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) 2 keV) ( E >2 keV) Soft X-ray Soft X-ray ( E