X-ray Spectroscopy ofAccreting White Dwarf Binaries

Koji Mukai (NASA/GSFC/CRESST & UMBC)

Chandra Workshop 2015 2

High Resolution Alone is Insufficient

2015 August 19

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

Chandra Workshop 2015 3

Shock Temperature and Mass

2015 August 19

Chandra Workshop 2015 4

Two Types of Intermediate Polars

2015 August 19

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

Chandra Workshop 2015 5

Aizu Model

2015 August 19

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

Chandra Workshop 2015 6

Global Fit for EX Hya

2015 August 19

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.

Chandra Workshop 2015 7

EX Hya: Selected Results

2015 August 19

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

Chandra Workshop 2015 8

V1223 Sgr: a Normal IP

2015 August 19

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 energy-dependent 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 IPsSpin modulation of EX Hya is probably mostly purely geometrical (both post-shock regions are simultaneously visible much of the time)

Chandra Workshop 2015 9

Normal IPs at CCD resolution

2015 August 19

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 ...

Chandra Workshop 2015 10

Photoionized Absorber

2015 August 19

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

Chandra Workshop 2015 11

Potential Science with ASTRO-H

2015 August 19

Density diagnostics using He-like Kalpha for V1223 Sgr

Probing the origin of hard X-rays in SS Cyg during outburst

Chandra Workshop 2015 12

Prospects for Dynamical Studies0.8 Mo 1.0 Mo 1.2 Mo 1.4Mo

Free-fall Velocity 5500 km/s 6900 km/s 8700 km/s 13800 km/s

¼ Vff 1375 km/s 1725 km/s 2175 km/s 3450 km/s

Keplerian Velocity 3900 km/s 4900 km/s 6200 km/s 9800 km/s

¼ Vkep 975 km/s 1225 km/s 1550 km/s 2450 km/s

Gravitational Redshift 50 km/s 80 km/s 130 km/s 320 km/s

2015 August 19

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 Chandrasekhar-mass white dwarfs

Chandra Workshop 2015 13

Gravitational Redshift

2015 August 19

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 dwarfbecome so massive, and what is its ultimate fate?

Chandra Workshop 2015 14

Summary 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

2015 August 19