Transcript SALT and Astrosat observations of magnetic cataclysmic variables

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
 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
Polars: Spectral Energy Distribution
• Most of the energy from these systems is a result of
accretion
• 3 main components:
cyclotron
radiation from
accretion
column
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soft X-ray
emission, from
heated surface of
primary
Beuermann (1998)
hard X-ray
emission, also
from accretion
column
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
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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
Example: V834 Cen, Porb= 101 min
SAAO 1.9-m photopolarimetry
Extend to spectropolarimetry
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
V834 Cen spectropolarimetry
20 keV,  = 0
AAT 3.9m
Results indicate
Multi-T shocks
20 keV,  = 0.7
Wickramasinghe Tuohy & Visvanathan ApJ 318, 326
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
11
HEAP12- HRI (KP Singh)
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
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– the mixture of
the two
(beat/synodic
period)
AO Psc
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
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.
SALT:
91 x 1m mirrors
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
SALT’s Instruments:
Cryostat &
detector
1. SALTICAM: UV-Vis CCD
Camera (built at SAAO:
Darragh O’Donoghue, PI)
An efficient “video” (~10 Hz) camera over
entire science FoV (8 arcmin).
Filter jukebox
Optics
Efficient in the UV/blue (capable down to
atmospheric cutoff at 320nm).
Optics
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.
SALTICAM in the lab
Resolving eclipses of Polars
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
Model fit:
locating hotspot 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
Intensity
DFTs
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
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
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
UZ For (Polar)
•
•
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
ASTROSAT: India’s first astronomy satellite
An ideal complement of instrumentsfor mCVs
2 UV(+Opt ) Imaging Telescopes
3 Large Area Xenon
Proportional
Counters (hard Xrays)
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)
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.
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
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LAXPC: Effective Area
Feb 13, 2012
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K.P. Singh
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 powerlaw 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