ASCA Deep Survey in the Lockman Hole Field

Abstract

Abstract We present the results of deep observations of the Lockman Hole field with the ASCA SIS, covering $926\,\mathrm{arcmin}^2$ in the 1–7 keV energy range, with a sensitivity 3–5 times deeper than the previous ASCA larger area surveys. Utilizing the results of the ROSAT Ultra Deep survey, we have detected 32, 25, and 15 sources in the 1–7, 1–2, and 2–7 keV bands, and only 4, 4, and 1 sources remain unidentified, respectively. The average hardness of the sources detected in the 1–7 keV band with fluxes of between $10^{-14}$ and $10^{-13}\,\mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$ corresponds to a photon index of $1.65\pm 0.10$. Type-2 AGNs show significantly harder spectra than type-1s, which can be explained by absorption column densities of $N_{\mathrm{H}} \simeq 10^{22-23}\,\mathrm{cm}^{-2}$ at the source redshifts. Fifteen sources are detected in the hard-band (2–7 keV), consisting of six type-1 AGNs, six type-2 AGNs, one star, one cluster of galaxies, and one unidentified source. Type-2 AGNs make up a major part of hard X-ray populations in the 2–7 keV band at a flux level of $3\times 10^{-14}\,\mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$ (2–7 keV). The redshift distributions of type-1 and type-2 AGNs suggest a lack of high luminosity (optically-defined) type-2 AGNs with intrinsic $N_{\mathrm{H}} \lt 10^{23}\,\mathrm{cm}^{-2}$, in the redshift range $z\simeq 1$–$2$ and/or with intrinsic luminosities larger than $3\times 10^{44}\,\mathrm{erg}\,\mathrm{s}^{-1}$ (2–10 keV at source frame).