8 mins read 07 Jun 2024

Population of Strange, Slow-Rotating, Astrophysical Objects Grows

A new population of astrophysical objects resembling neutron stars but with very long rotation periods has emerged. Are astronomers on the brink of discovering a new type of celestial object, or are we uncovering more about the strange and unfamiliar behaviours of compact remnants?

Credit: Carl Knox/OzGrav.

On October 9th, 2022, astronomers worldwide were dazzled and awed by what they dubbed the B.O.A.T. (brightest of all time)—an incredibly powerful gamma-ray burst from two billion light-years away that by chance, happened to be directed at Earth. This burst, caused by a massive star collapsing into a black hole, was so intense that it even caused parts of our atmosphere to oscillate. 

Astronomers across the electromagnetic spectrum, from space-based Gamma-ray and X-ray telescopes to ground-based radio telescopes, turned their attention to this remarkable event. In Australia, the CSIRO ASKAP radio telescope, located at Inyarrimanha Ilgari Bundara (the Murchison Radio Astronomy Observatory) in Western Australia, was observing the B.O.A.T. on October 15th. 

When a team led by Sydney University’s Dr Manisha Caleb (and including scientists from the University of Sydney, Australia’s national science agency CSIRO, ICRAR, Swinburne and ANU) processed the data, they noticed something peculiar.

In a small corner of their field of view, they spotted an object emitting four bright pulses, each lasting between 10 and 50 seconds. This is not strange as many sources of radio waves flicker on and off in radio astronomy observation. However, further analysis revealed these pulses were repeated every 54 minutes. 

“It is highly unusual to discover a neutron star candidate emitting radio pulsations in this way. The fact that the signal is repeating at such a leisurely pace is extraordinary,” said Dr Caleb.

To rule out terrestrial interference created by humans (electronics, satellites, broadcasts, etc.), the team conducted follow-up observations with ASKAP at first, then with MeerKAT radio interferometer in South Africa. The pulses persisted, arriving within 319 milliseconds of their predicted times at MeerKAT, confirming their astrophysical origin.

Multiple antennas of the CSIRO ASKAP Telescope. Credit: Dragonfly Media / CSIRO.

Multiple antennas of the SARAO MeerKAT Telescope. Credit: Space in Africa.

With two of the world’s most powerful telescopes at their disposal, Manisha and her team delved deeper, with their findings providing fascinating insights. They determined the object's exact rotation period, estimated its distance at around 16,000 light-years, and analysed the polarisation of the light from both telescopes. They discovered the object had three distinct emission states: bright pulses lasting tens of seconds, faint pulses lasting hundreds of milliseconds, and periods of no pulses at all. The new object has been named ASKAP J1935+2148. 

“What is intriguing is how this object displays three distinct emission states, each with properties entirely dissimilar from the others. The MeerKAT radio telescope in South Africa played a crucial role in distinguishing between these states. If the signals didn’t arise from the same point in the sky, we would not have believed it to be the same object producing these different signals,” Dr Caleb said.

These findings have left astronomers puzzled, and eager to uncover the mystery behind this enigmatic object.

Neutron Star or White Dwarf?

Artist rendition of a white dwarf (left) and neutron star (right). Credit: NASA, ESA AND G. BACON (STSCI).

When stars die, their fate depends on their initial mass. Stars like our Sun shed their outer layers and quietly leave behind hot, dense cores called white dwarfs, roughly the size of Earth. However, more massive stars, about 8 to 25 times the Sun's mass, end their lives dramatically in supernova explosions. These violent events cause their cores to collapse into neutron stars - incredibly dense objects only about 20 kilometres in diameter.

As a massive star collapses, its rotation speeds up due to the conservation of angular momentum (going from a progenitor with a very large radius to the tiny radius of a neutron star), similar to how an ice skater spins faster by pulling in their arms. These rapidly rotating neutron stars have powerful magnetic fields (as the magnetic flux from the progenitor is preserved), generating beams of radio waves from their poles. When these beams sweep past Earth, we observe them as pulsars.

Discovered in 1967, about 3,600 pulsars are known today, mostly in our Galaxy (there is a handful of this population in the Milky Way’s Globular Clusters and a few more in the Large and Small Magellanic Clouds). Our current models tell us that for pulsars to emit radio waves, they must spin fast enough and have strong magnetic fields (the slowest radio pulsar spins once every 23 seconds). Over time, both of these decay and the pulsar’s emissions stop broadcasting, becoming a cooling neutron star. Astronomers describe the evolution of pulsars by mapping them on the P/P-dot diagram, showing a pulsar's period versus its spin-down rate. From these, we are also able to characterise its magnetic field strength and approximate age. 

P/P-dot diagram that shows the main pulsar population in grey, and the magnetar population in pink. These populations exist on the right hand side of the death line. However, several newly discovered sources - including ASKAP J1935+2148 exist within the death valley indicating their long rotation periods. Credit: Caleb et al. 2024.

The P/P-dot diagram reveals intriguing aspects of pulsar populations. In the top right, we find neutron stars with extremely powerful magnetic fields, known as magnetars. In the bottom left are millisecond pulsars, which rotate hundreds of times per second. Most known pulsars are located above a distinct diagonal line, called the 'death line.' Below this line, we typically don't find pulsars because their magnetic fields and rotation periods fall below the thresholds necessary for generating radio emissions.

ASKAP 1935+2148, with a rotation period of about 54 minutes, falls into the "death valley" of this diagram—where pulsar emissions typically cease. However, it still has a strong magnetic field, puzzling astronomers. 

“The standard dipole emission model for pulsars and neutron stars is not applicable to this object,” she said.  “We need to invoke more complex magnetic field configurations, environments and instabilities to be able to explain what we see. The exact mechanism, however, remains unclear for now.”

“This object challenges the 60-year-old model that radio-emitting neutron stars with such long periods should not exist, making them unique cosmic laboratories for testing physics under extreme gravity.”

One possibility is that it might be a rotating magnetic white dwarf (of which about 600 are known), but such objects have never been observed to emit radio waves, so this model isn’t an exact fit as far as we know.

"We were able to provide a lower limit on the radius of an object to sustain the emission in the case of a magnetic dipole scenario, and this rules out an isolated rotation-powered white dwarf origin. However, the object could still be a white dwarf in a binary system with another star. Observations and detections at other wavelengths like optical or UV would help us nail down a white dwarf origin." 

Slow Rotating Transients Population Grows

The location of ASKAP J1935+2148. From our perspective, it is near the supernova remnant SGR 1935+2154, however Caleb and co-authors have found that it is infact in the foreground from this object. Credit: Caleb et al. 2024.

ASKAP J1935+2148 joins a growing group of astrophysical objects observed at radio frequencies that don't quite fit the models of neutron stars or white dwarfs. These objects have long rotation periods, and their radio luminosity is too high to be explained by the usual mechanisms of radio pulsars. This suggests alternative explanations for the emission processes that generate these radio pulses.

These objects fall below the 'death line' on the P/P-dot diagram, now raising questions about our understanding of neutron stars, the role of magnetic white dwarfs, or excitingly, the existence of a new class of astrophysical objects altogether.

“It might even prompt us to reconsider our decades-old understanding of neutron stars or white dwarfs; how they emit radio waves and what their populations are like in our Milky Way galaxy,” Dr Caleb said.

“The ongoing searches and discoveries strongly indicate a significant hidden population in our Galaxy,” she said. “This implies that our picture of neutron stars must change and we must revisit our understanding of whether all radio-emitting neutron stars can continue to emit beyond the death line or if these objects  represent a new and distinct population.”

Identifying more of these objects is challenging due to their small population and uncertain relationships. Wide field-of-view telescopes and high-cadence observations are crucial for spotting these transient phenomena (transients is the term astronomers use when describing short-lived events in space, such as supernovae, neutron star mergers, or mysterious objects that flicker on and off).  Telescopes like ASKAP are ideal for this, as they can survey the sky quickly and repeatedly, helping astronomers catch short-lived events and mysterious objects that flicker on and off.

Diagram showcasing the portion of the sky covered by the RACS survey, as well as the image noise. Red artefacts are results of objects detected by the survey with the Milky Way band clearly visible (and outlined). Credit: McConnell et al. 2020.

“We were simultaneously monitoring a source of gamma rays and seeking a fast radio burst when I spotted this object slowly flashing in the data,” said co-author on the paper Dr Emil Lenc from CSIRO. “ASKAP is one of the best telescopes in the world for this sort of research, as it is constantly scanning so much of the sky, allowing us to detect any anomalies.”

Professor Tara Murphy, leading radio astronomer and head of the School of Physics at the University of Sydney, said: “Until the advent of our new telescopes, the dynamic radio sky has been relatively unexplored. Now we're able to look deeply, and often, we are seeing all kinds of unusual phenomena. These events give us insights into how physics works in extreme environments.”

“The fact that we have discovered one means that there are potentially thousands out there,” concluded Dr Caleb. “We just need to look! We need to process archival radio data and potentially even design targeted surveys to find more of these objects.” 

Read the article in the journal Nature Astronomy