NICER Catches Milestone X-ray Burst

At about 10:04 p.m. EDT on Aug. 20, NASA’s Neutron star Interior Composition Explorer (NICER) telescope on the International Space Station detected a sudden spike of X-rays caused by a massive thermonuclear flash on the surface of a pulsar, the crushed remains of a star that long ago exploded as a supernova.

The X-ray burst, the brightest seen by NICER so far, came from an object named SAX J1808.4-3658, or J1808 for short. The observations reveal many phenomena that have never been seen together in a single burst. In addition, the subsiding fireball briefly brightened again for reasons astronomers cannot yet explain.

The data reveal a two-step change in brightness, which scientists think is caused by the ejection of separate layers from the pulsar surface, and other features that will help them decode the physics of these powerful events.

The explosion, which astronomers classify as a Type I X-ray burst, released as much energy in 20 seconds as the Sun does in nearly 10 days.

J1808 is located about 11,000 light-years away in the constellation Sagittarius, spins at a dizzying 401 rotations each second, and is one member of a binary system. Its companion is a brown dwarf, an object larger than a giant planet yet too small to be a star. A steady stream of hydrogen gas flows from the companion toward the neutron star, and it accumulates in a vast storage structure called an accretion disk.

Hydrogen raining onto the pulsar's surface forms a hot, ever-deepening global “sea.” At the base of this layer, temperatures and pressures increase until hydrogen nuclei fuse to form helium nuclei, which produces energy — a process at work in the core of our Sun.

The helium settles out and builds up a layer of its own. Eventually, the conditions allow helium nuclei to fuse into carbon. The helium erupts explosively and unleashes a thermonuclear fireball across the entire pulsar surface.

As the burst started, NICER data show that its X-ray brightness leveled off for almost a second before increasing again at a slower pace. The researchers interpret this “stall” as the moment when the energy of the blast built up enough to blow the pulsar’s hydrogen layer into space.

The fireball continued to build for another two seconds and then reached its peak, blowing off the more massive helium layer. The helium expanded faster, overtook the hydrogen layer before it could dissipate, and then slowed, stopped and settled back down onto the pulsar’s surface. Following this phase, the pulsar briefly brightened again by roughly 20 percent for reasons the team does not yet understand.

See NASA's NICER Catches Record-setting X-ray Burst