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VLBI Imaging of a High Luminosity X-ray Hotspot

This research study conducted by Leith Godfrey and colleagues at the Australian National University focuses on the VLBI imaging of a high luminosity X-ray hotspot in the galaxy PKS1421-490. They investigate the electron energy distribution and the mechanism responsible for producing a low-frequency flattening in the hotspot radio spectrum.

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VLBI Imaging of a High Luminosity X-ray Hotspot

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  1. VLBI Imaging of a High Luminosity X-ray Hotspot Leith Godfrey Research School of Astronomy & Astrophysics Australian National University Geoff Bicknell, ANU Jim Lovell, UTAS Dave Jauncey, ATNF Dan Schwartz, Harvard-Smithsonian CfA

  2. Northern Hotspot PKS1421-490 Jet +Lobe BLRG at z = 0.6 40 kpc BLRG Core Counter lobe 20GHz Australia Telescope Compact Array (ATCA)

  3. VLBI Scale Hotspot PKS1421-490 400 pc Northern Hotspot 40 kpc BLRG Core 2.3GHz Long Baseline Array (LBA) Counter lobe 20GHz Australia Telescope Compact Array (ATCA)

  4. 400 pc Most Luminous X-ray Hotspot Observed with Chandra • L2-10keV  3  1044 ergs s-1 • Comparable to luminosity of entire X-ray jet in PKS0637-752 • Peak I > 300 times Cygnus A hotspots • 75% of total source flux density @ 8GHz • Beq ~ 3 mG • 5 - 10 times greater than ‘typical’ Beq in bright hotspots (eg. Kataoka & Stawarz 2005)

  5. Major Results • Turnover in electron energy distribution • Mechanism for producing turnover • Thermalization of proton/electron jet?

  6. First we consider the spectrum of the entire radio galaxy: 400 MHz - 90 GHz Northern Hotspot Jet +Lobe BLRG Core (negligible flux) Counter lobe

  7. Whole Source Spectrum (F-  = 0.35  Unusually flat! (Fermi acceleration  > 0.5)  ~ 6 GHz

  8. VLBI Flux Densities Indicate Flattening in Hotspot Radio Spectrum  = 0.35  (extrapolation => hotspot spectrum must be steeper at higher ) 

  9. Is the Flattening Consistent with a Cutoff in the Hotspot Electron Energy Distribution? YES! Total source spectrum = HS + Non-HS HS = Hotspot model Spectrum (Power-law electron distribution with low energy cut-off) Non-HS = Non-hotspot model spectrum (lobe & jet)  [Hz]

  10. Are the VLBI Hotspot Flux Densities Consistent with this Cutoff? YES! Total source spectrum = HS + Non-HS HS = Hotspot model Spectrum (Power-law electron distribution with low energy cut-off) VLBI Hotspot Flux Densities Non-HS = Non-hotspot model spectrum (lobe & jet)  [Hz]

  11. Model Parameter Values min = 3  1 GHz Hotspot model Spectrum (Power-law electron distribution with low energy cut-off)  = 0.53  0.05  = 0.640.1  [Hz]

  12. Is Absorption Responsible for the Low-Frequency Flattening? • Synchrotron Self Absorption? • Requires B ~ 20 G • 4 orders of magnitude greater than minimum energy field! • Free-Free Absorption? • Requires unrealistically high cloud density for plausible cloud size and temperature.

  13. Modeling Hotspot X-ray Emission • One-zone synchrotron self Compton model. • Broken power-law electron energy distribution between min and max Parameters to note: • Bssc ~ Beq = 3  1 mG • min = 650  200 Infra-red, optical and X-ray points taken from Gelbord et al. (2005)

  14. Several Estimates in the Range min ~ 700  300 • min ~ 400 - Cyg A (Carilli 1991; Lazio et al. 2006) • min ~ 500 - 3C196 (Hardcastle 2001) • min ~ 650 - PKS1421-490 (this work) • min ~ 800 - 3C295 (Harris et al. 2000) • min ~1000 - 3C123 (Hardcastle 2001)

  15. Mechanism for Producing min ~ 700  300

  16. Consider transfer of jet energy to internal energy of hotspot plasma Shock Front n2,e , n2,p ,w2 n1,e , n1,p ,w1 Hotspot Jet Shock junction conditions give an expression relating the relativistic enthalpy density on each side of the shock Relativistic Enthalpy Density Internal energy density Rest mass energy density + pressure = +

  17. Model Assumptions Assume jet enthalpy density is dominated by rest mass energy of protons Jet: electrons = relativistic gas Hotspot: protons = ideal gas Protons and electrons equilibrate

  18. Assumed Form of EED Peak Lorentz factor from thermalization of electron/proton jet • av/p • Assume particular EED to calculate  •  ~ 0.75 ln(max/min) • For a = 2 • Typical  ~ 2 - 6 • Typical jet ~ 5 - 10 • IC/CMB modeling of quasar X-ray jets • eg. Kataoka & Stawarz 2005 • => Typical p ~ 400 - 3000

  19. Summary • We find min ~ 600 in a high luminosity hotspot. • Several current estimates of min in hotspots are distributed in the range min ~ 700  300. • This may arise naturally from thermalization of electron/proton jets if bulk Lorentz factors are of order jet ~ 5.

  20. 400 pc Doppler Beamed Hotspot? • Peak I > 300 times Cygnus A hotspots • LX-ray > 10 times all other observed hotspots. • Hotspot = 75% of total flux density @ 8GHz • B ~ Beq = 3 mG • 5 - 10 times greater than ‘typical’ B in bright hotspots (eg. Kataoka & Stawarz 2005) • Hotspot/counter-hotspot flux density ratio Rhs~300 at 20GHz.  ~ 2 - 3

  21. Bright Backflow argues against Doppler beaming • Backflow flux density in LBA image is 5 times that of whole counter lobe • Can’t appeal to Doppler beaming for backflow Turbulent backflow (Cocoon)? 400 pc 700 pc

  22. LBA 2.3GHz image B-field Alignment Argues Against Doppler Beaming • B almost perpendicular (800) to jet direction • Shock is not highly oblique • Post-shock velocity cannot be highly relativistic B E-vectors ATCA 20GHz polarized intensity and E-vectors

  23. Model Assumptions Jet: Assume enthalpy density is dominated by rest mass energy of protons Hotspot: electrons = relativistic gas protons = ideal gas Protons and electrons equilibrate Hence,

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