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SALT & ASTROSAT Observations of Magnetic Cataclysmic Variables David Buckley

SALT & ASTROSAT Observations of Magnetic Cataclysmic Variables David Buckley SALT Science Director & C.V. Raman Senior Fellow. Observing cataclysmic variables with SALT & ASTROSAT. Multiwaveband observations at high time resolution  X-rays, UV, and polarised optical cyclotron emission

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SALT & ASTROSAT Observations of Magnetic Cataclysmic Variables David Buckley

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  1. SALT & ASTROSAT Observations of Magnetic Cataclysmic Variables David Buckley SALT Science Director & C.V. Raman Senior Fellow

  2. Observing cataclysmic variables with SALT & ASTROSAT Multiwaveband observations at high time resolution  X-rays, UV, and polarised optical cyclotron emission  accretion-driven flaring and eclipses on time scales of seconds Science/questions: • size and location of impacting material and impact region • size of the two stars • heating mechanism •  gives (T,) of plasma in impact region • polarisation gives B

  3. Model of a Polar (AM Herculis system)

  4. Accretion column analogy on the Sun

  5. cyclotron radiation from accretion column soft X-ray emission, from heated surface of primary hard X-ray emission, also from accretion column Polars: Spectral Energy Distribution • Most of the energy from these systems is a result of accretion • 3 main components: 6 Beuermann (1998)

  6. Example: XMM-Newton Spectrum of V1432 Aql Rana, Singh, Buckley & Barrett 2005, ApJ Model Compenents: • Black body emission (88±2 eV) • Absorbers:1.7±0.3 x 1021 cm-2, fully covering the source & 1.3 ±0.2 x 1023 cm-2, covering 65% • Multi-temperature plasma model • Gaussian for 6.4 keV line emission 7

  7. Determining Magnetic Field Strength & Geometry in Polars: Fitting cyclotron model fits to All-Stokes broadband polarimetry Single pole system • Fit cyclotron parameters (plasma temp & density, cyclotron opacity, B & ) • using Potter’s Stokes imaging technique • fits model to data using a genetic • algorithm • Extend to spectropolarimetry Example: V834 Cen, Porb= 101 min SAAO 1.9-m photopolarimetry

  8. Spectropolarimetric possibilities for mCVs Time resolved, all-Stokes mode (simultaneous circular + linear): Polars + Intermediate Polars e.g. MN Hya: a ~3.4h Polar Circ. Pol. Cyclotron emission harmonics Intensity

  9. 20 keV,  = 0 V834 Censpectropolarimetry AAT 3.9m Results indicate Multi-T shocks 20 keV,  = 0.7 WickramasingheTuohy & VisvanathanApJ 318, 326

  10. Intermediate Polars • magnetic field ~106 G intermediate polar/DQ Her system • accretion takes place through a truncated disk and then via accretion “curtains” onto the white dwarf • magnetic field controls the flow in the final stages 18 Feb 2012 HEAP12- HRI (KP Singh) 11

  11. Intermediate Polar example: AO Psc Cropper et al (2002) • AO Psc: Optical spectrum like that of Polars, but without any identifiable polarisation • Variability on three different timescales now known to be • the orbital 3.591 h, • the spin period of the white dwarf 805.4 s • the mixture of the two (beat/synodic period) AO Psc 12

  12. SALT Capabilities for Magnetic CV Observations • Instrument modes are well suited to CVs • High time resolution (sub-sec) observations • photometry & spectroscopy • UV (λ > 320 nm) sensitivity • Polarimetric capability (e.g. magnetic CVs) • All-Stokes imaging polarimetry • Spectropolarimetry • Low Res (R~50) imaging spectropolarimetry • Advantages of SALT design and modus operandii • 100% queue scheduled service observing • Easy to schedule Targets of Opportunity • Easy to schedule phase or time critical observations • Easy to conduct regular long-term observations

  13. SALT Design Principle • New paradigm in cost effective design pioneered by • the HET in Texas. • fixed altitude (37 ± 6º zenith distance) • track objects at prime focus • optical analogue of Arecibo radio tel.

  14. SALT: 91 x 1m mirrors

  15. SALT Visibility Window Annulus of visibility for SALT: Annulus represents 12.5% of visible sky Declination range: +10º to -75º (70% of full sky coverage) Observation time available = time taken to cross annulus (east & west at mid Decs) Observation times from ~1h to 6h

  16. Cryostat & detector SALT’s Instruments: 1. SALTICAM: UV-Vis CCD Camera (built at SAAO: DarraghO’Donoghue, PI) An efficient “video” (~10 Hz) camera over entire science FoV (8 arcmin). Efficient in the UV/blue (capable down to atmospheric cutoff at 320nm). Capable of broad and intermediate-band imaging (Johnson-Cousins; SLOAN & Strömgren filters, plus UV and H) High time-resolution (to ~90 ms) photometry. Fulfills role as both an acquisition camera and science imager/photometer. Filter jukebox Optics Optics SALTICAM in the lab

  17. Resolving eclipses of Polars

  18. SALT’s First-Science An example: a light curve of an eclipsing magnetic CV (Polar) taken with SALTICAM Each data point a 0.1 sec exposure Ingress/Egress = 1.2 to 1.5 sec

  19. Model fit:locating hot-spot positions

  20. SALTICAM Observations of Intermediate Polars • SALT commissioning program primarily aimed to look for wavelength dependencies in the spin and beat modulations of IPs • Also looking at the flickering & aperiodicbehaviour of IPs (with Alexei Kniazev & Mikhail Revnivtsev) • Power spectra clues to missing inner disk? • Disrupted power law

  21. INTEGRAL/SWIFT source 1GRJ 14536-5522(Steve Potter, Martin Still, Koji Mukai, DB) • Flickering and QPOs seen in SALTICAM photometry • Polarimetry revealed system to be a Polar • Discovery of short period (2 – 5 min) circular polarimetry QPO variations

  22. Intensity Circ Pol New INTEGRAL/SWIFT source 1GRJ 14536-5522 • HIPPO (SAAO 1.9m instrument) All-Stokes photopolarimetry Trailed periodograms Intensity Circ. Pol. DFTs

  23. The Robert Stobie Spectrograph (RSS)(built at Wisconsin, Rutgers & SAAO) • An efficient and versatile Imaging Spectrograph • capable of UV-Vis spectroscopy from 310 – 900nm using VPHGs (red extension to 1.7μm, using a dichroic,is under construction. Completion in 2014?) • high time resolution ablility (~0.1 s) • specto- and imaging polarimetric capability • Fabry Perot imaging (incl. with pol.) • Multiple Object Spectroscopy • Can observe ~50objects at once Named in memory of Bob Stobie, previous SAAO Director & one the instigators of SALT. RSS reinstalled on SALT (Apr 2011)

  24. RSS Polarimetry • Imaging polarimetry • Spectropolarimetry

  25. Probing accretion columns polarimetrically with SALT

  26. Phase resolved QS Tel spectra ESO/MPI 2.2m example Two poles accreting Cyclotron humps move in position and shape and size as a function of phase Schwope et al. 1995 A&A 293, 764

  27. Stratified accretion shock models Allow testing of more realistic shock models(e.g. Potter et al.) with stratified temperature and density profiles dependent on parameters like: White dwarf mass, accretion rate magnetic field strength..

  28. Recent SALT experiments with a photon counting camera • The Berkeley Visible Image Tube (BVIT) installed at SALT Auxiliary Focus • A very high time resolution imaging photometer. • Enables a new time domain for astronomical observations with full imaging capability • Time resolution to ~μsec • BVIT is a simple instrument with minimal observational setup requirements and a high degree of post acquisition data flexibility. • Based on Microchannel Plate & strip anode detector • Prototype built with low QE S20 photocathode (peak of ~10% QE peaking at ~400nm) • Now upgraded to Super GenII, with ~20x improvement in count rate UZ For (Polar)

  29. ASTROSAT: India’s first astronomy satellite An ideal complement of instrumentsformCVs 2 UV(+Opt ) Imaging Telescopes 3 Large Area Xenon Proportional Counters (hard X-rays) Soft X-ray Telescope CZTI (hard X-rays) Radiator Plates For SXT and CZT Scanning Sky Monitor (SSM) Folded Solar panels SSM (2 – 10 keV)

  30. ASTROSAT – Key Strengths • Simultaneous UV to hard X-ray continuum (pure continuum) measurements • Large X-ray bandwidth, better hard X-ray sensitivity with low background • UV imaging capability better than GALEX • Transient detector via SSM • Satellite: 1.55 tons; 650 kms, 8 deg inclination. 3 gyros and 2 star trackers for attitude control by reaction wheel system with a magnetic torquer. Launch in ~mid 2013.

  31. UVIT: Two Telescopes • f/12 RC Optics • Focal Length: 4756mm • Diameter: 38 cm • Simultaneous Wide Angle ( ~ 28’) images in FUV (130-180 nm) in one and NUV (180-300 nm) & VIS (320-530 nm) in the other • MCP based intensified CMOS detectors • Spatial Resolution : 1.8” • Sensitivity in FUV: mag. 20 in 1000 s • Temporal Resolution ~ 30 ms, full frame ( < 5 ms, small window ) • Gratings for Slit-less spectroscopy in FUV & NUV • R ~ 100 33

  32. LAXPC: Effective Area Feb 13, 2012 K.P. Singh 35

  33. SALT-ASTROSAT Program Simultaneous Optical, UV to hard X-ray spectral measurements with ASTROSAT & SALT Objectives • Resolving all the spectral components (continuum): UV and soft X-rays (thermal) from accretion disk, hard X-ray reflection component, intrinsic power-law comp • Variability: • WD Rotation Period • Binary Periods • Eclipses • Absorption Dips • Shock Temperatures, plasma diagnostics and masses of the WD • Magnetic field strengths • Plan to coordinate SALT & ASTROSAT observations of mCVs during GTO phase (6 months) • Also aim to attempt contemporaneous observations during initial PV phase of ASTROSAT (e.g. AGN, XRBs, flare stars)

  34. FINAL REMARKS • Magnetic CVs offer multi-wavelength opportunities • Emission from near IR to X-rays (even radio, if sufficient sensitivity) • SALT has ideal instruments and capabilities for studying objects at high time resolution and polarimetrically • ASTROSAT will have excellent capabilities to study the accretion physics by virtue of X-ray observation • Simultaneous SALT-ASTROSAT observations of mCVs (& other similar multiwavelength emitters) provides excellent opportunities to extend our knowledge. • Time is ripe for new India-South African bilateral program to exploit these possibilities

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