Great interest for modern astrophysics and cosmology is a special class of phenomena called gamma-ray bursts. For several decades, and especially actively in recent years, science has been accumulating observational data regarding this large-scale cosmic phenomenon. Its nature has not yet been fully elucidated, but there are sufficiently substantiated theoretical models that claim to explain it.
The concept of the phenomenon
Gamma radiation is the hardest region of the electromagnetic spectrum, formed by high frequency photons from approximately 6∙1019 Hz. The wavelengths of gamma rays can be comparable to the size of an atom, and can also be several orders of magnitude smaller.
Gamma-ray burst is a short-lived and extremely bright burst of cosmic gamma-rays. Its duration can be from several tens of milliseconds to several thousand seconds; most often registeredflashes lasting about a second. The brightness of bursts can be significant, hundreds of times higher than the total brightness of the sky in the soft gamma range. Characteristic energies range from several tens to thousands of kiloelectronvolts per radiation quantum.
Sources of flares are evenly distributed over the celestial sphere. It has been proven that their sources are extremely far away, at cosmological distances of the order of billions of light years. Another feature of bursts is their varied and complex development profile, otherwise known as the light curve. Registration of this phenomenon occurs almost every day.
Study history
The discovery occurred in 1969 while processing information from the US military Vela satellites. It turned out that in 1967, the satellites recorded two short pulses of gamma radiation, which the team members could not identify with anything. Over the years, the number of such events has increased. In 1973, Vela's data was declassified and published, and a scientific study of the phenomenon began.
In the late 1970s and early 1980s in the Soviet Union, a series of KONUS experiments established the existence of short bursts of up to 2 seconds duration, and also proved that bursts of gamma radiation are randomly distributed.
In 1997, the phenomenon of "afterglow" was discovered - the slow decay of the burst at longer wavelengths. After that, scientists for the first time managed to identify the event with an optical object - a very distant redshift galaxy.z=0, 7. This made it possible to confirm the cosmological nature of the phenomenon.
In 2004, the Swift orbital gamma-ray observatory was launched, with the help of which it became possible to quickly identify gamma-range events with X-ray and optical radiation sources. Currently, several more devices are operating in orbit, including the Gamma-ray Space Telescope. Fermi.
Classification
Currently, based on the observed features, two types of gamma-ray bursts are distinguished:
- Long, characterized by a duration of 2 seconds or more. There are about 70% of such outbreaks. Their average duration is 20–30 seconds, and the maximum recorded duration of the GRB 130427A flare was more than 2 hours. There is a point of view according to which such long events (there are now three of them) should be distinguished as a special type of ultra-long bursts.
- Short. They develop and fade in a narrow time frame - less than 2 seconds, but on average last about 0.3 seconds. The record holder so far is the flash, which lasted only 11 milliseconds.
Next, we will look at the most likely causes of GRBs of the two main types.
Hypernova echoes
According to most astrophysicists, long bursts are the result of the collapse of extremely massive stars. There is a theoretical model that describes a rapidly rotating star with a mass of more than 30 solar masses, which at the end of its life gives rise to a black hole. The accretion disksuch an object, a collapsar, arises due to the matter of the stellar envelope rapidly falling onto the black hole. The black hole swallows it up in a few seconds.
As a result, powerful polar ultrarelativistic gas jets are formed - jets. The speed of the outflow of matter in jets is close to the speed of light, the temperature, and the magnetic fields in this region are enormous. Such a jet is capable of generating a flux of gamma radiation. The phenomenon was called a hypernova, by analogy with the term "supernova".
Many of the long bursts of gamma-rays are quite reliably identified with supernovae with an unusual spectrum in distant galaxies. Their observation in the radio range indicated the possible existence of ultrarelativistic jets.
Neutron star collisions
According to the model, short bursts occur when massive neutron stars or a neutron star-black hole pair merge. Such an event has received a special name - "kilon", since the energy emitted in this process can exceed the energy release of new stars by three orders of magnitude.
A pair of supermassive components first forms a binary system emitting gravitational waves. As a result, the system loses energy, and its components rapidly fall on each other along spiral trajectories. Their merger generates a rapidly rotating object with a strong magnetic field of a special configuration, due to which, again, ultrarelativistic jets are formed.
Simulation shows that a black hole eventually forms with an accretionary plasma toroid falling onto the black hole in 0.3 seconds. The existence of ultrarelativistic jets generated by accretion lasts the same amount of time. The observational data are generally consistent with this model.
In August 2017, gravitational wave detectors LIGO and Virgo detected a neutron star merger in a galaxy 130 million light-years away. The numerical parameters of the kilonova turned out to be not quite the same as the simulation predicts. But the gravitational wave event was accompanied by a short burst in the gamma-ray range, as well as effects in the X-ray to infrared wavelengths.
Strange flash
On June 14, 2006, the Swift Gamma Observatory detected an unusual event in a not-too-massive galaxy located 1.6 billion light-years away. Its characteristics did not correspond to the parameters of both long and short flashes. The gamma-ray burst GRB 060614 had two pulses: first, a hard pulse less than 5 seconds long, and then a 100-second "tail" of softer gamma rays. Signs of a supernova in the galaxy could not be detected.
Not so long ago similar events were already observed, but they were about 8 times weaker. So this hybrid surge does not yet fit into the theoretical model.
There have been several hypotheses about the origin of the anomalous gamma-ray burst GRB 060614. In-First, we can assume that it is really long, and strange features are due to some specific circumstances. Secondly, the flash was short, and the "tail" of the event for some reason acquired a large length. Thirdly, it can be assumed that astrophysicists have encountered a new type of bursts.
There is also a completely exotic hypothesis: on the example of GRB 060614, scientists encountered the so-called "white hole". This is a hypothetical region of space-time that has an event horizon, but moves along the time axis opposite to a normal black hole. In principle, the equations of the general theory of relativity predict the existence of white holes, but there are no prerequisites for their identification and no theoretical ideas about the mechanisms of formation of such objects. Most likely, the romantic hypothesis will have to be abandoned and focus on recalculating models.
Potential danger
Gamma-ray bursts in the universe are ubiquitous and occur quite often. A natural question arises: do they pose a danger to the Earth?
Theoretically calculated the consequences for the biosphere, which can cause intense gamma radiation. So, with an energy release of 1052 erg (which corresponds to 1039 MJ or about 3.3∙1038kWh) and a distance of 10 light years, the effect of the burst would be catastrophic. It has been calculated that on every square centimeter of the Earth's surface in the hemisphere that would have the misfortune to be hit by gamma raysflow, 1013 erg, or 1 MJ, or 0.3 kWh of energy will be released. The other hemisphere will not be in trouble either - all living things will die there, but a little later, due to secondary effects.
However, such a nightmare is unlikely to threaten us: there are simply no stars near the Sun that can provide such a monstrous energy release. The fate of becoming a black hole or a neutron star does not threaten stars close to us either.
Of course, a gamma-ray burst would pose a serious threat to the biosphere and at a much greater distance, however, it should be borne in mind that its radiation does not propagate isotropically, but in a rather narrow stream, and the probability of falling into it from the Earth is much less than in general not notice.
Learning Perspectives
Cosmic gamma-ray bursts have been one of the biggest astronomical mysteries for almost half a century. Now the level of knowledge about them is much advanced due to the rapid development of observational tools (including space ones), data processing and modeling.
For example, not so long ago an important step was taken in clarifying the origin of the burst phenomenon. When analyzing data from the Fermi satellite, it was found that gamma radiation is generated by collisions of protons of ultrarelativistic jets with protons of interstellar gas, and the details of this process were refined.
It is supposed to use the afterglow of distant events for more accurate measurements of the distribution of intergalactic gas up to distances determined by the redshift Z=10.
At the same timeMuch of the nature of bursts is still unknown, and we should wait for the emergence of new interesting facts and further progress in the study of these objects.
