On August 20, an X-ray instrument on the International Space Station (ISS) captured the brightest X-ray explosion ever recorded, foreign media reported. The explosion comes from pulsars thousands of light-years away, and the energy it releases in 20 seconds is equivalent to the amount of energy the sun releases in 10 days. Now, a NASA team has found what they believe is what caused the phenomenon.
A pulsar is a neutron star that is left behind after a larger star releases most of the material in a striking supernova. The remaining core is still active, especially at its poles, where it emits X-rays with a focused beam. Because these objects rotate so fast, some of their beams periodically sweep across the Earth to produce regular X-ray pulses, which is the name of the pulsar.
In this case, the pulse came from a pulsar called SAX J1808.4-3658 (or J1808) that rotates 401 times per second 11,000 light-years from Earth, in the constellation of Pissyta. But the signal semen previously detected by ISS is not an ordinary pulse — it is not only the brightest signal ever detected by NASA’s neutron star internal component detection (NICER) telescope, but it also shows some other strange features.
First, the explosion behaves extremely strongly at first, and then suddenly brightens up for two seconds after a pause of about a second, until it reaches its peak, and the flash disappears after a few seconds of stay. Then it briefly lit up about 20% in the dark, then disappeared for the next 40 seconds or so.
This particular pattern is unusual for this type of explosion, which astronomers call type I X-ray explosions. Now, the researchers have come up with a reason they think can explain the event, at least for most of it.
According to the team, this strange signal can be attributed to the environment in which it is located. The pulsar is not alone — J1808 is part of a binary system that contains brown dwarfs, which are too big to be planets but too small to be stars.
In the researchers’ view, because J1808 and the brown dwarf are so close, J1808 pulls hydrogen out of the brown dwarf and inhales it into the accretion disk that surrounds the pulsar. Over time, some of the gases in the disc become too dense and unstable, which causes an out-of-control process and eventually an explosion.
The light from the pulsar then struggles through the denser gas cloud and captures the energy of the heating and ionizing part of the gas. In turn, the gas captures more energy in the feedback loop. Eventually the gas begins to circle the pulsar and falls to the surface of the pulsar.
The falling hydrogen surrounds the pulsar like an ocean, dropping deeper and deeper. The deeper part of this is exposed to higher temperatures and pressures, which allow the hydrogen core to begin to fuse into helium. This process, which occurs in the sun’s core, generates energy.
Zaven Arzoumanian, co-author of the paper, points out that helium disperses and forms a layer of its own, “once the helium layer is a few meters deep, the condition allows the helium nuclei to fuse into carbon.” Helium then explodes, releasing a thermonuclear fireball and passing through the entire pulsar surface. “
In short, the initial light is due to the expansion of the hydrogen layer, and a second pause is because pulsars blast the hydrogen layer into space. After that, the larger helium layer was blown away, marking a higher peak. As the helium expands faster, it exceeds the hydrogen layer before decelerating, stopping, and returning to the surface of the pulsar. This will produce a flash weakening effect.
But there is still a mystery that the team has yet to solve, and that is what causes the signal to disappear after a brief lightening.
The study was published in Astrophysical Journal Letters.