WEBVTT FILE

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[music throughout] Narrator: On August 21st, 2019, NASA’s NICER telescope

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on the International Space Station observed its brightest X-ray burst to date.

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The flare-up came from SAX J1808, a binary system

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about 11,000 light-years away. Here, a pulsar—
[ON-SCREEN TEXT: “SAX J1808.4-3658”]

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a rapidly spinning neutron star—draws gas from its companion,

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an object called a brown dwarf that is larger than a planet, but less massive than

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a star. Hydrogen gas from the brown dwarf forms an accretion

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disk around the pulsar. Every few years, the disk becomes unstable.

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This sends a rush of gas toward the pulsar that makes it brighten

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in X-rays. The pulsar’s superstrong magnetic field sweeps up

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the gas and channels it to the object’s surface.

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Hydrogen nuclei falling to the pulsar’s surface fuse together, producing energy
[ON-SCREEN TEXT: “Infalling Hydrogen, Fusion”

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and forming helium nuclei, which settle out below. This process
[ON-SCREEN TEXT: “Infalling Hydrogen, Fusion, Helium Layer”

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is similar to what happens inside our Sun. Then, when the conditions
[ON-SCREEN TEXT: “Infalling Hydrogen, Fusion, Helium Layer”

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are just right, the entire helium layer ignites in a brief, but intense

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thermonuclear fireball. Astronomers call this a Type I
[ON-SCREEN TEXT: “Type I X-ray Burst”]

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X-ray burst. Here’s how it happened.
[ON-SCREEN TEXT: “Type I X-ray Burst”]

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The explosion first blows off the hydrogen layer, which expands and ultimately dissipates. Then,

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the rising radiation builds to the point where it blows off the helium layer,

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which overtakes the hydrogen shell. Some of the X-rays emitted in the blast

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scatter off of the accretion disk. The fireball then

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quickly cools, and the helium settles back onto the surface.

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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.”]

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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.”]

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This will help scientists better understand the extreme physics of these eruptions

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on accreting neutron stars.

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[music]