X-rays from Magnetic Cataclysmic Variables and ASTROSAT

X-rays from Magnetic Cataclysmic Variables and ASTROSAT

K.P Singh Tata Institute of Fundamental ResearchMumbai, India

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Talk Outline

• Introduction to CVs

• Types and classes of CVs

• Non-magnetic systems

• Magnetic systems: Polars and Intermediate Polars

• X-ray Light Curves of Polars and IPs

• Wide-Band, Low-Resolution X-ray spectra

• ASTROSAT

• Conclusions

18 Feb 2012 HEAP12- HRI (KP Singh)

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CVs: what are they?• Cataclysmic Variables are

– semi-detached binaries accreting

– from a red dwarf red dwarf main-sequence-like secondary star

– to a more massive white dwarf white dwarf primary star

• Binary: Roche potential: the gravitational potential around two orbiting point masses – resultant force on a test mass:

credit: csep10.phys.utk.edu

F tot = F M1(grav )+ F M

2(grav )+ FCoM ( centrifugal)

Centre of Mass

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Roche Lobe Overflow• Semi-detached secondary star fills its Roche

lobe so that it is distorted into a pear shape.

• At Lagrangian 1 (L1) point, gravitational and centrifugal forces cancel and material is lost from the secondary star into the primary Roche lobe.

• Material falls towards the white dwarf in a stream

• The 4 other stationary pointsL2 – L5 are important for orbit theory

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credit: csep10.phys.utk.edu

credit: www.genesismission.org


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The CV Zoo: subtypes• Cataclysmic Variables (non-magnetic)

– Novae large eruptions 6–9 magnitudes

– Recurrent Novae previous novae seen to repeat

– Dwarf Novae regular outbursts 2–5 magnitudes

› SU UMa stars occasional Superoutbursts

› Z Cam stars show protracted standstills

› U Gem stars all other DN

– Nova-like variables

› VY Scl stars show occasional drops in brightness

› UX UMa stars all other non-eruptive variables

• Intermediate Polars/DQ Her stars

• Polars/AM Her stars

magnetic systems


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(1) Non-Magnetic CVs

• magnetic field on primary <106 G (100 T) non-magnetic CV

• accretion takes place through a disk

• via boundary layer on white dwarf


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(2) Magnetic CVs: Polars

• NO DISK: accretion takes place via a stream and accretion column directly onto white dwarf

• Largest circular polarization varying with the orbital period: magnetic field > 107 G (1000 T) polar/AM Her system

• the magnetic field controls the flow fromsome threading region


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Polars: Synchronisation

• All of the variability in Polars occurs at a single period: the orbital period

– radial velocity curves of the secondary

– X-ray light curves from the primary

– polarisation variations

the white dwarf/red dwarf are locked into the same orientation: synchronised rotation

• The mechanism for synchronisation is the dissipation due to the magnetic field of the primary being dragged through the secondary

• As relative spin rate of primary decreases, locking can occur due to the dipole-dipole magnetostatic interaction between primary and (weaker) secondary magnetic field

• Some Polars not quite in synchronism; in these systems it typically takes 5–50 days for white dwarf orientation to repeat itself

• Very useful systems to study the effect of orientation of magnetic field on the accretion process


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Polars: Radial Accretion

• Infalling material is forced to follow the magnetic field lines

• Gas is initially in free-fall but then it encounters a shock front

• Shock converts kinetic energy into thermal energy (bulk motion into random motion) temperature increases to ~50 keV

• Velocity drops by 1/4 and density increases by 4

• Material radiates by cyclotron and bremsstrahlung and gradually settles on white dwarf

hard X-rays

white dwarf

shock

cold

supersonic

flow

hot

postshock

flow

soft X-ra

ys/

extreme UV

optical/IR

cyclotron

radiation


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X-ray Spectrum

• Polars/AM Her stars were found to be strong soft X-ray emitters (~1033 erg/s) in early surveys

• X-ray emission characterized by thermalized free-fall velocities from a white dwarf so emission is from a hot region close to the white dwarf surface: post-shock

• Cyclotron emission must also be from a hot region (otherwise narrow cyclotron emission lines rather than continuum)

AM Her

Rothschild et al (1981)


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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)18 Feb 2012

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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

Absorption due to ISM = 4.5 x 1020 cm-2 (fixed from ROSAT Obs; Staubert et al. 1994)


XMM: Rana, Singh et al. 2005, ApJ

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RXTE Spectrum of V1432 Aql

Model = Absorption (Multi-temperature Plasma + Gaussian)

• Bremsstrahlung model (temperature of >90 keV ; highest in Polars and IPs)• Mass of the WD related to the shock temperature • Mass of the WD in V1432 Aql is 1.2±0.1 solar mass.


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AM Her (Polar) Pspin (X-ray)=11139 sGirish, Rana & Singh 2007

Two-pole accretion based on opticalPolarizationInclination=52+-5 degrees


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UZ For (Eclipsing Polar): P=7591.8 s

• Two accretion regions evident

• Weak accretion stream

UZ For

STJ photometry: Perryman et al (2001)

Sky


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(3) Magnetic CVs: Intermediate Polars• magnetic field ~106 G intermediate polar/DQ Her system

• accretion takes place through a hollowed-out disk and then via accretion columns onto the white dwarf

• magnetic field controls the flow in the final stages


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Intermediate Polars: models• Intermediate Polars spin variability can be explained in

several ways

– visibility of the accretion region on the white dwarf

– visibility of the accretion “curtains”

– reprocessing of flux on the disk (optical/UV)

• From studies of the relative phasing in different wavelength bands and including the absorption effects now known to be a combination of the above models leading to the complex behavior in Intermediate Polar light curves

stream

Adapted from Hellier (2001)


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AO Psc (Intermediate Polar)

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)


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AO Psc (IP) Power Spectra

• By performing a Fourier Transform of the previous data, the main periodicities can be identified

– orbital period

– white dwarf spin

– beat (very faint in this system)

• Also evident are harmonics when the variations are non-sinusoidal (2, 3, 2)

Ginga:Norton et al.


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TV Col (Intermediate Polar) =1909.7s =5.5 h

Rana, Singh et al. 2004, AJ, 127, 489

First clear detection of Orbital modulation


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TV Col (IP): Spin and Orbital Phase LCs

Rana, Singh et al. 2004, AJ, 127, 489

Absorption Dips due to stream


INTEGRAL Discovered IP: IGR J17195-4100


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New spin period: 1053.9 sNew Binary Orbital period: 3.5 hoursGirish & Singh, MNRAS, 2012

Multi-temperature plasma, partial absorber and flourescence – sometimes a weak soft X-ray component. Accretion Curtain but perhaps no disc in this IP !Girish & Singh, MNRAS, 2012

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X-ray Spectrum: EX Hya (IP)• X-ray spectra of Intermediate Polars generally show

just the multi-temperature thermal bremsstrahlung component from the hot radial accretion flow – no soft reprocessed component from the white dwarf

• Main explanation is likely to be the larger area over which accretion takes place, but also photoelectric absorption is important

Fujimoto & Ishida 1997


ASTROSAT (1.55 tons; 650 kms, 8 deg inclination orbit by PSLV. 3 gyros and 2 star trackers for attitude control by reaction wheel system with a Magnetic torquer ) Launch: end of 2012

Soft X-ray Telescope

3 Large Area Xenon Proportional Counters

2 UV(+Opt ) Imaging Telescopes

CZTI

SSM (2 – 10 keV)

Folded Solar panels

Radiator PlatesFor SXT and CZT

Scanning Sky Monitor (SSM)

Feb 13, 2012K.P. Singh

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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

Energy Range : 3-80 keV ( 50 Mylar window, 2 atm. of 90 % Xenon + 10 % Methane )

Effective Area : 6000 cm² (@ 20 keV)

Energy Resolution : ~10% FWHM at 22 keV

Onboard purifier for the xenon gas

Field of View : 1° x 1° FWHM (Collimator : 50µ Sn + 25µ Cu + 100µ Al )Blocking shield on sides and bottom : 1mm Sn + 0.2 mm Cu

Timing Accuracy : 10 μsec in time tagged mode

(oven-controlled oscillator).

Large Area Xenon Proportional Counter (LAXPC): Characteristics

Feb 13, 2012 K.P. Singh

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LAXPC: Effective Area

CZT Imager characteristics

Area 1024 cm 2

Pixels 16384

Pixel size 2.4 mm X 2.4 mm (5 mm thick)

Read-out ASIC based (128 chips of 128 channels)

Imaging method Coded Aperture Mask (CAM)

Field of View 17 X 17 deg2 (uncollimated)6 X 6 (10 – 100 keV) – CAM

Angular resolution 8 arcmin

Energy resolution 5% @ 100 keV

Energy range 10 – 100 keV - Up to 1 MeV (Photometric)

Sensitivity 0.5 mCrab (5 sigma; 10 4 s)


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SXT Characteristics

Telescope Length: 2465 mm (Telescope + camera + baffle + door)Top Envelope Diameter: 386 mm Focal Length: 2000 mmEpoxy Replicated Gold Mirrors on Al substrates in conical Approximation to Wolter I geometry.Radius of mirrors: 65 - 130 mm; Reflector Length: 100 mmReflector thickness: 0.2 mm (Al) + Epoxy (~50 microns) + gold (1400 Angstroms)Reflector Smoothness: 8 – 10 AngstromsMinimum reflector spacing: 0.5 mmNo. of reflectors: 320 (40 per quadrant) Detector : E2V CCD-22 (Frame-Store) 600 x 600

Field of view : 41.3 x 41.3 arcmin

PSF: ~ 4 arcminsSensitivity (expected): 15 Crab (1 cps/mCrab)

SXT Effective Area vs. Energy (after subtraction of shadowing effects due to holding structure)

February 13, 2012

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Scanning Sky Monitor (SSM)

• Detector : Proportional counters with resistive anodes

• Ratio of signals on either ends of anode gives position.

• Energy Range : 2 - 10 keV

• Position resolution : 1.5 mm

• Field of View : 10 x 90 (degs) (FWHM)

• Sensitivity : 30 mCrab (5 min integration)

• Time resolution : 1 ms

• Angular resolution : ~ 10 arc min


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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

Simultaneous UV to hard X-ray spectral measurements with ASTROSAT : MCVs

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 and Mass of the WD

Cataclysmic Variables with Astrosat

SXT

LAXPC

C

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CVs: Open Issues• Many aspects deserve further investigation: here are some

– boundary layer in non-magnetics– the base of the post-shock accretion flow in magnetics and

the way this diffuses into the white dwarf– heating of the atmosphere around the accretion region in

magnetics, and effect on overall energy distribution– low accretion rate regimes in magnetics, whether this

results in a bombardment solution (no shock)– disk-magnetosphere interaction in IPs: important in a number

of contexts– disk-stream interactions in non-magnetics– magnetosphere-stream interactions in Polars– irradiation of the stream and secondary by X-ray flux– more astrophysics in the post-shock flow models (such as the

separation of electron and ion fluids)

• Combinations of high quality data (e.g. eclipse mapping of spectra) and new astrophysical fluid computations will transform the field and allow ever more intricate understandings of accretion phenomena to be achieved


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CVs in the grander scheme of things

• Cataclysmic variables are fairly common systems.

• CVs produce the low-level background of discrete sources in galactic X-ray emission – fainter but much more numerous than neutron-star or Black hole X-ray binaries

• They are highly important laboratories for studies of accretion

– disk behaviour › instabilities, › stream impacts, › warps, › tidal resonances, › spiral waves etc.

– magnetically dominated accretion › accretion columns, › emission from post-shock flow, › shocks, instabilities etc.

• Multi-wavelength emission (polarized in many cases) allows a multi-wavelength approach, providing very strong observational constraints on the interpretation of data


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CVs in the grander scheme of things (contd.)

• Important for investigations on how material interacts with a magnetic field:– threading region in Polars, – inner region of disk in Intermediate Polars, – Dwarf Nova oscillations in non-magnetic CVs

• In general, the balance of: – visibility of underlying system &– the emission (X-ray, optical) has been fundamental to making

enormous progress in understanding a wide range of astrophysics

• It is a field which incorporates fluid dynamics, MHD, a full range of emission processes, stellar evolution, gravitational radiation etc.

• A large number of important observational techniques have been developed in the context of CVs and then used elsewhere:– Doppler tomography, – eclipse mapping of disks and streams, – Stokes imaging, – timing analyses

• Progenitors of Type I Supernovae – Cosmological Distance Ladder



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Thank you !

Documents

X-rays from Magnetic Cataclysmic Variables and ASTROSAT