Year: 2011
Type: article
Abstract: We present far-UV HST /COS spectra of four nearby BL Lac objects. BL Lac spectra are dominated by a smooth, power-law continuum which arises in a relativistic jet. However, the spectra are not necessarily featureless; weak, broad- and/or narrow-line emission is sometimes seen in high-quality optical spectra. We present detections of Lyα emission in HST /COS spectra of Mrk 421 ( z = 0.030) and PKS 2005−489 ( z = 0.071) as well as an archival HST /GHRS observation of Mrk 501 ( z = 0.0337). Archival HST /STIS observations of PKS 2155−304 ( z = 0.116) show no Lyα emission to a very low upper limit. Using the assumption that the broad-line region (BLR) clouds are symmetrically placed around the active galactic nucleus (AGN), we use these measured Lyα emission features to constrain either the relativistic Γ values for the ionizing continuum produced by the jet (in the ionization-bounded case) or the mass of warm gas (in the density-bounded case). While realistic Γ values can be obtained for all four cases, the values for Mrk 421 and PKS 2155−304 are high enough to suggest that covering factors of BLR clouds of ∼1%–2% might be required to provide consistency with earlier values of Doppler boosting and viewing angles suggested for this class of BL Lacs. This discrepancy also exists in the case of M 87, where the amount of Doppler boosting in our direction is expected to be minimal, again suggestive of a small covering factor of BLR clouds. If, as these small covering factors might suggest, the assumptions of a density-bounded model could be more correct, then the observed Lyα luminosities require that BL Lac/FR 1 nuclei possess very little warm gas (10 −4 to 10 −5 M ☉ ) as suggested by Guilbert et al. If these clouds are in pressure balance with a hotter (∼10 6 K) gas, the BLR contains too little mass to power the AGN by accretion alone.
Source: The Astrophysical Journal
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
Award: G1130972977
OpenAlex ID: W1986166120