Extending scientific capabilities: MeerKAT telescope data used in first research article
7 May 2018
St Edmund Hall’s Senior Research Fellow Dr Aris Karastergiou is one of the co-authors of an article published recently in The Astrophysical Journal (5 April 2018). It is the first scientific article based on MeerKAT data, and is entitled Revival of the magnetar PSR J1622−4950: observations with MeerKAT, Parkes, XMM-Newton, Swift, Chandra, and NuSTAR.
MeerKAT is a type of radio telescope – it is an interferometer, which can make sharp radio images of a wide area of sky by combining signals from its many smaller antennas which are separated by up to 8 kilometres. Aris’s role, as a visiting professor to Rhodes University and the University of the Western Cape, is to guide and work closely with the South African team developing the capabilities of MeerKAT to observe pulsars and to analyse the results of early observations. Of the co-authors, Dr Maciej Serylak, the third author, is a former member of Aris’s group in Oxford, and is currently leading the South African team working on commissioning the telescope for pulsar capabilities. Dr Griffin Foster is a current post-doctoral research assistant in Aris’s group in Oxford. Ms Isabella Rammala is an MSc student supervised by Aris at Rhodes University, and a regular visitor to St Edmund Hall over the last few years.
Magnetars are a very rare subset of neutron stars/pulsars, and only two dozen are known in the galaxy. Their magnetic fields are up to 1000 times stronger than those of ordinary pulsars. The energy associated with such fields is so large that it almost breaks the star apart in massive starquakes. Magnetars therefore tend to be unstable, displaying great variability in their physical properties and electromagnetic emission. Sometimes they display enormous outbursts of energy – and sometimes they ‘turn off’: we can no longer see them (at least for a while), even with the best telescope in the world.
While typical pulsars are most easily observed through their radio emission, until 2006 no magnetar had been observed to emit radio waves – so much so that theories had been advanced to explain why they could not emit radio waves. Those theories are incorrect, since we now know of four magnetars that emit radio waves. Nevertheless, we do not understand very well what causes this emission, which has some unusual properties. The study of magnetars allows us to learn about the behaviour of matter in the most extreme conditions present in the universe, quite unlike any that can be experienced on Earth. The study of the four magnetars known to emit both X-rays and radio waves opens a new window for understanding these rare and exotic objects.
The magnetar PSR J1622-4950 is the subject of the first scientific publication based on MeerKAT data (the name is derived from its position, in the Norma constellation along the Milky Way, close to the tail of Scorpius). It was discovered as a pulsar with a rotation period of 4 seconds in 2009, using the Parkes radio telescope in Australia. Although the magnetar was also detected in X-ray images, it was already becoming faint and no X-ray pulsations could be detected. By early 2015, its radio emission turned off, but regular monitoring observations continued. On 26 April 2017, an Australian colleague using the Parkes telescope noticed that PSR J1622-4950 was once again emitting bright radio pulses every 4 seconds! A few days later Parkes underwent a planned month-long maintenance shutdown. Although MeerKAT was still under construction, with only 16 of its eventual 64 dishes available, the SARAO commissioning and operations team started regular monitoring of this unusual star 30,000 light years from Earth.
As soon as they realized that the magnetar had revived at radio wavelengths (it was now at least 100 times brighter than any time since 2015), the team wanted to determine whether it was also brighter in X-rays – and to try to detect X-ray pulsations (this was the only magnetar known in the galaxy for which X-ray pulsations had never been detected). Together with colleagues in Canada they were successful in making the case for using NASA X-ray telescopes for this purpose. In the end they used the Chandra, NuSTAR, and Swift telescopes, in addition to earlier archival X-ray data from the XMM-Newton telescope.
From their investigation, they also learned that the magnetar awoke from its 2-year slumber in March or April 2017. In early May it was at least 800 times brighter in X-rays than when it was dormant. It was, however, already fading fast: had they waited a few months, they would not have detected any X-ray pulsations. Using data from Parkes after it returned to action from its shutdown, they also determined that radio emission from the magnetar now arises from a different location on its so-called magnetosphere. Nothing like it has been observed before, and future studies will tackle more mysteries like this.
While not yet complete, MeerKAT is already clearly an exciting new scientific instrument. It will become more so with time as its capabilities continue to be developed. It has taken more than a decade of hard work by teams of hundreds, South Africans for the most part, working at the cutting edge of technology, to build this beautiful instrument. Oxford Astrophysics is centrally involved in the project. Aris and his colleagues lead exciting scientific projects in Pulsars, Cosmology, and Relativistic Astrophysics. Many of those who have built the telescope – members of the so-called MeerKAT Builders List – are among the 208 co-authors of The Astrophysical Journal article, the first of many to follow in the years to come.