A magnetar is a pulsar ( neutron star ) with extremely intense magnetic fields , which at 10 11 to 10 12 Tesla are about a thousand times stronger than usual with neutron stars. An estimated 10% of all neutron stars are magnetars.
They were discovered by several satellites in 1979 as the most powerful extrasolar gamma-ray bursts known to date , known as Soft Gamma Repeaters (SGR). The magnetar theory for SGRs was developed in 1992 by Robert C. Duncan and Christopher Thompson . The confirmation of particularly high magnetic fields came in 1998 from Chryssa Kouveliotou and colleagues. In 2003 Duncan, Thompson and Kouveliotou received the Bruno Rossi Prize for this .
According to current theories, neutron stars are formed during the gravitational collapse of stars with a nuclear mass of around 1.4 to 3 solar masses in a supernova . They have a typical radius of only about 10 to 15 km and an extremely strong magnetic field with a flux density of the order of 10 8 Tesla (T). The high flux density is based on the laws of electrodynamics , according to which the product of the star cross section and the magnetic field remains constant when the precursor star collapses.
Immediately after the collapse , neutron stars rotate due to the pirouette effect ( conservation of angular momentum ) with periods in the millisecond range, individual convection zones with 10 ms. If the rotation period of the star as a whole is less than 10 ms (and the precursor star already had a relatively strong magnetic field), a magnetar is created: a dynamo effect sets in that converts the enormous kinetic energy of the convection vortex into magnetic field energy within about 10 s. This creates a magnetic field that, at 10 11 T, is around a thousand times as strong as that of a normal neutron star. If, on the other hand, the rotation period of the star as a whole is greater than that of the convection zones, or if the precursor star had a weak magnetic field, a normal neutron star or pulsar is created .
The bulk density which such a magnetic field over its energy density in combination with the mass-energy equivalence in accordance can be assigned, is located in the region of a few dozen kilograms per cubic millimeter (kg / mm 3 ). Such a magnetic field is so strong that it changes the structure of the quantum vacuum , so that the matter-free space becomes birefringent .
If the axis of the magnetic field is inclined to the axis of rotation , a periodic radio wave is emitted, the power of which is typically 10 8 times as great as the total radiation power of the sun. The energy required for this is taken from the rotational energy , which is largely consumed within 10,000 years; the rotation period is then several seconds. Ordinary pulsars are slowed down considerably less and therefore rotate much faster.
A magnetar is possibly created by the merging of two neutron stars in a close binary star system . The magnetar then forms its strong magnetic field through a rapid differential rotation as a result of the merging process.
Example CXOU J164710.2-45516
Using the example of the magnetar CXOU J164710.2-45516 in the star cluster Westerlund 1 in the southern constellation Altar , which is 16,000 light years away , it became clear how a magnetar arises from a binary star system: The precursor star had about 40 times the solar mass. At first, two heavy stars orbited each other very closely. The heavier star first used up its fuel and puffed up. Its outer layers passed over to the star with less mass, which rotated faster and faster, which made it the magnetar precursor. In the star cluster Westerlund 1, the companion star Westerlund 1-5 not only had a relatively low mass and high luminosity, but also the high speed that can be expected after the recoil of a supernova. Its chemical composition - besides hydrogen and nitrogen, a lot of carbon - is unusual for stars. If the companion star is big enough, it returns parts of its matter to the first star and explodes as a supernova. The matter transfer before the end is the condition for the magnetar formation. As a result, the precursor star loses the mass that would otherwise make it a black hole, instead it becomes a magnetar. His companion - like Westerlund 1-5 - is thrown away by the force of the explosion - with parts of the matter of the neighboring star. This explains its composition.
Isolated neutron stars that have no companion in a binary star system are counted as magnetars if at least three of the following properties are observed:
- The rotation period is in the range from 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 in the order of 10 32 - 10 36 erg / s = 10 25 W - 10 29 W .
- Emission of short peaks with a duration of 0.1 to 10 seconds in the range of X-ray and gamma radiation with 10 34 - 10 47 erg / s = 10 27 W - 10 40 W.
Magnetars also emit X-rays with a luminosity of 10 27 to 10 29 W outside of bursts of radiation . This involves thermal radiation from the surface of the neutron star below 1 k eV and a second component in the range from 10 to 100 keV, which, however, has not yet been detected in all magnetars. The higher-energy, hard component is pulsed due to the rotation of the neutron star. Two hypotheses have been developed for this component of X-rays:
- Relativistic particles move along the magnetic field lines and hit the magnetic poles of the neutron star. In this case, the observed X-rays would be bremsstrahlung .
- Electron / positron pairs scatter on photons in the magnetosphere and transfer their energy to them. In this case, most of the X-ray radiation would have to arise at a distance of a few star radii above the magnetic poles.
Bursts of radiation
More than a dozen X-ray sources are known in our Milky Way that are considered candidates for magnetars. These objects show gamma and X-ray bursts with a duration of a few tenths of a second at irregular intervals . In this short time, as much high-energy radiation energy is released as the sun radiates across the entire spectrum in about 10,000 years . This short and extreme radiation pulse is followed by a relaxation phase lasting several minutes , in which the radiation decreases and has periodic fluctuations in the range of several seconds, the period of rotation of the magnetar.
These large outbreaks are usually followed by other smaller ones in the hours or years thereafter. These radiation sources are therefore also called Soft Gamma Repeaters (SGR). A statistical analysis of these eruptions shows a striking relationship with that of earthquakes. In fact, it is believed that these are fractures in the outer crust of the magnetar, which, like all neutron stars, consists of a plasma of electrons and crystalline iron and other atomic nuclei. Forces of the magnetic field that act on this solid crust are considered to be the cause.
The larger outbreaks are attributed to the large-scale rearrangement processes of a magnetic field that has become unstable, as they occur qualitatively similarly on the solar surface and generate the so-called flares there . After that, the observed high-energy radiation would be emitted by a fireball of hot plasma on the surface of the magnetar, which is locally bound by the strong magnetic field for a few tenths of a second, which requires field strengths above 10 10 T. The intensity of the emitted radiation is also associated with the fact that the radiation can penetrate this fireball unhindered, as the strong magnetic field prevents the free electrons from oscillating with the electromagnetic wave .
Soft gamma repeaters and anomalous X-ray pulsars ( AXP) show a constant X-ray radiation of 10 26 to 10 29 W with a rotation period of 2 to 12 s. Their rotation slows down at a rate of 10 −13 to 10 −10 . Sporadic outbreaks show of fractions of seconds to minutes with energies of 10 31 to 10 40 J . After the eruptions, the constant X-ray brightness usually remains above the resting level for years.
It is assumed that magnetars only show such eruptions in the first 10,000 years after their formation and have then stabilized their magnetic fields. The still hot neutron star will continue to shine as an anomalous X-ray pulsar for a few thousand years until its temperature is no longer sufficient. The Milky Way may be home to several million such inconspicuous magnetars.
Possible magnetars as a source
On December 27, 2004 at 22:30:26 CET a spectacular radiation outbreak ( superflare ) of the Soft Gamma Repeater SGR 1806-20 was observed, which is 50,000 light years away from the galactic center of the Milky Way . The power of hard gamma radiation arriving on earth exceeded that of the full moon in the visible spectral range for about 0.1 s. In terms of radiant power, this was the brightest object outside the solar system that has ever been observed. Within 0.1 s, as much energy was emitted as the sun would convert in 100,000 years. This energy was about a hundred times more powerful than any combined magnetar burst ever observed in the Milky Way. After about 0.2 s, the gamma flash changed into soft gamma and X-ray radiation. If this eruption had occurred 10 light years apart, it could have triggered a mass extinction or extinction on Earth .
In the case of large eruptions, quasi-periodic oscillations in the range of X-rays and gamma rays with frequencies in the range from 10 to 1000 Hz are observed. These oscillations are interpreted as seismic oscillations of the crust of the neutron star and can be analyzed with the help of asteroseismology to study the structure of neutron stars. With this, the equation of state of matter under the high pressures inside the degenerate stars can be determined and a reliable upper limit for the mass of the neutron stars can be derived.
The neutron star SGR J1550-5418, which is about 30,000 light years away, is the fastest rotating magnetar currently known with a rotation period of 2.07 s. It also emits gamma-ray flashes in rapid succession (more than one hundred flashes were registered in less than 20 minutes), as observations with the Fermi Gamma-ray Space Telescope show. Observations with the X-ray telescope of the Swift satellite also show that the neutron star is surrounded by circular radiation echoes. Apparently, dust in its surroundings reflects part of the radiation from the gamma-ray flashes.
Other sources of radiation bursts that do not match the Magnetar model
There are at least two sources with rapid gamma-ray / X-ray bursts whose magnetic field is too weak for a magnetar. SGR 0418 + 5729 has a magnetic field of no more than 7 · 10 8 T and showed pulsations with a period of 9.1 s during an outbreak. The observed slowdown in the rotation speed of SGR 0418 + 5729 also speaks for a magnetic field strength far below the 10 10 to 10 11 T, which are used as a basis for the definition of a magnetar. The unusual combination of pulsating gamma / x-ray bursts and a weak magnetic field could be the result of accretion from a circumstellar ring onto a rotating quark star . With Swift J1822.3-1606, too , a dipole field derived from the deceleration of rotation is below the critical field density. From the X-rays during the cooling, the age of Swift J1822.3-1606 has been estimated to be 500,000 years.
Criticism & alternative models for the radiation bursts
The interpretation of the origin of SGRs and AXPs through the decay of an ultra-strong magnetic field in a neutron star, a magnetar, has not gone without criticism. If the radiation bursts originated from magnetars, the following observations should be made:
- Permanent radio radiation from the soft gamma repeaters and AXPs should be detected because of the high magnetic field density. The observations, on the other hand, show only temporary outbreaks of radio radiation.
- There should be no SGR with magnetic field densities below 7 · 10 8 T.
- There should be no pulsars with magnetic field densities comparable to the magnetars without evidence of the radiation bursts of the SGRs and AXPs. However, this is exactly what has been observed
- The young radio pulsar PSR J1846-0258 with an age of 880 shows strong outbreaks in the range of X-rays and behaves like AXP. Its loss of rotational energy covers the need for radiated electromagnetic radiation.
There are alternative hypotheses, according to which the radiation bursts are the result of a quark star in combination with an accretion disk or the drift model. Accordingly, the pulsed radiation is created near the light cylinder by plasma enclosed in magnetic field loops. In these models, a magnetar is not required, but a rapidly rotating neutron star with a magnetic field of around 10 8 T. Massive white dwarfs with a strong magnetic field and masses of 1.4 solar masses could also generate outbreaks that are ascribed to the magnetars. Due to the larger radius of the white dwarfs compared to neutron stars, they have more angular momentum, which can be released due to the cooling of the white dwarf when it shrinks and provides the energy required for the radiation bursts.
Magnetars show anti-glitches in contrast to all other isolated neutron stars. A glitch is an abrupt period shortening of the period of rotation of pulsars, which is interpreted as a transfer of torque from the interior of the neutron star to its crust. The abrupt period extensions of the magnetars, which are called 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 temporally related to a radiation outbreak.
Magnetars in superluminous supernovae
A small group of supernovae radiates about a hundred times more energy than a normal Type I supernova ; they achieve both a higher maximum brightness and a broader light curve and are called superluminous supernovae . Three hypotheses have been developed for these overly bright eruptions:
- An intense interaction of the supernova shell with the circumstellar matter that was shed from the precursor star in a previous stage.
- More 56 Ni is created in a pair instability supernova , with the decay of these radioactive isotopes determining the late stages of the light curve.
- After the birth of a magnetar in the supernova, its speed of rotation is quickly slowed down, and the energy released in the process drives the superluminous supernova.
Magnetars as a source of gamma-ray bursts of long duration
The millisecond magnetar model is also seen as a possible energy source for long-lasting gamma-ray bursts . This leads to the gravitational collapse of a massive star, from which a proto-neutron star emerges with a rotation period of approx. One millisecond and a strong magnetic field with a magnetic flux density of over 10 11 T. An energy of 10 45 J can be extracted from this within 100 seconds . Under certain conditions it emerges along the star's axis of rotation and accelerates a jet to relativistic speeds. If such jets are directed towards the earth, they are registered here as long-lasting gamma-ray bursts. After crossing the Tolman-Oppenheimer-Volkoff boundary, the magnetar will likely collapse into a black hole within a short time due to relapsing matter .
Possible precursors of magnetars
It is possible that magnetars are formed as a late consequence of a collision between two massive stars. Tau Scorpii is a candidate for such a potential precursor. In 2019, a research group was able to show with a simulation that the special magnetic properties of Tau Scorpii could actually be attributed to such a merger of two stars. According to the simulation, the collision of the stars leads to the formation of an accretion disk , which orbits the newly formed star. The high speed of the particles in the star and accretion disk creates strong magnetic fields . Magnetic fields created in this way can possibly persist for so long that they persist even after a supernova . The resulting neutron star would then be a magnetar.
- SGR 1900 + 14
- CXOU J164710.2-45516
- SGR 1806-20
- SGR 1550-5418
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