The collapse left the densest object known-a solar mass object squeezed into the size of a city. These star remnants are about 20 kilometers (12.5 miles) in diameter. The weight of a neutron star cube on the earth is about 1 trillion kilograms (or 1 billion tons)-about equivalent to a mountain.
Since neutron stars began to exist in the form of stars, they have been found scattered throughout the galaxy where we found stars. Like stars, they can exist alone or in a binary system of companion stars.
Most neutron stars are observed as pulsars. Pulsars are rotating neutron stars, and radiation pulses are observed at very regular intervals, usually between milliseconds and seconds. Pulsars have a very strong magnetic field, which can eject a jet of particles along two magnetic poles. These accelerating particles produce very powerful light beams. Usually, the magnetic field is not aligned with the spin axis, so these particle beams and rays will be swept when the star rotates. When the light beam passes through our sight, we will see a pulse-in other words, when the light beam sweeps across the earth, we will see the pulsar turn on and off.
One way to look at pulsars is to compare them to lighthouses. At night, the lighthouse sends out a beam of light and sweeps across the sky. Even if the light is on all the time, you can only see it when the beam is pointing directly in your direction. The video below is an animation of a neutron star, which shows that the magnetic field rotates with the star. Halfway through, the angle of view changes, so we can see the light beam sweeping our line of sight-this is the pulse mode of pulsars.
Another kind of neutron star is called a magnetar. In a typical neutron star, the magnetic field is trillions of times that of the earth; However, in magnetic stars, the magnetic field is stronger than 1000 times.
1967, Jocelyn Bell, a doctoral student under the guidance of antony hewish, detected the radio signal at Murad Radio Observatory in Cambridge, England by using interplanetary scintillation array. The signal has very regular pulses with an interval of 1.3 seconds. In fact, the repetition time of the signal at 1.3 seconds is so accurate that it was initially thought to be due to the noise in the telescope. However, it turns out that this is radio radiation from a pulsar now called PSR B1919+12.
Most pulsars are found by their radio signals. Accretion neutron stars in binary systems are mainly observed in X-rays. Magnetism can be observed in both X-rays and gamma rays. Without multi-wavelength observation, we wouldn't know the number of neutron stars as we do now.
This page focuses on pulsars and magnetars as multi-wavelength sources. Check out our binary page to learn more about binary and accretion-driven binary.
Pulsars are not only observed for the first time in radio waves, but also most pulsars we know are first discovered as wireless power sources. Some pulsars discovered by radio have also been found to produce pulses of visible light, X-rays and gamma rays. However, some pulsars with no radio correspondence have been found in X-rays and gamma rays. There are several different mechanisms to power pulsars.
The "rotational power" pulsar is finally powered by the spin of the neutron star. Radio, optical, X-ray and gamma-ray pulsar beams can be generated when high-energy electrons interact in the magnetic field above the magnetic pole of neutron stars. The ultimate energy comes from the rotation of neutron stars. The final loss of rotational energy leads to the slow rotation period of pulsars.
When a neutron star first formed in a supernova, its surface was very hot (over 654.38+0 million degrees). Over time, the surface will cool, but when the surface is still hot enough, it can be seen with an X-ray telescope. If some parts of the neutron star are hotter than others, such as the magnetic poles, we can see the thermal X-ray pulses on the surface of the neutron star when the hot spots pass through our sight.
Magnetic stars are neutron stars with extreme magnetic fields-even more extreme than neutron stars in pulsars. These sources show stable X-ray pulsation and soft gamma ray burst. In fact, the first discovered magnetar is called soft gamma ray repeater (SGR), which is considered as a subclass of gamma ray bursts (see our page about gamma ray bursts to know what they are).
Gamma ray bursts have been discovered for more than 25 years, but there are still many uncertainties about their origins. They were first discovered in the late1960s as part of the nuclear test ban verification; The American satellite received a gamma ray burst. Many people worry that these explosions may be caused by nuclear explosions in the Soviet Union, but they are convinced that these explosions come from outside the atmosphere. "Official" was found in 1973 (proposed by Klebsedal, Olsen and Strong). Since then, more than 2,500 bursts have been detected, among which BATSE (Compton Gamma Ray Observatory Burst and Transient Source Experiment) exceeded 1800. Before solving the problem of what gamma-ray bursts are, we need to determine what they are through observation.
Roughly speaking, gamma ray bursts are energy bursts that mainly appear in gamma rays and come from outside the earth. The flux on the earth is between 10-8 erg/cm 2/s and 10-3 erg/cm 2/s, the explosion duration is between 10 ms and 1000 s, and the photon energy is usually between 100 keV and 2 MeV. Flux as a function of time varies from pulse to pulse, but the peak in a pulse usually follows a "Fred" curve (rapid rise and exponential decay). This is an animated gif, which shows the explosion simulation we see on the galaxy map (left) and its brightness as a function of time (right). In a word, gamma ray bursts are very uneven, so it is difficult to extract characteristic behaviors that are easy to classify (see the typical time profile of GRB).
Can we at least know how far the gamma ray burst is? Until recently, the answer was "no", uncertain. From the beginning of 1970, it is obvious that gamma ray bursts come from all parts of the sky with roughly equal probability. Because other aspects of gamma-ray bursts (such as fast rise time [less than1ms in some cases] and high photon energy) seem to be consistent with the origin of neutron stars, most people before 1990 thought that gamma-ray bursts came from neutron stars in the Milky Way, but those instruments simply didn't have enough sensitivity to detect the detection ability deep enough to see the deviation from the center and plane of the Milky Way. However, since 1990, the Burst and Transient Source Experiment (BATSE) of Compton Gamma Ray Observatory has observed nearly one gamma ray burst every day, and these gamma ray bursts are almost isotropic (the sky map of the first 92/kloc-0 bursts). It is believed that if neutron stars in galaxies are really the source of gamma-ray bursts, BATSE should be able to see them far enough away, so that their distribution should be more like pancakes than spheres. Another evidence comes from the number of sources with at least a given flux. If the universe is Euclidean and the sources are evenly distributed, there will be many sources at the distance R that are proportional to R 3, and the darkest source will have a flux proportional to 1/R 2. Therefore, in the Euclidean universe with a uniformly distributed source of intrinsic luminosity, the curve between log N(N = the number of sources with flux greater than f) and log F should have a slope of -3/2. At the highest flux, this slope can be seen, but at lower flux, the slope becomes smaller, showing continuous rolling, and becomes about -0.8 at the lowest flux that BATSE can see.
What does this mean? If the intrinsic luminosity is constant, the attenuation at a lower flux corresponds to a larger distance, which means that the distribution is marginal in a sense. For example, if the source is distributed on a thin plane instead of a sphere, the slope is-1, while for the source on a straight line, the slope is -0.5. Even if the source distribution is spherical, if the source becomes less dense at a further distance, or the flux decreases at a speed exceeding 1/r 2, the slope will turn over. Because of the isotropic distribution, many people think that gamma-ray bursts are cosmological. When the typical red shift z= 1, the red shift will reduce the flux in the right way to explain the log N-log F flip. But until 1997, there was no "conclusive evidence" to tell us that these bursts were cosmological. In fact, there was a reasonable gamma-ray burst model, in which the bursts came from the extended halo around our galaxy.
All this changed in 1997, when researchers made a major breakthrough by using the Italian-Dutch satellite BeppoSAX. One of the crux of our understanding of gamma-ray bursts is that they are always a "once and for all" phenomenon. After a brief flash of gamma rays lasted for a few seconds, that was all she wrote. There is no detectable emission in other frequencies (such as optical and radio), which means that the location of these sources cannot be determined. This is where BeppoSAX comes in. This satellite can detect the X-ray emission of gamma-ray bursts six to ten times a year, and locate the emission in about two minutes (thirty minutes or a little smaller than the appearance size of billiards at the far end of the football field). This is better than localization using BATSE 100 times. It makes people find that most gamma-ray bursts observed by BeppoSAX have X-ray afterglow. This is the first picture, showing a bright spot (left) and then gradually disappearing (right). Many also have optical and radio afterglow! The afterglow of optics and radio makes the position determined to be angular seconds or better (the apparent size of eyelashes at the far end of the football field! )。 Further observation shows that there are gamma-ray bursts in galaxies, at least in projection and possibly in reality. Not only that, many of these galaxies have detected red shifts, and some of them are very large: one detected red shift is greater than 3.4! So this solves at least part of the problem: the explosion observed with BeppoSAX is absolutely cosmological. If gamma ray bursts are cosmological, their energy release must be enormous. At present, people think that most storms are emitted at close range (just like lighthouses), and only 10 5 1 erg is emitted in gamma rays. This is still puzzling. The constraints of these models are very strict, and no detailed model can bypass all the constraints. However, whatever these are, the energy release itself ensures that the central engine is one of the biggest explosions around! At present, the two most popular views are that (1) the explosion is caused by the inhalation of two neutron stars or a neutron star and a black hole, or (2) the explosion is caused by a massive star (which may have 20 solar masses or more) entering a rapidly rotating massive black hole. Generally speaking, a longer pulse train belongs to type (2), but there is still a problem for a shorter pulse train. Either way, it seems inevitable that all the energy will enter interstellar space and produce serious shock waves.
Another (slightly less mysterious) explosion is thought to come from a neutron star, which is a soft gamma ray repeater explosion. These usually last 0. 1 sec to 3 seconds, and have spectral peaks in the range of 10 keV to 30 keV. In the past, soft gamma-ray repeaters have been identified as supernova remnants, but these identifications are now considered suspicious, except for a single source in large magellanic cloud (SGR 0525-66) (see Gaensler et al. 200 1, ApJ, 559, 963). Caution is particularly appropriate because there are only four (! ) SGR(SGR 0525-66, SGR 1900+ 14, SGR 1806-20 and SGR 1627-4 1) is known, where the numbers give b/KLOC. Although these sources are few, people's interest in them has been focused on them, because (1) they have different observation characteristics from any other known astronomical phenomena, (2) they have some attractive connections with gamma ray bursts, and (3) the current SGRs model involves 10 14 Gauss to 10.
SGR 0525-66 had a special epidemic on March 5th 1979, which caused great concern and was usually called "March 5th incident". This is the highest intensity gamma ray event seen so far. It starts with a hard peak that lasts for a quarter of a second, rises less than a millisecond, and then continues to emit soft radiation for 200 seconds. Transmission during this extended tail has a clear period of 8 seconds and is consistent with rotational modulation. Due to the high intensity and rapid occurrence of this event, nine different satellites in the whole solar system recorded this event, and the relative time between satellites made it possible to determine the direction of the event very accurately. It is determined that the direction of the event is consistent with the N49 supernova remnant in large magellanic cloud, and its distance is more than 50,000 parsec. At this distance, the peak luminosity of the initial hard peak exceeds 10 45 erg per second. That is to say, a quarter of an hour before the outbreak, the energy emitted by this source is equivalent to the energy radiated by the sun in 3000 years! This is also an event that makes some astronomers think that SGR is related to classical gamma bursts. If the hard peak is analyzed separately, its duration, light curve and energy spectrum are no different from those of classical GRB. In fact, if the event is ten times as far away as it is (so we will miss the extended soft emission), we will think it is another yawning gamma ray burst.
Observations on other explosions of SGR 0525-66 (not as spectacular as the event on March 5) and explosions of SGR 1900+ 14 and SGR 1806-20 initially showed that all these explosions were related to supernova remnants, but as mentioned above, this was challenged. Even if they are related to residues, the source is not in the center of residues; Instead, they lean to one side, and the distance means a speed of 500- 1500 kilometers per second. The typical peak luminosity of SGR bursts is from 10 40 to 10 42 erg per second. This information can be summarized as follows:
Therefore, if SGR is related to supernova remnants, then they come from young neutron stars. The next question is what is the energy source of the explosion? People naturally think of accretion or rotation, but they also consider strong magnetic fields.
If SGR is related to supernova remnants, they are moving at high speed because they are not in the center of the remnants. Then accretion will cause serious problems, because it is inferred that the high speed of the three SGR means that neutron stars cannot absorb enough mass from the interstellar medium. In addition, it turns out that, for example, the accretion of asteroids is expected to last for tens of thousands of seconds, instead of the observed tenth of a second. There is a bigger problem with rotation. A neutron star that rotates with an 8-second period, such as the one that produced the March 5 event, has only about three times the available rotational energy of 10 44 erg. However, the event on March 5th released about 4 times 10 44 erg, and then the X-ray energy released in the form of continuous emission was 3 times 10 44 erg, so there was not enough rotational energy to complete the work.
From about 1992, chris thompson and Rob Duncan began to propose another energy source, that is, a very strong magnetic field. Part of the reason why they are attracted is that the event on March 5 means that the rotation period of neutron stars is very long (8 seconds) compared with the expected rotation period (less than 1 second). If the neutron star is rotating by magnetic braking, as is generally believed, the magnetic field needs to be close to 10 15 gauss to reach such a long period in the 5000-year history of N49 supernova remnant! Thompson and Duncan noticed that this means that the total magnetic energy of a star is about 10 47 erg, which is easy. They also found that the model was consistent with other characteristics of SGR epidemic.
Therefore, it is possible that the magnetic field of some neutron stars is 10 15 gauss. So what? Now that we have determined that the fields of some neutron stars are 10 12 to10/3 gauss, which sounds incredible, what's the big deal about the other two orders of magnitude?
The difference lies in the subatomic level. In a magnetic field, charged particles (such as electrons or protons) will rotate around the magnetic field at a preferred frequency (cyclotron frequency) which is proportional to the strength of the magnetic field. This principle is used in magnetic resonance imaging, where the preferred frequency is radio wavelength. When a magnetic field with neutron star intensity is introduced, the electron cyclotron frequency is in X-ray. When the magnetic field is 4.4 14 times 10 13 gauss, the electron cyclotron energy (cyclotron frequency times Planck constant) is equal to the electron rest mass energy. This field has been proved to be the key field of quantum electrodynamics, so (basically) there are many strange processes. We don't pray for this super-strong field to enter the laboratory, we only have the prediction of quantum mechanics to guide us. Therefore, if we can determine the existence of such a field in astronomy, then by studying these celestial bodies, we can test our quantum mechanics theory in a new physical system.
But first, we must obtain more direct evidence to prove the existence of such a high field. The latest supporting evidence appeared in 1998, when several soft gamma ray repeaters were active, and finally (1) spin period and (2) the rate of change of spin period could be measured, which allowed to estimate the magnetic fields of these sources with the simplest approximation. You guessed it, it seems that a magnetic field larger than about 10 14 gauss is needed, although there are some subtleties. What is really good is a feature in the energy spectrum of these super-strong fields. We didn't, but many people made a lot of efforts. Tomek Brick and I put forward what I think is the most promising one. It is related to the so-called vacuum vibration, and its spectral feature is the reduction of X-ray spectrum. When the intensity is high, it will move to lower energy. If we see such signatures, we will have strong direct evidence to prove the existence of these super-strong fields, and theorists like me will have a wonderful new playground!