Pulsars are astronomical laboratories where nature plays with extreme gravity and electromagnetic phenomena. Formed in the deaths of massive stars, these bodies shine and influence their environment in ways that are disproportionate to their size. Some of them rotate hundreds or thousands of times each second. Called millisecond pulsars, they are some of the most precisely timed phenomena we know.

For a number of reasons, astronomers are sure the fastest spinning pulsars are driven by matter they strip off companion stars, but direct observations proved hard to obtain. A new set of data could help with that. A. Papitto and colleagues found a pulsar locked in mutual orbit with a star, transitioning between feeding off gas from its companion to rapid stable rotation. This shows not only that matter transfer between a star and pulsar could drive the rapid rotation, but that the transition between feeding and a stable, millisecond rotation occurs in a very short time.

Pulsars are the very dense remains of stars that started more massive than our Sun. After the collapse of a supernova, they pack a star's worth of mass into a body about 20 kilometers (12.4 miles) in diameter. These objects emit powerful beams of light. Due to their rapid rotation, those beams appear to us as regular pulses. An isolated pulsar emits light with several seconds between flashes, but some can spin up to hundreds or thousands of times faster (the millisecond pulsars).

According to theoretical models, these millisecond pulsars are produced when they accrete—feed on matter—from a companion star. This idea is supported observationally. X-ray binaries emit bursts of energetic radiation from nuclear reactions that occur when matter falls onto a pulsar from its companion. However, the connection between accretion and millisecond pulsars was inferred mostly from indirect evidence, which is why the current observations are important.

The researchers associated a transient bright X-ray flare detected on March 28, 2013 with a millisecond pulsar known as PSR J1824-2452I, which is located in the globular star cluster M28. This pulsar rotates about 254 times each second as measured in radio light; follow-up observations showed that rate was matched by variations in the X-ray emissions.

However, during the X-ray outbursts themselves, the millisecond pulsar apparently emitted no radio light. That's consistent with the accretion model: as matter falls in, the normal pulsations will be disrupted by nuclear reactions on the pulsar's surface, though the authors caution that other phenomena could also be responsible for the lack of radio pulsations.

Regular variations in the arrival of light from the system showed that the pulsar is in mutual orbit with a small companion star about 20 percent of the Sun's mass. The binary completes one orbit every 11 hours; that data is consistent in both the X-ray and radio observations.

Now we have the full story. The pulsar PSR J1824-2452I is in a very close binary system, where its strong gravity strips gas from its companion star. When enough of that gas built up, the high temperatures on the pulsar's surface ignited a runaway thermonuclear reaction, emitting a strong burst of X-ray light (the one detected by the Swift observatory on March 28). Over the next month, this X-ray emission faded until the millisecond pulsar activity resumed, which was detected in May. Furthermore, analysis of archival data from the Chandra X-ray telescope showed that the pulsar likely underwent a similar outburst in 2008, with a period of quiet between then and 2013.

Together, these observations indicate a relatively swift transition between accretion and normal rotation. The association of a specific millisecond pulsar with a particular X-ray outburst is remarkably strong evidence in favor of the accretion model for pulsar spin-up. Not only that, but the rapidity of transition after the burst could provide some important insights into the magnetic field of the pulsar. This field is evidently powerful enough to clear away the detritus from a nuclear explosion and resume regular pulsar activity.

Nature, 2013. DOI: 10.1038/nature12470 (About DOIs).