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Astron. Astrophys. 335, 443-448 (1998)

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3. Discussion

The radio and X-ray core emission in conjunction with the lack of prominent optical emission lines are properties similar to a BL Lac type object but the lack of strong optical core emission distinguishes this galaxy from a typical BL Lac.

Our high resolution MERLIN observation at 6 cm (Fig. 3) shows a compact region of about 100 parsecs. These results further support the compact nature of the core emission of the VLA images by Harris et al. (1984) and the archival 3.6 cm data (Fig. 2). The position angle of the second component (PA= [FORMULA]) agrees, within the errors, with those in the 21 cm map (PA=145o) and the 3.6 cm image (PA= [FORMULA]).

We have computed the values for [FORMULA] (0.43) and [FORMULA] (0.99) with an assumption that the upper limit of the optical core flux is 0.24 mJy (1/10 of the total flux of the galaxy in the V band). The X-ray flux from an HRI measurement is 0.56 µJy at 1 keV. Given these values, 26W20 lies in the center of the X-ray selected BL Lac objects on the [FORMULA] vs. [FORMULA] plot as shown in Fig. 8 of Tananbaum et al. 1997. If the core luminosity is significantly less than our upper limit, this could place 26W20 at an extreme position outside of the BL Lac group.

Given the range of [FORMULA] and [FORMULA] for a typical X-ray selected BL Lac, the core optical flux is between 3.2 [FORMULA] and 1.55 mJy. Therefore, an excess of optical flux in the core will not significantly change the color or magnitude of the galaxy as a whole.

No evidence for the presence of any hot, extended gas was found in the sub-cluster containing 26W20. From density measurements of the sub-cluster environment, the values fall within the range of ambient densities of other clusters harboring TRGs.

We propose that the BL Lac properties observed in 26W20 are evidence of energetic emission from the core due to beamed emission from a component of the jet. Therefore, the large apparent (i.e. if emission is isotropic) X-ray luminosity of [FORMULA] ergs s-1, concomitant with the absence of emission lines, means either there is no gas in the galaxy to form emission line regions or the X-rays are beamed and not available to ionize the gas.

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© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998