Vojtech Simon v X-ray monitoring of cataclysmic variables – dependence on the instruments Talk: International Workshop on Astronomical X-Ray Optics, Prague, Czech Republic, 2015 v Astronomical Institute, Academy of Sciences, Ondrejov, Czech Republic v Czech Technical University in Prague, Faculty of Electrical Engineering, Prague, Czech Republic 1 2

Monitoring enables to: identify the type of system place the events (e.g. outbursts) in the context of the long-term activity of the system form the representative ensemble of events (e.g. outbursts) in (a) a given system, (b) in a type of systems This is important for our understanding of the physical processes involved. Transitions between the activity states (e.g. outbursts, high/low states) are often fast and unpredictable – monitors are needed. The importance of the X-ray monitoring (I) The importance of the X-ray monitoring (I) 2

X-ray monitors onboard various satellites  Monitors are typically sensitive to radiation within 2 < E < 10 keV – soft X-ray emission components thus often remain unstudied.  Most cataclysmic variables radiate beyond this monitored spectral region. 3

Occasional pointing in any spectral band is not enough:  many pieces of information on the time evolution are lost in any spectral band spectral band  time allocation has to be justified (search for unexpected behavior of the object is usually not approved) of the object is usually not approved) Determining a comprehensive picture about the processes operating in a given system (or a group of systems) requires analysis of an ensemble of events. 4 X-ray monitoring and pointed observations X-ray monitoring and pointed observations

Donor WD Accretion disk 5 Cataclysmic variables as X-ray emitters Patterson & Raymond (1985) Warner (1995) Most X-rays (bremsstrahlung) from boundary Most X-rays (bremsstrahlung) from boundary layer (encircling the equator of the white layer (encircling the equator of the white dwarf (WD)). dwarf (WD)). Large structural changes of the boundary Large structural changes of the boundary layer (e.g. between quiescence and outburst) layer (e.g. between quiescence and outburst) Mildly magnetized WD: B ~ 10 6 Gauss Mildly magnetized WD: B ~ 10 6 Gauss Bremsstrahlung X-ray emission from an Bremsstrahlung X-ray emission from an impact of the accretion columns onto the impact of the accretion columns onto the magnetic poles of the WD magnetic poles of the WD Strongly magnetized WD: B > 10 7 Gauss Strongly magnetized WD: B > 10 7 Gauss thermal (soft X-rays – WD heated by impact) thermal (soft X-rays – WD heated by impact) bremsstrahlung (accretion column – hard X) bremsstrahlung (accretion column – hard X) “Non-magnetized” WD: “Non-magnetized” WD:

Problems in the long-term coverage in X-rays Detectability of the object strongly depends on its activity:  Low-mass X-ray binaries (LMXBs – systems with the neutron-star or the black-hole accretor ): intensity of X-ray emission strongly increases during active states (outbursts, episodes of the high states) often no large variations between soft and hard X-ray intensity (for E < 12 keV)  Cataclysmic variables (CVs – white-dwarf accretor): intensity of X-ray emission strongly depends on the X-ray band and the state of activity in a given CV X-ray data are often fragmentary – many CVs are too faint for the available X-ray monitors (with only a very few exceptions) 6

Mission: RXTE (Rossi X-Ray Timing Explorer) (1996 – 2012) Three shadow cameras (6 x 90 degrees FOV) Energy range: 1.5 – 12 keV: 1.5 – 3 keV 3 – 5 keV 5 – 12 keV Time resolution: 90 s integration time – 80% of the sky every 90 min – one-day means are usually used to increase the sensitivity Spatial resolution: 3 x 15 arcmin Sensitivity: ~13 mCrab for one-day means) 7 Levine et al. (1996) ASM/RXTE – monitor for medium/hard X-rays

Mission: ISS (since 2010) Slit cameras in 6 units (160 x 1.5 degrees FOV) Energy range: 2 – 20 keV: 2 – 4 keV 4 – 10 keV 10 – 20 keV Time resolution: – the source is observed twice per 92 min orbit – one-day means are usually used to increase the sensitivity 8 Matsuoka et al. (2009) Mihara et al. (2011) MAXI/ISS – monitor for medium/hard X-rays

Mission: NASA Swift (since 2004) Aperture: Coded mask Field of view: 1.4 sr (partially-coded) 9 BAT/Swift – monitor for very hard X-rays Telescope PSF: 17 arcmin Energy range: 15 – 150 keV (15 – 50 keV is used for monitoring of X-ray sources) Krimm et al. (2013)

Thermal-viscous instability of the accretion disk – Outbursts (episodes of the mass Thermal-viscous instability of the accretion disk – Outbursts (episodes of the mass accretion onto the WD) accretion onto the WD)  Optical outburst – the disk switches to the hot state – mass accretion from the disk  X-ray outburst – accretion onto the magnetic poles of the WD Dwarf nova & intermediate polar GK Per/2E Based on: Simon (2002) GINGA spectra (Ishida et al. 1992) GINGA spectra (Ishida et al. 1992) quiescence outburst ASM/ RXTE Moving averages Optical band (AAVSO) BAT/ Swift One-day means Outburst: Outburst: - higher X-ray intensity - larger absorption ASM BAT

X-ray intensity saturates near the peak of the optical outburst (in the time of the largest mass inflow through the disk) An increase of absorption of X-rays cannot explain this saturation. Structural changes of the accretion regions at the poles of the WD Dwarf nova & intermediate polar GK Per / 2E ASM/ RXTE BAT/Swift Optical

A very hard bremsstrahlung A very hard bremsstrahlung X-ray spectrum ( Suzaku data X-ray spectrum ( Suzaku data (Hayashi & Ishida 2014)) (Hayashi & Ishida 2014)) Site: Post-shock accretion Site: Post-shock accretion columnmagnetic column at the magnetic poles of the WD poles of the WD Very hard emission in the Very hard emission in the BAT/ Swift band BAT/ Swift band V1223 Sgr (intermediate polar) – a very hard X-ray source For comparison: X-ray spectrum of MV Lyr (novalike in the high state) <0.5 keV blackbody emission ROSAT PSPC data (Greiner 1998) Greiner (1998) Hayashi & Ishida (2014) BAT/Swift band 12

V1223 Sgr (intermediate polar) – X-ray activity  Accreting regions at the polar caps of the WD: – sources of very hard X-ray emission (bremsstrahlung)  The cause of the shallow low state – decrease of the mass inflow to the disk from the donor (not only changes of the disk structure) – this places the constraints on the model of Beuermann et al. (2004) 13 NSVS & ASAS data Thermalemission(disk) BAT/Swift Bremsstrahlung (accr. regions)

Optical Hard X-ray ( E >2 keV) ( E >2 keV) Soft X-ray Soft X-ray ( E <0.5 keV) Outbursts: thermal-viscous instability of the accretion disk Large dependence of the outburst profile on the bandpass  Strong brightening only in very soft X-rays, not in hard X-rays ! Large structural changes of the boundary layer during the outburst Wheatley et al. (2003) SS Cyg / 2E (dwarf nova) Optical and X-ray emissions come from different regions of the system Spikes during bottom part of transition part of transition 14

Large dependence of the outburst profile on the X-ray band (wavelength of emission)  During the outburst, a strong brightening only in very soft (E < 0.3 keV) X-rays, not in hard X-rays (even a decrease) ! Ishida et al. (2009) SS Cyg / 2E (dwarf nova) OutburstQuiescence 15

Peak luminosity in the soft X-ray band:  strongly affected by absorption  this band is neglected by most monitors Supersoft X-ray sources – the expected SED 1 keV0.1 keV 1.24 keV0.41 keV0.25 keV Ness et al. (2013) Popham & Di Stefano (1996) Modelled spectral energy distribution (SED) Observed X- ray spectrum – strong influence of absorption 16

Supersoft X-ray sources – V Sge / 2E Greiner & van Teeseling (1998) X-ray spectrum (normalized) Optical Optical high state Optical low state Optical luminosity in antiphase with X-ray luminosity: optical high state – X-ray faint and hard optical low state – X-ray bright and very soft 17

Optical Kuulkers et al. (2006) AM Her / 2E (polar) - SED during the high state The observable result of several processes operating in the accretion region MAXI / ISS BAT / Swift 1…Cyclotron emission – dominant in the optical and IR band 1…Cyclotron emission – dominant in the optical and IR band 2…Bremsstrahlung – medium and hard X-ray emission 2…Bremsstrahlung – medium and hard X-ray emission 3…Thermal emission from the surface of the WD heated by the impact – 3…Thermal emission from the surface of the WD heated by the impact – soft X-ray excess (not in the band observed by most monitors!) soft X-ray excess (not in the band observed by most monitors!) ASM / RXTE

Based on: Simon (2011) 19 AM Her – variable emission output in the high states Cyclotron+stream emission emissionBremsstrahlung AFOEV data ASM/ RXTE data (1.5 – 12 keV) Evolution of the optical and hard X-ray intensities in the individual high-state episodes (smoothed through the orbital modulation) Relation between intensities from two processes in a high-state episode Optical – dominant cyclotron em. Optical – dominant cyclotron em. Hard X-ray – bremsstrahlung em. Hard X-ray – bremsstrahlung em. Dramatically different properties of the emitting region(s) on the WD in the high-state episodes.

20 Based on: Simon (2011) AM Her – a relation between the optical and X-ray intensities in two high-state episodes Two consecutive episodes of the high state:  Intensities of the optical and hard X-ray (1.5 – 12 keV) emissions are anticorrelated.  A higher luminosity of the bremsstrahlung emission may not be always accompanied by a higher optical (cyclotron+stream) emission in a given episode of the high state.  This relation of intensities is representative for the whole HS episode. Optical emission Optical emission Hard X-ray Hard X-ray emission (RXTE) HS1 HS1 HS2 HS2

AM Her / 2E – monitoring of the bremsstrahlung component in a polar Successful monitoring of the hard X- Successful monitoring of the hard X- ray emission in the high states of a ray emission in the high states of a polar polar Simultaneous observing with two Simultaneous observing with two monitors: monitors: – data in two bands are available – data in two bands are available Optical high states: X-ray emission is Optical high states: X-ray emission is detectable only in these phases detectable only in these phases Relation between the optical and hard Relation between the optical and hard X-ray intensities on long timescales X-ray intensities on long timescales  Information about the total balance of the emission components is still the emission components is still missing missing (the monitors cannot observe the (the monitors cannot observe the SOFT X-ray peak) SOFT X-ray peak) Optical band AAVSO data Medium/hard X-rays MAXI / ISS Very hard X-rays BAT / Swift Cyclotron emission Bremsstrahlung emission Tail of bremsstrahlung emission 21

AE Aqr DQ Her CV types and their X-ray spectra Non-magnetic CVs (no spin modulation of WD, various states) Intermediate polars (from spin modul.) Polars SS Cyg (quiescence) k T of bremsstrahlung component ? (data: various sources – mostly from Warner (1995) Spectra in various levels of activity  Bremsstrahlung – dominant in most CV types but the structures of the emitting regions largely differ Bremsstrahlung produces softer X-ray emission in ”non-magnetic” CVs (accretion via boundary layer on the equatorial belt of the WD) Intermediate polars: the hardest spectra Intermediate polars: the hardest spectra (radial inflow onto the poles from the disk)  k T in intermediate polars largely differs from system to system from system to system Polars: radial inflow from the stream 22

Analysis of the data of faint X-ray CVs from monitors Binning of the X-ray data enables to analyze faint sources (but it strongly smooths the profiles of the features) moothing the X-ray data through the orbital modulation Smoothing the X-ray data through the orbital modulation Determining the mean levels of X-ray intensity in some states of activity – possible e.g. for the high states of polars Simultaneous monitoring of the same object with several monitors: – possibility to determine the hardness ratio of X-ray emission 23

Profiles of features of the long-term activity of cataclysmic Profiles of features of the long-term activity of cataclysmic variables are measurable by the monitors. Search for the common variables are measurable by the monitors. Search for the common features is needed. features is needed. Spectral variations are measurable by some monitors (or by a Spectral variations are measurable by some monitors (or by a combination of observing by several monitors). combination of observing by several monitors). We emphasize the very important role of the spectral region of We emphasize the very important role of the spectral region of the X-ray monitor. the X-ray monitor. The available monitors can detect only cataclysmic variables with The available monitors can detect only cataclysmic variables with magnetized white dwarfs (WDs) – the mode of accretion is very magnetized white dwarfs (WDs) – the mode of accretion is very important. important.  Only radial flow onto the WD causes sufficiently hard X-ray spectrum to be observable by the monitors. spectrum to be observable by the monitors. 24 Conclusions

Acknowledgements: Acknowledgements: This study was supported by grants J and S provided by the Grant Agency of the Czech Republic. This research has made use of the observations provided by the ASM/RXTE team (Levine et al., 1996, ApJ, 469, L33) and public data from Swift/BAT transient monitor provided by the Swift/BAT team (Krimm et al., 2013, ApJS, 209, 14). This research has also made use of the observations from the ASAS project (Pojmanski, G.,1997,AcA,47,467), AAVSO International database (USA) (Henden 2013, 2014, 2015) and the AFOEV database (France). I thank the variable star observers worldwide. This publication also made use of the data from the Northern Sky Variability Survey created jointly by the Los Alamos National Laboratory and University of Michigan. The NSVS was funded by the Department of Energy, the National Aeronautics and Space Administration, and the National Science Foundation. I also thank Prof. Petr Harmanec for providing me with the code HEC13. The Fortran source version, compiled version and brief instructions on how to use the program can be obtained at http: //astro.troja.mff.cuni.cz/ftp/hec/HEC13/ 25