Active galactic nuclei 2 - AGN
Carlo Ferrigno
Department of astronomy - University of Geneva
https://cms.unige.ch/isdc/ferrigno/
2017 May 10
• X-ray emission
• Jets and winds
• Cosmological relevance (next lecture)
Summary
Unification
•
The line of sight determines the type of object at order zero.The broad-band SED
•
Radio emisioin: In Seyfert, the radio is only a few percents and con-centrated in the centre (low-power jets?).In radio loud galaxies, ex- tended jets.
•
Infrared emission is thought to de- rive from reprocessing of primary in the dusty torus. Sy 1 have stronger continuum that Sy 2.•
Optical and UV-emission: big blue bump Accretion disk and broad lines in the BLR for Sy 1. Scattered light and Galaxy contribution for obscured•
Bolometric luminosity can be esti-X-ray spectrum
•
Schematic view of main spectral components (Ricci 2011, PhD thesis.)•
Black-body disk emission in the UV band, at the left of the above plot!T
I=
2×
105M
108
M
1/4M ˙ M
yr
−1!
1/4R
I1014
cm
−3/4X-ray luminosity function
From Silverman et al. (2008).
•
The X-ray luminosity varies by 4 orders of magnitude.•
Masses are from 106 to 109M
Compton corona
From Haardt (1997).
•
Fast variability→
a fewr
g size for the emission region•
Inverse Compton corona with possible different shapes (seed photons from disc).Measured properties
From Dadina (2008).
•
Continuum properties are nearly equal between Sy 1 and Sy 2 galaxiesReprocessed emission
•
Power-law continuum due to Comptonization in hot Corona•
Thompson scattering of power-law photons onto disk: Comptom hump at∼
30 keV•
Photoabsorption of power-law photons in disk or torus: fluerescent FeK line at 6.4 keV.•
Modulation of continuum due to light bending of compact corona (lamp post model)Ionized reflection
From García et al. (2013).
•
For different ionization parameters, reflection onto an accretion disc creates different spectra.•
It might cause the “soft excess” observed in many objects.Soft excess - Comptonization model
From Petrucci et al. (2013).
•
Extrapolation of power-law does not work: either you blur reflection or you have multi-phase Comptonization.Soft excess - Comptonization model
From Petrucci et al. (2013).
•
Hot corona makes hard X-ray continuum.•
Warm corona makes Soft Excess.Soft excess - Comptonization model
From Petrucci et al. (2013).
•
Hot corona makes hard X-ray continuum.•
Warm corona makes Soft Excess.•
Absorption shapes the final spectrum..Broad Iron line
From Guainazzi et al. (2010).
•
A robust measurement of broad FeK line broadening.•
It is possible to decompose a narrow component from far cold gas from aK α diagnostics
Line profiles
From Fabian et al. (2002).
•
Narrow line is present•
Broadened line is very strong•
A steep emissivity profile•
However Majority of AGNs and QSOs do NOT show evidence for broadened lines ! Maybe truncated disks? Or ionized discs? Or viewing angle?•
Narrow line are ubiquitous: reflec- tion on the distant torus ? However too large velocity (4000-7000 km/s ) as compared to expected one (750 km/s). In the broad-line region?Location of Iron K
From Ponti et al. (2013).
•
The neutral Fe Kα
emission line in MrK 509 can be decomposed into a nar- row (σ
= 0.027 keV) component (found in the Chandra HETG data) plus a re- solved (σ
= 0.22 keV) component.•
It reverberates the hard X-ray continuum without any measurable lag sug- gesting that the region producing the resolved Fe Kα
component is located within a few light days to a weekPolarization
From Schnittman & Krolik (2009).
•
X-ray polarizion from spherical corona around a Kerr BH.•
signatures of polarization could help ruling out some geometries (IXPE mis-• X-ray emission
• Jets and winds
• Blazars
• Cosmological relevance
Winds
•
Wind can originate form the disk due to thermal or magnetic instabilitiues•
there are also UFOs (Ultra Fast Outflows).Winds a recent view
•
Markarian 509 was observed for 100 days with XMM-Newton, INTEGRAL and a fleet of observatories.•
Observations have shown that the outflow consists of giant bullets propelled at millions of kilometres per hour.•
The bullets are stripped away from a dusty reservoir of matter waiting to fall into the black hole. The surprise is that the reservoir is situated more than 15 light years away from the black hole.•
The observations also show that the accretion disc features a ’skin’ of gas with a temperature of millions of degrees. This is where the X-rays and gamma rays come from to drive the more distant gas outwards.Jets
•
enormous jets•
dust torus, much larger than the central engineSynchrotron spectrum
•
At lowν
electrons absorb syn-chrotron emission: self absorption.
•
The turnover describes the surface at whichτ =
1. In generalτ ∝ R
.•
For a power-law distribution of elec- tronsE
−p the total spectral shape isHigh frequency :
P
ν∝ ν
−(p−1)/2 (1) Low frequency :P
ν∝ B
−1/2ν
5/2 (2)•
More compact regions are optically thick, more diffuse regions are opti- cally thin.•
Synchrotron radiation is polarized:measuring its abgle allows us to find the magnetic field direction.
•
motion of jets can be directly ob- servedSee monitoring with VLBI on
http://www.physics.purdue.edu/astro/MOJAVE/movies.html
Superluminal motion and Doppler boosting
•
apparent motion at super-luminal speed for high speed and almost radial direction•
apparent velocity isv
app= ∆x
⊥∆t
obs= cβ sin φ∆t
e(
1− β cos φ)∆t
e•
frequency boosting•
Remembering thatS
νν
3 is a rela- tivistic invariant and using a power- law spectrum with indexα
•
The observed intensity ration of two identical jets isJets and lobes
•
This VLA image of the radio-loud quasar 3C 175 shows the core, an appar- ently one-sided jet, and two radio lobes with hot spots of comparable flux densities.•
The jet is intrinsically two-sided but relativistic, so Doppler boosting brightens the approaching jet and dims the receding jet.•
Both lobes and their hot spots are comparably bright and thus are not moving relativistically.MOJAVE Survey results
http://www.physics.purdue.edu/astro/MOJAVE/movies.html
•
Distribution of observed apparent velocities is from 0 to 15c.•
Quasars up to apparentβ ∼
50. BL Lac up toβ ∼
6•
similar speed of components within jets.•
bent trajectories, not cannon balls•
most luminosity is in unresolved regions smaller than 0.005 mas•
high energy gamma-ray emitters have faster and more compact jetsHigh-energy jet emission
M 87 Credit NASA/STSci/UMBC/ Pert- man et al.
Synchrotron emission dominates the EM spectrum:
•
similar morphologies in radio, X-ray and optical•
X-ray spectral index typically steeper than in the radio•
Correlated variability• γ
e∼
107−
108•
Life time of electron emitting syn- chrotron radiation ist ∼
1.
6×
107yrB
10−3
G
−2γ
e−1•
Blobs are regions with compact fireballas, outer regions filled with accelerated electrons in shocks.High-energy jet emission: Compton contribution
3C 273 in Radio, Optical and X0rays
•
In FRII galaxies, there is an excess of high-energy (X toγ
-ray) withrespect to synchrotron extrapolation
•
Only some FR I galaxies have this problem•
Inverse Compton of synchrotron ra- diation or from external field (CMB or interstellar field)•
compact jet→
SSC ?•
It is an open problem to determine the nature of the high-energy spec- tral energy distribution•
Ratio of powers is ratio of seed en- ergy densities:P
IC/P
S= U
rad/U
B.Blazars
•
“Blazars” indicates the set of Optical Violent variables and BL Lac (1978).Later, also the Flat spectrum Radio Quasars have been added
•
Dominant population of extragalacticγ
-ray sourcesBlazar’s SED
•
collimation and amplification due to relativistic boost•
Two peaks of similar intensity: one synchrotron, one inverse Compton•
Problem is about the seed photon•
Doppler factor isδ =
1γ (
1− β cos φ)
•
In the knot’s reference frame, radi- ation is emitted isotropically, in the observer’s frame it is beamed in a cone with opening angleγ
−1•
The energy peak is shiftedν
O= δν
e•
the intensity is boosted byI
v= δ
3I
ν0•
Apparent luminosity isL ∝ νI
ν∝ δ
4•
Blazars are due to jets pointing at us, major ejections cause outbursts.Huge swing of luminosity !
•
Interesting to think about a hadronicJet launching
•
Jets appear to be the consequence of accretion, rotation, and magnetic fields.•
Magneto-hydrodynamical model is necessary because speed near BH is rel- ativistic. Collimation by magnetic field.•
The dominant paradigms for jet launching follow the ideas outlined in Bland- ford & Znajek (1977) and Blandford & Payne (1982).Blandford-Payne
•
In the Blandford & Payne (BP) model the power for the jet comes from the ac- cretion disk.•
A large-scale poloidal magnetic field is anchored in and rotates with the disk.If the fieldlines are angled outward sufficiently with respect to the disk, there can be a net outward force on the matter. As matter is accelerated along the rotating field lines, its angular momentum increases still further, increasing the acceleration and driving an outflow.
B2 / 8π >> P, ρv2
B2 / 8π < ρv2
disk, not force free force free
region
not force free B
Alfven surface
Ω
l = Ω0r02
l = Ω0rA2
Blandford-Znajek mechanism
From Blandford & Znajek (1977).
•
Extracts rotational energy from the BH to create jets !•
the essential ingredient is a Kerr black-hole embedded in a magnetic field generated n the disk•
The key to the BZ process is that within the ergosphere it is possible to have an electromagnetic flux with negative energy at infinity. This neg- ative energy flux enters the hole, thereby reducing both the hole?s mass-energy and angular momen- tum•
the compensating EM flux at theexterior accelerates electrons, which then interact with photons and pro- duce EM cascade (IC+pair)
Magneto Rotational instability (MRI)
•
The accretion disk orbiting a massive object becomes turbulent.•
Fluid elements are linked by magnetic field and electric forces, but the differ- ential Keplerian rotation creates stress. At first approximation, we can treat the fluid elements as viscous oscillators which can become unstable.Feedback on surroundings
Fornax A: Radio (VLA) overlaid on optical (STSci/POSS-II); Credit: NRAO/AUI and J.M. Uson
•
Enourmous influence on space surrounding Galaxies: cosmological feed-Active Galactic Nuclei 0–35a
Bibliography
Blandford, R. D. & Payne, D. G. 1982, Mon. Not. R. astr. Soc, 199, 883 Blandford, R. D. & Znajek, R. L. 1977, Mon. Not. R. astr. Soc, 179, 433 Dadina, M. 2008, A&A, 485, 417
García, J., Dauser, T., Reynolds, C. S., et al. 2013, ApJ, 768, 146
Guainazzi, M., Bianchi, S., Matt, G., et al. 2010, MNRAS, 406, no Petrucci, P.-O., Paltani, S., Malzac, J., et al. 2013, A&A, 549, A73 Ponti, G., Cappi, M., Costantini, E., et al. 2013, A&A, 549, A72 Schnittman, J. D. & Krolik, J. H. 2009, ApJ, 712, 45
Silverman, J. D., Green, P. J., Barkhouse, W. A., et al. 2008, ApJ, 679, 118
