An extremely X-ray weak blazar at z = 5

We present the discovery and properties of DES J014132.4-542749.9 (DES0141-54), a new powerful radio-loud active galactic nucleus (AGN) in the early Universe (z = 5.0). It was discovered by cross-matching the first data release of the Dark Energy Survey (DES DR1) with the Sidney University Molonglo Survey (SUMSS) radio catalog at 0.843 GHz. This object is the first radio-loud AGN at high redshift discovered in the DES. The radio properties of DES0141-54, namely its very large radio-loudness (R > 10⁴), the high radio luminosity (L_{0.8 GHz} = 1.73 × 10²⁸ W Hz^-1), and the flatness of the radio spectrum (α = 0.35) up to very high frequencies (120 GHz in the source's rest frame), classify this object as a blazar, meaning, a radio-loud AGN observed along the relativistic jet axis. However, the X-ray luminosity of DES0141-54 is much lower compared to those of the high redshift (z ≥ 4.5) blazars discovered so far. Moreover its X-ray-to-radio luminosity ratio (log( L_{[0.5-10] keV} / L_{1.4 GHz}) = 9.96±0.30 Hz) is small also when compared to lower redshift blazars: only 2% of the low-z population has a similar ratio. By modeling the spectral energy distribution we found that this peculiar X-ray weakness and the powerful radio emission could be related to a particularly high value of the magnetic field. Finally, the mass of the central black hole is relatively small (M_BH = 3-8 × 10⁸ M_⊙) compared to other confirmed blazars at similar redshift, making DES0141-54 the radio-loud AGN that host the smallest supermassive black hole ever discovered at z ≥ 5.