August 20 this year, the NICER instrument (Neutron Star Interior Composition Explorer), installed on the International Space Station, recorded the brightest x-ray flash in history. The source of this outbreak is a pulsar, located at a distance of about ten thousand light years from us. And in 20 seconds this pulsar released as much energy into the surrounding space as our Sun radiates in a period of time of 10 days.
A pulsar is one of the varieties of neutron stars, the remnants of supernova explosions. After this, the former stellar core remains active and this activity is most strong in the region of the poles, where powerful fluxes of X-ray radiation are beaten from the neutron star region into space. Quite often, neutron stars rotate at high speed and the x-rays emitted by them pass in the direction of the Earth, forming impulses, because of which pulsars got their name.
In this case, the x-ray flash came from a pulsar called SAX J1808.4-3658 (J1808). This pulsar is located at a distance of 11 thousand light-years from Earth in the constellation Sagittarius, it rotates at a speed of 401th revolution per second. In addition to the fact that the flash produced by this pulsar became the brightest, the parameters of its x-ray radiation have some other strange features.
The flash began with a powerful pulse, followed by a pause of approximately 1 second. Then the x-ray flux began to gain intensity and after another 2 seconds it reached a peak value, peak brightness. This intensity remained unchanged for several seconds, after which the brightness gradually went down, dropping to 20 percent of the peak over the next 40 seconds.
Such a complex X-ray pulse is very unusual for Type I flares, but scientists seem to have found a suitable explanation for this. It all lies in the fact that the J1808 pulsar is not a “loner”, it is part of a binary system, and its companion is a brown dwarf, a space object too large to be considered a planet, but too small to become a full-fledged star.
And the neutron star J1808 constantly pulls gaseous hydrogen from the environment of the brown dwarf. This hydrogen accumulates in the accretion disk of the pulsar and when its concentration increases to a certain limit, a powerful explosion occurs. The pulsar light is trying to “push” through a dense gas cloud, part of which is heated and ionized. When the amount of energy absorbed by the gas becomes large, the cloud expands, and part of the gas falls on the surface of the pulsar, which leads to a powerful thermonuclear explosion, accompanied by the release of oceans of energy into the surrounding space.
With all this, several types of thermonuclear reactions occur in the pulsar region. The first of these is the fusion reaction of hydrogen nuclei into a helium nucleus, the same reactions “activate” our Sun. But, in the immediate vicinity of the surface of a neutron star, reactions take place during which helium nuclei merge into a carbon nucleus. The energy and speed of these reactions is such that as a result of the explosion, the upper hydrogen layer of the pulsar simply blows off into the surrounding space.
The first peak of the x-ray flare was the result of the expansion of the upper hydrogen layer of the pulsar, and during a subsequent pause, this layer detached from the surface and rushed into the surrounding space. The second, brightest and longest peak of radiation occurred when an expanding cloud of hydrogen was overtaken by the impact of a higher-speed wave of helium, dropped from the surface of the pulsar a little later than the hydrogen wave. And, after all this, the pulsar stabilized again, and the brightness of the x-ray flash began to decline.
However, in the form of an X-ray flash of the pulsar J1808, there are still a few features for which scientists have not found suitable explanations. Investigation of the reasons for the appearance of these features, taking into account the high enough quality of the data collected by the NICER tool, will not take long, and all this will allow scientists to open the veil of some secrets and mysteries associated with neutron stars and pulsars.