A Radio and X-ray Study of Particle Acceleration in Centaurus A’s Jet

1. A Radio and X-ray Study of Particle Acceleration in Centaurus A’s Jet Joanna Goodger University of Hertfordshire Supervisors: Martin Hardcastle and Judith Croston Collaborators: Kraft, Jordan, Juett, Evans, Forman, Sarazin, Sivakoff, Birkinshaw, Worrall, Brassington, Harris, Jones, Murray, Raychaudhury, Woodley

2. Overview • Introduction to Centaurus A and its jet • Previous X-ray results • Radio results • Current X-ray work • What next… 1` 1.1kpc

3. Centaurus A’s Jet X-ray emission from the knots and diffuse emission indicate regions of current particle acceleration Coincident radio emission tells us about the structure, dynamics and evolution Previous work has detected motions in some but not all of the radio knots prompting monitoring in the radio and X-ray 1kpc The presence of stationary and moving knots could indicate both local shocks and compressions in the fluid flow. Studying the variability in simultaneous radio and X-ray observations gives us insights into the nature of these knots.

4. Particle Acceleration Hardcastle et al. 2007, ApJ 670, L81 q < 60” Flat aX consistent with shock acceleration coupled with rapid downstream expansion losses while aRX > aX suggests that particle acceleration is localized to small sub-regions of the jet. S

5. Particle Acceleration Hardcastle et al. 2007, ApJ 670, L81 60”< q < 190” Emission is dominated by the extended filamentary emission, consistent with a diffuse acceleration mechanism. (Kraft et al. 2006, ApJ 641, 158) S

6. Particle Acceleration Hardcastle et al. 2007, ApJ 670, L81 q > 190” X-ray spectral steepening not synchrotron losses due to spatial scales being too large; believed to be a progressive loss of efficiency in high energy particle acceleration. S 1 kpc Several Regimes of Particle Acceleration along the jet

7. Particle Acceleration Worrall et al. 2008, ApJ 670, L81 A significant difference in inner and outer radial region knots (below, left and right) has prompted further investigation into a spectral index dependence on jet radial position. NH (1022) cm-2 Photon Index (a+1)

8. 1991 2002 2003 2004 2006 2007 Radio Data Six epochs of VLA data showing clear motion of knots and diffuse material downstream in the jet. Proposal submitted recently for another epoch in early 2009. Multi-frequency radio data quasi-simultaneous to Chandra Very Large Project taken in the Summer of 2007

9. Compact Knots A6A A5A Compact Knots: Some knots move along the jet (A1B) with velocities of ~0.6c Others appear stationary (A1A, A2A etc). Some increase in flux with time (SJ1), however there is lower level variability in other knots A2A A4 A1E A2B A1B A1A SJ1 A3B SJ2 A3A A1D SJ3 A1C

10. Radio Proper Motions A1B - Moving A1A - Stationary These are examples of a stationary knot (left) and a moving knot (right). The observed motions are consistent with those previous published in Hardcastle et al. 2003.

11. Radio Variability SJ1: Stationary, but shows strong variability, increasing in flux by a factor of two in the last 4 epochs. SJ1

12. Radio Variability Other knots show less dramatic changes increasing or decreasing by 10-20% over 15 years. A1B A1C A2A A1A

13. Polarization Polarization vectors for the group A knots from 2002

14. Polarization A2 Group A1 Group Q U To determine if the polarization varies over the monitoring program, we constructed residuals of the Q and U stokes parameters (2007 – 2002) In the A2 group, the vectors appear to swing from positive (black) to negative (white) at opposite ends of the group; either due to motion, real change in polarisation or variability in the flux of the knot.

15. Polarization A1E Total Intensity Q/U - Constant Total Intensity - Increasing Fractional Polarization Intensity - Constant Q/U Ratio change in fractional polarization with constant position angle. Fractional Pol.

16. Polarization A2B Q/U Total Intensity - Constant Total Intensity - Constant Fractional Polarization Intensity - Increasing Q/U Ratio change position angle with constant fractional polarization. Fractional Pol.

17. Polarization A2D Q/U Total Intensity - Constant Total Intensity - Constant Fractional Polarization Intensity - Varying Q/U ratio changing position angle and constant fractional polarization. Fractional Pol.

18. X-ray knots F group G group C group A group B group EX1 and EX2 Counterjet knots

19. X-ray Counterparts 11/40 X-ray knots have radio counterparts 11/20 radio knots have X-ray counterparts

20. X-ray Variability 2004 2007 The lack of radio counterpart and its proximity to the edge of the jet suggest that AX2a is a coincidently positioned LMXB

21. X-ray Variability SX1 SX1’s flat spectrum and lack of radio counterpart mean we cannot rule it out as a LMXB unrelated to the jet.

22. X-ray Variability CX3 gradually increases in flux from 2000, being undetected in the 1999 observation. Unfortunately, we have not detected a radio counterpart. This is also one of the less compact X-ray knots.

23. X-ray Variability A1A A1A X-ray variability is in sync with the observed radio variability, increasing gradually on long time scales.

24. Summary • Long term variability (eg. A1A) • Consistent, in sync, changes in the flux in both X-ray and radio suggest changes in the magnetic field strength and/or particle density • No changes in particle acceleration as this would produce stronger X-ray variability. • Combined with non-movement in the radio, this is consistent with a model where these stationary knots are local standing shocks. • Short term variability (eg. CX3 and SX1) • Flares in X-ray or radio on timescales shorter than the light crossing time, suggest change in unresolved sub-regions or beaming. • Changes in the polarization would also fit this picture. • No radio flaring of this type has been observed, and no flaring X-ray knot has a radio counterpart.

25. Future Work To investigate the properties of the inner jet further we plan to • Refine our method for measuring the proper motions of the radio knots • Determine the broadband spectra, using our multi-frequency radio data, of the knots and diffuse material in the jet • Further investigate the spectral properties of the knots as a function of their position in the jet • Investigate the spectral properties of the narrow innermost jet in radio and X-ray • Search for optical and infra-red counterpart to the knots • (Get my PhD)