ASKAP J1935+2148

MeerKAT is an amazing array of 64 radio telescopes in South Africa. Astronomers want to expand this to the Square Kilometer Array, which will actually consist of thousands of telescopes in South Africa and Australia. But it’s already seeing great stuff. For example, this June they found a weird thing that flashes like a pulsar, but extremely slowly: roughly once an hour, instead of many times a second like an ordinary pulsar!

That’s insane! What is this thing? We don’t know, and that’s exactly why it’s cool.

Imagine an enormous star flinging off its outer layers after it runs out of fuel and its core collapses under its own gravity. If the core doesn’t collapse to form a black hole or completely explode, it can shrink down to a ball of neutronium just 20 kilometers across. Just as a ballerina spins faster as she pulls in her arms, this ball spins really fast—like 1000 rotations a second. And since neutronium conducts electricity, it can blast out radio waves as it spins, creating a blinking radio signal, called a pulsar.

Pulsars are so precisely periodic that when Jocelyn Bell first spotted one, she considered the possibility that it was a signal from space aliens!

We did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission. It is an interesting problem—if one thinks one may have detected life elsewhere in the universe, how does one announce the results responsibly?

Like the rest of us, pulsars slow down as they age. But this also means their signal weakens, and eventually quits. So we usually don’t see pulsars in the gray region of the following chart—to the right of the line called the ‘pulsar death line’.

Pulsars are the gray dots to the left of this line. The pink squares are called ‘magnetars’. These are the squalling infants in the world of pulsars: young and highly magnetized neutron stars that do crazy stuff like put out big bursts of X-rays now and then.

But then there are weirder things. In Australia there’s an array called Australian Square Kilometer Array Pathfinder or ASKAP, which was built to test technologies for the forthcoming Square Kilometer Array. It was searching for radio waves connected to a gamma ray burst in 2022 when it stumbled on something that blasts out radio waves about once an hour. It lost track of this object, so folks brought in the more powerful MeerKAT array and found it again.

Now this mysterious radio source is called ASKAP J1935+2148. It’s well to the right of the pulsar death line. What could it be?

It blasts out radio waves once every 54 minutes, which is incredibly slow for a pulsar. Pulsars usually pulse somewhere between 1000 times a second and once every few seconds.

ASKAP J1935+2148 puts out three kinds of pulses in a seemingly random way:

• bright pulses of radio waves that are strongly circularly polarized, • weak pulses that are somewhat less polarized, and &ull; no pulse at all.

But if you fill in the missing pulses, you’ll see the pulses keep the same period with an accuracy of 1/10 of a second! So I imagine it must be something quite heavy slowly spinning around, which has a patch that switches between three modes of radio emission.

It could be a really weird pulsar, but nobody knows how a pulsar spinning so slowly could put out radio waves. It could be a pulsar-like white dwarf. Three of these are known, people argue about how they work, and they pulse more slowly than ordinary pulsars—but not as slowly as this.

In short, it’s a mystery! And that means we’ll learn something cool.

For a nice account of this, try Astrobites:

• Magnus L’Argent, This ultra-long period radio signal can’t make up its mind, Astrobites, 2 July 2024.

For even more details, try the original paper:

• M. Caleb, E. Lenc, D. L. Kaplan, T. Murphy, Y. P. Men, R. M. Shannon, L. Ferrario, K. M. Rajwade, T. E. Clarke, S. Giacintucci, N. Hurley-Walker, S. D. Hyman, M. E. Lower, Sam McSweeney, V. Ravi, E. D. Barr, S. Buchner, C. M. L. Flynn, J. W. T. Hessels, M. Kramer, J. Pritchard and B. W. Stappers, An emission-state-switching radio transient with a 54-minute period, Nature Astronomy 8 (2024), 1159–1168.

That’s where the nice figure of the ‘pulsar death line’ came from.