- Slides: 28
Active Galactic Nuclei: Jets and other Outflows To discuss two aspects of AGN Activity (About phenomena on parsec & kpc Scales) Gopal Krishna NCRA-TIFR, Pune, INDIA Paul J. Wiita GSU, Atlanta, USA KASI-APCTP Joint Workshop (KAW 4), Daejeon, Korea (May 17 -19, 2006)
Three topics n Peculiar radio (synchrotron) spectrum: Sn n 1/3 Electron energy spectrum: either mono-energetic, or having a low energy cut-off (LEC) Salient examples : n n Galactic (Sgr A* and the "ARC") Extragalactic (extreme IDV quasar PKS 0405 -385) n Possible implication of LEC for the bulk motion of quasar jets n Interplay of thermal and relativistic plasma outflows from AGN Based on: Gopal Krishna, Dhurde & Wiita (Ap. J, 615, L 81, 2004) Gopal Krishna, Wiita & Dhurde (MNRAS, 2006, in press) Gopal Krishna, Wiita & Joshi, (Submitted, 2006)
Early Evidence for LEC in the Nuclear Cores Lack of Faraday depolarization (from VLBI) gmin ~ 100 (Wardle 1977; Jones & O'Dell 1977) More direct recent evidence for LEC From turnover in the radio spectrum of the eastern hotspot of Cygnus A (nt ~ 0. 1 GHz gmin~300) (Joseph et al 2006; Biermann et al. 1995; Carilli et al. 1991) Near theoretical estimate for hadronic interactions (gmin ~ 100) Spectral turnover due to LEC can be more readily seen for superluminal VLBI radio knots (Because nt is pushed to GHz range due to strong Doppler shift) (Gopal-Krishna, Biermann & Wiita 2004) For a wide range of B and , LEC is the main cause of spectral flattening/ turnover (Since LEC becomes effective at higher frequency than SSA) (Gopal-Krishna, Biermann & Wiita 2004)
Bulk Lorenz factor of the jet ( j) from the inverted spectrum of the Extreme Intra-day Variable (IDV) Blazar PKS 0405 -385 n =1/3 (Protheroe, 2003) Ref: Duschl & Lesch, 1994 up to nt 230 GHz Ref: Protheroe, 2003
Other Indications of Ultra-High j on Parsec / Sub-Parsec Scale n To avoid excessive photo-photon losses, variable Te. V emission demands Ultra-relativistic jets (Krawczynski et al. 2002) with 15 < j < 100 (Mastichiadis & Kirk, 1997; Krawczynski, et al. 2001) n Correcting the spectrum for Gamma-ray absorption by the IR background strongly implies j > 50 (e. g. , Henri & Saugé, 2006) n Evidence for Tb (apparent) > 1013 K in IDV blazars would also suggest j> 50 (for simple quasi-spherical geometry of the source) (e. g. , Protheroe, 2003; Macquart & de Bruyn, 2005) n n For several EGRET blazars, recent VLBI shows: vapp > 25 c (hence j > 25) (Piner et al. 2006) GRB models usually require jets with j ~ 100 -1000 (e. g. , Sari et al. , 1999; Meszaros, 2002) Note: Jet formation model ( j >30) by Vlahakis & Konigl, 2004)
Problem Posed by Ultra-High j (> 30) n As many as 35% - 50% of the VLBI knots in Te. V blazars are found to be stationery or moving subluminally. (Piner & Edwards 2004) n The fraction is much lower for a normal blazar population (e. g. , Jorstad & Marscher, 2003) (Hence, no serious inconsistency with j ~20 -30) However, a serious inconsistency for Te. V blazars
How to Reconcile Ultra-Relativistic Jets with the Slow Moving Radio Knots? n n n Viewing angle ( ) of the jet is within ~ 1 o (from our line of sight) (NOT a general explanation: Since only ¼ j 2 (~10 -4) VLBI knots can appear subluminal) Motion of the knots reflects pattern speed, not physical speed (However, see Homan et al. 2006) A dramatic deceleration of jet between sub-pc and parsec scale (Georganopoulos & Kazanas, 2003) DIFFICULTIES n n n Why deceleration in Te. V blazars only (and not in EGRET blazars)? Evidence, in fact, points to acceleration on parsec scale (Piner 2006) Spine-sheath structure of jets: (e. g. , Ghisellini et al. 2004) Fast spine produces Te. V variability via IC and only the slower outer layer is picked in radio VLBI (observational evidence: Giroletti et al 2004) DIFFICULTIES n n Why a two-component jet needs to be invoked only for Te. V blazars? Why don't the shocks produce radio knots even in the fast spine?
n n Possible resolution of the Paradox: Conical (Ultra-Relativistic) Jets Substantial opening angles are seen for some well-resolved VLBI jets. Good example of conical VLBI jet is M 87 ( >10 o) (Junor et al. , 1999) n n Consequence of conical jet: For an ultra-relativistic jet, a huge variation of j (i. e. , of Doppler boosting factor & apparent motion) would occur across the jet’s cross section Needed: Weighted averaging of app by the distribution of fluxboosting A( ) over the jet's cross section (Gopal Krishna et al, 2004) n n Remember that while A( ) varies monotonically with , app( ) does not. Moreover, if the line-of-sight to the core passes through the jet’s cone, then large vector cancellation of app can occur over the jet’s cross section.
Pseudo-colour rendition of the nucleus of M 87 at 43 GHz on 3 March 1999. (Junor et al, 1999)
Relevant analytical expressions (Gopal Krishna et al. 2004) Sobs= n ( ). Sem( )d A( )Sem [where, n=3 for radio knots and A( )=mean amplification factor] (Fomalont et al. 1991)
Conical Jets w/ High Lorentz Factors Weighted app vs for = 100, 50, 10 and opening angle = 0, 1, 5 and 10 degrees, With blob 3 boosting Probability of large app can be quite low for high if opening angle is a few degrees
High Gammas Yet Low Betas n n n app vs for jet and prob of app > for opening angles = 0, 1, 5, 10 degrees and = 50, 10 (continuum 2 boosting) Despite high in an effective spine population statistics are OK Predict transversely resolved jets show different app
Some key Implications n Thus, even a radio knot moving with the ultra-relativistic spine of the jet would frequently appear to move subluminally (we believe this is the case of Te. V blazars). n n This will happen even for viewing angles ( ) significantly larger than 1/ j (Hence, not so unlikely) Effective beaming angle is the same as the jet’s opening angle (5º to 10º) ( >> 1/ j). Usually, this is associated with canonical jets ( =0) of j=5 to 10. Hence, ultra-relativistic conical jets are also consistent with FR I radio galaxies being the parent population of BL Lacs.
Dynamical interaction between thermal and relativistic outflows from AGN (Evidence from Radio Morphology) n In several RGs, the inner edges of the two radio lobes are sharply truncated n n Thus, strip-like central gaps are seen in the radio bridges Typical dimensions of central gaps: Width~30 kpc ( 0. 5 Mpc) Inference: The huge strip-like gap seen between the radio lobe pair betrays the presence of a “Superdisk" made of denser material (Gopal-Krishna & Wiita 2000; Gopal-Krishna & Nath 2001) Since the sharp edges can only be seen from a favorable viewing angle, superdisk should be a fairly common feature Previous Interpretations of the Radio Gaps, in general: n Back-flowing synchrotron plasma in the radio lobes is blocked by the ISM of the parent galaxy (ISM arising from stellar winds and/or captured disk galaxies) n Buoyancy led outward squeezing of the lobe plasma by the ISM n
4 C 14. 27 3 C 33 3 C 192 Ref: DRAGN Atlas (P. Leahy) 3 C 381 3 C 401
Need for an Alternative Interpretation n n Radio gaps in some RGs are extremely wide: upto 0. 5 Mpc (PKS 0114 -476) Often the parent galaxy is seen at one edge of the radio gap (In some cases, even outside the gap, i. e. , within a lobe): (3 C 16, 3 C 19) (Saripalli et al. 2002) (DRAGN atlas (P. Leahy)
A Plausible mechanism for the radio gaps n Dynamical Interaction of the radio lobes with a powerful thermal wind outflowing from the AGN (GK, Wiita & Joshi 2006) Emerging Pieces of Evidence: n Thermal winds (vw>103 km/s) and mass outflow of ~1 M /yr are generic to AGN (e. g. , Soker & Pizzolato 2005; Brighenti & Mathews 2006) n For example, in ADIOS model, accretion energy mostly ends up in a thermal wind (Blandford & Begelman 1999) n Thus, relativistic jet pair and non-relativistic wind outflow seem to co-exist (e. g. , Binney 2004; Gregg et al. 2006) n Evidences: Absorption of AGN's continuum, seen in UV and X-ray bands (review by Crenshaw et al. 2003) n Wind outflow probably PRECEDES the jet ejection and lasts for w > ~ 108 yrs (e. g. , Rawlings 2003; Gregg et al. 2006) n Mechanical luminosity of the wind can greatly exceed AGN’s bolometric luminosity (Churazov et al. 2002; Peterson & Fabian 2005) n Wind outflow is quasi-spherical, while the jets are well collimated (e. g. , Levine & Gnedin 2005)
The Basic Model: Sequence of Events n Wind outflow from AGN blows an expanding bubble of metal-rich, hot gas n Later, the AGN ejects a pair of narrow jets of relativistic plasma n The jets rapidly traverse the wind bubble and often come out of the bubble n From then on, the high-pressure backflow of relativistic plasma in the radio lobes begins to impinge on the wind bubble, from outside n This sideways compression of expanding wind bubble by the two radio lobes transform the bubble into a fat pancake, or superdisk n AGN's hot wind escapes through the superdisk region, normal to jets n The superdisk is "frozen" in the space. It manifests itself as a strip-like central emission gap in the radio bridge n Meanwhile, the galaxy can continue to move within the cosmic web It can move ~ 100 kpc in ~ 300 Myr, with a speed of ~ 300 km/s n Thus, in about 108 years the parent galaxy can even reach the edge of the radio emission gap (sometimes, even cross over into the radio lobe: eg. , 3 C 16, 3 C 19) n Now onwards, the two jets propagate through very different types of ambient media (wind material and radio lobe plasma)
The Basic Model: Sequence of Events
Modelling the dynamics of the bubble and the jets (Gopal Krishna, Wiita & Joshi 2006) (Uses the analytical works of Levine & Gnedin 2005; Scannapieco & Oh 2004; Kaiser & Alexander 1997) Asymptotic (equilibrium) radius of the wind bubble:
For the jet starting a time tj after the onset of the AGN wind: Catch-up time (tc): when jet catches up with the bubble’s surface: Catch up length of the jet After catching up [tc>t >(tj+ j)]: Assumption: Jet stops advancing when the AGN switches off.
Gopal Krishna, Wiita & Joshi, 2006
Finding Jet Parameters n n n Determining bulk Lorentz factors, , and misalignment angles, , are difficult for all jets Often just set =1/ , the most probable value Flux variability and brightness temperature give estimates: S = change in flux over time obs Tmax= 3 x 1010 K app from VLBI knot speed is spectral index
Conical Jets Also Imply n n n Inferred Lorentz factors can be well below the actual ones Inferred viewing angles can be substantially underestimated, implying deprojected lengths are overestimated Inferred opening angles of < 2 o can also be underestimated IC boosting of AD UV photons by ~10 jets would yield more soft x-rays than seen (“Sikora bump”) but if >50 then this gives hard x-ray fluxes consistent with observations So ultrarelativistic jets with >30 may well be common
Inferred Lorentz Factors inf vs. for =100, 50 and 10 for =5 o P( ) and < inf>
Inferred Projection Angles n n Inferred angles can be well below the actual viewing angle if the velocity is high and the opening angle even a few degrees This means that de-projected jet lengths are overestimated
Conclusions n Part I: Modest opening angles (5º – 10º) of AGN jets can explain the jet Lorenz factor paradox of Te. V blazars n n n Thus, the frequently observed subluminal motion of VLBI knots can be reconciled with the ultra-high bulk Lorenz factors ( j >30 – 50) inferred from rapid Te. V and radio flux variability. Some further consequences of this picture are discussed in our second paper (Gopal Krishna, Wiita & Durde, MNRAS, 2006, in press. ) Part II: Dynamical interaction between thermal (wind) and non -thermal (jet) outflows resulting from the AGN activity, gives rise to fat pancake or superdisk shaped regions. n n The metal-rich in which hot wind material filling the superdisk escapes to hundreds of kpc, roughly orthogonal to the radio axis. Superdisks manifest their presence by causing strip-like emission gaps in the middle of radio bridges.