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Hard X-ray Emitting White Dwarfs in Symbiotic Stars: a Progress Report

Hard X-ray Emitting White Dwarfs in Symbiotic Stars: a Progress Report. Koji Mukai (NASA/GSFC/CRESST & UMBC) Koji.Mukai@nasa.gov Jamie Kennea (PSU), Juan Luna (CfA), Jeno Sokoloski (Columbia) X-ray Univserse 2008 Granada, Spain 2008 May 29. Symbiotic Stars.

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Hard X-ray Emitting White Dwarfs in Symbiotic Stars: a Progress Report

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  1. Hard X-ray Emitting White Dwarfsin Symbiotic Stars: a Progress Report Koji Mukai (NASA/GSFC/CRESST & UMBC) Koji.Mukai@nasa.gov Jamie Kennea (PSU), Juan Luna (CfA), Jeno Sokoloski (Columbia) X-ray Univserse 2008 Granada, Spain 2008 May 29

  2. Symbiotic Stars A Symbiotic Star is binary containing a red giant with emission lines. In many cases, there is a white dwarf accreting from the giant, producing a blue continuum that ionizes the wind and other circumstellar matter. This talk concentrates on those symbiotic stars with a white dwarf companion (others, e.g., GX 1+4, have a neutron star companion) Some Symbiotic Stars are also recurrent novae: once every several decades or so, they undergo thermonuclear runaway. X-ray Universe 2008

  3. CVs and Symbiotic Stars • Cataclysmic Variables (CVs) • Are semi-detached binaries with a white dwarf accretor • Have a K-M ~main sequence mass donor • Always accrete via Roche-lobe overflow • Have a typical orbital period of hours • Are usually dominated by accretion luminosity in the visible band • Have been well studied in the X-rays (see the rest of this session) • Symbiotic Stars • Also have a white dwarf accretor • Have M giant mass donors • Are usually thought to be wind accretors (but question of high accretion rate) • Have a typical orbital period of years • Are usually dominated by the photospheric emission of the M giant in the visible band • Have not been well studied in the X-rays X-ray Universe 2008

  4. ROSAT View: Muerset et al. • ROSAT observations of symbiotic stars (Muerset et al. 1997) revealed several distinct types of X-ray emission. • Supersoft (e.g., RR Tel) - detectable below 0.5 keV, photospheric emission from nuclear burning white dwarfs • “Beta-type” (e.g., AG Peg) - peaks suggests optically thin thermal emission with kT~1 keV, interpreted as due to colliding winds • Others (e.g., GX 1+4) - harder emission, in this case from accretion onto a neutron star X-ray Universe 2008

  5. BAT detections of the Gang of 4 4 symbiotic stars have now been detected in the Swift/BAT survey (2 were also detected with INTEGRAL) Neither the supersoft or the “beta type” emission seen with ROSAT can explain these BAT detections So what is going on? X-ray Universe 2008

  6. The curious case of CH Cyg CH Cyg, in addition to the “beta type” emission, showed a much harder component in ROSAT data (which, in fact, had been known from earlier satellites) X-ray Universe 2008

  7. ASCA Spectrum of CH Cyg X-ray Universe 2008

  8. ASCA Spectrum of CH Cyg Ezuka & Ishida (1998) found that the ASCA spectrum of CH Cyg to be • Clearly separated into two spectral components • Above about E~2 keV, there is a highly absorbed, hard component. • There is a clear detection of Fe K complex, indicating thin thermal origin • A single kT (~5 keV) fit would work • But the effective bandpass was narrow (E~2-10 keV) so only weak constraints can be placed on models • Reminescent of 2-10 keV X-rays seen in CVs, indicating accretion onto the white dwarf as the probable origin • Below about E~2 keV, there is an unabsorbed, soft component • Several emission lines (Ne, Mg, Si) are detected, again indicating thin thermal origin • Origin of this component has since been a subject of lively debate X-ray Universe 2008

  9. Recent & Ongoing Projects In the rest of this talk, we will present selected item from: • Kennea et al. (2008) summarizes works done with Swift • Analysis of BAT light curves and spectra • Follow-up using pointed XRT observations for source identification and joint BAT-XRT spectral fits • Hours~days timescale spectral variability • Additional works: • Suzaku observation of CD -57 3057 aka SS 73-17 (Smith et al. 2008), Chandra and Suzaku observation of the same (approved) • Chandra DDT observation of RT Cru (Luna & Sokoloski 2007) • Suzaku observation of CH Cyg (Mukai et al. 2007), RT Cru, & T CrB (Luna et al., work in progress) • RXTE/PCA monitoring of T CrB and CH Cyg • Chandra TOO observation of CH Cyg (approved) X-ray Universe 2008

  10. BAT 9-month Light Curve All 4 symibotics are variable. RT Cru appears to have been much brighter in hard X-rays in 2003 (INTEGRAL; Chernyakova et al 2005) CH Cyg went into a low state in the spring/summer of 2005 X-ray Universe 2008

  11. Two Types of Low States in CH Cyg During the 2001 March Chandra/HETG observation, both the soft and the hard component was weak (which in turn allowed detection of spatially extended X-ray emission from the jet; Galloway & Sokoloski 2004) From 2005 summer to present, CH Cyg appears to have been in a second type of low state: the soft X-ray probably is still bright while the hard X-rays appear to be low X-ray Universe 2008

  12. Clues from the Fe K lines • Both in high state (ASCA) and in 2001 low state (Chandra), the Fe complex is dominated by the He-like (6.7 keV) line • the luminosity reflects accretion rate • In Suzaku data, the 6.4 keV line dominates: the direct line-of-sight is blocked and we only see the scattered component X-ray Universe 2008

  13. RXTE Spectra of CH Cyg Currently CH Cyg is very faint for RXTE/PCA, but untillvery recently, it appears to have been dominated by the 6.4 keV line. This may be changing (still a factor of ~10 lower than in ASCA data) X-ray Universe 2008

  14. BAT+XRT Spectra of T CrB X-ray Universe 2008

  15. BAT+XRT Joint Fits • Swift XRT has problems with CH Cyg because it is extremely bright in the optical • The other three can be fitted with a bremsstrahlung continuum plus a single Gaussian near 6.5 keV, with a strong partial covering absorber. • Note BAT spectrum is average over 9 months, XRT data are from relatively short, pointed observations • More complex models are not warranted given the quality of the data • kT~37 keV for RT Cru, 17.2 keV for T CrB, and 17 keV for CD-57 3057 • BAT only fit gives kT~5 keV for CH Cyg X-ray Universe 2008

  16. Spectral Variability of RT Cru Swift/XRT spectra of RT Cru at three flux levels - the hard part is more or less steady while the soft part strongly varies X-ray Universe 2008

  17. XRT Count Rate vs. Hardness X-ray Universe 2008

  18. The Origin of Variability • All three symbiotic stars - RT Cru and CD-57 3057 definitely and probably also T CrB - show strong partial covering absorber • All three (again, not so definitely for T CrB) become softer when brighter • We do not see the beta-type emission in these three • We do not know why all 4 hard X-ray bright symibotics show so strong an intrinsic absorber. • This is not a general characteristic of symbiotic stars. If so, we would never see any supersoft component from symbiotic stars • Absorber must be close to the hard X-ray source, i.e. the white dwarf (remember these binaries have AU-type separation!) Circumbinary absorber shouldn’t be able to vary on hour timesclaes. X-ray Universe 2008

  19. The Lack of Coherent Periods • Although these symbiotic stars are all variable on short time scales, they do not show coherent periods • in X-rays or • in the optical Chandra light curves of RT Cru The lack of coherent periods strongly suggest the accreting objects in the hard X-ray symbiotic stars as a class are not magnetic white dwarfs X-ray Universe 2008

  20. Spectra of the Hard Component • The hard X-rays are optical thin, thermal emission from the accreting plasma, as also seen in magnetic and non-magnetic CVs • These symbiotic stars have harder spectra than non-magnetic CVs • Yet they are unlikely to have a magnetic white dwarf • Non-magnetic CVs can have high temperatures if the accreting white dwarf is massive - SS Cyg at ~1.1Msun is the hardest among dwarf novae • By scaling from SS Cyg in outburst (ask me for details later), we estimate ~1.35 Msun for T CrB and CD-57 3057, and ~1.4 Msun for RT Cru • This agrees with the independent estimates for T CrB, which is also a recurrent nova. • Maybe the hard X-ray survey is a good way to find near Chandrasekhar mass white dwarfs in symbiotic stars X-ray Universe 2008

  21. Spectroscopy of the hard component Suzaku XIS+HXD/PIN spectra of T CrB X-ray Universe 2008

  22. Spectroscopy of the hard component Suzaku spectrum of RT Cru X-ray Universe 2008

  23. Fe Kalpha Complex XIS clearly resolves the Fe K complex into three lines X-ray Universe 2008

  24. Conclusions so far and Questions • Swift/BAT survey has identified 4 symbiotic stars as bright hard X-ray (>10 keV) sources. • Why these 4, and not others? RS Oph also is a recurrent nova with similarly massive white dwarf • The hard X-rays are thin thermal emissions, i.e., from plasma accreting onto the white dwarf • Are our inference for very massive white dwarfs correct? • They all have strong intrinsic (partial covering and variable) absorbers • Where exactly is the absorber? It’s unlikely to be geometric if all hard X-ray bright symbiotic stars have such an absorber • They are variable on timescales of hours to years, including two types of low states for CH Cyg • How many others like these are there? • Current BAT sensitivity is about ~2x10-11, so pointed XMM-Newton (etc.) observations or the e-ROSITA survey can detect many more X-ray Universe 2008

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