Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Variability and Proper Motion of X-ray Knots in the Jet of Centaurus A   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 years, collective proper motion of $11.3\pm3.3$ mas yr$^{-1}$, or
$0.68\pm0.20c$, 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 $0.10c$. 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 $\simeq 80\rm\
\mu G$ 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.

21
The categories of this paper are: adap-org
4
The categories of this paper are: cond-mat
35
The categories of this paper are: mtrl-th
0
The categories of this paper are: hep-ph
26
The categories of this paper are: bayes-an
7
The categories of this paper are: hep-th
14
The categories of this paper are: math-ph
34
The categories of this paper are: chem-ph
27
The categories of this paper are: comp-gas
36
The categories of this paper are: cmp-lg
15
The categories of this paper are: nucl-ex
22
The categories of this paper are: chao-dyn
13
The categories of this paper are: quant-ph
9
The categories of this paper are: nlin
32
The categories of this paper are: atom-ph
10
The categories of this paper are: q-bio