PKS 0521-365

PKS 0521-365 was initially listed as a statistically weak ( ) EGRET detection [Fichtel et al. 1994; von Montigny et al. 1995a; Lin et al. 1995]. However, the positional coincidence of the gamma-ray and radio sources resulted in its being classified as a high-confidence identification in the second EGRET catalog [Thompson et al. 1995]. PKS 0521-365 is one of the brightest radio sources in the sky at 2.3 GHz [Wall 1994].

The radio source was identified optically as an N galaxy by Bolton, Clarke, & Ekers [1965]. Danziger et al. [1979] measured its red shift to be z=0.0554, making it the second lowest red shift AGN in the EGRET list after Mrk 421. PKS 0521-365 has not yet been detected at TeV energies, unlike Mrk 421. The CANGAROO team [Kifune et al. 1996] find a 3 upper limit of cm s between 1994 December and 1995 January from PKS 0521-365 at energies above 1 TeV. This limit corresponds to approximately 1/5 of the flux at 1 TeV expected from an extrapolation of the EGRET flux at 200 GeV [Bowden et al. 1993]. For comparison, the flux of Mrk 421 above 500 GeV is cm s [Punch et al. 1992].

Danziger et al. [1979] discovered an optical jet and followed up with VLA observations which revealed an asymmetric radio structure with an unresolved component centred on the optical galaxy. More detailed observations of the jets [Keel 1986; Macchetto et al. 1991; Falomo 1994] showed a strong correspondence between the features seen at optical and radio wavelengths.

VLBI observations of the unresolved component in the VLA images were first made by Broderick et al. [1972] who found a compact component of 0.9 Jy at 2.3 GHz with a 25 million wavelength baseline. Preston et al. [1989] modelled their sparse 2.3 GHz data with a 1.2 Jy circular Gaussian component of 1.4 mas FWHM. The new VLBI images (Figures 4.3 - 4.7) show that the compact radio source is dominated by a slightly resolved component, together with a jet-like component which extends towards the north-west at a position angle of 310 . The core is the flat-spectrum component in the south-east. The position angle of the arcsecond-scale optical and radio jets is 312 . The mas-scale and arcsecond-scale structures are aligned to better than 2 .

Figures 4.3, 4.4, and 4.5 show the images resulting from data obtained at 4.8 GHz between 1992 November 23 and 1993 May 14. Figure 4.6 shows an image of slightly higher resolution at 8.4 GHz, from data obtained in 1993 October 21. Finally, Figure 4.7 is another image made from the data at 4.8 GHz from 1992 November 23, after data on baselines to Shanghai and Hartebeesthoek were added to the Australian array data. The resulting high-resolution image resolves out the jet-like feature seen in Figure 4.3 but reveals the structure of the radio source within 5 mas of the core.

 

  
Figure: Low contour, 1%. Peak, 1.3 Jy/beam. Beam, 4.7 2.3 mas @ 1.5 .

  
Figure: Low contour, 0.5%. Peak, 1.8 Jy/beam. Beam, 3.7 2.3 mas @ 0.6 .

 

  
Figure: Low contour, 1%. Peak, 2.0 Jy/beam. Beam, 4.3 2.1 mas @ -6.3 .

  
Figure: Low contour, 1%. Peak, 1.6 Jy/beam. Beam 2.0 1.3 mas @ -10.1 .

  
Figure: Low contour, 2%. Peak, 0.7 Jy/beam. Beam, 1.2 0.7 mas @ 3.7 .

 

 

Table: Limits on jet to counterjet surface brightness ratio (R) and core-jet separation (d) for each of the PKS 0521-365 epochs.

The observed (and source frame) brightness temperature of the core, from Figure 4.7, is  K; de-convolved component FWHM of 0.8 0.6 mas and total flux density of 1.2 Jy at 4.8 GHz.

Limits on the jet to counterjet surface brightness ratio (R) and core-jet separations (d) are given at each epoch in Table 4.2. Given the error-bars on the core-jet separations and the lower limits on R, these results are consistent with the idea of mild relativistic speeds and moderate beaming proposed by Falomo et al. [1995] for PKS 0521-365, but also with a model which has a highly relativistic flow closely aligned to our line of sight. However, the brightness temperature is well below the nominal inverse Compton limit of K and therefore does not favour PKS 0521-365 as a highly beamed radio source. Observations at later epochs will allow further constraints on any apparent motion in the jet of PKS 0521-365, giving a more complete indication of the importance of relativistic beaming.