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X-ray dark planet
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As X-ray detectors have become more sensitive, they have observed that some planets (an X-ray dark planet) and other normally X-ray non-luminescent celestial objects under certain conditions emit, fluoresce, or reflect X-rays. X-ray astronomy X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray emission or apparent emission from celestial objects. While X-ray emission is expected from sources which are extremely hot gas or plasma that is millions of kelvins in temperature, such as a coronal cloud, X-rays have also been detected from discrete sources on or near other celestial objects that themselves do not appear to emit X-rays. In addition, celestial objects have been detected which reflect or fluoresce X-rays due to nearby X-ray emitting sources. X-rays can be generated by a variety of processes and these in turn can be modeled to understand the detection of X-rays from unexpected sources. Images of the Sun during the recent quiet period, in wavelengths normally used to observe the coronal cloud, show quite a dark object. By comparison with the X-ray dark Sun, there is the Sun in super soft X-rays which overlap the extreme ultraviolet (EUV). Black body emitter A black body emitter with an apparent surface temperature of ~5800 K (representing the Sun) emits X-rays, but the intensity is orders of magnitude below the visible. If correlated with surface (photosphere) temperature, the lower the photosphere temperature, the greater the number of orders of magnitude lower is the X-ray emission. Betelgeuse, for example, is one of the most visibly luminous objects in the celestial sphere at an apparent surface temperature of 3,500 K. Yet Betelgeuse appears to be always X-ray dark. The X-ray flux from the entire stellar surface is five orders of magnitude below the quiet Sun X-ray surface flux. Of over 230 stars currently identified as possessing planets, about a third have been detected in X-rays. Stars with close-in giant planets are on average more X-ray active by a factor ≈ 4 than those with planets that are more distant. The "cometary planet" is observed with the Hubble Space Telescope Cosmic Origins Spectrograph (COS) which probes the planets chemistry at ultraviolet wavelengths. The giant planet has been given the name "Osiris". HD 209458 is a visual spectral type G0V star V376 Pegasi (V376 Peg) at equatorial coordinates for the epoch 2000 J2000.0 right ascension (RA) declination (Dec) according to SIMBAD. V376 Peg is not a known X-ray source. But, XMM Newton detected an X-ray source with a count rate of 3.3 x 10 cts/s at the stellar position of HD 209458. A coronal temperature of 3 x 10 K at a distance of 47 pc can have an X-ray luminosity of L<sub>x</sub> 1.1 x 10 erg s . Jupiter Jupiter's strong, rapidly rotating magnetic field (light blue lines in the figure) generates strong electric fields in the space around the planet. Charged particles (white dots), trapped in Jupiter's magnetic field, are continually being accelerated (gold particles) down into the atmosphere above the polar regions, so auroras are almost always active on Jupiter. Electric voltages of about 10 megavolts, and currents of 10 megaamperes—a hundred times greater than the most powerful lightning bolts—are required to explain the auroras at Jupiter's poles, which are a thousand times more powerful than those on Earth. In a new composite image, data from the Chandra X-ray Observatory is superimposed on the latest image of Jupiter from the Hubble Space Telescope. The powerful X-ray auroras observed near the poles of Jupiter may be caused by the interaction of sulfur and oxygen ions in the outer regions of the Jovian magnetic field. Scientists would like to better understand the details of this process. Unlike Earth's aurorae, which are transient and only occur at times of heightened solar activity, Jupiter's aurorae are permanent, though their intensity varies from day to day. They consist of three main components: the main ovals, which are bright, narrow (< 1000 km in width) circular features located at approximately 16° from the magnetic poles; The auroral emissions were detected in almost all parts of the electromagnetic spectrum from radio waves to X-rays (up to 3 keV). Saturn Saturn's X-ray spectrum is similar to that of X-rays from the Sun indicating that Saturn's X-radiation is due to the reflection of solar X-rays by Saturn's atmosphere. For comparison the Chandra X-ray Observatory image is on the left and the Hubble visual image is on the right of Saturn on April 14, 2003. The visual image is much brighter, and shows the beautiful ring structures, which were not detected in X-rays. Earth Bright, low energy (0.1-10 keV) X-ray arcs, observed with the Chandra X-ray Observatory (maximum intensity in red) were generated during auroral activity between December 16, 2003, and April 13, 2004. In the composite image at the right the distance from the North pole to the black circle is 3,340 km. The first picture of the Earth in X-rays is taken in March 1996, with the orbiting Polar satellite. Energetically charged particles from the Sun cause aurora and energize electrons in the Earth's magnetosphere. These electrons move along the Earth's magnetic field and eventually strike the Earth's ionosphere, producing the X-ray emission. On Earth, auroras are triggered by solar storms of energetic particles, which disturb Earth's magnetic field. As shown by the swept-back appearance in the illustration, gusts of particles from the Sun also distort Jupiter's magnetic field, and on occasion produce auroras. X-rays from the dark Moon are geocoronal X-rays caused by collisions of heavy ions of carbon, oxygen and neon in the solar wind with hydrogen atoms located tens of thousands of miles above the surface of Earth. During the collisions, the solar ions capture electrons from hydrogen atoms, X-rays are fluoresced as the captured electrons drop to lower energy states. During the Skylark rocket flights on April 4, 1967, and April 20, 1967, from Woomera, Australia, an X-ray flux from the Earth is observed and found to be due to solar X-rays reflected in the atmosphere. Venus X-rays from Venus are produced by fluorescence stimulated from solar X-rays bombarding oxygen and other atoms in the atmosphere between 120 and 140 km above the surface. Optical reflection is from clouds 50 to 70 km above the surface. The Chandra X-ray Observatory image on the right is the first X-ray image ever made of Venus. It shows a half crescent due to the relative orientation of the Sun, Earth and Venus. Mars X-radiation from the Sun excites oxygen atoms in the Martian upper atmosphere, about 120 km above its surface, to emit X-ray fluorescence. A faint X-ray halo that extends out to 7,000 km above the surface of Mars has also been found. The Chandra X-ray Observatory image on the right is the first look at X-rays from Mars. Moon Some of the detected X-rays, originating from solar system bodies other than the Sun, are produced by fluorescence. Scattered solar X-rays provide an additional component. Chandra detected X-ray fluorescence from oxygen, magnesium, aluminum, and silicon atoms on the surface of the Moon. This fluorescence is produced by solar X-rays bombarding the surface. In the Röntgensatellit (ROSAT) image of the Moon, pixel brightness corresponds to X-ray intensity. The bright lunar hemisphere shines in X-rays because it re-emits X-rays originating from the Sun. The background sky has an X-ray glow in part due to the myriad of distant, powerful active galaxies, unresolved in the ROSAT picture. The dark side of the Moon's disk shadows this X-ray background radiation coming from deep space. A few X-rays only seem to come from the shadowed lunar hemisphere. Instead, they originate in Earth's geocorona or extended atmosphere which surrounds the orbiting X-ray observatory. The measured lunar X-ray luminosity (L<sub>x</sub>) of 120 kilowatts (1.2 x 10 erg/s) makes the Moon one of the weakest known non-terrestrial X-ray source. To the right are two images of the constellation Orion. On the left is Orion as seen in X-rays only. The brightest object in the visual image is the full Moon, which is also in the X-ray image. The X-ray colors represent the temperature of the X-ray emission from each star: hot stars are blue-white and cooler stars are yellow-red. Early attempts to detect X-rays from the Moon made X-ray astronomy history of their own. An Aerobee-Hi sounding rocket instrumented with Geiger counters sensitive to wavelengths centered at about 0.3 nm is launched from the White Sands Missile Range on June 12, 1962. This flight detected X-ray emission from the Milky Way. The extrasolar X-ray source is designated Scorpius X-1. The Galactic Center is < 20° RA and < 20° Dec from Sco X-1, the two X-ray sources are separated by ~20° of arc and may not have been resolvable in the June 1962 flight. Galilean satellites The Chandra X-ray Observatory has detected X-rays from Io and Europa during observations of the Jovian system on November 25-26, 1999, over the nominal energies from 300 to 1890 eV, with a clustering between 500 and 700 eV. Estimated energy fluxes are 4 x 10 erg cm s for Io and 3 x 10 erg cm s for Europa. Ganymede may have been detected at a third the X-ray luminosity of Io. The Io plasma torus is also an X-ray emitter. Comet Lulin NASA's Swift Gamma-ray Explorer satellite is monitoring Comet Lulin as it closed to 63 Gm of Earth. For the first time, astronomers can see simultaneous UV and X-ray images of a comet. "The solar wind—a fast-moving stream of particles from the sun—interacts with the comet's broader cloud of atoms. This causes the solar wind to light up with X-rays, and that's what Swift's XRT sees", said Stefan Immler, of the Goddard Space Flight Center. This interaction, called charge exchange, results in X-rays from most comets when they pass within about three times Earth's distance from the sun. Because Lulin is so active, its atomic cloud is especially dense. As a result, the X-ray-emitting region extends far sunward of the comet. Comet McNaught-Hartley X-ray emission is detected from comet McNaught-Hartley (C/1999 T1). The comet is observed with the advanced CCD imaging spectrometer aboard the Chandra X-ray Observatory on January 8-14, 2001. Comet Hyakutake Extreme ultraviolet (XUV) and X-ray emissions have been detected from comet Hyakutake. Observations were obtained from ROSAT and Rossi X-ray Timing Explorer. Using a three-dimensional adaptive magnetohydrodynamic model to calculate the plasma flow in the coma of comet Hyakutake, a resulting X-ray emission from the density distribution of solar wind ions in the coma agrees with the observed soft X-ray emission.<ref name=Haberli/> See Also
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