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Narrator: When a massive star
explodes as a supernova, its core may be crushed into one of two types
of compact remnant: a black hole, or a neutron star.
Neutron stars are the size of a city, but
contain more mass than our sun. They rotate rapidly, host
powerful magnetic fields, and produce beams of radiation that emit a wide
range of energy. When we detect pulses as the beams sweep over
Earth, the object is known as a pulsar. Paul Ray: They can spin at
many times per second on their axis; the fastest pulsars spin over 700 times
per second. And that rapidly spinning massive object, generates
extremely strong magnetic fields and accelerates particles to
high energies. And we see that those accelerated particles
emitting energy in the form of gamma rays, x-rays, and radio waves.
And when that beam sweeps past the line of sight to the Earth we see it pulse on
and that's why they're named pulsars. Narrator: The most
sensitive tool for observing pulsars in gamma-ray light is NASA's
Fermi Gamma-ray Space Telescope. Fermi scans the entire
sky for high-energy sources and has found many previously undetected
gamma-ray emitters. Scientists have identified many of these,
but for some, the source of the gamma rays remains unknown.
Roger W. Romani: I got interested a couple years ago in trying to find the
limits of what Fermi can discover, how extreme these objects can be, and
in order to do that I focused on the set of objects that are
relatively bright and well measured by Fermi and
found that virtually all of them have now been identified. At present, when I started
this project,there were only six objects which we hadn't figured out what they were yet.
Despite intense searches, at radio, with radio wave lengths,
which is the standard way in which people find pulsars--and also looking at the gamma rays
themselves--no pulsations had been seen. So something was unique
about these six objects and I thought, that's where the discovery space is going to be.
If we can track down what those are, we will have a good chance of finding something new.
We took this small set of six objects and attacked them
with a number of wave bands, but I think the thing that helped us make the greatest progress
was looking in the optical, in visible light. Now this may seem a little bit unusual
for studying the high-energy gamma-ray universe, but, it turns out that
many of these objects seem to have optical counterparts. And if you can figure out what the
visible light counterpart of an object is, you're long ways along the
track to understanding what it's all about. Paul Ray: It was Roger
Romani's optical observations that discovered a counterpart
to the gamma-ray source that showed a binary period that was
indicative of this potentially being a binary millisecond pulsar. Alice Harding: It brightened, and it
dimmed, and brightened, and so this looked like we were
looking at possibly something which was
irradiated by a companion pulsar. And that every
time you're looking at the bright face you see a bright optical source and when
it rotates away from you, and you see the dark face, you don't see anything.
Roger Romani: We managed to get enough observations of the object to piece together its orbital
period, and found, remarkably, that it was an incredibly
heated object--blue white on one side, deep, deep red on the other--
that was orbiting around something invisible with an orbital period of about
one and a half hours. Now, that's faster then any spin powered
pulsar ever known, and indicates that it's a really, really tight system
and that the gamma rays are blasting the companion at point-blank range.
Our colleagues in Germany managed to use the orbital period
that we measured to search in the gamma rays directly and, with a computational
tour de force, managed to find the pulse signal of the pulsar
directly in the gamma rays themself. Holger Pletsch: What I'm
doing is blind searches for pulsars so that we
try to find pulsars that have not been seen before. So you don't know
how fast the pulsar is spinning, where exactly it is sitting in the sky.
To do that, you have basically to try, every possible combination of
parameters--if they match your data output stream. So the problem is
that the number of possible combinations is tremendously high, so the
straightforward brute force approach is impossible. The computation power you would
need would be in excess of what is available in the whole planet.
So our work is to invent more efficient methods to do that.
The basic method is analogous to zooming.
It's similar to changing your objectives
of your microscope, in favor of one of higher magnification, so you look at
one interesting point on the slide, and then you zoom in on that.
And then you further zoom in if it still is interesting. To find the pulsations
in the gamma ray data requires about
5,000 cpu days. So if you do it
on your laptop you need 5,000 days, but if you
have 5,000 laptops, you only need one day. And so
that is the path we took because we have a computing cluster that is called Atlas
at the Albert Einstein Institute in Hannover and that computing facility
we used for this analysis and it was immediately clear.
This is a detection, so it's, it cannot be a noise fluctuation
because it's so loud in the data.
Narrator: A pulsar, that was a strong gamma ray source yet showed no radio signature
intrigued researchers. Among them was Paul Ray of the
Naval Research Laboratory. He and his team thought they might have a solution to the
puzzling lack of radio emission. Paul Ray: When we first discovered the
system I looked back at our archival radio observations and none of them
showed detections of this pulsar. We think that nearly all pulsars do emit
radio waves. The radio beam is emitted for most pulsars
from a region above the polar cap of the star, and that means it's a tightly concentrated
flashlight-type beam. In a system like this where there's wind being blown
off the companion star, there's a lot of obscuring material along the line of sight.
It might be that it is a radio pulsar and we just couldn't see it.
And the one way to confront that is to use a higher radio frequency,
that's more penetrating, that's less affected by the scattering in the intervening
material. And so we went and made an observation with the Robert C. Byrd Green Bank
Telescope run by the National Radio Astronomy Observatory in West Virginia,
at a much higher frequency than typical radio observations. And it was in one
of those observations that we first saw the signal from the system. And it
appears that it is most of the time obscured by the material
from its companion. Narrator: A combination of radio
optical, and gamma-ray data allowed astronomers to assemble a
complete picture of the system. It turned out to be a rare black widow
binary where a rejuvenated pulsar is gradually evaporating a low
mass companion star. Alice Harding: They get this name because they are in
very close systems, with the companion star being close enough to the
neutron star, that the neutron star is irradiating the companion.
So the neutron star is producing a wind of energetic particles and
magnetic fields, and also all the gamma rays that are radiated. All this
hits the companion star and heats it up to very
high temperatures, but only on one side. So the side that's
towards the neutron star gets blasted by this pulsar wind.
Paul Ray: And it has been whittled away over billions of years to where it now is
only about 8 times the mass of Jupiter. Alice Harding: This whole system is about the size
of the Earth-Moon system, so it's very compact.
Paul Ray: We see the pulsar at the center, spinning, and emitting beams of
radio and gamma rays. The radio waves are represented by the green,
and the gamma rays are represented by the magenta. That radiation that
impinges on the star is blowing off clouds of ionized material
that are collecting around the system, and that's what obscures the radio emission.
So we see that most of the time, the radio represented in green only
makes it to that obscuring material, and not through it. While the gamma rays
which are much more penetrating go right through.
Roger W. Romani: It turns out that in as far as it's a pulsar, it's not so very unusual.
What's unusual about it is this binary system, and the binary
system seems to have--through its history--allowed this neutron star
pulsar to accrete enormous amounts of mass. The measurements to date
suggest that it is very heavy indeed, and heavy neutron stars push
the absolute extreme of the densest matter in our visible universe. I say
this because many people think of black holes as being exotic and the most extreme objects
known, but after all, a black hole has collapsed to the point where
nothing is visible, it's black. A neutron star is an object that's
on the hairy edge of becoming a black hole, yet is still visible in our universe.
Hence the study of the these ultra-massive neutron stars
gives us the opportunity to study the most extreme matter in our visible universe.
If this fellow is as heavy as he seems, he pushes
that study to a new horizon, to a region of density
and pressure which has never previously been seen.
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