X-ray observatories observe same targets
13 September 2000XMM-Newton has this summer passed into its operational phase and NASA's Chandra X-ray observatory has just celebrated its first year in orbit. The world's foremost X-ray astronomy missions will now each be contributing to a greater understanding of the X-ray universe.
During XMM-Newton's calibration phase, the two spacecraft worked together in a series of joint observations, simultaneously viewing the same celestial objects. Two other X-ray missions, the Dutch/Italian BeppoSAX and American RXTE (Rossi X-ray Timing Explorer), were also used in this series of combined observations.
Joint observations of this kind are not entirely new. In the past there have been campaigns involving RXTE, BeppoSAX and Japan's ASCA. They stem from the desire of the science community to have better calibrations.
One of the main problems on past missions was that different scientific parameters were measured for the same sources. Because they were not simultaneous in time, it could not be ascertained where this was due to sources changing over time, or just calibration errors.
The first co-ordinated observations occurred at the end of May as part of the calibration of XMM-Newton's science instruments, in particular of the Reflection Grating Spectrometer (RGS) and the EPIC cameras.
The targets included some active galactic nuclei (AGN), the central regions of active galaxies in which exceptionally large amounts of energy are being generated in a small region of space. Sources - such as the blazar PKS 2155-304 observed by XMM-Newton and Chandra - are extremely bright but can vary very rapidly in intensity. Variations of PKS 2155-304 on time scales as short as an hour have been observed.
Despite this variability, the point-like brightness of these sources (with few features such as emission lines and absorbtion edges) made them good targets for the effective area calibrations of XMM-Newton's gratings and imaging cameras.
During three XMM-Newton revolutions in mid-June, the ESA and NASA X-ray observatories separately viewed several other sources including 3C 273, the brightest and first quasar ever to be discovered. 3C 273, situated in the Virgo constellation, at a distance of some 2 billion light years has an enormous absolute brightness. At this great distance, the quasar's luminosity is about 2 trillion times that of our Sun. The object also displays a bright optical jet that appears to shoot out of the nucleus.
To view 3C 273, XMM-Newton and Chandra were joined at certain times by the Italian-Dutch BeppoSAX and the RXTE X-ray spacecraft, all observing this intriguing quasarin the same time period.
All these observations were organised by the XMM-Newton science mission planners at VILSPA and their Chandra counterparts at the Harvard-Smithsonian Center for Astrophysics in the United States. They gave the XMM-Newton instrument teams their much needed calibration data.
But they also offered XMM-Newton Principal Investigators greater insights into the targets themselves, with the first science results of their observations to be published in the next few weeks.