We present an analysis of the multiwavelength behaviour of the blazar OJ 248 at z = 0.939 in the period 2006–2013 .
We use low-energy data ( optical , near-infrared , and radio ) obtained by 21 observatories participating in the GLAST-AGILE Support Program ( GASP ) of the Whole Earth Blazar Telescope ( WEBT ) , as well as data from the Swift ( optical–UV and X-rays ) and Fermi ( \gamma -rays ) satellites , to study flux and spectral variability and correlations among emissions in different bands .
We take into account the effect of absorption by the damped Lyman \alpha intervening system at z = 0.525 .
Two major outbursts were observed in 2006–2007 and in 2012–2013 at optical and near-IR wavelengths , while in the high-frequency radio light curves prominent radio outbursts are visible peaking at the end of 2010 and beginning of 2013 , revealing a complex radio-optical correlation .
Cross-correlation analysis suggests a delay of the optical variations after the \gamma -ray ones of about a month , which is a peculiar behaviour in blazars .
We also analyse optical polarimetric and spectroscopic data .
The average polarization percentage P is less than 3 % , but it reaches \sim 19 \% during the early stage of the 2012–2013 outburst .
A vague correlation of P with brightness is observed .
There is no preferred electric vector polarisation angle and during the outburst the linear polarization vector shows wide rotations in both directions , suggesting a complex behaviour/structure of the jet and possible turbulence .
The analysis of 140 optical spectra acquired at the Steward Observatory reveals a strong Mg II broad emission line with an essentially stable flux of 6.2 \times 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } and a full width at half-maximum of 2053 km s ^ { -1 } .