Cataclysmic variables as hard X-ray emitters seen by INTEGRAL

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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, Fricova 298,
25165  Ondrejov, Czech Republic (e-mail
galis_at_ta3.sk) 2 ISDC Data Centre for
Astrophysics, Chemin d'Écogia 16, CH-1290
Versoix, Switzerland 3 Instituto di astrofisica
spaziale e fisica cosmica, INAF, via Gobetti 101,
40129 Bologna, Italia
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Cataclysmic variables (CVs)

• A special group of eruptive variable stars (gt1800
  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 (lt10 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).

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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.

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INTEGRAL CVs
INTEGRAL/IBIS significance maps of the area
around V1223 Sgr

• 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.

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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 shortterm 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 INTEGRALs OSA7 software.

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Intermediate polar V1223 Sgr

• V 1223 Sgr is the most significantly detected CV
  by INTEGRAL/IBIS .
• Bright X-ray source (4U 184931) 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.
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Intermediate polar 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.

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Another example ... Intermediate polar V709 Cas

• This X-ray source was recognized as an IP
  following its detection in the ROSAT All Sky
  Survey as RXJ0028.85917 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.

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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.
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Broad-band (3-100) keV spectra

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

?2/d.o.f 14.9/10
?2 /d.o.f 12.4/10
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Broad-band (3-100) keV spectra

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

?2/d.o.f 9.9/10
?2 /d.o.f 7.7/8
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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
  51022 cm-2) and we fit the ? 30 keV excess by
  Gaussian profile the spectrum is well described
  with ?2/d.o.f 6/8.

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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.

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Thank you for your attention