VLBI Imaging of a High Luminosity X-ray Hotspot

Leith Godfrey Research School of Astronomy & Astrophysics Australian National University

Geoff Bicknell, ANUJim Lovell, UTASDave Jauncey, ATNFDan Schwartz, Harvard-Smithsonian CfA

20GHz Australia Telescope Compact Array (ATCA)

BLRG at z = 0.6

PKS1421-490

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 HotspotPKS1421-490

BLRG Core

Counter lobe

Northern Hotspot

40 kpc

400 pc

• 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)

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]

= 0.640.1

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:

• Bssc ~ Beq = 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 ~ 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)

Mechanism for Producing min ~ 700 300

Consider transfer of jet energy to internal energy of hotspot plasma

Shock Front

JetHotspot

n2,e , n2,p

,w2

n1,e , n1,p

,w1

Relativistic Enthalpy Density

Rest mass energy density

+ pressureInternal 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 ~ 2 - 6

• Typical jet ~ 5 - 10– IC/CMB modeling of quasar X-ray jets– eg. Kataoka & Stawarz 2005

• => Typical p ~ 400 - 3000

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?

~ 2 - 3

400 pc

• 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.

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

(800) 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-vectorsB

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,