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 (L0:8 GHz = 1:73 x 1028WHz-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 L1:4 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 (MBH = 3-8 x 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.