Astronomers Catch A Pulsar Powering Up Before Flaring

An accreting pulsar. Credit: NASA/JPL-Caltech.

Pulsars are a particular type of neutron star, one of the end products of supernovae. These extreme objects pulsate many times per second and can release huge amounts of energy in dramatic outbursts. For the first time, astronomers have followed in exceptional detail a pulsar accreting material before producing an energetic flare. The observations are reported in the Monthly Notices of the Royal Astronomical Society.

The object is SAX J1808.4−3658, which rotates on its axis 400 times per second. It is part of an accreting neutron star system, where the pulsar steals material from a companion. The material organizes itself in an accretion disk, periodically falling on the star.  

This collision is what produces the flares. As the material hits the dense and hot neutron star, a blast of energy is thrown out. This is equivalent to the energy produced by the Sun in 10 years being released in just a few weeks.

Theories suggest this process might happen over the course of just a few days, but these observations suggest the process is longer, at least for this system. It took 12 days for the material to spiral in and collide with the pulsar.

“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,” lead author Adelle Goodwin, from the Monash University, said in a statement.

“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.”

One possibility for the length of accretion in this system is the composition of the disk. As the pulsar is stealing material from a companion star, this is usually hydrogen since it’s the most abundant element in stars. This disk, however, is 50 percent helium – the second most abundant element. The difference might be key to stretching the time of the “power-up.”

SAX J1808.4−3658  is located 11,000 light-years away in the constellation Sagittarius.

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