x ray astronomy

Black Holes: Monsters in Space

This artist’s concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Supermassive black holes are enormously dense objects buried at the hearts of galaxies. (Smaller black holes also exist throughout galaxies.) In this illustration, the supermassive black hole at the center is surrounded by matter flowing onto the black hole in what is termed an accretion disk. This disk forms as the dust and gas in the galaxy falls onto the hole, attracted by its gravity.

Also shown is an outflowing jet of energetic particles, believed to be powered by the black hole’s spin. The regions near black holes contain compact sources of high energy X-ray radiation thought, in some scenarios, to originate from the base of these jets. This high energy X-radiation lights up the disk, which reflects it, making the disk a source of X-rays. The reflected light enables astronomers to see how fast matter is swirling in the inner region of the disk, and ultimately to measure the black hole’s spin rate.

Image credit:
NASA/JPL-Caltech

Astronomers have discovered what happens when the eruption from a supermassive black hole is swept up by the collision and merger of two galaxy clusters. This composite image contains X-rays from Chandra (blue), radio emission from the GMRT (red), and optical data from Subaru (red, green, and blue) of the colliding galaxy clusters called Abell 3411 and Abell 3412. These and other telescopes were used to analyze how the combination of these two powerful phenomena can create an extraordinary cosmic particle accelerator.

Image credit: X-ray: NASA/CXC/SAO/R. van Weeren et al; Optical: NAOJ/Subaru; Radio: NCRA/TIFR/GMRT/ Chandra X-ray Observatory

Infrared, X-ray & Optical Images of Centaurus A

Centaurus A is the fifth brightest galaxy in the sky – making it an ideal target for amateur astronomers – and is famous for the dust lane across its middle and a giant jet blasting away from the supermassive black hole at its center.  Cen A is an active galaxy about 12 million light years from Earth.

Credit: X-ray: NASA/CXC/SAO; Optical: Rolf Olsen; Infrared: NASA/JPL-Caltech

What is a black hole?

When a star runs out of nuclear fuel, it will collapse. If the core, or central region, of the star has a mass that is greater than three Suns, no known nuclear forces can prevent the core from forming a deep gravitational warp in space called a black hole.

A black hole does not have a surface in the usual sense of the word. There is simply a region, or boundary, in space around a black hole beyond which we cannot see.

This boundary is called the event horizon. Anything that passes beyond the event horizon is doomed to be crushed as it descends ever deeper into the gravitational well of the black hole. No visible light, nor X-rays, nor any other form of electromagnetic radiation, nor any particle, no matter how energetic, can escape. The radius of the event horizon (proportional to the mass) is very small, only 30 kilometers for a non-spinning black hole with the mass of 10 Suns.

Can astronomers see a black hole? Not directly. The only way to find one is to use circumstantial evidence. Observations must imply that a sufficiently large amount of matter is compressed into a sufficiently small region of space so that no other explanation is possible. For stellar black holes, this means observing the orbital acceleration of a star as it orbits its unseen companion in a double or binary star system.

Searching for black holes is tricky business. One way to locate them has been to study X-ray binary systems. These systems consist of a visible star in close orbit around an invisible companion star which may be a neutron star or black hole. The companion star pulls gas away from the visible star.

As this gas forms a flattened disk, it swirls toward the companion. Friction caused by collisions between the particles in the gas heats them to extreme temperatures and they produce X-rays that flicker or vary in intensity within a second.

Many bright X-ray binary sources have been discovered in our galaxy and nearby galaxies. In about ten of these systems, the rapid orbital velocity of the visible star indicates that the unseen companion is a black hole. The X-rays in these objects are produced by particles very close to the event horizon. In less than a second after they give off their X-rays, they disappear beyond the event horizon.

However, not all the matter in the disk around a black hole is doomed to fall into the black hole. In many black hole systems, some of the gas escapes as a hot wind that is blown away from the disk at high speeds. Even more dramatic are the high-energy jets that radio and X-ray observations show exploding away from some stellar black holes. These jets can move at nearly the speed of light in tight beams and travel several light years before slowing down and fading away.

Do black holes grow when matter falls into them? Yes, the mass of the black hole increases by an amount equal to the amount of mass it captures. The radius of the event horizon also increases by about 3 kilometers for every solar mass that it swallows. A black hole in the center of a galaxy, where stars are densely packed, may grow to the mass of a billion Suns and become what is known as a supermassive black hole.

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Second gravitational wave makes it official: merging black holes don’t burst!

“Yesterday’s announcement showed the world that, at 03:38 UTC on December 26, 2015, a second black hole-black hole merger occurred, between a 14 and an 8 solar mass black hole. Thankfully, the Fermi GBM data set is online and searchable, and you can look yourself for whether any events were triggered in the vicinity of that event. While there were potential events offset by hours on either side, neither X-rays or gamma-rays were seen coincident with GW151226. The satellite designed to monitor exactly those types of signals, the only one that gave a positive result previously, came up empty this time.”

When the first gravitational wave signal ever, GW150914, was directly detected, NASA’s Fermi GBM team shocked the world by announcing the detection of a high-energy burst of electromagnetic radiation. This was a huge surprise, because merging black holes shouldn’t have a bright gamma ray or X-ray flash associated with them! A statistical reanalysis and the ESA’s INTEGRAL satellite both failed to confirm it, but it would take a second event to know for certain. With GW151226 now in the books, a look through the Fermi GBM data shows what we suspected all along: black holes DON’T burst when they merge!

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The above images are spectacular representations of what Chandra X-Ray Telescope has brought to the astronomy table. The earth’s atmosphere filters out a great majority of x-rays, therefore, by having a telescope in orbit outside of the atmosphere, it gives astronomers a new perspective on the makeup of various celestial bodies. As seen in these images, high energy particles often emit high levels of x-rays which are typically invisible to us if we simply take a picture in the visible spectrum. Having an x-ray observatory like Chandra opens a brand new (beautiful) window to the universe.

Earth is surrounded by X-rays being emitted from a mysterious source

If humans could see in wavelengths beyond visible light, the whole cosmos would look like a glowing, sparkling ball of light. Some of that extra light we can’t see is made of X-rays that regularly crash into Earth’s atmosphere. Now a new NASA study has returned has found that some of the X-rays are coming from a source related to the Sun.

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X ray Echoes from Circinus X 1 : Circinus X-1 is an X-ray binary star known for its erratic variability. In the bizarre Circinus X-1 system, a dense neutron star, the collapsed remnant of a supernova explosion, orbits with a more ordinary stellar companion. Observations of the X-ray binary in months following an intense X-ray flare from the source in 2013 progressively revealed striking concentric rings - bright X-ray light echoes from four intervening clouds of interstellar dust. In this X-ray/optical composite, the swaths of Chandra Observatory X-ray image data showing partial outlines of the rings are in false colors. Remarkably, timing the X-ray echoes, along with known distances to the interstellar dust clouds, determines the formerly highly uncertain distance to Circinus X-1 itself to be 30,700 light-years. via NASA

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