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3 Optical spectral properties of 1ES 1927+654


  \begin{figure}
\par\includegraphics[width=7cm,clip]{H3775F11.eps} \end{figure} Figure 9: POSS II red image of 1ES 1927+654. The 90 percent error circle of the Chandra position is overplotted. The field of view is $2\; \times$2 arcmin. The Chandra position is within one arcsec consistent with the (1). The objects labeled with (2) to (6) are spectroscopically identified as G or K stars.


  \begin{figure}
\par\includegraphics[angle=-90,width=8.8cm,clip]{H3775F12.ps}\par\includegraphics[angle=-90,width=8.8cm,clip]{H3775F13.ps} \end{figure} Figure 10: Upper panel: optical spectrum of 1 ES 1927+654 taken in June 2001. Typical AGN (emission) and galaxy (absorption) lines are marked in the plot. The high [O III] $\lambda $5007 to $\rm H\beta $ ratio, about 15, excludes a Narrow-Line Seyfert 1 classification. Lower panel: spectrum of the H$\alpha $ region obtained in June 2002 with the same telescope and instrument and a spectral resolution of 3 Å. The H$\alpha $ region is well described by narrow emission lines ( ${\textit FWHM} \sim 170$ km s-1) plus a flat continuum (solid line). A possible broad H$\alpha $ component with a maximum line flux of $\sim $5% of the narrow H$\alpha $ emission line is overplotted (dotted line).

The optical spectrum of 1ES 1927+654 was taken in June 2001 using the Carelec spectrograph with the 1.93 m telescope at the Haute-Provence observatory. The spectral resolution is 6 Å (3 pixels) and the signal-to-noise ratio is about 30.

The optical spectrum in Fig. 10 shows narrow emission lines and several galaxy absorption line features of Ca H+K $\lambda\lambda3933/3968$, CH G $\lambda4304$, Mg I $\lambda5173$, and Na I $\lambda5893$ indicating the presence of a host galaxy continuum. The observed FWHM of most emission lines is about 330 km s-1, which corresponds to the instrumental resolution.

The lower FHWM value of $270\pm20$ km s-1 for the H$\beta$ line is probably due to the presence of an underlying absorption component, which can barely be seen when the spectrum is displayed at full resolution. The flux ratios of [O III]5007/H $\beta=14.6$, [O I]6300/H $\alpha=0.2$, [N II]6584/H $\alpha=0.6$, and [S II]/H $\alpha=0.6$ point to a Seyfert 2 classification using the standard diagnostics of Veilleux & Osterbrock (1987).

The rest frame equivalent widths of the absorption lines CH G $\lambda4304$ and Mg I $\lambda5173$ of $0.18\pm0.03$ Å and $0.14\pm0.03$ Å, respectively, have been used to decompose the spectrum into the power-law featureless continuum and the host galaxy contribution. Assuming an optical spectral index of $-1.5 \le \alpha_{\rm opt} \le 2.0$ for AGN and typical rest frame equivalent widths of 3.3 to 5.8 Å for CH G $\lambda4304$ and of 2.4 to 4.8 Å for Mg I $\lambda5173$ (see Goodrich & Osterbrock 1983) we have determined the fraction of the power-law featureless continuum to be between 92 and 97% at 4800 Å. The optical continuum emission of 1 ES 1927+654 seems therefore to be dominated by the AGN.

From the H$\alpha $ to H$\beta$ flux ratio we have determined the optical extinction AV=2.0 in the narrow line region, assuming Case B recombination (see Veilleux et al. 1997). The actual visible extinction may in fact be even lower by a few percent, baring in mind that the H$\beta$ emission line is affected by an underlying absorption, which can not be precisely measured here. While the the optical spectrum does not clearly indicate the presence of a significant broad H$\alpha $ component, we have nevertheless tried to fit the H$\rm\alpha$ region with an additional component (see Fig. 10, lower panel), to estimate its possible strength. The addition of the broad component does not significantly improve the fit, indicating that the presence of a broad $\rm H\alpha$ component is not required by the data. The line flux of any possible broad component, has to be lower than 5 percent of the narrow H$\alpha $ component. The equivalent width of the narrow H$\alpha $ line is 2.4 Å. The central wavelength of this possible broad H$\alpha $ emission component is also slightly shifted to the red. We note that the lower signal-to-noise spectrum of Perlman et al. (1996) also did not show a broad $\rm H\alpha$ component.

Following Goodrich et al. (1994), the non-detection of a significant broad H$\alpha $ emission line suggests an extinction value of the Broad Line Region (BLR) of at least AV = 3.7. Near-infrared spectra of 1ES 1927+654 are required to estimate the upper limit of the extinction of the BLR by the detection of broad emission line components at longer wavelength, e.g. at Br$\gamma$ or Br$\alpha $.

The AV values discussed above refer to the sum of the Galactic and intrinsic extinction. The Galactic AV value in the direction of 1ES 1927+654 is 0.291 as found in NED. Therefore, the corrected AV for the Narrow Line Region in the distant AGN is at most 1.71 (or slightly lower if we could correct for the underlying absorption line at H$\beta$).


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