Hard X-ray Emitting White Dwarfs in Symbiotic Stars: a Progress Report Koji Mukai (NASA/GSFC/CRESST & UMBC) [email protected] Jamie Kennea (PSU), Juan.

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

Koji Mukai (NASA/GSFC/CRESST & UMBC)

[email protected]

Jamie Kennea (PSU), Juan Luna (CfA), Jeno Sokoloski (Columbia)

X-ray Univserse 2008

Granada, Spain

2008 May 29

mailto:[email protected]




2008 May 29 X-ray Universe 2008 2

Symbiotic StarsA 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.


CVs and Symbiotic StarsCataclysmic 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


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


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?


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)


ASCA Spectrum of CH Cyg


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


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)


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


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


Clues from the Fe K linesBoth 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


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)


BAT+XRT Spectra of T CrB


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


Spectral Variability of RT CruSwift/XRT spectra of RT Cru at three flux levels - the hard part is more or less steady while the soft part strongly varies


XRT Count Rate vs. Hardness


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.


The Lack of Coherent Periods

Chandra light curves of RT Cru

Although these symbiotic stars are all variable on short time scales, they do not show coherent periods•in X-rays or•in the optical

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


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


Spectroscopy of the hard component

Suzaku XIS+HXD/PIN spectra of T CrB


Spectroscopy of the hard component

Suzaku spectrum of RT Cru


Fe Kalpha Complex

XIS clearly resolves the Fe K complex into three lines


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

Hard X-ray Emitting White Dwarfs in Symbiotic Stars: a Progress Report Koji Mukai (NASA/GSFC/CRESST & UMBC) [email protected] Jamie Kennea (PSU), Juan.

Documents

xray universe

session symbiotic stars

hard xray

thermal emission

photospheric emission

harder emission

beta type emission

kev xrays