The strong radio source PKS 1830-211 was first proposed to be a gravitational lensed QSO by Pramesh Rao & Subrahmanyan (1988). High resolution radio images obtained from several interferometric arrays have revealed that the source has a ring-like structure with two bright components on sub-arcsecond scales (Jauncey et al. 1991). This suggests a close alignement of the lensed source behind the lensing object. Actually, two absorption systems have been detected at (Wiklind & Combes 1996) and (Lovell et al. 1996), so it seems likely that the image of the background QSO (with a redshift ) is lensed by two different extragalactic objects (probably gas-rich spirals). The background source can be modeled as a core-knot-jet structure, similar to other flat-spectrum QSOs which are known to be -ray emitters (Nair et al. 1993). The -ray spectrum of 2EG is remarkably similar to several spectra of QSOs detected by EGRET, like 0234+285 and 0454-463 (von Montigny et al. 1995). These high-energy spectra are much steeper than those expected for galactic sources like pulsars. This fact, along with the spatial coincidence, strongly suggests the identification of 2EG with PKS 1830-211.
The presence of variability in the time history of 2EG could provide additional support to the proposed identification. This time history is presented in graphical form in Fig. 4 (click here) for a yr time span. We have used the data from the second EGRET catalog corrected and completed by McLaughlin et al. (1996). Systematic errors over the statistical uncertainties of EGRET flux measurements are difficult to estimate. These errors can be due to uncertainties in the instrumental calibration as a function of energy, uncertainties in angle within the instrument, and errors in the galactic diffuse radiation model. McLaughlin et al. (1996) have quantified these systematic errors assuming that -ray pulsars are nonvariable sources, obtaining an additional uncertainty of 6.5% 1.0% which is included in the error bars in Fig. 4 (click here).
Figure 4: Time history of 2EG over a period of 3.5 yr
A -variability analysis of the entire light curve (see Romero et al.
1994) shows that the source behaviour is "probably variable''. This
is mainly due to the large errors in the flux values during the viewing
periods from mid-1992 to mid-1993. However, if we restrict our analysis to
the lapse August 1991 - February 1992 we find clear evidence for
significant variability. The flux increased from
cm-2 s-1 in August 1991 to cm-2
s-1 at the end of 1991, and then decreased to
cm-2 s-1 in February 1992. This implies a flux change by a factor
of at least 2.5 over a time scale of 6 months. This behaviour seems
to be incompatible with a pulsar (see Ramanamurthy et al.
1995) or SNR-type source. Conversely, rapid -ray variability
in PKS 1830-211 could be produced both intrinsically or by gravitational
microlensing. In this latter case, the background -ray region in
the innermost part of the QSO is magnified by a compact massive object in
one of the intervening galaxies. The variability time scale is given by the
time spent by the line of sight to the source in crossing the microlens
Einstein radius, i.e.
where is the microlens Einstein radius, is a distance in Gpc obtained from the source-lens, lens, and source angular-diameter distances in a Robertson-Walker Universe, M is the mass of the lens in units of solar masses, and v3 is the velocity V of the lens in units of 103 km s-1 (see Romero et al. 1995 and references therein for details). Assuming a redshift for the background source and for the microlens, we find that if (we have considered H0=100 km s-1 and q0=1/2). Consequently, a MACHO-like object in the halo of the foreground galaxy could produce the observed variability.
Flux variations will occur in this scenario just if the angular radius of the source in the lens plane is smaller than the Einstein angular radius of the microlens. This imposes the constraint that the size of the -ray emitting region should be cm pc, in good accordance with the sizes expected for the -spheres in blazars (e.g. Blandford & Levinson 1995). Since the -spheres are much smaller than the compact radio cores, no correlation with lower frequency variability should be expected for the -ray microlensing events. In fact, the sizes of the optical and radio emitting regions in the lens plane should largely exceed the Einstein ring sizes for small compact objects and, consequently, no significant amplifications of the lensed images should happen at these wavelengths. On the other hand, intrinsic -ray variability seems to occur in the initial phases of high radio outbursts (e.g. Valtaoja & Teräsranta 1995). This fact could be used to discriminate between future intrinsic and extrinsic -ray variability events in PKS 1830-211.