Xrays from Magnetic Cataclysmic Variables and ASTROSAT K

X-rays from Magnetic Cataclysmic Variables and ASTROSAT K. P Singh Tata Institute of Fundamental Research Mumbai, India

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 2 HEAP 12 - HRI (KP Singh)

CVs: what are they? • Cataclysmic Variables are – semi-detached binaries accreting – from a red dwarf main-sequence-like secondary star – to a more massive white dwarf primary star • Binary: Roche potential: the gravitational potential around two orbiting point masses – resultant force on a test mass: Centre of Mass 2 credit: csep 10. phys. utk. edu 18 Feb 2012 3 HEAP 12 - HRI (KP Singh)

Roche Lobe Overflow • Semi-detached secondary star fills its Roche lobe so that it is distorted into a pear shape. 2 • At Lagrangian 1 (L 1) 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 points L 2 – L 5 are important for orbit theory 18 Feb 2012 4 credit: www. genesismission. org HEAP 12 - HRI (KP Singh) credit: csep 10. phys. utk. edu

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 magnetic systems • Polars/AM Her stars 18 Feb 2012 5 HEAP 12 - HRI (KP Singh)

(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 18 Feb 2012 6 HEAP 12 - HRI (KP Singh)

(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 from some threading region 18 Feb 2012 7 HEAP 12 - HRI (KP Singh)

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 18 Feb 2012 8 HEAP 12 - HRI (KP Singh)

Polars: Radial Accretion • Infalling material is forced ic to follow the magnetic field old n o c ers lines p u • 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 ke. V • Velocity drops by 1/4 and density increases by 4 • Material radiates by cyclotron and bremsstrahlung and gradually settles on white dwarf 18 Feb 2012 9 s w flo /IR l a n tic op lotro n cyc iatio rad ck o sh rd ha t ho k oc h s st flow po s ay r X s/ y a -r UV X t sof reme t ex HEAP 12 - HRI (KP Singh) rf ite wh a dw

Rothschild et al (1981) 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) 18 Feb 2012 10 AM Her HEAP 12 - HRI (KP Singh)

Polars: Spectral Energy Distribution • Most of the energy from these systems is a result of accretion • 3 main components: cyclotron radiation from accretion column 18 Feb 2012 11 soft X-ray emission, from heated surface of primary Beuermann (1998) hard X-ray emission, also from accretion column

XMM-Newton Spectrum of V 1432 Aql XMM: Rana, Singh et al. 2005, Ap. J Model Compenents: • Black body emission (88± 2 e. V) • 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 ke. V line emission Absorption due to ISM = 4. 5 x 1020 cm-2 (fixed from ROSAT Obs; Staubert et al. 1994) 18 Feb 2012 12 HEAP 12 - HRI (KP Singh)

RXTE Spectrum of V 1432 Aql • Bremsstrahlung model (temperature of >90 ke. V ; highest in Polars and IPs) • Mass of the WD related to the shock temperature • Mass of the WD in V 1432 Aql is 1. 2± 0. 1 solar mass. 18 Feb 2012 13 HEAP 12 - HRI (KP Singh) Model = Absorption (Multi-temperature Plasma + Gaussian)

AM Her (Polar) Pspin (X-ray)=11139 s Girish, Rana & Singh 2007 Two-pole accretion based on optical Polarization Inclination=52+-5 degrees 18 Feb 2012 14 HEAP 12 - HRI (KP Singh)

UZ For (Eclipsing Polar): P=7591. 8 s • Two accretion regions evident • Weak accretion stream UZ For Sky 18 Feb 2012 15 HEAP 12 - HRI (KP Singh) STJ photometry: Perryman et al (2001)

(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 18 Feb 2012 16 HEAP 12 - HRI (KP Singh)

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) stream Adapted from Hellier (2001) • 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 18 Feb 2012 17 HEAP 12 - HRI (KP Singh)

AO Psc (Intermediate Polar) 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 AO Psc – the orbital 3. 591 h, – the spin period of the white dwarf 805. 4 s – the mixture of the two (beat/synodic period) 18 Feb 2012 18 HEAP 12 - HRI (KP Singh)

AO Psc (IP) Power Spectra Ginga: Norton et al. • 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 ) 18 Feb 2012 19 HEAP 12 - HRI (KP Singh)

TV Col (Intermediate Polar) =1909. 7 s =5. 5 h First clear detection of Orbital modulation 18 Feb 2012 20 HEAP 12 - HRI (KP Singh) Rana, Singh et al. 2004, AJ, 127, 489

TV Col (IP): Spin and Orbital Phase LCs Absorption Dips due to stream 18 Feb 2012 21 HEAP 12 - HRI (KP Singh) Rana, Singh et al. 2004, AJ, 127, 489

INTEGRAL Discovered IP: IGR J 17195 -4100 New spin period: 1053. 9 s New Binary Orbital period: 3. 5 hours Girish & Singh, MNRAS, 2012 18 Feb 2012 22 HEAP 12 - HRI (KP Singh)

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

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 18 Feb 2012 24 HEAP 12 - HRI (KP Singh)

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 2 UV(+Opt ) Imaging Telescopes 3 Large Area Xenon Proportional Counters Soft X-ray Telescope CZTI Radiator Plates For SXT and CZT Scanning Sky Monitor (SSM) Folded Solar panels SSM (2 – 10 ke. V)

UVIT: Two Telescopes • f/12 RC Optics • Focal Length: 4756 mm • Diameter: 38 cm • Simultaneous Wide Angle ( ~ 28’) images in FUV (130 -180 nm) in one and NUV (180300 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 26 Feb 13, 2012 K. P. Singh

Large Area Xenon Proportional Counter (LAXPC): Characteristics Energy Range : 3 -80 ke. V ( 50 Mylar window, 2 atm. of 90 % Effective Area : Energy Resolution : ~10% FWHM at 22 ke. V Xenon + 10 % Methane ) 6000 cm² (@ 20 ke. V) 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 : 1 mm Sn + 0. 2 mm Cu Timing Accuracy : (oven-controlled oscillator). 10 μsec in time tagged mode

LAXPC: Effective Area Feb 13, 2012 28 K. P. Singh

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 deg 2 (uncollimated) 6 X 6 (10 – 100 ke. V) – CAM Angular resolution 8 arcmin Energy resolution 5% @ 100 ke. V Energy range 10 – 100 ke. V - Up to 1 Me. V (Photometric) Sensitivity 0. 5 m. Crab (5 sigma; 10 4 s)

SXT Characteristics Telescope Length: 2465 mm (Telescope + camera + baffle + door) Top Envelope Diameter: 386 mm Focal Length: 2000 mm Epoxy Replicated Gold Mirrors on Al substrates in conical Approximation to Wolter I geometry. Radius of mirrors: 65 - 130 mm; Reflector Length: 100 mm Reflector thickness: 0. 2 mm (Al) + Epoxy (~50 microns) + gold (1400 Angstroms) Reflector Smoothness: 8 – 10 Angstroms Minimum reflector spacing: 0. 5 mm No. of reflectors: 320 (40 per quadrant) Detector : E 2 V CCD-22 (Frame-Store) 600 x 600 Field of view : 41. 3 x 41. 3 arcmin PSF: ~ 4 arcmins Sensitivity (expected): 15 Crab (1 cps/m. Crab) Feb 13, 2012 30 K. P. Singh

SXT Effective Area vs. Energy (after subtraction of shadowing effects due to holding structure) February 13, 2012 31

Scanning Sky Monitor (SSM) • Detector : Proportional counters with resistive anodes • Ratio of signals on either ends of anode gives position. • Energy Range : 2 - 10 ke. V • Position resolution : 1. 5 mm • Field of View : 10 x 90 (degs) (FWHM) • Sensitivity 30 m. Crab (5 min integration) • Time resolution : 1 ms • Angular resolution : ~ 10 arc min Feb 13, 2012 32 : K. P. Singh

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 C SXT LAXPC

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 18 Feb 2012 36 HEAP 12 - HRI (KP Singh)

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 18 Feb 2012 37 HEAP 12 - HRI (KP Singh)

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 18 Feb 2012 38 HEAP 12 - HRI (KP Singh)

Thank you ! Feb 13, 2012 39 K. P. Singh