Astron. Astrophys. 358, 793-811 (2000)

6. Microlensing of a realistic jet structure

So far, we have only investigated a very simple model of the source structure, consisting of a core plus a single jet-component. To see how a real source behaves, when microlensed by similar MFs as in the lens galaxy of B1600+434, we simulated light curves of a more complex jet structure.

6.1. The jet structure of 3C120

We used the inner jet-structure seen in 3C120 (Gómez et al. 1998; Gómez, Marscher & Alberdi 1999) as a `template', because it is one of the best-studied nearby jet structures. The inner jet consists of a core plus at least 15 distinguishable jet-components (Gómez et al. 1999), which move superluminally, with velocities typically around 4.5c (H₀=65 km s^-1 Mpc^-1; Gómez et al. 1998). The jet was observed at 22 GHz. The total flux density of 3C120 at this frequency is 5.70.9 Jy (O'Dell et al. 1978), whereas the fitted inner-jet components contain about 2.5 Jy (Gómez et al. 1999). We therefore assume that the inner jet contains about 40% of the total flux density of 3C120.

Note that 22 GHz corresponds to approximately X-band (8.5 GHz) observations, when placed at the source redshift of B1600+434 (z=1.59). We thus scale the size of the jet structure down by the ratio of angular diameters distances for 3C120 and the lensed source of B1600+434 and its flux density by the ratio of luminosity distances squared.

At a redshift of z=1.59, 3C120 would be a 1.8 mJy source. To obtain a source of about 25 mJy total, as observed for B1600+434, we scale each jet component's size by a factor =3.7 in radius and assume that about 60% of its flux density is contained in an extended structure, which is not sensitive to microlensing. We assume their surface brightness temperature to be conserved and associate the radius of the source with half the FWHM of the component size determined by Gómez et al. (1999). The resulting jet structure has a flux density of about 25 mJy at 8.5 GHz and an angular size of the inner jet less than 1 mas. If we only use the inner jet and not assume the additional 60% of extended emission, one should scale the inner-jet-components by a factor of =5.7 to obtain a total flux density of about 25 mJy.

6.2. Microlensed light curves of 3C120

The jet structure is randomly placed on the magnification pattern. We recalculate the jet structure and the resulting normalized light curves at epochs separated by 3.3 days, which is the average sampling of the light curves of B1600+434. For image A, we use the MF AS7, which corresponds to a halo filled with stellar remnants, such as black holes and neutron stars. For image B, we use the MF BS2.

We calculate light curves with a total time span of 35 weeks, corresponding to the length of the observed VLA light curves of B1600+434. We subsequently scale the light curves by a factor 0.4 (=3.7 for 40% of the flux density from the inner jet) or 1.0 (=5.7 for 100% of the flux density from the inner-jet). We repeat these simulations using an apparent velocity three times larger (=3).

In Fig. 10, one simulated light curve is shown for the images A and B, for each size scale factor () and velocity scale factor (). The modulation-index and estimated variability time scale between strong microlensing events are listed in Table 6.

Fig. 10. Eight simulated microlensing light curves of 3C120. The parameters for each light curve are listed in Table 6.

Table 6. Scaling parameters for 3C120 and results from eight arbitrary simulated light curves. LCs show light curves using the MF B2, whereas LCs show light curves using the MF A7.

6.3. A comparison with B1600+434

Not only does the modulation-index correspond well with that seen for B1600+434-A and B, also the time scale of variability is in the order of several weeks to months (depending on the choice of ; Table 6). The similarity between some of the simulated light curves and those observed for B1600+434 is remarkable, knowing that we have not resorted to extreme assumptions.

We therefore regard these simulations as `proof of principle', showing that microlensing of multiply-imaged compact flat-spectrum radio sources, of which more and more are being discovered - for example in the CLASS/JVAS survey (e.g Browne et al. 1998) - can be a very common occurrence, enabling us to study both the structure of these high-z radio sources, as well as the MF of compact objects in the intermediate-z lens galaxies.

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000
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