Understanding the ultrafast TeV variability of blazars Dimitrios - - PowerPoint PPT Presentation

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

GRBs

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• 14

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Kouveliotou et al. 2003

Relativistic jets in GRBs, (micro)quasars

AGN jets

Stirling et al. 2001

Cygnus X-1

Blazar spectral sequence

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Blazars: AGN Jets observed at small angle

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A diversity of objects: BL lacs (HBLs, LBLs), FSRQs

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Characteristic “double bump” spectrum



likely result of synchrotron and SSC

Fossati et al. 1998

Typical Blazar variability

730 740 750 760 770 780 790 JD − 2,453,000 13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 Apparent Magnitude

3C279

• Dec. 26, 2005 − Feb. 28, 2006

I R V B U

10

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! [Hz] 10

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!F! [Jy Hz]

3C279

P1 (June 1991 flare) P2 (Dec. 92 / Jan. 93) June 2003

• Jan. 15, 2006

3C279; e.g., Collmar et al. 2007

ultrafast TeV blazars

PKS 2155-304 Aharonian

et al. 2007

Markarian 501 Albert

et al. 2007

Both vary on timescales 3-5 min or tv <<Rg/c!!!

The jet-driving mechanism: MHD Energy Extraction

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 …

Strong fields extract the rotational energy

• f the black hole and/or inner accretion

disk Jet is launched Poynting flux dominated

Blazar emission through shocks or … blobs

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(Internal) shocks

Rees 1978; Spada et al. 2001; Guetta et al. 2004

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The bulk Lorenz factor of the jet is variable

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A fast shell with γ2>γ1 collides with a slower one dissipating their relative kinetic energy

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Emitting blobs

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Undetermined dissipative mechanism

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Quasi spherical regions in the jet that contain relativistic particles and magnetic fields

γ2υ2 γ1υ1 γυ

Blazar emission through shocks or … blobs

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(Internal) shocks

Rees 1978; Spada et al. 2001; Guetta et al. 2004

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The bulk Lorenz factor of the jet is variable



A fast shell with γ2>γ1 collides with a slower one dissipating their relative kinetic energy

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Emitting blobs

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Undetermined dissipative mechanism

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Quasi spherical regions in the jet that contain relativistic particles and magnetic fields

γ2υ2 γ1υ1 γυ Magnetic dissipation has been typically ignored as source of energy

see, however, Romanova & Lovelace 1992; Jaroschek et al. 2004; Sikora et al. 2005; Giannios & Spruit 2006

Variability and source size

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The variability constrains the size of the source

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a source with typical scale of

Rem cannot vary faster that

timescales Rem~ctvar

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For material moving with bulk Γ, the radius of emission is constrained to be Rem ~2 Γ2ctvar

Rem

f(t)

t

Observer

Variability and source size



The variability constrains the size of the source



a source with typical scale of

Rem cannot vary faster that

timescales Rem~ctvar



For material moving with bulk Γ, the radius of emission is constrained to be Rem ~2 Γ2ctvar

Rem

f(t)

t Rs/c

Observer

Variability and source size



The variability constrains the size of the source



a source with typical scale of

Rem cannot vary faster that

timescales Rem~ctvar



For material moving with bulk Γ, the radius of emission is constrained to be Rem ~2 Γ2ctvar

Rem

f(t)

t Rs/c

Observer

Variability and relativistic motion

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Relativistic effects can preserve variability timescales of the central engine but do NOT shorten them

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For given radius Rem of emission of a shell, the typical observed duration of the emission is tvar=Rem/Γ2c

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High Γs Allow for large distance of emission and short tvar

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The collision radius of two shells with Γ2 ~3Γ ejected with time difference δtbh ≥Rg/c from the inner disk/BH is Rem~Γ2cδtbh

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The observed variability is comparable (or longer) to that imprinted by the central engine

tvar ~ Rem Γ2c ~ δtbh ≥ Rg c

Numerical verification

• bserver’s time

• Fig. 2c

• bserver’s time

• Fig. 2d

• Fig. 3
• bserver’s time

Kobayashi & Piran 1997; Mimica et al. 2005; for analytical arguments see Nakar & Piran 2002

Implications from ultrafast TeV flaring

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Models that associate the variability to black hole activity do not work

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Internal shocks at odds for these flares

Indicates variability originating from the jet

Tavecchio & Ghisellini 2008; Giannios et al. 2009

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For the TeV (result of SSC or EIC) emission to escape pair creation Γem>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 (vapp ~a few c) at most

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• n 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

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Or accelerates (rarefies only locally) Lyutikov & Lister 2010

 The jet is slower (Γj~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

with Γj~10 and a blob moving with Γco~10, θ’~π/2 w.r.t. the jet

 In the lab frame, the

blob moves with Γbl~Γj Γco~100 moving with the jet

Giannios, Uzdensky & Begelman 2009

Where are these compact minijets come from?

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Suppose the jet remains Poynting-flux dominated σ>>1 at the blazar emission region (R~100-1000 Rg)

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Magnetic dissipation through relativistic magnetic reconnection

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Material enters at subrelativistic speed and

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leaves the reconnection region at the Alfvén speed Γout~σ1/2

Relativistic Petschek Reconnection Lyubarsky 2005

What causes the field reversals?

(Current Driven) MHD jet instabilities? Moll 2009 Field reversals at the base of the jet?

Energetics and lengthscales

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Emission

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Synchrotron, SSC, EIC?

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Minijet typical size estimated by the observed (isotropic) energy of the TeV flares Lftf~3x1049 erg

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Transparency condition hν syn ≈ ΓjΓ

minijetγ e 2hν cyc ≈1.2L j,47 1/ 2 σ2 3/ 2r 2 −1 keV,

hνSSC ≈120L j,47

1/ 2 σ2 5/ 2r 2 −1 GeV

l"~ 1014 L f ,47

1/ 3 t f ,300 1/ 3 r 2 2/ 3

Γj,1

1/ 3σ2 1/ 2L j,47 1/ 3

cm ~ Rg τγγ ~ Nsyn

target σT

5 l"≈ 0.8 L f ,47

1/ 3 t f ,300 1/ 3 L j,47 2/ 3

σ2

1/ 2r 2 4 / 3Γ 1 10/ 3

Impications: Flares from misaligned jets

Γj

θ’

Γj

Jet viewed on axis Off-axis jet

Jet frame lab frame

Mrk 501, PKS 2155-304 M87

Implications: TeV flares from M87

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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

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Savolainen et al. (2009; MOJAVE team): FERMI sample of blazars shows θsrc~90o

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Is the GeV emission preferentially emitted perp. to the jet propagation?

Refinements to the model

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Reconnection minijets are not spherical blobs

Nalewajko et al. 2010 in prep.

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Reconnection region may well be more complicated

Lin et al. 2005; Drake et al. 2006; Bemporad 2008; Daughton et L. 2009…

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Breaking into many islands

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Containing termination shocks for minijets

Samtaney et al. 2009

Detailed Geometry

B I

z y x 2ψ2 l2 l3 l2

Nalewajko et al. 2010, in prep.

Radiative transfer calculations

38 39 40 41 42 43 44 45 46 47 12 14 16 18 20 22 24 26 28

• 2

2 4 6 8 10 12 log ! L! [erg s-1] log ! [Hz] log E [eV] I + OPP II + OPP

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

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Dissipation of magnetic energy may also be responsible for (part of?) the blazar emission