Transcript Markus Boettcher

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
Motivation
Until a few years ago, all TeV blazars were highfrequency-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 g-rays.
Similar in physical parameters to other TeV
blazars (HBLs)? (→ SSC dominated?)
Or more similar to LBLs? (→ EC required?)
Leptonic Blazar Model
Relativistic jet outflow with G ≈ 10
g-q
g1
n
g2 g
Compton
emission
nFn
Radiative cooling
↔ escape =>
Qe (g,t)
Synchrotron
emission
nFn
Qe (g,t)
Injection,
acceleration of
ultrarelativistic
electrons
g-q or g-2
g-(q+1)
g1 gb g2
gb g1 g2
g
n
Seed photons:
g b:
tcool(gb) = tesc
Synchrotron (within same region [SSC] or
slower/faster earlier/later emission regions
[decel. jet]), Accr. Disk, BLR, dust torus (EC)
Spectral modeling results along the
Blazar Sequence: Leptonic Models
Low magnetic fields
(~ 0.1 G);
High electron
energies (up to TeV);
High-frequency peaked
BL Lac (HBL):
The “classical” picture
Large bulk Lorentz
factors (G > 10)
No dense
circumnuclear
material → No
strong external
photon field
Synchrotron
SSC
Spectral modeling results along the
Blazar Sequence: Leptonic Models
High magnetic fields (~ a few G);
Radio Quasar (FSRQ)
Lower electron energies (up to
GeV);
Lower bulk Lorentz factors (G ~ 10)
Plenty of
circumnuclear
material →
Strong external
photon field
External
Synchrotron Compton
The Quasar 3C279 on Feb. 23, 2006
nFn, g ~ 5x1013 Jy Hz
nFn,sy ~ 1013 Jy Hz
Feb. 23:
• High optical flux
nsy ~ 5x1013 Hz
=> esy ~ 4x10-7
• Steep optical
spectrum (a = 1.7
-> p = 4.4)
ng ~ 1025 Hz =>
eg ~ 105
Accretion disk: LD ~ 2x1045 erg/s;
eD ~ 10-5
• High X-ray flux
• Soft X-ray
spectrum
Parameter Estimates: SSC
• Optical index a = 1.7 => p = 4.4 => cooling break
(3.4 -> 4.4) would not produce a nFn peak => peak
must be related to low-energy cutoff, gp = g1
• Separation of synchrotron and gamma-ray peak
=> gp = (eg/esy)1/2 ~ 1.6x105
• nsy = 4.2x106 gp2 BG D/(1+z) Hz
=> BG D1 ~ 7x10-5
Parameter Estimates: External Compton
• External photons of es ~ 10-5 can be Thomson scattered
up to eg ~ 105 => Accretion disk photons can be source
photon field.
• Location of gamma-ray peak
=> gp = (eg/[G2es])1/2 ~ 104
•
G1-1
nsy = 4.2x106 gp2 BG D/(1+z) Hz
=> BG ~ 1.8x10-2 G12 D1-1
• Relate synchrotron flux level to electron energy density,
eB = u’B/u’e
=> eB ~ 10-8 G17 R163
a) G ~ 15, B ~ 0.03 G, eB ~ 10-7
b) eB ~ 1, B ~ 0.25 G, G ~ 140 R16-3/7
Attempted leptonic one-zone model fit, EC dominated
X-rays severely
underproduced!
(Bӧttcher, Reimer & Marscher 2009)
Alternative: Multi-zone leptonic model
X-ray through gamma-ray spectrum reproduced by SSC; optical
spectrum has to be produced in a different part of the jet.
Linj = 2.3*1049 erg/s
gmin = 104
gmax = 106
q = 2.3
B = 0.2 G
G = D = 20
RB = 6*1015 cm
u’B/u’e = 2.5*10-4
(Bӧttcher, Reimer & Marscher 2009)
Requires far sub-equipartition magnetic fields!
Hadronic Model Fits
(Bӧttcher, Reimer &
Marscher 2009)
• Optical and g-ray spectral index can be decoupled
• X-rays filled in by electromagnetic cascades
• However: Requires very large jet luminosities, Lj ~ 1049 erg/s
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
gmin = 450
gmax = 4.5*105
q = 2.2
B = 0.007 G
LB/Le = 5.7*10-2
G = D = 30
Wide peak separation and low X-ray flux
require unusually low magnetic field!
RB = 1017 cm
W Comae
• Much more natural parameters for EC model
• For Compton scattering in Thomson regime, external
photons must have E ~ (mec2)2/EVHE ~ 0.1 – 1 eV => IR
Linj = 2*1044 erg/s
gmin = 700
gmax = 105
q = 2.3
RB = 1.8*1015 cm
B = 0.25 G
-> Equipartition!
Dtvar ~ 35 min. allowed with
external IR photon field
G = D = 30
(Acciari et al. 2008)
W Comae
Major VHE g-ray flare detected by VERITAS in June 2008.
Similar modeling conclusions to March 2008:
High flux state on MJD 54624
SSC fit:
B = 0.24 G
LB/Le = 2.3*10-3
EC fit:
B = 0.35 G
LB/Le = 0.32
(Acciari et al. 2009, in prep.)
3C66A
Major VHE g-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
G = D = 30
RB = 3*1016 cm
=> dtvar,min = 13 hr
3C66A
Fit with external IR radiation field (next = 1.5*1014 Hz)
yields more natural parameters:
B = 0.3 G
LB/Le = 0.1
G = D = 30
RB = 2*1016 cm
=> dtvar,min = 8.9 hr
Summary
1.
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 inermediateand 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
Intermediate objects:
Low-frequency peaked BL Lacs (LBLs):
Peak frequencies at IR/Optical and GeV
gamma-rays
Intermediate overall luminosity
Sometimes g-ray dominated
(Hartman et al. 2000)
Quasars:
Low-frequency component from radio to
optical/UV
High-frequency component from X-rays
to g-rays, often dominating total power
Peak frequencies lower than in BL Lac
objects (Boettcher & Reimer 2004)
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 highenergy gamma-rays
Constraints from Observations
Estimates from the SED:
nFn (C) / nFn (sy) ~ u’rad / u’B
→ Estimate u’rad
nFn (C)
nsy = 3.4*106 (B/G) (D/(1+z)) gp2Hz
→ Estimate peak of electron
spectrum, gp
nFn (sy)
If g-rays are from SSC:
nC/nsy = gp2
If g-rays are from EC (BLR or IR):
nC ~ G2 eext gp2
nsy
nC
From synchrotron spectral index a:
Electron sp. Index p = 2a + 1
Constraints from Observations
If g-rays are Compton emission in Thomson regime:
u’sy
nFn (C)
nFn (sy)
u’disk ≈
u’rad =
u’BLR ≈
SSC
LD
4pr2G2c
LD tBLR G2
4prBLR2c
EC(disk)
EC(BLR)
u’IR ≈ G2 uIR
EC(IR)
u’jet ≈ Grel2 u’jet
EC(jet)
’ = in the co-moving frame
of the emission region
nFn (C) / nFn (sy) ~ (dE/dt)T / (dE/dt)sy = u’rad / u’B
W Comae
Low-flux state around MJD 54626 is poorly constrained
because of lack of g-ray detections
Low-flux state on MJD 54626
SSC fit:
B = 1.0 G
LB/Le = 0.40
(can easily be ruled out by
any g-ray-detection!)
EC fit:
B = 0.35 G
LB/Le = 0.35
W Comae
Intermediate state around MJD 54631.5 (XMM-Newton
ToO) ; also poorly constrained g-ray spectrum
Intermediate-flux state on MJD 54631.5
SSC fit:
B = 0.7 G
LB/Le = 0.10
EC fit:
B = 0.45 G
LB/Le = 0.78