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Turbulence and Magnetic Field Amplification in the Supernova Remnants

Turbulence and Magnetic Field Amplification in the Supernova Remnants. TI, Yamasaki & Inutsuka (2009) in preparation. Tsuyoshi Inoue (NAOJ). Ryo Yamazaki (Hiroshima Univ.) Shu-ichiro Inutsuka (Kyoto Univ.). Introduction. Supernova r emunants ( SNRs ):

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Turbulence and Magnetic Field Amplification in the Supernova Remnants

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  1. Turbulence and Magnetic Field Amplification in the Supernova Remnants TI, Yamasaki & Inutsuka (2009) in preparation Tsuyoshi Inoue (NAOJ) Ryo Yamazaki (Hiroshima Univ.) Shu-ichiroInutsuka (Kyoto Univ.)

  2. Introduction • Supernova remunants(SNRs): • The site of CR acceleration • SNRs emit radiations with various wavelengths (radio to TeVg) --> Physical conditions of SNRs are very important to understand them. • Uchiyama+07, Uchiyama & Aharonian 08 discovered that There are bright X-ray hot spots in SNR RXJ1713 and CasA. The hot spots decrease its luminosity with the timescale of a few years. Synchrotron cooling time：tsynch years --> B ~ 1mG (~ 200 × BISM ) is necessary!

  3. Motivation of This Work • A possible mechanism of B amplification in SNRs • (Balsara+01, Giacalone & Jokipii 07) • Density fluctuations of preshock medium cause magnetic field • amplification in the post shock medium (SNR). • They assumed the caused by turbulence of ISM. • However, the realistic strength of turbulence cannot amplify B • to the order of 1mG (Balsara+01). --> In order to make hot spot (1mG level of B), larger r fluctuations are needed. • Large amplitude r fluctuations are ubiquitously exist in ISM: • Small-scale HI clouds generated by thermal instability • HI clouds have density n ~ 10-100 cm-3 >> diffuse medium n ~ 1 cm-3 --> Stronger amplification than the case of turbulent medium can be expected, if we consider complex (and realistic) ISM!

  4. Interstellar Medium • ISM is an open system in which radiative cooling/heating are effective • （Field+ 69, Wofire+95, 03） • Heating source : photoelectric heating by dust grains (PAHs) • Cooling sources : line emissions (T>103 K: Ly-a, T<103 K: CII/OI fine structure) Thermal equilibrium curve Cooling region Diffuse warm gas T ~ 8000 K, n ~ 0.1-1 cm-3 HI cloud Diffuse gas unstable HI clouds Pressure T ~ 100 K, n ~ 10-100 cm-3 Heating region Number density --> Diffuse warm gas and HI clouds can coexist under pressure equilibrium. • Thermal instability: the equilibrium that connects warm/cold phase is unstable. --> Unstable gas evolves into diffuse gas and HI clouds (Field 65, Balbus 95).

  5. Evolution of Thermally Bistable ISM T.I. & Inutsuka 08, Hennebelle & Audit 07 • ISM is affected by (late-stage) supernova shock waves once/several Myr. (McKee & Ostriker 77) • Recent high-resolution MHD simulations have shown that ISM evolves as follows: Cooling region 1. Shock compression 2. Cooling 3. Development of thermal instability Pressure Heating region • Timescale of this evolution is a few Myr • （timesacle of cooling/heating）. Number density --> In general, ISM is two-phase medium composed of warm gas and HI clouds.

  6. Generation of Two-phase Medium • To generate two-phase medium as a consequence of thermal instability, • we performed MHD simulation. • basic equations: Thermal conduction Heating/cooling fuction • Initial condition: • thermally unstable equilibrium • + B filed (Bx=6mG, By=0) Result (density structure) • Results: Red region: diffuse warm gas (n~1 cm-3, T ~ 5000 K) Blue region: HI clouds (n~30 cm-3, T ~ 100 K) *Scale of HI clouds are determined by scale of thermal instability (~ 0.1 pc) *Small scale HI clouds are ubiquitously observed in ISM (Heiles & Troland 2003)

  7. Early-stage Supernova Shock wave • To investigate SNR formed by piling up the two-phase medium, • we induced shock wave at a boundary of computational domain. • Set the hot plasma (n=0.1 cm-3) • at one of the boundary. • Basic equations: ideal MHD equations. *Strength of induced shock wave corresponds to early-stage supernova shock of the age ~ 1,000 years.

  8. Result of Model 1 T.I. et al 09 in preparation • Model 1: Perpendicular shock, vshock ~ 1300 km/s） Number Density Magnetic Field Strength B field is strongly amplified to the level of 1 mG in some region!

  9. Result of Model 2 T.I. et al 09 in preparation • Model 1: Parallel shock, vshock ~ 1300 km/s） Number Density Magnetic Field Strength B field is strongly amplified to the level of 1 mG in some regions!

  10. Evolutions of Maximum B and b |B|max plasma b at B is max Model 1 (perp. shock) Model 2 (para. shock) b B [mG] Model 1 (perp. shock) Model 2 (para. shock) • Maximum magnetic field strength saturate at B ~ 1mG. • Maximum magnetic field strength is determined by the condition • b= pthermal/pmag ~ 1.

  11. Mechanism of B amplification T.I. et al 09 in preparation • Amplification mechanism of B • By using the eq. of continuity and induction eq., • we can obtain: --> B is amplified if velocity filed has shear along B filed line. Para. shock case (model 2) Perp. shock case (model 1) • Strong velocity shear is generated when • HI clouds are swept by the shock. Shock vshock in HI cloud << vshock in warm gas B --> post shock gas flows to round HI cloud B v --> velocity shear is inducedmost strongly at transition layer between HI cloud and surrounding diffuse gas v Shock (scale of the transition layer l ~ ~ 0.05 pc T.I.+06)

  12. Comparison with Observation • X-ray image of SNR(RXJ1713) by Uchiyama+07 10 arcsec = 0.048 pc • Scale of hot spots~ 0.05 pc • Hot spots seem to be located far behind the shock front

  13. Comparison with Observation • Result of our Simulation 0.05 pc • Scale of the regions where magnetic field is amplified to 1 mG • agrees well with the scale of the X-ray hot spots. • Regions with high B are located far behind the shock as the X-ray hot spots. --> Regions with high B can make X-ray hot spots, if electrons are accelerated around there by secondary shock.

  14. Summary • In general, ISM has large amplitude fluctuations due to thermal instability, • which generates small scale HI clouds (l~0.1 pc). • Shocked shell generated by piling up two-phase medium is turbulent. • Velocity shear along B line amplifies B at transition layer between HI cloud • and diffuse warm gas. • In the case Vshock ~ 1000 km/s, B strength grows to the order of 1 mG（b~ 1）. • Scale of the regions where B is on the oreder of 1 mG is 0.05 pc, which agrees • well with the scale of X-ray hot spots. • The scale 0.05 pc is determined by the scale of transition layer between • HI cloud and surrounding diffuse gas at which velocity shear is most • strongly induced.

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