A team of astronomers has detected for the first time in history amillisecond pulsar in a critical phase of its evolution, when it goes from emitting x-ray pulses to emitting radio waves. His work is published today in the magazineNature.
Pulsars are magnetized neutron stars, the dead cores of massive stars that exploded as a supernova when they ran out of fuel. They rotate at high speed, emitting pulses of electromagnetic radiation hundreds of times per second, like a lighthouse. And they are classified according to how they generate these emissions. Radio pulsars derive their energy from the rotation of their magnetic field, while X-ray pulsars feed on an accretion disk formed by matter ripped from a companion star.
Current theories suggest thatneutron stars speed up their rotation as they accumulate mass from their companion staron your accretion disk. When material from the disk falls towards the star, it heats up and emits X-rays. After several billion years, the accretion rate slows and the pulsars turn on again, but this time emitting radio waves. Astronomers thought that there is an intermediate phase in which neutron stars oscillate between these two states, but until now no direct and conclusive evidence has been found to support this theory.
Thanks tojoint work of ESA’s Integral and XMM-Newton space observatories, combined with subsequent observations from NASA’s Swift and Chandra satellites and a series of radio telescopes on the ground, scientists have finally surprised a pulsar in this transition phase. “The search has come to an end: we have discovered a millisecond pulsar that, in a matter of weeks, has gone from being an accretion pulsar, bright in X-rays, to a rotating pulsar, bright in radio wavelengths. It is the missing link in the evolution of pulsars ”, says Alessandro Papitto, from the Institute of Space Sciences of Barcelona and co-author of the work.
A pulsar in transition
This pulsar, identified asIGR J18245-2452, was first observed in the X-ray band by Integral on March 28, 2013. It is found in the globular cluster M28, in the constellation Sagittarius. Observations made with XMM-Newton allowed determining that its rotation period was 3.9 milliseconds, which means that it rotates on its own axis more than 250 times per second, clearly classifying it as aPulsar millisecond X-ray.
However, after comparing its rotation period and other characteristics with those of the other pulsars in M28, it was found that it perfectly matched the description of a pulsar observed in 2006 – only this one emitted radio waves. Astronomers continued to observe the object with X-ray telescopes, but also began a series of observations in the radio band, looking for signs of a new personality change. What they did not expect was that this change in behavior would occur in a matter of weeks.
“We thought that this transition would only occur once in the billions of years that the evolutionary process of this type of system lasts, but in less than a month the pulsar went from emitting X-rays to radio waves and returned back to the state initial, demonstrating that change can occur in an extremely short time scale ”, explains Enrico Bozzo, from the University of Geneva, Switzerland, and co-author of the publication.
Although the transformation occurred much faster than anticipated, its characteristics are still consistent with current theories. The transition is thought to be based on the interaction between the pulsar’s magnetic field and the pressure of material falling toward it from its low-mass companion star. When the flow of matter from the companion star intensifies, its high density blocks the emission of radio waves and the pulsar is only visible in the X-ray band, thanks to the radiation emitted by matter from the accretion disk when it heats up as it falls towards the neutron star. However, when the accretion speed decreases, the pulsar’s magnetic field expands and expels any remaining matter, resuming the emission of radio waves.
By studying archival data for this particular pulsar, astronomers have discovered that these cycles can be repeated in a matter of a few years. “The discovery ofthis transitional pulsar puts an end to decades of work searching for an object like this, and it will help us to better understand the evolutionary process of pulsars ”, points out Erik Kuulkers, scientist of the Integral Project for ESA.
