Intermediate polars are a major fraction of all cataclysmic variables detected by INTEGRAL in hard X-ray. These objects have recently been proposed to be the dominant X-ray source population detected near the Galactic centre, and they also contribute significantly to X-ray diffuse Galactic ridge emission. Nevertheless, only 25% of all known intermediate polars have been detected in hard X-ray. This fact can be related to the activity state of these close interacting binaries.A multi-frequency (from optical to X-ray) investigation of intermediate polars is essential for understanding the physical mechanisms responsible for the observed activity of these objects

Gális, R.

Hric, L.

Kundra, E.

Münz, F.

Acta Polytechnica

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

Acta Polytechnica Vol. 51 No. 6/2011Cataclysmic Variables — X-rays and Optical Activity in V1223 Sgr andV709CasR. Ga´lis, L. Hric, E. Kundra, F. Mu¨nzAbstractIntermediate polars are a major fraction of all cataclysmic variables detected by INTEGRAL in hard X-ray. These objectshave recently been proposed to be the dominant X-ray source population detected near the Galactic centre, and theyalso contribute significantly to X-ray diffuse Galactic ridge emission. Nevertheless, only 25% of all known intermediatepolars have been detected in hard X-ray. This fact can be related to the activity state of these close interacting binaries.A multi-frequency (from optical to X-ray) investigation of intermediate polars is essential for understanding the physicalmechanisms responsible for the observed activity of these objects.Keywords: Cataclysmic variables, optical and X-ray variability, mass transfer and accretion.1 IntroductionCataclysmic variables (CVs) manifest strong activityin a whole spectrum from radio up to γ-rays. CVsare close binary systems consisting of a hot whitedwarf (WD) and a red main-sequence star of spec-tral type M or K, which fills the volume of its innerRoche lobe and transfers matter to the vicinity ofthe WD [1]. The mass transfer between componentscauses the observed activity of the CVs, which variesfrom relatively small light variations (flickering) toenormous photometric changes (outbursts of novae)on time scales that cover a wide range from very fastvariability of fractions of seconds (flickering) to long-term variations of several years or decades (activitycycles).According to strength of the WD magnetic field,the transferred matter creates an accretion disk (clas-sical CVs) or follows magnetic lines and falls to thesurface of the WD (magnetic CVs). Magnetic CVsare a small sub-set of all catalogued CV systems,and fall into two categories: polars and intermedi-ate polars (IPs). In IPs, the WD magnetic field(106 − 107 G) is not strong enough to disrupt thedisc entirely (as in the case of polars) and it simplytruncates the inner part of the disc [2]. An accretionflow is channelled down towards the magnetic polesand onto the WD surface.Increased interest in CVs was aroused after thediscovery of the very hard X-ray emission (up to80 keV) of many IPs. When the transferred ma-terial impacts the WD atmosphere, a strong shockwill form above its surface [3]. The temperaturein the post-shock region (PSR) can be very highand the plasma is cooled mainly via optically thinbremsstrahlung radiation in the hard X-ray band [4].As we showed in our previous analysis [5], the broad-band spectra (3–100keV) of the studied IPs can bewell fitted by a thermal bremsstrahlung model withPSR temperature kT ≈ (20–25) keV. Reflection ofthe bremsstrahlung photons at an optically thick coldmedium can also contribute to the hard X-ray spec-trum [6]. In the case of V2400 Oph, a significantemission excess detected at ≈ 26 keV can be causedby the reflection of X-rays from the surface of theWD [5].In total, 32 CVs (and 2 symbiotic systems) weredetected in the data of INTEGRAL [7–9, 5]. This ismore than was expected, and it represents ≈ 5% ofall detections of this space observatory. The sampleof IPs detected in the (20–40) keV energy band has23 members, representing more than 70% of all CVsdetected by INTEGRAL [5]. IPs are the most lumi-nous and the hardest X-ray sources among accretingWDs. In hard X-rays, these objects seem to be moreluminous (up to a factor of 10) than polars [10]. Instrongly magnetized (B ≥ 107G) polar systems, cy-clotron radiation is an important cooling mechanism,which suppresses high temperature bremsstrahlungemission, whilst it should be negligible for IPs. Thiscould explain why most of the CVs observed in thehard X-ray band are IPs.Short-term soft X-ray modulations have been ob-served in the orbital period, in the spin period ofthe WD, or a beat between the two. No significantmodulation has been found so far in the 20–30keVlight curves [11]. Most CVs seem to have persis-tent soft gamma ray fluxes. Nevertheless, the sampledetected by INTEGRAL represents only 25% of allknown IPs [5]. Some IPs are not detectable even if wehave significant exposure time (more than 4Msec) forthese sources. This fact can be related to the activ-13Acta Polytechnica Vol. 51 No. 6/2011Fig. 1: Left panel: INTEGRAL/IBIS flux curves of V1223 Sgr in the corresponding energy bands. The arrows represent3σ upper limits. Right panel: INTEGRAL/OMC light curve of V1223 Sgr in the optical spectral band. The crossesrepresent average magnitude values in the corresponding seasonsity state of these interacting binaries. To understandthis relation, it is necessary to study the correlationbetween the activity state and the X-ray emission ofIPs.2 Observations, analysis andresultsWe used all publicly available observational datafrom INTEGRAL/JEM-X and INTEGRAL/IBIS tostudy possible variability of the selected IPs in thehard X-ray and soft γ-ray spectral bands. In addi-tion, observations from INTEGRAL/OMC data wereused to look for long-term variability of these objectsin optical band. The observational data used in ouranalysis was processed by INTEGRAL’s Offline Stan-dard Analysis Package OSA7. The libraries grid.pyand ms_suite.py developed by FM were also usedfor the data processing and preparation of the finalmosaics.2.1 Intermediate polar V1223 SgrIP V1223Sgr is a bright X-ray source (4U 1849-31)with possible X-ray flare activity. A short-term bursthas also been detected from this system in the opti-cal [12]. These outbursts are probably a result of diskinstabilities or an increase in mass transfer, but thereis no correlation between optical and X-ray burst ac-tivity. Moreover, episodes of deep low state (decreaseby several magnitudes) of V1223Sgr in the opticalband have also been detected [13].The observational material for IP V1223Sgr con-sisted of 132 (INTEGRAL/JEM-X) and 1375 (IN-TEGRAL/IBIS) individual pointings obtained inthe course of almost three years (MJD 52710.38–53809.25). The overall INTEGRAL/IBIS mosaics(total exposure time 1 405.5 ksec) showed that V 1223Sgr is detectable up to 80 keV with ≈ 5 σ significance.The broad-band (3–100) keV spectrum was very wellfitted by a bremsstrahlung model with temperaturekT = 23.7+1.4−1.3 keV [5].During the monitored period, the mean fluxes ofthis object were (103.0 ± 4.0) × 10−12 erg cm−2 s−1,(46.4± 1.4)× 10−12 erg cm−2 s−1,(15.1± 1.5)× 10−12 erg cm−2 s−1and (12.3±2.0)×10−12 erg cm−2 s−1 in (15–25) keV,(25–40) keV, (40–60) keV and (60–80) keV band, re-spectively.The inspection of the data showed that the ob-servations were obtained in the course of 7 separateseasons. As the next step we split the data accordingthese seasons to investigate long-term X-ray/γ-rayvariability. The flux curves are displayed in Figure 1(left panel). It is clear that during the monitoredperiod the fluxes of V1223Sgr were long-term vari-able (especially in the softer bands), with a significantdrop around MJD ≈ 53 650.The optical light curve of V1223Sgr (INTE-GRAL/OMC) is shown in Figure 1 (right panel).We can see that optical brightness of this source waslong-term variable, too. Moreover, these light vari-ations are strongly correlated with the changes inthe (15–25)keV, (25–40)keV and (40–60)keV spec-tral bands with correlation coefficients 0.81, 0.82 and0.89, respectively.Our detailed period analysis of INTEGRAL/OMC data did not yield any significant period,only partial detection of the orbital period Porb =3.37hrs [14]. This was probably caused by complexintrinsic variability (flickering) of V1223 Sgr, possi-ble period variations or a the drastic change in thebrightness of this object during monitored period.However, a particular analysis is not possible due toinappropriate time distribution of the data.14Acta Polytechnica Vol. 51 No. 6/2011Fig. 2: Left panel: INTEGRAL/JEM-X flux curves of V 1223 Sgr in the corresponding energy bands. The arrowsrepresent 3σ upper limits. Right panel: INTEGRAL/IBIS phase diagram of V1223 Sgr in (15–25) keV band folded withorbital period (3.37 hrs)Fig. 3: INTEGRAL/IBIS light curves of V709Cas in thecorresponding energy bands. The arrows represent 3σupper limitsWe also prepared overall mosaics using all avail-able data from INTEGRAL/JEM- X. The totalexposure time was 80.6 ksec. The medial fluxesof V1223Sgr during the monitored period were(2.8 ± 0.4) × 10−12 erg cm−2 s−1, (3.7 ± 0.3) ×10−12 erg cm−2 s−1, (2.6± 0.4)× 10−12 erg cm−2 s−1and (4.9 ± 1.1) × 10−12 erg cm−2 s−1 in (3–6) keV,(6–10) keV, (10–15) keV and (15–25) keV band, re-spectively. We also spit the data to 7 seasons to inves-tigate the long-term X-ray variability of V1223Sgr.The flux curves are displayed in Figure 2 (left panel).As we can see the corresponding errors are too largefor subsequent analysis and therefore we can concludethat the INTEGRAL/JEM-X fluxes of V1223Sgrwere persistent within their errors in the monitoredperiod.Typically, soft X-ray modulations were observedin the orbital period, in spin period of WD, or a beatbetween the two in IPs. However, the IPs are closebinary systems with orbital periods in the order ofhours and these objects, are not detectable on thesetime scales by INTEGRAL/IBIS. As the next step,we attempted to investigate the possible short-termvariability of V1223Sgr in INTEGRAL/IBIS data.We prepared a unique method of folding the particu-lar phase interval on the basis of proper time intervalsfrom the individual science windows. Our methodapplied Good Time Intervals (GTIs) according tothe (orbital or other) phase bin and created phaseresolved mosaics (assuming sufficient exposure) ofa periodic source. A phase diagram of the fluxesV1223Sgr in the (15–25) keV band folded with theorbital period and constructed using the data fromtime interval MJD (52 917.17–52926.84) is shown inFigure 2 (right panel).2.2 Intermediate polar V709CasThis X-ray source was recognized as an IP afterit had been detected in the ROSAT All Sky Sur-vey as RXJ0028.8+5917 and was identified with a14th magnitude blue star, V709Cas. The broad-band spectrum (3–100) keV of this object was fit-ted by the bremsstrahlung model with temperaturekT = 29.6±2.5keV, covering factor CF = 0.31±0.04,column density nH = (58 ± 15) × 1022 cm−2 andiron line energy 6.5 ± 0.2 keV [15]. The analysisshowed that accounting for Compton scattering doesnot significantly change the obtained mass of the WD(MWD ≈ 0.9M) in the case of V709Cas [15]. Thebroad-band (3–100) keV spectrum from INTEGRALwas well fitted by a thermal bremsstrahlung modelwith post-shock temperature kT = 24.4+1.5−1.4 keV [5].Our analysis of all available observational datafor V709Cas showed that this source is detectableup to 100 keV. The hard X-ray/soft γ-ray fluxes arenot persistent, and the flux curves indicate that thebrightness of this IP increased by a factor of ≈ 2from MJD 52700 to MJD 53700 in (15–25) keV en-ergy band (Figure 3).15Acta Polytechnica Vol. 51 No. 6/20113 ConclusionsWe analysed all available observational data from IN-TEGRAL for IP V1223Sgr and V709Cas. Our anal-ysis of the data from INTEGRAL/IBIS showed thatthe fluxes of these objects are long-term variable,mainly in the (15–25) keV and (25–40) keV bands.Moreover this hard X-ray/soft γ-ray variability is cor-related with the changes in the optical spectral bandin the case of V 1223Sgr. Our analysis revealed adeep flux drop around MJD ≈ 53 650 observed inboth the X-ray band and the optical band for thisintermediate polar.A significant part of the optical emission from IPsis produced by a hot spot, where the matter from adonor star interacts with the outer rim of the accre-tion disk. X-ray emission is produced by the inter-action of the accreting matter with the WD surface.The emission in both the optical and the X-ray bandsis therefore related to the mass transfer, and the ob-served variations are therefore probably caused bychanges in the mass accretion rate.We are preparing a photometric campaign to ob-tain long-term homogeneous observations (to coverwhole activity cycles) as well as sets of observationswith high time resolution (to cover the orbital cyclesin detail) of selected CVs, mainly as a follow up to theINTEGRAL observations. Simultaneous analysis ofmulti-frequency observation (from optical to X-ray)enables a complex study of the physical mechanismrelated to the mass transfer in these interacting bi-naries.AcknowledgementThe International Gamma-Ray Astrophysics Obser-vatory (INTEGRAL) is a European Space Agencymission with instruments and a science data cen-tre funded by the ESA member states (especially bythe PI countries: Denmark, France, Germany, Italy,Spain, Switzerland), the Czech Republic and Poland,and with the participation of Russia and the USA.This study was supported by a project of the SlovakAcademy of Sciences, VEGA Grant No. 2/0078/10.References[1] Warner, B.: Cataclysmic variable stars. Cam-bridge : Cambridge University Press, 1995.[2] Patterson, J.: Publ. Astron. Soc. Pacific, 1994,106, 209.[3] Aizu, K.: Prog. Theor. Phys. 1973, 49, 1 184.[4] King, A. R., Lasota, J. P.: Mon. Not. R. Astron.Soc. 1979, 188, 653.[5] Ga´lis, R., Eckert, D., Paltani, S., Mu¨nz, F.,Kocka, M.: Baltic Astronomy, 2009, 18, 321.[6] van Teeseling, A., Kaastra, J. S., Heise, J.: As-tron. Astrophys. 1996, 312, 186.[7] Bird, A. J., Malizia, A., Bazzano, A., Bar-low, E. J., et al.: Astrophys. J. Suppl. Ser. 2007,170, 175.[8] Kniazev, A., Revnivtsev, M., Sazonov, S., Bu-renin, R., Tekola, A.: The Astronomer’s Tele-gram, 2008, 1 488.[9] Masetti, N., Parisi, P., Palazzi, E., Landi, R., etal.: The Astronomer’s Telegram, 2008, 1 620.[10] Chanmugam, G., Ray, A., Singh, K. P.: Astro-phys. J. 1991, 375, 600.[11] Barlow, E. J., Knigge, C., Bird, A. J., Dean, A.,et al.: Mon. Not. R. Astron. Soc. 2006, 372,224.[12] van Amerongen, S., van Paradijs, J.: Astron.Astrophys. 1989, 219, 195.[13] Garnavich, P., Szkody, P.: Publ. Astron. Soc.Pacific, 1988, 100, 1 522.[14] Jablonski, F., Steiner, J. E.: Astrophys. J. 1987,323, 672.[15] Suleimanov, V., Poutanen, J., Falanga, M.,Werner, K.: Astron. Astrophys. 2008, 491, 525.R. Ga´lisDepartment of Theoretical Physics and AstrophysicsInstitute of PhysicsFaculty of SciencesP. J. Sˇafa´rik UniversityPark Angelinum 9, 040 01 Kosˇice, SlovakiaL. HricE. KundraAstronomical Instituteof the Slovak Academy of Sciences059 60 Tatranska´ Lomnica, SlovakiaF. Mu¨nzFaculty of ScienceMasaryk UniversityKotla´rˇska´ 2, 611 37 Brno, Czech Republic16

Cataclysmic Variables — X-rays and Optical Activity in V1223 Sgr and V709 Cas

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