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Description
The COSMOS-Legacy survey is a 4.6-Ms Chandra program that has imaged 2.2 deg<sup>2</sup> of the COSMOS field with an effective exposure of ~160 ks over the central 1.5 deg<sup>2</sup> and of ~80 ks in the remaining area. The survey is the combination of 56 new observations obtained as an X-ray Visionary Project with the previous C-COSMOS survey. In the reference paper, the authors describe the reduction and analysis of the new observations and the properties of 2273 point sources detected above a spurious probability of 2 x 10<sup>-5</sup>. The authors also present the updated properties of the C-COSMOS sources detected in the new data. The whole survey includes 4,016 point sources (3,814, 2,920 and 2,440 in the full, soft, and hard band). The limiting depths are 2.2 x 10<sup>-16</sup>, 1.5 x 10<sup>-15</sup>, and 8.9 x 10<sup>-16</sup> erg/cm<sup>2</sup>/s in the 0.5-2, 2-10, and 0.5-10 keV bands, respectively. The observed fraction of obscured active galactic nuclei with a column density > 10<sup>22</sup> cm<sup>-2</sup> from the hardness ratio (HR) is ~ 50<sup>+17</sup>_-16_%. Given the large sample, the authors compute source number counts in the hard and soft bands, significantly reducing the uncertainties of 5% - 10%. For the first time, they compute number counts for obscured (HR > -0.2) and unobscured (HR < -0.2) sources and find significant differences between the two populations in the soft band. Due to the unprecedented large exposure, the COSMOS-Legacy area is three times larger than surveys at similar depths and its depth is three times fainter than surveys covering similar areas. The area-flux region occupied by COSMOS-Legacy is likely to remain unsurpassed for years to come. The half-a-field shift tiling strategy was designed to uniformly cover the COSMOS Hubble area in depth and point-spread function (PSF) size by combining the old C-COSMOS (Elvis et al., 2009, ApJS, 184, 158) observations with the new Chandra ones (see Figure 1 in the reference paper). The main properties of the new ACIS-I Chandra COSMOS-Legacy observations are summarized in Table 1 therein. The observations took place in four blocks: 2012 November to 2013 January; 2013 March to July; 2013 October to 2014 January; and 2014 March. The mean net effective exposure time per field was 48.8 ks after all the cleaning and reduction operations. This table was created by the HEASARC in July 2016 based on <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/ApJ/819/62">CDS Catalog J/ApJ/819/62</a>, file table5.dat. Some of the values for the name parameter in the HEASARC's implementation of this table were corrected in April 2018. This is a service provided by NASA HEASARC .
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