Slow-spinning neutron star: Dish radio telescopes with Milky Way behind and zigzag lines coming in from distant bright stars.

EarthSky | Slowly spinning neutron star: A new class of objects?

Slowly spinning neutron star: Bowl radio telescope with the Milky Way behind and zigzag lines coming in from distant bright stars.
The artist’s concept of the signal from the slowly spinning neutron star (in magenta), as opposed to those from faster spinning sources. Image via Danielle Futselaar / SARAO.

Past Manisha Caleb, University of Sydney. Originally published May 30, 2022 at The conversation.

Surprise! A slowly spinning neutron star

The discovery of a neutron star to transmit unusual radio signals is to rewrite our understanding of these unique star systems.

My colleagues and I (the MeerTRAP team) made the discovery when they observed Vela X-1 region of the Milky Way about 1,300 light years away from the earth using MeerKAT radio telescope in South Africa. We saw a strange flash or “pulse” that lasted about 300 milliseconds.

The lightning had certain properties of a radio-transmitting neutron star. But this did not look like anything we had seen before.

We are curious and searched through older data from the region in the hope of finding similar pulses. Interestingly, we identified more such pulses that had previously been missed by our pulse detection system in real time (because we usually only search for pulses that last about 20-30 milliseconds).

A quick analysis of the arrival times of the pulses showed that they were repeated approximately every 76 seconds – while most neutron star pulses circulate through within a few seconds, or even milliseconds.

Our observation showed that PSR J0941-4046 had some of the characteristics of one pulsar or even a magnetar. Pulsars are the extremely dense remnants of collapsed giant stars that usually emit radio waves from their poles. When they rotate, the radio pulses can be measured from the ground, a bit like how you see a lighthouse periodically flashing in the distance.

However, the longest known rotation period for a pulsar before this was 23.5 seconds, which means that we may have found a whole new class of radio-transmitting objects. Our findings are published 30 May 2022, in Nature astronomy.

Diagram: Orb with magnetic power lines and radio rays coming out of both poles.
Neutron stars are the collapsed nuclei of massive stars. Those that emit rays of electromagnetic radiation are classified as pulsars. read more about neutron stars. Image via Shutterstock.

An anomaly among neutron stars?

To use all information available to us from MeerTRAP and ThunderKAT project on MeerKAT, we managed to determine the position of the object with excellent accuracy. After this, we conducted our more sensitive follow-up observations to study the source of the pulses.

The newly discovered object, named PSR J0941-4046, is a strange radio-transmitting galactic neutron star that rotates extremely slowly compared to other pulsars. The pulsar pulse frequencies are incredibly consistent, and our follow-up observations allowed us to predict the arrival time of each pulse to a 100-millionth of a second.

In addition to the unexpected pulse rate, PSR J0941-4046 is also unique because it is located in the neutron star’s “graveyard”. This is a region in space where we do not expect to detect any radio emissions at all, as it is theoretical that the neutron stars here are at the end of their life cycle and therefore not active (or less active). PSR J0941-4046 challenges our understanding of how neutron stars are born and evolve.

It is also fascinating because it appears to produce at least seven distinctly different pulse shapes, while most neutron stars do not show such variation. This diversity in pulse form, and also pulse intensity, is probably related to the object’s unknown physical emission mechanism.

A special type of pulse shows strongly quasi-periodic structure, indicating that some form of oscillation drives the radio radiation. These pulses can give us valuable information about the interior of PSR J0941-4046.

These quasi-periodic pulses have some resemblance to enigmatic ones fast radio bursts, which are short radio bursts of unknown origin. However, it is not yet clear whether PSR J0941-4046 emits the type of energies observed in fast radio bursts. If we discover that it does, it may be that the PSR J0941-4046 is an “ultra-long period magnets.”

Magnets are neutron stars with very powerful magnetic fields. Only a handful are known to transmit in the radio part of spectrum. Although we have not yet identified an ultra-long period magnet, they are believed to be a possible source of rapid radio bursts.

Lucky to see it

It is unclear how long PSR J0941-4046 has been active and transmitted on the radio spectrum, as radio surveys do not usually search for such long periods.

We do not know how many of these sources can be found in the galaxy. In addition, we can only detect radio emissions from PSR J0941-4046 during 0.5% of its rotation period, so it is only visible to us for a fraction of a second. It’s quite lucky that we were able to discover it from the beginning.

Discovering similar sources is challenging. And that challenge means that there may be a larger undiscovered population waiting to be discovered. Our finding also contributes to the possibility of a new class of radio transients: the ultra-long-neutron star. Future searches for similar objects will be crucial to our understanding of the neutron star population.

Arc-shaped white lines surround a bright spot in space with rays coming out of both poles.
The artist’s concept of neutron star showing magnetic power lines and radio rays. Via Shutterstock.

Manisha Caleblecturer, University of Sydney.

This article is republished from The conversation under a Creative Commons license. Read original article.

Summary: Slowly spinning neutron star PSR J0941-4046 challenges our understanding of how neutron stars are born and develop.

Source: Discovered by a radio-transmitting neutron star with an ultra-long spin period of 76s


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