X-ray sources in the 1.75 Ms Ultra Narrow Deep Field observed by XMM-Newton

In this work we present the results of the survey carried out on one of the deepest X-ray fields observed by the XMM-Newton satellite. The 1.75 Ms Ultra Narrow Deep Field (XMM175UNDF) survey is made by 13 observations taken over 2 years with a total exposure time of 1.75 Ms (1.372 Ms after flare-filtered) in a field of $30' \times 30' $ centered around the blazar 1ES 1553+113. We stacked the 13 observations reaching flux limits of $4.03 \times 10^{-16} $, $1.3 \times 10^{-15}$, and $9.8 \times 10^{-16}\, erg\, s^{-1}\, cm^{-2}$ in the soft $(0.2 - 2\, \mathrm{keV})$, hard $(2 - 12\, \mathrm{keV})$, and full $(0.2 - 12\, \mathrm{keV})$ bands, respectively. Using a conservative threshold of Maximum Likelihood significance of $ML \geq 6$, corresponding to $3\sigma$, we detected 301 point-sources for which we derived positions, fluxes in different bands, and hardness ratios. Thanks to an optical follow-up carried out using the 10.4m the Gran Telescopio Canarias (GTC) on the same field in the $u'g'r'i'z'$ bands, combined with WISE/2MASS IR data; we identified 244 optical/IR counterpart candidates for our X-ray sources and estimated their X-ray luminosities, redshift distribution, X-ray/optical $-$ X-ray/IR flux ratios, and absolute magnitudes. Finally, we divided this subsample in 40 non-active sources and 204 AGNs, of which 139 are classified as Seyfert galaxies and 41 as Quasars.