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Pulse phase resolved spectroscopy of accreting HMXB pulsars 1A 1118-61 and Vela X-1 with Suzaku

Pulse phase resolved spectroscopy of accreting HMXB pulsars 1A 1118-61 and Vela X-1 with Suzaku. Chandreyee Maitra HEAP 2012. Plan of the talk. Accreting HMXB pulsars Pulse profiles & energy dependence X-ray spectra: Continuum model X-ray spectra: Line model (CRSFs)

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Pulse phase resolved spectroscopy of accreting HMXB pulsars 1A 1118-61 and Vela X-1 with Suzaku

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  1. Pulse phase resolved spectroscopy of accreting HMXB pulsars1A 1118-61 and Vela X-1 with Suzaku Chandreyee Maitra HEAP 2012

  2. Plan of the talk • Accreting HMXB pulsars • Pulse profiles & energy dependence • X-ray spectra: Continuum model • X-ray spectra: Line model (CRSFs) • The case of two HMXBs: 1A 1118-61 & Vela X-1 • Discussion & Conclusion • Future prospects: Studies with ASTROSAT & X-ray polarimetric studies.

  3. Accreting HMXB pulsars • Wind accretion (Dips & Flares)‏ • Be & Supergiant companions ‏ • Lx ~ 1033-1038 ergs s-1 • Pulsation period in the range 0.1<P<104 s • Strong magnetic field ~ 1012 G • Flow couples to the magnetic field at Alfven radius. • Chanelling of matter along the magnetic field lines • Formation of accretion column above polar caps & pulsations • Source of X-ray radiation → Thermal Bremsstrahlung & Comptonization (Negueruela based on Davidson & Ostriker 1973‏

  4. Pulse Profiles & Energy dependence • Shape of the pulse profile depends on the anisotropic emission of radiation from the polar caps and modified by absorption & scattering & light bending. • General trend of high energy pulses to have simpler shapes and for modulation of the pulse profiles to increase with energies (White, Swank & Holt 1983; Bildsten et al. 1997)‏ • Multi peaked profile at lower energies and and a single asymmetric peak at higher energies ( > 10 keV) have been found in many pulsars: Her X-1 (Nagase 1989) 4U 0115+63( Tsygankov et al. 2007) ,and more recently in GRO J1008-57 (Naik et al. 2011), GX 304-1(Devasia et al. 2011) and 1A 1118-61 (Doroshenko et al. 2011; Devasia et al. 2011; Nespoli et al. 2011)‏

  5. Energy dependent pulse profiles of GX 301-4 (Devasia et al. 2011)‏ Energy dependent pulse profiles of GRO J1008-57 (Naik et al. 2011)‏

  6. X-ray spectra :Continuum model Becker & Wolff (2005, 2007) → modelled spectra arising from a pulsar accretion column including bulk motion & thermal comptonization : Continuum modelling: phenomenological models mostly Exhibits pulse phase dependence

  7. X-ray spectra Linemodel:cyclotron resonance scattering features (CRSF)‏ Continuum X-ray photons produced resonantly scattered with the quantized plasma electrons producing the CRSF . Classically : spiralling motion of an electron in the magnetic field r~ΛB =ħ/mev Quantization of electron energy ﬩ to the B-field lines into Landau Levels Bcrit= (m2c3/ħe) = 44.14 x 1012 G Line modeling: Emperical: Gaussian or Lorenzian profiles: F(E)= CONT(E).exp(-Ƭ(E))‏ Physical models: (depends on KTe, Ƭ, B, μ)‏ (Araya & Harding 1999,2000; Schonherr et. al 2007; Nishimura 2011, Mukherjee & Bhattacharya 2011)

  8. Observations of CRSF: & results obtained • Ecyc=11.6 keVB12 --> effected due to gravitational redshift. • CRSF observed: Ecyc=(1+Z)Eobs --> a methord to estimate the magnetic fields of X-ray pulsars • Magnetic field of ~ 17 XRP's measured. ( Wilms 2010;Heindl et al. 2004; Staubert et al. 2003; Coburn et al. 2002; Santangelo at al. 2000) by RXTE, Integral, Beppo SAX & recently Suzaku • Mainly HMXB pulsars --> 50 % transient --> B ~ 1-5 * 1012 G. • Nearly harmonic spacing --> deviations found can be explained by relativistic corrections (4U 0115+63 Santangelo et al. 1999)‏ Vela X-1 ( kreykenbohm 2002) and A0535+26 (Caballero 2009) • Variation of the parameters observed with pulse phase . (4u 1538-52 , Cen X-3, GX 301-2; Robba et al. 2001; Naik 2011;Suchy et al. 2011)

  9. 41 keV 4U 0115+63 RXTE Heindl et al. 1999 GX301-2 Suzaku Suchy et al. 2011

  10. Variation of the cyclotron parameters with pulse phase --> cross section & depth of the CRSF is thought to depend significantly on the viewing angle of the accretion column • Variation of the CRSF parameters with phase used to model the structure of magnetic field around neutron stars  can probe magnetic field distortions & deviations from a dipole geometry. Can be used to infer the properties of the plasma & constraints between parameters • To perform phase resolved spectroscopy: need to separate the effect of variation of accretion rate (Lx) from that due to field structure alone. • Pulse phase resolved spectroscopy requires high sensitivity ( Suzaku ; in future ASTROSAT)‏ • Also requires good continuum modelling : Broad-band spectroscopy useful : Suzaku, ASTROSAT

  11. Essential Requirements to probe these features in detail → Broad-Band Energy Coverage & High sensitivity .... Suzaku launched in 2005 covers 0.2-600 KeV and good sensitivity → This would allow reliable broad-band continuum modeling specially required for the study to CRSFs.‏

  12. The Case of 2 HMXBs- 1A 1118-61 & Vela X-1 • 1A 1118-61: Hard X-ray transient pulsar; optical counterpart Be star; 3rd Giant Outburst in 2009 by Swift (Mangano et al. 2009). Pulsations detected at 407.68± 0.02 s ; Observations by Suzaku twice: during the peak of the outburst and ~20 days later • Vela X-1: Wind fed HMXB containing a supergiant companion and a massive neutron star. Pulsations detected at 283.23 ± 0.15 s; Observations with Suzaku on June 2008. Long observations ~ 103 ks

  13. Pulse profiles: 1A 1118-61 Peak of the outburst • Double peaked structure with secondary peak decreasing with energy. → disappears at ~ 12 keV • At high energies profile shows a single peak with a large pulse fraction. • A narrow low energy peak (< 2 keV) is seen coincident with the pulse minima at higher energies. → phase dependent soft-excess originating from the reprocessing in the accretion mound /column ?

  14. Pulse profiles: Declining phase • Pulse profile similar to the peak of the outburst ( secondary peak merges with the rising part of the main peak & soft excess peak detected)‏ • Additional pulse component seen after main peak < 2 keV & between 6-7 keV

  15. 5 peaked structure at lower energies (< 12 keV) strength of peaks and dips varying with energy Double peaked structure at higher energies. Explained due to energy dependent anisotropic emission from the polar caps and the accretion mound/column & a phase locked absorption in the accretion stream Pulse phase resolved spectroscopy carried out → probes the accretion column at different viewing angles w.r.t. our L.O.S. Pulse profiles: Vela X-1

  16. Intensity dependent Pulse profiles of vela X-1. To avoid intensity Dependence averaged Data over intensity band < 50 c s-1 Light curve along with hardness ratio for the entire duration of observation of Vela X-1. Stretch with constant hardness ratio is chosen to ensure results are free from systematic phase averaged spectral variability.

  17. Spectral analysis: Phase averaged • Fitted with a partial covering cutoff-powerlaw model with interstellar absorption and a narrow gaussian Fe kα line at 6.4 keV for both sources • For Vela X-1 additionally a Thermal comptonization model is used which give similar statistics to the more phenomenological model. • For 1A 1118-61 :A broad cyclotron absorption feature found in the spectra at ~ 49 keV during the peak & ~ 52 keV declining the declining phase respectively . For Vela X-1 fundamental and harmonic at ~ 25 & 52 KeV. Line Ratios !=2 !!! • Fitted with a lorenzian profile ”cyclabs” in Xspec. • 1A 1118-61 → Result may imply slight increase in the centroid of the line energy with decreasing Lx in agreement with variation in cyclotron line energy with the height of the accretion column. (Mihara et al. 2004; Nakajima et al. 2006). CRSF feature is also broader during the peak (More later...)‏

  18. Phase averaged spectra during the peak o the outburst : 1A 1118-61 Phase averaged spectra during the declining phase of the outburst : 1A 1118-61 Phase average spectra of Vela X-1

  19. Phase averaged parameters of spectral fitting 1A 1118-61 Vela X-1

  20. Spectral analysis: phase resolved continuum 1A 1118-61 Phase resolved parameters during the peak of the outburst Phase resolved parameters during the declining phase

  21. Vela X-1

  22. Spectral analysis: phase resolved continuum • Energy dependence of the pulse profiles → dependence of the spectrum with pulse phase • Abrupt increase in absorption column density and/or covering fraction during the dips in the pulse profile. → additional absorption component at that pulse phase which obscures radiation → phase locked accretion stream?? • Spectral hardening during the pulse peaks observed for Vela X-1 and during the peak of the outburst for 1A 1118-61 → deep and more direct view into the emission region ?? • For the comptonization model → At the dips in the pulse profiles optical depth increases and KT minimum. KT highest at ascending and descending edges of peaks; Efold of ”Highecut” and KT of ”CompTT” show similar trend.

  23. Spectral analysis: phase resolved CRSF Two models give consistent results for the variation of the CRSF parameters. Deepest and widest lines at the ascending and descending edges of the peak Two models give consistent results for the variation of the CRSF parameters. Deepest and widest lines at the ascending and descending edges of the peak Two models give consistent results for the variation of the CRSF parameters. Deepest and widest lines at the ascending and descending edges of the peak Cyclotron energy follows the pulse profile and the value maximum at the peaks of the profile. Indication of anti-correlation and hysterisis between the energy and width of the profile.

  24. Ratio of the line energies in Vela X-1 as obtained by fitting with ‘Highecut’ and ‘CompTT’ model. “Circle” denotes the parameters obtained with the ‘Highecut’ model and “star” denotes the same obtained with the ‘CompTT’ model.

  25. Discussion & Conclusion • Owing to the broad-band capability and high sensitivity of Suzaku we have been able to investigate the complex energy dependence of the pulse profiles in detail. • Dips in the pulse profile can be explained due to additional absorption component at that phase  accretion stream phase locked to neutron star. • Pulse phase resolved spectroscopy of the CRSFs of 1A 1118-61 and Vela X-1  most detailed results obtained so far. • Variation of both fundamental and harmonic CRSF of Vela X-1 with phase line ratios deviate from 2. • Detailed theoretical modeling required  for interpretation of radiation in these systems which can fit pulse phase resolved spectra.

  26. Studies with ASTROSAT Spectra of five accretion powered pulsars as would be detected by LAXPC. The lines are model spectra without the CRSFs, and the data show the simulated spectra for LAXPC.

  27. Polarimetric studies • Thomson scattering polarimeter (5-30 keV) : pulse phase resolved polarimetric studies probing the structure & geometry of the magnetic fields of neutron stars; beaming mechanism, its relation with luminosity mass accretion rate etc.

  28. Thank you

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