Radio-optical scrutiny of compact AGN: correlations between properties of pc-scale jets and optical nuclear emission

Abstract

Aims. We study the correlations between the VLBA (Very Long Baseline Array) radio emission at 15 GHz, extended emission at 151 MHz, and optical nuclear emission at 5100 Å for a complete sample of 135 compact jets. Methods. We use the partial Kendall's tau correlation analysis to check the link between radio properties of parsec-scale jets and optical nuclear luminosities of host active galactic nuclei (AGN). Results. We find a significant positive correlation for 99 quasars between optical nuclear luminosities and total radio (VLBA) luminosities of unresolved cores at 15 GHz originated at milliarcseconds scales. For 18 BL Lacs, the optical continuum emission correlates with the radio emission of the jet at 15 GHz. We suggest that the radio and optical emission are beamed and originate in the innermost part of the sub-parsec-scale jet in quasars. Analysis of the relation between the apparent speed of the jet and the optical nuclear luminosity at 5100 Å supports the relativistic beaming model for the optical emission generated in the jet, and allows the peak values of the intrinsic optical luminosity of the jet and its Lorentz factor to be estimated for the populations of quasars (2 × 10 20 W Hz −1 and γ = 52), BL Lacs (9 × 10 21 W Hz −1 and γ = 20), and radio galaxies (1.5 × 10 21 W Hz −1 and γ = 9). The radio-loudness of quasars (the ratio of 15 GHz flux density and optical nuclear flux at 5100 Å) is found to increase at high redshifts, which is interpreted as progressively higher Doppler factors in radio regime compared to those in optical. A strong positive correlation is found between the intrinsic kinetic power of the jet (measured from the flux density at 151 MHz) and the apparent luminosities of the total and the unresolved core emission of the jet at 15 GHz. This correlation is interpreted in terms of an intrinsically more luminous parsec-scale jet producing more luminous extended structure, which is detectable at low radio frequencies, 151 MHz. A possibility that the low frequency radio emission is relativistically beamed in superluminal sources and therefore correlates with radio luminosity of the jet at 15 GHz cannot be ruled out. Monitoring of superluminal AGN in a wide range of frequencies is required to check the contribution of each effect.

Radio-optical-gamma-ray properties of MOJAVE AGN detected byFermi/LAT

“…Arshakian et al (2010b) found a positive correlation in the L opt -L VLBA plane for AGN from the MOJAVE-1 sample. They concluded that the correlation is due to the population of quasars and that the optical emission is nonthermal and generated in the parsec-scale jet.…”

“…The sample comprises quasars, BL Lacs, radio galaxies and some unidentified sources having typically a core-jet structure on milliarcsecond scales. At high redshifts, the sample has a selection bias towards BL Lacs and quasars with fast and bright one-sided jets oriented closer to the line of sight, while at low redshifts (z < 0.1) there is a selection bias towards slow and intrinsically bright jets of radio galaxies (for more details see Lister & Homan 2005;Arshakian et al 2010b).…”

“…A procedure to correct the photometric fluxes for a stellar emission is discussed in Arshakian et al (2010b). Typical errors associated with optical nuclear fluxes are between 15% and 25%.…”

“…We use the optical nuclear fluxes at 5100 Å given in Arshakian et al (2010b) and the total radio flux densities at 15 GHz measured (during the period from 1994 to 2003 as part of the VLBA 15 GHz monitoring survey) at the epochs at which the unresolved component flux density has its maximum value. γ-ray luminosities were computed using Eq.…”

“…Long-term radio -optical monitoring of radio galaxies showed that a variable optical emission on scales from a few months to a few years is generated in the innermost 0.4 pc region of the radio jet (Arshakian et al 2010a;León-Tavares et al 2010). A significant positive correlation at milliarcseconds scales was reported between the optical nuclear emission and (i) the radio emission of the unresolved cores at 15 GHz in superluminal quasars, and (ii) the extended jet emission in BL Lac objects by Arshakian et al (2010b). Those results suggested that the radio and optical emissions are beamed and originate in the innermost part of the sub-parsec-scale jet in quasars.…”