Models for non-HBL VHE Gamma-Ray Blazars

Markus Böttcher

Ohio University, Athens, OH, USA

“TeV Particle Astrophysics”SLAC, Menlo Park, CA, July 13 – 17, 2009

Until a few years ago, all TeV blazars were high-frequency-peaked BLLac objects (HBLs).

Recently, the Intermediate BL Lac objects W Comae and 3C66A (VERITAS), the low-frequency-peaked BL Lac object (LBL) BL Lacertae, and even the FSRQ 3C279

(MAGIC) and were detected in TeV -rays.

Similar in physical parameters to other TeV blazars (HBLs)? (→ SSC dominated?)

Or more similar to LBLs? (→ EC required?)

Motivation

Leptonic Blazar ModelRelativistic jet outflow with ≈ 10

Injection, acceleration of ultrarelativistic

electrons

Qe (,

t)

Synchrotron emission

F

Compton emission

F

-q

Radiative cooling ↔ escape =>

Seed photons:

Synchrotron (within same region [SSC] or slower/faster earlier/later emission regions

[decel. jet]), Accr. Disk, BLR, dust torus (EC)

Qe (,

t)

-(q+1)

b

-q or -2

b 1

b: cool(b) = esc

Spectral modeling results along the Blazar Sequence: Leptonic Models

High-frequency peaked BL Lac (HBL):

No dense circumnuclear material → No strong external

photon field

SynchrotronSSC

Low magnetic fields (~ 0.1 G);

High electron energies (up to TeV);

Large bulk Lorentz factors ( > 10)

The “classical” picture

Spectral modeling results along the Blazar Sequence: Leptonic Models

Radio Quasar (FSRQ)

Plenty of circumnuclear

material → Strong external

photon field

SynchrotronExternal Compton

High magnetic fields (~ a few G);

Lower electron energies (up to GeV);

Lower bulk Lorentz factors ( ~ 10)

The Quasar 3C279 on Feb. 23, 2006

Feb. 23:

• High optical flux

• Steep optical spectrum ( = 1.7 -> p = 4.4)

• High X-ray flux

• Soft X-ray spectrum

sy ~ 5x1013 Hz => sy ~ 4x10-7 ~ 1025 Hz =>

~ 105

Fsy ~ 1013 Jy Hz

F ~ 5x1013 Jy Hz

Accretion disk: LD ~ 2x1045 erg/s; D ~ 10-5

Parameter Estimates: SSC

• Optical index = 1.7 => p = 4.4 => cooling break (3.4 -> 4.4) would not produce a F peak => peak must be related to low-energy cutoff, p = 1

• Separation of synchrotron and gamma-ray peak

=> p = (/sy)1/2 ~ 1.6x105

• sy = 4.2x106 p2 BG D/(1+z) Hz

=> BG D1 ~ 7x10-5

Parameter Estimates: External Compton• External photons of s ~ 10-5 can be Thomson scattered up

to ~ 105 => Accretion disk photons can be source photon field.

• Location of gamma-ray peak

=> p = (/[2s])1/2 ~ 104 1-1

• sy = 4.2x106 p2 BG D/(1+z) Hz

=> BG ~ 1.8x10-2 12 D1

-1

• Relate synchrotron flux level to electron energy density, eB = u’B/u’e

=> eB ~ 10-8 17 R16

3

a) ~ 15, B ~ 0.03 G, eB ~ 10-7

b) eB ~ 1, B ~ 0.25 G, ~ 140 R16-3/7

X-rays severely underproduced!

Attempted leptonic one-zone model fit, EC dominated

(Bӧttcher, Reimer & Marscher 2009)

Requires far sub-equipartition magnetic fields!

Alternative: Multi-zone leptonic model

Linj = 2.3*1049 erg/s

min = 104

max = 106

q = 2.3

B = 0.2 G

= D = 20

RB = 6*1015 cm

u’B/u’e = 2.5*10-4

X-ray through gamma-ray spectrum reproduced by SSC; optical spectrum has to be produced in a different part of the jet.

(Bӧttcher, Reimer & Marscher 2009)

• Optical and -ray spectral index can be decoupled

• X-rays filled in by electromagnetic cascades

• However: Requires very large jet luminosities, Lj ~ 1049 erg/s

Hadronic Model Fits

(Bӧttcher, Reimer & Marscher 2009)

W Comae• Detected by VERITAS in March 2008 (big flare on March 14)

• One-zone SSC model requires extreme parameters:

Acciari et al. (2008) Linj = 2.8*1045 erg/s

min = 450

max = 4.5*105

q = 2.2

B = 0.007 G

= D = 30

RB = 1017 cmWide peak separation and low X-ray flux

require unusually low magnetic field!

LB/Le = 5.7*10-2

W Comae• Much more natural parameters for EC model

Linj = 2*1044 erg/s

min = 700

max = 105

q = 2.3

RB = 1.8*1015 cm

B = 0.25 G

-> Equipartition!

= D = 30

tvar ~ 35 min. allowed with external IR photon field

• For Compton scattering in Thomson regime, external photons must have E ~ (mec2)2/EVHE ~ 0.1 – 1 eV => IR

(Acciari et al. 2008)

W ComaeMajor VHE -ray flare detected by VERITAS in June 2008.

Similar modeling conclusions to March 2008:

SSC fit:

B= 0.24 G

LB/Le = 2.3*10-3

EC fit:

B = 0.35 G

LB/Le = 0.32

High flux state on MJD 54624

(Acciari et al. 2009, in prep.)

3C66AMajor VHE -ray flare detected by VERITAS in October 2008

Pure SSC fit requires far sub-equipartition magnetic field:

B = 0.1 G

LB/Le = 8.0*10-3

= D = 30

RB = 3*1016 cm

=> tvar,min = 13 hr

3C66AFit with external IR radiation field (ext = 1.5*1014 Hz)

yields more natural parameters:

B = 0.3 G

LB/Le = 0.1

= D = 30

RB = 2*1016 cm

=> tvar,min = 8.9 hr

Summary1. The MAGIC detection of 3C279 poses severe

problems for leptonic models. Hadronic models provide a viable alternative, but require a very large jet power.

2. Recent VHE gamma-ray detections of inermediate- and low-frequency peaked BL Lac objects extends the TeV blazar source list towards new classes of blazars.

3. IBLs appear to require source parameters truly intermediate between HBLs and LBLs: In leptonic models, a non-negligible contribution from external Compton on an external IR radiation field yields more natural parameters than a pure SSC interpretation.

Blazar Classification

Quasars:

Low-frequency component from radio to optical/UV

High-frequency component from X-rays to -rays, often dominating total power

Peak frequencies lower than in BL Lac objects

(Hartman et al. 2000)

High-frequency peaked BL Lacs (HBLs):

Low-frequency component from radio to UV/X-rays, often

dominating the total power

High-frequency component from hard X-rays to high-

energy gamma-rays

Intermediate objects:

Low-frequency peaked BL Lacs (LBLs):

Peak frequencies at IR/Optical and GeV gamma-rays

Intermediate overall luminosity

Sometimes -ray dominated

(Boettcher & Reimer 2004)

Estimates from the SED:

F (sy)

F (C)

F (C) / F (sy) ~ u’rad / u’B

Constraints from Observations

C/sy = p2

sy C

sy = 3.4*106 (B/G) (D/(1+z)) pHz

→ Estimate u’rad

→ Estimate peak of electron spectrum, p

If -rays are from SSC:

If -rays are from EC (BLR or IR):

C ~ ext p2

From synchrotron spectral index:

Electron sp. Index p

F ~

If -rays are Compton emission in Thomson regime:

F (sy)

F (C)

F (C) / F (sy) ~ (dE/dt)T / (dE/dt)sy = u’rad / u’B

u’rad =

’ = in the co-moving frame of the emission region

u’sy

u’BLR ≈

u’IR ≈

u’disk ≈

u’jet ≈

SSC

EC(disk)

EC(BLR)

EC(IR)

EC(jet)

LD

4r22cLD BLR 2

4rBLR2c

2 uIR

rel2 u’jet

Constraints from Observations

W ComaeLow-flux state around MJD 54626 is poorly constrained

because of lack of -ray detections

SSC fit:

B= 1.0 G

LB/Le = 0.40

(can easily be ruled out by any -ray-detection!)

EC fit:

B = 0.35 G

LB/Le = 0.35

Low-flux state on MJD 54626

W ComaeIntermediate state around MJD 54631.5 (XMM-Newton

ToO) ; also poorly constrained -ray spectrum

SSC fit:

B= 0.7 G

LB/Le = 0.10

EC fit:

B = 0.45 G

LB/Le = 0.78

Intermediate-flux state on MJD 54631.5