The Weak Blue Bump of H2106-099 and Active Galactic Nuclei Dereddening

Abstract

We present multifrequency spectra of the Seyfert 1 galaxy H2106-099, from radio to hard X-rays, spanning over a decade of observations. The hard X-ray (2-20 keV) spectrum measured with Ginga was not unusual, with a log slope (F_υ ∝ E^α), of -0.80±0.02 on 1988 May 18 and -1.02±0.10 on 1988 May 22/23 UT, and with no significant observed variations in total flux. Other measurements showed variability and unusual spectral features: The V band flux was observed to change by a factor of 1.8 (>10 σ)in 6 weeks. Only moderate optical Fe II emission is present, but strong [Fe VII] and [Fe X] epochs. The Balmer lines show greater than 25% variations in flux relative to the mean, and He I changed by more than 100% relative to the mean in 6 yr. The most surprising finds came from the composite UV through near-IR spectrum: If the spectrum is dereddened by the galactic extinction value derived from 21 cm observations, a residual 2175 Å absorption feature is present. Additional dereddening to correct the feature yields E(B - V) = 0.07 mag due to material outside our Galaxy, most probably associated with the active galactic nucleus (AGN) or its host galaxy. No other clear indications of reddening are observed in this object, suggesting that blue bump strength measurements in low and intermediate redshift AGNs could be incorrect if derived without UV observations of the region near 2175 A in the AGN frame. After all reddening corrections are performed, the log slope of H2106 -099 from the near-IR (∼12500 Å) to the UV (∼1400 Å), -0.94±0.05, is steep compared to other AGNs, suggesting that the blue bump in this object is intrinsically weak. Weak blue bumps are, therefore, not always an artifact caused by reddening. The spectral indices of this object in the optical-UV region are steeper than those predicted by optically thin free-free emission models; therefore, some other mechanism must dominate the continuum in this region.