THE WEAK BLUE BUMP OF H2106-099 AND ACTIVE GALACTIC NUCLEI DEREDDENING

We present multifrequency spectra of the Seyfert 1 galaxy H2106-099, f rom 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, wi th a log slope (F-v proportional to E(alpha)), of -0.80 +/- 0.02 on 19 88 May 18 and -1.02 +/- 0.10 on 1988 May 22/23 UT, and with no signifi cant observed variations in total flux. Other measurements showed vari ability and unusual spectral features: The V band flux was observed to change by a factor of 1.8 (>10 sigma) in 6 weeks. Only moderate optic al Fe II emission is present, but strong [Fe VII] and [Fe X] epochs. T he Balmer lines show greater than 25% variations in flux relative to t he 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 A absorption fe ature 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 intermediat e redshift AGNs could be incorrect if derived without UV observations of the region near 2175 Angstrom in the AGN frame. After all reddening corrections are performed, the log slope of H2106-099 from the near-I R (similar to 12500 Angstrom) to the UV (similar to 1400 Angstrom), -0 .94 +/- 0.05, is steep compared to other AGNs, suggesting that the blu e bump in this object is intrinsically weak. Weak blue bumps are, ther efore, not always an artifact caused by reddening. The spectral indice s of this object in the optical-UV region are steeper than those predi cted by optically thin free-free emission models; therefore, some othe r mechanism must dominate the continuum in this region.