Astronomers have, for the very first time, managed to observe a pulsar in a stage of transition, indicating the mechanism for the mysterious millisecond pulsars that have long puzzled the scientific community. Researchers have established the pulsar to alternate between X-ray and radio wave emissions, based upon a rise and fall in the gases that stream onto its surface from its companion star.
Pulsars (A.K.A. pulsating stars) are magnetized, rotating neutron stars that are capable of releasing electromagnetic radiation. These beams of electromagnetic radiation are only identified when pointing towards the Earth, and is responsible for the “pulsed” appearance. Detected radio emissions are the result of the pulsar’s rotating magnetic field; these radio waves are focused in two separate beams, derived from each of the pulsar’s magnetic poles.
Essentially, pulsars convert the kinetic energy of their rotation into radiation. Over a protracted period of rotation, as energy is continuously released as radiation, the rotation speed begins to subside.
Older stars rotate with periods of a few seconds, whilst younger pulsars demonstrate a much more rapid spin. Therefore, it came as a great surprise to astronomers, during the 1980s, that millisecond pulsars were in existence; these pulsars were of an older age, but possessed phenomenally rapid periods.
According to a new research study, which was published in the journal Nature, scientists believe they can explain the millisecond pulsar through use of a “recycling” scenario. Sometimes a pulsar might belong to a binary system, and is capable of accreting matter from an adjacent star. The pulsar “feeds” off its stellar companion, using an accretion disk, which causes it to gain angular momentum; this serves to ramp up the rotational speed of these old pulsars, reducing their period to a matter of milliseconds.
During the accretion process, X-rays are emitted and can be measured from accreting pulsars in binary systems. Now, the research team believe they have established a definitive relationship between X-ray releasing millisecond pulsars and radio-emitting millisecond pulsars, first observed during the 1980s.
To do this, the group, led by Alessandro Papitto (Institut de Ciències de l’Espai, Barcelona), investigated the PSR J1824-2452I pulsar, situated in the Messier 28 globular star cluster.
According to Enrico Bozzo of the ISDC Data Center for Astrophysics at the University of Geneva, who was one of the study’s co-authors, the pulsar’s X-ray emissions were detected using the European Space Agency’s (ESA) INTEGRAL Earth satellite:
“The INTEGRAL data are provided almost in real-time to the scientific community. This is a crucial feature for planning follow-up observations with other facilities: so we looked at our pulsar in greater detail with XMM-Newton.”
XMM-Newton was used to accurately calculate the pulsar’s period, amounting to a mere 3.9 milliseconds, spinning at an incredible 250 times each second.
The team then noticed that the spin period and orbital parameters matched perfectly to those of another previously observed pulsar, investigated a few years back; however, observations of this pulsar had only been published in a graduate thesis, showing it to emit radio wavelengths, as opposed to X-rays. Papitto noted his team’s extraordinary finding:
“… when we compared it to our pulsar, we realised immediately that this source, which had once been bright in radio waves and was now shining in X-rays, was no ordinary pulsar.”
Do Pulsar’s Show a Behavioral Shift?
Armed with this knowledge, the research team set about monitoring the emission of both X-rays and radio waves from their pulsar. The pulsar transitioned from X-ray emitting, during April of 2013, to radio wave emitting in May of the same year.
The scientists believe that the pulsar yoyos between an accretion-powered, X-ray releasing millisecond pulsar, and their rotation-driven, radio wave emitting equivalents. After looking at data from NASA’s Chandra X-ray Observatory, the astronomers confirmed that the pulsar was also X-ray bright during 2006; within the ensuing years, they expect the pulsar to flip back to this state, once more.
The group describe the “rhythmical interplay” between pressure of accreted matter and the magnetic field. During periods of stronger accretion, the accretion matter’s high density prevents acceleration of particles that can emit radio waves – the millisecond pulsar remains only visible in the X-ray range. Reduced accretion, meanwhile, creates an expansion of the pulsar’s magnetosphere, pushing matter away – the pulsar under these conditions is visible when looking for radio waves.
By: James Fenner