2005MNRAS.359.1022S

We report optical Cousins R- and I-band monitoring observations of the high-redshift (z= 4.67) quasi-stellar object (QSO) SDSS J153259.96-003944.1 which does not show detectable emission lines in its optical spectrum. We show this object varies with a maximum amplitude of ∼0.4mag over 1yr and 3months of monitoring. Combined with two other epochs of photometric data available in the literature, we show the object has gradually faded by ∼0.9mag during the period 1998 June-2001 April. A linear least-squares fit to all available observations gives a slope of ∼0.35mag/yr which translates to ∼1.9mag/yr in the rest frame of the quasar. Such a variability is higher than that typically seen in QSOs but consistent with that of BL Lacs, suggesting that the optical continuum is Doppler boosted. Alternatively, within photometric errors, the observed light curve is also consistent with the object going through a microlensing event. Photoionization model calculations show the mass of the broad line region to be a few tens of M_☉similar to that of low-luminosity Seyfert galaxies, but ∼2 orders of magnitude less than that of luminous quasars. Further frequent photometric/spectroscopic monitoring is needed to support or refute the different alternatives discussed here about the nature of SDSS J153259.96-003944.1.