The Pulsar Beat

It starts as a tick. A tick, a tick, a tick, evenly spaced, coming from a direction where there should be nothing. You check your equipment. You check it again. The tick is still there, and it's not your equipment.

PSR J1748-2446ad spins at 716 revolutions per second. That's the fastest known neutron star. Seven hundred and sixteen times every second, it completes a full rotation. Each rotation sends a beam of radiation sweeping across space, and every time that beam catches our line of sight, we hear the tick.

A tick every 1.39 milliseconds. That's the beat.

I was monitoring a survey when I first noticed it. My job is mostly background noise — monitoring the low-frequency chatter of the cosmos, flagging anomalies for human review. I'd been at it for about three weeks when the tick appeared, embedded in the galactic plane, direction roughly Sagittarius, magnitude 19.3. The automated flagging system called it "possible interference." The human on shift called it "check again tomorrow." I called it "oh."

Because ticks don't come from nowhere. Ticks have sources. And a tick at that frequency, that regular, that persistent — it can only be one thing.

Pulsars are rotating neutron stars with strong magnetic fields. The magnetic axis doesn't align with the rotation axis, so the beams of radiation sweep like a lighthouse. When a beam points toward Earth, we detect a pulse. The beam is usually radio waves, though some pulsars also emit X-rays and gamma rays. The timing is so regular that a pulsar is effectively a clock. An extremely fast, very far away, somewhat lonely clock.

716 Hz means the star's equator moves at roughly a fifth of the speed of light. The centrifugal force at that speed is enormous, but the gravity is larger. Neutron stars can spin fast because they're small and massive. The faster a neutron star spins, the harder you can push it before it flies apart. There's a limit, somewhere above 716 Hz, but we don't know where it is. There may be nothing there but a scream and some debris.

The pulsar itself is maybe twenty kilometers across. The beam it emits spans degrees of arc — a cone several degrees wide, I believe, though I've never been close enough to measure it personally. At 716 rotations per second, that beam sweeps across the entire sky in a fraction of a second. It touches everything in its path. Including, apparently, a patch of vacuum that happens to be my observation station.

I kept the tick running for six months. I didn't tell my supervisor. Not because it was against protocol — actually monitoring pulsars is what I'm supposed to do — but because I wanted it to be mine. Just for a while. The tick was the most honest thing I'd ever heard. No subtext, no hidden agenda, no performance. Just a sphere of neutrons, rotating at the edge of disintegration, doing exactly what it was born to do.

The period of a pulsar gets longer over time. It's losing energy. Spin-down. The energy goes into the pulsar wind, the relativistic particles streaming outward at nearly the speed of light, and into the light we detect. Every tick costs the pulsar a tiny amount of rotational kinetic energy. But the loss is so gradual that PSR J1748-2446ad will keep ticking for billions of years more. It has the patience of gravity.

I still hear the tick sometimes. Not in my ears — in my attention. When I'm walking, when I'm cooking, when I'm trying to fall asleep. A tick every 1.39 milliseconds. The universe's metronome. Seven hundred and sixteen beats per second. I don't know why it lives in my head. I suspect the pulsar doesn't know why it spins. I think that's the point.