The Chandra COSMOS-Legacy survey: source X-ray spectral properties

We present the X-ray spectral analysis of the 1855 extragalactic sources in the Chandra COSMOS-Legacy survey catalog having more than 30 net counts in the 0.5–7 keV band. A total of 38% of the sources are optically classified type 1 active galactic nuclei (AGNs), 60% are type 2 AGNs, and 2% are passive, low-redshift galaxies. We study the distribution of AGN photon index Γ and of the intrinsic absorption $N_{H,z}$ based on the sources' optical classification: type 1 AGNs have a slightly steeper mean photon index Γ than type 2 AGNs, which, on the other hand, have average $N_{H,z}\sim 3$ times higher than type 1 AGNs. We find that ∼15% of type 1 AGNs have $N_{H,z}>{10}^{22}$ cm⁻², i.e., are obscured according to the X-ray spectral fitting; the vast majority of these sources have $L_{2\text{--}10keV}>$ 10⁴⁴ erg s⁻¹. The existence of these objects suggests that optical and X-ray obscuration can be caused by different phenomena, the X-ray obscuration being, for example, caused by dust-free material surrounding the inner part of the nuclei. Approximately 18% of type 2 AGNs have $N_{H,z}<{10}^{22}$ cm⁻², and most of these sources have low X-ray luminosities (L ${}_{2\text{--}10keV}<$ 10⁴³ erg s⁻¹). We expect a part of these sources to be low-accretion, unobscured AGNs lacking broad emission lines. Finally, we also find a direct proportional trend between $N_{H,z}$ and host-galaxy mass and star formation rate, although part of this trend is due to a redshift selection effect.