WEBVTT FILE

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[music]

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[music]

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Narrator: When a massive star

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explodes as a supernova, its core may be crushed into one of two types

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of compact remnant: a black hole, or a neutron star.

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Neutron stars are the size of a city, but

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contain more mass than our sun. They rotate rapidly, host

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powerful magnetic fields, and produce beams of radiation that emit a wide

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range of energy. When we detect pulses as the beams sweep over

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Earth, the object is known as a pulsar. Paul Ray: They can spin at

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many times per second on their axis; the fastest pulsars spin over 700 times

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per second. And that rapidly spinning massive object, generates

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extremely strong magnetic fields and accelerates particles to

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high energies. And we see that those accelerated particles

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emitting energy in the form of gamma rays, x-rays, and radio waves.

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And when that beam sweeps past the line of sight to the Earth we see it pulse on

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and that's why they're named pulsars. Narrator: The most

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sensitive tool for observing pulsars in gamma-ray light is NASA's

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Fermi Gamma-ray Space Telescope. Fermi scans the entire

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sky for high-energy sources and has found many previously undetected

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gamma-ray emitters. Scientists have identified many of these,

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but for some, the source of the gamma rays remains unknown.

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Roger W. Romani: I got interested a couple years ago in trying to find the

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limits of what Fermi can discover, how extreme these objects can be, and

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in order to do that I focused on the set of objects that are

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relatively bright and well measured by Fermi and

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found that virtually all of them have now been identified. At present, when I started

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this project,there were only six objects which we hadn't figured out what they were yet.

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Despite intense searches, at radio, with radio wave lengths,

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which is the standard way in which people find pulsars--and also looking at the gamma rays

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themselves--no pulsations had been seen. So something was unique

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about these six objects and I thought, that's where the discovery space is going to be.

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If we can track down what those are, we will have a good chance of finding something new.

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We took this small set of six objects and attacked them

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with a number of wave bands, but I think the thing that helped us make the greatest progress

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was looking in the optical, in visible light. Now this may seem a little bit unusual

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for studying the high-energy gamma-ray universe, but, it turns out that

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many of these objects seem to have optical counterparts. And if you can figure out what the

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visible light counterpart of an object is, you're long ways along the

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track to understanding what it's all about. Paul Ray: It was Roger

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Romani's optical observations that discovered a counterpart

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to the gamma-ray source that showed a binary period that was

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indicative of this potentially being a binary millisecond pulsar. Alice Harding: It brightened, and it

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dimmed, and brightened, and so this looked like we were

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looking at possibly something which was

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irradiated by a companion pulsar. And that every

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time you're looking at the bright face you see a bright optical source and when

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it rotates away from you, and you see the dark face, you don't see anything.

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Roger Romani: We managed to get enough observations of the object to piece together its orbital

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period, and found, remarkably, that it was an incredibly

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heated object--blue white on one side, deep, deep red on the other--

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that was orbiting around something invisible with an orbital period of about

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one and a half hours. Now, that's faster then any spin powered

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pulsar ever known, and indicates that it's a really, really tight system

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and that the gamma rays are blasting the companion at point-blank range.

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Our colleagues in Germany managed to use the orbital period

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that we measured to search in the gamma rays directly and, with a computational

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tour de force, managed to find the pulse signal of the pulsar

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directly in the gamma rays themself. Holger Pletsch: What I'm

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doing is blind searches for pulsars so that we

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try to find pulsars that have not been seen before. So you don't know

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how fast the pulsar is spinning, where exactly it is sitting in the sky.

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To do that, you have basically to try, every possible combination of

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parameters--if they match your data output stream. So the problem is

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that the number of possible combinations is tremendously high, so the

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straightforward brute force approach is impossible. The computation power you would

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need would be in excess of what is available in the whole planet.

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So our work is to invent more efficient methods to do that.

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The basic method is analogous to zooming.

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It's similar to changing your objectives

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of your microscope, in favor of one of higher magnification, so you look at

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one interesting point on the slide, and then you zoom in on that.

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And then you further zoom in if it still is interesting. To find the pulsations

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in the gamma ray data requires about

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5.000 cpu days. So if you do it

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on your laptop you need 5,000 days, but if you

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have 5,000 laptops, you only need one day. And so

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that is the path we took because we have a computing cluster that is called Atlas

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at the Albert Einstein Institute in Hannover and that computing facility

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we used for this analysis and it was immediately clear.

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This is a detection, so it's, it cannot be a noise fluctuation

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because it's so loud in the data.

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Narrator: A pulsar, that was a strong gamma ray source yet showed no radio signature

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intrigued researchers. Among them was Paul Ray of the

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Naval Research Laboratory. He and his team thought they might have a solution to the

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puzzling lack of radio emission. Paul Ray: When we first discovered the

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system I looked back at our archival radio observations and none of them

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showed detections of this pulsar. We think that nearly all pulsars do emit

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radio waves. The radio beam is emitted for most pulsars

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from a region above the polar cap of the star, and that means it's a tightly concentrated

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flashlight-type beam. In a system like this where there's wind being blown

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off the companion star, there's a lot of obscuring material along the line of sight.

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It might be that it is a radio pulsar and we just couldn't see it.

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And the one way to confront that is to use a higher radio frequency,

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that's more penetrating, that's less affected by the scattering in the intervening

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material. And so we went and made an observation with the Robert C. Byrd Green Bank

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Telescope run by the National Radio Astronomy Observatory in West Virginia,

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at a much higher frequency than typical radio observations. And it was in one

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of those observations that we first saw the signal from the system. And it

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appears that it is most of the time obscured by the material

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from its companion. Narrator: A combination of radio

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optical, and gamma-ray data allowed astronomers to assemble a

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complete picture of the system. It turned out to be a rare black widow

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binary where a rejuvenated pulsar is gradually evaporating a low

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mass companion star. Alice Harding: They get this name because they are in

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very close systems, with the companion star being close enough to the

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neutron star, that the neutron star is irradiating the companion.

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So the neutron star is producing a wind of energetic particles and

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magnetic fields, and also all the gamma rays that are radiated. All this

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hits the companion star and heats it up to very

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high temperatures, but only on one side. So the side that's

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towards the neutron star gets blasted by this pulsar wind.

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Paul Ray: And it has been whittled away over billions of years to where it now is

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only about 8 times the mass of Jupiter. Alice Harding: This whole system is about the size

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of the Earth-Moon system, so it's very compact.

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Paul Ray: We see the pulsar at the center, spinning, and emitting beams of

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radio and gamma rays. The radio waves are represented by the green,

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and the gamma rays are represented by the magenta. That radiation that

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impinges on the star is blowing off clouds of ionized material

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that are collecting around the system, and that's what obscures the radio emission.

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So we see that most of the time, the radio represented in green only

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makes it to that obscuring material, and not through it. While the gamma rays

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which are much more penetrating go right through.

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Roger W. Romani: It turns out that in as far as it's a pulsar, it's not so very unusual.

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What's unusual about it is this binary system, and the binary

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system seems to have--through its history--allowed this neutron star

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pulsar to accrete enormous amounts of mass. The measurements to date

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suggest that it is very heavy indeed, and heavy neutron stars push

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the absolute extreme of the densest matter in our visible universe. I say

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this because many people think of black holes as being exotic and the most extreme objects

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known, but after all, a black hole has collapsed to the point where

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nothing is visible, it's black. A neutron star is an object that's

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on the hairy edge of becoming a black hole, yet is still visible in our universe.

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Hence the study of the these ultra-massive neutron stars

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gives us the opportunity to study the most extreme matter in our visible universe.

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If this fellow is as heavy as he seems, he pushes

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that study to a new horizon, to a region of density

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and pressure which has never previously been seen.

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[music]

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[beeping]

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[beeping]

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