the galactic center of the Milky Way in 50,000 light years distance. The incident on the earth power of hard gamma radiation exceeded for 0.1 s of the full moon in the visible spectral range. So it was with regard to the radiation power to the brightest object outside the solar system that has ever been observed. Within 0.1 s so much energy has been radiated as the sun translates into 100,000 years. This energy was about a hundred times stronger than that of all Magnetarausbrüche together that have ever been observed in the Milky Way. After approximately 0.2 s the gamma flash went over in soft gamma - and X-rays. Had this outbreak occurred at a distance of 10 light years, it could have caused a mass extinction or mass extinctions on Earth.

In the large eruptions quasi- periodic oscillations are observed in X-ray and gamma radiation with frequencies in the range of 10 to 1000 Hz. These oscillations are interpreted as seismic vibrations of the crust of the neutron star and can be analyzed by means of asteroseismology to investigate the structure of neutron stars. Thus, the equation of state of matter under the high pressures inside the degenerate star can be determined and a reliable upper limit for the mass of the neutron star can be derived.

The approximately 30,000 light- years distant neutron star SGR J1550 - 5418 is equipped with a rotation period of 2.07 s, the fastest rotating magnetar known at the time. He also sends in rapid succession gamma -ray bursts (there were more than one hundred Strikes detected in less than 20 minutes), as observations with the Fermi Gamma - ray Space Telescope show. Observations with the X-ray telescope satellite Swift also show that the neutron star is surrounded by circular radiation echoes. Clearly reflected dust in its environment a portion of the radiation from the gamma -ray bursts.

There are at least two sources with rapid Gammastrahlen-/Röntgenausbrüchen whose magnetic field is too weak for a magnetar. SGR 0418 5729 has a magnetic field of no more than 7 x 108 T and pointing during an outbreak pulsations with a period of 9.1 s also the observed slowing of the rotation speed of SGR 0418 5729 indicates a magnetic field strength far below the 1010 to 1011 T, which can be specified when defining a magnetar basis. The unusual combination of vibrant Gamma-/Röntgenausbrüchen and a weak magnetic field could be the result of accretion from a circumstellar ring on a rotating quark star. Also in Swift J1822.3 - 1606 is a product derived from the rotation deceleration dipole field below the critical field density. From the X-ray radiation during cooling the age of Swift J1822.3 - 1606 has been estimated at 500,000 years.

The interpretation of the origin of SGRs and AXPs by the decay of an ultra- strong magnetic field in a neutron star, a magnetar is not without criticism. If the flares would run out of magnetars, the following observations should be made:

• It should be demonstrated permanent radio emission from the soft gamma repeaters and AXPs because of the high magnetic field density. In contrast, the observations show only temporary bursts of radio radiation.
• There should be no SGR type with magnetic field densities below 7 × 108 T
• There should be comparable to the magnetars with no evidence for the flares of SGRs and AXPs no pulsars with magnetic field densities. This is exactly what has been observed, however
• The young radio pulsar PSR J1846 -0258 with an age of 880 years shows strong outbursts in the field of X-ray radiation and behaves like AXP. His loss of rotational energy covers the need for radiated electromagnetic radiation.

There are alternative hypotheses, according to which the ray bursts are respectively the drift model, the result of a quark star in combination with an accretion disk. Accordingly, the pulsed radiation near the light cylinder is caused by trapped in magnetic loops plasma. In these models, not a magnetar is required, but a rapidly rotating neutron star with a magnetic field of about 108 T. Also massive white dwarfs with a strong magnetic field and mass of 1.4 solar masses might outbreaks that are attributed to the magnetars produce. Due to the larger radius of the white dwarfs in comparison to neutron stars they have more angular momentum, which can be released due to the cooling of the white dwarf during shrinking and the energy required for the ray bursts provides.

Magnetars show anti - glitches in contrast to all other isolated neutron stars. A glitch is a sudden period shortening of the period of rotation in pulsars, which are interpreted to a transfer of torque from the interior of the neutron star to its crust. The abrupt period extensions of magnetars, which are referred to as anti - glitches, on the other hand probably have their origin in the magnetosphere or are a consequence of wind braking. Both hypotheses are based on the observation that the Anti- glitches are temporarily associated with a radiation burst.

Magnetars in over luminous supernovae

A small group of supernovae shines about one hundred times more energy than normal supernovae of type I. They achieve both a higher maximum brightness and a wider light curve. For those over light eruptions three hypotheses have been developed:

• An intense interaction of the supernova envelope with the circumstellar matter, which was dropped in a previous stage of the progenitor star.
• The result is more 56Ni in a pair instability supernova, the decay of these radioactive isotopes determines the late stages of the light curve.
• After the birth of a magnetar in the supernova its rotation speed is decelerated quickly and the released energy stimulates the over -luminous supernova.

The Magnetarmodell explains better than the two alternative models the frequently observed asymmetric light curve near the maximum and the variance of the maximum brightness.

The Millisekundenmagnetarmodell is also regarded as a potential energy source for the bursts long. This leads to a gravitational collapse in a massive star, which shows a proto - neutron star with a rotation period of the order of a millisecond and a strong magnetic field with a magnetic flux density of more than 1011 t. Because of this proto- neutron star, an energy of up to 1045 J within a period of 100 s can be extracted. This energy occurs under certain conditions along the axis of rotation of a massive star and a jet accelerated to relativistic speeds. If such jets directed to the earth, they are registered here as gamma-ray bursts of long duration. The magnetar collapses probably due to back- falling matter within a short time into a black hole after exceeding the Tolman -Oppenheimer - Volkoff limit.