Discovery of an Extreme MeV Blazar with the Swift Burst Alert Telescope

The Burst Alert Telescope (BAT) on board Swift detected bright emission from 15-195 keV from the source SWIFT J0746.3+2548 (J0746 in the following), identified with the optically faint (R~19), z=2.979 quasar SDSS J074625.87+244901.2. Here we present Swift and multiwavelength observations of this source. The X-ray emission from J0746 is variable on timescales of hours to weeks in 0.5-8 keV and of a few months in 15-195 keV, but there is no accompanying spectral variability in the 0.5-8 keV band. There is a suggestion that the BAT spectrum, initially very hard (photon index Γ~0.7), steepened to Γ~1.3 in a few months, together with a decrease of the 15-195 keV flux by a factor ~2. The 0.5-8 keV continuum is well described by a power law with Γ~1.3 and spectral flattening below 1 keV. The latter can be described with a column density in excess of the Galactic value with intrinsic column density N^z_H~10²² cm^-2, or with a flatter power law, implying a sharp (ΔΓ>~1) break across 16 keV in the quasar's rest frame. The spectral energy distribution of J0746 is double-humped, with the first component peaking at IR wavelengths and the second component at MeV energies. These properties suggest that J0746 is a blazar with high gamma-ray luminosity and low peak energy (MeV), stretching the blazar sequence to an extreme.