Abstract:
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
∼0.4 mag over 1 yr and 3 months of monitoring. Combined with two other epochs of photometric data available in the literature, we show the object has gradually faded by ∼0.9 mag
during the period 1998 June–2001 April. A linear least-squares fit to all available observations
gives a slope of ∼0.35 mag yr−¹ which translates to ∼1.9 mag yr−¹ 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.
