Pluto is emitting x-rays, and we don’t know why. 

X-rays aren’t something we’d expect from Pluto, since the planet has no clear way of generating them. It’s a small, cold world with little magnetic field. Some solar x-rays might scatter off Pluto in our direction, but the level of x-rays is higher than what could be produced by scattering. So what gives?

The most likely explanation is that the x-rays are produced through an interaction between the solar wind and Pluto’s atmosphere.  As the New Horizon’s flyby found, Pluto’s atmosphere is actually quite stable, so interactions with the solar wind could produce x-rays. Similar interactions between the solar wind and the comas of comets have been seen to produce x-rays. But the level of x-rays from Pluto is even higher than we’d expect from such an interaction, so that isn’t the whole story.

While these x-rays are a mystery, it’s important to keep in mind that the amount of data is actually quite small. There’s enough data from the Chandra spacecraft to know that it’s not a random fluke, but it’s difficult to get much detail from such a small sample. What this study does show is that Pluto does produce x-rays, and perhaps we should give it a closer look.

Paper: C.M. Lisse, et al. The puzzling detection of x-rays from Pluto by Chandra. Icarus. (2016) DOI: 10.1016/j.icarus.2016.07.008

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When we observe a supernova, we can identify its type by looking at its spectrum. Supernovae that have strong hydrogen lines are known as type II, and are caused by the core collapse of a star at the end of its life. Stars without a strong hydrogen line are known as type I, and there’s a variation of type I supernovae with a strong silicon line known as type Ia. This last variety is particularly useful for astronomers, because they always have about the same absolute magnitude. This means they can be used as “standard candles” to determine the distance of a galaxy. While we know type Ia supernovae are not caused by the core collapse of a large star, we aren’t entirely sure what triggers them.  

There are two main models for type Ia supernovae. The first is that they are caused when a white dwarf star closely orbits a companion star. The outer layer of the companion star can be gravitationally captured by the white dwarf, which causes a runaway fusion process that results in a supernova. The other is that they are triggered by the collision of two white dwarfs. Understanding the origin of these supernovae is important, particularly in light of the fact that there is some evidence they might not be as standard as we thought.

One of the challenges with studying these supernovae is that they typically occur in other galaxies. There hasn’t been a clear supernova in our own galaxy for several hundred years, even though there’s evidence that one occurs in our galaxy about every 50 years. This is likely due to the fact that much of our galaxy is blocked by gas and dust, making them difficult to observe.  But recently observations from the Chandra x-ray observatory have found the remnant of a supernova in our galaxy that occurred about 110 years ago. Since it’s in a region of the galaxy rich in dust, the visible light from the supernova was obscured, which is why astronomers at the time didn’t see it. Looking through archive data from Chandra, a team found the remnant has been increasing in brightness at x-ray and radio wavelengths as the remnant interacts with interstellar gas and dust. Such an increase is what you’d expect from the collision model of type Ia supernovae, but not the binary model.

While this supports the idea that type Ia supernovae can occur from white dwarf mergers, it doesn’t necessarily mean that they can’t occur through a dance of binary stars as well. It’s possible that both mechanisms can produce supernovae of this type. It will take more observations to determine whether mergers are the only cause.

Paper: Sayan Chakraborti, et al. Young Remnants of Type Ia Supernovae and Their Progenitors: A Study of SNR G1.9+0.3. The Astrophysical Journal, Volume 819, Number 1 (2016)  arXiv:1510.08851 [astro-ph.HE]

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When a black hole interacts with surrounding gas and dust, it can create jets of material that fly away from the black hole at nearly the speed of light. When those jets interact with the cosmic microwave background, something brilliant can occur. 

The cosmic microwave background (CMB) is an afterglow of the big bang. We see it as a faint glow of microwaves in all directions, and that’s because the entire universe is filled with the remnant light of the big bang. Photons from the CMB are streaming through the universe in all directions. When electrons in a black hole jet collide with these photons, they can give the photons an energy boost. Since the electrons collide at nearly the speed of light, the photons are boosted into x-rays.

Recently the Chandra x-ray observatory found an example of such a CMB enhanced jet. The x-rays from this jet began their journey more than 11 billion years ago. At this time the CMB was stronger, and so the x-rays emitted by this jet are about 150 times brighter than a jet forming in the present universe would be. It’s a great example of how the background light of the universe can be rekindled by black holes.

Interestingly, this active black hole isn’t bright at radio frequencies. Often the electrons of a black hole jet interact with magnetic fields to create strong radio emissions, and usually such black hole jets are detected by radio waves first. If more black holes can be x-ray loud but radio quiet, it could mean that there are many more distant x-ray jets that have simply been overlooked.

Paper: A. Simionescu, et al. Serendipitous discovery of an extended X-ray jet without a radio counterpart in a high-redshift quasar.  arXiv:1509.04822 [astro-ph.HE]

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Most people think of black holes as gravitational traps that suck in surrounding matter, or dark regions of intense gravity that can only be observed by the way they warp light around it. But the dynamics of material around a black hole is quite complicated, and we’re still working out the details. Take, for example, the recent observations of a black hole flare.

Much of the light we observe from the region around a black hole is in the form of x-rays. These are caused by superheated material in the accretion disk or corona around the black hole itself. Occasionally these x-rays will briefly flare up, as the black hole in our galaxy did in 2013. Such a flare could be caused by material being captured by the black hole, but it could also be caused by dynamical interactions of the surrounding material.

We’re seeing the latter effect in new results from the NuSTAR telescope. In 2014 the black hole known as Mrk 335 was seen to flare up. Within a week NuSTAR was able to observe the spectrum of the flare, and team determined from its shift of brightness and color that the flare was moving at about 20% the speed of light. This means it was an ejected corona, and not captured material.

The ability to see this level of detail around a black hole is important, because it is this type of dynamic behavior that allows us to refine our complex models of black hole dynamics.

Paper: Dan Wilkins et al. Flaring from the supermassive black hole in Mrk 335 studied with Swift and NuSTAR. Monthly Notices of the Royal Astronomical Society, Oxford University Press, in press. (2015)

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Most galaxies have a supermassive black hole in their center. Some of these we can observe directly, or in the case of the supermassive black hole in our galaxy through the motions of nearby stars. But many supermassive black holes are surrounded by gas and dust, and so aren’t easily observed. But new observations from NuSTAR has peered through the dust in five galaxies to observe their supermassive black holes.NuSTAR is an x-ray telescope, so it’s capable of detecting the kind of high-energy x-rays that can penetrate the dust surrounding a supermassive black hole. Using data from the satellite, the team was able to detect five black holes. While this approach only works for “active” black holes that emit lots of x-rays, it allows us to verify that black holes exist in these galaxies, and may help explain why some black holes are so shrouded in dust while others are not.

This method also allows us to confirm some of the indirect methods we’ve used to infer the presence of black holes in a galaxy. It means our models are on the right track. Or you could say our models are five by five.

Paper: G. B. Lansbury et al. NuSTAR Reveals Extreme Absorption in z < 0.5 Type 2 Quasars. ApJ, accepted for publication; arXiv: 1506.05120 (2015)

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In the center of our galaxy there is a supermassive black hole known as Sgr A*. Through observations of stars orbiting the black hole, we know it has a mass of about 4 million Suns. Normally this black hole is pretty quiet, but in 2013 there was an unexpected x-ray burst.

What’s interesting about this burst is the shortness of its duration. In just a couple of hours Sgr A* brightened by a factor of 400 before fading to normal levels. Given the duration it is possible that an asteroid-sized object was captured by the black hole, creating a burst of energy before crossing its event horizon. Simulations of such captures indicate that the body would be ripped apart by tidal forces and greatly heated before final capture, which would produce an x-ray burst lasting an hour or so. Another idea is that it could be due to magnetic field lines in the black hole’s accretion disk snapping into realignment after being twisted around the black hole. We see similar effects with the magnetic field lines of the Sun and other stars.

Time lapse of the x-ray burst.

It’s difficult to be sure, because Sgr A* is hidden behind a wall of gas and dust in the center of our galaxy known as the zone of avoidance. This means visible light from that region is blocked, and we can only observe Sgr A* with radio waves and x-rays or gamma rays. It isn’t possible to detect small bodies such as asteroids or planets orbiting the black hole.

If other small bodies are heading for the black hole, we won’t know until after it’s happened.

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The image shows two colliding galaxies known as NGC 2207 and IC 2163. It’s a false-color image, where infrared is shown as dark red, visible is shown as normal, and x-ray is shown as purple. The first impression you might have is that the image looks awfully purple, and that means there are lots of x-ray sources in these two galaxies.

The reason for this is that the two galaxies are colliding. Young galaxies tend to have lots of gas and dust around to make stars, so they can produce stars at a fairly high rate, such as we see in dusty starburst galaxies. But over time the rate of stellar production goes down as the free gas and dust tends to get used up. In the Milky Way, for example, new stars form at a rate of only 1 or 2 a year.

In the x-ray only image, the ULXs are clearly seen.
Credit: NASA/CXC/SAO/S.Mineo et al

In these galaxies that rate is about 24 solar-mass stars per year. We know this because of the high number of ultraluminous x-ray sources (ULXs), seen as bright violet dots within the image. The stars associated with these sources are only about 10 million years old, and such sources don’t stay bright for long on astronomical scales.

Images such as this further support what we’ve long thought, that galaxy collisions can stir up star production in galaxies.

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There’s been a few articles in the popular press recently announcing the discovery of dark matter particles coming from the Sun. This is not the case. No science team is claiming they’ve discovered dark matter. The story traces it origin to a paper being published in MNRAS, which could be an indirect detection of dark matter, but could also be a few other things. It is an interesting paper, nonetheless.

What the paper actually looks at is possible evidence of axions coming from the Sun. Axions are hypothetical particles that were first proposed in 1977 to address certain issues in quantum chromodynamics (QCD), which describes the behavior of quarks and such that make up protons and electrons (among other things). There has been no evidence for axions, but if they exist, then the Sun should produce axions through nuclear interactions similar to the way it produces neutrinos.

According to the model, axions would be be low mass, chargeless particles that don’t interact strongly with light. This would make them a suitable candidate for cold dark matter. Of course this would also make them particularly difficult to observe directly. But unlike other dark matter candidates, axions do interact slightly with light and electromagnetic fields. So we might be able to see them by their interaction with Earth’s magnetic field.

In the paper the authors demonstrate that high energy axions striking the Earth’s magnetic field could produce x-rays. When they modeled this idea they found that axion-induced x-rays would have a seasonal variation due to the varying tilt of Earth’s axis (and thus its magnetic field). They then looked at x-ray data from the XMM-Newton spacecraft, and found that it had in fact detected a seasonal variation of x-rays. This variation is consistent with the axion model.

Of course before we can say that it’s definitely axions, we need to look for other possible explanations. Not surprisingly there are several. For example, the seasonal variations could be due to subtle and complex interactions between the solar wind and Earth’s magnetic field. So axions are a good candidate for this cyclic variation, but not the only candidate.

Where things could get interesting is through a more detailed study of the seasonal x-ray variations. If they are produced by axions, then the x-rays should have distinct signatures in their spectrum that would distinguish them from other models. So like many cutting edge discoveries it has potential without clear confirmation.

Paper: G. W. Fraser, et al. Potential solar axion signatures in X-ray observations with the XMM-Newton observatory. Monthly Notices of the Royal Astronomical Society, 20 October, 2014

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In an earlier post I talked about ultraluminous x-ray sources, and how they are typically powered by stellar-mass black holes. The source of these intense x-rays is the superheated material surrounding the black hole. By observing the intensity of the x-rays, we can get a handle on just how much mass a black hole is actively accumulating. The x-ray intensity has also been used as a measure of the black hole’s mass, since there is a rough limit to just how much matter a black hole of a given size can accrete. It’s known as the Eddington limit, but as we’ve seen, that limit isn’t absolute.

The basic idea of the Eddington limit is that the very x-rays produced as a black hole accretes matter also work to prevent matter from being accreted. That’s because the emitted light can exert an outward pressure on the infalling matter, thus reducing the amount that reaches the black hole. At some point the intensity of light would be so great that the surrounding matter would be pushed away from the black hole, preventing matter from accreting altogether. This is what’s known as the Eddington limit.

Which brings us to the case of a black hole known as P13. Like most ultraluminous black holes, P13 has a binary companion, in this case a blue supergiant of about 20 solar masses. The black hole captures material from its companion, which is how it gets the matter it accretes. As the black hole and star orbit each other, their light is Doppler shifted due to their orbital motion. From this we know that the two orbit each other with a period of about 64 days.

That information by itself isn’t enough to determine the mass of the black hole, but in a recent paper in Nature, new data is used to determine the mass of P13. As the authors point out, the intense x-rays of the black hole heat the facing side of the companion star. This affects the spectrum emitted by the star, which can be observed. My modeling the x-ray heating and matching it to the observed spectrum, the team could get a measure of their distance of separation. From this they determined the mass of P13 to be less than 15 solar masses.

That’s a great result by itself, but what’s surprising is that this is much smaller than the value expected given its luminosity. According to the Eddington limit, a black hole of this size shouldn’t be nearly so bright. Given past violations of the Eddington limit, that might not be too surprising, but the rate of accretion is huge. This black hole is eating an Earth’s worth of mass every three years.

Just how that occurs is not clear. What is clear is that P13 is a hungry hippo indeed.

Paper: C. Motch, et al. A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source. Nature 514, 198–201 (2014)

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An ultraluminous x-ray source (ULX) is an  intense, localized sources of x-rays. They are generally powered by solar-mass black holes, similar to the way quasars and blazars are powered by supermassive black holes. We’ve generally thought only black holes could provide enough power to generate such powerful x-rays, but now it seems that isn’t always the case. New results have been published in Nature that show some of them might be powered by accreting neutron stars.

The team looked at x-ray data from NuSTAR taken of M82, which is a galaxy with several ultraluminous x-ray sources. What they noticed that instead of being a constant level of brightness, there was some flickering of brightness in the data. Some variation in brightness is expected, but in this case the flickering was both regular and rather fast. They tracked the source of the flickering down to a ULX known as M82X-2, and found that it flickered with a period of about 1.4 seconds.

A black hole wouldn’t cause such a flicker, but a neutron star would.  When they do, we call them pulsars. So despite having the x-ray intensity of a black hole, this particular x-ray source is a neutron star. Just how a neutron star could emit such intense energy is not clear. One idea is that it is accreting massive amounts of material, the heating of which drives the x-ray production. But if that’s the case, this particular neutron star may be eating its way toward becoming a black hole.

The interesting thing about this new work is that it was only possible because of a recent supernova in M82, causing several telescopes to focus their attention there. Because of the wealth of high-resolution data, the team was able to distinguish this flicker. So it seems other ultraluminous x-ray sources may be worth a closer look.

Paper: M. Bachetti, et al. An ultraluminous X-ray source powered by an accreting neutron star. Nature 514 (7521): 202 (2014)

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Yesterday I wrote about the cosmic x-ray background, and I noted that x-ray astronomy, particularly with high-energy x-rays is difficult. But what is it about x-rays that makes x-ray astronomy so challenging?

X-rays are electromagnetic waves (light) with very short wavelengths. Visible light has wavelengths between about 400 to 700 nanometers, while x-rays have wavelengths of 0.01 to 10 nanometers. Their wavelengths are roughly the size of an atom. Because of their very short wavelength and high energy, x-ray photons act almost like hard particles. At longer wavelengths they are known as soft x-rays, while at shorter wavelengths they are known as hard x-rays. Hard x-rays in particular are very good at penetrating material. This is what makes them useful used as medical x-rays or CT scans.

This ability to penetrate is problematic for astronomy. A telescope works by focusing light from a larger area to a smaller one. An optical telescope uses lenses to refract light or curved mirrors to reflect it. Radio waves bounce easily off conductive materials such metal, so large metal dishes can be used as radio mirrors to focus radio waves to the detectors. But because x-rays tend to penetrate material rather than reflect off them, you can’t focus them as you would visible light or radio waves.

But x-rays can scatter off certain materials if they strike the surface at a very low angle. Basically an x-ray photon can make a glancing blow off the surface of the material, which changes the direction of the photon slightly. So to make an x-ray telescope, you need to make a column of reflective surfaces nested within each other, as seen in the image above. Each layer scatters x-ray photons slightly, but the amount of deflection by each layer is different so that all x-rays entering the telescope are focused toward the detector.

If there x-ray photons were only scattered once, then they would only focus after a very far distance. So you add another column of reflective surfaces to scatter the x-ray photons again. With two scattering the length of your telescope can be reduced to a more manageable length of about 10 meters.

Of course there are other challenges. Because the x-rays come in at such a shallow angle, any defect in your reflecting surface can scatter them in the wrong direction, which reduces the sensitivity of your telescope. So the reflectors need to be extremely precise. Metals such as platinum can be used for lower energy x-ray reflectors, but high-energy x-ray reflectors require multiple layers of different materials as a scattering surface. This means that after you make the reflecting surfaces, you then have to vacuum deposit a few-atoms thick layer of a low density material such as silicon or carbon, then a layer of a high density material such as platinum, and keep doing that about a hundred times. You have to do this for each mirror. In the case of the NuStar x-ray telescope, for example, there are 130 concentric focusing mirrors.

One last thing. X-rays don’t penetrate through the Earth’s atmosphere much, so this x-ray telescope, which is about the length of a bus, has to be launched into space.

This is why x-ray astronomy is such a challenge.

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You are likely familiar with the cosmic microwave background. This background is a thermal remnant of the big bang. Because of the expansion of the universe, this remnant energy has a temperature of about 2.7 K, which means it exists primarily in the microwave wavelengths. We see this cosmic background as a diffuse, low-energy glow of microwave radiation.

But there is another background that exists, known as the cosmic x-ray background. Just as the cosmic microwave background is a diffuse microwave glow, the cosmic x-ray background is a diffuse x-ray glow. You can see an image of this x-ray background in the image above. It is a false-color image, where red, green and blue represent low, medium and high x-ray energies.

Unlike the microwave background, the x-ray background is not a remnant of the big bang. Instead it is generated through several processes. Most of the background is produced by localized sources such as active galactic nuclei, but other sources are the the local bubble of interstellar media that surrounds the Sun and other stars in our local spiral arm of Orion. But there is a small portion of the background that remains unexplained.

One of the difficulties in understanding the x-ray background is the sheer challenge of observing x-rays at high resolution. X-rays tend to penetrate materials, so you can’t simply make a mirror to focus x-rays the way we do visible light or radio waves. X-ray telescopes must have special materials to reflect x-rays, and they need to have a very long focal length.

But I’ll talk about that next time.

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