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Spectra of partially self-absorbed jets

Spectra of partially self-absorbed jets. Christian Kaiser University of Southampton. Overview. Blandford-Königl (BK) model Energy losses and gains of electrons Model spectra with losses and gains Comparison with the VLBA jet of Cygnus X-1 Future observational diagnostics.

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Spectra of partially self-absorbed jets

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  1. Spectra of partially self-absorbed jets Christian Kaiser University of Southampton

  2. Overview • Blandford-Königl (BK) model • Energy losses and gains of electrons • Model spectra with losses and gains • Comparison with the VLBA jet of Cygnus X-1 • Future observational diagnostics

  3. Blandford & Königl (1979) • THE model for flat radio spectra with extreme surface brightness temperature. • Flat spectra: • 718 citations since publication (2.3 per month!) • ONLY applicable for jets at large angle to line of sight!

  4. The basics • Magnetised plasma with electrons with an energy distribution of: • Peaked spectrum. Absorbed: Optically thin:

  5. The basics • Need to adjust jet properties to get the peaks ‘right’. • Important ingredients: • Structure of magnetic field • Energy evolution of electrons • In BK model: • B-field perpendicular to jet • No energy losses of electrons

  6. No energy losses? “We assume that relativistic electrons can be accelerated continuously within the jet,…” “There must […] be ongoing particle acceleration to compensate for the cooling associated with adiabatic decompression…” Hmmm…

  7. Energy distributions with radiative losses • Synchrotron emission leads to a high-energy cut-off • Self-absorption mitigates the losses (somewhat). McCray (1969)

  8. Radiative losses and gains • Radiative losses halted for electrons with Lorentz factors where the optical depth This does not affect adiabatic losses!

  9. Two models • Ballistic jet: • Free expansion, conical shape • Only radiative losses • Limiting case • Adiabatic jet: • Confined by external medium so that • Both adiabatic and radiative losses

  10. Model spectra

  11. Model spectra • Of course, still get optically thin/thick regions at extremes • Energy losses can steepen optically thin spectrum • …or lead to peaks at high frequencies Ballistic jet Adiabatic jet

  12. Comparing with observations • VLBA jet of Cygnus X-1 • Can measure flux and extent at one frequency • NO information on second frequency • NO information on high frequency cut-off Stirling et al. (2001)

  13. Comparing with observations • Both models can be made to fit, but… • Adiabatic model way out of equipartition (106 times more energy in magnetic field) • VERY thin jets (opening angle ~5”) • Problem: long extent of observed jet needs • High optical density far out • Strong magnetic field

  14. Future observational diagnostics • Jet extent at two frequencies (simultaneous) Factor 2 in observing frequency

  15. Future observational diagnostics • High-frequency cut-off probes close to black hole • In Cygnus X-1 example, down to 5 RS in infrared

  16. Summary • Even with radiative and adiabatic losses self-absorbed jets produce flat spectra • No need for mysterious re-acceleration • Finding the high frequency cut-off will probe very close to black hole • BUT: Narrow jets may tell us of the need for more physics?

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