2005MNRAS.356..568N

We present the results of a 200-ks Chandra observation of part of the Groth Strip region, using the ACIS-I instrument. We present a relatively simple method for the detection of point sources and calculation of limiting sensitivities, which we argue is at least as sensitive and more self-consistent than previous methods presented in the literature. A total of 158 distinct X-ray sources are included in our point-source catalogue in the ACIS-I area. The number counts show a relative dearth of X-ray sources in this region. For example, at a flux limit of 10^–15 erg/cm2/s, around 20 per cent more soft-band sources are detected in the HDF-N and almost 50 per cent more in the ELAIS-N1 field, which we have analysed by the same method for comparison. We find, however, that these differences are consistent with Poisson variations at <2σ significance, and therefore there is no evidence for cosmic variance based on these number counts alone. We determine the average spectra of the objects and find a marked difference between the soft-band-selected sources, which have Γ= 1.9 typical of unobscured active galactic nuclei (AGN), and the hard-band-selected sources, which have Γ= 1.0. Reassuringly, the sample as a whole has a mean spectrum of Γ= 1.4±0.1, the same as the X-ray background. None the less, our results imply that the fraction of sources with significant obscuration is only ∼25 per cent, much less than predicted by standard AGN population synthesis models. This is confirmed by direct spectral fitting, with only a handful of objects showing evidence for absorption. After accounting for absorption, all objects are consistent with a mean intrinsic spectrum of Γ= 1.76±0.08, very similar to local Seyfert galaxies. The survey area is distinguished by having outstanding multiwaveband coverage. Comparison with these observations and detailed discussion of the X-ray source properties will be presented in future papers.