Variability and Proper Motion of X-ray Knots in the Jet of Centaurus A

Accepted to ApJ, 14 pages, 8 figures, 2 tablesWe 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.Peer reviewe

A 1D fluid model of the Centaurus A jet

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. Final published version available at: https://doi.org/10.1093/mnras/stz348.We implement a steady, one-dimensional flow model for the X-ray jet of Centaurus A in which entrainment of stellar mass loss is the primary cause of dissipation. Using over 260 ks of new and archival Chandra/ACIS data, we have constrained the temperature, density and pressure distributions of gas in the central regions of the host galaxy of Centaurus A, and so the pressure throughout the length of its jet. The model is constrained by the observed profiles of pressure and jet width, and conserves matter and energy, enabling us to estimate jet velocities, and hence all the other flow properties. Invoking realistic stellar populations within the jet, we find that the increase in its momentum flux exceeds the net pressure force on the jet unless only about one half of the total stellar mass loss is entrained. For self-consistent models, the bulk speed only falls modestly, from ~0.67c to ~0.52c over the range of 0.25-5.94 kpc from the nucleus. The sonic Mach number varies between ~5.3 and 3.6 over this range.Peer reviewe

Spectral Analysis of X-Ray Emission Mechanisms in Local Astrophysical Environments

In order to establish contributions of different X-ray emission mechanisms from local astrophysical environments, we perform a theoretical analysis of observed cometary X-ray emission spectra. We develop a model from first principles that generates updated spectra of solar wind charge-exchange (CX) emissions together with accurate scattering and fluorescence spectra of solar X-rays by atoms, molecules, and dust/ice particles. This model also explores scattering and fluorescence spectra for different solar conditions, including spectra induced by solar X-ray flares of different classes and durations. We compare the modeled spectra with cometary and planetary observations from the Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer (ACIS) and determine the primary emission mechanisms for both the 0.3–1.0 keV and 1.0–2.0 keV photon energy ranges. These comparisons establish upper limits on cometary dust/ice mass production rates and grain size distributions. Our results also demonstrate the utility of charge-exchange emissions as a remote diagnostics tool of both astrophysical plasma interaction and solar wind composition. In addition, we observe potential soft X-ray emissions via ACIS around 0.2 keV that are correlated in intensity to the hard X-ray emissions between 0.4-1.0 keV. We fit our CX model to these emissions, but our lack of a unique solution at low energies makes it impossible to conclude if they are cometary CX in origin. Finally, we discuss probable emission mechanism sources for these soft X-rays and explore new opportunities these findings present in understanding emission processes via Chandra

The Cocoon Shocks of Cygnus A: Pressures and Their Implications for the Jets and Lobes

We use 2.0 Msec of Chandra observations to investigate the cocoon shocks of Cygnus A and some implications for its lobes and jet. Measured shock Mach numbers vary in the range 1.18–1.66 around the cocoon. We estimate a total outburst energy of $\simeq 4.7\times {10}^{60}\,\mathrm{erg}$, with an age of $\simeq 2\times {10}^{7}\,\mathrm{years}$. The average postshock pressure is found to be $8.6\pm 0.3\times {10}^{-10}\,\mathrm{erg}\,{\mathrm{cm}}^{-3}$, which agrees with the average pressure of the thin rim of compressed gas between the radio lobes and shocks, as determined from X-ray spectra. However, average rim pressures are found to be lower in the western lobe than in the eastern lobe by sime20%. Pressure estimates for hotspots A and D from synchrotron self-Compton models imply that each jet exerts a ram pressure gsim3 times its static pressure, consistent with the positions of the hotspots moving about on the cocoon shock over time. A steady, one-dimensional flow model is used to estimate jet properties, finding mildly relativistic flow speeds within the allowed parameter range. Models in which the jet carries a negligible flux of rest mass are consistent with the observed properties of the jets and hotspots. This favors the jets being light, implying that the kinetic power and momentum flux are carried primarily by the internal energy of the jet plasma rather than by its rest mass