Transcript Document 7451945

High energy radiation from extragalactic
jets and prospects for GLAST
Greg Madejski
Stanford Linear Accelerator Center and
Kavli Institute for Particle Astrophysics and Cosmology
In collaboration with: Jun Kataoka, Tad Takahashi, Marek Sikora, Lukasz
Stawarz, Yuri Kovalev, Stefan Wagner, …
• Context: blazars as active galaxies with dominant relativistic jet
• Main questions: structure, acceleration, collimation, and content of the jet,
-> all inferred from radiative processes via broad-band spectra
• Recent Suzaku observations of blazars – signature of “bulk-Compton” process?
• Future: GLAST
Context: Broad-band spectra and time variability
(example: archetypal GeV blazar 3C279, 1996 flare)
(Data from Wehrle et al. 1998)
* GeV emission dominates the observed flux
* Correlated variability on day time scales is common
* Variability in X-ray and g-ray bands puts constraints on the minimum relativistic
boost (Gj) of the innermost region (via g-g absorption to e+/e- pair production)
* Lorentz factors Gj of jets inferred from VLBI (multi-parsec scales) can be
compared against Gj inferred from variability (sub-parsec scales)
-> Gjet as a function of distance from the black hole? - > constraints on acceleration
process of the jet
EGRET All Sky Map (>100 MeV)
3C279
Cygnus
Region
Vela
Geminga
Crab
Cosmic Ray
Interactions
With ISM
LMC
PSR B1706-44
PKS 0208-512
PKS
0528+134
“Working picture” and approach
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Radiation processes probably are
adequately understood (synchrotron for
LE peak, inverse Compton for the HE
peak) – but hadronic processes are not
completely ruled out
Pressing questions:
How are the jets accelerated and
collimated?
What is the process of energy
dissipation (=acceleration of radiating
particles) in the jet?
What is the content of the jet as a
function of distance from the BH?
Approach for all three: study the timeresolved spectra, infer the radiative
processes and thus jet structure,
connection to the accretion process
and the black hole ->
MULTI-BAND MONITORING IS
REQUIRED
Focus here: jet content
Diagram from Padovani and Urry
The “blazar sequence”
* Work by G. Fossati, G. Ghisellini,
L. Maraschi, others (1998 and on)
* Multi-frequency data on blazars
reveals a “progression” –
* As the radio luminosity increases:
- Location of the first and second peaks
moves to lower frequencies
- Ratio of the luminosities between
the high and low frequency
components increases
- Strength of emission lines increases
High-luminosity, MeV - GeV-peaking objects (FR-IIs beamed towards us)
Lower-luminosity, TeV-peaking objects (FR-Is beamed towards us)
Suzaku observations of blazars
•Suzaku observed a number of blazars - the list of EGRET blazars includes PKS 1510-089,
BL Lacertae, SWIFT0746+25, 0836+714 (covered below), but also many TeV blazars
•Generally, the X-ray spectra of EGRET blazars are very hard power laws (photon indices
a often < 0.5), extending into the HXD PIN energy range (contrast to TeV blazars)
* Very difficult to explain such hard spectra!
Suzaku XIS + HXD PIN spectrum of 0836+714
Synchrotron + Compton model by Tavecchio et al. 2000
(old BeppoSAX data)
Suzaku observations of PKS 1510-089: hard
power law + “soft excess”
•
Suzaku observed PKS 1510089, a blazar at z ~ 0.3 for
120 ks (joint observation of
Jun Kataoka & GM; Kataoka+
2007)
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Spectrum is a hard power law
(energy index a ~ 0.2), but
with a soft X-ray “excess”
below ~ 1.5 keV, best
described as a steep power
law or a thermal component
with kT ~ 0.2 keV
•
Explanation as a “tail” of the
synchrotron emission unlikely
– extrapolation does not work
•
Too hot to be the tail of the
“blue bump”
Suzaku obervations of PKS 1510-089: possible
evidence for “bulk-Compton” bump?
•
This soft excess might be the tentative evidence for the “Sikora bump” – arising by
the inverse Compton scattering of BEL light by the cold electrons comoving in the
relativistic jet (Ebump ~ E(Lya) x G2 ~ 1 keV)
•
Even if it is not “bulk-Compton”…
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From its isotropic luminosity of LBC < 3 x 1044 erg/s - we can set a limit on the
energy flux Le,cold carried by the cold electrons and the e+/e- pair content of the jet:
-since LBC = (4sT/3mec2) UBEL rBLR Gj3 Le,cold
we have
Le,cold
< 2.7x1043 (rBLR/0.1pc) (Gj/10)-3 (LBEL/1045erg/s)-1 erg/s
- Significantly less than the required kinetic luminosity
of the jet
- Now the total power delivered by the jet
must be 8x1044 erg/s
-With more realistic parameters, ne/np in the jet is < 5
--> Jet contains more pairs than protons, but cannot be
dynamically dominated by e+/e- pairs
-For details, see Kataoka+ 07 (-> AstroPh in a week)
GLAST LAT has much higher sensitivity to weak
sources, with much better angular resolution
Planned launch: December 2007
GLAST
EGRET
GLAST Large Area Telescope – principles of
operation
*
g-rays interact with the hi-z material in the foils,
pair-produce, and are tracked with silicon
strip detectors
* Energy range is ~ 30 MeV – 300 GeV, with the
peak effective area of ~ 10,000 cm2
* This allows an overlap with TeV observatories
* The instrument “looks” simultaneously into ~ 2
steradians of the sky
Schematic of operation
of GLAST LAT
GLAST observing strategy & performance:
angular resolution, broad-band
sensitivity (1 year)
(for more details, see poster by Paneque)
100 s
1orbit
1 day
3EG 
limit
0.01 
0.001
LAT 1
yr
2.3 10-9
cm-2 s-1
Sensitivity of GLAST to measure flux & variability of g-ray sources
•
GLAST can measure flux of
3C279 in outburst to better than
10% in a day, can determine the
index to DG ~ 0.1
(assumed G = 2)
•
Expected light curve of 3C279
from GLAST LAT
(flux history is assumed to be the
same as that measured by
EGRET for 3C279 in 1996)
SUMMARY
SUZAKU HAS PROVEN TO BE AN EXCELLENT INSTRUMENT TO STUDY BLAZARS,
BUT THE MOST IMPORTANT INFERENCES WILL BE VIA MULTI-BAND
MONITORING OF THOSE HIGHLY VARIABLE SOURCES
MOST CRUCIAL WILL BE JOINT OBSERVATIONS WITH GLAST, TOGETHER WITH
OPTICAL OBSERVATIONS
WE HAVE A GOOD IDEA ABOUT RADIATIVE PROCESSES IN BLAZARS HIGHLY ENERGETIC PARTICLES RADIATING IN A RELATIVISTIC JET –
BUT MANY QUESTIONS REMAIN:
•
WHAT IS THE CONNECTION OF THE JET TO THE BLACK HOLE?
•
WHAT ACCELERATES AND COLLIMATES THE JET?
•
HOW IS BULK KINETIC ENERGY OF THE JET CONVERTED TO ENERGY OF
RADIATING PARTICLES?
•
WHAT IS THE RELATIONSHIP OF BLAZARS TO RADIO GALAXIES?
GLAST AND SUZAKU WIILL OPEN NEW ERA FOR STUDIES OF
ASTROPHYSICAL JETS, ALLOWING NEW
INSIGHTS INTO THE STRUCTURE
OF THOSE EXTREME PARTICLE ACCELERATORS
Recent outburst of 3C454.3
SSC or ERC?
Example of an object
where SSC may
dominate: Mkn 421
Example of an object
where ERC may
dominate: 3C279
(data from Macomb et al. 1995)
(data from Wehrle et al. 1998)
Modelling of radiative processes in blazars
• In the context of the synchrotron models, emitted photon frequency is
ns = 1.3 x 106 B x gel2 Hz
where B is the magnetic field in Gauss
and gel is the electron Lorentz factor
• The best models have B ~ 1 Gauss, and gel for electrons radiating at the
peak of the synchrotron spectral component of ~ 103 – 106,
depending on the particular source
• Degeneracy between B and gel is “broken” by spectral variability
+ spectral curvature, at least for HBLs (Perlman et al. 2005)
• The high energy (Compton) component is produced by the same
electrons as the synchrotron peak and ncompton = nseed x gel2 Hz
• Still, the jet Lorentz factor Gj is ~ 10, while Lorentz factors of
radiating electrons are gel ~ 103 – 106
• Thus, one of the central questions in blazar research is:
HOW ARE THE RADIATING PARTICLES ACCELERATED?
Interpretation of the observational data for blazars
PARTICLE ACCELERATION
* The most popular models invoke the Fermi acceleration process in shocks forming via collision of
inhomogeneities or distinct plasma clouds in the jet (“internal shock” model, also invoked for GRBs)
* This can work reasonably well: the acceleration time scale tacc to get electron up to a Lorentz factor gel
can be as short as ~ 10-6 gel B-1 seconds, while the cooling time (due to synchrotron losses) is
~ 5 x 108 gel-1 B-2 seconds, perhaps up to 10 times faster for Compton cooling, so accelerating
electrons to gel up to ~ 106 via this process is viable (but by no means unique!)
INTERNAL SHOCK SCENARIO MODEL
* This model assumes that the central source produces multiple clouds of plasma and ejects them with
various relativistic speeds: those clouds collide with each other, and the collision results in shock
formation which leads to particle acceleration
* A simple "toy model" that reproduces observations well assumes two clouds of equal masses, with
Lorentz factors G1 and G2 with G1 < G2 (G1 and G2 >> 1)
* From G2 and G1 one can infer the efficiency (fraction of kinetic power available for particle acceleration)
* Recent simulations reproducing well the X-ray light curves of Mkn 421 (and applicable to other objects)
(Tanihata et al. 2003) imply that the dispersion of G cannot be too large
* However, the small dispersion of G implies a low efficiency – (< 0.1%) so there might be a problem
- as huge kinetic luminosities of particles are required…
* MY OWN PREJUDICE IS THAT THE JETS ARE LAUNCHED AS MHD OUTFLOWS,
AND ARE INITIALLY DOMINATED BY POYNTING FLUX
* WE NEED TO UNDERSTAND DISSIPATION/PARTICLE ACCELERATION
AS THE DISK – JET CONNECTION
AS WELL
Content of the jet
• Are blazar jets dominated by kinetic energy of particles
from the start, or are they initially dominated by magnetic
field (Poynting flux)? (Blandford, Vlahakis, Wiita, Meier, Hardee, …)
• There is a critical test of this hypothesis, at least for
quasar-type (“EGRET”) blazars:
• If the kinetic energy is carried by particles, the radiation
environment of the AGN should be bulk-Comptonupscattered to X-ray energies by the bulk motion of the jet
• If Gjet = 10, the ~10 eV, the H Lya photons should appear
bulk-upscattered to 102 x 10 eV ~ 1 keV
• X-ray flare should precede the g-ray flare (“precursor”)
• X-ray monitoring concurrent with GLAST observations is
crucial to settle this
• A lack of X-ray precursors would imply that the jet is
“particle-poor” and may be dominated by Poynting flux
GLAST LAT Science Performance Requirements Summary
Parameter
Requirement
Peak Effective Area (in range 1-10 GeV)
>8000 cm2
Energy Resolution 100 MeV on-axis
<10%
Energy Resolution 10 GeV on-axis
<10%
Energy Resolution 10-300 GeV on-axis
<20%
Energy Resolution 10-300 GeV off-axis (>60º)
<6%
PSF 68% 100 MeV on-axis
<3.5°
PSF 68% 10 GeV on-axis
<0.15°
PSF 95/68 ratio
<3
PSF 55º/normal ratio
<1.7
Field of View
>2sr
Background rejection (E>100 MeV)
<10% diffuse
Point Source Sensitivity(>100MeV)
<6x10-9 cm-2s-1
Source Location Determination
<0.5 arcmin
Sensitivity of GLAST LAT
Recent outburst of 3C454.3
• The points represent the energy flux for which will
get 2 sigmas that that specific energy range (1/4 of a
decade). Since we show 15 points, that implies an
overall signal significance of ~ sqrt(15)*2 ~ 8 sigmas.
• 1 GeV = 1.6 e-3 erg
• GLAST performance: angular
resolution, effective area as a
function of energy and off-axis
angle
100 s
1 orbit
1 day
3EG 
limit
0.01 
0.001
LAT 1 yr
2.3 10-9
cm-2 s-1
Radio, optical and X-ray images of the jet in M 87
* Jets are common in AGN – and radiate in radio, optical and X-ray wavelengths
* Blazars are the objects where jet is pointing close to the line of sight
* In many (but not all) blazars, the jet emission dominates the observed spectrum
Suzaku observations of blazars
* Suzaku observed a number of blazars in the SWG and GO phases
* The list of EGRET blazars includes PKS 1510-089
0836+714, BL Lacertae, and SWIFT0746+25
* Generally, the X-ray spectra of EGRET blazars are very hard power laws (G < 1.5), extending
into the HXD PIN energy range
* This is in contrast to the TeV blazars, showing softer power laws (G > 2), gradually
steepening towards higher energy (DG ~ 0.3 / decade) – Costamante’s talk
Suzaku XIS + PI spectrum of PKS 1510-089
(see poster by Kohmura)
Synchrotron + Compton model by Tavecchio et al. 2000
(old BeppoSAX data)
Example of recent Suzaku data for blazars: SWIFT0746
* Simultaneous multi-wavelength observations are very important, but
not all the ground-based data for recent campaigns are “in” yet
* The hard X-ray spectra measured by Suzaku (here: photon index 1.2)
already pose challenges to hadronic models sometimes invoked to
explain the “high energy” peaks of blazars
MODELING OF BLAZAR EMISSION
From Sikora, Begelman, and Rees 1994
•
Most viable models are “leptonic”
– synchrotron emission for the
low-energy peak, Compton
emission for the high energy peak
•
Source of the “seed” photons for
inverse Compton scattering can
depend on the environment internal to the jet (the
“Synchrotron self-Compton”) or
external to the jet (“External
Radiation Compton”)
•
The latter (ERC) is probably
applicable to blazars hosted in
quasars such as 3C279
•
Modelling is pretty robust, gives
B ~ 1 Gauss, gmax of radiating
electrons ~ 105
•
Most likely the jets are initially
dominated by magnetic energy
(Poynting flux); Sikora, GM,
Begelman, Lasota (2005)
Suzaku and EGRET blazars
* 0836+714 (=4C71.07) is one of most distant EGRET blazars, z = 2.172 shows a power law with G = 1.4
* The X-rays are likely produced by Compton upscattering of external, broad emission line photons or IR
* Hard X-ray band probes the „bulk” of the radiating particles (low n radio – contaminated by extended comp.)
* Suzaku-inferred power law index index G = 1.4 implies power law index of radiating particles
p ~ 1.8, even for such a hard spectrum
* Very important towards the determination of the jet content, and – because of the radiation
environment of the host galaxy – the distance from the black hole where the jet forms
XIS
BeppoSAX + multi-l 0836 data from Tavecchio et al. (2000)
HXD PIN
Suzaku 50 ks observation of 1510-089
Another example of Suzaku-observed blazar: BL Lacertae
BL Lacertae shows complex X-ray spectrum:
Hard power law above ~ 2 keV (onset of the
Compton component, probably SSC)
+ “soft excess,” probably the “tail” of the
synchrotron component
Light curve (1 day):
Modest variability (10%)
Synchrotron
ssc
ERC
Suzaku spectrum:
Absorption less than Galactic
(plotted as the green model)
-> 2 component spectrum
(soft E<2 keV + hard E>2 keV)
SSC+ERC model of simultaneous
data from 1998 (Madejski et al. 1999)
Example of VLBI superluminal expansion of a
(potential) GLAST blazar: 0716+714
•
All known EGRET blazars show
powerful radio jets
•
0716+714 is just one example, most bright EGRET blazars are
monitored with VLBI
(Jorstad et al. 2001)
•
Lorentz factors G of jets inferred
from VLBI (multi-parsec scales)
can be compared against G
inferred from variability (subparsec scales)
•
-> Gjet as a function of distance
from the black hole? - >
constraints on acceleration
process of the jet
Toy model: particle acceleration via internal shocks
Broad line region
providing the ambient UV
Accretion disk and black hole
Time ->
Diagram for the internal shock scenario – colliding shells
model: G2 > G1, shell 2 collides with shell 1