X-ray monitoring of cataclysmic
variables – dependence on
the instruments
v
v
Vojtech Simon
v
1
2
Astronomical Institute, Academy of Sciences, 251 65 Ondrejov,
Czech Republic
Czech Technical University in Prague, Faculty of Electrical
Engineering, Prague, Czech Republic
Talk: International Workshop on Astronomical X-Ray Optics, Prague,
Czech Republic, 2015
The importance of the X-ray monitoring (I)
Monitoring enables to:
identify the type of system
place the events (e.g. outbursts) in the context of the long-term
activity of the system
form the representative ensemble of events (e.g. outbursts) in
(a) a given system,
(b) in a type of systems
This is important for our understanding of the physical processes
involved.
Transitions between the activity states (e.g. outbursts, high/low
states) are often fast and unpredictable – monitors are needed.
2
X-ray monitors onboard various satellites
 Monitors are typically sensitive to radiation within 2 < E < 10 keV – soft
X-ray emission components thus often remain unstudied.
 Most cataclysmic variables radiate beyond this monitored spectral region.
3
X-ray monitoring and pointed observations
Occasional pointing in any spectral band is not enough:
 many pieces of information on the time evolution are lost in any
spectral band
 time allocation has to be justified (search for unexpected behavior
of the object is usually not approved)
Determining a comprehensive picture about the processes operating
in a given system (or a group of systems) requires analysis of an
ensemble of events.
4
Cataclysmic variables as X-ray emitters
Donor
WD
Accretion disk
“Non-magnetized” WD:
Most X-rays (bremsstrahlung) from boundary
layer (encircling the equator of the white
dwarf (WD)).
Large structural changes of the boundary
layer (e.g. between quiescence and outburst)
Mildly magnetized WD: B ~ 106 Gauss
Bremsstrahlung X-ray emission from an
impact of the accretion columns onto the
magnetic poles of the WD
Strongly magnetized WD: B > 107 Gauss
thermal (soft X-rays – WD heated by impact)
bremsstrahlung (accretion column – hard X)
Patterson & Raymond (1985)
Warner (1995)
5
Problems in the long-term coverage in X-rays
Detectability of the object strongly depends on its activity:
 Low-mass X-ray binaries (LMXBs – systems with the neutron-star or
the black-hole accretor ):
intensity of X-ray emission strongly increases during active states
(outbursts, episodes of the high states)
often no large variations between soft and hard X-ray intensity (for
E < 12 keV)
 Cataclysmic variables (CVs – white-dwarf accretor):
intensity of X-ray emission strongly depends on the X-ray band
and the state of activity in a given CV
X-ray data are often fragmentary – many CVs are too faint for the
available X-ray monitors (with only a very few exceptions)
6
ASM/RXTE – monitor for medium/hard X-rays
Mission: RXTE (Rossi X-Ray Timing Explorer) (1996 – 2012)
Three shadow cameras
(6 x 90 degrees FOV)
Energy range: 1.5 – 12 keV:
1.5 – 3 keV 3 – 5 keV
5 – 12 keV
Time resolution: 90 s integration time
– 80% of the sky every 90 min
– one-day means are usually used
to increase the sensitivity
Spatial resolution: 3 x 15 arcmin
Sensitivity: ~13 mCrab for one-day means)
Levine et al. (1996)
7
MAXI/ISS – monitor for medium/hard X-rays
Mission: ISS
(since 2010)
Slit cameras in 6 units
(160 x 1.5 degrees FOV)
Energy range: 2 – 20 keV:
2 – 4 keV 4 – 10 keV 10 – 20 keV
Time resolution:
– the source is observed twice per
92 min orbit
– one-day means are usually used
to increase the sensitivity
Matsuoka
et al. (2009)
Mihara et
al. (2011)
8
BAT/Swift – monitor for very hard X-rays
Krimm et
al. (2013)
9
Mission:
NASA Swift (since 2004)
Aperture: Coded mask
Field of view: 1.4 sr (partially-coded)
Telescope PSF:
17 arcmin
Energy range: 15 – 150 keV
(15 – 50 keV is used for
monitoring of X-ray sources)
Optical band (AAVSO)
Dwarf nova &
intermediate polar
GK Per/2E 0327.7+4344
outburst
ASM/RXTE
Moving
averages
quiescence
ASM
BAT/Swift
One-day means
BAT
GINGA spectra (Ishida et al. 1992)
Based on: Simon (2002)
Outburst:
- higher X-ray intensity
- larger absorption
Thermal-viscous instability of the accretion disk – Outbursts (episodes of the mass
accretion onto the WD)
 Optical outburst – the disk switches to the hot state – mass accretion from the disk
 X-ray outburst – accretion onto the magnetic poles of the WD
10
Dwarf nova & intermediate polar GK Per / 2E 0327.7+4344
Optical
ASM/RXTE
Optical
Optical
ASM/RXTE
BAT/Swift
BAT/Swift
X-ray intensity saturates near the peak of the optical outburst (in
the time of the largest mass inflow through the disk)
An increase of absorption of X-rays cannot explain this saturation.
Structural changes of the accretion regions at the poles of the WD
11
V1223 Sgr (intermediate polar) – a very hard X-ray source
A very hard bremsstrahlung
X-ray spectrum (Suzaku data
(Hayashi & Ishida 2014))
BAT/Swift band
Hayashi &
Ishida (2014)
Site: Post-shock accretion
column at the magnetic
poles of the WD
Very hard emission in the
BAT/Swift band
For comparison:
X-ray spectrum of MV Lyr
(novalike in the high state)
Greiner (1998)
<0.5 keV blackbody emission
ROSAT PSPC data (Greiner 1998)
12
V1223 Sgr (intermediate polar) – X-ray activity
NSVS &
ASAS data
Thermal
emission
(disk)
Bremsstrahlung
(accr. regions)
BAT/Swift
 Accreting regions at the
polar caps of the WD:
– sources of very hard X-ray
emission (bremsstrahlung)
 The cause of the shallow
low state – decrease of
the mass inflow to the disk
from the donor
(not only changes of the
disk structure) – this
places the constraints on
the model of Beuermann
et al. (2004)
13
SS Cyg / 2E 2140.7+4321
(dwarf nova)
Optical and X-ray emissions come from different regions of the system
Outbursts:
thermal-viscous
instability of the accretion disk
Optical
Soft X-ray
(E<0.5 keV)
Hard X-ray
(E>2 keV)
Spikes during bottom
part of transition
Large dependence of the outburst
profile on the bandpass
 Strong brightening only in very
soft X-rays, not in hard X-rays !
Large structural changes of
the boundary layer during the
outburst
Wheatley et al. (2003)
14
SS Cyg / 2E 2140.7+4321
Quiescence
(dwarf nova)
Outburst
Ishida et al. (2009)
Large dependence of the outburst profile on the X-ray band
(wavelength of emission)
 During the outburst, a strong brightening only in very soft
(E < 0.3 keV) X-rays, not in hard X-rays (even a decrease) !
15
Supersoft X-ray sources – the expected SED
Peak luminosity in the soft X-ray band:
 strongly affected by absorption
 this band is neglected by most monitors
1 keV
0.1 keV
1.24 keV
0.41 keV
0.25 keV
Ness et
al. (2013)
Observed Xray spectrum
Modelled
spectral
energy
distribution
(SED)
–
strong
influence of
absorption
Popham & Di Stefano (1996)
16
Supersoft X-ray sources – V Sge / 2E 2018.0+2056
X-ray spectrum
(normalized)
Optical luminosity in antiphase with X-ray
luminosity:
optical high state – X-ray faint and hard
optical low state – X-ray bright and very soft
Greiner & van Teeseling (1998)
17
Optical
low state
Optical
high state
MAXI / ISS
Optical
AM Her / 2E 1814.9+4951 (polar) - SED during the high state
3
2
BAT / Swift
ASM / RXTE
1
The observable result of several processes operating in the accretion region
1…Cyclotron emission – dominant in the optical and IR band
2…Bremsstrahlung – medium and hard X-ray emission
3…Thermal emission from the surface of the WD heated by the impact –
soft X-ray excess (not in the band observed by most monitors!)
Kuulkers et
al. (2006)
18
AFOEV data
Cyclotron+stream
emission
AM Her – variable emission
output in the high states
Based on: Simon (2011)
Bremsstrahlung
emission
ASM/RXTE data (1.5 – 12 keV)
Evolution of the optical and hard X-ray
intensities in the individual high-state
episodes
(smoothed through the orbital modulation)
Relation between intensities from two
processes in a high-state episode
Optical – dominant cyclotron em.
Hard X-ray – bremsstrahlung em.
Dramatically different properties of
the emitting region(s) on the WD in
the high-state episodes.
19
AM Her – a relation between the optical and X-ray
intensities in two high-state episodes
Optical
emission
Hard X-ray
emission (RXTE)
HS1
HS2
HS1
HS2
Two consecutive episodes of the high state:
 Intensities of the optical and hard X-ray (1.5 – 12 keV) emissions are anticorrelated.
 A higher luminosity of the bremsstrahlung emission may not be always
accompanied by a higher optical (cyclotron+stream) emission in a given
episode of the high state.
 This relation of intensities is representative for the whole HS episode.
Based on:
Simon (2011)
20
AM Her / 2E 1814.9+4951 – monitoring of the bremsstrahlung
component in a polar
Cyclotron emission
Optical band
AAVSO data
Successful monitoring of the hard Xray emission in the high states of a
polar
Simultaneous observing with two
monitors:
– data in two bands are available
Bremsstrahlung
emission
Optical high states: X-ray emission is
detectable only in these phases
Medium/hard
X-rays MAXI / ISS
Very hard X-rays
BAT / Swift
Tail of bremsstrahlung
emission
Relation between the optical and hard
X-ray intensities on long timescales
 Information about the total balance of
the emission components is still
missing
(the monitors cannot observe the
SOFT X-ray peak)
21
CV types and their X-ray spectra
Non-magnetic CVs
(no spin modulation
of WD, various states)
SS Cyg
(quiescence)
AE Aqr
DQ Her
? Intermediate polars
Spectra in various levels of activity
 Bremsstrahlung – dominant in most CV
types but the structures of the emitting
regions largely differ
(from spin modul.)
Bremsstrahlung produces softer X-ray
emission in ”non-magnetic” CVs
(accretion via boundary layer on
the equatorial belt of the WD)
Polars
Intermediate polars: the hardest spectra
(radial inflow onto the poles from the disk)
 kT in intermediate polars largely differs
from system to system
kT of bremsstrahlung component
(data: various sources – mostly from Warner (1995)
Polars: radial inflow from the stream
22
Analysis of the data of faint X-ray CVs
from monitors
Binning of the X-ray data enables to analyze faint sources (but it
strongly smooths the profiles of the features)
Smoothing the X-ray data through the orbital modulation
Determining the mean levels of X-ray intensity in some states of
activity – possible e.g. for the high states of polars
Simultaneous monitoring of the same object with several
monitors:
– possibility to determine the hardness ratio of X-ray emission
23
Conclusions
Profiles of features of the long-term activity of cataclysmic
variables are measurable by the monitors. Search for the common
features is needed.
Spectral variations are measurable by some monitors (or by a
combination of observing by several monitors).
We emphasize the very important role of the spectral region of
the X-ray monitor.
The available monitors can detect only cataclysmic variables with
magnetized white dwarfs (WDs) – the mode of accretion is very
important.

Only radial flow onto the WD causes sufficiently hard X-ray
spectrum to be observable by the monitors.
24
Acknowledgements:
This study was supported by grants 13-39464J and 13-33324S provided by
the Grant Agency of the Czech Republic. This research has made use of
the observations provided by the ASM/RXTE team (Levine et al., 1996,
ApJ, 469, L33) and public data from Swift/BAT transient monitor provided
by the Swift/BAT team (Krimm et al., 2013, ApJS, 209, 14). This research
has also made use of the observations from the ASAS project (Pojmanski,
G.,1997,AcA,47,467), AAVSO International database (USA) (Henden 2013,
2014, 2015) and the AFOEV database (France). I thank the variable star
observers worldwide. This publication also made use of the data from the
Northern Sky Variability Survey created jointly by the Los Alamos
National Laboratory and University of Michigan. The NSVS was funded
by the Department of Energy, the National Aeronautics and Space
Administration, and the National Science Foundation. I also thank
Prof. Petr Harmanec for providing me with the code HEC13. The Fortran
source version, compiled version and brief instructions on how to use the
program can be obtained at http: //astro.troja.mff.cuni.cz/ftp/hec/HEC13/
25