An international team has discovered a pulsar called IGR J18245-2452 that oscillates between two types: radio pulsars and X-ray pulsars. This is an important discovery in the study of neutron stars because it shows the evolutionary link between these two types of emissions. The discovery of this phase in the evolution of pulsars confirms a theory that’s more than thirty years old.
Neutron stars are one of the possible results of the last stage of evolution of a star. If it explodes in a supernova and the mass that remains is at least 1.4 solar masses, it collapses into an object smaller than a white dwarf. More than 3 solar masses would originate a black hole. In a neutron star, this mass is compacted in a sphere which may have a diameter of about 10-20 km (6-12 miles). Typically, the enormous magnetic field of neutron stars causes an emission of very regular electromagnetic pulses and for this reason they are called pulsars (PULSating stAR).
According to the theory which has now finally found evidence thanks to this research, the radio pulsars with the highest rotation speed were once X-ray pulsars that have captured material from a gas disk produced by a companion star and this resulted in that acceleration. Such material is reduced in the course of a long time, even a billion years, leaving the radio pulsar with a very rapid rotation.
It’s been speculated that a pulsar could oscillate between the two types of emissions during an intermediate stage of its life. The amount of material that reaches the pulsar varies in time and there’s a period in which that determines the oscillation between the two states. When its amount increases, its higher density switches off the radio emission and the pulsar is only visible to X-rays. When instead the quantity of material decreases, the pulsar’s magnetic field expands pushes away the remaining material causing a new emission of radio waves.
At last a pulsar in this transition stage was found. This research has been successful thanks to the combined use of a series of telescopes that include some ground radiotelescopes in various nations, ESA’s INTEGRAL and XMM-Newton space telescopes and NASA’s Swift and Chandra space telescopes.
The INTEGRAL space telescope has identified IGR J18245-2452 at the end of March 2013 observing the inside of the Messier 28 globular cluster, nearly 18,000 light-years away from Earth. In that moment, the star was an X-ray pulsar that rotated on its axis over 250 times per second.
The data collected on IGR J18245-2452 allowed to search among the astronomical data of the previous years, which revealed that from 2006 onwards a pulsar was observed with very similar characteristics with the only difference that it was a radio pulsar. This discovery led to focus the observations on that pulsar and after a few weeks it moved from the X-ray phase to being a radio pulsar again confirming the fact that it’s in the intermediate stage between the two types of pulsars predicted by the theory.
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