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,