150 keV Emission from PKS 2149−306 with BEPPOSAX

Abstract

A BeppoSAX observation of the z=2.34 quasar PKS 2149-306 produced a strong signal in the high-energy PDS instrument up to a maximum observed energy of nearly 50 keV, 150 keV in the quasar frame. The BeppoSAX spectrum spans almost three decades (0.3-150 keV, quasar frame) and shows an extremely hard (α=0.4+/-0.05) X-ray spectrum above 3 keV (comparable to XJB the X-ray background slope), and either a softer (α=1.0 +0.6 -0.3) low-energy component, or an ionized absorber at zero redshift. No evidence is seen of an Fe-K emission line (EW<167 eV at 6.5 keV quasar frame), a Compton hump (R<0.3). A bremsstrahlung fit gives kT(rest)=46 +32 -16 keV, similar to the X-ray background value, and a high-energy cut off power law requires E cut 120 keV (quasar frame). The SED of PKS 2149-306 shows two peaks at ~10 12+/-0.5 Hz and ~10 21+/-1.0 Hz (~0.3 mm and ~4 MeV), strongly resembling a low-energy cut off BL Lac object (LBL). The ratio of the two peaks shows an extreme Compton dominance (C D =1.4+/-0.4), as in flat-spectrum radio quasars (FSRQs). The presence of an additional ``optical/UV big bump'' component may provide photons that cool the jet, suppressing the radio emission.