Supernova remnants and pulsars

Fangjun Lu

卢方军Institute of High Energy Physics

Chinese Academy of Sciences（中国科学院高能物理研究所）

Classification of SNe

SNe are divided into two catergies from their optical spectra. Physically, they are either due to thermonulcear explosion or due to the core collapse of a massive star.

Structure of a supernova remnant

Type II Type Ia

Outline

• Properties of the progenitor star of a type Ia SN

• Dynamic evolution of the reverse shock• Interaction of the pulsar with its

enviroments.

Two scenarios of Type Ia SNe

The single degenerate scenario is widely favored today because it can naturally explain the uniform peak luminosity of type Ia SNe. But, little observational evidence has been detected to distinguish the two scenarios.

degenerate star

Collision and merge of two white dwarfs

The whitedwarf accretes mass from the companion star to reach the upper mass limit (1.4 solar mass) and then explode.

The explosion process and the expected consequence of a single degeneration SN

In the single degeneration scenario, the binary system contains a normal star, and so signature of such an event should come from the interaction of the explosion and the companion star: Fast moving companion star Mass stripped from the companion star during the explosion.

In his publication Astronomiae

instauratae progymnasmata, Tycho

Brahe writes that during an after-

dinner stroll on November 11th, 1572,

“I suddenly and unexpectedly beheld

near the zenith an unaccustomed star

with a bright radiant light.” He goes

on to describe how it rivaled Venus

(magnitude -4.6 at the time) and how

sharp-eyed “country folk” could spot

it in the daytime sky.

Tycho Brahe recorded an SN in 1572

The Tycho SNR is one of the best places to find such information.

Nearby (3 kpc)BrightYoung (1572-2009)Small absorption (6×1021 cm-2)A well proved Ia SNR

X-ray Radio

Nature 2004, 431 1069

Tycho G

Tycho G, a star similar to the Sun and with a distance to the Tycho SNR, is found to have proper motion and radial velocity significantly higher than the other stars in the field. It is suggested to be the companion star in the progenitor binary system.

0.2-10 keV X-ray image and spectrum of the Tycho SNR

Dominated by the thermal emission from SN ejecta

4-6 keV

Intriguing properties of the X-ray arc:(1)Half way from the remnant center;(2)Bright, narrow and sharp as these at the outer rim;(3)Convex to the remnant center (SN site);(4)Nonthermal spectrum.

X-ray arc

Result of the interaction of blast wave with a cloud.

Comparison between the remnant in different energy bands. A shadow is cast beyond the X-ray arc.

4-6keV

Sulfur

Silicon

Iron

Azimuth mean brightness profiles of the Tycho SNR beyond the radius of the X-ray arc in different energy bands. A shadow is obvious in all energy bands.

Azimuth mean brightness profiles of the Tycho SNR within the radius of the X-ray arc in different energy bands. Local enhancements immediately within the arc are evident.

Is the arc a feature in the outer layer or in the interior of the remnant?

Lin

e of sigh

t

If it represents the blast wave in the outer layer:

(1)The arc could be quite diffused;(2)It should be convexed outward,

like the filaments in the outer boundary.

If it represents the interactions of the shock with a cloud inside the remnant:

(1)The arc could be as sharp as those in the outer boundary;

(2)The cloud will block the SN ejecta in this direction.

Possible origins of the cloud

• A cloud unrelated to the progenitor system

of the SN.

• Materials ejected by the binary system

during its evolution.

Matter stripped from the companion star

during the SN explosion.

A cloud unrelated to the progenitor system?

In this case it should be a

cold molecular cloud to be

consistent with the long

lifetime. However, no

molecular cloud or optical

emission filament has been

detected spatially coincide

with the X-ray arc. This

origin is unlikely.

12CO observations of Tycho region(Lee et al. 2004, ApJ 605, L113)

Arc Position

Materials ejected by the binary system during its evolution?

The mass donor is suggested to be very

similar to the Sun but a slightly evolved,

it can not contribute to the cloud during

its lifetime.

The planetary nebula surrounding the

exploded white dwarf could be a source

of the cloud material. However, the

planetary nebulae observed always have

a spherically or axially symmetric

morphology.

The singleness of the X-ray arc makes

the possibility of the planetary nebula

origin of the cloud very low. Images of planetary nebulae

Matter stripped from the companion star during the SN explosion?

Suggested by many theoretic works (Wheeler et al. 1975; Fryxell & Arnett 1981; Taam & Fryxell 1984; Chugai 1986; Marietta et al. 2000; Meng et al. 2007, etc.) but no direct observational evidence has been detected (Leonard 2004).

Reason to attribute the cloud as the stripped mass

The opening angle of the arc to the explosive centre and the lack of X-ray line emission in the volume shadowed by the arc is well consistent with the simulations of type Ia SN explosion (Marietta et al. 2000).

20°

Binary parameters of the progenitor sysem

The nonthermal X-ray most probably represents the interaction between the ejecta and a bulk of materials in the interior of the remnant, which is most likely between SN ejecta and the stripped envelop of the companion star.

We obtained a stripped mass of ≤0.0083Mʘ,which is

consistent with that observed for two extragalactic Ia SNe (Leonard 2007) and close to the simulations by Pakmor et al. (2008).

The orbital period of the progenitor binary system is about a few days, and the separation of the two component stars is about 1/10 of the distance from the Earth to the Sun.

Summary (1)

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.

• I will show pulsar wind nebula properties in various conditions:– “Freely” expands;– Within a supernova remnant;– Moves supersonically in space;– In high velocity interstellar wind.

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.

11

3/2 3/2 1/2

8.7 10( )

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 2010)

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 pulsar wind usually has a torus+jets geometry

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. (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)

Sgr A*

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.

G359.95-0.04HESS J1745-290

TeV gamma-ray astronomy Wei Cui 2009 RAA, 9 841

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).

A deep Chandra image of the GC region.

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.

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 (3)• 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.

• The filamentary PWNe in the Galactic center shows that there is a high velocity radial Galactic wind.

• The PWNe can be thus used as tools to explore the physics conditions of the environments.

Thanks!

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.

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.