Understanding the ultrafast TeV variability of blazars Dimitrios Giannios Princeton, Department for Astrophysical Sciences With D. Uzdensky, M. C. Begelman, K. Nalewajko, M. Sikora TeVPA, Paris, 21/7/2010
Relativistic jets in GRBs, (micro)quasars AGN jets GRBs Cygnus X-1 25 20 15 MilliARC SEC 10 5 0 -5 4 2 0 -2 -4 -6 -8 -10 -12 -14 MilliARC SEC Kouveliotou et al. 2003 Stirling et al. 2001
Blazar spectral sequence Blazars: AGN Jets observed at small angle A diversity of objects: BL lacs (HBLs, LBLs), FSRQs Characteristic “double bump” spectrum likely result of synchrotron and SSC Fossati et al. 1998
Typical Blazar variability 3C279 3C279 Dec. 26, 2005 − Feb. 28, 2006 13.0 14 10 P1 (June 1991 flare) P2 (Dec. 92 / Jan. 93) 13.5 June 2003 Jan. 15, 2006 13 10 Apparent Magnitude 14.0 ! F ! [Jy Hz] 14.5 12 10 15.0 I R 11 10 15.5 V B U 16.0 10 10 16.5 9 11 13 15 17 19 21 23 25 730 740 750 760 770 780 790 10 10 10 10 10 10 10 10 10 JD − 2,453,000 ! [Hz] 3C279; e.g., Collmar et al. 2007
ultrafast TeV blazars Markarian 501 Albert PKS 2155-304 Aharonian et al. 2007 et al. 2007 Both vary on timescales 3-5 min or t v <<R g /c!!!
The jet-driving mechanism: MHD Energy Extraction Strong fields extract the rotational energy of the black hole and/or inner accretion disk Jet is launched Poynting flux dominated Blandford & Znajek 1977 Blandford & Payne 1982 Begelman & Li 1992 Meier et al. 2001 Koide et al. 2001 van Putten 2001 Nakamura & Meier 2004 Barkov & Komissarov 2008 …
Blazar emission through shocks or … blobs γ 2 υ 2 γ 1 υ 1 γυ (Internal) shocks Rees 1978; Spada et al. 2001; Guetta et al. 2004 The bulk Lorenz factor of the jet is variable A fast shell with γ 2 > γ 1 collides with a slower one dissipating their relative kinetic energy Emitting blobs Undetermined dissipative mechanism Quasi spherical regions in the jet that contain relativistic particles and magnetic fields
Blazar emission through shocks or … blobs γ 2 υ 2 γ 1 υ 1 γυ (Internal) shocks Rees 1978; Spada et al. 2001; Guetta et al. 2004 The bulk Lorenz factor of the jet is variable A fast shell with γ 2 > γ 1 collides with a slower one dissipating their relative kinetic energy Magnetic dissipation has Emitting blobs been typically ignored as Undetermined dissipative mechanism source of energy Quasi spherical regions in the jet see, however, Romanova & Lovelace 1992; that contain relativistic particles Jaroschek et al. 2004; Sikora et al. 2005; Giannios & Spruit 2006 and magnetic fields
Variability and source size Observer The variability constrains the size of the source a source with typical scale of R em cannot vary faster that timescales R em ~c t var R em For material moving with bulk f ( t ) Γ , the radius of emission is constrained to be R em ~2 Γ 2 ct var t
Variability and source size Observer The variability constrains the size of the source a source with typical scale of R em cannot vary faster that timescales R em ~c t var R em For material moving with bulk f ( t ) Γ , the radius of emission is constrained to be R em ~2 Γ 2 ct var t R s /c
Variability and source size Observer The variability constrains the size of the source a source with typical scale of R em cannot vary faster that timescales R em ~c t var R em For material moving with bulk f ( t ) Γ , the radius of emission is constrained to be R em ~2 Γ 2 ct var t R s /c
Variability and relativistic motion Relativistic effects can preserve variability timescales of the central engine but do NOT shorten them For given radius R em of emission of a shell, the typical observed duration of the emission is t var = R em / Γ 2 c High Γ s Allow for large distance of emission and short t var The collision radius of two shells with Γ 2 ~3 Γ ejected with time difference δ t bh ≥ R g /c from the inner disk/BH is R em ~ Γ 2 c δ t bh The observed variability is comparable (or longer) to that imprinted by the Γ 2 c ~ δ t bh ≥ R g t var ~ R em central engine c
Numerical verification Fig. 2c 1 0.9 0.8 0.7 0.6 luminosity 0.5 Fig. 3 1 0.4 0.3 0.9 0.2 0.1 0.8 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.7 observer’s time 0.6 ejection time Fig. 2d 1 0.5 0.9 0.4 0.8 0.7 0.3 0.6 luminosity 0.2 0.5 0.4 0.1 0.3 0 0.2 0 0.2 0.4 0.6 0.8 1 1.2 observer’s time 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 observer’s time Kobayashi & Piran 1997; Mimica et al. 2005; for analytical arguments see Nakar & Piran 2002
Implications from ultrafast TeV flaring Models that associate the variability to black hole activity do not work Internal shocks at odds for these flares Indicates variability originating from the jet Tavecchio & Ghisellini 2008; Giannios et al. 2009 For the TeV (result of SSC or EIC) emission to escape pair creation Γ em >50 is needed Begelman, Fabian & Rees 2008; Mastichiadis & Moraitis 2008
“Slow” proper motions for these blazars However, PKS 2155-304, Mrk 501 show “moderately” superluminal ejections (v app ~a few c) at most on sub-pc (VLBI) scales Piner & Edwards 2004; Ghiroletti et al. 2004; Piner et al. 2008
Proposed solutions The jet decelerates on sub-pc scales (just after the blazar zone) Georganopoulos & Kazanas 2004; Levinson 2007 Or accelerates (rarefies only locally) Lyutikov & Lister 2010 The jet is slower ( Γ j ~10) but contains blobs that move relativistically within the jet (needles or jets in a jet) Ghisellini et a. 2009; Giannios, Uzdensky & Begelman 2009; 2010; Nalewajko et al. 2010 in Prep.
Jets in the jet: kinematics Consider a jet moving moving with the jet with Γ j ~10 and a blob moving with Γ co ~10, θ ’~ π /2 w.r.t. the jet In the lab frame, the blob moves with Γ bl ~ Γ j Γ co ~100 Giannios, Uzdensky & Begelman 2009
Where are these compact minijets come from? Suppose the jet remains Poynting-flux dominated σ >>1 at the blazar emission region (R~100-1000 R g ) Magnetic dissipation through relativistic magnetic reconnection Material enters at subrelativistic Relativistic Petschek Reconnection speed and Lyubarsky 2005 leaves the reconnection region at the Alfvén speed Γ out ~ σ 1/2
What causes the field reversals? Field reversals at the (Current Driven) MHD jet base of the jet? instabilities? Moll 2009
Energetics and lengthscales Emission Synchrotron, SSC, EIC? − 1 keV, 1/ 2 σ 2 2 h ν cyc ≈ 1.2 L j ,47 3/ 2 r h ν syn ≈ Γ j Γ minijet γ e 2 − 1 GeV 1/ 2 σ 2 5/ 2 r h ν SSC ≈ 120 L j ,47 2 Minijet typical size estimated by the observed (isotropic) energy of the TeV flares L f t f ~3x10 49 erg 1/ 3 t f ,300 1/ 3 r 2/ 3 l "~ 10 14 L f ,47 2 cm ~ R g 1/ 3 σ 2 1/ 2 L j ,47 1/ 3 Γ j ,1 Transparency condition 1/ 3 t f ,300 1/ 3 L j ,47 2/ 3 5 l " ≈ 0.8 L f ,47 target σ T τ γγ ~ N syn 1/ 2 r 4 / 3 Γ 10/ 3 σ 2 2 1
Impications: Flares from misaligned jets Mrk 501, PKS 2155-304 Jet viewed on axis Γ j M87 Γ j Off-axis jet θ ’ lab frame Jet frame
Implications: TeV flares from M87 A minijet that emits off the jet axis explains the intra- day flares much better than a jet viewed off axis Giannios, Uzdensky & Begelman 2010 Acciari et al. 2009
Evidence for beaming: anisotropic GeV emission in the jet rest frame Savolainen et al. (2009; MOJAVE team): FERMI sample of blazars shows θ src ~90 o Is the GeV emission preferentially emitted perp. to the jet propagation?
Refinements to the model Reconnection minijets are not spherical blobs Nalewajko et al. 2010 in prep. Reconnection region may well be more complicated Lin et al. 2005; Drake et al. 2006; Bemporad 2008; Daughton et L. 2009… Breaking into many islands Containing termination shocks for minijets Samtaney et al. 2009
Detailed Geometry I B z y x 2 ψ 2 l 2 l 2 l 3 Nalewajko et al. 2010, in prep.
Radiative transfer calculations log E [eV] -2 0 2 4 6 8 10 12 47 I + OPP 46 II + OPP 45 log ! L ! [erg s -1 ] 44 43 42 41 40 39 38 12 14 16 18 20 22 24 26 28 log ! [Hz] Nalewajko et al. 2010, in prep.
Concluding Ultrafast varying blazars challenge current views for jet emission Magnetic fields likely launch and accelerate the relativistic jets Dissipation of magnetic energy may also be responsible for (part of?) the blazar emission
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