The buried Balmer-edge signatures from quasars

Abstract

In our previous paper, we reported the detection of a Balmer edge absorption feature in the polarized flux of one quasar (Ton 202). We have now found similar Balmer edge features in the polarized flux of four more quasars (4C09.72, 3C95, B2 1208+32, 3C323.1), and possibly a few more, out of 14 newly observed with the Very Large Telescope and the Keck Telescope. In addition, we also re-observed Ton 202, but we did not detect such a dramatic feature, apparently due to polarization variability (the two observations are 1 yr apart). The polarization measurements of some quasars are affected by an interstellar polarization in our Galaxy, but the measurements have been corrected for this effect reasonably well. As the broad emission lines are essentially unpolarized and the polarization is confined only to the continuum in the five quasars including Ton 202 in both epochs, the polarized flux is considered to originate interior to the broad emission-line region. The Balmer edge feature seen in the polarized flux is most simply interpreted as an intrinsic spectral feature of the quasar ultraviolet/optical continuum, or the ‘Big Blue Bump’ emission. In this case, the edge feature seen in absorption indeed indicates the thermal and optically thick nature of the continuum emitted. However, we also discuss other possible interpretations.