Photoionized features in the X-ray spectrum of EX Hydrae

0.2 MB·English

by  G. J. M. Luna

Draftversion January29,2010
PreprinttypesetusingLATEXstyleemulateapjv.11/10/09
PHOTOIONIZED FEATURES IN THE X-RAY SPECTRUM OF EX HYDRAE
G. J. M. Luna1, J. C. Raymond1, N. S. Brickhouse1, C. W. Mauche2, D. Proga3, D. Steeghs4, R. Hoogerwerf5
Draft version January 29, 2010
ABSTRACT
We presentthe firstresultsfroma long(496ks)Chandra HighEnergyTransmissionGratingobser-
vationoftheintermediatepolarEXHydrae. Inadditiontothenarrowemissionlinesfromthecooling
0 post-shock gas, for the first time we have detected a broad component in some of the X-ray emission
1 lines, namely OVIII λ18.97,MgXII λ8.42, SiXIV λ6.18,and FeXVII λ16.78. The broadand narrow
0 components have widths of ∼ 1600 km s−1 and ∼ 150 km s−1, respectively. We propose a scenario
2 wherethe broadcomponentis formedin the pre-shockaccretionflow,photoionizedby radiationfrom
n the post-shock flow. Because the photoionized region has to be close to the radiation source in order
a to produce strong photoionized emission lines from ions like O VIII, Fe XVII, Mg XII, and Si XIV,
J our photoionization model constrains the height of the standing shock above the white dwarf sur-
9 face. Thus, the X-ray spectrum from EX Hya manifests features of both magnetic and non-magnetic
2 cataclysmic variables.
Subject headings: Binaries: cataclysmic variables — stars: individual (EX Hydrae) — techniques:

] spectroscopy — X-rays: stars
R
S
. 1. INTRODUCTION observedX-ray spectrum is dominated by emission lines
h
formed in a region photoionized by the radiation field
p EX Hydrae (EX Hya) belongs to a sub-class of mag-
fromtheshockedregion. Incontrast,theboundarylayer
- netic cataclysmic variables (CVs), known as interme-
o diate polars (IPs), wherein the white dwarf (WD) at the inner edge of the accretion disk of non-magnetic
r magnetic field (≈ 0.1–10 MG) channels the accreting CVs covers a relatively large area and therefore the spe-
t
s material from the inner edge of the truncated accre- cific accretion rate is low, allowing the X-ray photons
a to escape freely. Mukai et al. (2003) observed this “bi-
tion disk through the accretion curtains and accretion
[ nomial” distribution in moderately exposed (∼ 100 ks)
columns to spots near the magnetic poles (Rosen et al.
observations obtained with Chandra using the High En-
1 1988). In this model, the magnetically channeled ma-
ergyTransmissionGrating(HETG),andgenerallyfound
v terial reaches highly supersonic velocities in the pre-
9 shock flow (v = [2GM /R ]1/2 ≃ 6000 km s−1, that the X-ray spectra of magnetic CVs are compatible
5 where v is ffthe free-falWl DvelocWitDy for a WD of mass with photoionized emission while those of non-magnetic
54 MWD =ff0.79M⊙ andradiusRWD =7.1×108cm)before CgaVss(kanroewcnonassisatecnotolwinigth-flaowcomlloisdioelnianllyX-iroanyizsepde,ctcroaollifintg-
passing through a strong stand-off shock near the WD
. ting packages such as XSPEC6). Interestingly, EX Hya
1 surface. At the shock, the kinetic energy is converted
was the “exception” in this binomialdistribution: its X-
0 into thermal energy, heating the gas to temperatures of
0 ∼20 keV(Fujimoto & Ishida 1997; Brunschweiger et al. rayspectrumwaswellfitwithacooling-flowmodel. This
canbe explainedifEXHyahasatallshock(Allan et al.
1 2009). Belowthe shock,inthe post-shockflow,the cool-
: ing material radiates its thermal and residual gravita- 1998), a lower accretion rate, and/or larger accretion
v spots than other IPs, and therefore a low specific ac-
tional energy through free-free, bound-bound, and free-
i cretion rate, leading to a dominant collisionally ionized,
X bound radiation, which is primarily detected in X-rays.
cooling spectrum.
r ThenatureoftheobservedX-rayspectrumofCVsde-
We have obtained a deeper observation of EX Hya
a pends primarily on the specific accretion rate (i.e., the
with the Chandra HETG in order to study a number of
accretion rate per unit area). The small spot size and
accretion-related phenomena. In this Letter, we present
hence high specific accretion rate of magnetic CVs im-
new results from this observation discussed in Section
pose a conical geometry that does not allow the X-ray
2. In Section 3 we present an analysis of the X-ray line
photons to escape without further interaction. Thus the
profilesthatrequireabroadcomponent,andaphotoion-
1Harvard-SmithsonianCenter for Astrophysics, 60 Garden St., ization model to explain it. Discussion and conclusions
Cambridge, MA, 02138, USA. E-mail: [email protected]; are presented in Section 4.
[email protected]; [email protected]
2Lawrence Livermore National Laboratory, L-473, 2. OBSERVATIONS
7000 East Avenue, Livermore, CA 94550, USA. E-mail: EX Hya was observed with Chandra using the HETG
[email protected]
3Department of Physics andAstronomy, Universityof Nevada, in combination with the ACIS-S for 496 ks. The ob-
4505 South Maryland Parkway, Box 454002, Las Vegas, NV, servation was obtained in four segments (ObsIDs 7449:
89154-4002, USA.E-mail: [email protected] start time 2007 May 13 22:15:35 UT, exposure time
4DepartmentofPhysics,UniversityofWarwick,CoventryCV4 130.65 ks; 7452: start time 2007 May 17 03:12:38 UT,
7AL,UK.E-mail: [email protected]
exposure time 49.17 ks, 7450: start time 2007 May 18
5InteractiveSupercomputing,Inc.,135BeaverStreet,Waltham,
MA02452,USA.E-mail: [email protected]
6 http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/
2 Luna et al.
21:56:57 UT, exposure time 162.73 ks; and 7451: start ing in: χ2(O VIII)=398.60; χ2(Mg XII)=1577.30;
time2007May2114:15:08UT,exposuretime153.07ks). χ2(Si XIV)=772.40 and χ2(Fe XVII)=141.77]. The
WeextractedHigh-EnergyGrating(HEG)andMedium- difference in χ2 between the fit of the double- and
Energy Grating (MEG) ± first-order spectra and Ancil- single-Gaussian models to the data was computed for
lary Responce Matrices (ARFs) using the CIAO7 script each spectrum in order to test if the broad component
fullgarf, while Response Matrix Functions (RMFs) could be produced purely by random, Poisson noise.
were extracted using mkgrmf script. We then combined We found that only 1–10 in 10,000 of the randomly
ARFs and spectra from each observation segment using generated single-Gaussian spectra could produce a
the add grating spectra script. For Si XIV λ6.18 and similar decrease in χ2 when compared to the observed
Mg XII λ8.42, fits were performed using the HEG and data, after the addition of a broad emission line (see
MEG±firstorderdata,whereasforFeXVIIλ16.78and Table 1 and Figure 3). The null hypothesis probability
O VIII λ18.97, fits were performed using only the MEG that the broad component could arise purely from
± first order data, due to the rapid decay of the HEG Poissonnoise is .0.1%; thus the broad components are
effective area for λ & 17 ˚A (Chandra Proposer’s Obser- detected at 99.9% confidence.
vatory Guide v11.0).
3.2. The photoionization model
3. EMISSIONLINEPROFILES The width of the broad component allows us to con-
3.1. Line profiles analysis strain the region where the line is formed. As listed in
Table 1, we measured large widths in the broad compo-
Inordertotestmodelsofthecoolingofthepost-shock
nents, which imply that the radiating plasma is moving
gas (e.g. Aizu 1973; Canalle et al. 2005), we measured with large velocities (v & 2000 km s−1). Of the three
the fluxes of the strong emission lines observed in the
regions of the magnetically controlled accretion flow —
Chandra HETG spectrum using a Gaussian to repre-
the accretioncurtains, where the flow rises up and away
sent each emission line and a first-order polynomial to
from the truncated accretion disk; the pre-shock flow,
represent the nearby continuum. We found that some
where the relatively coolflow falls with high velocity to-
of the strong emission lines were poorly fit with such a
wardtheWDsurface;andthepost-shockflow,wherethe
model, and that acceptable fits to these lines, in par-
hot,low-velocityflowsettlesontotheWDsurface—the
ticular O VIII λ18.97, Fe XVII λ16.78, Mg XII λ8.42,
pre-shock flow is the most likely source of the observed
and Si XIV λ6.18, could be obtained by adding a sec-
broadcomponentsoftheX-rayemissionlinesofEXHya.
ond Gaussian to represent the broad line wings. All the
In principle either photoionization or collisional exci-
fit parameters were free to vary and their limits deter-
tation could produce the observed broad components;
mined to 90% confidence. Figure 1 shows the studied
however,if the width is interpreted as thermal broaden-
emissionlines togetherwith the best-fit models. Central
ing, the implied temperature is too high. The FWHM
wavelength,full widthhalfmaximum(FWHM), andob-
of the emission line can be translated into temperature
servedflux of the narrowand broad components of each
through:
line are listed in Table 1.
An instrumental origin for these broad emission lines ∆λ 2
seems unlikely. First, the broad component is detected Tion =µ×(cid:18)7.7×1F0W−H5M×λ(cid:19) (1)
in four spectral orders simultaneously (HEG and MEG
±firstorders),whereasanybroadeninginthedispersion where T is the ion temperature in eV, µ is the ion
ion
direction of a diffraction arm should not affect the dis- mass,∆λ isthe observedFWHMofthe lineandλ
FWHM
persion in another arm. Second, as shown in Figure 2, a istheobservedcentralwavelength. Themeasuredwidths
comparison with a similarly deeply exposed observation imply temperatures T ∼ 80 keV, much higher than
ion
of the pre-main-sequence star TW Hya (exposure time: the peak electron temperature measured of ∼ 20 keV
489ks,Brickhouse et al.2009)withthesameinstrument (Fujimoto & Ishida 1997; Brunschweiger et al. 2009).
configuration does not show similar broad features. Ontheotherhand,aphotoionizationoriginfortheob-
The statistical significance of the presence of a servedbroademissionlinesispossibleifthereisaregion
broad component in the line profiles of O VIII λ18.97, in the system that fullfills certain conditions. First, ion-
Fe XVII λ16.78, Mg XII λ8.42, and Si XIV λ6.18 in ization parameters (ξ =L /nr2, where L is the X-ray
X X
the HETG spectra of EX Hya was assessed by fitting luminosity,nisthenumberdensity,andristhedistance
10,000 Monte Carlo realizations of the spectra in the between the source and targetof the photoionizing flux)
neighborhood of each line, using a model consisting of ofafewhundredarerequiredtoaccountforthepresence
a continuum plus a single narrow Gaussian, with the of OVIII, MgXII, and SiXIV (e.g., Kallman & McCray
same exposure time, response, and ancillary matrices 1982). Second,theemissionmeasure(EM =n2V,where
as our observation, following the method described in V is the volume) of the photoionized plasma must be
Protassovet al. (2002) and Reeves et al. (2003). We fit large enough to produce the observed flux.
the simulated spectra using two models: a continuum The conditions above exclude the accretion curtains
plus (i) a single narrow Gaussian [with a total of 4 and favor the pre-shock photoionized flow as the source
free parameters, resulting in: χ2(O VIII)=503.70; of the broad X-ray emission lines. Consider first the ac-
χ2(Mg XII)=1671.69; χ2(Si XIV)=868.04, and cretioncurtains: usingtheobservedOVIIIluminosityat
χ2(Fe XVII)=162.72] and (ii) narrow and broad an inner disk radius of 10RWD (e.g., Beuermann et al.
Gaussians [with a total of 7 free parameters, result- 2003)andwithanionizationparameterofafewhundred
results in a density of around 3×1010 cm−3, and hence
7 Chandra Interactive AnalysisofObservations (CIAOv. 4.1) avolumeof2×105R3 ,whichishundreds oftimes the
WD
X-rays line profiles in EX Hydrae. 3
TABLE 1
Line Fit Parameters.
NarrowComponent BroadComponent
λb FWHMc λb FWHMc
Linea (˚A) (kms−1) Fluxd (˚A) (kms−1) Fluxd
OVIIIλ18.97 .... 18.972+−00..000066 315+−3245 7.13+−00..43 18.972+−00..001111 1922+−213602 2.77+−00..33
FeXVIIλ16.78 ... 16.777+−00..002142 117+−4520 2.75+−00..1185 16.783+−00..001198 1963+−1521417 0.55+−00..1291
MgXIIλ8.42..... 8.422+−00..000066 360+−4314 1.08+−00..0098 8.424+−00..000066 1329+−117068 0.53+−00..1009
SiXIVλ6.18 ..... 6.183+−00..000066 410+−2537 1.09+−00..0130 6.185+−00..000066 1297+−6798 0.84+−00..0035
a OVIIIandFeXVIIwerefitusingonlytheMEGarm;MgXIIandSiXIVwerefitusingboththeHEGandMEGarms.
b Absolute wavelength accuracy: HEG=0.006 ˚A, MEG=0.011 ˚A. Relative wavelength accuracy: HEG=0.0010 ˚A, MEG= 0.0020 ˚A
(Chandra Proposer’sObservatoryGuidev11.0)
c FWHM≈2.3548σ.
d Unitsof10−4 photons cm−2 s−1.
350 300 400
(a) (b) (c) (d)
300 250
300
300
250
200
bin 200 bin bin bin
nts/ nts/ 150 nts/ 200 nts/ 200
ou 150 ou ou ou
C C C C
100
100 100 100
50 50
0 0 0
15 15 15 15
10 10 10 10
5 5 5 5
χ2 0 χ2 0 χ2 0 χ2 0
∆ -5 ∆ -5 ∆ -5 ∆ -5
-10 -10 -10 -10
-15 -15 -15 -15
18.8 18.9 19.0 19.1 19.2 16.55 16.66 16.77 16.89 17.00 8.25 8.34 8.43 8.51 8.60 6.05 6.11 6.18 6.24 6.30
Wavelength [Angstrom] Wavelength [Angstrom] Wavelength [Angstrom] Wavelength [Angstrom]
100 300 400
(e) 120 (f) (g) (h)
250
80
100 300
200
n 60 n 80 n n
bi bi bi bi
unts/ unts/ 60 unts/ 150 unts/ 200
Co 40 Co Co Co
100
40
100
20
20 50
0 0 0
15 15 15 15
10 10 10 10
5 5 5 5
χ2 0 χ2 0 χ2 0 χ2 0
∆ -5 ∆ -5 ∆ -5 ∆ -5
-10 -10 -10 -10
-15 -15 -15 -15
18.70 18.83 18.95 19.08 19.20 16.55 16.66 16.77 16.89 17.00 8.25 8.34 8.43 8.51 8.60 6.05 6.11 6.18 6.24 6.30
Wavelength [Angstrom] Wavelength [Angstrom] Wavelength [Angstrom] Wavelength [Angstrom]
Fig.1.—Observedemissionlineprofileswithoverlaiddouble-Gaussianmodelsof(a,e)OVIIIλ18.97(MEG±1),(b,f)FeXVIIλ16.78
(MEG±1),(c,g)MgXIIλ8.42(HEG±1),and(d,h)SiXIVλ6.18(HEG±1). Dataareshownbytheblackhistogramsandthenarrow,
broad, and net components of the model profiles are shown by the green, blue, and red histograms, respectively. The two emission lines
(FeXXI–XXIVλ8.31andFeXXIVλ8.38)inthevicinityofMgXIIλ8.42wereincludedinthefit. Beloweachpanelweplottheresiduals
relativetothenetdouble-Gaussianmodel(red)andtoitsnarrowcomponent(green). ∆χ2 standsforthedifferencebetweenthedataand
themodel,squared,dividedbythevariance,withthesignofthedifferencebetween thedataandthemodel.
4 Luna et al.
cludeonlyone-dimensionalradiativetransferandneglect
1.2
time-dependent ionization, compressional heating of the
1.0 EX Hya infalling gas due to the assumed dipole geometry, and
densitystratificationinthepre-shockflow. However,our
bin0.8 TW Hya model should give the correct intensities for the strong
nts/ lines. We note that the detection of a broad component
Cou0.6 in Fe XVII λ16.78, but not in λ15.01 supports a pho-
Scaled 0.4 tλo1i6o.n7i8z/aFtieonXoVrIiIgiλn1.5L.0ie1d>ahl25etaatl.d(e1n9s9it0i)esproefd1ic0t11FecmXV−3II,
0.2 with the ratio of both lines increasing with density.
It should be noted, however, that the tabulated (pre-
Hya 00..03 shock) densities are comparable to the post-shock den-
W 0.2 sities derived by Mauche et al. (2001, 2003) from the
T 0.1
Hya--00..10 Fe XVII and Fe XXII emission-line ratios, whereas the
X -0.2 post-shock density should be higher by a factor of four
E 18.8 18.9 19.0 19.1 due to shock compression and by an additional factor
Wavelength [Angstrom]
due to compression as the shocked gas cools. The latter
would be an order of magnitude for a steady flow, but
Fig.2.—Upperpanel: ObservedOVIIIλ18.97emissionlinepro- could be much smaller due to the thermal instability of
fileinEXHya(black)andTWHya(red),scaledtothesamepeak
intensity. Lower panel: Relative difference between the EX Hya radiative shocks (Chevalier & Imamura 1982).
tahnedETXWHHyyaalpinreofipleros,fisleh.owTinhgetlahcekexocfeassbermoaisdsicoonminpotnheenwtiinngsthoef 4. DISCUSSIONANDCONCLUSIONS
TWHyalineprofiledemonstrates thatthebroadcomponentseen Our photoionization models (A to D) yield similar
inEXHyaisnotofinstrumentalorigin. fluxes for O VIII, in good agreement with the observed
value, but the predicted Fe XVII, Mg XII, and Si XIV
volumeavailableatthatdistancefromtheWD.Consider fluxes are smaller by a factor of 2 to 30. Although our
nextthepre-shockflow: atadistance0.1RWD,thesame simple model is not accurate in the prediction of all
ionization parameter and luminosity imply a density of the observed fluxes, it is appropriate in the sense that
around3×1014cm−3,andhenceavolumeof0.002RW3 D, it shows that very small spots require higher pre-shock
which is roughly the size of the accretion shock region. densities than those observed by Mauche et al. (2001,
Given these considerations, we constructed a model 2003) for the post-shock flow, and very large accretion
of the X-ray line emission from the photoionized flow spots give small ionization parameters that cannot pro-
above the stand-off shock. We assume a WD of mass duce the observed flux. Furthermore, comparison of the
MWD =0.79M⊙ (Beuermann & Reinsch 2008), a corre- free-fall velocity with the line widths indicates that the
sponding radius RWD = 7.1×108 cm (using the mass- accretion column must not be too far from the limb of
radius relationship of Pringle & Webbink 1975) and a the WD as seen from Earth. The simple model pre-
luminosity L = 2.6×1032 erg s−1 at a distance of 64 dicts that the Doppler width (v ≈ 2000 km s−1) of the
X
pc (Beuermann et al. 2003). These parameters and the ionized materialis smaller than the ∼6000 km s−1 free-
assumed size of the accretion spots determine the den- fall velocity (Section 1). If the columns were observed
sityattheshock,andthisdensityinturndeterminesthe face-on, this larger Doppler shift would be seen. The
cooling time and shock height. smaller observed line width suggests that the accreting
We assume a density n0 at the base of the pre-shock polesremaininthevicinityoftheWDlimbasseenfrom
flowandadipolemagneticfield,hencethatthepre-shock Earth,assupportedbythesmallspin-phaseradialveloc-
densityscaleswiththeradiusrasn (R /r)−5/2. With ity variations observed by Hoogerwerf et al. (2005) and
0 WD
this assumption, the flux of the illuminating radiation the small equivalent width (EW ∼ 36 eV) of the Fe Kα
and hence the ionization parameter drops rapidly just line: George & Fabian (1991) calculated that such an
above the shock on a length scale comparable to the EW would be observed if the angle between the line of
shock size. The atomic physics packages described in sight and the reflecting surface is roughly 80◦, i.e. the
Miller et al. (2008) are used to compute the ionization accreting poles are close to the limb.
and thermal equilibrium and the attenuation of radia- We can decide which models are more adequate us-
tion along and perpendicular to the column axis. We ing additional arguments. First, the height of the shock
have explored four different models — designated A, B, shouldbelargerthan≈0.015R toaccountfortheob-
WD
C and D — that were chosen to match the observed servedmodulationin the X-raylightcurves(Allan et al.
luminosity, L , and differ only in the area of the accre- 1998; Hoogerwerf et al. 2006), but less than ∼2R to
X WD
tionspot. Theresultingparametersofeachmodel,listed account for the ∼ 20 keV thermal bremsstrahlung tem-
in Table 2, are: the pre-shock density (n ), the optical perature (Fujimoto & Ishida 1997; Brunschweiger et al.
0
depth for electron scattering (τ ) , the size of the accre- 2009). Second, the predicted pre-shock densities should
e
tion spot (r ), the shock height (h ), the height be less than the values derived by Mauche et al. (2001,
spot shock
of the O VIII peak emissivity (d) and the observed flux 2003) from the Fe XVII and Fe XXII emission-line ra-
oftheobservedbroadlines. Thecorrespondingradiative tios. Third, a significant optical depth for electron scat-
recombinationcontinua(RRCs)shouldbecomparablein teringimplies modulations(pulse fraction&10%)inthe
strengthtothepredictedlinefluxes,buttheywillbesig- lightcurvesinthe6–8keVrange,whichwereobservedin
nificantly smeared in wavelength and therefore difficult Ginga dataanalyzedby Rosen et al.(1991). Allan et al.
to detect. The models are quite simple in that they in- (1998), however,foundonlyanupper limitof4%for the
X-rays line profiles in EX Hydrae. 5
600 (a) 1400 (b) 1500 (c) 250 (d)
1200
500 200
uency 400 uency 1800000 uency 1000 uency 150
Freq 230000 Freq 460000 Freq 500 Freq 100
100 200 50
0 0 0 0
-100 -50 0 50 100 150 -25 00 25 50 -50 00 50 100 -150 -50 50 150
∆χ2 ∆χ2 ∆χ2 ∆χ2
Fig. 3.— Histograms of the distribution of ∆χ2 for (a) O VIII, (b) Fe XVII, (c) Mg XII, and (d) Si XIV resulting from fitting 10,000
Monte Carlo simulated spectra with a continuum plus (i) a single narrow Gaussian and (ii) narrow and broad Gaussians. In the case of
OVIII, MgXII, and SiXIV, only 1 out of 10,000 randomly generated spectra produce a∆χ2 (i.e. difference inχ2 between the fit of the
double- and single-Gaussian models) equivalent to that found in our data; for Fe XVII, 10 out 10,000 produce a ∆χ2 equivalent to that
foundinourdata. The∆χ2 valuemeasuredintheobserveddataismarkedineachpanelbyadashedline.
TABLE 2
Photoionization modelresults.
Parameter ModelA ModelB ModelC ModelD ObservedFluxa
n0b ............. 0.41 2.7 12. 30. ···
τe............... 0.03 0.19 0.73 1.8 ···
rspotc ........... 0.098 0.056 0.031 0.018 ···
hshockd .......... 0.62 0.19 0.062 0.019 ···
de............... 1.03 0.24 0.080 0.025 ···
OVIIIλ18.97f.... 2.34 2.09 2.45 3.08 2.77+−00..33
FeXVIIλ16.78f... 0.21 0.16 0.17 0.16 0.55+−00..1291
MgXIIλ8.42f .... 0.07 0.08 0.10 0.09 0.53+−00..1009
SiXIVλ6.18f..... 0.03 0.05 0.08 0.09 0.84+−00..0035
a Foreaseofcomparisonwiththemodelpredictions,thiscolumnreproducesthebroadcomponent fluxlistedinTable1.
b Densityatthebaseofthepre-shockflowinunitsof1014 cm−3.
c RadiusoftheaccretionspotonthesurfaceoftheWDinunitsofthewhitedwarfradius,RWD=7.1×108 cm.
d HeightoftheshockabovetheWDsurfaceinunitsofthewhitedwarfradius.
e HeightofthepeakOVIIIemissivityabovetheWDsurfaceinunitsofthewhitedwarfradius.
f Modelpredictedfluxforthebroadcomponentinunitsof10−4 photons cm−2 s−1.
pulse fraction in the light curve in the 6–8 keV range contributiontothelightcurvemodulationinthe6–8keV
observed with ASCA. Moreover, as detailed by Rosen energy range and absorptionis small, in agreementwith
(1992), the favourable lines of sight to detect a high en- the absence of ∼100% pulse fraction in the low-energy
ergy modulation due to electron scattering would also spinlightcurvesandupper limitof4%inthe pulse frac-
imply in significant photoelectric absorption in the pre- tionofthelightcurvesinthe6–8keVrangeasshownby
shockcoldmaterial,whichwillextinguishthelow-energy Allan et al. (1998).
flux andmanifestitselfasflat-bottomedsoftX-raysspin Summarizing, we detect for the first time in X-rays a
light curves. broadcomponentinthe emissionline profilesofEXHya
Based on the above conditions, we discard models C and suggest that its origin is related to photoioniza-
and D because: (i) their densities are higher than those tion of a region in the pre-shock flow by radiation emit-
obtained from the Fe XVII and Fe XXII (Mauche et al. ted in the collisionally ionized post-shock cooling flow.
2001,2003)lineratios;(ii)thecombinationofhighdensi- Such an origin conforms with that of the broad opti-
tiesandsmallaccretionspotsizeimpliessignificantopti- cal, UV, and FUV emission lines observed in EX Hya
caldepthforelectronscattering(seeTable2),predicting (Hellier et al. 1987; Greeley et al. 1997; Mauche 1999;
flat-bottomedsoftX-raysspinlightcurves,whicharenot Belle et al. 2003): lines whose widths, fluxes, and radial
observed(Hoogerwerf et al.2006;Allan et al.1998);(iii) velocities vary predominately on the white dwarf spin
modelDimpliesasmallshockheight,suchthatthecolli- phase, which are interpreted as arising in the pre-shock
sionallyionizedlinesinthepost-shockflowwillformeven flow. Evenmoresatisfying,intwocases—OVIλλ1032,
closertothe whitedwarf. Suchemissionwouldnotshow 1038 in the FUV (Mauche 1999) and N V λλ1239, 1243
the observed sinusoidal modulation in the light curves inthe UV (Belle et al.2003)—narrowcomponents that
(Allan et al. 1998), but would show the presence of two rotate with the white dwarf are observedin the predom-
spikes (observedin the MgXI lightcurve,but not in the inately broad emission lines. In fact, the most obvious
more highly ionized MgXII; Hoogerwerf et al. 2006). differencebetweentheOVIandNVemissionlinesinthe
Models A and B are both acceptable in terms of: (i) FUVandUVwavebandsandtheOVIIIλ18.97,FeXVII
their densities agree with values obtained from observa- λ16.78,MgXIIλ8.42,andSiXIVλ6.18emissionlines in
tions (Mauche et al. 2001, 2003); (ii) the optical depth theX-raywavebandistherelativestrengthofthenarrow
for electron scattering in Models A and B is low and its and broad components; in the former case, the narrow
6 Luna et al.
component is the tail on the dog, whereas in the latter We thank the anonymous referee for useful comments
caseitisthebroadcomponentthatisthetail. Although which help to improve the final manuscript. Support
rathersimple,ourphotoionizationmodelfortheoriginof for this work was provided by NASA through Chan-
the broad component of the X-ray emission lines yields dra awards G07-8026X issued by the Chandra X-ray
appropriate values for the height of the shock and the Observatory Center, which is operated by the SAO for
size of the accretion spot. Altogether, our observational and on behalf of NASA under contract NAS8-03060.
findings and the results from our model agree with a CWM’s contribution to this work was performed un-
scenario where the geometry imposed by the specific ac- der the auspices of the U.S. Department of Energy by
cretionrate(Section 1)allowsmostofthe photonsfrom Lawrence Livermore National Laboratory under Con-
collisional excitation to escape freely, and a small frac- tractDE-AC52-07NA27344. NSBissupportedbyNASA
tion of those photons to photoionize the pre-shock gas. Contract NAS8-03060 to SAO for the Chandra X-ray
In this regard, the X-ray spectrum from EX Hya mani- Center.
fests both features of the “binomial” distribution found
by Mukai et al. (2003).
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