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Black holes and pulsars are two of the most mysterious (and coolest-sounding) cosmic objects – and we study both of them here at the Astrophysics Science Division (ASD). Here is Blueshift’s inside look at two recent discoveries made using data from the Rossi X-ray Timing Explorer (which I personally like to think of as the “little satellite that could”). One of these discoveries has to do with black holes and how they emit x-rays.

The other, about pulsar eclipses, was made by an ASD scientist, Dr. Craig Markwardt, who talked to Blueshift about what he found.  
One reason we get to study things like black holes and pulsars here in the ASD is that we do a lot of high-energy astronomy. By this, I don’t mean that our astronomers walk around overly-caffeinated (actually, they probably do) – I’m talking about x-rays and gamma rays.

High-energy x-rays and gamma rays are created when matter is heated to extremely hot temperatures. You can find hot matter around black holes and neutron stars (and pulsars, a specific kind of neutron star), especially when one of these objects is in a binary system with another star. The strong gravity of the black hole or neutron star will pull matter off its companion, heating it up to the point where x-rays and gamma rays emit.
There are a lot of satellites orbiting Earth whose job it is to observe these x-rays and gamma rays – there are big observatories like Chandra and plenty of small-to-medium sized ones like Suzaku, XMM, Swift, and CGRO. The one we’re talking about today is the Rossi X-ray Timing Explorer (or RXTE, for short).

I have a soft spot for this satellite, because I spent my first few years at NASA doing education, outreach, and websites for it. It doesn’t take pretty pictures (well, not in the stereotypical sense) – so it’s a hard sell in a world that loves nothing better than to see the newest Hubble image! But just because it doesn’t capture images doesn’t mean that it doesn’t collect valuable scientific data. RXTE gives us valuable information about the timing of the sources it observes (like the aforementioned black holes and neutron stars). When an x-ray photon hits RXTE’s instruments, it creates an electrical signal; the strength, shape, and timing of that electrical signal can be used to determine the energy, time, and position in the sky of the x-ray photon with great accuracy.

Though RXTE is considered a senior citizen in the satellite world (it launched 15 years ago, and only Hubble has been running longer), it has nevertheless been in the news recently.
As we mentioned, when a black hole pulls matter from its companion star, the matter is heated and emits x-rays. In addition, strong magnetic fields will occasionally eject some of this hot matter in the form of jets, which blast away from the black hole, sometimes at half the speed of light! Though astronomers have the big picture, it’s the details that are hard to figure out – do the x-rays emitted come from the jets, or from the disk of matter pulled from the companion star? Or from somewhere else entirely?

Dr. David Russell, a postdoc at University of Amsterdam, and his team, used optical, infrared, and radio data along with RXTE’s x-ray data to look at XTE J1550-564, which is in a binary system with a black hole in it. It was actually discovered in 1998 by RXTE, when it became extraordinarily bright in X-rays. In 2000, this object had another outburst.

Using the data from the 2000 outburst, Russell’s team was able to construct a detailed picture of what was happening to this black hole. The energy of the x-rays detected varied, as did the region of the black hole from which they were detected. When the x-rays seemed to come from the disk of hot matter around it, the jets would shut down. When the disk cooled, the jets switched on again, and the x-rays emitted seemed to be coming from close to the black hole. As the outburst faded, the strongest x-rays were coming from the blobs of matter that were hurled into space by the jets.

Here’s a diagram I adapted from this one, which shows the order of events in this outburst. The accompanying article goes into more technical detail about the x-ray emission.

The bottom line is that we are learning new things – and from old satellites! “We’re really beginning to get a handle on the ‘ecology’ of these extreme systems, thanks in large part to RXTE,” Russell told NASA.
The recent pulsar discovery is just as fascinating. Our moon can eclipse our sun – but other systems can have eclipses too – even a star system with pulsar in it. Swift J1749.4-2807 is the first x-ray pulsar we’ve observed being eclipsed by its companion star. When this binary system had an x-ray outburst, it was observed by RXTE. RXTE observed three eclipses, and was able to confirm that the system had a millisecond pulsar in it (click to enlarge the diagram).
What’s the significance of this discovery? Besides a chance to learn something new about x-ray binary systems like this, RXTE’s observations can be used to test a prediction of Einstein’s relativity. If you’d like to learn more, NASA released a new press release on this discovery. We also spoke to Dr. Craig Markwardt for an inside look.

Q: What’s the difference between a millisecond pulsar and a regular pulsar? Or rather, is there a relationship between them? Are they different types of objects or does one become the other?
A: All pulsars are neutron stars that are spinning and make emission that is pulsed at periodic intervals. But it’s not so simple. We distinguish between different kinds of neutron stars, based on their observed properties and how they were formed. Some pulsars live by themselves – isolated pulsars – and some have a companion star. Some spin slowly – as slow as once per 500 seconds – and some spin rapidly, almost as fast as one thousand rotations per second. We call those fast pulsars “millisecond pulsars.” In this case we discovered the source in the x-rays, so we give it the longer name “accretion powered millisecond x-ray pulsar.” We think that all millisecond pulsars are spun up by accreting matter from an orbiting companion. The accreted matter actually applies a torquing “twist force” to the pulsar, which helps it spin faster.

Q: What made you decide to study this particular source using RXTE? Its name has Swift in it, so presumably it was discovered using Swift? Did you discover it? How did your decision to study this source come about?
A: We discovered this source completely by chance, which is how much x-ray astronomy is done. In 2006, the Swift observatory detected a short burst of x-rays from this source for the first time, which is how it got its name. When a monitoring program detected that the source was active again in 2010, we observed with RXTE, which is the only observatory that can do rapid x-ray timing. That was the first time, four years after the source’s discovery, that we detected the fast x-ray pulsations that identify the source as a rapidly spinning neutron star.

Q: Can you tell us what’s special about this source?
A: Swift J1749 is one of a handful of millisecond x-ray pulsars, but what is really special about this source is that it’s the first pulsar that is eclipsed by its companion star.

Q: What is an x-ray eclipse, and how did you figure out this source was doing it?
A: We normally think of a solar eclipse as the moon passing in front of the sun and blocking out the sun’s light. In this case, the pulsar’s x-ray light is being blocked by its companion star, which passes in front of the pulsar every orbital revolution. We found that the measured x-ray intensity decreased to “zero” once per orbital period, and that dip occurred at exactly the expected time for an eclipse. This occurs when the pulsar is at its farthest point in its orbit and the companion star as at its closest point.

Q: What do these eclipses mean, or tell us about the source?
A: The presence of an eclipse tells us that the orbit of the system is being viewed nearly edge-on. Other systems that don’t have eclipses are seen more face on, and never have a chance to eclipse. The more important measurement is the duration of the eclipse. Using the duration of the eclipse, we can estimate the size and mass of the companion star, as well as the other geometric angles of the orbital system. Our report is one of the most precise determinations of the orbital properties of an x-ray binary system.
What we are really excited about is the prospect to be able to “weigh” a neutron star. Having measured almost all of the orbital properties of the system, we need to measure only one more thing in order to estimate the mass of the pulsar in this system (which is a neutron star). This is exciting because such a measurement tells about the density of highly compact neutron star matter, which we don’t normally get to do in labs on earth. Those kinds of measurements can also tell us about how strong force nuclear reactions behave. Because this system has been accreting for a very long time, perhaps even a billion years, we expect that this pulsar will be more massive than “typical” neutron stars.

Q: Are you planning any follow-up to this?
A: As I said, we need to measure only one more quantity in order to weigh this pulsar, and that quantity is the Doppler shift of the companion star. The best way to do this is to measure an infrared spectrum of the star and measure the Doppler shift of emission lines. We are hopeful
that this will be a doable project soon.
In the meantime, the x-ray emission from this system has quieted down, meaning that for now, the accretion has stopped. We will be waiting for the next outburst so that we can measure the eclipses again, and hopefully more precisely.
Here’s a little video to show you what such an eclipse might look like!
More images are available at the Scientific Visualization Studio. Thanks to Craig Markwardt for his time!
Courtesy of: NASA's Goddard Space Flight Center

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