The buried Balmer-edge signatures from quasars Kishimoto, Makoto; Antonucci, Robert; Boisson, Catherine ...
Monthly Notices of the Royal Astronomical Society,
11/2004, Letnik:
354, Številka:
4
Journal Article
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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.
We have found for the first time a Balmer-edge feature in the polarized flux spectrum of a quasar (Ton 202). The edge feature is seen as a discontinuity in the slope, rather than as a discontinuity ...in the absolute flux. As the polarized flux contains essentially no broad emission lines, it is considered to arise interior to the broad emission line region, showing the spectrum with all the emissions outside the nucleus scraped off and removed. Therefore, the polarized flux spectrum is likely to reveal features intrinsic to the big blue bump emission. In this case, the existence of the Balmer-edge feature, seen in absorption in the shorter wavelength side, indicates that the big blue bump is indeed thermal and optically thick.
Over its 13 years of operation (1990 -- 2002), the Faint Object Camera (FOC) on board the Hubble Space Telescope (HST) observed 26 individual active galactic nuclei (AGNs) in ultraviolet (UV) imaging ...polarimetry. However, not all of the observations have been reduced and analyzed or set within a standardized framework. We plan to reduce and analyze the AGN observations that have been neglected in the FOC archives using a consistent, novel, and open-access reduction pipeline of our own. We then extend the method to the full AGN sample, thus leading to potential discoveries in the near future. We developed a new pipeline in Python that will be able to reduce all the FOC observations in imaging polarimetry in a homogeneous way. Most of the previously published reduced observations are dispersed throughout the literature, with the range of different analyses and approaches making it difficult to fully interpret the FOC AGN sample. By standardizing the method, we have enabled a coherent comparison among the different observational sets. In this first paper of a series exploring the full HST/FOC AGN sample, we present an exhaustively detailed account of how to properly reduce the observational data. Current progress in data-analysis is implemented in and has provided state-of-the-art UV polarimetric maps. We compare our new maps to the benchmark AGN case of NGC~1068 and successfully reproduce the main results previously published, while pushing the polarimetric exploration of this AGN futher, thanks to a finer resolution and a higher signal-to-noise ratio (S/N) than previously reported. We also present, for the first time, an optical polarimetric map of the radio-loud AGN IC~5063 and we examine the complex interactions between the AGN outflows and the surrounding interstellar medium (ISM).
The dichotomy between radio-loud (RL) and radio-quiet (RQ) active galactic nuclei (AGN) is thought to be intrinsically related to radio jet production. This difference may be explained by the ...presence of a strong magnetic field (B-field) that enhances, or is the cause of, the accretion activity and the jet power. Here, we report the first evidence of an intrinsic difference in the dust polarized emission cores of four RL and five RQ obscured AGN using 89 \(\mu\)m polarization with HAWC+/SOFIA. We find that the thermal polarized emission increases with the nuclear radio-loudness, \(R_{20} = L_{\rm 5GHz}/ L_{\rm 20\mu m}\). The dust emission cores of RL AGN are measured to be polarized, \(\sim5-11\)%, while RQ AGN are unpolarized, \(<1\)%. For RQ AGN, our results are consistent with the observed region being filled with an unmagnetized or highly turbulent, disk and/or expanding outflow at scales of \(5-130\) pc from the AGN. For RL AGN, the measured \(89\) \(\mu\)m polarization arises primarily from magnetically aligned dust grains associated with a \(5-130\) pc-scale dusty obscuring structure with a toroidal B-field orientation highly offset, \(65\pm22^{\circ}\), with respect to the jet axis. Our results indicate that the size and strength of the B-fields surrounding the AGN are intrinsically related to the strength of the jet power -- the stronger the jet power is, the larger and stronger the toroidal B-field is. The detection of a \(\le130\) pc-scale ordered toroidal B-field suggests that a) the infalling gas that fuels RL AGN is magnetized, b) there is a magnetohydrodynamic wind that collimates the jet, and/or c) the jet is able to magnetize its surroundings.
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