[music throughout] Narrator: On August 21st, 2019, NASA’s NICER telescope
on the International Space Station observed its brightest X-ray burst to date.
The flare-up came from SAX J1808, a binary system
about 11,000 light-years away. Here, a pulsar— [ON-SCREEN TEXT: “SAX J1808.4-3658”]
a rapidly spinning neutron star—draws gas from its companion,
an object called a brown dwarf that is larger than a planet, but less massive than
a star. Hydrogen gas from the brown dwarf forms an accretion
disk around the pulsar. Every few years, the disk becomes unstable.
This sends a rush of gas toward the pulsar that makes it brighten
in X-rays. The pulsar’s superstrong magnetic field sweeps up
the gas and channels it to the object’s surface.
Hydrogen nuclei falling to the pulsar’s surface fuse together, producing energy [ON-SCREEN TEXT: “Infalling Hydrogen, Fusion”
and forming helium nuclei, which settle out below. This process [ON-SCREEN TEXT: “Infalling Hydrogen, Fusion, Helium Layer”
is similar to what happens inside our Sun. Then, when the conditions [ON-SCREEN TEXT: “Infalling Hydrogen, Fusion, Helium Layer”
are just right, the entire helium layer ignites in a brief, but intense
thermonuclear fireball. Astronomers call this a Type I [ON-SCREEN TEXT: “Type I X-ray Burst”]
X-ray burst. Here’s how it happened. [ON-SCREEN TEXT: “Type I X-ray Burst”]
The explosion first blows off the hydrogen layer, which expands and ultimately dissipates. Then,
the rising radiation builds to the point where it blows off the helium layer,
which overtakes the hydrogen shell. Some of the X-rays emitted in the blast
scatter off of the accretion disk. The fireball then
quickly cools, and the helium settles back onto the surface.
It was all over in 20 seconds, but NICER data clearly show [ON-SCREEN GRAPHICS/TEXT: Graph showing a light curve with two peaks. X-axis: “Time (seconds).” Y-axis: “X-ray counts (x 1,000).” Two highlighted areas are labeled “Hydrogen expansion” and “Helium expansion.” A third area is labeled “Unexplained rebrightening.”]
important details that haven’t been seen together in other bursts. [ON-SCREEN GRAPHICS/TEXT: Graph showing a light curve with two peaks. X-axis: “Time (seconds).” Y-axis: “X-ray counts (x 1,000).” Two highlighted areas are labeled “Hydrogen expansion” and “Helium expansion.” A third area is labeled “Unexplained rebrightening.”]
This will help scientists better understand the extreme physics of these eruptions
on accreting neutron stars.
[music]