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Huge Gas Clouds Discovered Orbiting Black Holes

Scientists have known for a while that there can be gas clouds circling supermassive black holes. These clouds were believed to be nothing more than a thin fog, shaped into a ring over time. A team led by Alex Markowitz of the University of California, San Diego studied data collected by NASA’s Rossi X-ray Timing Explorer (RXTE) and confirmed that this gas isn’t just a mist; it can form clouds so dense that they actually interfere with the amount of  the black hole’s radiation that can be observed by our telescopes.  The results were published in the Monthly Notices of the Royal Astronomical Society. Gas clouds that stray too close to the black hole (such as G2 in our own galaxy) are doomed to be torn apart. For the most part, these dense clouds are at a safe distance, ranging from a few light weeks to a few light years away from the AGN. However, the researchers did observe clouds in the galaxy NGC 3783 that were in the process of being pulled apart from tidal forces.

Markowitz’s team analyzed data collected over RXTE’s 16 year mission. RXTE, which ended operations in 2012, detected variations in x-ray emissions from a variety of sources, including galactic nuclei. Active galactic nuclei (AGN) are incredibly bright spots at the center of a galaxy. As the supermassive black hole accumulates gas and dust, it begins to emit radiation which can be detected across the electromagnetic spectrum, from radio up to gamma rays. RXTE, naturally, studied the x-ray wavelengths. Of the 55 AGN studied, they discovered twelve periods of time when the x-ray emissions were temporarily muted. The duration of these instances varied; while some lasted only a few hours, others were obscured for years at a time. The scientists concluded that these variations were caused by incredibly dense gas clouds. When these clouds passed between the AGN and the telescope, it obscured the signal. Though computer models have predicted this could be the case, this is the first confirmation on the topic.
Stars fuse atoms together, creating progressively heavier elements. These fusion reactions release a tremendous amount of energy, fueling the star. However, when iron is fused, it requires more energy than it puts out, creating a death sentence for the star. Eventually, the core collapses in on itself to create a black hole or neutron star while the outer layers erupt in a spectacular explosion known as a supernova, scattering all of the elements it has created into the Universe. Though the term wouldn’t be coined until the 1930s, the first supernova was observed in 185 CE.  Over nearly 1830 years of observation, astronomers have studied countless supernovae and their remnants with a variety of increasingly sophisticated telescopes. A team of astronomers from UC Berkeley led by Steven Boggs have made a historical step forward by actually imaging the core of a supernova remnant named Cassiopeia A (Cas A) using NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR), showing what it looked like in its final moments. The results were published in Nature.

Light from Cas A, located about 11,000 light years away in the Cassiopeia constellation, first hit Earth about 300 years ago. It has been a popular target for studying supernovae, given its relative proximity. Though previous x-ray and infrared analysis have revealed the shockwave pattern, NuSTAR was able to penetrate through to the core and detect titanium-44, a radioactive isotope that was formed upon the star’s collapse. By peering into the core of the Cas A, astronomers will be able to better understand the nuclear explosion inside the supernova to learn where and how elements are fused. This will lead to more accurate computer models, allowing astrophysicists to conduct better experiments about the physics at the core of the star that cause supernovae to occur. Because stars are spheres, it had previously been assumed the supernova should have equal expansion and distribution of elements. Images from the Chandra X-Ray Telescope revealed back in 2008 that Cas A had unequal congregations of charged silicon ions. The new NuSTAR images reveal that other elements, namely iron and titanium, are also unequally distributed and are differently heated, even though the heavier elements should have been fused in the same area of the star. This hints that the core undergoes conformational change prior to the supernova; an idea that will be explored with further study. NuSTAR’s method of analyzing the high-energy x-ray emissions of titanium-44 in Cas A is being applied to other supernova remnants as well. This will help determine if Cas A’s explosion was typical for all supernovae, or if there are variations involved that are yet unknown. The remnants to be studied have been carefully selected based on age and distance. Older remnants with radioactive isotopes will not be emitting the high-energy x-rays necessary to be imaged by NuSTAR, while supernovae that are too far away will not have a clearly visible structure.

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