Abstract
Abstract We report results from Chandra observations analyzed for evidence of variability and proper motion in the X-ray jet of Centaurus A. Using data spanning 15 yr, collective proper motion of 11.3 ± 3.3 mas yr−1, or 0.68 ± 0.20c, is detected for the fainter X-ray knots and other substructure present within the jet. The three brightest knots (AX1A, AX1C, and BX2) are found to be stationary to an upper limit of . Brightness variations up to 27% are detected for several X-ray knots in the jet. For the fading knots, BX2 and AX1C, the changes in spectral slope expected to accompany synchrotron cooling are not found, ruling it out and placing upper limits of ≃80 μG for each of their magnetic field strengths. Adiabatic expansion can account for the observed decreases in brightness. Constraints on models for the origin of the knots are established. Jet plasma overrunning an obstacle is favored as the generator of stationary knots, while moving knots are likely produced either by internal differences in jet speed or the late stages of jet interaction with nebular or cloud material.
Highlights
Kiloparsec-scale X-ray jets have been recognized as a hallmark of extragalactic radio sources for several decades (e.g., Feigelson et al 1981; Harris et al 1997; Turner et al 1997; Hardcastle et al 2001; Worrall et al 2001)
Recent and archival Chandra observations were analyzed for evidence of variability and proper motion in the X-ray jet of Centaurus A (Cen A)
Collective proper motion for the fainter substructure in the jet was measured by fitting the cross-correlation between epochs, finding a projected speed of 11.3 ± 3.3 mas yr−1 over the projected jet length 0.26–1.35 kpc from the active galactic nucleus (AGN)
Summary
Kiloparsec-scale X-ray jets have been recognized as a hallmark of extragalactic radio sources for several decades (e.g., Feigelson et al 1981; Harris et al 1997; Turner et al 1997; Hardcastle et al 2001; Worrall et al 2001). Hardcastle et al (2001) and Goodger et al (2010) detected radio proper motion for 3 of the 40 knots identified in the jet Those moving knots showed comparatively little X-ray emission, indicating that high-energy electron acceleration is less efficient in these structures than at those with zero apparent motion. Goodger et al (2010) put forward collisions with stationary objects as the most likely scenario for the origin of the majority of the knots in Cen A, and they proposed compressions in the fluid flow that do not result in particle acceleration to X-ray-emitting energies as a feasible explanation for the few moving, radio-only knots. The results are compared with predictions from various knot formation models
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