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Jets and acceleration

Jets and acceleration. Mark Birkinshaw University of Bristol. Five questions on acceleration in jets. Where is acceleration occurring? Multi-wavelength imaging – locations of radiating particles What acceleration is occurring?

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Jets and acceleration

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  1. Jets and acceleration Mark Birkinshaw University of Bristol

  2. Five questions on acceleration in jets • Where is acceleration occurring? • Multi-wavelength imaging – locations of radiating particles • What acceleration is occurring? • Spectral energy distributions – energy spectra of radiating particles • How is acceleration occurring? • Multi-wavelength polarimetry – configurations of fields • Multi-wavelength variability – timescales of radiating population changes • How efficiently is acceleration occurring? • Energetics • Is the radiating population the majority population in jets? • Polarimetry, dynamics Handicap: evidence of acceleration is only in e+/e- (one possible exception) Mark Birkinshaw, U. Bristol

  3. Centaurus A Low-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) Mark Birkinshaw, U. Bristol

  4. 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) Mark Birkinshaw, U. Bristol

  5. 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. Mark Birkinshaw, U. Bristol

  6. Centaurus A 100 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. Aharonian et al. 2010 Abdo et al. 2010 Mark Birkinshaw, U. Bristol

  7. 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. Mark Birkinshaw, U. Bristol

  8. M 87, radio and X-ray 3C 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. Mark Birkinshaw, U. Bristol

  9. M 87, X-rays 3C 274 = M 87 Chandra 0.5-2.5 keV; centre Non-thermal contains strong jet component Obvious radio jet/X-ray gas relationship Mark Birkinshaw, U. Bristol

  10. 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) Mark Birkinshaw, U. Bristol

  11. 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 Mark Birkinshaw, U. Bristol

  12. 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. image Harris et al. 2006 Mark Birkinshaw, U. Bristol

  13. 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. Mark Birkinshaw, U. Bristol

  14. 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. Mark Birkinshaw, U. Bristol

  15. M87, polarization 3C 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. Owenet al. (1999) Apparent magnetic field directions. Mark Birkinshaw, U. Bristol

  16. 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. Mark Birkinshaw, U. Bristol

  17. Some low-power jets 3C 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. Mark Birkinshaw, U. Bristol

  18. Some low-power jets NGC 6251 Spitzer No obvious dust Radio jet detected in IR for the first time. IR colour: red = 8 m, green = 4.5 m, blue = 3.6 m Mark Birkinshaw, U. Bristol

  19. 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 Mark Birkinshaw, U. Bristol

  20. Some low-power jets 3C 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 ~ 105even outside knots. Stokes I, % polarization, outer/inner apparent B vectors; Perlman et al. 2006 Mark Birkinshaw, U. Bristol

  21. Some low-power jets Many 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? Mark Birkinshaw, U. Bristol

  22. 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. Mark Birkinshaw, U. Bristol

  23. PKS 2152-699 Higher-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. Mark Birkinshaw, U. Bristol

  24. 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). Mark Birkinshaw, U. Bristol

  25. 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? Mark Birkinshaw, U. Bristol

  26. 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) Mark Birkinshaw, U. Bristol

  27. PKS 0637-752 Quasar at z = 0.651; 1 arcsec = 7 kpc Godfrey et al. (2012). Quasi-periodic oscillations out to major bend. Knot structure suggests some sort of instability/oscillation (reconfinement shocks?) causing regular spacing. Mark Birkinshaw, U. Bristol

  28. 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 Mark Birkinshaw, U. Bristol

  29. PKS 0920-397 1 arcsec = 6.5 kpc X-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) Mark Birkinshaw, U. Bristol

  30. 3C 279 3C 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) Mark Birkinshaw, U. Bristol

  31. 3C 279: circular polarization 3C 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. Mark Birkinshaw, U. Bristol

  32. 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 Mark Birkinshaw, U. Bristol

  33. 3C 15, optical/radio 3C 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 Mark Birkinshaw, U. Bristol

  34. 3C 15, optical/radio 3C 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 Mark Birkinshaw, U. Bristol

  35. 3C 346, optical/radio 3C 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 Mark Birkinshaw, U. Bristol

  36. 3C 346, optical/radio 3C 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 Mark Birkinshaw, U. Bristol

  37. Generalizations: broad-band spectra Low-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 offsets High-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 Mark Birkinshaw, U. Bristol

  38. 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. Mark Birkinshaw, U. Bristol

  39. 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 Mark Birkinshaw, U. Bristol

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