Suzaku observation of a White Dwarf as a new Candidate of Cosmic-ray Origin

1. Suzaku observation of a White Dwarf as a new Candidate of Cosmic-ray Origin Yukikatsu Terada (Saitama Univ), T. Hayashi (Tokyo Metro.Univ), M. Ishida (ISAS/JAXA), K. Makishima (U.Tokyo;RIKEN), K. Mukai(NASA/GSFC), T. Dotani, S. Okada, A. Bamba, R.Nakamura (ISAS/JAXA), S. Naik(Physics Research lab, India), and K Morigami (Saitama Univ) Based on Terada Y. et. al, 2008 PASJ 60, 387-397

2. Magnetized White Dwarfs are one of candidates !! Neutron Stars and White Dwarfs Magnetic field B : 3-4 order of mag. smaller Rotation velocity v ∝ P -1 : 4 order of mag. Smaller System Length L : 3 order of mag. larger Space density 3 order of mag. larger What is the Cosmic-ray origin ? Hillas diagram Emax = e v B L ~6x1016(P/1s)-1(B/1012G)(L/106cm)2 eV for neutron stars case If MWDs can accelerate particles (even if weak), they become important CR source having large space density.

3. Terada PhD and references therein Wichramasinghe and Ferrario 2000 Manchester etal 2001 Edwards etal 2001 Makishima etal 1999 Rotating compact objects, NS vs WDs Dynamo of bicycles 102～103 Gauss, 100 ms～sec, 10 cm Induced potentiala few Volts Spin axis Magnetic field radiation Neutron star 1012～1013 Gauss, msec～sec, 10 km Induced potential1016～1018 Volts Some white dwarfs have induced potential of 1015～ 1016 Volts !!  Enough to accelerate CRs.

4. ASCA Reynolds etal 1991 Bhat etal 1991 Schlegel etal 1995 Previous searchs for non-thermal electrons in white dwarfs Radio observations of WD binaries AM Her, AE Aqr, DQ Her, ST LMi, BG Cmi, AR UMa → 1 ～ 20 mJy Synchrotron emission? (max. optical flares) Pavelin etal 1994, Bond 2002, Mason&Gray 2007 AM Hercules(B ~30MG, P ~0.129days) Old reports on TeV γ band Cyclotron Meintjes etal 1992, 1994, Bhat etal 1991 Black body Flare: (>2 TeV) (5.6±2.1)×10-11 photons/s/cm2 Quiescence: (>0.5TeV) < 6×10-12 photons/s/cm2 e- thermal bremsstrahlung Non thermal? Thermal Needs confirmation with recent observatory! WD MS Accretion Energy ~ 1033 erg/s, Thermal X-ray ~ 1032 erg/s Non thermal emission ~ 1031 erg/s Optical Energy (keV) Non-thermal electrons should exist in WD!

5. Accretion White Dwarf Late star Main purpose search for non thermal tail in hard X-ray band. Energy (keV) Suzaku observation of a White dwarf： AE Aquarii Magnetic Cataclysmic variables: AE Aqurii • Fastest rotator among MCVs with ~33 sec period • (close to break-up condition; Casares etal 1996) • Stable spin down for 20 years (de Jager 1991, Mauche 2006), • spin down energy 5x1033 erg/s • Radio synchrotron flares (Bastian etal 1988, A.Simon etal 1990) • Pulsed TeV gamma-rays (Brink etal 1990, Meintjes etal 1992,1994) • Low plasma temperature; inhibited accretion (ASCA) •  Magnetic propeller effect (XMM; Itoh etal 2006) Suzaku observation Observation 2005/10/30 21:39 – 11/02 1:02 70 ksec (XIS) / 53 ksec (HXD) 2006/10/25 05:34 – 10/26 9:11 47 ksec (XIS) / 41 ksec (HXD) X-ray CCD (XIS): separate thermal emission Hard X-ray Detector(HXD): Deep non thermal search

6. Discovery of Hard X-ray Pulses! Pulse profile in 2005 Periodogram around Spin period 0.5—10 keV 10—30 keV New! Well-known thermal modulation + Sharp pulse in over 4 keV PXIS=33.0769s, PPIN=33.0764+-0.005s

7. The Pulses enhances at flares Light curves Flares Pulse profiles at flares and quiescence Norm Crab like??? Similar to the nature of non-thermal radio/TeV emissions

8. Having Non-thermal spectral property? Peak Phase averaged spectra Phase resolved spectra No Thermal lines in the Pulse spectrum in over 2 keV band.  Photon Index of 2.0 +- 0.3 2.9keV+0.53keV MEKAL (XIS) + PL Index 1.1 +- 0.6 (PIN) or 50 keV MEKAL (PIN) Although it is statistically insufficient, non thermal origin is more feasible.

9. Luminosity Index Becker and Trumper 1999 Possenti et al 2002 AE Aqr AE Aqr First discovery of White dwarf equivalent of Pulsars? Hard X-ray Power Law component vs NS spectra X-ray Flux (erg/s) Where is the acceleration site in the system? Spin down energy (erg/s) Companion star accretion Dense accretion materials (Difference from NS cases) n = 1011cm-3 (Itoh+ ‘06) >> Goldreigh&Julian(’69)density 104 cm-3 (Ikhasanov&Bierman’06)  Electric Potential: short-circuited by materials. Propeller Effect: Low density at near the WD (<1010cm) → one possible site ? Hard X-ray Pulse WD

10. What is the emission mechanism? ① Non-thermal bremsstrahlung ② Inverse Compton scattering ③ Curvature radiation  Too small efficiency. ④ Synchrotron radiation  Most feasibleamong them. • Lorents factor of γ～104 (B/105)-1 to generate 30 keV X-ray. • High efficiency with Life time of 8 μsec (B/105)-2(γ/104)-1 • Can generate anisotropic emission to generate sharp signal • Photon index 1.1 is similar to the NS case of 1.4 ⑤ Others Any ideas ?

11. Summary • Suzaku observed a White dwarf binary, AE Aquarii, to • search for possible non-thermal emission from the system. • Suzaku discovered the Hard X-ray pulsation in over 4 keV • band (both with the XIS and the HXD). The pulse intensity • enhances at the soft-X flares. • There are no thermal lines in the phase resolved spectra • of the Pulse. The photon index is 1.1 and the luminosity • comes at 0.09% of the Spin down energy (6x1033erg/s). • One possibility of the origin of the pulse is Synchrotron • emission from low density region near the white dwarf. • AE Aquarii should be a white dwarf equivalent of a pulsar. • White dwarfs should play an important contribution to Cosmic • ray origin as silent but numerous particle-acceleration site. Please check Terada Y. et. al, 2008 PASJ 60, 387-397 (Free access of PASJ WWW site. http://pasj.asj.or.jp/v60/v60n2.html)