The Radio Properties For BL Lac Object S5 2007+777

The object S5 2007+777 is a classic radio-selected BL Lac object with a kpc-scale extended X-ray jet.The Doppler factor derived from X-ray emission in literatures is around 13.0,then the intrinsic jet scale will be around 1 Mpc.In this work,we present the jet structure,brightness temperature,proper motion and other radio properties of this source by collecting EVN data and VLBA 2cm survey archival data.We found that all EVN and VLBA images show a core-jet structure,and the jet direction is similar in all VLBI images,but slightly different with kpc-scale jet.The Doppler factor constrained from the brightness temperature of radio core is around 5.0,smaller than the value estimated from X-ray emission,however consistent with those derived with other methods at radio band.After considering all VLBI data,we found that there is no significant proper motion in the source.These results indicate that the Doppler factor of this source may not be very high.However,even with the estimated Doppler factor of 5.0,the jet linear size may still be large,around 0.5 Mpc.This implies that this source is probably possessing a very large intrinsic jet,similar to giant radio galaxies.As far as we know,there are only five BL Lac objects with X-ray jets of kpc scale up to now.Among them,we mainly studied the properties of S5 2007+777 and compared S5 2007+777 with OJ287,and found that the two objects have some similar properties.