Transcript X-ray emission from young pulsar, PWN and SNRs

Pulsar wind nebulae and their
interaction with the environments
Fangjun Lu
卢方军
Institute of High Energy Physics
Chinese Academy of Sciences
Most of the spin-down power of a
pulsar is not released through
direct radiation
(Li, Lu, & Li 2008, ApJ 682, 1166)
http://chandra.harvard.edu/
Most (90%) of the spin-down power of a
pulsar is released via a relativistic wind.
The basic configuration of a PWN
Pulsar
•
Pulsar wind
Therefore, the properties of
a pulsar
Terminal shock
wind nebula are highly determined
Pulsar wind nebula
by the composition and geometry
of
Interface with the
the pulsar wind as well Interstellar
as the
Medium
interaction
thetheenvironments.
The
magnetized pulsarwith
wind leaves
pulsar with almost the
speed of c (γ~103-106).
• A termination shock forms at the radius where the wind rampressure balances the pressure of the environments, and over there
the particles are randomized ( and probably accelerated) and begin
to emit synchrotron photons.
• The PWN is a magnetized particle bubble surrounded by the ISM.
( Rees & Gunn 1974)
Outline
• A statistical study of the X-ray emission of
young pulsars and pulsar wind nebulae.
• Pulsar wind nebulae in various conditions:
–
–
–
–
–
“Freely” expands;
Within a supernova remnant;
Moves supersonically in space;
In highly organized magnetic field;
In high velocity interstellar wind.
A statistical study with a sample of 27 pulsars
Li, Lu & Li 2008, ApJ 682, 1166
A statistical study with a sample of 24 PWNe
Li, Lu & Li 2008, ApJ 682, 1166
Relations between the X-ray luminosity and
spin-down power
The more energetic pulsars intend to release a
bigger fraction of their spin-down power in their
PWNe.
Li, Lu & Li 2008, ApJ 682, 1166
Relations between the X-ray luminosity and
the photon index
Apparently, there is a negative correlation between the Xray luminosity and spectral index for pulsars, while a positive
correlation holds for the PWNe.
Li, Lu & Li 2008, ApJ 682, 1166
Pulsare
PWNe
Younger (more energetic) pulsars tend to have harder spectra from
their magnetosphere, while softer spectra from their wind nebulae
Li, Lu & Li 2008, ApJ 682, 1166
What do these results tell us about
the physics of pulsars and PWNe,
besides these empirical formulae?
Physical implications
•
The photon indices between 2 and 3 for pulsars pose question on all
the current pulsar models. Because the cascade never produces X-ray
emission with photon index higher than 2.
•
The spectral index of synchrotron photons is between (p+1)/2 to
(p+2)/2 from the slow-cooling regime to the fast-cooling regime. The
photon indices of the PWNe strongly favor an electron index of p~2.2,
well consistent with the theoretical predictions on particle acceleration
in relativistic collisionless shocks. So the termination shock does
accelerate electrons. The γfactor of the wind is probably much smaller
than 106.
•
The PWNe of the more energetic pulsars tend to have higher magnetic
field in the nebulae, which gives higher synchrotron efficiency and
softer X-ray spectrum of the nebula. But it is not the case for the X-ray
emission of the pulsars.
PWNe in various environment conditions
Interfaces
The morphology of a PWN is
determined by the geometry of the
pulsar wind and the distribution of
the ISM surrounding the PWN.
t sy 
8.7  10
B
3/ 2
 sin
3/ 2
11
 
or t x ~ 40 yr B 41 .5 keV
1/ 2
m
 0 .5
(s)
The X-ray image represents the
distribution of fresh wind particles,
and the X-ray spectral evolution
traces the particle flow. The radio
images could be affected by the
distribution of the aged particles.
SNR G54.1+0.3: A PWN without significant
confinement
Pulsar
G54.1+0.3 is powered by
the 136 ms pulsar in the
center. The Chandra X-ray
image shows bright ring
surrounding the pulsar, and
two elongations roughly
perpendicular to the ring.
The wind of the central
pulsar can be divided into
two components: equatorial
flow and polar flow.
(Lu et al. 2002)
The downstream velocity could be derived as 0.4 c by fitting the
brightness variations, which means that the wind is particle dominated.
(Lu et al. 2002)
(Lang, Wang, Lu, & Clubb 2010)
The X-ray and radio images look very much similar to each other, and
the magnetic field is well organized.
The radio and X-ray
extents are almost the
same for G54.1+0.3.
(Lang, Clubb, Lu, & Wang 2009)
• Similarity of the radio and X-ray morphology: there
is no significant accumulation of old particles.
• Similar radio and X-ray sizes: the size of the nebula
is determined mainly by the diffusing of the
particles rather than by their lifetime. The quick
diffusion lowers both the particle number density
and the magnetic field (if we assume equipartition)
and so the radio synchrotron brightness decreases
very rapidly.
G54.1+0.3 is very weakly
confined by the environment
PSR B1951+32
The PWN of PSR B1951+32 is in the center of SNR CTB 80.
S II
O III
(Hester & Kulkarni 1989)
Hα
Log (OIII/H α) & H α
In the center of CTB 80 (surrounding the pulsar), small nebulae
have been detected in optical emission lines.
Radio image of core of CTB 80
Chandra X-ray image
Migliazzo et al. (2002)
Li, Lu & Li (2005)
The PWN of PSR B1951+32 shows a shell-like structure in both radio and Xrays, suggesting strong confinement by the SNR ejecta. The high brightness
region is well within the OIII and S II line filaments, while the Hα filaments
define the edge of the low brightness region.
X-ray tail
Termination
shock
Shell
Pulsar
Contours: X-ray Greyscale: Radio
The PWN is produced by the supersonically moving pulsar in SNR ejecta.
(Li, Lu, & Li 2005)
We find intriguing spectral hardening in regions of the radio and X-ray shell, which
can only be explained by the new particle acceleration. This shows that the pulsar
wind bubble can expand supersonically and generate shocks, even in such an old
(5×105 yr) system. The optical filaments are also produced by the shock wave
propagating into the ejecta and the interstellar medium.
(Li, Lu, & Li 2005)
G359.94-0.04: A cometary PWN near the
Galactic Center
Chandra X-ray image of the Galactic
center.
X-ray contours overlaid on the IR
image
(Wang, Lu, & Gotthelf 2006)
(van der Swaluw et al. 2003)
(Gaensler et al. 2004)
When a pulsar moves supersonically in the ISM, a bow shock will be running
into the ISM, a termination shock will be ahead of the pulsar, and most of the
the pulsar wind will be confined to the direction opposite to the pulsar proper
motion. The radius of the termination shock is determined by the spin-down
power and proper motion velocity of the pulsar as well as the ISM density.
Linear brightness profile
Photon index evolution
The cometary morphology is a sign of the ram-pressure confined
PWN. The spectral steepening with increasing distance from the
point source further confirms such an identification.
HESS TeV Observation of
the Galactic Center Region
G359.95-0.04
HESS J1745-290
Evidence for G359.95-0.04 (rather than Sgr A*) as
the counterpart of HESS J1745-290
• Many PWNe are observed as TeV sources.
• G359.95-0.04 is consistent with HESS J1745-290
positionally .
• G359.95-0.04 can contribute comparable TeV flux
through inverse compton scattering to the ambient seed
photons (Wang et al. 2006, Hinton & Aharonian 2007, ApJ 657, 302).
• HESS J1845-290 does not show any variation, especially
when Sgr A* (the other candidate) experiences flares
(Hinton et al. 2007, @ICRC 30).
Radio map of the GC region
• Non-thermal filaments almost perpendicular to the Galactic Plane.
• The origin of the high energy particles has been a long time mystery.
X-ray G0.13-0.11: A pulsar wind nebula in
strong organized magnetic field
Wang, Lu & Lang 2002
G0.13-0.11 leads a bunch of NTFs (the GC arc)
Spectral index variation of the GC arc.
Spectral index variation of the GC arc
Reflects the aging of the high energy particles with increase
distance from the pulsar. G0.13-0.11 is probably a PWN in strong
organized magnetic field.
The X-ray emission of G359.54+0.18
Lu, Wang & Lang 2003
Pulsar wind particles could also illuminate the non-thermal
radio filaments in the Galactic Center region.
A deep Chandra image of the GC region.
Morphological properties
(Lu, Yuan, & Lou 2008)
Their spectra
are nonthermal.
F1-F10
(Lu, Yuan, & Lou 2008)
Main morphological and spectral properties of
the filaments
• Most of the filaments contain point-like sources at
their heads.
• All
the filaments
cometaryare
morphology.
The
X-ray show
filaments
most
• The
spectra
are
non-thermal,
with
photon
indices
probably
close
to
Sgr
A*
and
32
34
1.0-2.5, Lx 10 to 10 erg/s, and absorption
powered
by23 pulsars
younger
column
density 10
cm-2.
5 yrs.counts, a spectral
• When
there
are enough
than
3*10
softening with distance from the point-like source
can be detected.
Star formation rate
15 pulsars younger than 3*105 yrs within 7 pc
from Sgr A*
=>
Star formation rate of 6*10-4 solar mass yr-1,
~100 times higher than the mean star formation
rate of the Galaxy.
ThereArches
are many
massive stars
in the GC region.
The
cluster
Quintuplet
cluster
mass function of stars in this region suggests a star
formation rate of 10-7 solar mass yr-1 pc-3, about 250
times higher than the mean of the Galaxy, consistent
with our estimate basing on the number of X-ray
PWNe in this region.
Galactic Plane
Sgr A*
Most of the X-ray filament tails point away from Sgr A*
• The fact that most of the X-ray filament tails point away from
Sgr A* suggests that there exists a radial flow from GC.
This flow (Galactic wind) blows the pulsar wind particles to
the anti-GC direction and shapes the cometary morphology.
These filaments thus represent PWNe in strong windy
environment.
• Since the pulsars are expected to move in random
directions with peculiar velocities of ~400 km/s, the speed of
the Galactic wind should be comparable to or greater than
~400 km/s.
Summary
• The luminosity of the PWN is positively correlated with the spindown power of the pulsar. So PWNe are usually detected for
young pulsars.
• The morphology of a PWN is determined by the interaction with
the environment. If the pulsar
– is in a low density cavity, then the structure of the PWN reflects
mainly the pulsar wind geometry.
– is surrounded by the SNR ejecta, the wind materials will be well
confined, and the expansion of the PWN can generate a strong
shock into the ejecta.
– moves supersonically in the ISM, the PWN will be like a comet with
the tail points to the opposite direction of the pulsar proper motion.
– locates in strong organized magnetic field, the PWN may well trace
the structure of the magnetic field.
• The filamentary PWNe in the Galactic center shows that the star
formation rate of the Galactic Center region is about 100 times
higher than the mean of the Galaxy. It is also shown that there is
a high velocity radial Galactic wind.