A BL Lacertae Object at a Cosmic Age of 800 Myr

We report the discovery of the BL Lacertae object FIRST J233153.20+112952.11 at redshift >6 using nearinfrared spectroscopy and broadband observations in the infrared, submillimeter, and radio wavelengths. The detection of the continuum break at similar to 0.92 mu m in the near-infrared spectrum of FIRST J233153.20+112952.11 provides an approximate redshift of 6.57, corresponding to an age of the universe of similar to 800 Myr. The rest-frame UV/optical spectrum of FIRST J233153.20+112952.11 shows no emission lines detected with equivalent width >1.5 angstrom. The nondetection of the C IV lambda 1549 broad emission line constrains the luminosity of the accretion disk to be less than or similar to 10(45) erg s(-1), which corresponds to the radiatively inefficient accretion regime of BL Lacertae objects. The spectral index of the rest-frame UV/optical continuum, alpha(nu,opt) = 1.43 +/- 0.23, is consistent with the expected spectral index of the synchrotron emission spectrum of the relativistic jet. The flat radio continuum (alpha(nu,r) approximate to 0) in a rest-frame frequency interval of 7-23 GHz is similar to that of the typical BL Lacertae objects and also consistent with originating from the synchrotron jet emission. The rest-frame UV/optical and radio fluxes of FIRST J233153.20+112952.11 show significant (15%-40%) variations. The shortest recorded rest-frame timescale of these variations is similar to 8 days. Assuming the local scaling relation between the radio luminosity and black hole mass, the 5 GHz radio luminosity of FIRST J233153.20+112952.11, 1.6 x 10(33 )erg s(-1) Hz(-1), suggests a billion solar mass central supermassive black hole.