Non-thermal emission and particle acceleration by reverse shock in SNR ejecta

Jiangtao Li(jiangtaoli@nju.edu.cn)

http://astronomy.nju.edu.cn/~ygchen/others/ljt/ljt_main.htm

Astronomy Department of Nanjing UniversityDepartment of Astronomy, UMASS

Astrophysics in the Multi-beam Radio Receiver Era, Nanjing, Jiangsu province, China.

Outline

• 1. Diffuse shock acceleration (DSA) and CR production.

• 2. SNR dynamics and morphology.

• 3. Non-thermal emission in reverse shock and Fe Kα line in the ejecta.

• 1. Diffuse shock acceleration (DSA) and CR production.

• 1.1 Temperature equilibrium in post-shock gas.• Mass-proportional heating and e-p equi-partition

• The heating of electrons: Coulomb or non-Coulomb

• 1.2 Production and amplification of magnetic field.• Origins of magnetic field in SNRs• How shock wave excite magnetic field?

• 1.3 Particle acceleration and CR production.• How to accelerate CRs?• Which particle is easier to be accelerated? • The effect of CR.

1.1 Temperature equilibrium in post-shock gas

• Mass-proportional heating and e-p equi-partition

• If all the shock energy is used to heat the particles:

• When CR acceleration or other energy consuming processes are important, could be much smaller than 3/16.

• When shock heating timescale is much smaller than the e-p (electron-proton) temperature equilibrium timescale, the temperature of electron and proton are proportional to their mass: this is called mass-proportional heating.

• The heating of electrons: Coulomb or non-Coulomb• Collisionless shock: In low density plasma, when the mean fre

e path of the particle is larger than the typical length scale of the shock front, shock is called collisionless.

• In collisionless shock, Coulomb collision is often not an effective way to transfer energy from proton to electron.

• Other processes, like different kinds of plasma wave may be more efficient way in e-p equilibrium. In quasi-perpendicular shock, Buneman instability will heat the electron to a temperature of , then the ion-acoustic instability will further heat the electron to a temperature ~300 times of the mass-proportional heating value, corresponds to ~20% of the shock energy going into heating electrons. An alternative scenario includes heating by lower hybrid waves. While in quasi-parallel shock, the heating is mainly due to whistler turbulence, but the electron heating is rather less than for the quasi-perpendicular case (Laming et al. 1996).

1.2 Production and amplification of magnetic field• Origins of magnetic field in SNRs:

• Ambient magnetic field

• Produced by strong shock wave

(Han J. L. et al. 1997, A&A, 322, 98)

• How shock wave excite magnetic field?

Current sheet produced by Weibel instability (Weibel instability is produced by the upstream and counterstream fluid) in the shock wave transition layers.

Electron and ion dominated processes in magnetic field production

1.3 Particle acceleration and CR production

• How to accelerate CRs?

• First order Fermi acceleration: Charged particle can be reflected by magnetic field between up- and down-stream in quasi-perpendicular shocks. These multiple reflections greatly increase its energy. The resulting energy spectrum of many particles undergoing this process turn out to be a power law with index >~2. The term "First order" comes from the fact that the energy gain per shock crossing is proportional to βs, the shock velocity in unit of the speed of light.

• Which particle is easier to be accelerated?• The injection problem: In the environment of a shock, only par

ticles with significant super-thermal energies can cross the shock and “enter the game” of acceleration.

• Thermal electrons is not energetic enough to cross the shock front and be accelerated efficiently.

• Protons heated by collisionless shock is much more energetic than thermal electrons, so they are the main source of CR.

• Positrons released by the radioactive decay of 56Ni and 56Co have typical energies of order 1 MeV, at which they could be accelerated as efficiently as protons of the similar energy.

• Charge exchange and the production of high energy neutral particles, Balmer double line.

• The effect of CR:

• Excite plasma instabilities in the upstream, make the magnetic field amplification and particle acceleration more efficient.

• Produce CR precursor, ionizing and heating the upstream gas before the shock wave.

• Others?

• 2. SNR dynamics and morphology

• 2.1 How the ambient magnetic field affect the multi-wavelength morphology of SNRs?

• The effect of magnetic field • Synchrotron radio and IC γ-ray emission

• 2.2 How CR acceleration affect the SNR dynamics?

2.1 How the ambient magnetic field affect the multi-wavelength morphology of SNRs?

• The effect of magnetic field

• First, affect the distribution and compression of the plasma;

• Second, make cosmic ray acceleration more efficient;

• Third, affect the electron injection and synchrotron emission.

Quasi-perpendicular particle injection case.(Orlando S. et al. 2007, A&A, 470, 927)

Direction of magnetic field determines the direction of the arcs, the gradient of magnetic field determines the strength of the arc (ratio between arc and off-arc regions), and the angle between magnetic field and the gradient of magnetic field determines asymmetry of the two arcs.

• Synchrotron radio and IC γ-ray emission• Key parameters: • ς(Θ0): The injection efficiency (fraction of accelerat

ed electrons)• σB(Θ0): The compression ratio of ISMF

• Emax(Θ0): Maximum energy of electrons

(Petruk O., Beshley V., Bocchino F. & Orlando S. et al. 2009, MNRAS)

Azimuthal variation: If Emax is constant over the SNR surface, the azimuthal variation of surface brightness in radio and IC γ–rays is opposite.Why?: This happens because the IC image is affected by large radiative losses of the emitting electrons behind a perpendicular shock, while the larger magnetic field increases the radio brightness there. Compensation: Variation of Emax over the SNR surface may (to some extent) hide this effect. The maximum energy should increase with obliquity in this case.

Sychrotron radioIC γ-ray

2.2 How CR acceleration affect the SNR dynamics?

• Acceleration of CRs will lead to lower postshock plasma temperatures as well as higher postshock densities. The ionization state is affected by both gas density and electron temperature, so also affected by CR acceleration. DSA (diffuse shock acceleration) may als

o enhance the thermal X-ray production. Shorten both the temperature equilibration and ionization equilibrium timescale.

• Reverse shock is not modeled.

• (Ellison D. C. et al. 2007, ApJ, 661, 879 )

• (Patnaude D. J., Ellison D. C. & Slane P. 2009, ApJ, 696, 1956)

• The density, temperature and ionization state affected by DSA of CR and CSM proton density.

• teq is the temperature equilibration timescale only due to Coulomb collision, when consider other energy transfer processes, the equilibrium timescale should be shorter.

• The presence of DSA of CR make the equilibrium easier to reach.

• Emission measure affected by DSA of CRs. Thermal emission enhanced at least in the Chandra and XMM-Newton band.

• 3. Non-thermal emission in reverse shock and Fe Kα line in the ejecta.

• 3.1 Why particle acceleration and non-thermal emission is preferred in forward shock in Type Ia SN?

• 3.2 Non-thermal emission predicted in forward shock. Which mechanism (synchrotron, IC or pion decay)?

• 3.3 Non-thermal emission detected in reverse shock.

• 3.4 The Fe Kα line.

3.1 Why particle acceleration and non-thermal emission is preferred in forward shock in Type Ia SN?

• Since the magnetic field in SN ejecta is often thought to be very weak, and shock excited magnetic field is not evidenced in SNRs by observation, the reverse shock has met a problem of magnetic field amplification.

3.2 Non-thermal emission predicted in forward shock, Which mechanism (synchrotron, IC or pio

n decay)?• Synchrotron e

mission dominate in radio and X-ray, while IC emission and pion-decay dominate in Ge

V-TeV. • (Ellison D. C.

et al. 2007, ApJ, 661, 879 )

3.3 Non-thermal emission detected in reverse shock

RCW86(Rho J., Dyer K. K., Borkowski K. J. & Reynolds S. P. 2002, ApJ, 581, 1116)

3.4 The Fe Kα line• In RCW86, Fe Kα is associa

ted with the synchrotron-emitting regions and not with the thermal filaments.

• It cannot be produced by thermal emission from a cosmic-abundance plasma; the ionization time is too short, as shown by both the low centroid energy (6.4 keV) and the absence of oxygen lines below 1 keV. Instead, a model of a plane shock in Fe-rich ejecta, with a synchrotron continuum, provides a natural explanation.

(Rho J., Dyer K. K., Borkowski K. J. & Reynolds S. P. 2002, ApJ, 581, 1116)

Kepler

• 6.4 keV Fe Kα line is detected in the ejecta of Kepler’s SNR, but no clear evidence for X-ray synchrotron emission is revealed.

• Collisional excitation of the 6.4 keV Fe Kα line is rejected, neither by thermal non-ionization equilibrium electrons nor nonthermal electrons.

• The possibility for photo-induced high energy recombination fluorescent line and high energy particle excitation (include CR excitation).

Summary

• Theoretically, diffuse shock acceleration (DSA) in collisionless shock of young supernova remnants (SNR) could provide significant CR acceleration and also non-thermal emission.

• The acceleration of CR can significantly affect the SNR dynamics and multi-wavelength morphology.

• Evidences for particle acceleration are not only detected in forward shock, but also in reverse shock. Fe Kα line, especially the 6.4 keV Fe Kα line detected in the ejecta, is probably an indicator of particle acceleration or other high energy processes in the reverse shock.

Thank you very much!

I’m expected to get PhD in Jun 2010, and now trying to find a job, please contact me via jiangtaoli@nju.edu.cn if you are interested in me. Any information or suggestions are welcome. For more information about me, please refer to:http://astronomy.nju.edu.cn/~ygchen/others/ljt/ljt_main.htm