Chandra X-Ray Space Observatory and Its Mission to Unlock the Universe
X-Rays: A Hidden Frontier
When you look around you, everything you see is through the visible portion of what we call the electromagnetic spectrum, or light. That visible part is but a narrow field of the total light spectrum, whose scope is wide and diverse. Other portions to this field included (but are not limited to) infrared, radio waves, and microwaves. One component of the spectrum which is just beginning to be used in space observations are x-rays. The main satellite that explores them is the Chandra X-Ray Observatory, and its journey to becoming that flagship started in the 1960s.
What is Sco-X1?
In 1962, Riccardo Giacconi and his team from American Science and Engineering entered an agreement with the Air Force to help monitor nuclear explosions in the atmosphere from the Soviets. In the same year, he convinced the Air Force (which was envious of the Apollo program and wanted in on it in some fashion) to launch a Geiger counter into space to detect x-rays from the moon in an effort to reveal its composition. On June 18, 1962, an Aerobee rocket was launched with the counter from White Sands Test Range in Nevada. The Geiger counter was in space for only 350 seconds, outside of Earth’s x-ray absorbing atmosphere and into the void of space (38).
While no emissions were detected from the moon, the counter did pick up a huge emission coming from the constellation Scorpius. They named the source of these x-rays Scorpius X-1, or Sco-X1 for short. This object was a deep mystery at the time. The Naval Research Laboratory knew that the Sun did emit x-rays in its upper atmosphere, but they were one-millionth as intense as the visible light emitted by the sun. Sco-X1 was thousands of times as luminous as the Sun in the x-ray spectrum. In fact, most of Sco’s emissions are solely x-rays. Riccardo knew more sophisticated equipment would be needed for further studies (38).
Chandra is Built and Launched
In 1963, Riccardo along with Herbert Gursky hand to NASA a 5-year plan that would culminate in the development of an x-ray telescope. It would take 36 years until his dream was realized in Chandra, launched in 1999. The basic design of Chandra is the same as it was in 1963, but with all the technological advances that have been made since then, including the ability to harness energy from its solar panels and to run on less power than two hair dryers (Kunzig 38, Klesuis 46).
Riccardo knew that x-rays were so energetic that they would simply embed themselves into traditional lenses and flat mirrors, so he designed a conical mirror, made of 4 smaller ones built in descending radius, that would let the rays “skip” along the surface which allows for a low angle of entry and thus better data collection. The long, funnel shape also allows the telescope to see further into space. The mirror has been polished well (so the biggest surface disturbance is 1/10,000,000,000 of an inch, or said another way: no bumps higher than 6 atoms!) for good resolution also (Kunzig 40, Klesuis 46).
Chandra also uses charged-coupled devices (CCD’s), frequently used by the Kepler Space Telescope, for its camera. 10 chips within it measure an x-ray's position as well as its energy. Just as it is with visible light, all molecules have a signature wavelength that can be used to identify the material present. Composition of the objects emitting the x-rays can thus be determined (Kunzig 40, Klesuis 46).
Chandra orbits the Earth in 2.6 days and is one-third the distance from the moon above our surface. It was positioned to increase exposure time and to decrease the interference from the Van Allen belts (Klesuis 46).
Findings of Chandra: Black Holes
As it turns out, Chandra has determined that supernovas emit x-rays in their early years. Depending on the mass of the star that goes supernova, several options will be left over once the stellar explosion is over. For a star that is more than 25 solar masses, a black hole will form. However, if the star is between 10 and 25 solar masses, it will leave behind a neutron star, a dense object made solely of neutrons (Kunzig 40).
A very important observation of galaxy M83 showed that ultra lumnoius X-ray sources, the binary systems that most stellar mass black holes are found in, can have quite an age variation. Some are young with blue stars and others are old with red stars. The black hole usually forms at the same time as its companion, so by knowing the age of the system we can gather more important parameters on black hole evolution (NASA).
A further study on galaxy M83 revealed a stellar-mass black hole MQ1 that was cheating on how much energy it was releasing into the surrounding system. This basis stems from the Eddington Limit, which should be a cap on how much energy a black hole can produce before cutting off its own food supply. Observations from Chandra, ASTA, and Hubble seem to show that the black hole was exporting 2-5 times as much energy as should be possible (Timmer, Choi).
Chandra can see black holes and neutron stars by an accretion disk that surrounds them. This forms when a black hole or a neutron star has a companion star that is so close to the object that it gets material sucked from it. This material falls into a disk that surrounds the black hole or neutron star. While in this disk and as it falls into the host object, the material can get so heated that it will emit x-rays that Chandra can detect. Sco-X1 has turned out to be a neutron star based on the x-ray emissions as well as its mass (42).
Chandra is not only looking at normal black holes but supermassive ones also. In particular, it makes observations of Sagittarius A*, the center of our galaxy. Chandra also looks at other galactic cores as well as galactic interactions. Gas can become trapped between galaxies and gets heated, releasing x-rays. By mapping where the gas is located, we can figure out how the galaxies are interacting with one another (42).
Initial observations of A* showed that it flared on a daily basis with some nearly 100 times as bright as normal. However, on September 14, 2013 a flare was spotted by Daryl Haggard, from Amherst College, and her team that was 400 times brighter than a normal flare and 3 times the brightness of the previous record holder. Then a year later a burst 200 times the norm was seen. This and any other flare are because of asteroids that fell to within 1 AU of A*, falling apart under tidal forces and heated up by the ensuing friction. These asteroids are small, at least 6 miles-wide and could come from a cloud surrounding A* (NASA "Chandra Finds," Powell, Haynes, Andrews).
After this study, Chandra looked again to A* and over a 5-week period watched its eating habits. It found that instead of consuming most of the material falling in, A* only will take 1% and release the rest into outer space. Chandra observed this as it looked at temperature fluctuations of the x-rays being emitted by the excited matter. A* may be not eating well because of the local magnetic fields causing material to be polarized away. The study also showed that the source of the x-rays was not from small stars surrounding A* but most likely from the solar wind emitted by massive stars around A* (Moskowitz, "Chandra").
Chandra led a study looking at supermassive black holes (SMBH) in galaxies NGC 4342 and NGC 4291, finding that the black holes there grew faster than the rest of the galaxy. At first scientists felt that tidal stripping, or lost mass through a close encounter with another galaxy, was at fault but this was disproven after x-ray observations from Chandra showed that the dark matter, which would have been partially stripped, remained intact. Scientists now think those black holes ate a lot early in their lives, preventing star growth through radiation and hence limiting our ability to fully detect the mass of the galaxies (Chandra “Black hole growth”).
This is just a part of mounting evidence that SMBHs and their host galaxies might not grow in tandem. Chandra along with Swift and the Very Large Array collected x-ray and radio wave data on several spiral galaxies including NCGs 4178, 4561 and 4395. They found that these did not have a central bulge like galaxies with SMBHs yet a very small one was found in each galaxy. This could indicate that some other means of galactic growth occurs or that we don't fully understand SMBH formation theory (Chandra “Revealing”).
Findings of Chandra: AGN
The observatory has also examined a special type of black hole called a quasar. Specifically, Chandra looked at RX J1131-1231, which is 6.1 billion-years-old and has a mass 200 million times that of the sun. The quasar is gravitationally lensed by a foreground galaxy, which gave scientists the chance to examine light that would normally be too obscured to make any measurements. Specifically, Chandra and the XMM-Newton X-ray observatories looked at light emitted from iron atoms near the quasar. Based on the level of excitement the photons were in scientists were able to find that the spin of the quasar was 67-87% the max allowed by general relativity, implying that the quasar had a merger in the past (Francis).
Chandra also helped in an investigation of 65 active galactic nuclei. While Chandra looked at the x-rays from them, the Hershel telescope examined the far-infrared portion. Why? In the hopes of uncovering star growth in galaxies. They found that both the infrared and x-rays grew proportionally until they got to high levels, where infrared tapered off. Scientists think this is because the active black hole (x-rays) heat the gas surrounding the black hole so much that potential new stars (infrared) cannot have cool enough gas to condense (JPL “Overfed”).
Chandra has also helped reveal properties of intermediate black holes (IMBH), more massive than stellar but less that SMBH's Located in galaxy NGC 2276, the IMBH NGC 2276 3c is about 100 million light years away and weighs in at 50,000 stellar masses. But even more intriguing is the jets that arise from it, much like SMBH's. This suggests that IMBH's may be a stepping stone to becoming a SMBH ("Chandra Finds").
Findings of Chandra: Exoplanets
Though the Kepler Space Telescope gets much credit for finding exoplanets, Chandra along with the XMM-Newton Observatory was able to make important findings on several of them. In the star system HD 189733, 63 light years away from us, a Jupiter-sized planet passes in front of the star and causes a dip in the spectrum. But fortunately, this eclipsing system impacts not only visual wavelengths but also x-rays. Based on the data obtained, the high x-ray output is because of the planet losing much of its atmosphere - between 220 million to 1.3 billion pounds a second! Chandra is taking this opportunity to learn more about this interesting dynamic, caused by the planet's proximity to its host star (Chandra X-ray Center).
Our little planet cannot affect the Sun much save for some gravitational forces. But Chandra has observed exoplanet WASP-18b having a huge impact on WASP-18, its star. Located 330 light years away, WASP-18b has about 10 Jupiters in total mass and is very close to WASP-18, so close in fact that it has caused the star to become less active (100x less than normal) than it otherwise would be. Models had shown the star to be between 500 million and 2 billion years old, which would normally mean it is quite active and has large magnetic and x-ray activity. Because of WASP-18b's proximity to its host star, it has huge tidal forces as a result of gravity and thus may pull on material that is near the star's surface, which affects how the plasma flows through the star. This in turn can wind down the dynamo effect that produces magnetic fields. If anything were to impact that movement then the field would be decreased (Chandra Team).
As it is with many satellites, Chandra has plenty of life in her. She is just getting into her rhythms and will surely unlock more as we delve deeper into x-rays and their role in our universe.
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© 2013 Leonard Kelley
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