An international team with the participation of the Institute of Space Sciences (ICE-CSIC), led by astronomers from Curtin University and the International Centre for Radio Astronomy Research (ICRAR) has discovered a new type of stellar object that challenges our understanding of the physics of neutron stars. The finding is published today in the journal Nature.

The object could be an ultra-long period magnetar, a rare type of neutron star with extremely strong magnetic fields that can produce powerful bursts of energy, but also a magnetic white dwarf, the old phase of a Sun-like star.  None of the current scenarios for those objects can yet successfully explain all the characteristics of this new source. Until recently, magnetars were observed to spin at periods of just a few seconds. The newly discovered object emits radio waves every 21 minutes, and if interpreted as a pulsar it would hold the record for the longest period radio magnetar ever detected.

The source, named GPM J1839−10, was discovered using the Murchison Widefield Array (MWA), a radio telescope on Wajarri Yamaji Country in outback Western Australia. It is 15,000 light-years away from Earth in the Scutum constellation and it is the second such long periodic radio objects ever detected and observed for the first time at all wavelengths in 2022 by ICE-CSIC researchers.

However, the first discovered long period radio emitter was transient, and stayed bright in the sky for a few months only. This new 21 minutes source is instead observable in archival observations dating back 1988. ICE-CSIC researchers Nanda Rea and Francesco Coti Zelati led follow-up observations of this new object using the Gran Telescopio CANARIAS (GTC), the world's largest optical telescope located in La Palma (Canary Islands, Spain), along with ESA's XMM-Newton X-ray telescope, and coordinated the physical interpretation of the results. “Discovering two such systems in such a short time is telling us they are very common in the Universe”, said Nanda Rea, ICE-CSIC professor, member of the Institut d’Estudis Espacials de Catalunya (IEEC) and second author of the Nature study.

"This remarkable object might challenge our understanding of neutron stars and magnetars, which are some of the most exotic and extreme objects in the Universe," said lead author Dr Natasha Hurley-Walker. 

Observations dating as far back as 1988

Initially, scientists could not explain what they had found. In January 2022, they published a paper in Nature describing an enigmatic transient object that would intermittently appear and disappear, emitting powerful beams of energy three times per hour.

Between July and September 2022, the team scanned the sky using the MWA telescope. They soon found what they were looking for in GPM J1839−10, which emits bursts of energy that last up to five minutes—five times longer than the first object.

Other telescopes followed up to confirm the discovery and learn more about the object’s unique characteristics. These included three CSIRO radio telescopes in Australia, MeerKAT in South Africa, the XMM-Newton space telescope and the Gran Telescopio Canarias.

Armed with GPM J 1839−10's celestial coordinates and characteristics, the team also began searching the observational archives of the world's premier radio telescopes, popping up in the archives of the Giant Metrewave Radio Telescope (GMRT) in India, and the Very Large Array (VLA) in the USA had observations dating as far back as 1988.

"That was quite incredible, our telescopes first recorded pulses from this object, but no one noticed it, and it stayed hidden in the data for 33 years”. "They missed it because they hadn't expected to find anything like it," added Dr. Hurley-Walker.

“Long-period pulsars have been overlooked in the radio surveys conducted so far. These surveys are designed to scan a wide area of the sky, but they only observe a particular sky region for a short amount of time, typically just a few minutes. This approach is highly effective at detecting pulsars with spin periods ranging from milliseconds to seconds. However, it unfortunately falls short in capturing a sufficient number of consecutive pulses from long-period pulsars” said Dr.  Coti Zelati, also a researcher at ICE-CSIC and a member of the IEEC.

A stellar object below the ‘death line’

Not all pulsars  produce radio waves. Some are believed to lie below the 'death line', a critical threshold where a star's magnetic field becomes too weak to accelerate the particles responsible for the radio waves. "We have studied in detail the emission of a possible pulsar or magnetar rotating so slow -  using detailed computer simulations - and the bright emission of these systems coupled with their slow rotations challenges all current scenario for pulsar radio emission, laying below such ‘death lines’”- says Prof. Rea.

The discovery has important implications for our understanding of the physics of neutron stars and white dwarfs, and the behaviour of magnetic fields in extreme environments. It also raises new questions about the formation and evolution of magnetars and could possibly shed light on the origin of mysterious phenomena such as fast radio bursts.

Prof. Rea’s research group at ICE-CSIC, in particular PhD students Celsa Pardo and Michele Ronchi, and Dr. Vanessa Graber, has conducted more simulations to predict how many of these objects we would expect to see depending on their still mysterious nature. The accompanying simulation paper has been recently submitted to the Astrophysical Journal Letter.  

The teams at Curtin University and at ICE-CSIC hope to discover more of these mysterious periodic radio bursts in the future, which could help finally understand the nature of these fascinating and enigmatic objects.