The jet apparent motion and central engine study of Fermi blazars

Abstract

ABSTRACT The study of blazar jet has been performed for several decades via VLBI technique, while its generation and propagation stay unclear. In the present work, we compiled a sample of 407 VLBI detected Fermi blazars (VFBs) and studied the correlations between apparent velocity (${\rm log}\, \beta _{\rm app}$) and jet/accretion disc properties. We found a positive correlation between γ-ray luminosity (${\rm log}\, L_{\rm \gamma }$) and ${\rm log}\, \beta _{\rm app}$; the correlation suggests that the apparent motion of jet knot is related to the jet power. The correlations between ${\rm log}\, \beta ^{\rm max}_{\rm app}$ and the jet radiation power (${\rm log}\, L_{\rm rad}$), and between ${\rm log}\, \beta ^{\rm max}_{\rm app}$ and the jet extended region luminosity at 5 GHz (${\rm log}\, L_{\rm 5GHz}^{\rm ext}$), which is an indicator of jet kinetic power, reveal that the knots apparent motion is correlated with both jet radiation power and the kinetic power. But this indication is not held for flat-spectrum radio quasar (FSRQs) in terms of the correlation ${\rm log}\, L_{\rm 5GHz}^{\rm ext}\, versus \, {\rm log}\, \beta ^{\rm max}_{\rm app}$. Besides, ${\rm log}\, \beta ^{\rm max}_{\rm app}$ has a moderate correlation with accretion disc luminosity (${\rm log}\, L_{\rm Disk}$) and the normalized accretion disc luminosity ${\rm log}\, (L_{\rm Disk}/L_{\rm Edd})$, which may suggest both the power of accretion disc and the accretion rate are critical to generate knots and to accelerate them. In addition, we found the VFBs have larger average values of γ-ray luminosity (${\rm log}\, L_{\rm \gamma }$), γ-ray photon index (αph), and variability index (${\rm log}\, VI$) than the rest of Fermi blazars. Through Gaussian mixture models method, we generated a criteria, ${\rm log}\, L_{\rm \gamma } \gt 45.40$, αph > 2.24, and ${\rm log}\, VI \gt 1.71$ to find VFB candidates, selected 228 VFB candidates from the rest of Fermi blazar.