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7 mins read 31 May 2024

A Millisecond Pulsar lurking in the Galactic Centre

 A team of astronomers, lead by CSIRO scientists, has used the Murriyang (Parkes radio telescope) and South Africa’s MeerKAT telescope to make the first detections of a millisecond pulsar in the Galactic Centre, looking near the mysterious radio filaments surrounding our Galaxy’s supermassive black hole.

Heywood et al. 2022 / South African Radio Astronomy Observatory (SARAO).

The heart of the Milky Way Galaxy is a complex, turbulent and violent place. Residing at its centre is the monstrous supermassive black hole with a mass of about 4.3 million Suns, known as Sgr A* (and pronounced ‘Sagittarius A-star’). Astronomers have noted there are approximately 10 million stars in the region, mostly forming a population of older red giant stars, as well as a small population of younger, massive OB-class and Wolf-Rayet stars. 

The Galactic Centre lies about 26,000 light-years away from our comfortable, outer suburb location where the Earth resides, so we are spared from all the activity that occurs in the region. A downside to this safe distancing is that our view towards the centre is inhibited due to the interstellar dust along the line of sight, which reduces (and makes impossible) visibility in the optical, ultraviolet, or soft X-ray bands. We can however study it better at radio frequencies, thanks to their longer wavelengths, that can pierce through the dust. Even at radio wavelengths, this region is such a bright radio source, that trying to study and image it can throw up its own challenges.

However, when astronomers turn their radio telescopes towards the region and successfully gather data, a fascinating, science-fiction-like scene unfolds. The Galactic Centre is an active place, filled with expanding spherical blast shells from historical detonation of massive stars, twisted magnetic fields, and charged particles that zip around at close to the speed of light. 

One particular feature of the Galactic Centre that, to this day, still baffles astronomers is the presence of large, thin, tube-like radio filaments that are oriented almost vertically in relation to the Galactic plane and can stretch for hundreds of light years. There remain open questions about what created these filaments, how long they have survived, and what they might tell us about the central engine of our home Galaxy. What we do know is that they are visible at radio frequencies, and only are associated with the Galaxy centre - that is, we don’t see them anywhere else in the Galaxy. Naturally, we can deduce that they are somewhat related to this region and potentially the supermassive black hole. 

Now, astronomers from Australia’s national science agency, CSIRO, have made another exciting discovery - the detection of a millisecond pulsar, lurking near a kink of one of these huge filaments (known as ‘The Snake’) localised near Sgr A*. This is the first-millisecond pulsar found in the Galactic Centre region.

Pulsars Powering Radio Filaments

The newly discovered PSR J1744-2946 next to the long, radio filament known as 'The Snake' near the Galactic Centre. Credit: Heywood et al. 2022.

The filament known as The Snake measures about 150 light-years in length, and is the longest of all filaments surrounding Sgr A*. Just offset from The Snake is another, non-thermal, radio-emitting, filamentary structure that is being dubbed ‘The Sunfish’. 

“These radio filaments are enormous, magnetic structures illuminated by energetic particles,” said CSIRO astronomer Marcus Lower, the lead author on the paper. “Nobody knows exactly where or how they came to be. But there is some evidence that they’re linked to either a burst of star formation or activity around the central black hole.”

Millisecond pulsars are a sub-category of pulsars with rotation periods of hundreds of revolutions per second. They’re thought to evolve from low-mass X-ray binary systems that feature a neutron star and a regular star. As the regular star ages, it expands and puffs out its outer layers, which are then accreted onto the neutron star, increasing momentum and rotational velocity. 

Millisecond pulsars are excellent astrophysical laboratories for astronomers, because of their long-term stability, meaning they can be used as cosmic clocks. Scientists use them in very high-precision timing experiments, such as the recent announcement of evidence for the gravitational wave background. They’re also used to test Einstein’s General Theory of Relativity (and so far, provide supporting evidence of this). 

What Marcus and the team have found is a new millisecond pulsar (called PSR J1744-2946) showcasing a spin period of 8.4 milliseconds (so rotating on its axis 119 times per second) that has a binary companion with a mass over 0.05 times that of the Sun. The orbital period of the system is 4.8 hours.

A discovery like this now raises questions as to any potential association between millisecond pulsar systems, and the source of radio emissions from the giant, narrow filaments and structures that surround the Galactic Centre, like The Sunfish.

“This appears to confirm a long-held suspicion that the filaments can be energised by pulsars,” said Marcus. “And since there's an enormous amount of filaments threading the Galactic Centre, then we should be able to find more pulsars by targeting our observations towards sources embedded in them.”

Getting Closer to Sgr A*

The image of the supermassive black hole at the centre of our Galaxy, as captured by the Event Horizon Telescope (EHT). Credit: EHT Collaboration.

One of the holy grails of astrophysics and General Relativity physics is the discovery of a millisecond pulsar orbiting a black hole, or better still, a supermassive black hole. That’s because of the remarkable stability and clock-like nature that millisecond pulsars provide allowing the measurement of their pulses at high precision to unveil a suite of information about the pulsar, their environments, their binary configurations and the material of the interstellar medium, between us and them. When their companion happens to be a black hole (with no electromagnetic counterpart signal) this can lead to scientists learning more about gravity under the extreme conditions of a black hole.

“Pulsars orbiting close to the black hole can be used to map out the space-time that surrounds it, and even test whether or not Hawking's no-hair theorem holds up,” said Marcus. 

One exciting aspect of this discovery presented in this pre-print is that this millisecond pulsar system is now officially the closest (on record) to the Galactic Centre and supermassive black hole, Sgr A*, which is a huge achievement and can hopefully be exploited to learn more about the dynamics of the region.

The reason for this is not because there are minimal pulsars around the Galactic Centre - in fact, astronomers believe that there should be many more - but because pulsar signals are extremely hard to detect the closer you get to the centre. That’s due to several factors, such as the pulsar’s signal being scattered with more material (in this case, not dust, but the column of electrons in the cool ionised interstellar medium) between us and the Galactic Centre. This medium tends to broaden the pulsar’s signal, and it is so thick heading towards the Galactic Centre that pulsar signals are often broadened into the background noise. 

This broadening occurs more at lower frequencies, so higher frequencies are needed to pierce through this electron column density to detect these pulsars. However, there’s a catch. Pulsar signals are often brighter at the lower frequencies, and so many instruments built into global radio telescopes, often use low-frequency receivers to find and time pulsars. 

Murriyang (CSIRO Parkes radio telescope). Credit: R. Mandow.

Thankfully, since the end of 2018, Murriyang (Parkes radio telescope) has been equipped with an ultra-wideband receiver that can measure radio signals across a huge 3.3 GHz bandwidth ranging from 700 MHz to 4000 MHz. 

“Murriyang's excellent high-frequency sensitivity was definitely key to discovering J1744-2946,” said Marcus. “However, it's only by luck that nobody else had thought to look for the source in the Snake”

It is with this powerful tool, that Marcus and the team were able to detect this new millisecond pulsar system, as it only appeared above the 2 GHz range. These high frequencies have made their way to Earth, cutting through the thick electron column density that has scattered the low-frequency signals from this pulsar. 

“We're still actively monitoring the pulsar and are eagerly awaiting a measurement of how fast its spin period is slowing down,” said Marcus. “This will tell us how much energy it is capable of injecting into the Sunfish and whether the filament is entirely powered by the pulsar alone.”

Read the article in the journal The Astrophysical Journal Letters