Cataclysmic variables as hard X-ray emitters seen by INTEGRAL

The X-ray Universe 2008, Granada, Spain, 27-30 May

R. Gális1,2, R. Hudec1, F. Münz3, M. Kocka1, D.Eckert2 & S. Paltani 2

1 Astronomical Institute AV CR, Fričova 298, 25165  Ondřejov, Czech Republic (e-mail: galis@ta3.sk)2 ISDC – Data Centre for Astrophysics, Chemin d'Écogia 16, CH-1290 Versoix, Switzerland3 Instituto di astrofisica spaziale e fisica cosmica, INAF, via Gobetti 101, 40129 Bologna, Italia

Cataclysmic variables (CVs)

A special group of eruptive variable stars (>1800 objects; Downes, 2006).

Strong activity manifests in the whole electromagnetic spectrum on the time-scales from seconds to millions of years.

CVs are close binary systems of a hot white dwarf (WD) and red MS star of spectral type M or K, which fills the volume of its inner Roche lobe and transfers matter to the vicinity of the WD.

According to strength of WD magnetic field this matter is creating a accretion disk or follows magnetic lines and falls to surface of the WD.

Magnetic CVs are a small sub-set of the total number of catalogued CV systems (<10%; Downes et al. 2001) and fall into two categories:

• polars (or AM Her type after the prototype system),

• intermediate polars (IPs or DQ Her type).

Intermediate polars

In IPs, the WD magnetic field (106 – 107 G) is not strong enough to disrupt disc entirely and simply truncates the inner part of disc. An accretion flow is channelled down towards the magnetic poles and onto the WD surface.

In a simple model of a column of gas impacting the atmosphere of the WD, a shock will form and hard X-ray/soft gamma-ray emission will result from thermal bremsstrahlung cooling by free electrons in the hot post-shock region (PSR) with kT 10s of keV.

INTEGRAL & CVs

In total, 21 CVs (and 2 symbiotic systems) detected (more than expected, 5% of INTEGRAL detections) (Barlow et al., 2006, Bird et al., 2007).

Moreover INTEGRAL discovered 4 new mCVs so the mission play important role in detection as well as in study of physical processes in CVs.

IPs represent only ~2% of the catalogued CVs, but they dominate the group of CVs seen by INTEGRAL/IBIS (11 confirmed or probable IPs).

Deep IPs survey – looking for all known IPs (65, Downes, 2006) on (20-40) keV INTEGRAL/IBIS mosaics. Some IPs remained invisible even after 300 ksec (AP Cru, 386 ksec, BP Cru, 386 ksec, DD Cir, 303 ksec).

This can be related to activity state of the sources – the hard X-ray emission of IPs can be (highly) variable.

INTEGRAL/IBIS significance maps of the area around V1223 Sgr

X-ray/gamma ray variability

Short-term X-ray modulations have been observed at the orbital period, spin period of the WD or a beat between the two.

No significant long-term variability has been found so far in the 20-30 keV light curves (Barlow et al. 2006). The majority of the CVs seems to have persistent soft gamma ray fluxes.

We used all available observational data from INTEGRAL/JEM-X and INTEGRAL/IBIS detectors to study possible variability of selected IPs in X-ray and gamma ray.

In addition, we used all available observations from INTEGRAL/OMC to look for short–term periodic modulations as well as long-term variability of these IPs in optical.

The inspection of the data showed that observations were obtained during several separate seasons. As the next step we splitted the data according these seasons and these data were analysed separately.

Observational data used in our analysis were processed by standard INTEGRAL’s OSA7 software.

Intermediate polar V1223 Sgr*

V 1223 Sgr is the most significantly detected CV by INTEGRAL/IBIS .

Bright X-ray source (4U 1849–31) with possible X-ray flare activity. Unusual high energy burst (a flare lasting for 3.5 hrs with a peak flux ∼

3 times that of the average) was observed by ∼ INTEGRAL/IBIS at MJD = 52743 (Barlow et al. 2006).

A short-term bursts have also been detected from this system in the optical by van Amerongen & van Paradijs (1989) and Šimon et al. (2005). These outbursts can be a result of disk instabilities or an increase in mass transfer but there is no correlation between optical and X-rays burst activity.

Moreover the episodes of deep low state (decrease by several magnitudes) of V 1223 Sgr were also detected in optical (Garnavich and Szkody 1988).

Our overall mosaics of all available data from INTEGRAL/IBIS (total exposure time 1.4Msec) showed that V1223 Sgr is detectable up to the (60 - 80) keV band.

* See poster C.3 for more details.

Intermediate polar V1223 Sgr

52600 52800 53000 53200 53400 53600 53800 54000JD (hel.) - 2 400 000 [d]

0

20

40

60

80

100

120

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Flu

x [

10

-12 e

rg s

-1 c

m-2

]

V1223 Sgr(15 - 25) keV

(25 - 40) keV

(40 - 60) keV

(60 - 80) keV

52700 52800 52900 53000 53100 53200 53300 53400 53500 53600 53700 53800 53900Julian D ate - 2 400 000

14.4

14.2

14.0

13.8

13.6

13.4

13.2

13.0

12.8

Ma

gn

itu

de

V

V1223 Sgr

Optical variations are correlated with the changes in (15 – 25) keV, (25 – 40) keV and (40 – 60) keV spectral bands with correlation coefficient 0. 81, 0. 82 and 0.89, respectively.

The fluxes from INTEGRAL/JEM-X were persistent within their errors in monitored time period.

Another example ... Intermediate polar V709 Cas

52600 52700 52800 52900 53000 53100 53200 53300 53400 53500 53600 53700 53800 53900JD (hel.) - 2 400 000 [d]

0

10

20

30

40

50

60

70

80

90

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Flu

x [

10

-12

erg

s-1

cm

-2]

V709 Cas(15 - 25) keV

(25 - 40) keV

(40 - 60) keV

(60 - 80) keV

This X-ray source was recognized as an IP following its detection in the ROSAT All Sky Survey as RXJ0028.8+5917 and was identified with the 14th magnitude blue star. This is hence one of optically brightest CV in the INTEGRAL/IBIS CV sample.

Detectable up to 60 keV by INTEGRAL/IBIS.

The hard X-ray / soft gamma ray fluxes are not persistent and the light curves indicate that the brightness of this IP increased by a factor 2 from MJD 52 700 to MJD 53 700 in (15-25) keV energy band.

Broad-band (3-100) keV spectra*

Hard X-ray/soft gamma-ray spectra can be well fitted by thermal bremsstrahlung model with temperature of PSR kT 10s of keV.

Reflection of the bremsstrahlung photons at the WD surface also contributes to the hard X-ray spectrum (van Teeseling, Kaastra & Heise 1996).

In the recent investigations by Canalle et al. (2005) and Saxton et al. (2007) have studied the role of the two-temperature plasma and considered the dipole magnetic funneling.

Suleimanov et al. (2008) showed that the influence of Compton scattering on the broad-band X-ray spectra of IPs is significant only at high accretion rates and large WD masses.

We used all available observational data from INTEGRAL/JEM-X and INTEGRAL/IBIS detectors to study broad-band spectra of the selected IPs: V 1223 Sgr, V 709 Cas, IGR J16167-4957, GK Per and V 2400 Oph.

* See poster C.2 for more details.

Broad-band (3-100) keV spectra

INTEGRAL/JEM-X + IBIS spectra of the studied IPs fitted by a thermal bremsstrahlung model.

keV4.24 5.14.1

kT

2/d.o.f = 14.9/10

keV7.23 4.13.1

kT

2 /d.o.f = 12.4/10

Broad-band (3-100) keV spectra

INTEGRAL/JEM-X + IBIS spectra of the studied IPs fitted by a thermal bremsstrahlung model.

keV5.22 5.44.3

kT

2/d.o.f = 9.9/10

keV5.19 0.27.1

kT

2 /d.o.f = 7.7/8

Broad-band (3-100) keV spectrum of V2400 Oph

The fit of composite spectrum (3-100 keV) by only a thermal bremsstrahlung model with temperature kT = (21 ± 1.6) keV) is not sufficient to completely describe the data (2/d.o.f = 29/10).

Significant excess around 30 keV can be connected with reflection from the surface of the WD. If we take into account the absorption (nH = 5×1022 cm−2) and we fit the 30 keV excess by Gaussian profile the spectrum is well described with 2/d.o.f = 6/8.

Conclusions and plans

The fluxes of some IPs (V 1223 Sgr, V 709 Cas, RT Cru, ...) are long-term variable, mainly in (15-25) and (25-40) keV bands. Moreover these hard X-ray / soft gamma ray variability is correlated with the changes in optical spectral band (V 1223 Sgr).

The broad-band spectra (3 -100 keV) of studied IPs can be well fitted by a thermal bremsstrahlung model with the temperature of post-shock region kT 20 - 25 keV.

In the case of GK Per and V2400 Oph there are strong evidences for emission excess around 30 keV, which can be caused by reflection of X-ray from an optically thick cold medium (the surface of the WD).

INTEGRAL proves an effective tool to find and to analyse CVs and symbiotic systems in observational window 3-100 keV. The number of detected CVs will probably increase in time.

Plan to detail analyse all CVs and symbiotic systems detected by INTEGRAL. Work in progress.

Thank you for your attention