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Recent Suzaku Results on Strongly Magnetized Neutron Stars

This paper discusses the recent findings from Suzaku observations on strongly magnetized neutron stars and their implications, including the measurements of surface magnetic fields using electron cyclotron resonances. The paper explores the possibility of nuclear ferromagnetism in magnetars and the existence of intermediate objects between magnetars and normal strong-field neutron stars. The findings highlight the importance of accurate determination of the continuum spectrum using wide-band data. This paper also presents the unique spectrum of an AXP, 4U 0142+61, which is distinct from other accreting/rotating X-ray pulsars.

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Recent Suzaku Results on Strongly Magnetized Neutron Stars

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  1. Recent Suzaku Results on Strongly Magnetized Neutron Stars Kazuo Makishima (University of Tokyo and RIKEN) T. Enoto, T. Mihara, M. Nakajima, K. Nakazawa, & Y. Terada X-ray Universe 2008

  2. 10 BeppoSAX Ginga (Gauss) Log B RXTE 8 ASCA 13 Suzaku 12 6 4 Number 2 0 2 20 10 100 5 50 Resonance Energy (keV) • Measurements of surface m.f. using electron cyclotron resonances; Ea = 11.6 (B/1012G) keV • Measured m.f. strengths clustered in (1~5)x1012 G(Makishima + 99). • “Field decay” hypothesis unlikely. • Resonance photosphere height depends on Lx (Nakajima Poster D.7). 1. Strongly Magnetized NSs • Is m.f. due to nuclear ferromagnetism (Makishima + 99)? • Magnetars really have 1014-15 G? • Any objects in between ordinary pulsars and magnetars? • Origin of the resonance width? X-ray Universe 2008

  3. PSR 1509 On-Off, 45 ks HXD-GSO HXD-PIN • [Accreting Binary Pulsars] • A0535+26Terada+ApJL (2006); Naik+ ApJ (2008) • Her X-1Enoto+ PASJ (2008); Terada+ AdSpRes (2007) • 4U 1626-67 • GRO J1008-57 (TOO) • [Rotation-Powered Pulsars] • The Crab pulsar • PSR 1509-58 Terada+ PASJ (2008) • [Magnetars] • 4U 0142+61Enoto+. in prep. • 1E 1841-045Morii+ AIPC (2008) • AXP CXOU J164710.2-455216 (TOO)Naik+ PASJ (2008) • SGR1900+14 (TOO) • SGR 1806-20 (TOO; burst+persistent)Esposito+ A&A (2008) 2. Suzaku Observations X-ray Universe 2008

  4. 42 keV 23 keV 36 keV 0.05 0.02 0.01 .002 2005 3. Crab Ratios HXD-GSO 2006 0.1 .01 Lx=4x1035 erg/s • A0535+26(Suzku,Terada+06） HXD-PIN • Her X-1 on-off(Suzaku, Enoto + 08)Lx=3x1037 erg/s 20 50 100 200 1 .1 .01 20 50 100 200 Energy (keV) • Continuum and absorption couple strongly. • Using regions near Ea1/2 and >3Ea1, define the continuum as • f(E)=AEα exp(-E/Ecut) • X0331+53(RXTE ; Nakjima + 08) • Lx=3x1038 erg/s 5 10 20 50 100 X-ray Universe 2008

  5. pivot 1 0.5 0.2 0.1 A0535+26 Her X-1 On-Off X0331 RXTE 0 50 100 150 200 Assuming spectrum = f(E)xexp[-S(E)], exp[-S] is plotted logarithmically. The vertical scale thus becomes -S itself. The energy axis is normalized to Ea1, and ordinatge is adjusted by ~20% The three objects exhibt very similar profiles. 4. Extraction of Absorption Profiles X-ray Universe 2008

  6. X0331 (PCA) Deconvolved νFν Her X-1 X0331 (HEXTE) A0535 (GSO) A0535 (PIN) Her X-1 10 20 50 100 keV F(E)=(aE-α+bEβ) exp(-E/Ecut) exp{-S1(E/Ea1)-S2(E/Ea2)} ・“NPEX” continuum : independent among the three. ・S1: Lorentzian xE2, common to the three as a function of E/Ea. ・S2 : likewise, but X0331 is allowed to have separate Ea2 and W2. 5. Joint Fitting of the 3 Pulsars Count Spectra D1=1.61 ± 0.06 W1/Ea1=0.36 ±0.02 D2=1.51 ± 0.13 χ2/ν=1.08 10 20 50 100 keV X-ray Universe 2008

  7. The 3 objects are in common physical conditions, despite the luminosity differences by ~3 orders of magnitude. cf. Not claiming that this applies to all accreting pulsars. • The NPEX continuum is justified; the1st and 2nd resonances heavily affect the continuum even in between Ea1 and Ea2. • Gaussian absorption cross section gives significantly worse joint fits than the Lorentzian x E2 cross section (Enoto et al. 2008); chi^2=347.43 vs. 307.01 for the same d.o.f=287. • The resonance width may not be explained by the thermal Doppler effects. Unexpectedly short lifetimes of the excited Landau levels (Enoto et al. 2008)? 6.Implications X-ray Universe 2008

  8. A0535+26 e+e- creation --> rush onto mag.poles --> non-thermal Bremss --> Compton back scatt. (Kotoku+07 PASJ) 0 0.5 1 1.5 2 Observed with Suaku on 2007 Aug. 13 for 93 ksec. 7. AXP 4U0142+61 (Enoto et al.) Soft pulsations are stable. Hard pulses vary on ~30 ks. Very hard component with Gamma~1. Improves INTEGRAL results (Kuiper+06). GSO XIS PIN P=8.688784 sec 1st 30 ks 2nd 30 ks3rd 30 ks 0.06 0.04 0.02 0 BGD-subtracted 10-70 keV folded pulse profile X-ray Universe 2008

  9. 8. Summary Spectra of accreting X-ray pulsars are strongly affected by electron cyclotron resonances, but the effects may be rather common among different objects. The continuum must be carefully determined using wide-band data. The spectrum of an AXP, 4U 0142+61. is clearly distinct from those of any accreting/rotating X-ray pulsars. A ultra-strong MF provides the most likely explanation. Existence of intermediate objects between magnetars and normal strong-field NSs is open (cf. Dr. Haberl’s talk). Further Suzaku observations are of great value. X-ray Universe 2008

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