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SALT & ASTROSAT Observations of Magnetic Cataclysmic Variables
David BuckleySALT 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 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
Model of a Polar (AM Herculis system)
Accretion column analogy on the Sun
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Polars: Spectral Energy Distribution
• Most of the energy from these systems is a result of accretion
• 3 main components:
cyclotron radiation from accretion column
hard X-ray emission, also from accretion column
soft X-ray emission, from heated surface of primary
Beuermann (1998)
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Example: XMM-Newton Spectrum of V1432 Aql
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
Rana, Singh, Buckley & Barrett 2005, ApJ
Example: V834 Cen, Porb= 101 min SAAO 1.9-m photopolarimetry
Determining Magnetic Field Strength & Geometry in Polars:Fitting cyclotron model fits to All-Stokes broadband polarimetry
Fit cyclotron parameters (plasma temp & density, cyclotron opacity, B & )
• using Potter’s Stokes imaging technique
• fits model to data using a geneticalgorithm
Extend to spectropolarimetry
Single pole system
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.
Intensity
Cyclotron emission harmonics
V834 Cen spectropolarimetry
Wickramasinghe Tuohy & Visvanathan ApJ 318, 326
AAT 3.9m
Results indicateMulti-T shocks
20 keV, = 0
20 keV, = 0.7
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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)
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Intermediate Polar example: AO Psc
AO Psc
• 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)
Cropper et al (2002)
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
New paradigm in cost effective design pioneered bythe HET in Texas.
• fixed altitude (37 ± 6º zenith distance)
• track objects at prime focus
• optical analogue of Arecibo radio tel.
SALT Design Principle
SALT:91 x 1m mirrors
Annulus of visibility for SALT:
Annulus represents12.5% 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
SALT Visibility Window
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.
Optics
SALTICAM in the lab
SALT’s Instruments:1. SALTICAM: UV-Vis CCD Camera (built at SAAO: Darragh O’Donoghue, PI)
Cryostat &detector
Filter jukebox Optics
Resolving eclipses of Polars
An example: a light curve of an eclipsing magnetic CV (Polar) taken with SALTICAM
Ingress/Egress = 1.2 to 1.5 sec
Each data point a 0.1 sec exposure
SALT’s First-Science
Model fit:locating hot-spot positions
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 & aperiodic behaviour of IPs (with Alexei Kniazev & Mikhail Revnivtsev)
– Power spectra clues to missing inner disk?– Disrupted power law
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
New INTEGRAL/SWIFT source 1GRJ 14536-5522• HIPPO (SAAO 1.9m instrument) All-Stokes photopolarimetry
Intensity Circ Pol
Trailed periodograms
DFTs
Intensity Circ. Pol.
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 ~50 objects at once
Named in memory of Bob Stobie, previous SAAO Director & one the instigators of SALT.
RSS reinstalled on SALT (Apr 2011)
RSS Polarimetry
• Imaging polarimetry• Spectropolarimetry
Probing accretion columns polarimetrically with SALT
Phase resolved QS Tel spectra
Schwope et al. 1995 A&A 293, 764
ESO/MPI 2.2m example
Two poles accreting
Cyclotron humps move in position and shape and size as a function of phase
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..
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)
ASTROSAT: India’s first astronomy satelliteAn ideal complement of instrumentsfor mCVs
Soft X-ray Telescope
3 Large Area Xenon Proportional Counters (hard X-rays)
2 UV(+Opt ) Imaging Telescopes
CZTI (hard X-rays)
SSM (2 – 10 keV)
Folded Solar panels
Radiator PlatesFor SXT and CZT
Scanning Sky Monitor (SSM)
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.
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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
Feb 13, 2012 K.P. Singh
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LAXPC: Effective Area
SALT-ASTROSAT ProgramSimultaneous 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)
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