Deep Extragalactic X-ray Surveys

1.2. Current Deep Surveys with Chandra and XMM-Newton

The superb Wolter telescopes and charge-coupled device (CCD) detectors on Chandra and XMM-Newton provide deep-survey researchers with

Sensitive imaging spectroscopy from 0.5-10 keV, with up to 50-250 times (depending upon the energy band considered) the sensitivity of previous X-ray missions.

X-ray source positions with accuracies of 0.3-1" (Chandra) and 1-3" (XMM-Newton). These high-quality positions are essential for matching to faint multiwavelength counterparts in deep surveys, thereby allowing efficient follow-up studies.

Large source samples (100-600 sources or more, per survey) allowing reliable statistical inferences to be drawn about faint extragalactic X-ray source populations.

The deep-survey capabilities of Chandra and XMM-Newton are complementary in several important respects. Due to its sub-arcsecond imaging which provides a small source detection cell, Chandra can achieve the highest possible approx 0.5-8 keV sensitivity with long exposures; the faintest Chandra sources detected have count rates of approx 1 count every 2-4 days. Even the deepest Chandra observations performed to date do not suffer from significant source confusion (Alexander et al. 2003b), in contrast to the case for XMM-Newton, where confusion becomes significant for gtapprox 100-200 ks exposures. XMM-Newton has a substantially larger photon collecting area than Chandra, allowing efficient X-ray spectroscopy at fluxes above its confusion limit. The field of view for XMM-Newton is also approx 2.5 times that of Chandra.

Table 1 lists the current deep Chandra and XMM-Newton surveys; these 21 surveys have a total exposure exceeding 80 days. The most sensitive surveys performed by Chandra and XMM-Newton, the 2.0 Ms Chandra Deep Field-North (CDF-N) and the 770 ks XMM-Newton Lockman Hole field, are shown in Figure 2. The surveys in Table 1 have generally been performed in regions of sky where (1) the extensive requisite multiwavelength supporting data already exist and/or some interesting astronomical object is present (e.g., Abell 370, 3C 295, or the SSA22 "protocluster"), and (2) there is little Galactic foreground X-ray absorption (e.g., Lockman 2004). At the flux levels probed by these surveys, even moderate-luminosity AGN, similar to Seyfert 1 galaxies in the local universe, can be detected to z gtapprox 4. The surveys in Table 1 span a significant range of solid angle coverage; however, they are all "pencil-beam" surveys in that even the widest cover only approx 5 × 10^-5 of the sky (about nine times the solid angle of the full Moon).

**Table 1.** Deep Extragalactic X-ray Surveys with *Chandra* and *XMM-Newton*

Survey	Max. Eff.	Solid Angle	Representative
Name	Exp. (ks)	(arcmin²)	Reference or Note

Chandra

Chandra Deep Field-North	1950	448	Alexander et al. (2003b)
Chandra Deep Field-South	940	391	Giacconi et al. (2002)
HRC Lockman Hole	300	900	PI: S.S. Murray
Extended CDF-S	250	900	PI: W.N. Brandt
Extended Groth Strip	200	1800	Nandra et al. (2005)
Lynx	185	286	Stern et al. (2002a)
LALA Cetus	174	428	Wang et al. (2004b)
LALA Boötes	172	346	Wang et al. (2004a)
SSA13	101	357	Barger et al. (2001a)
Abell 370	94	357	Barger et al. (2001b)
3C 295	92	274	D'Elia et al. (2004)
SSA22 "protocluster"	78	428	Cowie et al. (2002)
ELAIS N1+N2	75	586	Manners et al. (2003)

XMM-Newton

Lockman Hole	770	1556	Hasinger (2004)
Chandra Deep Field-South	370	802	Streblyanska et al. (2004)
Chandra Deep Field-North	180	752	Miyaji et al. (2003)
13 hr Field	130	665	Page et al. (2003)
Subaru XMM-Newton Deep	100	4104	PI: M.G. Watson
ELAIS S1	100	1620	PI: F. Fiore
Groth-Westphal	81	727	Miyaji et al. (2004)
Marano Field	79	2140	Lamer et al. (2003)
COSMOS	75	7200	PI: G. Hasinger

The solid angles quoted above represent the total sky coverage at bright X-ray flux limits. These surveys have wide ranges of sensitivity across their respective solid angles due to instrumental effects and differing satellite pointing strategies. Furthermore, follow-up studies of detected X-ray sources have only been done for subregions of some of these surveys. Please consult the cited papers for details.

Figure 2. (a) The 2.0 Ms Chandra Deep Field-North, the deepest Chandra observation to date. This image has been constructed from 0.5-2 keV (red), 2-4 keV (green), and 4-8 keV (blue) adaptively smoothed images. The two most prominent red diffuse patches are galaxy groups/clusters (Bauer et al. 2002a). The regions covered by the HDF-N (Ferguson, Dickinson & Williams 2000) and GOODS-N (Giavalisco et al. 2004) surveys with HST are outlined and labeled. This field subtends approx 448 arcmin² ( approx 60% the solid angle of the full Moon), and approx 580 sources are detected. Adapted from Alexander et al. (2003b). (b) The 770 ks XMM-Newton Lockman Hole field, the deepest XMM-Newton observation to date. This image has been constructed from 0.5-2 keV (red), 2-4.5 keV (green), and 4.5-10 keV (blue) images. This field subtends approx 1556 arcmin², and approx 550 sources are detected. Adapted from Hasinger (2004).