In a terrifying departure from my modus operandi, I will be presenting a piece of creative non-fiction at the REO Town Reading Series on Thursday April 22nd! I’ll be telling a story about the brightest little pulsar in our galaxy. Join Mary Fox, Stevie Pipis, Selena Gambrell Anderson, and me with host and curator Matt Rossi on FB live starting at 7:30pm EDT.
UPDATE: Here is the video link for you to watch at your leisure! Thank you to everyone who tuned in. Transcript of my piece:
I’m going to tell you a story about the brightest little pulsar in the Milky Way, named Swift J0243.6+6124, and don’t worry, you don’t have to remember that name, and I’ll tell you what a pulsar is.
On October 3rd 2017 there was a very bright burst of light near the Perseus constellation. It wasn’t close enough to see by eye, but things that go ‘bump’ in the night tend to be interesting to astronomers, so we had telescopes at the ready. The Swift X-ray observatory was the telescope first to detect it, which is why ‘swift’ gets to be first part of its name. The telephone number in the rest of the name, and yes we actually call it a telephone number, is its location in galactic coordinates, which is like latitude and longitude for the night sky. For the next 150 days, taking us into the spring of 2018, we saw this shiny new source get unimaginably bright, then dim again. Not only were there X-rays, but gamma rays, visible light, and radio waves also shone from it during that time, in their own ways.
In the night sky, when something transient shines brightly in X-rays, this light often comes from near the remnant of a dead star, like a black hole. But calling it a dead star doesn’t quite convey its character; calling it a zombie star is far more accurate. Like zombie people, this zombie star is eating living stars, and it’s still quite active and spinning around, and in general, like zombie people, one could argue that its time as a zombie is far more interesting than its time when it was alive.
Many zombie stars have a little star friend that they grew up with. They orbit around each other as they’ve always done, but like we’ve seen too many times in the movies, the little star friend can’t bring itself to leave, which ultimately leads to its untimely demise. The zombie star feeds on its star friend, slowly draining the outer material from the star and forming a disk around itself, like it’s greedily filling its plate. This syphoned star stuff, waiting to be gobbled down by the zombie star, gets very hot, like 100 times hotter than the surface of the sun. It shines its heat light in X-rays, so when we detect these particular colors and shapes of X-rays from a point in the galaxy, we know that this whole zombie scenario is happening.
Now let’s check in with the ‘unimaginably bright’ aspect of this source. First, I need to tell you what we imagined the limit was. As a zombie star is eating, if it eats more stuff, it shines more brightly. But as your intuition may have already told you, things shining light in space cannot be infinitely bright. There reaches a point where the radiation of the light shining out, pushes back and doesn’t allow more material to fall in. If there’s enough material, the system can even sustain this luminosity. In the 1920s, this luminosity limit was worked out in detail by Sir Arthur Stanley Eddington, a British astronomer, physicist, and mathematician. An important detail to keep in mind is that this luminosity limit depends on the mass, or weight, of the object doing the eating.
When we as astronomers are observing X-ray light from these voracious black holes, we often don’t know how massive they are, so we just pretend that they’re all 10 times the mass of the Sun. In the sciences, when we’re trying to figure something out but don’t have all the information (which is how it goes in astronomy, more so than other fields), we simplify, make a plausible assumption, and move on. Some of the sources we see in the night sky are brighter than this luminosity limit for a black hole weighing 10 times the mass of the Sun, and we call them ultra-luminous. Also, if you’ve ever wondered what’s more than ‘ultra’, according to astronomers, it’s ‘hyper’. So, two things could be happening here: either it’s actually more massive than 10 times the Sun, so it’s shining normally for its actual mass, or there’s some funky, fancy stuff going on that is temporarily letting it be brighter than it should be.
But! Here’s the twist. You didn’t know there’d be a twist, but there is. When we were observing the X-rays from J0243, we instantly noticed pulses in the brightness of the X-ray light, nearly perfectly precise like the ticking of a clock. Black holes can’t pulse. There is no mechanism to get a signal this precise from them. However, neutron stars do, and some of them are so good at pulsing that we call them pulsars. Neutron stars are another type of zombie star, left over when a medium-big star is massive enough to die in a supernova, but not massive enough to collapse into a black hole.
A pulsar is a neutron star with two bright spots on the surface at the north and south magnetic pole, off-kilter with how it spins. It’s like if a neutron star were a light house, in that it’s always shining beams of light, but you only see it when they point in your direction. Pulsars are effectively cosmic clocks that are stunningly accurate out to many decimal places, and NASA is testing a technology to use them like reference points for interplanetary GPS.
So, knowing that J0243 is a pulsar, only a fraction the mass we first assumed for the luminosity limit, it is very super ultra hyper bright. It sounds like it shouldn’t be possible, but nature begs to differ. What’s probably happening is that the pulsar’s magnetic fields, which are thousands of times stronger than anything we can create on Earth, are holding things in place like Spanx, so it can eat even more and radiate even more. I think there’s an analogy here about Spanx holding stuff in, letting the star shine extra bright, but I prefer a more body-positive ethos so I’m going to let that slip away.
What does this mean?
I’ve been studying X-rays from this source, and one really interesting thing about it has to do with its iron levels. Though gas in space tends to be mostly hydrogen, there are trace amounts of other chemicals like oxygen, silicon, magnesium, and iron. If the gas gets hot enough (spoiler: it does), the iron can light up and fluoresce like neon lights. This shows up in cool shapes that we can see with extremely fancy and expensive X-ray detectors on satellites in space. If we want to know where this gas with iron is, like geometrically in the system, we can’t just take a picture because the whole system (zombie star, star friend, plate of stuff it’s eating) are way too small and way too far away.
Instead we need to figure it out from the physics, like a more boring, yet higher stakes, version of Clue. We know the weapon and the victim and the murderer, but not the room, and the room is important. The big weird wrench in the gears here is that the iron doesn’t know that it’s around a pulsar. It doesn’t see the pulsations. And like, How can you not know with such a stupidly bright pulsar, but anyways. The two possibilities are that it has a wall of stuff blocking its direct line-of-sight with the pulsar, or it has a cozy cocoon of dust and gas wrapped around it, diffusing the pulses into a strong, steady light.
I don’t have an answer to this yet. Scientific research is trial and error and error, ad nauseam, and I am in the thick of it. A very helpful colleague suggested some calculations I can do, but I haven’t done that yet, and instead I wrote this.
