Spectral analysis of four 'hypervariable' AGN: a microneedle in the
  haystack?
  We analyze four extreme active galactic nuclei (AGN) transients to explore
the possibility that they are caused by rare, high-amplitude microlensing
events. These previously unknown type-I AGN are located in the redshift range
0.6-1.1 and show changes of > 1.5 mag in the g-band on a time-scale of ~years.
Multi-epoch optical spectroscopy, from the William Herschel Telescope, shows
clear differential variability in the broad line fluxes with respect to the
continuum changes and also evolution in the line profiles. In two cases, a
simple point-source, point-lens microlensing model provides an excellent match
to the long-term variability seen in these objects. For both models, the
parameter constraints are consistent with the microlensing being due to an
intervening stellar mass object but as yet there is no confirmation of the
presence of an intervening galaxy. The models predict a peak amplification of
10.3/13.5 and an Einstein time-scale of 7.5/10.8 yr, respectively. In one case,
the data also allow constraints on the size of the CIII] emitting region, with
some simplifying assumptions, to to be ~1.0-6.5 light-days and a lower limit on
the size of the MgII emitting region to be > 9 light-days (half-light radii).
This CIII] radius is perhaps surprisingly small. In the remaining two objects,
there is spectroscopic evidence for an intervening absorber but the extra
structure seen in the light curves requires a more complex lensing scenario to
adequately explain.
