Variability in the Inverse‐Compton X‐Ray Flux from the Jet in Quasar 3C 345

Abstract

We present the results of the -rst systematic study of correlated variability in the X-ray emission and the parsec-scale radio structure of the superluminal II quasar 3C 345. This quasar is one of a class of core-dominated Nat-spectrum radio sources that are believed to emit X-rays via the synchrotron self- Compton process. By studying its variability in X-rays, we can test this hypothesis for 3C 345 by model- ing the expected inverse-Compton Nux from the parsec-scale jet, using parameters derived from multifrequency VLBI imaging. Since the predicted X-ray Nux is very sensitive to the physical parameters of the compact radio-emitting regions, a study of variability, with quasi-simultaneous X-ray and VLBI data, is of particular interest because it is less sensitive to assumptions in the adopted model. The soft X-ray Nux density of 3C 345 was observed by the ROSAT PSPC instrument at seven epochs during 1990E1993, during which time its Nux density varied by a factor of 2, but with no change in spectral index. The X-ray points closely track the high-frequency radio Nux light curve. Using a series of VLBI images, we followed the time evolution of spectral shapes and angular sizes of the nucleus and the strongest jet component C7,II at a distance of ^0.5 mas (2 pc) from the nucleus, and derived physical parameters for the components. We -nd that component C7, not the nucleus, is the dominant generator of the observed X-rays during 1992E1993. For the nucleus, we applied the inhomogeneous-jet model of Koe nigl and found -nd that it underpredicts the X-ray Nux for any plausible combination of physical parameters derived from observation. A homogeneous sphere, with Nux density peaking at a few GHz, is adequate for modeling the evolution of C7. This sphere model requires that C7 dominated the X-ray emission, unless its Doppler factor is ?10. This contrasts with the situation at epoch 1990.55, for which comparable data (Unwin et al. 1994) showed that the nucleus and C5 (the dominant jet feature at that time) both contributed to the X-ray emission. We discuss the lack of detectable c-rays from 3C 345 in relation to other c-ray loud blazars with which it shares many observational properties. Combining the superluminal speed (from VLBI) and Doppler factor deduced from the synchrotron self-Compton calcu- lation, we solve for the jet kinematics at the position of C7, and we -nd that the jet bends away from the line of sight (from h ^ 2i to ^10i) and accelerates from c ^ 5 to over the range of (deprojected)