X-ray astronomy with lobster-eye telescope v Vojtech Simon v v 1 Astronomical Institute, Academy of Sciences, 251 65 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

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

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

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

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

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 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) 6

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 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) 7

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: 17 arcmin Energy range: 15 – 150 keV (15 – 50 keV for monitoring of X-ray sources)

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

Crab nebula – calibrating source in the X-ray band Observing by ASM/RXTE monitor: 1.5 – 12 keV 1 Crab (E=1.5-12 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

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 1705-44 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

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 1731-260 (an X-ray binary) (Narita et al. 2001) AM Her (a polar-cataclysmic variable) (Matt et al. 2000) CAL 83 (a supersoft X-ray source) (Greiner et al. 1991) 12

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 1731-260 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

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

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

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 1731-260 Detectable by monitors in the long high state Duration of the high state: years – decades Flux = 0.1 Crab 16

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)

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 J1701-462 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

X-ray spectrum of active state (outburst) XTE J1701-462 19 ASM/RXTE BAT/Swift MCD (multicolor disk) Boundary layer PCA/RXTE spectrum Lin et al. (2009)

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 J1701-462 Prominent cyclic fluctuations in 1.5 – 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

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

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.

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

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

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 < 0.2 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

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

Acknowledgements: This study was supported by grant 13-39464J, 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