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)

Most (90%) of the spin-down power of a pulsar is released via a relativistic

wind.

http://chandra.harvard.edu/

( Rees & Gunn 1974)

Pulsar

Pulsar wind

Terminal shock

Pulsar wind nebula

Interface with the Interstellar Medium

• The magnetized pulsar wind leaves the pulsar with almost the speed of c (γ~103-106).

• A termination shock forms at the radius where the wind ram-pressure 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.

The basic configuration of a PWN

Therefore, the properties of a pulsar wind nebula are highly determined

by the composition and geometry of the pulsar wind as well as the

interaction with the environments.

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 X-ray luminosity and spectral index for pulsars, while a positive correlation holds for the PWNe.

Li, Lu & Li 2008, ApJ 682, 1166

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

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

The morphology of a PWN is

determined by the geometry of the

pulsar wind and the distribution of

the ISM surrounding the PWN.

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sinsym

t sB

Interfaces

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.

5.05.1440~

keVByrtxor

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)

The X-ray and radio images look very much similar to each other, and the magnetic field is well organized.

(Lang, Wang, Lu, & Clubb 2010)

The radio and X-ray extents are almost the same for G54.1+0.3.

(Lang, Clubb, Lu, & Wang 2009)

G54.1+0.3 is very weakly confined by the environment

• 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.

The PWN of PSR B1951+32 is in the center of SNR CTB 80.

PSR B1951+32

S II O III

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.

(Hester & Kulkarni 1989)

Radio image of core of CTB 80

Migliazzo et al. (2002)

Chandra X-ray imageLi, Lu & Li (2005)

The PWN of PSR B1951+32 shows a shell-like structure in both radio and X-rays, 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.

Contours: X-ray Greyscale: Radio

Termination shock

Pulsar

X-ray tail

Shell

(Li, Lu, & Li 2005)

The PWN is produced by the supersonically moving pulsar in SNR ejecta.

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)

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.

(van der Swaluw et al. 2003) (Gaensler et al. 2004)

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

Pulsar wind particles could also illuminate the non-thermal radio filaments in the Galactic Center region.

Lu, Wang & Lang 2003

A deep Chandra image of the GC region.

Morphological properties

(Lu, Yuan, & Lou 2008)

Their spectra are non-thermal.

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 show cometary morphology. • The spectra are non-thermal, with photon indices

1.0-2.5, Lx 1032 to 1034 erg/s, and absorption column density 1023 cm-2.

• When there are enough counts, a spectral softening with distance from the point-like source can be detected.

The X-ray filaments are most probably close to Sgr A* and powered by pulsars younger than 3*105 yrs.

Star formation rate15 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.

Arches cluster Quintuplet clusterThere are many massive stars in the GC region. The 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.

Most of the X-ray filament tails point away from Sgr A*

Sgr A*

Galactic Plane

• 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 spin-

down 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.