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