Happy 20^th anniversary to Swift!

In 2004, on November 20^th, the Neil Gehrels Swift Observatory (or just Swift for short) spacecraft roared into space onboard a Delta II rocket. A lot’s happened since then.

In the 1990s, astronomers had a problem. One of the biggest mysteries we had was the nature of gamma-ray bursts (or GRBs). These are intense and immensely powerful blasts of super-high-energy radiation. We knew they were happening out in the distant Universe, but the blasts didn’t last long; some radiated for several minutes but many were just seconds long, and some only milliseconds in duration!

We didn’t know what was causing them. Gamma-ray telescopes at the time weren’t good at pinpointing locations, and could only give a very vague placement in the sky for a burst, which made it impossible to follow up with telescope that can see in visible or ultraviolet light, a critical step for understanding the sources.

Swift was designed to solve that problem. It was built with three telescopes. One is the Burst Alert Telescope, or BAT (and boy oh boy did we have fun with that name, as you’d imagine a huge cohort of nerdy scientists would), which is very clever indeed. It has a large array of gamma-ray detectors that can see the high-energy photons, but not where they come from. However, in front of them is a roughly semicircular mask, called a Coded Aperture Mask, that is made of a substance opaque to gamma rays but also has hundred of square holes cut out of it in a random pattern.

When a burst goes off, the closed off squares create a unique shadow pattern on the detectors that can be used to locate the GRB on the sky by backtracking the shadow pattern. The whole spacecraft can then swivel very rapidly (in under a minute typically) and point its other two telescopes, one that sees in visible and ultraviolet and the other in X-rays. These look for the afterglow, the visible but fading glow from the huge GRB fireball. That can last from seconds to minutes, and be used to nail down the GRB’s position. 

And it worked! As I write this (in December 2024, and my apologies for missing the actual launch anniversary!) it’s detected a staggering 1,793 bursts. That’s about 90 per year, though that’s a little misleading; as time went on and the scientists and engineers understood the spacecraft better, they were able to adjust it to spot more GRBs, so it sees more now than it used to.

Moreover, when it spotted a GRB it would rapidly and automatically transmit the coordinates on the sky to a ground-based network. Member telescopes could then interrupt their observations to respond  as quickly as possible to the alert, swinging around to observe the fading afterglow. It’s impossible to exaggerate the importance of this we learned a huge amount about GRBs due to this capability.

The explosions, it turns out, are from the moment when a black hole is born! This can happen when a massive star explodes as a supernova, and its core collapses to form a black hole, or when two super-dense neutron stars collide and explode (creating a kilonova) and form a black hole. Either way, gamma-ray bursts are the birth cries of black holes. And Swift was always listening for those announcements. [For more on this please see my episode of Crash Course Astronomy: Gamma-ray Bursts.]

The name of the spacecraft was unusual for the time when most were named after people or were acronyms; Swift was named after the bird which can move with amazing agility to catch insects on the fly. Appropriate.

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