Of great interest to 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 clarified, but there are sufficiently substantiated theoretical models that claim to explain it.
The concept of the phenomenon
Gamma radiation is the most rigid region of the electromagnetic spectrum, formed by high-frequency photons - from about 6 ∙ 10 19 Hz. The wavelengths of gamma rays can be comparable with the size of the atom, and can also be shorter by several orders of magnitude.
A gamma-ray burst is a short-term and extremely bright burst of cosmic gamma radiation. Its duration can range from several tens of milliseconds to several thousand seconds; most often flashes lasting about a second are recorded. The brightness of bursts is significant, hundreds of times higher than the total brightness of the sky in the soft gamma range. Typical energies range from several tens to thousands of kiloelectron-volts per radiation quantum.
The sources of flares are distributed evenly across the celestial sphere. It is proved that their sources are extremely far, at cosmological distances of the order of billions of light years. Another feature of the bursts is the diverse and complex development profile, otherwise called the light curve. Registration of this phenomenon occurs almost every day.
Study history
The discovery occurred in 1969 when processing information from the American military satellites Vela. It turned out that in 1967 the satellites recorded two short pulses of gamma radiation, which the team could not identify with anything. Over the course of several years, the number of such events has increased. In 1973, Vela data were 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 CONUS experiments revealed the existence of short bursts lasting up to 2 seconds, and it was also proved that gamma-ray bursts are randomly distributed.
In 1997, the phenomenon of “afterglow” was discovered - a slow decay of a burst at longer wavelengths. After that, scientists for the first time managed to identify the event with an optical object - a very distant galaxy with a redshift of z = 0.7. This allowed us to confirm the cosmological nature of the phenomenon.
In 2004, the Swift orbital gamma observatory was launched, with the help of which it became possible to quickly identify gamma-ray 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. Such outbreaks account for about 70%. Their average duration was 20–30 seconds, and the maximum recorded flash duration of GRB 130427A was more than 2 hours. There is a point of view according to which such long events (there are three of them now) should be singled out as a special type of ultra-long bursts.
- Short. They develop and fade in a narrow time frame - less than 2 seconds, on average, they last about 0.3 seconds. The record holder is the outbreak, which lasted only 11 milliseconds.
Next, we will consider the most likely causes of gamma-ray bursts of the two main types.
Hypernov echo
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 generates a black hole. The accretion disk of such an object - a collapsar - arises due to the stellar-shell matter rapidly falling on a black hole. A black hole absorbs it in a few seconds.
As a result, powerful polar ultrarelativistic gas jets — jets — are formed. The velocity of the outflow of matter in jets is close to the speed of light, temperature, and magnetic fields in this region are huge. Such a jet is capable of generating a gamma radiation flux. The phenomenon is called hypernova, by analogy with the term "supernova."
Many of the long bursts of gamma radiation are reliably identified with supernovae having an unusual spectrum in distant galaxies. Their observation in the radio range indicated the possible existence of ultrarelativistic jets.
Collisions of neutron stars
According to the model, the occurrence of short bursts occurs during the merger of massive neutron stars or a pair of neutron stars - black holes. Such an event received a special name - “kilo”, 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 paths. Their merger generates a rapidly rotating object with a strong magnetic field of a special configuration, thanks to which ultrarelativistic jets are again formed.
The simulation shows that, as a result, a black hole is formed with an accretion 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. Observational data are generally consistent with this model.
In August 2017, the LIGO and Virgo gravitational wave detectors recorded the fusion of neutron stars in a galaxy 130 million light-years distant. The numerical parameters of the kilon are not quite as predicted by the simulation. But the gravitational-wave event was accompanied by a short burst in the range of gamma rays, as well as effects in the wave ranges from x-ray to infrared.
Strange flash
On June 14, 2006, the Swift Gamma Observatory recorded an unusual event in a not-so-massive galaxy, located at a distance of 1.6 billion light-years. Its characteristics did not match the parameters of both long and short flashes. The GRB 060614 gamma-ray burst had two pulses: first, a hard pulse with a duration of less than 5 seconds, and then a 100-second “tail” of softer gamma rays. Signs of a supernova in the galaxy could not be found.
Not so long ago, similar events were already observed, but they were approximately 8 times weaker. So this hybrid surge does not yet fit into the framework of the theoretical model.
The hypothesis about the origin of the abnormal gamma-ray burst GRB 060614 has arisen somewhat. Firstly, it can be assumed that it is really long, and strange features are due to any specific circumstances. Secondly, the outbreak was short, and the “tail” of the event for some reason acquired a greater length. Thirdly, it can be assumed that astrophysicists are faced with a new type of bursts.
There is a completely exotic hypothesis: for example, GRB 060614, scientists are faced with the so-called "white hole". This is a hypothetical space-time region with an event horizon, but moving 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, however, 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 set aside and focus on recounting models.
Potential hazard
Gamma-ray bursts in the Universe are widespread and occur quite often. A logical 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 10 52 erg (which corresponds to 10 39 MJ or about 3.3 ∙ 10 38 kW ∙ h) and a distance of 10 light years, the burst effect would be catastrophic. It is estimated that 10 13 erg, or 1 MJ, or 0.3 kWh of energy will be released on every square centimeter of the Earth’s surface in that hemisphere that would have the misfortune of falling under the gamma stream. The other hemisphere will not be too well - all life there will perish, 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 also does not threaten the stars close to us.
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 rather with a rather narrow stream, and the probability of getting into it from the Earth is much less than not being noticed.
Learning prospects
Cosmic gamma-ray bursts - this is one of the largest astronomical puzzles for almost half a century. Now the level of knowledge about them is much advanced due to the rapid development of surveillance (including space), 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 the data of the Fermi satellite, it was found that gamma radiation is generated during collisions of protons of ultrarelativistic jets with protons of interstellar gas, and the details of this process are clarified.
It is supposed to use the afterglow of distant events for more accurate measurements of the distribution of the intergalactic gas up to distances determined by the redshift Z = 10.
At the same time, much in the nature of bursts is still unknown, and we should wait for the appearance of new interesting facts and further progress in the study of these objects.