1. X-ray astronomy with lobster-eye telescopev
Vojtech Simon v v 1
Astronomical Institute, Academy of Sciences, Ondrejov,
Czech Republic
Czech Technical University in Prague, Faculty of Electrical
Engineering, Prague, Czech Republic 2
Talk: International Workshop on Astronomical X-Ray Optics, Prague,
Czech Republic, 5 – 8 Dec 2016

2. The importance of X-ray monitoring
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

3. Structure of low-mass X-ray binaries (LMXBs)
Donor – thermal radiation
(optical, IR)
Close vicinity of the compact object
Comptonizing cloud (inverse
Compton scattering – hard X-rays)
(boundary layer)
Outer disk region – thermal
radiation (UV, optical, IR)
Inner disk region – thermal radiation
(soft X-rays (E up to several keV))
Jets: synchrotron (radio, IR?)
Donor,
lobe-filling star
Mass stream
Compact object
(neutron star,
black hole)
Stream impact onto disk
Accretion disk 3

4. Structures of high-mass X-ray binaries (HMXBs)
Donor – thermal radiation
(UV, optical, IR) – often
dominant in the optical
Close vicinity of the compact
object: inverse Compton process,
bremsstrahlung (X-rays)
Disk (if exists) embedding the
compact object – thermal rad.
(spectral band in which it is
detected differs from system
to system)
Colliding winds: inverse Compton
process, brehmsstrahlung (X-rays)
Jets: synchrotron (radio, IR?)
Accretion modes:
Roche lobe overflow
Wind accretion
Periastron passage
Accretion disk
Apastron
Periastron
Okazaki (2005)
Donor,
filling
its lobe
Compact
object (NS,
BH, WD)
Donor,
underfilling
its lobe
Compact
object (NS,
BH, WD)
Donor,
underfilling
its lobe
Lobe size of the compact object 4

5. The importance of the long-term coverage
Transient X-ray sources:
wide-field monitoring of the sky is necessary (most transients are
discovered only by the first detection of their outburst)
outbursts are usually unpredictable – only their mean recurrence
time (cycle-length) can be determined from a long (years to
decades) series of observations
(Quasi)persistent X-ray sources:
objects often in the high state (luminous in X-rays)
transitions between the high/low states (and fluctuations in the high
state) are usually fast (~days) and unpredictable
Superorbital X-ray variations:
modulation - timescale of weeks and months 5

6. ASM/RXTE – previous 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 – 5 keV – 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) 6

7. MAXI/ISS – current 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 – 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) 7

8. BAT/Swift – current monitor for very hard X-rays
Krimm et
al. (2013) 8
Mission: NASA Swift (since 2004)
Aperture: Coded mask
Field of view: 1.4 sr (partially-coded)
Telescope PSF: arcmin
Energy range: – 150 keV
(15 – 50 keV for monitoring
of X-ray sources)

9. Previous or current X-ray monitors:
shadow cameras
slit cameras
coded mask
prepared Lobster-eye: novel type of optics for soft X-rays
Types of sources promising for detection by the testing type of Lobster
X-ray binaries of various subtypes:
High-mass X-ray binaries (HMXBs)
Low-mass X-ray binaries (LMXBs)
X-ray transients (XTs) - subtype of HMXBs and LMXBs
Persistent X-ray sources - subtype of HMXBs and LMXBs 9

10. Crab nebula – calibrating source in the X-ray band
Observing by ASM/RXTE monitor: – 12 keV
1 Crab (E= keV): F = 2.67 x 10-8 erg/cm2/s
Stable flux during the time interval of about 13 years of observing by
the same monitor
The moving averages show that the flux is almost stable within the
observing uncertainties. 10

11. Systematics of low-mass X-ray binaries (LMXBs)
Thermally
unstable disk
4U 1608–52
Increase of the time-averaged mass transfer rate
Transient source
Soft X-ray transient (SXT)
Thermally
unstable disk 4U
Transient source
Thermally stable disk 11
Ser X-1
Persistent source
Sequence according to van Paradijs (1996)
Change of the long-term activity from large-amplitude, isolated outbursts starting from the baseline quiescent state to the dominant relatively small fluctuations in the high state.
Only systems with the neutron star accretor are used.
ASM / RXTE
(1.5 – 1 2 keV) 5

12. Typical X-ray spectra of various types of X-ray binaries
Intensity is
set equal to
unity for the
peak flux.
Medipix
Medipix
Sources: KS (an X-ray binary) (Narita et al. 2001)
AM Her (a polar-cataclysmic variable) (Matt et al. 2000)
CAL (a supersoft X-ray source) (Greiner et al. 1991) 12

13. Typical X-ray spectrum of a low-mass X-ray binary in outburst (high state)
Simon (2012)
The highest intensity in
the soft X-ray band
Decrease of intensity
with growing energy –
it is better to construct
monitors observing in KS
Smooth lines:
fits by HEC13
ASCA
spectrum
Narita et
al. (2001)
Medipix
Medipix
Spectral changes can be measured by monitor (ASM/RXTE data (bands A, B, C) for
comparison) 13

14. Flux ~ 11 Crabs Sco X-1 1.5 – 12 keV The brightest X-ray binary
Comparison star
The brightest
X-ray binary
in the soft
X-ray band
Persistent
(emits X-ray
and optical
radiations
everytime)
Time evolution in the optical
1.5 – 12 keV
1 Crab = 76 ct/s
Flux ~
11 Crabs
The most suitable object for testing the X-ray instrument 14

15. Sco X-1 Medipix Histogram of soft X-ray flux
Flux of the system is variable,
but Sco X-1 is always very
bright in soft X-rays.
X-ray spectrum
The sensitivity region of Medipix
will be close to the peak of
luminosity of Sco X-1. 15

16. Examples of types of X-ray activity
Soft X-ray transient (SXT)
Aql X-1
X-ray flux is highly variable.
Detectable by monitors only in
outburst
Duration of outburst: weeks-months
Recurrence time (interval between
outbursts): months to years
Flux =
0.5 Crab
1 Crab = 76 ct/s
(Quasi)persistent X-ray
binary source KS
Detectable by monitors in the long
high state
Duration of the high state: years –
decades
Flux =
0.1 Crab 16

17. Aql X-1 – evolution of the hardness ratios during an outburst
1 Crab = 76 ct/s
Flux =
0.5 Crab
Evolution of the hardness ratios
with intensity during one intense outburst:
Hysteresis especially in HR2
(discrepancy between the rising
and decaying branches of the
outburst)
The hardest spectrum occurs in
the rising phase of the outburst. 17
ASM/RXTE hardness ratios:
HR1 = Flux (3-5 keV) / Flux (1.5-3 keV)
HR2 = Flux (5-12 keV) / Flux (3-5 keV)

18. XTE J1701-462 ASM/RXTE (1.5 – 12 keV) BAT/Swift (15 – 50 keV)
Evolution of the outburst seen by the X-ray monitors (one-day means)
XTE J
A very long (~600 d) outburst of the neutron star SXT
Simon (2014, New Astronomy)
Flux =
0.5 Crab
Residuals of the fit smoothed by
the two-sided moving averages
with various filter half-widths Q
conjunction
Number of data in each mean
ASM/RXTE (1.5 – 12 keV) BAT/Swift (15 – 50 keV) 18

19. X-ray spectrum of active state (outburst) XTE J1701-46219
ASM/RXTE
BAT/Swift
MCD (multicolor
disk)
Boundary layer
PCA/RXTE spectrum
Lin et al. (2009)

20. XTE J1701-462 Dependence of the intensity variations on the X-ray band
1.5-3 keV
3-5 keV
5-12 keV
ASM/
RXTE
XTE J
Prominent cyclic fluctuations
in – 12 keV (HEC13 fits
with identical parameters)
Vertical lines: times of the
minima of IA (1.5 – 3 keV) in
the fit.
Intensity variations in the BAT
data (15 – 50 keV) –
– no cyclic modulation, only
rapid fluctuations
BAT/Swift
15–50 keV
Simon (2014, New Astronomy) 20

21. Analysis of data of faint X-ray sources
from the monitor
Binning of X-ray data enables to analyze faint sources (although
it smooths the profiles of the features).
Smoothing the X-ray data through the orbital modulation of the
source
Determining the mean levels of X-ray intensity in some states of
activity – possible e.g. for the high states of the sources
Simultaneous monitoring of the same object with several
monitors:
– possibility to determine the hardness ratio of X-ray emission 21

22. Composed view of X-ray sky in soft X-rays (1.5 – 12 keV)
All-Sky Monitor (ASM/RXTE)
The brightest
X-ray source
Z Cam – dwarf nova
Calibration source
V1500 Cyg – classical nova
View toward the Galactic center 22
Most detected objects are binary systems with mass-accreting neutron star or black hole.

23. Distribution of X-ray binaries in the Galaxy
Most low-mass X-ray binaries
concentrate toward the galactic
equator and the galactic bulge.
High-mass X-ray systems –
large accumulation toward the
equator, weak toward the bulge. 23

24. Field of view of lobster
Field of the Galactic
center (most low-
mass X-ray binaries
concentrate here).
Several such objects
will be in the field of
view of lobster.
Field of
view
Field of
view 24

25. Conclusions (I)
X-ray emission of X-ray binaries – continuum dominates (often
inverse Compton mechanism) – small changes of the band
of sensitivity (e.g. by E of several keV) are not important for the
performance of LOBSTER.
These objects are most luminous in soft X-rays (E ~ 2 keV).
X-rays are heavily absorbed for E < keV (interstellar
absorption by the neutral Hydrogen) – thus usually
unobservable by the X-ray telescopes
X-ray flux of the X-ray spectra of X-ray binaries only gradually
decreases from the peak at E ~ 1 – 3 keV toward the higher
energies. 25

26. Conclusions (II)
The accompanying X-ray spectral variations (changes of the
hardness ratios) (e.g. when various states of activity are
compared) are measurable by the monitor.
We emphasize the important role of the spectral region of the
X-ray monitor:
Example: the very hard X-ray band like the one in BAT/Swift
sometimes even maps a different activity than the monitor
observing in soft X-rays (e.g. several keV).
Identification of objects in the LOBSTER data – also possible to
compare their position and state of activity with the simultaneous
observations with other currently observing X-ray monitors (e.g.
BAT/Swift, MAXI/ISS). 26

27. Acknowledgements:
This study was supported by grant J, 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).
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/ 27