Lecture Note
X-Rays and γ-Rays from Relativistic Jets in Quasars
-
University:
Boston University -
Course:
CAS AS 311 | Planetary Physics Academic year:
2023
-
Views:
58
Pages:
27
Author:
Clint W.
X-Rays & γ-Rays from
Relativistic Jets in Quasars Phenomenology: What Is a Quasar?
•
•
•
Ultraluminous (1045-1047 ergs/s), active nucleus of a galaxy (most
luminous non-transient objects in the universe)
Broad (tens of thousands of km/s) emission lines with high
redshifts
Visible/uv continuum has broad “bump” in ν-Fν spectrum
Subclass (5-10%): Blazars
•
•
•
•
•
Emission dominated by relativistic jet (flow Lorentz factors can
be at least as high as 40 but usually <10) (jet is 2-sided, but
relativistic beaming causes receding side to be too faint to
detect)
Radio – visible (in some cases even to soft X-rays): incoherent
synchrotron radiation (random pitch angles; magnetic field is fairly
turbulent – near-zero circular polarization, modest linear polarization)
X-ray – γ-ray: (inverse) Compton scattering of radio-visible (-Xray for sources of TeV γ-rays) “seed” photons
Apparent superluminal motion of bright features on radio images
Extremely rapid variations in brightness relative to light-travel
time across emission region (Doppler effect for approaching jet) Hubble Space Telescope Images of Host Galaxies of
Quasars Model of a Quasar
•
•
Supermassive (3x108±1 solar masses) accreting black hole
Dominant components of observed emission depend on
direction of line of sight:
→ Viewed within about 40° of disk equator: non-blazar
→ Viewed within about 20° of jet axis: blazar
→ more extreme for smaller angle, higher Lorentz factor
of flow
clouds Quasar Emission
•
Accretion disk: thermal blackbody emission, temperature depends on
distance from black hole (“big blue bump”→ soft X-rays, harder Xrays from electron scattering in “corona” above disk)
•
Clouds near base of jet: broadest emission lines (not clear whether
motions of clouds are inflow, outflow, rotation, or chaotic); clouds
farther out: “narrow” emission lines
→ Jet: synchrotron + inverse Compton (various sources of seed
photons: accretion disk, jet, clouds)
clouds The Jet in the Radio Galaxy M87
•
•
Hubble Space Telescope observations of motions of stars
in nucleus: Central black hole of 3x109 solar masses
Jet appears broad close to black hole (becomes narrowly
focused by ~ 1 light-year)
Observations by Junor,
Biretta, & Livio 1999, Nature
Images taken from NASA’s
Space Telescope Science
Institute web page,
www.stsci.edu Why Study Quasars?
→ Physics under extreme conditions:
• Prolific acceleration of particles
• Prolific production of high-energy photons (up to
10 TeV)
• Exotic forms of matter (perhaps more positrons
than protons in jet)
• Relativistic gas dynamics
• Relativistic turbulence
• Relativistic plasma processes
• Accretion onto supermassive black holes
• Relativistic effects and illusions
• Probes of the high-redshift universe Some Issues Addressed by Our Research
• Phenomenology of relativistic jets: Superluminal
motion, Lorentz factors, rapid variability in
brightness & polarization, changes in direction,
magnetic field topology, disturbances in flow, …
• Relativistic shock waves as sites of particle
acceleration and magnetic field amplification
• Similarity between observed features and
relativistic gas dynamical simulations
• Origin of X-rays and γ-rays
• Comparison of bright γ-ray blazars with general
population of radio-“loud” quasars
• Evidence for accretion onto black holes Superluminal Motion (An Illusion)
to observer
θ
βc
Velocity transverse to line of sight: βc sin θ; Γ = (1-β2)-½
But, in observer’s frame, source keeps getting closer, so
arrival times of radio waves are compressed
Time required for source to move distance Δr is t´=(Δr/Γ)/(βc)
in source frame (length contraction) → Δr = Γβct´
Transformation to observer’s frame: t = t´/[Γ(1-β cos θ)]
(Doppler effect)
So, βapp= (Δr sin θ)/t = Γβc(t´/t) sin θ = (βct sin θ)/(1-β cos θ)
→ Maximum value: βapp= Γβc [occurs at angle θ = sin-1(1/Γ)] The Radio Galaxy 3C 111 (z=0.0485)
Clear example of one-sided jet structure
with superluminal apparent motion
at 5c (1.5 milliarcsec/yr)
Jet propagates through galaxy and into
intergalactic space, where it feeds
giant twin radio “lobes”
Scale: 1 mas = 0.92 pc = 3.0 lt-yr (Ho=70)
Stationary core
Observations
made at 43
GHz with the
Very Large
Array (VLBA)
of the National
Radio
Astronomy
Observatory The Quasar 3C 454.3 (z=0.859)
Superluminal knot moves through
extended stationary feature, then
vanishes
Scale: 1 mas = 7.4 pc = 24 lt-yr
(Ho=70) The BL Lac Object 1823+568 (z=0.66 ?)
Superluminal apparent motion
at ~12c (0.36 milliarcsec/yr)
Persists for 1.5 yr, then stops!
(although weak remnant
may continue moving)
→ Disturbance energizes
stationary feature (standing
shock?)
Scale: 1 mas = 6.6 pc = 21 lt-yr
(Ho=70) The Quasar CTA 102 (z=1.037)
Scale: 1 mas = 7.8 pc = 25 lt-yr (Ho=70)
“Monster” radio outburst in 1997 produced very
bright superluminal (13c) knot, with continued
activity as stationary feature 2 mas away from
core dissolves
Polarization changes rapidly; similarities with
JCMT polarization direction at some epochs,
not at others BL Lac (z=0.069)
Jet does a hula dance:
~ 2-yr cycle of 24° swing in
direction of jet near
core
→ interpreted as
precession
Scale: 1 mas = 1.3 pc = 4.1
lt-yr (Ho=70) The “Microquasar” GRS 1915+105: A
Superluminal Object in Our Galaxy
Apparent velocity 2-3c, but jet makes a
large angle to line of sight; Γ ≅ 5
Mirabel & Rodriguez (1994 Nature)
Binary system, giant star + black hole of
14 solar masses
Ejection of superluminal knots follows
drop in X-ray flux, modeled as
sudden break off & accretion of
inner gaseous disk
Both figures from Felix Mirabel’s web page:
Moriond.in2p3.fr/J01/transparents/Mirabel/mirabel.pdf Cartoon of Quasar vs. Microquasar
Can this analogy be confirmed by observation of an active galactic
nucleus with some features similar to those of microquasars?
Figure from Felix Mirabel’s web page The Radio Galaxy 3C 120 (z=0.033)
Scale: 1 mas = 0.64 pc = 2.1 lt-yr (Ho=70)
Superluminal apparent motion, 4-6c (1.82.8 milliarcsec/yr)
X-ray spectrum similar to BH binaries
Radio core at least 1 lt-yr from black hole
Superluminal ejections follow X-ray dips
→ Similar to microquasar GRS 1915+105 Relativistic Gas Dynamics of Jets: Numerical
Simulations
• Beam of relativistic gas injected from left, flow Lorentz factor Γ=4
• Perturbation initiated in form of momentary increase to Γ=11
• Shock wave forms at head of disturbance
• Notice compressions & rarefactions in wake of disturbance
Click to view movie
Synchrotron emission
for jet pointing 10°
from line of sight,
convolved by circular
point-spread function X-Rays from Relativistic Jets in Quasars
Observations by Marscher et al. using NASA’s Rossi X-ray Timing Explorer (RXTE):
Several blazars observed 2-3 times per week (except for 8-week sun constraint)
X-ray flux is highly variable with wide range of amplitudes and timescales
Example (started in March 2001): 3C 273 (z=0.158), brightest quasar in visible light: The Quasar PKS 1510-089 (z=0.361)
Scale: 1 mas = 4.8 pc = 16 lt-yr (Ho=70)
Superluminal apparent motion greater than 40c
(2.1 milliarcsec/yr) (fastest confirmed speed)
Bright X-ray source with highly variable flux
Bright source of 0.1-3 GeV γ-rays The Quasar PKS 1510-089 (cont.)
X-ray flares well-correlated with radio (14.5 GHz) in 1997 & 1998, with radio variations
leading X-ray by ~ 2 weeks (“reverse time lag”); ejections of superluminal knots tend
to coincide with X-ray flares
→ X-rays come from superluminal knots in jet (not near accretion disk)
→ delays signify frequency stratification (e-s accelerated at shock front, highest energy
e-s “die” quickly, lower-energy ones live longer → lower ν emitted over larger
volume)
Discrete Cross-Correlation
Coefficient (X-ray vs. radio)
X-ray
Radio
Visible The Quasar 3C 279 (z=0.538)
Superluminal apparent speeds, 5c – 13c
Scale: 1 mas = 6.0 pc = 19.5 lt-yr (Ho=70)
Magnetic field usually ⊥ jet axis
→ transverse shocks
Sudden change in speed at beginning
of 1996
Bright, highly variable X-ray & γ-ray
emission The Quasar 3C 279 (cont.)
Variations in X-ray flux usually well-correlated with visible brightness, with little or no
time lag; ejections of superluminal knots tend to coincide with X-ray flares
→ X-rays and visible light come from superluminal knots in jet (not near accretion disk)
Note: radio light curve is a superposition of a number of components in jet, hence there
is only sometimes a correspondence between radio flares & ejections of new knots
X-ray
Visible
Radio
Discrete Cross-Correlation
Coefficient (X-ray vs. visible) γ-Rays from Blazars
~70 blazars detected by Thomson, Hartman, et al. using NASA’s
Compton Gamma Ray Observatory (CGRO, EGRET detector,
0.1-3 GeV)
Questions: Are γ-ray blazars different from quasars not detected
by CGRO?
Are γ-rays produced near the black hole or in radio-emitting
section of jet (→ intimately related to physics of γ-ray
production)?
Our project: Observations at multiple epochs (93-97) of radio jets
of 42 γ-ray blazars with the VLBA to obtain apparent velocities
to estimate flow Lorentz factors that can be compared with
those of the general radio-“loud” quasar population
Secondary goal: Try to determine whether superluminal
ejections are related to times of especially high γ-ray flux γ-Rays from Blazars: Main Results
Distribution of apparent velocities of radio knots in γ-ray blazars
peaks at 10-12c while that of general radio-loud quasars is
skewed toward 1-3c
Times of high γ-ray flux statistically associated with epochs of
ejection of superluminal radio knots (needs confirmation from
future γ-ray mission GLAST, which will provide much more
detailed γ-ray light curves)
→ γ-rays are emitted from radio-emitting section of jet, probably
from superluminal knots (not from near black hole; from
theoretical considerations, any γ-rays emitted from close to the
BH would be lost to e+-e- pair production off X-rays from
accretion disk) Conclusions
•
•
•
•
•
The ultimate source of the luminosity and production of relativistic jets of
quasars and other active galactic nuclei with jets is accretion onto
supermassive black holes (see Meier et al. 2001, Science and
www.jpl.nasa.gov/releases/2002/release_2002_21.html for magnetohydrodynamical (MHD) simulations of jet production)
X-rays and γ-rays from blazars are produced via (inverse) Compton
scattering by same electrons that emit the synchrotron radiation seen at
radio to visible (to X-ray in the TeV-emitting blazars) frequencies
Site of X-ray and γ-ray emission is in the same shock waves that produced
superluminal radio knots, many light-years from the black hole
Multi-frequency monitoring & space-VLBI (iARISE) during the next
generation γ-ray mission (GLAST) is surely the best way to confirm the more
tentative results of our study of γ-ray blazars and, of course, to reveal new
phenomena
Relativistic jets display complex phenomena that resemble cylindrically
symmetric relativistic gas dynamical simulations; as high-resolution fully 3-D
gas and MHD calculations become available, such comparisons should
allow us to make important inferences regarding the nature of jets
X-Rays and γ-Rays from Relativistic Jets in Quasars
of 27
Report
Tell us what’s wrong with it:
Thanks, got it!
We will moderate it soon!
Free up your schedule!
Our EduBirdie Experts Are Here for You 24/7! Just fill out a form and let us know how we can assist you.
Take 5 seconds to unlock
Enter your email below and get instant access to your document