The NUSTAR Orbiting Telescope – Spinning Black Holes and Exploding Stars
Talk at University of Alberta by Dr. Fiona Harrison, California Institute of Technology (May 30, 2017)
This blog describes a public talk given by Fiona Harrison, a
professor of physics at Caltech, on May 30, 2017 at the University of
Alberta. She is principal investigator
for NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR), an orbital telescope
that examines the cosmos in the high X-ray band. Though there are other X-ray telescopes, NuSTAR
is designed to be sensitive to much higher energy X-rays than its predecessors,
such as Chandra and XMM-Newton. It is
therefore able to make discoveries in areas that were heretofore not accessible
to astrophysical observations.
About NuSTAR
- Though there are other X-ray telescopes, NuSTAR is designed to be sensitive to much higher X-rays than its predecessors, such as Chandra and XMM-Newton. It is therefore able to make discoveries in areas that were heretofore not accessible to astrophysical observations.
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Chandra and XMM-Newton operate in the1-10 KEV
range.
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NuSTAR operates in the 3-79 KEV range.
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KEV is a measure of photon energy, which can
also be expressed in wavelength measure.
Basically, higher KEV means higher energy, which means shorter
wavelength and higher frequency.
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Fiona Harrison also used the term “blue end” of
the X-ray spectrum, borrowing terminology from visual astronomy.
- There have been some instruments in the high X-ray or gamma ray range, but NuSTAR is the first to have reasonably high resolution. In fact, it has a resolution of about 30 seconds of arc, and a field of view (FOV) of about 12 minutes. Previous high X-ray band telescopes were only accurate to about 0.1 degrees, or 6 minutes.
- For context, the moon is about half a degree of arc, or 30 minutes of arc. Each minute has 60 seconds of arc. So, NuSTAR can see down to the resolution of about one-sixtieth the width of the full moon
- This might not seem all that impressive, as a good backyard telescope (8 inch) can get down to about half an arc-second. But, getting high resolution in the X-ray band is difficult, because the photons have to be reflected at a high angle (glancing reflection), whereas light in the visible band can be collected via a parabolic mirror or a lens. So, the underlying geometry and optics of the situations are very different
- As “big science” telescopes go, NuSTAR was actually quite inexpensive, at about $160 million. For comparison, the Hubble telescope has cost several billions and the James Webb is estimated to come in at close to ten billion, by launch time in 2018. The ESA X-ray telescope, XMM-Newton cost about 700 million Euros, which would be well over a billion in U.S. dollars.
- So, NuSTAR takes its place alongside some other “ground-breaking” orbital telescopes (perhaps “space-breaking” is a more apt term), such as:
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COBE, which opened up the micro-wave band.
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Spitzer, which opened up the infrared band.
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Hubble, which greatly extended our range of
observations in the visual band.
- It is worth noting that a lot of new technological developments were needed to build NuSTAR, in order to give high resolution in the far X-ray band.
- Fiona Harrison has a background in experimental physics, as she is a Robert A Millikan Prize Fellow. You may recall that name from high school physics, in association with some of the first really precise measurements of the speed of light, that were instrumental in leading Einstein to the Theory of Special Relativity.
The Launch
- NuSTAR was launched in June 2012. Since the Earth’s atmosphere is opaque to most wavelengths (visual and radio being the primary exceptions), telescopes operating in these wavelengths must be launched into orbit, and return their data as digital information.
- The telescope orbits at about 600 km above the surface of the Earth.
- The launch was from the Kwajalein Atoll, in the south Pacific.
- It was an interesting launch. NuSTAR was launched into orbit from a rocket, which was itself dropped from an airplane, in flight. A short clip of this launch was reminiscent of a cruise missile being launched from an aircraft.
- About 10 days after the launch, the telescope was “unfurled”.
- Since the payload area of most rockets is at the top, and cone-shaped, the payloads are stored in a folded state. Once the desired orbit has been achieved, they are unfurled, to assume their proper functional shape. That can include booms holding telemetry instrumentation, solar panels to provide power, reaction wheels and minor rocket nozzles for station keeping, and the telescope itself.
- In the case of NuSTAR, this was extra tricky. X-ray telescopes have to be long, as the mirrors can only reflect X-rays at a glancing angle. NuSTAR required a full 10 meters, and the unfolding of the telescope took about half an hour, a very tense half-hour for the engineers and scientists involved in the project.
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NuSTAR’s New Science
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The ability to see in the far X-ray in high
resolution has given us new views of the cosmos. Here are some examples.
Spinning Black Holes
- We have now seen Sagittarius* in much more detail. That’s the center of the Milky Way galaxy, which harbours a super-massive black hole. This has allowed us to see remnants of dead stars, that are near that giant black hole.
- NuSTAR has given us the detailed view needed to measure the spin of black holes, based on their effects on nearby matter.
- An accretion disk surrounds a black hole, where matter is accelerated as it swirls around and eventually falls into the black hole.
- As particles in the accretion disk are sped up to nearly the speed of light, X-rays are generated
- There is something called the innermost stable orbit, whose size is related to the amount of spin that the black hole has.
- It should be noted that the spin of a black hole is an odd concept to understand. After all, a black hole is mathematically described as a singularity, or a point source. How can a point source spin?
- But matter that falls into the black hole generally has some net angular momentum, so it must spin, in some sense of the term.
- An effect of general relativity called frame dragging decreases the innermost stable orbit, and that is how we can infer the spin of the black hole.
- In practice, this is measured via the spectrum of the X-ray photons observed by our telescopes.
- However, dust and gas clouds around the black hole can also create a similar X-ray spectrum at the lower ranges.
- NuSTAR’s ability to observe in the higher X-ray energies made the difference. It allowed the spin of the black hole to be measured unambiguously.
- As we observe more and more of these black hole spin states, we will gain greater insights into cosmic evolution.
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Exploding Stars
- In this case, we are talking about high mass stars (10 solar masses and up), that end their lives as supernovae, which is to say fantastic explosions, that rip the star apart.
- Essentially, they run out of nuclear fuel, and when that happens, the star’s balance between the outward force of nuclear fusion and the inward force of gravity is broken. The star then collapses, and when the in falling matter collides, there is a bounce-back of incredible energy, the super-nova.
- This process is instrumental in creating the elements that are heavier than iron, by-products that are needed for life, among other important considerations.
- Depending on the initial mass of the star, a neutron star or a black hole are also created. Both are incredibly dense objects.
- We know that these events occur – they have been seen occasionally within the Milky Way galaxy during recorded history (e.g. the Crab Nebula was formed by one in the early years of the second millennium). They have been observed and studied in other galaxies as well.
- There has been extensive computer modelling of super-novae, based on theoretical considerations. But, computer models have not yet been developed that actually get the star to explode – it is hard for the models to have matter fall in to the center of the star in just the right way to kick off the explosion.
- There are a few contending models, that use different mechanisms to kick off the explosion. One of these is a “sloshing” model, whereby the matter of the star sloshes around for a while. There are other models as well.
- It was hard to see just what was going on in the center of the star, before the explosion, so it was difficult to choose between the models.
- The most energetic photons come from the deepest levels of the star
- NuSTAR can now detect these very high energy photons, so we are beginning to see into the heart of the exploding star.
- So far, it turns out that the data best supports the sloshing model.
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Miscellaneous Notes and Observations
- The big question – should one write x-ray or X-ray? The consensus seems to be to capitalize if you are using it as a noun (“high energy X-rays were detected”), versus lower-case when used as a verb (“the doctor x-rayed the patients leg”). That being said, there is still a lot of disagreement on the subject.
- The talk was part of the Helen Sawyer Hogg Public Lecture series, put on by the Canadian Astronomical Society and the Royal Astronomical Society of Canada. Helen Sawyer Hogg was a significant figure in popularizing astrophysics in the 20th century in Canada.
- It was noted that the X-ray band that NuSTAR uses, is about the same as that used in medical X-rays. X-Rays in those energy levels can penetrate the dust and gas of interstellar space, which block lower energy X-rays. This is also the energy level needed to penetrate soft tissue, for medical imaging.
- In fact, there were a number of people in the audience that were involved in medical X-ray research, so there was a lively interest in the overlap between astrophysical detection and medical imaging technologies.
- Your blogger has a “two degrees of separation” relationship with NuSTAR, as a close relative did a PhD in astrophysics under Vicki Kaskpi of McGill University and a MSc at the University of Alberta under Sharon Morsink, the host of the talk. Vicki Kaspi was a consultant to NASA on the NuSTAR project. That close relative is now engaged in data science, since jobs are relatively plentiful and they pay well.
- You can check out the NuSTAR website for more details (and possibly more accurate details 😀).
- There are a lot more items on that website, that describe other important findings that NuSTAR has been instrumental in making. Many of these are related to highly compressed astrophysical objects, such as super-novae, neutron stars and black holes.
- But NuSTAR has improved our view of other objects as well. For example, here’s a picture of the sun, via NuSTAR and some other X-ray telescopes, just because it looks cool.
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Sources
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Notes from the talk at the University of
Alberta.
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And now that you have read about some real cutting-edge science, you
should think about reading some Science Fiction (because all work and no play
can make you a dull person, or so they say).
Here’s a novel that features a neutron star (and a pretty girl, who is
also an engineer, among many other interesting characters):
The Witches' Stones, Book 1 - Rescue from the
Planet of the Amartos
Young
Earth woman and spaceship mechanic, Sarah Mackenzie, has unwittingly triggered
a vast source of energy, the Witches' Stones, via her psychic abilities, of
which she was unaware. She becomes the focal point of a desperate contest
between the authoritarian galactic power, known as The Organization, and the
democratic Earth-based galactic power, known as The Terran Confederation. The
Organization wants to capture her, and utilize her powers to create a
super-weapon; the Terra Confederation wants to prevent that at all costs. The
mysterious psychic aliens, the Witches of Kordea also become involved, as they
see her as a possible threat, or a possible ally, for the safety of their own
world.
A small
but fast scout-ship, with its pilot and an agent of the Terra Confederation,
Coryn Leigh, are sent to rescue her from a distant planet at the very edge of
the galaxy, near space claimed by The Organization. Battles, physical and
mental, whirl around the young woman, as the agent and pilot strive at all
costs to keep her from the clutches of the Organization.
