Suzaku observation of a White Dwarf as a new Candidate of Cosmicray Origin

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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.TokyoRIKEN), 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
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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.
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Rotating compact objects, NS vs WDs
Dynamo of bicycles
102103 Gauss, 100 mssec, 10 cm
Induced potential a few Volts
Spin axis
Magnetic field
radiation
Neutron star
10121013 Gauss, msecsec, 10 km
Induced potential 10161018 Volts
Some white dwarfs have induced potential of 1015
1016 Volts !! ? Enough to accelerate CRs.
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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, MasonGray 2007
AM Hercules (B 30MG, P 0.129days)
Old reports on TeV ? band
Cyclotron
Meintjes etal 1992, 1994, Bhat etal 1991
Black body
Flare (gt2 TeV) (5.62.1)10-11
photons/s/cm2 Quiescence (gt0.5TeV) lt
610-12 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!
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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 2139 11/02 102 70 ksec (XIS)
/ 53 ksec (HXD) 2006/10/25 0534 10/26 911
47 ksec (XIS) / 41 ksec (HXD)
X-ray CCD (XIS) separate thermal emission Hard
X-ray Detector(HXD) Deep non thermal search
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Discovery of Hard X-ray Pulses!
Pulse profile in 2005
Periodogram around Spin period
0.510 keV
1030 keV
New!
Well-known thermal modulation Sharp pulse in
over 4 keV
PXIS33.0769s, PPIN33.0764-0.005s
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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
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Having Non-thermal spectral property?
Peak
Phase averaged spectra
Phase resolved spectra
2.9keV0.53keV MEKAL (XIS) PL Index 1.1
- 0.6 (PIN)
No Thermal lines in the Pulse spectrum in over 2
keV band. ? Photon Index of 2.0 - 0.3
or 50 keV MEKAL (PIN)
Although it is statistically insufficient, non
thermal origin is more feasible.
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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) gtgt
GoldreighJulian(69)density 104 cm-3
(IkhasanovBierman06) ? Electric Potential
short-circuited by materials. Propeller Effect
Low density at near the WD (lt1010cm) ? one
possible site ?
Hard X-ray Pulse
WD
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What is the emission mechanism?
? Non-thermal bremsstrahlung ? Inverse Compton
scattering ? Curvature radiation
? Too small efficiency.
? Synchrotron radiation
? Most feasible among them.

• Lorents factor of ?104 (B/105)-1 to generate 30
  keV X-ray.
• High efficiency with Life time of 8 µsec
  (B/105)-2(?/104)-1
• Can generate anisotropic emission to generate
  sharp signal
• Photon index 1.1 is similar to the NS case of 1.4

? Others
Any ideas ?
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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)