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

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2. Observations and data reduction

2.1. Radio

2.1.1. Low resolution

The archival 21 cm VLA radio data of 26W20 have been reprocessed (Fig. 1). This map was made with a hybrid configuration of antennas and a 30k [FORMULA] taper. At 1413 MHz, the core has a flux density of 165 mJy; the tail, 87 mJy; and the lobe, 171 mJy. The total flux density of the source is 435 [FORMULA] 40 mJy. The TRG model requires the galaxy to be moving with a sizeable fraction of its velocity vector perpendicular to our line of sight. In this model, the twin jets point initially parallel and anti-parallel with the line-of-sight. They are then bent back over short spatial scales in such a way that the twin tails appear superimposed.

2.1.2. High resolution

Imaging of the VLA archival data (Fig. 2) at 3.6 cm shows an unresolved core with a peak flux density of 48 mJy/beam. Using the FWHM as the angular extent, the scale size of the core is [FORMULA] kpc.

[FIGURE] Fig. 2. VLA tapered [FORMULA] cm image of the core of 26W20. The restoring beam is [FORMULA] [FORMULA] [FORMULA] in PA [FORMULA].

We recently mapped 26W20 with the MERLIN array at 6 cm (Fig. 3). With an integrated flux density of 45 mJy, 70% of the emission measured by the VLA is contained within a compact region of scale size [FORMULA] 100 pc. From the image it is seen that we were able to detect a second component at nearly the same position angle as the jet/tail in the VLA maps.

[FIGURE] Fig. 3. MERLIN [FORMULA] cm image of the core of 26W20. The synthesized beam is [FORMULA] [FORMULA] 0.03 [FORMULA] in PA [FORMULA].

2.1.3. Radio spectrum

Table 2 lists the flux densities for all the observations. Fig. 4 shows the radio spectrum of the core using two beam sizes of [FORMULA] and [FORMULA]. The spectral index, [FORMULA], is 0.25 [FORMULA] for the [FORMULA] beam.


Table 2. Flux densities for the core of 26W20.
The uncertainties are given in parenthesis.
1 Obs-date:1980Jun01; 50 k [FORMULA] taper
2 Obs-date:1980Jun01; HCD, fig 6
3 Obs-date:1996Dec; MERLIN
4 Obs-date:1992Dec26; Integrated flux using a [FORMULA] box.
5 Obs-date:1992Dec26; Integrated flux using a 1 [FORMULA] box.
6 Obs-date:91/92; MPIfR, Mack et al. (1993). In that compilation of 10 TRGs, a table is given of previous observations; but most or all appear to use large beams and so do not provide useful data for our purposes.

[FIGURE] Fig. 4. Radio spectrum using different beam sizes.

2.2. X-ray

Due to the close proximity of 26W20 to the cluster Abell 754, there are several observations with ROSAT and EINSTEIN for which 26W20 is in the field of view. The galaxy was first detected with the EINSTEIN observatory and imaged with the IPC and HRI [FORMULA] ergs s-1; HCD). Five ROSAT observations using the HRI and the Position Sensitive Proportional Counter (PSPC) are available spanning two years (Table 3; three off-axis PSPC, one on-axis PSPC and one on-axis HRI). All luminosity measurements are calculated for the energy range 0.5 to 3.0 keV with one sigma errors associated with the count statistics. Data analysis was performed with the PROS package under IRAF.


Table 3. X-ray source parameters.
(1) On-axis observations.
(2) Off-axis observations. The count rates have not been corrected for vignetting.

2.2.1. Spatial extent

The X-ray core emission is unresolved with the ROSAT HRI. The radial profile (Fig. 5) shows the close fit of the count distribution to the ROSAT point response function.

[FIGURE] Fig. 5. A radial profile of the ROSAT HRI data. Counts are binned in [FORMULA] intervals.

There is no conclusive evidence for the presence of any significant extended emission in the local environment of 26W20. The low level emission seen in the [FORMULA] to [FORMULA] region at about 1% of the peak intensity can be attributed to instrumental effects. As shown in Fig. 1 of The HRI Calibration Report (David et al. 1995), there is a discrepancy in the fit to the point response function in the [FORMULA] to [FORMULA] range as determined using a long HZ43 observation.

2.2.2. Spectral fit

Analysis of archival X-ray data from the ROSAT PSPC and our HRI data (including EINSTEIN) is presented in Fig. 6 and Table 3. The energy index and the galactic column density were allowed as free parameters. The values of the fitted column density agreed closely with the galactic value of [FORMULA] cm-2. Using the long, on-axis PSPC observation (13 ksec), we find for a power law model fit with 29 degrees of freedom, [FORMULA], log [FORMULA] =20.90 [FORMULA] 0.04 and [FORMULA]. For a Raymond-Smith thermal model, the fitted spectral parameters are kT= [FORMULA] keV; log [FORMULA], and [FORMULA]. On the basis of the goodness of fit, the core emission is described more favorably by a non-thermal model.

[FIGURE] Fig. 6. X-ray variability and spectral parameters of 26W20 with one sigma errors. The curves connect to the average EINSTEIN values of 1979/1980 (off the left side of the graph): [FORMULA] =2.2 [FORMULA] ergs s-1, [FORMULA] =0.8, log [FORMULA].

The best fit spectral parameters for each individual observation are shown in Fig. 6. The slight increase in the energy index is within the errors.

2.2.3. Variability

All ROSAT luminosities (Fig. 6 ; Table 3) have been calculated with a power law model with an energy index of 1.32 and a log of the galactic [FORMULA] of 20.90. These values are the best fit parameters from the longest on-axis PSPC observation. A decrease of the luminosity from PSPC measurements is relatively mild over two years from 1991 to 1993. However, an 18% increase in the luminosity occurred within five days between two PSPC observations in 1992. An HRI image about two weeks later confirms this increase. We suspect that our under sampled measurements do not clearly represent the temporal behavior of the core luminosity. Note that the ROSAT luminosities are substantially higher than the average of the two EO observations.

2.2.4. Ambient medium

If 26W20 is a TRG, there should be an ambient gas through which the galaxy is moving. Upper limits of the particle density from a hot, X-ray emitting gas were calculated using the ROSAT HRI data. The particle densities estimated from different methods are displayed in Table 4.


Table 4. Ambient density measurements.
The cgs unit of flux used is (ergs cm-2 s-1) and pressure is (dyne cm-2). The energy range is (0.5 - 3.0 keV).
1) A King model for A754 with kT=7 keV (Abramopoulos and Ku; 1983) provides an upper limit to the cluster gas. Due to an unknown projection, 26W20 could be significantly further from the cluster center than the projected separation.
2) Fluxes were measured for two different size regions surrounding 26W20. The larger volume represents the scale where the radio structure changes from straight jet/tail to a lobe. It is also the scale size suggested by the galaxy surface density. The smaller [FORMULA] region would be appropriate for the core radius if the local gas had a King distribution. The values are 3 sigma upper limits for a uniform gas.
3) Since non-thermal pressure within the radio jet and lobe should be in balance with the external medium, particularly for the lobe, a lower limit to the pressure was calculated from the radio emission. This is a strict lower limit since a) we assumed a filling factor of 1, b) we integrated electron energies which correspond to only the observed part of the radio spectrum and c) we assumed no contribution to the energy density from relativistic protons.

The particle density measurements of the sub-cluster environment surrounding 26W20 include an acceptable range of values in agreement with the radio non-thermal pressure measurements. From the King model of the cluster, it is evident that particle densities in this region are attributed to the local environment of the sub-cluster and not to the main cluster, Abell 754. We can then compare our estimated upper limit of 0.76 [FORMULA] cm-3 to that for TRGs in other clusters. Our upper limit falls within the range of ambient densities (0.02 - 6) [FORMULA] cm-3 as reported by Feretti et al. (1992) near TRGs in Abell clusters.

As a final check to detect the presence of any hot, extended gas in the sub-cluster, 26W20 and an adjacent source were subtracted from the HRI map. Then the image was rebinned to [FORMULA] pixels and smoothed with a Gaussian of FWHM= [FORMULA]. There appeared to be no conclusive evidence for the existence of any hot extended gas.

2.3. Optical data

Two, ten minute, blue channel CCD optical spectra were taken on a Multiple Mirror Telescope (MMT) open night on April 9, 1997. Fig. 7 is the summed wavelength calibrated spectrum for both exposures. The measured redshift of z=0.0537 agrees with previous measurements (HCD). The slit has a width of [FORMULA] and a length of [FORMULA]. The dispersion of the grating is 300 lines/mm. The 3K [FORMULA] 1K CCD has a spatial resolution of [FORMULA] /pixel. The PA of the slit is [FORMULA] aligned along the radio jet.

[FIGURE] Fig. 7. Optical spectrum of 26W20 with the MMT. Features include the MgI absorption line at 5468 Å, the CaII (H-K) break at 4162 Å and the NaD line at 6205 Å. Residuals from sky subtraction are seen at about 6900 and 7600 Å. None of the usual AGN emission lines are found.

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Online publication: June 18, 1998