 Plasma is shock-heated when accretion flow hits the white dwarf surface
 Shock temperature can exceed 50 keV
 kT>10 keV plasma primarily emits
Bremsstrahlung continuum
 Plasma models must handle such high kT
 Pre-shock accretion flow further modifies the spectral shape via absorption
 Understanding of the broad-band continuum is essential for both measurements using, and interpretation of, high-resolution X-ray spectroscopy
 Also, broad-band, moderate resolution data have their uses:
 WD mass can ben inferred from kTmax
 Reflection amplitude provides an extra handle on geometry
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2015 August 19 Chandra Workshop 2015 3

 Non-magnetic CVs and low luminosity IPs exhibit Cooling Flow like X-ray spectra
 Normal IPs were characterized by Mukai et al. (2003) as having photoionized X-ray spectra
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X-ray emission region in magnetic CVs are better understood – Aizu Model
 Isobaric cooling flow is a decent (but not perfect) approximation
 Analytical solutions (inc. those used in CV-specific models) often approximates plasma cooling using Bremsstrahlung cooling only
 Additional cooling channels can be present (cyclotron, Compton)
Accretion disk boundary layer (non-magnetic case) is a far trickier problem
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 Luna et al. (2015) used ½ M sec HETG data on EX Hya.
 A cooling flow model does a decent job fitting the entire
HETG data
 In detail, however, there are indications that the emission measure distribution deviates from that predicted by the isobaric cooling flow
 Pandel et al. (2005) analyzed XMM-Newton (EPIC+RGS) data for a sample of dwarf noave (i.e., non-magnetic) and found cooling flow-like solutions but with the EM distributions were not always the same.
 Pandel et al. and Byckling et al. (2010) found kTmax consistent with strong shocks from Keplerian flow in non-magnetic CVs.
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 Orbital motion of the white dwarf detected in X-rays (Hoogerwerf et al. 2004); but not the spin modulation
 Broad component detected in several lines, notably OVIII (Luna et al. 2010), presumably from photoionized pre-shock flow
 Use of Fe L lines (Fe XVII and Fe XXII) as potential density diagnostics
 Fluorescent 6.4 keV Fe line seen, but weak
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 At high accretion rate, the shock is close to the white dwarf surface
 Most X-ray photons cannot escape throught the sides of the post-shock region – complex absorption ensues
 This causes the energydependent spin modulation of
X-ray intensity, a defining characteristic of IPs
 Done & Magdziarz (1998) developed a complex absorption model
 Complex absorption x cooling flow can explain the X-ray spectra of normal IPs
Spin modulation of EX Hya is probably mostly purely geometrical (both post-shock regions are simultaneously visible much of the time)
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 High S/N EPIC data can be modeled, above 1 keV, using pwab(mkcflow+ga)
 V709 Cas appears to have discrete features (photoionized lines of Ne IX and O VII?)
 NY Lup appears to have a soft, blacbody-like component from the heated surface of the secondary
 The most notable feature in V1223 Sgr is neither ...
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 The OVII edge has now been detected in
V1223 Sgr with
HETG (Mukai et al. 2001), EPIC, and with RGS
 Also seen in several other
IPs
 The same, pre-shock flow has the physical characteristics ripe for both OVII edge in absorption and resonant lines in emission
 Can this be the reason why the detection of RRC has proved elusive?
 An ionized version of the pwab model is in the works
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 Density diagnostics using He-like
Kalpha for V1223 Sgr
2015 August 19
 Probing the origin of hard X-rays in
SS Cyg during outburst
Chandra Workshop 2015 11

Free-fall Velocity
¼ Vff
Keplerian Velocity
¼ Vkep
Gravitational Redshift
0.8 Mo 1.0 Mo 1.2 Mo 1.4Mo
5500 km/s 6900 km/s 8700 km/s 13800 km/s
1375 km/s 1725 km/s 2175 km/s 3450 km/s
3900 km/s 4900 km/s 6200 km/s 9800 km/s
975 km/s
50 km/s
1225 km/s 1550 km/s 2450 km/s
80 km/s 130 km/s 320 km/s
 At 6 keV, 300 km/s is 6 eV: these velocities are within reach of the SXS
 In the post-shock region, plasma decelerates further as it cools: lower energy lines are not expected to have anywhere near ¼ Vff or ¼ Vkep (studying Fe lines an advantage)
 A fraction of the 6.4 keV line is from the white dwarf surface: gravitational redshift may well be measurable
 The steep mass dependence makes it a great tool for near Chandrasekharmass white dwarfs
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One exciting possibility is to use the gravitational redshift of the white dwarf mass in symbiotic stars such as T CrB – for near-
Chandrasekhar mass, this may be the best and most accurate method for WD mass determination
If indeed T CrB harbors a near-Chandrasekhar mass white dwarf, which is suggested using less direct method, then we will ask: how did the white dwarf become so massive, and what is its ultimate fate?
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 Accreting white dwarfs all have the same central engine – a cooling flow like, multi-temperature, optically thin(-ish) emission region
 High quality data can probe the deviation from the isobaric cooling flow model
 Complex absorbers can create an illusion of dichotomy
 The best grating data available so far is on EX Hya, an atypical (low luminosity) IP
 Most other X-ray data on accreting white dwarf binaries lack the
S/N and/or the spectral resolution to enable similarly detailed studies
 ASTRO-H is going to change this, with high S/N, high resolution
(with SXS) and broad-band (with HXI) data
 The calorimeter brings dynamical studies into the realm of possibility
 As a special case, gravitational redshift is potentially within reach of the SXS if the white dwarf is massive
 Also, chemical abundances of CVs and symbiotic stars may yield unexpected insights
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