the biggest accelerators in space and on earth jets and acceleration mark birkinshaw university of...

The biggest accelerators in space and on Earth

Jets and acceleration

Mark BirkinshawUniversity of Bristol

19 March 2013 Mark Birkinshaw, U. Bristol 2


Five questions on acceleration in jets

1. Where is acceleration occurring?• Multi-wavelength imaging – locations of radiating particles

2. What acceleration is occurring?• Spectral energy distributions – energy spectra of radiating particles

3. How is acceleration occurring?• Multi-wavelength polarimetry – configurations of fields• Multi-wavelength variability – timescales of radiating population changes

4. How efficiently is acceleration occurring?• Energetics

5. Is the radiating population the majority population in jets?• Polarimetry, dynamics

Handicap: evidence of acceleration is only in e+/e- (one possible exception)



Centaurus ALow-power source

Radio: small-scale jet, knot motions

Infra-red: jet and dust

Optical: too absorbed

X-ray: fine-scale structure, bright core

γ-ray: to Eγ > 100 GeV

UHECR: above 1018 eV (possibly)

Combi & Romero (1997)



Centaurus A

Well-defined jet to NE in radio and X-ray

Bright inner lobes, bounded by X-ray sheath to SW

Radio is synchrotron radiation; X-rays from jet and sheath also synchrotron (Croston et al.)

Emitting electrons: loss times ~ 105 years for radio-emitting electrons, ~ 10 years for X-ray emitters.

Kraft et al. (2003)



Centaurus A

Worrall et al. (2008)

Well-defined jet to NE in radio and X-ray, only weak X-ray spectral variations

Emitting electrons: loss times ~ 105 years for radio, ~ 10 years for X-rays

Therefore extensive local acceleration both at bright knots and in diffuse region, to γ > 107 in the nT-scale fields

X-rays not edge-brightened in most places, so not acceleration in shear layer

Internal turbulence driving acceleration? Expect declining turbulent power density down jet.



Centaurus A100 GeV γ-rays from centre and lobes (Fermi).

TeV γ-rays from core/inner jet (HESS).

iC from electrons with γ ~ 104 in lobes (iC faster than synchrotron, since B ~ 0.1 nT).

HESS emission could be from core, jet, or inner lobes. Abdo et al. 2010

Aharonian et al. 2010



Lessons from Centaurus A • Radio knot motions at speeds a few × 0.1c – kinetic energy

source for acceleration; low-prominence knots further out where jet should be slower

• Inferred (minimum energy) fields in knots and sheath ~ 1 nT• Inferred electron γ ~ 103 to 107. γmin not known.• Local electron acceleration to TeV energies• X-ray/radio offsets – multiple particle acceleration sites• Different knot properties, different motions – related to nature of

particle acceleration?• Γ rays = SSC from cores? Highest required γ ~ 108 only.



M 87, radio and X-ray3C 274 = M 87

Chandra 0.5-2.5 keV greyscale.

VLA P-band contours.~ 4 arcsec resolution. Radio size ≈ 60 kpc.

Residual read-out streak.



M 87, X-rays3C 274 = M 87

Chandra 0.5-2.5 keV; centre

Non-thermal contains strong jet component

Obvious radio jet/X-ray gas relationship



M87• High variability of HST-1

• Relativistic internal motions

• Polarization/intensity correlations implying a sheared flow

• Steep power-law spectra of brightest X-ray peaks (synchrotron)

• Break frequencies drop with distance from core

• γmax ~ 107 or more in knots VLA, HST, Chandra, Chandra + smoothed HST; Marshall et al. (2002)



M 87, HST-1• 80 pc from core (projected; = 1 arcsec)• Flaring in radio, optical, X-ray• Superluminal subcomponents (4c; Cheung et al. 2007);

collimated shock in M87 jet• Related to TeV emission? (Aharonian et al. 2006; HESS

detection); light curve peaked in 2005; resolved core not varying with TeV flare

• Second TeV flare in 2008 (Acciari et al., 2009), detected by Fermi (Abdo et al. 2009)

• Chang et al. find HST-1 to be optically thin, brightest region moving at 0.6c, mostly resolved on 0.1pc scales.

• Like whole jet, over-pressured relative to adjacent X-ray medium, even at minimum energy



M 87 HST-1, VHE γ-ray, X-ray

Chandra stacked image; light curve.

HST-1 brighter than core over about 3 years.

No HST-1 flare with 2008 flare in VHE gamma-rays (Acciari et al. 2008).

Acceleration to γ ~ 106 in HST-1, no evidence of higher γ.

Harris et al. 2006

image



M87, HST-1VLBI structure

Chang et al. (2010) mapping of HST-1 finds outward motion at about 0.6c, components optically thin and about 0.1pc in size, plus much extended emission which appears not to be parallel to the jet direction.

No polarization data.

Not compact enough for gamma rays – likely γ rays from core.



M87, inner jet• VLBI for M 87, M 84

resolves scales of order 100 Rs and may show base of collimation zone.

• M 87 shows edge-brightening of inner jet, which may be common.

• Counter-jet brightness ratio gives v ~ 0.5c, plausible alignment

Averaged multi-epoch image (Ly et al., 2007)

Wide opening angle (white lines) from core, still some ambiguity in core location.



M87, polarization3C 274 polarization from Perlman et al. (1999).

Low polarization at core in radio, high in optical.

HST-1 polarization transverse.

D-east patterns differ.

Magnetic field mostly parallel to jet, except in (some) knots. Fractional polarization drops in knot peaks in optical. Shock + shear model. Owen et al. (1999)

Apparent magnetic field directions.



Lessons from M87 • Knot motions at up to c, relativistic internal motions in knots • Variability (e.g., HST-1) consistent with synchrotron outburst in

moderately relativistic flow• No obvious counter-jet, or counterjet HST-1. What triggered HST-

1?• VLBI inner structure has edge brightening (e.g., Ly et al. 2007),

possible signature of shear acceleration? Or B amplification?• Radio and X-ray structure suggests convective plumes lifting core

material, so slow entrainment happens.• Transverse field in knots suggests shock compression, and hence

good sites for first-order Fermi acceleration• Acceleration is also needed between the major knots: turbulence?

Shocklets? Reconnections? If shear acceleration, then sharpest velocity gradients migrate towards centre of jet.



Some low-power jets3C 31

Residual R map, after subtracting E galaxy profile. 11 Jy feature to N is counterpart of the brighter radio jet. Core structure from AGN and disk.

Croston et al.

More convincing in Spitzer 8 m data

Bliss et al.



Some low-power jetsNGC 6251

SpitzerNo obvious dustRadio jet detected in IR for the first time.

IR colour: red = 8 m, green = 4.5 m, blue = 3.6 m



Some low-power jets

3C 66B; radio, IR, optical, X-ray jets; jet peak offsets and different brightness gradients (Hardcastle et al., Tansley et al.)

10 kpc



Some low-power jets3C 66B

Optical polarization (~ 30%). Synchrotron emission with significant magnetic order.

Diffuse and knot-related X-rays. Short radiative lifetimes of electrons; efficient acceleration to γ ~ 105 even outside knots.

Stokes I, % polarization, outer/inner apparent B vectors; Perlman et al. 2006



Some low-power jetsMany jet spectra are similar: here M 87 and 3C 66B.

Break frequencies in IR or optical. Using equipartition fields, break energies in the 300 GeV - 1 TeV range.

Spectral break by > 0.5, indicative of acceleration physics/jet dynamics interaction?



Lessons from low-power jets

• Knot magnetic fields ~ 10 nT common.• Electrons at spectral breaks have E 300 GeV.• Lifetimes of electrons emitting synchrotron X-rays ~ 30 years in

knots, so spectra are from locally-accelerated particles.• Synchrotron spectra of jets are very similar between different

sources, with breaks > 0.5, not synchrotron ageing• X-ray spectra steeper than radio spectra – not inverse-Compton

radiation.• Synchrotron jets, close to equipartition.• Still unclear if SEDs of emission between the knots are the same

as emission in the knots.



PKS 2152-699Higher-power source.Overall view from Worrall et al. (2012): the radio lobes are expanding into, and shocking, a thermal atmosphere. The jet kinks around a gas cloud a few kpc from the core (without being disrupted), and continues to hot-spots embedded in the lobes. Jet knot fields ~ 20 nT.The N lobe is tilted towards us, with inclination about 10º.



PKS 2152-699

S hotspot: radio (left; intensity and fractional polarization E); X-ray (centre); ground-based optical (right). The radio, optical, and X-ray emissions are coincident and consistent with a broken power-law spectrum, which below about 1015 Hz has a spectral index of 0.7 (Worrall et al. 2012).



PKS 2152-699

N hotspot: radio (left; intensity and fractional polarization E); X-ray (centre); ground-based optical (right). The optical emission is concentrated on the SW edge of the complex (towards the core). Some X-rays come from this region – the acceleration zone with synchrotron emission?



PKS 2152-699

Strongest radio emission from the leading edge of the hotspot, where strong polarization indicates field compression. Optical emission from this edge is probably synchrotron.The X-rays and optical emission to the SW could also have an inverse-Compton origin in the Georganopoulos & Kazanas (2003) model. Hotspot fields ~ 20 nT, similar to the jet knots. (Worrall et al. 2012)



PKS 0637-752

Quasar at z = 0.651; 1 arcsec = 7 kpcGodfrey et al. (2012). Quasi-periodic oscillations out to major bend. Knot structure suggests some sort of instability/oscillation (reconfinement shocks?) causing regular spacing.



PKS 0637-752

High-power, one-sided jet, quasar at z = 0.651; 1 arcsec = 7 kpc.

First X-ray jet detected by Chandra.

X-ray/radio ratio “fairly” constant out to major bend.

Radio is synchrotron emission, B ~ 10 nT

iC/CMB explanation for X-rays: jet relativistic to 50+ kpc (VLBI Γ ~ 18)

Chandra, HST, ATCA; Lovell et al. 2003



PKS 0920-397

1 arcsec = 6.5 kpcX-rays from inverse-Compton radiation, scattering the boosted CMB. Varying radio/X-ray ratios down jet imply multiple velocity components in jet, or varying jet orientations to the line of sight. (Marshall et al. 2013)



3C 2793C 279 (z = 0.536, FSRQ). VLBA: circular polarization (contours) on total intensity grey-scale.Peak circular polarization +0.8%.Sign of polarization changes at 7 mm (Vitrishchak et al., 2008), suggesting Faraday conversion in an inhomogeneous core (structured inner jet)?Jet circular polarization known in about 10 AGN (e.g., Homan & Lister 2006).

Vitrishchak et al. (2008)



3C 279: circular polarization3C 279 (Homan et al. 1998; Wardle et al. 1998)• Little effect from surrounding medium on circular polarization –

it originates – from the synchrotron emission itself (very ordered field, electron-proton

plasma), or– by internal conversion from linear polarization via changing

perpendicular B fields, or internal Faraday rotation (any plasma)• V < 1% in cores, a few % at jet edges, in the detected objects• 3C 279’s circular polarization originates in optically-thick parts

of source by Faraday conversion• Circular polarization tends to increase with increasing frequency

– source inhomogeneity (Vitrishchak et al. 2008)• Plausible Faraday conversion models exist (Homan et al. 2009),

but put less pressure on γmin than first thought.



Lessons from quasar jets• Emission not single-component synchrotron, since spectra too

complicated, often with “notch” in the optical• Emission not SSC if system near equipartition (Chartas et al.

2000)• iC emission/CMB is possible, but issues

– X-ray decreasing/radio increasing down many jets– Knot/inter-knot contrast is higher than expected in X-ray (should be less

than in radio; combination of expansion and ageing?)– Sources have huge sizes if beamed– Why no entrainment and slowing changing the properties?– Why no big infra-red bump from iC of cold electrons?

• Polarization measurements of high-energy component (optical in some SEDs) would resolve issue

• Circular polarization may give composition



3C 15, optical/radio3C 15 at 3 cm and 606 nm.

Comparison of radio polarimetry (left, on X-ray image) and optical polarimetry (right).

Vectors of apparent magnetic field similar, but do differ – colour scales show ratio of percentage polarizations (left) and position angle difference (right).

Dulwich et al. 2007



3C 15, optical/radio3C 15 polarization

Can be interpreted as spine/sheath structure with different properties in two regions. Cylindrical symmetry must be broken to get fields other than transverse and parallel.

Toy model shown gives one (of many) possibilities.

Dulwich et al. 2007



3C 346, optical/radio3C 346 at 1.4 cm and 606 nm.

Comparison of radio polarimetry (bottom) and optical polarimetry (top).

E vectors (rotated 90º to show apparent magnetic field direction) differ – colour scales show ratio of percentage polarizations (top) and position angle difference (bottom).

Dulwich et al. 2005



3C 346, optical/radio3C 346 polarization (1.4 cm)

Can be interpreted as oblique shock, where the jet turns at a shock plane and the magnetic field changes character because of the compression, if v ≈ 0.9c.

Apparent jet deflection of 70º is three times the true deflection because of projection effects (upstream jet at 15º to line of sight).

Dulwich et al. 2005



Generalizations: broad-band spectraLow-power jets:• Synchrotron spectra, radio to X-ray, with break in the IR or

optical, corresponding to a TeV electron energy• Spectrum breaks by > 0.5 – not 0.5: diagnostic of

acceleration physics, electron diffusion, and dynamics• Similar spectra in knots and diffuse emission, but n.b. knot

offsetsHigh-power jets: BL Lacs/FSRQ cores• Synchrotron self-Compton emission leads to X-ray/gamma-ray

“second peak”, from compact bases of jets• Extended jets have X-ray/gamma-ray spectrum that is as flat as

the radio spectrum, from external inverse-Compton • Both mechanisms rely on relativistic boosting



Generalizations: jet composition• May initially be electromagnetic, e+/e- plasma, or p/e-

• Expect entrainment to load with normal plasma quickly• On large scales the energy/momentum ratio affects dynamics

and suggests p/e- plasma, but only have kinematics from VLBI• Particle acceleration is efficient to electron energies of many

TeV (lifetimes of years), based on X–ray data, both in and between knots

• Much of discussion is based on minimum-energy arguments, but is this appropriate in highly-active core regions? It works in lobes, but is this the same?

• Value of γmin is crucial for energy calculations, but not known.



Locations of acceleration• Jet knots – example of HST-1 in M87 perhaps most extreme• In-between jet knots – turbulence developed by shear is

resonable choice, direct motion to/fro across shear layer also possible

• Hotspots – strongest local dump of kinetic energy, so obvious location for acceleration, but don’t always see X-rays at expected level: the upper limit of the acceleration process is far from clear

• Re-acceleration of particles by local compressions in/near jet also possible

• Efficiency of conversion of jet kinetic energy to radiation is low: remainder of energy heats/displaces intergalactic medium

the biggest accelerators in space and on earth jets and acceleration mark birkinshaw university of...

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