Non-thermal emission from Magnetic White Dwarf binary AR Scorpii

Transcription

1 Non-thermal emission from Magnetic White Dwarf binary AR Scorpii Jumpei Takata (Huazhong University of Science and Technology, China) On behalf of Y. Hui (HUST), C.-P. Hu, K.S. Cheng (HKU, HK), L.C.C. Lin (CHEA), A.K.H. Kong (NTHU, Taiwan), P.S. Pal, P.H.T. Tam (SYSU, China), C.Y. Hui (CNU, Korea)

2 Outline 1. Introduction - Magnetic white dwarf - Non-thermal pulsed emission of AR Scorpii 2. Model the for pulsed emission Relativistic electrons trapped by the WD s closed magnetic field lines

3 1. Introduction : Magnetic WD White dwarf is the end point of the stellar evolution of a progenitor with a mass of <8M sun Over 97% of all stars. Magnetic WD has a surface magnetic field strength Bs~ G, determined from the polarization/zeeman splitting. Blue : Polar Red : IP Solid : Single No. of DA MWDs Zeeman-split of Ca H/K absorption lines indicates B=0.5MG (Kawka and Vennes 2011) -2 Log B (MG) 2

4 1. Introduction : Magnetic WD Highly magnetic WD is found in the binary system (Cataclysmic variable). Spinning of the two stars of Polar is synchronized by the strong magnetic field. WD in IP has a shorter spin period than orbital period. IP Orbit Period (minute) AR Scorpii Spin Period (second)

5 1 Introduction : Intermediate Polar Magnetic Cataclysmic Variables Magnetic WD (B>10 6 G) + low mass star (M-type) Polar and Intermediate Polar. Thermal X-ray emissions from accretion column on WD surface. Intermediate Polar (IP) WD s spin is shorter than the orbital period. Usually, accretion disk around WD. X-ray/optical pulsation (beat frequency or spin frequency). 1 orbit 1 spin (from ppt of K.P Singh) AO Psc (Cropper et al. 2002)

6 1 Introduction : AR Scorpii (Marsh et al. 2016) Spin period of WD: 117s Orbital period: 3.56 hours Optical modulation with a beat frequency ~ 1/118s Companion : M-type star ν B UV Op'cal IR Radio Orbital phase Beat phase Pulsed emission from radio to UV bands. Beat frequency; Broadband spectrum described by a synchrotron radiation of non-relativistic electrons. Weak X-ray emission Pulse fraction <30% No accretion feature. Flux L 32 ~ 10 erg/s M-star Frequency WD

7 1 Introduction :Optical polarization Similarity and dissimilarity with NS pulsar High linear polarization at the pulse peaks A large swing of the position angle (~180 degree) through one pulse. (Buckley et al. 2017) AR Sco Pulse profile à These features indicate the non-thermal emission (synchrotron radiation) from the relativistic electrons. PosiDon angle Linear Buckley et al RotaDon phase

8 1. Introduction : X-ray observation One orbit opdcal (Takata et al. 2018) X-ray WD OpDcal/X-ray maximum OpDcal/X-ray minimum The optical/x-ray emission is mainly produced at the stellar surface. Modulation is cause by the change of observable surface area of the dayside of M-star. No indication (absorption and high N H ) of accretion in X-ray data.

9 1. Introduction : Optical/X-ray pulsed emission EPIC (opdcal) mhz A significant pulsation in soft X-ray bands. The peak positions are aligned with the peaks of UV bands. Orbital resolved pulse profiles also show the aligned pulses. Indication of the non-thermal pulsed emission. The X-ray pulse fraction is only ~14%.

10 1. Introduction : Broadband spectrum (Thermal plasma) Phase average Pulsed component (This work)

11 What is uniqueness of AR Scorpii? No indication of accretion disk (propeller phase, Beskrovnaya & Ikhsanov 2017) Optical/X-ray maximum at the SUPC of the companion orbit. Broadband pulsed emission from radio to soft X-ray bands. Sharp double peaks and the pulse peaks are aligned in optical/soft X-rays New type of WD binary

12 2. Model (Takata et al. 2017) Heating process of the stellar surface: -- Dynamo effect can generate stellar magnetic field of ~ G (Reiners et al. 2009) -- Magnetic field of WD at the companion surface -- Magnetic interaction between WD and companion star cause a dissipation of the magnetic energy δ = 0.01:Skin depth η = B φ B p ~ 1

13 2. Model : System (Takata et al. ApJ in press) Dipole field B 8 s ~ 10 G M-type star interacts with the closed magnetic field line of WD cm Light Cylinder Radius : cm

14 2. Model : Emission region B 8 s ~ 10 G B ~ 10 2 G Dissipated magnetic energy is converted into the particles on the companion surface Synchrotron process on the M-type star surface??? à Lorentz factor ~100 to explain the optical. à But the cooling time scale (~400s) is longer than the dynamical time scale (~2s) (~ 0.15 cm R lc ) Accelerated electrons could migrate to inner part of the WD magnetosphere.

15 Emission from the trapped electrons Van Allen radiation Jupiter's synchrotron radiation Synchrotron radiations of the magnetospheres of the Earth and Jupiter

16 2. Model : Magnetic mirror 2 2 v v + v = = constant v 2 = B constant (the first adiabadc invariance) magnetic mirror at = v 0

17 2. Model : Magnetic mirror Equation of motion with a synchrotron energy loss Lorentz factor γ : ParDcle Lorentz factor P : Perpendicular momentum Momentum perpendicular to magnetic field Synchrotron loss + first adiabatic invariance The electrons injected from the M-type star surface with a pitch angle > ~0.05 radian are trapped by the magnetic mirror.

18 γ 0 =50 Mirror point Migrating electrons can lose their energy at the mirror point, where the pitch angle increases. Lorentz factor Time from injection from M-star / WD spin period Mirror point Most of the initial energy is loose by the radiation at the first mirror point after the injection. Radial distance from WD/ Separation

19 No magnetic mirror for an electron injected with a smaller pitch angle Time from injection from M-star / WD spin period The electrons injected from the M- type star surface with a pitch angle > ~0.05 radian are trapped by the magnetic mirror. Radial distance from WD/ Separation

20 2. Model :Emission direction The emission direction is along the direction of the electron motion (velocity direction). Unit vector of the motion Co-rotation motion with WD Motion along the magnetic field line Gyration motion

21 2. Model : Time of arrival Viewing angle from the spin axis (z-direction) Time of arrival of a photon Time of electron injection from M- star surface Travel time of the electron to emission point Flight time of photon

22 2. Model : Time of arrival Spin Phase Earth viewing Lorentz factor Mirror point Radial distance from WD/ Separation Spin Phase

23 2. Model : Formation of pulse Inclined rotator. Emission is mainly produced at the first magnetic mirror point. t=0 t=0.5p s mirror point (emission region) Electrons injected earlier. But, need a longer time to arrive the first mirror point. Electrons injected later. But, need a shorter time to arrive the first mirror point.

24 2. Model : Formation of pulse The electrons injected at the different times can arrive at the first mirror point, simultaneously. Enhance the emissions at a specific spin phase. Double peak profile (contribution form north and south hemispheres. ) Earth viewing angle Intensity sky map Model predicts the modulation with the beat frequency. Spin phase Model pulse profile Two beat phase

25 2. Model :Beat frequency In the model, the pulse peak corresponds to the phase when the magnetic axis directs towards the companion star. à The companion star orbits around WD. à Every ~118s, the magnetic axis directs towards the companion star.

26 Power spectrum of the timing analysis (Model)

27 2. Model : Polarization To explain the swing direction ζ ζ! o! 90, if Ω µ > > WD WD or! o! 90, if Ω µ < < WD WD 0 0

28 Summary AR Scorpii is the new type of IP. Pulsed emission from radio to X-ray bands. Optical/UV/X-ray pulses are in phase Synchrotron radiation. Heating and acceleration process on the M-star surface. Interaction with the magnetic field of WD and M-star. The synchrotron emission of electrons trapped in the WD magnetosphere Emission from the first mirror point.