Pulsar

A pulsar ( portmanteau of Engl. Pulsating source of radio emission " pulsating radio source " ) is a rapidly rotating neutron star. The symmetry axis of its magnetic field is different from the axis of rotation, which is why synchrotron radiation to emit along the dipole. If the soil in the radiation field, one receives as a lighthouse of recurrent signals. Pulsars emit mainly in the radio frequency range, sometimes up to or only in X-rays. Of the more than 1700 known sources could be observed intensity variations only in a few also in the visible range. The rotation period of a pulsar without companion is between 0.01 and 8 seconds. Rotation time increases per second to about 10-15 s ( ie it is in the course of time, slower) and limits the life of about ten million years.

There are also so-called millisecond pulsars ( about five percent of pulsars ) with orbital periods of one to ten milliseconds and longer service life.

Designation

Pulsars bear the acronym PSR ( Pulsating Source of radio emission ) and a grid reference. The letters B and J differ between the B1950.0 and the J2000.0 coordinate system. So is the pulsar PSR B1919 21 in the sky at about the right ascension and declination of 21 19:19 °.

History

Jocelyn Bell and Antony Hewish discovered her doctor father the first Pulsar in the search for radio sources on 28 November 1967. For this study, all sources have been detected in a wide field, which had within a short time large variations in intensity of solar radiation. The signals of the PSR B1919 21 designated later as the pulsar were characterized by unusual regularity of the radiated waves, such that Bell and Hewish it first for an artificial signal - possibly an extraterrestrial civilization - held ( Little Green Man 1). Antony Hewish was awarded in 1974 for the discovery of pulsars with the Nobel Prize for Physics.

The first physicist who suspected rotating neutron stars immediately after their discovery behind pulsars, was Thomas Gold in 1968/69. However, a conference refused at first its corresponding lecture as too absurd from and considered this as not even worthy of discussion. Later his opinion but was confirmed.

Russell Hulse and Joseph H. Taylor, Jr. discovered the pulsar PSR 1974 16 1913, a system of two each in less than eight hours orbiting pulsars. Their orbital period is shortened permanently in a way that can only be explained by the emission of gravitational waves according to the general theory of relativity. Hulse and Taylor received it in 1993 also received the Nobel Prize for Physics. By May 2006, nearly 1700 pulsars were known, including a Doppelpulsarsystem ( the system PSR J0737 - 2003 discovered 3039 ).

With an age of about 900 years, PSR B0531 21 in the Crab Nebula is the youngest known pulsar.

One particular in the formation of the pulsar is moving on a highly elliptical orbit around a sun PSR J1903 0327 big star, which rotates at 465 revolutions per second.

1982, the first millisecond pulsar was discovered with the designation PSR 1937 21. The stability of its rotation period of 1.5578 ms is at least 3:10 -16 and exceeds the accuracy of atomic clocks. This accuracy can be used for precise location of the earth, thereby providing further evidence for gravitational waves.

Emergence of a pulsar

After a supernova of a massive star, a neutron star is left in a hot, ionized gas nebula. The neutron star consists of one part of matter of the original star ( 1.44 to 3 solar masses ) in a small space (diameter 20 km). In addition, the entire supernova remnant from neutron star and nebulae retains its angular momentum, and the magnetic field of the original star is compressed in the neutron star. Furthermore, there are differences in the electrical potential of the order of 1011 volts.

A pulsar gets its radiant energy

• From accretion, see X-ray binary star,
• From the magnetic field, see Magnetar,
• And in the normal case, the rotational energy.

By conservation of angular momentum and the strong reduction of the spatial extent of the rotation of the neutron star accelerates so much so that the rotation period instead of several days is now only seconds or fractions of seconds. The result is a very compact celestial bodies with a strong magnetic field ( typical flux densities of 108 Tesla), which quickly rotates within the ionized gas nebula.

Construction of a pulsar and origin of the pulsed radiation

Pulsars have like all neutron stars have a density in the range of atomic nuclei, equivalent to around 1012 kg / cm ³ and are superfluid and superconducting.

The magnetic field direction of the neutron star close to the axis of rotation to identify a particular angle. If the magnetic field direction deviates from the axis of rotation, the magnetic field lines move quickly through the ionized gas clouds and cause the emission of electromagnetic waves in the direction of the magnetic field. Due to the rotation emphasize the magnetic field lines, and with them, the electromagnetic waves as the light of a lighthouse on the environment. Is the earth or the solar system within the double cone, which is swept out by the direction of the electromagnetic radiation pulsed radiation can be measured.

A pulsar emits electromagnetic waves over a wide wavelength range, the predominant Shares may in the frequency range of radio waves ( radio pulsar ), visible light, or even in the field of X-ray radiation ( Röntgenpulsar ) lie. Younger pulsars tend to emit higher energy radiation.

Assessments

Under simplified assumptions can be the rotational speed and rotational energy of a pulsar estimate. The output body is sunlike and have a constant density, as well as the contracted neutron star.

Output variables:

• Solar radius: 7 × 108 m
• Solar mass: 2 × 1030 kg
• Rotation time: 25.4 days; Angular velocity: 3 × 10-6 s-1

Model output:

• Neutron star radius 1.6 × 104 m
• Mass: unchanged 2 × 1030kg

The moment of inertia Θ ( Θ = 2/5 · M · R ²) decreases quadratically when the radius R decreases, at constant mass M. Since the angular momentum L (L = Θ · ω ) is retained, the rotational speed must ω to the ratio of the moments of inertia of the sun and neutron star enlarge. By the same factor is increased, the rotational energy E ( Erot = 1/2 · ω · L).

Used in the following values ​​:

• Ratio of the moments of inertia of the sun and neutron star: 2 × 109
• Rotational energy of the sun: 1.5 × 1036 J
• Rotational energy of the neutron star: 3 × 1045 J
• Rotation time: 0.001 s = 1 ms

In the simple estimate the rotational speed would be a multiple of the speed of light at the equator of the surface. Since this is impossible, can only contract a star when it repels mass and reduces its angular momentum. The rotational energy is in the range around 1040 J.

Millisekundenpulsare

Pulsar with a rotation period of less than about 20 milliseconds are called Millisekundenpulsare. The record holder is PSR J1748 - 2446ad in the globular cluster Terzan 5 with a rotation frequency of 716 Hertz ( ≈ 1.4 milliseconds per rotation). The Millisekundenpulsare differ in addition to the faster rotation of normal pulsars by their weak magnetic field of up to 108 G, its slow rotation decrease, their high characteristic age as well as their preferred occurrence in binary systems by 75 % compared to other pulsars with less than 1%. The maximum rotation frequency of neutron stars should be at around 1,500 hertz, since at higher rotational speeds, a strong emission of gravitational waves begins.

For the formation of Millisekundenpulsare two scenarios are known:

• When recycling old pulsars in binary systems is the accretion of matter that flows from the companion to the neutron star, the angular momentum transferred to the erloschenden Pulsar, thus achieving rapid rotation. The immediate predecessor of the Millisekundenpulsare mass X-ray binaries are low and medium. Because the rotational axis of the pulsar due to the accretion is perpendicular to the orbital plane the radiation hits the companion and heats it to the extent that the star loses mass. This Millisekundenpulsare are referred to as Black Widow Pulsars, because they dissolve the long-term companion star completely.
• A direct channel is the accretion -induced collapse of a white dwarf - ONeMg. Exceeds the white dwarf by collecting matter the Chandrasekhar limit of 1.4 solar masses, it does not come to a supernova of type Ia, but it is immediately produced a rapidly rotating neutron star.

Through intensive radio surveys in globular clusters, a large number have been found at Millisekundenpulsaren in recent years. The high incidence is associated with the high density of stars in this star aggregates in connection with neutron stars can catch a companion and accrete from this matter. In this phase, less than stellar mass X-ray binaries ( LMXB ), the rotation of the neutron star is accelerated to the typical values ​​for Millisekundenpulsare. Surprisingly, normal young pulsars with a rotation period of a few tenths of a second and magnetic fields in the globular clusters in addition to a large number of Millisekundenpulsaren been discovered around the 1011 Gauss. This was unexpected, because in the old globular clusters, massive stars no longer exist, can carry more than one supernova to the birth of a normal pulsar. One hypothesis is that the globular clusters, these pulsars have gravitationally captured and bound. Pulsars usually have a high proper motion caused by asymmetric supernova explosions or by the destruction of a binary star system in the supernova phase. The idea of capture of a companion and the subsequent recycling of the pulsar by accretion of matter from the companion is confirmed by the partially observed high orbital eccentricity of pulsars in globular clusters. The lanes in close binary systems should be circularized after a few 10 million years due to tidal effects and therefore these pulsars must have been revived recently.

In contrast to the normal pulsars the Millisekundenpulsare show a very low noise in the pulse arrival times, as these rapidly rotating neutron stars show no instabilities by differential rotation. Therefore, the Milliskundenpulsare are good candidates to search through the light-time effect for companions who lead a Ortänderung because of Kepler's laws to a variation of the pulse arrival times. This neutron stars, white dwarfs, brown dwarfs, exoplanets, and possibly the asteroid belt have been discovered around Millisekundenpulsare. Exoplanets and the asteroid belt may have formed from the accretion disks, which have accelerated the Millisekundenpulsare again.

Proper motion

Young pulsars show on average a proper motion of 400 km / s, with peak values ​​of up to more than 1000 km / s can be achieved. These velocities are too high to be interpreted as a result of a breakup of a binary star during a supernova explosion. For large motions, the following hypotheses have been put forward, all of which are attributed to an asymmetry in the supernova:

• A unipolar asymmetry in the structure of the progenitor star of the supernova and the pulsar. This hypothesis is not supported by current stellar models.
• An asymmetric radiation from the emission of neutrinos during the supernova. Already a deviation of 1 % can lead to a proper motion of 300 km / s.
• The gravitational forces of a non-uniformly ejected shell can give the newborn neutron star in the first seconds of its formation a kick out of some 100 km / s.

Irregular pulse profiles

Period jumps

Pulsars show next to a continuous increase in the rotational period also period jumps ( engl. glitch ) in which the rotation of the neutron star accelerates within a very short period of time. Then, the rotational period increases faster than before, until the original value has been reached before the jump. The discontinuous change in the rotational period shall cease at Millisekundenpulsaren and young neutron stars with an age of less than 500 years, with nearly all pulsars. The period jumps are as a transfer of angular momentum from the superfluid interior of the neutron star interprets the slower rotating crust. However, this model is hard to explain anti - glitches, where the rotation period of the neutron star is shortened dramatically. The period jumps are detected even in unusual X-ray pulsars. The jump activity, the cumulative period change per year continually decreases with the age of pulsars and is a method that allows the study of the interior of the neutron star.

Nulling

As nulling the temporary complete disappearance of pulses is referred to in some pulsars. Within a period of two pulses, the transition from a normal pulse can be made to the off and just as quickly can happen switching. Most pulsars affected by nulling take a break of 5 percent, which appear randomly distributed. The record holder is likely to be J1502 - 5653, in which 93% of the observation time no pulse is detected. The cause of the Nullings and fast switching between the two states is the subject of scientific debate. During a training phase, the slowing down of the rotational period of the pulsar decreases. Therefore, the emission mechanism should actually disabled and therefore can not be the result of radiation in a different direction in space, the nulling.

An extreme form of Nullings could Rotating radio transients represent. In these pulsars only single pulses with an interval 10-10000 seconds detectable while the rotational periods of 0.4 to 7 seconds is less. It involves pulsars, since individual neutron stars between the two forms Pulsar and Rotating radio transient towards and forth. The low probability of detection of Rotating Radio Transient suggests that there are as previously thought five to six times higher neutron stars in the Milky Way. Therefore, the core-collapse supernova should occur more frequently or are there alternative formation channels.

Giant pulses

In the field of radio wave pulses show a variation of their intensity by a factor of 10, a small number of pulsars, including the pulsar in the Crab Nebula, show variations in the intensity of individual pulses that exceed a factor of 10 by several orders of magnitude. The phenomenon of giant pulses seems to occur only in very young and, therefore, rapidly rotating pulsars. In comparison to the radio emission, the intensity of gamma and X-rays remains unchanged during the giant pulses. It is believed that the giant pulses have the same cause as the nulling.