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

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

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

400 pc L 2-10keV  3  ergs s -1 –Comparable to luminosity of entire X-ray jet in PKS Peak I > 300 times Cygnus A hotspots 75% of total source flux 8GHz B eq ~ 3 mG – times greater than ‘typical’ B eq in bright hotspots (eg. Kataoka & Stawarz 2005) Most Luminous X-ray Hotspot Observed with Chandra

Major Results 1.Turnover in electron energy distribution 2.Mechanism for producing turnover –Thermalization of proton/electron jet?

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

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

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

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

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

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

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.

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

Several Estimates in the Range  min ~ 700  300  min ~ Cyg A (Carilli 1991; Lazio et al. 2006)  min ~ C196 (Hardcastle 2001)  min ~ PKS (this work)  min ~ C295 (Harris et al. 2000)  min ~ C123 (Hardcastle 2001)

Mechanism for Producing  min ~ 700  300

Consider transfer of jet energy to internal energy of hotspot plasma Shock Front JetHotspot n 2,e, n 2,p  ,  w 2 n 1,e, n 1,p  ,  w 1 Relativistic Enthalpy Density Rest mass energy density + pressure Internal energy density += Shock junction conditions give an expression relating the relativistic enthalpy density on each side of the shock

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

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  ~ Typical  jet ~ –IC/CMB modeling of quasar X-ray jets –eg. Kataoka & Stawarz 2005 => Typical  p ~ Assumed Form of EED

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.

Doppler Beamed Hotspot?  ~ pc Peak I > 300 times Cygnus A hotspots L X-ray > 10 times all other observed hotspots. Hotspot = 75% of total flux 8GHz B ~ B eq = 3 mG – times greater than ‘typical’ B in bright hotspots (eg. Kataoka & Stawarz 2005) Hotspot/counter-hotspot flux density ratio R hs ~300 at 20GHz.

Bright Backflow argues against Doppler beaming 400 pc 700 pc 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)?

B-field Alignment Argues Against Doppler Beaming B almost perpendicular (80 0 ) to jet direction –Shock is not highly oblique –Post-shock velocity cannot be highly relativistic ATCA 20GHz polarized intensity and E-vectors LBA 2.3GHz image E-vectors B

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