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X-rays from Magnetic Cataclysmic
Variables and ASTROSAT
K.P Singh
Research
India
Tata Institute of Fundamental
Mumbai,
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
HEAP12- 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:
Ftot = F M ( grav )+ F M 2 ( grav)+ FCoM ( centrifugal)
1
Centre of Mass
2
credit: csep10.phys.utk.edu
18 Feb 2012
3
HEAP12- 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 (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 points
L2 – L5 are important for
orbit theory
18 Feb 2012
4
credit:
www.genesismission.org
HEAP12- HRI (KP Singh)
credit:
csep10.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
HEAP12- 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
HEAP12- 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
HEAP12- 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
HEAP12- HRI (KP Singh)
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
18 Feb 2012
9
HEAP12- HRI (KP Singh)
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
HEAP12- 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 V1432 Aql
XMM: Rana, Singh et al. 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
Absorption due to ISM = 4.5 x 1020 cm-2
(fixed from ROSAT Obs;
Staubert et al. 1994)
18 Feb 2012
12
HEAP12- HRI (KP Singh)
RXTE Spectrum of V1432 Aql
• 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.
18 Feb 2012
13
HEAP12- 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
HEAP12- 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
HEAP12- 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
HEAP12- 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
HEAP12- 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
HEAP12- 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
HEAP12- HRI (KP Singh)
TV Col (Intermediate Polar)
=1909.7s
=5.5 h
First clear detection of
Orbital modulation
18 Feb 2012
20
HEAP12- 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
HEAP12- HRI (KP Singh)
Rana, Singh et al. 2004, AJ,
127, 489
INTEGRAL Discovered IP: IGR J17195-4100
New spin period:
1053.9 s
New Binary Orbital period: 3.5 hours
Girish & Singh, MNRAS, 2012
18 Feb 2012
22
HEAP12- 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
HEAP12- 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 keV)
UVIT: Two Telescopes
• f/12 RC Optics
• Focal Length: 4756mm
• Diameter: 38 cm
• Simultaneous Wide Angle (
~ 28’) images in FUV (130180 nm) in one and NUV
(180-300 nm) & VIS (320530 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 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
:
(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 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)
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
: E2V 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/mCrab)
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 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
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
HEAP12- 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
HEAP12- 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
HEAP12- HRI (KP Singh)
Thank you !
Feb 13, 2012
39
K.P. Singh