On the role of BH spin and accretion in powering relativistic jets in AGN

1. Onthe role of BH spin and accretion in powering relativistic jets in AGN Laura Maraschi M.Colpi,G.Ghisellini,A.Perego,F.Tavecchio INAF - Brera Observatory, Milan, Italy Krakow, May 23-26 2011

2. The core of a radio loud AGN Matter accretes onto the BH dissipating energy in an accretion disk or rad. inefficient accretion flow At the center highly relativistic powerful jets are launched, radiating up to gamma-rays

3. OUTLINE  A spin threshold for the Blandford-Znajek mechanism ?  The physical link between jet power and accretion power  Relevant results from FERMI observations Grand Unification prospects for AGN

4. The Blandford - Znajek mechanism (1977) It concerns the extraction of the free energy associated with the BHspin through a surrounding magnetosphere “Translated” by Macdonald and Thorne (1982) Direct numerical simulations of BH magnetosphere Kommissarov 2001, 2004 (confirms BZ approx. ) From BZ to jet properties Mc Kinney 2005, 2006 (G = 10 theta = 5°…..) Efficiency still under investigation: e.g. Tchekhovskoy et al. 2010 ON THE WHOLE THE MECHANISM APPEARS SUCCESSFUL

5. The Blandford Znajek mechanism (1977) H F plunge illustrated by Macdonald and Thorne (1982)

6. The B – Z “formula” WhereWF is the angular velocity of the mag.field lines and WHthat of the BH LBZ > 0 only if WF < WH

7. Indeed the BZ 77 solution finds ΩF = ½ ΩH Confirmed by Kommissarov (2001, 2004) In these papers the magnetosphere configuration is “assumed”

8. Conjecture about the existence of a threshold for the BZ mechanism

9. The magnetsphere must be advected by the accretion flow: suppose, before the “plunge” ΩF = ΩISCO Near the horizon ΩF > ΩISCO CompareΩH with ΩISCO as a function of the BH spin “a”

10. Comparing ΩISCO with ΩH Crossing at a=0.4

11. The analytically predicted B-Z power is negative fora < 0.4 No BZ

12. We are well aware that this argument is qualitative…………………..even simplistic However there is a global physical reason to expect that, if the Black Hole is slowly rotating, the e.m. torque exerted by the faster rotating field lines, frozen in the inflowing matter, will change sign (BZ 1977 MT 1982) The condition for this sign change needs to be explored quantitatively

13. Mc Kinney and Gammie 2004 performed a GRMHD simulation of a spinning black hole surrounded by a gas torus ! They study the infall of magnetized gas from the torus onto the BH……… Among other interesting results they mention that “in their runs the BZ process does not operate for a < 0.5” This is encouraging !

14. The Net EM Luminosity produced via BZ Ratio of EM to mass inflow power (negative) ! From Mc Kinney and Gammie 2004

15. The power scaling factor for the BZ mech. indicates a physical link between Jet power and Accretion power. Assuming equipartiton between kinetic and magnetic energy densities in the plunging region B2 rH2 c ~ 2 Mdot c2 Krolik (1999) Maraschi (2001) Levinson (2010) However the BZ power also depends on the BH spin ( as a2 up to a6 ?). A tight connection can result only if the range of BH spin is small….

16. FERMI surveyed the whole sky in gamma-rays with unprecedented sensitivity The blazar data from the first 3 months already allowed to derive interesting results For FSRQs the acc. disc is directly observed, and the gamma-ray luminosities can be compared with the accretion luminosity, while for BL Lacs an upper limit can be obtained

17. Jet power vs. accretion disk luminosity (SED modelling) i) For FSRQs the correlation is significant (subtracting z dependence) ii) Pjet > Ldisk Ldisk~0.1 Pacc Pjet ~ Pacc Ghisellini et al. 2010

18. Jet power vs. Disk lum. in FSRQs previous smaller independent samples: Different selection,still Pjet = 10 Ldisk For powerful blazars, Pjet ~ 10 Ldisk ~ Pacc M01, M & Tavecchio 2003, M et al. 2008

19. Model independent: Jet vs. Disk observed luminosities of bright FERMI blazars For FSRQs Lg ~10-100Ld Lg (true) ~ Ld

20. FSRQs disappear and BL Lacs appear below a disk luminosity of 10 45 erg s -1 = 10 -2 LEdd for M = 109 BLLacs are subEddington and radiatively inefficient accretors Lacc prop to m2 in this regime HOWEVER Ljet prop to m, according to the B-Z formula, thus Ljet prop to L acc1/2 as indicated by the grey stripe in the figure

21. The existence of a spin threshold for the BZ process would allow to understand: i) the close relationship between jet power and accretion power found in the FERMI survey ii)the existence of two AGN populations radio-loud and radio quiet respectively with spin above and below the threshold (Sikora 2007) leading to a Grand Unification of AGN on the basis of 2 parameters “a” and “Mdot”

22. Optically thick disc FSRQ m FR II BL Lac FR I Optically thin hot flow The fundamental AGN plane No jet a < 0.5 a > 0.5 Powerful jet a

23. The radio loudness – accretion rate plane From Sikora et al. 2007 These general properties can be understood:

24. Conclusions The two populations are distinguished by different values of “a” above and below ≈ 0.5 respectively. The R parameter increases at low λ due to reduced optical efficiency of disk accretion. A prediction is that radio emission in “radio quiet” objects is due to the Blandford Payne process which does not lead to highly relativistic jets

27. The critical divide: L g(ob)~ 1047 L g(em)~ 1045 ~ 0.01 L(Edd) for M~ 109 GMT 09

28. An important consequence from the BZ mechanism also for BL Lacs If the jet power and accretion power are related (as BZ predicts) BL Lacs , which appear at lower gamma-ray luminosity, must also have accretion power lower than FSRQ (in particular largely sub Eddington)

29. Interpretation of the “divide” For a “typical” mass of 109 solar masses, an average beaming factor of order 100, the dividing luminosity corresponds to 0.01 Eddington. Jets in AGN accreting above this limit (FSRQs) have “steep” gamma-ray spectra. AGN accreting below this limit turn into BL Lacs with “hard” gamma-ray spectra because accretion becomes radiatively inefficient and does not provide enough photons for External Compton scattering

30. The FRII – FRI divide The power limit that separates the two morfology classes depends on the luminosty of the host galaxy Ledlow and Owen

31. FRI—FRII: a mass dependent luminosity and morphology division Ledlow and Owen diagram in terms of jet power G&C 2001

32. Jet vs Disk luminosity (FERMI 3 m) Lg ~10-100Ld Both are “observed” quantities but Lg is beamed (factor 100) Lg ~ Ld

33. Back to the BZ formula: Omega H must be larger than Omega F B must be carried by infalling matter and is frozen into the accretion disk Suppose, before “the plunge”, Omega F = Omega (ISCO)

34. The division between FRIs and FRIIs in the Ledlow and Owen diagram also corresponds to a limit of ~ 1% Eddington accretion (deriving the jet power from the extended radio emission and the BH mass from the galaxy magnitude)

35. The coincidence between these two totally independent limits confirms in a “model independent” way the scenario in which the different properties and SEDs of FSRQs and BL Lac objects are due to different Eddington ratios

36. Gamma-ray luminosity vs disk luminosity (2 observables) Lg ~10-100Ld (model independent but Lg is beamed, Ld is not) modelling needed

37. Gamma-ray luminosity vs disk luminosity (2 observables) Lg ~10-100Ld (model independentbut Lg is beamed, Ld is not) modelling needed

38. Average SED models of the FSRQs and BLLacs in the 3 months Fermi Blazar sample • The blue bump • is directly • observed in • FSRQs • the accretion disk lumnosity can be derived For BL Lacs upper limits can be derived

39. What are these results telling about the jet production mechanism ? The Blandford Znajek (BZ) mechanism is at present the most popular model for the production of relativistic jets: It is purely el.mag., thus in some sense “simple” ! But it does not specify the origin of the field thus it is not completely realistic….. Present theoretical work concerns mostly GRMHD simulations to study its astrophysical realization

40. GRMHD simulations of a Poynting dominated • jet with selfconsistent interaction between • the BH and the accretion disk (Mc Kinney 06) • show that  • ~ 10 is reached at R ~ 100 Rs --Theta ~ 5 deg surrounded by a wider cone with smaller G • consistent with “spine – layer” jet structure inferred from observations Winds from the inner regions of the acc. Disk reach G < 3 (not adequate) BZ (ideal) is described by a simple formula !!

41. The near equality Pjet ~ Pacc requires high efficiency This seems to be a problem(Mc Kinney 05) Possible interesting solution is proposed by Garofalo (09) considering a special mechanism of field amplification in the plunging region AND “counterrotation” ….  The plunging region is wider

42. Suppose WF corresponds to the rotation frequency at the innermost stable orbit Omega (ISCO) of the accretion disc (could be larger but not smaller in the plunge) If the BH rotation frequency is SMALLER than WISCO, LBZ is negative and no jet can be produced This argument is qualitative but strongly Suggests the existence of a threshold in a below which the B-Z process cannot occur

43. The existence of a threshold for the BZ • mechanism would allow to understand • i) The correlation between jet power • and accretion power • in jetted (radio-loud) AGN • The Radio Loud/Radio Quiet dicothomy • (contrary to the continuous dependence • on “a” discussed by Tchekovskoy Narayan • Mc Kinney 2009)

44. Conclusions For FSRQs jet power and accretion power are indeed correlated and are comparable This is consistent with the Blandford & Znajek mechanism for the origin of jets, but requires very high efficiency. Previous estimates (Gammie 04, Mc Kinney 05) are relatively low unless a~1. An attractive possibility, proposed by Garofalo 09 is that in the brightest blazars the BH and disk are counterrotating

45. • The accretion rate in Eddington units, m is a fundamental parameter for: • the radiative properties of the accretion flow associated with the jets (bright disks or RIAF) • the shape of the jet SED through the intensity of the radiation field surrounding the jet (radiative energy losses and em. mech.)The shape and luminosity of the gamma-ray emission! • the jet power and its survival to large scales, that is the main morphological difference between FRI and FRII radio sources (Celotti and Ghisellini 2001) • •

46. Jet power where Electrons, mag. field and bulk Lorentz factor from radiative models, protons need assumption. Power can be estim. at diff. scales along the jet

47. Assuming Pjet~Pacc always and using mass estimates we derive accretion rates in Eddington Units

48. Jet power to disk Luminosity ratio for FSRQs Pjet~10 Ldisk~Pacc

49. The spectral sequence of blazar SEDs Fossati et al. 1998; Donato et al. 2001 RED FSRQ The Spectral Energy Distributions of blazars show systematic trends: peaks at higher frequencies with decreasing luminosity th Th BL Lacs BLUE

50. JET POWERS AND SEDs F In FSRQs the jet’s SEDs are “red” due to the high photon density provided by the optically thick accretion disk. At accretion rates below some threshold (10-2,10-3 Edd.) the optical disk disappears because the accretion flow becomes radiatively inefficient: the jet propagates in a photon poor ambient and its SED is “blue”