An extremely X--ray weak blazar at z=5
  We present the discovery and properties of DESJ014132.4-542749.9
(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>10$^{4}$), the high radio luminosity
(L$_{0.8 GHz}$=1.73$\times$10$^{28}$ W Hz$^{-1}$), and the flatness of the
radio spectrum ($\alpha$=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
DESJ0141-54 is much lower compared to those of the high redshift (z$\geq$4.5)
blazars discovered so far. Moreover its X-ray-to-radio luminosity ratio
(log($\frac{L_{[0.5-10]keV}}{L_{1.4GHz}}$)=9.96$\pm$0.30 Hz) is small also when
compared to lower redshift blazars: only 2\% of the low-z population has a
similar ratio. By modeling the spectral energy distribution we found that this
peculiar X--ray weakness and the powerful radio emission could be related to a
particularly high value of the magnetic field. Finally, the mass of the central
black hole is relatively small (M$_{BH}$ = 3-8 $\times$10$^8$ M$_{\odot}$)
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$\geq$5.
