Sizing up the weirdest objects in the universe

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How big is a neutron star?

Astrophysicists are combining multiple methods to reveal the secrets of some of the weirdest objects in the universe.

For Symmetry Magazine:

Neutron stars are arguably the strangest objects in the cosmos. Born from the deaths of massive stars, they combine strong gravity with temperatures and densities higher than anything we can make in the lab.

While we’ve known about neutron stars for the better part of a century, astrophysicists still aren’t entirely sure how large they are. That uncertainty is related to two other unanswered questions: What’s in the middle of neutron stars, and how massive can they grow?

[read the rest at Symmetry Magazine]

Weird X-Rays Spur Speculation about Dark Matter Detection

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Weird X-Rays Spur Speculation about Dark Matter Detection

From Scientific American:

Many major discoveries in astronomy began with an unexplained signal: pulsars, quasars and the cosmic microwave background are just three out of many examples. When astronomers recently discovered x-rays with no obvious origin, it sparked an exciting hypothesis. Maybe this is a sign of dark matter, the invisible substance making up about 85 percent of all the matter in the universe. If so, it hints that the identity of the particles is different than the prevailing models predict.

The anomalous x-rays, spotted by the European Space Agency’s orbiting XMM–Newton telescope, originate from two different sources: the Andromeda Galaxy and the Perseus cluster of galaxies. The challenge is to determine what created those x-rays, as described in a study published last month in Physical Review Letters. (See also an earlier study published in The Astrophysical Journal.) The signal is real but weak and astronomers must now determine whether it is extraordinary or has a mundane explanation. If that can be done, they can set about the work of identifying what kind of dark matter might be responsible. [Read more at Scientific American ]

The Cassiopeia A supernova remnant. [Credit: NASA/CXC/SAO]

The Cassiopeia A supernova remnant. [Credit: NASA/CXC/SAO]

Nearly every atom of your body was forged in a supernova explosion and dispersed into space. But how do massive stars explode? The details are complicated, pushing the limits of computer simulations and our ability to observe with telescopes. In the absence of very close-by events, the best data come from supernova remnants: the still-glowing gas ejected during the explosion. A new set of observations of X-ray emissions from radioactive titanium in the Cassiopeia A supernova remnant show that it was a lumpy space princess highly asymmetrical explosion. That agrees with theory, but the researchers also turned up an odd disconnect between the titanium and other materials.

Cassiopeia A (abbreviated Cas A) is a historical oddity. The supernova was relatively close to Earth—a mere 11,000 light-years distant—and should have been visible around CE 1671, yet no astronomers of any culture recorded it. That’s in stark contrast to famous earlier explosions: Tycho’s supernova, Kepler’s supernova, and of course the supernova that made the Crab Nebula. This mysterious absence has led some astronomers to speculate that some unknown mechanism diffused the energy from the explosion, making the supernova far less bright than expected. [Read more…]

Supernovas: mysterious and lumpy space explosions

No question: supermassive black holes get a lot of the glory, thanks to their obvious presence at the centers of many galaxies. However, stars more than 20 times the mass of our Sun leave behind smaller, stellar-mass black holes after their violent supernova deaths. Despite this model’s wide acceptance, astronomers have only identified about 50 stellar-mass black holes in the Milky Way to date, but there must be many more lurking. A new study may have revealed why: the black holes are shrouded by a thick donut of gas that blocks much of their X-ray light from reaching us.

J. M. Corral-Santana and colleagues based this hypothesis on a detailed study of a relatively faint, fluctuating X-ray source in the Milky Way. Their observations in X-ray and visible light revealed the signs of a binary system: an ordinary star in orbit around a black hole, similar to other systems, but with some key differences. For one, the star and black hole were so close together that the orbital period of the system was only 2.8 hours. For another, the matter being drawn off the star was obscuring the black hole when viewed from Earth. The authors hypothesized that many other black holes may be similarly hidden, and future searches should take that possibility into account. [Read more…]

The case of the missing black holes

The region near a black hole is one of the most extreme environments in the Universe, but historically it’s been hard to study directly. Using the XMM-Newton and NuSTAR telescopes, astronomers have measured the rotation of gas near the supermassive black hole at the center of the Great Barred Spiral Galaxy. They found that this black hole is spinning nearly as fast as it can be, and that the matter orbiting the black hole is similarly moving near the speed of light—to the extent that the results of Einstein’s general relativity must be used to understand how it’s moving. The key measurement involved observing X-rays reflected off the matter whirling around the black hole, a significant observation of a relativistic phenomenon.

A new X-ray observation of the region surrounding the supermassive black hole in the Great Barred Spiral Galaxy may have answered one of the big questions. G. Risaliti and colleagues found the distinct signature of X-rays reflecting off gas orbiting the black hole at nearly the speed of light. The detailed information the astronomers gleaned allowed them to rule out some explanations for the bright X-ray emission, bringing us closer to an understanding of the extreme environment near these gravitational engines. [Read more…]

Measuring the spin of a black hole using X-rays

(Yes, I’m inundating you all with writing. It’s a busy week, and I still have a few more things forthcoming to share with you.)

Supernova remnant SNR 1987a, what’s left after a bright blue star exploded in the Large Magellanic Cloud. [Credit: NASA/ESA/P. Challis and R. Kirshner (Harvard-Smithsonian Center for Astrophysics]

Supernova 1987a was the death of a massive blue star in the Large Magellanic Cloud, one of the satellite galaxies of our Milky Way. Because of its relative proximity and occurrence during the era of modern astronomy, the supernova remnant SNR 1987a (as it is known) is one of the best-studied of all. As a result, it provides a good testbed for the theory of star explosions. A new X-ray observation has measured the amount of radioactive titaniumcreated in the remnant, and showed it’s enough to power a lot of the light emission in the years following the supernova.

The decay of 44Ti produces high-energy X-ray photons at three distinct wavelengths. The researchers in the current study aimed the INTEGRAL (INTErnational Gamma-RAy Laboratory) satellite at SNR 1987a for about 4.5 million seconds (a total of over seven weeks) to obtain clear X-ray spectra. This process was complicated by the presence of a pulsar and a black hole binary system that, from our perspective, appear near SNR 1987a in the sky—these bodies also emit X-ray light. The astronomers identified the telltale spectral signature of titanium decay, and extrapolated from the number of photons (the flux) to determine the mass of the titanium before the decay process began. [Read more….]

Radioactive titanium powers a supernova afterglow