A deep Chandra survey of the Groth Strip – I. The X‐ray data

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 cm −2 s −1 , 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.