|FIGURE 1: A rogues' gallery of nearby X-ray sources. They are (clockwise from upper left): a) a Gaussian smoothed image of Venus; b) Titan occults part of the Crab Nebula; c) Mars; d) a slightly smoothed image of Comet Ikeya-Zhang; e) Jupiter from HRC, where the two red/yellow objects in the lower right are ACIS views of Io and Europa montaged onto the figure; and f) True-color Saturn smoothed to 53. References are given in the text.|
The Moon was actually the first target in the search for Xrays outside of the Earth and the Sun. A sounding rocket failed to detect scattered solar X-rays from the Moon between 1.5 and 6 keV (Giacconi 1962). The on-board Geiger counter did discover Sco X-1, so the mission was not a complete loss. X-rays were detected from the Moon by Apollo using a variety of experiments. ROSAT became the first telescope to image the Moon in X-rays. This observation was proof of the cosmological nature of the Xray background as it was blocked by the dark side of the Moon. Schmitt (1991) argued that the Moon's X-ray luminosity arises from scattering of solar X-rays. They also detected faint X-ray emission from the dark portion of the Moon, which they attributed to solar wind electrons striking the lunar surface.
Spectra from recent Chandra observations indicate that the emission from the dark side of the Moon may actually be geocoronal emission occurring between the Earth and the Moon as solar wind ions capture electrons from hydrogen atoms in the extreme upper reaches of the Earth s atmosphere (Wargelin et al. 2004, in preparation). The Chandra observations of the illuminated portion of the Moon detect X-ray emission lines from oxygen, magnesium, aluminum and silicon. The X-rays are now thought to be produced by fluorescence when solar X-rays bombard the lunar surface.
Comets are arguably the most surprising X-ray sources in space. Their X-ray emission was discovered by ROSAT during the apparition of Hyakutake in 1996 (Lisse 1996). With a luminosity of about 1015 erg/sec, Hyakutake was the third brightest X-ray source in the solar system after the Sun and Jupiter. Much as in the case of the Moon, the low spectral resolution of this previous generation of X-ray instruments left the production mechanism uncertain.
Chandra observed comet C/Linear 1999 S4 in a series of eight 1 ks snapshots on July 14, 2000 (Lisse 2001). The strong oxygen feature in the spectrum obtained by the Advanced CCD Imaging Spectrometer (ACIS) immediately ruled out all models except charge exchange as the sole source of the X-ray production. The best fit models to the data include charge exchange lines of O VIII, O VII, N VIII, N VI, C VII and C VI overlying a soft thermal spectrum. Additional comets have since been observed and the observations have resulted in more lines detected, a better understanding of comet morphology, and the differences induced by cometary density, composition, and the speed of the solar wind as well as cometary rotation.
For all comets, the emission seems to track the solar wind at the location of the comet. A corollary to this is that comets around young stars (with mass loss rates which are many orders of magnitude greater than that of the sun) should have X-ray emission several orders of magnitude greater than solar system comets. In addition, young stars may have hundreds of comets proximate to them at any given time.
Most of the solar system's planets have been observed by CXO. These observations are challenging because of the relative motion of the bodies and their optical brightness, which can be greater than the design limit of the optical blocking filters of the ACIS.
Venus & Mars
Venus has been unobservable by previous X-ray telescopes, owing both to its proximity to the Sun and its extreme optical luminosity. To offset the latter problem, Venus was observed by Dennerl et al. (2002) using both the ACIS-I array and the Low Energy Transmission Grating with the ACIS-S array. Lines of carbon, nitrogen and oxygen were clearly seen. The X-ray emissionextends above the cloud tops and unlike the optical emission it is clearly limb brightened. The morphology, luminosity and spectra are consistent with fluorescent X-ray scattering of solar X-rays by atoms about 110 km above the surface.
Mars, undetected by ROSAT, was observed in July 2001. Dennerl et al. (2002) used the ACIS-I detector. Mars is similar to Venus in that oxygen was detected with a morphology, luminosity and energy spectrum consistent with fluorescence scattering of X-rays at 80 km above the Martian surface. Unlike Venus, a weak X-ray signal was detected extending several radii beyond the planet, which may be the signature of charge exchange between the solar wind and a large, low density, atmosphere of neutrals. This may be direct evidence that Mars is still losing atmosphere to deep space.
Unlike Mars and Venus, Jupiter had been previously detected in X-rays. Polar emission was thought to be the product of oxygen and sulfur ions from Io being transported through the Jovian magnetosphere and then precipitating through the auroral regions. Jupiter has been subject to multiple observing campaigns. Using the High Resolution Camera (HRC), Gladstone (2002) easily detected emission of about 1016 erg/sec from the entire surface of Jupiter with a strong concentration toward the poles. While the general surface emission is presumably from scattered solar X-rays, the polar emission is between the poles and the auroral regions imaged by HST and various other spacecraft. This means the material responsible for this emission cannot be from Io as previously thought and must emanate from more than 30 RJup away. This leaves the Sun as the probable source of the ions which collide with the Jovian atmosphere near the poles. The polar emission is seen to pulse with a 45 minute cadence. A search for a 45 minute resonance in the observation or within Sun-Jupiter interaction has not found a suspected source for the periodicity.
Elsner et al. (2002) revisited the early ACIS observations. While the data from Jupiter itself was uninterpretable, they found X-ray emission from the moons Io and Europa. They believe this emission comes from bombardment of the satellites surfaces by hydrogen, oxygen and sulfur in the Io plasma torus which itself is detectable at about 10% the total luminosity of Jupiter itself.
Saturn was observed in April 2003 (Ness et al. 2004). Previously Ness & Schmitt (2000) had reported a ROSAT detection of 22 photons coincident with Saturn in a 5.3 ks interval. (Only 7.6 photons were expected.) From this, they derived a flux of about 2 x 10-14 ergs/cm2/sec. In the Chandra observations, Ness detects over 100 counts above the background. While the Chandra observation puts the detection of Saturn on firm footing, the observed flux was 20% the flux expected based on the ROSAT result. Analysis of these data are ongoing but there is no evidence of polar concentration nor temporal variability in the signal from Saturn.
There was also a unique observation of Titan passing in front of the Crab nebula, only a few arc seconds from the Crab pulsar (Mori et al. 2004, in preparation).
Recent & Upcoming Observations
Two other planets have also been observed by Chandra?. The Earth was observed in cycle 4 by Gladstone, Elsner & Waite using the HRC. Following from their Jovian experience they hypothesize a particle source region near Earth s magnetopause and auroral entry of heavy solar wind ions due to high-latitude reconnection. A weak detection is seen in the publicly available data. Uranus was observed by Metzger in 2002. No source is obvious in the data. A similar null result was seen from the asteroid 1998 WT 24.
More work is being done on nearby objects. All of the observations taken to date are photon starved. None of the observations have been able to take advantage of Chandra's full resolution due to counting statistics. One cannot clearly identify the multiple X-ray production mechanisms present in most bodies. During Chandra's cycle 4, Jupiter was revisited by both HRC and by ACIS (using a mode specialized to overcome the optical light leak). Current Cycle 5 approved observations include a more detailed study of the Earth s north polar cusp, and a two rotation period observation of Saturn in time for the first Cassini encounter. As a first attempt at cometary tomography, 2P/Encke was observed for an entire rotation period.
I have not been the PI on any of these projects, but I thank the PI's for letting me play in their sandbox.
Scott J. Wolk
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