Deep Surveys with Chandra and XMMNewton

Further progress was only possible with deep Chandra and XMM-Newton surveys selected in the 0.5-10 keV band. A comprehensive account of deep surveys with Chandra and XMM-Newton and their source content is given in a recent review by Brandt and Hasinger [7]. The Chandra and XMM-Newton observatories, the current workhorses for X-ray astronomy provide powerful capabilities to pursue the X-ray background studies. Those capabilities are largely complementary between the two observatories. Chandrs's excellent sub-arcsecond point spread function allows very long, highly sensitive exposures in the «0.5-8 keV band. The faintest Chandra sources detected have count rates of about one count every 3 days. The Chandra images are far enough away from the background and confusion limits, that even stacking analyses with effective observing times of more than 10 Ms are feasible [7]. XMM-Newton, on the other hand, has a substantially larger telescope collecting area and CCD chips with higher quantum efficiency both in the soft and in the hard band, albeit with a significantly larger point spread function than Chandra. The field of view of XMM-Newton is also more than twice as large as that of the Chandra CCDs. XMM-Newton is thus ideal to cover large area surveys and to obtain high quality spectra of moderately bright X-ray sources.

The Chandra X-ray Observatory has performed deep X-ray surveys in a number of fields with ever increasing exposure times [62,63,82] and has completed a 1 Ms exposure in the Chandra Deep Field South (CDF-S, [71]) and a 2 Ms exposure in the Hubble Deep Field North (HDF-N, [1]). A similar set of deep fields has also been observed with XMM-Newton [34,35,57,65,83]. In Fig. 25.4 (left), the color composite Chandra image of the CDF-S is shown. This was constructed by combining images smoothed with a Gaussian with a = 1" in three bands (0.3-1 keV, 1-3 keV, 3-7 keV), which contain approximately equal numbers of photons from detected sources. Blue sources are those undetected in the soft (0.5-2keV) band, most likely due to intrinsic absorption from neutral hydrogen with column densities Nh > 1022 cm~2. Figure 25.1 (right) shows a similar image for the 2Ms observation of the HDF-N [1]. Very soft sources appear red. A few extended low surface brightness sources are also readily visible in the image.

The deepest XMM-Newton survey is shown in Fig. 25.2 (right panel). The different colors of X-ray sources indicate different spectral hardness and thus predominantly different amounts of intrinsic absorption. The fact that there are so many faint green and blue sources immediately confirmed one of the major predictions of the population synthesis models, i.e., that there is a significant fraction of absorbed sources, required to produce the hard spectrum of the X-ray background.

Fig. 25.4 left: Color composite image of the Chandra Deep Field North of 2 Ms. The image was obtained combining three energy bands: 0.3-1 keV, 1-3 keV, 3-7 keV (respectively, red, green, and blue), from [1]. right: Deep survey image obtained with XMM-Newton (370 ks) of the Chandra Deep Field South using the same energy bands as in Fig. 25.2 right [84]

Fig. 25.4 left: Color composite image of the Chandra Deep Field North of 2 Ms. The image was obtained combining three energy bands: 0.3-1 keV, 1-3 keV, 3-7 keV (respectively, red, green, and blue), from [1]. right: Deep survey image obtained with XMM-Newton (370 ks) of the Chandra Deep Field South using the same energy bands as in Fig. 25.2 right [84]

0 0

Post a comment