For the first time, astronomers have observed a neutron star collecting material before blasting out an immensely bright X-ray burst—a burst thousands of times brighter than the Sun.
Observing such a phenomenon was not a small task—it involved a team of 15 scientists from various institutes using several different observatories (their findings were published in the Monthly Notices of the Royal Astronomical Society). Their observations mark the first time anyone has ever observed a neutron star in this level of detail. They observed the neutron star in multiple wavelengths, made precision measurements, and effectively revealed a great deal of information on neutron stars and their behavior.
The researchers set out to catch a neutron star before it began bursting out X-rays, and to understand the physics behind the process. For a long time, physicists have only been able to speculate about the duration of a neutron star’s ‘switching on’ process, many of which believed it happened over two or three days. The team managed to find a neutron star right before it began the outburst, discovering it took a total of 12 days for material to gather and then collide with the star.
“These observations allow us to study the structure of the accretion disk, and determine how quickly and easily material can move inwards to the neutron star,” said Ph.D. candidate Adelle Goodwin of the Monash School of Physics and Astronomy, the lead scientist of the paper. “Using multiple telescopes that are sensitive to light in different energies we were able to trace that the initial activity happened near the companion star, in the outer edges of the accretion disk, and it took 12 days for the disk to be brought into the hot state and for material to spiral inward to the neutron star, and X-rays to be produced.”
Accreting Neutron Star
The neutron star the team discovered, known as SAX J1808.4-3658, is about 11,000 light-years from Earth in the constellation Sagittarius. It is an ‘accreting’ neutron star rotating at 400 times per second, making it a pulsar. In an ‘accreting’ neutron star system, the pulsar absorbs material from its companion star, forming a swirling disk of matter that slowly spirals inwards towards the center. After the matter collides with the pulsar, it outbursts an immense energy blast of X-rays. SAX J1808.4-3658, in particular, has an accretion disk with unusually high levels of helium, comprising 50% of it (most are filled with hydrogen). The researchers believe this may be the reason why the pulsar took 12 days to 'power-up,' as the helium effectively slows down the heating of the disk.
The team’s observations provide a mountain of insight into the mysterious world of pulsars and are certain to encourage further research. Abu Dhabi of the New York University, a co-author on the study, states that "this work enables us to shed some light on the physics of accreting neutron star systems, and to understand how these explosive outbursts are triggered in the first place, which has puzzled astronomers for a long time."