1. Vojtech Simon v v Monitoring of low-mass binary X-ray sources Astronomical Institute, Academy of Sciences 251 65 Ondrejov, Czech Republic Talk: AXRO, Dec 10–13, 2012 v

2. Transient X-ray sources: Transient X-ray sources: wide-field monitoring of the sky is necessary (most transients are wide-field monitoring of the sky is necessary (most transients are discovered only by the first detection of their outburst) discovered only by the first detection of their outburst) outbursts are usually unpredictable – only their mean recurrence outbursts are usually unpredictable – only their mean recurrence time (cycle-length) can be determined from a long (years to time (cycle-length) can be determined from a long (years to decades) series of observations decades) series of observations (Quasi)persistent X-ray sources: (Quasi)persistent X-ray sources: transitions between the high/low states (and fluctuations in the high transitions between the high/low states (and fluctuations in the high state) are usually fast (~days) and unpredictable state) are usually fast (~days) and unpredictable Superorbital X-ray variations: Superorbital X-ray variations: timescale of weeks and months timescale of weeks and months stability of the cycle depends on the parameters of the system stability of the cycle depends on the parameters of the system 2 The importance of the long-term coverage (I) The importance of the long-term coverage (I)

3. Occasional pointing in any spectral band is not enough: many pieces of information on the time evolution are lost in any many pieces of information on the time evolution are lost in any spectral band spectral band time allocation has to be justified (search for unexpected behavior time allocation has to be justified (search for unexpected behavior of the object is usually not approved) 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. 3 We will discuss the activity in various X-ray bands and how monitoring helps. The importance of the long-term coverage (II) The importance of the long-term coverage (II)

4. Donor, lobe-filling star Mass stream Compact object (NS, BH) Accretion disk Donor – thermal radiation Donor – thermal radiation (optical, IR) (optical, IR) Typical structure of low-mass X-ray binaries (LMXBs) Outer disk region – thermal Outer disk region – thermal radiation (UV, optical, IR) radiation (UV, optical, IR) Jets: synchrotron (radio, IR?) Jets: synchrotron (radio, IR?) 4 Inner disk region – thermal radiation Inner disk region – thermal radiation (soft X-rays (E up to several keV)) (soft X-rays (E up to several keV)) Comptonizing cloud around the Comptonizing cloud around the compact object (inverse Compton compact object (inverse Compton scattering – hard X-rays) scattering – hard X-rays)

5. Types of the long-term activity Outbursts vs. quiescence – mass accumulates in the outer regions Outbursts vs. quiescence – mass accumulates in the outer regions of the accretion disk during quiescence of the accretion disk during quiescence – strong accretion of matter from the disk – strong accretion of matter from the disk onto the central compact object during onto the central compact object during outburst outburst High/low state transitions – transient decrease of the mass transfer High/low state transitions – transient decrease of the mass transfer rate rate Cyclic (superorbital) changes of luminosity (variations of the disk Cyclic (superorbital) changes of luminosity (variations of the disk shape and interaction of the disk with the inflowing mass stream shape and interaction of the disk with the inflowing mass stream from the donor) from the donor) 5 Low-mass binary X-ray sources – excellent targets for monitoring Low-mass binary X-ray sources – excellent targets for monitoring

6. What can we expect from data from X-ray monitors? What can we expect from data from X-ray monitors? 6 Various physical processes produce specific large-amplitude variations of X-ray Various physical processes produce specific large-amplitude variations of X-ray luminosity on a timescale of days, weeks, to years and decades. luminosity on a timescale of days, weeks, to years and decades. The characteristic features (e.g. outbursts, high/low state transitions) can be The characteristic features (e.g. outbursts, high/low state transitions) can be investigated even in a single-band X-ray light curve (monitors often work with a investigated even in a single-band X-ray light curve (monitors often work with a single band (typically in soft X-rays, a few keV)). single band (typically in soft X-rays, a few keV)). Even some model predictions are already available Even some model predictions are already available Dividing the observed spectral region into several bands (e.g. used in ASM/RXTE) Dividing the observed spectral region into several bands (e.g. used in ASM/RXTE) or simultaneous usage of various monitors (e.g. ASM/RXTE & BAT/Swift) helps to or simultaneous usage of various monitors (e.g. ASM/RXTE & BAT/Swift) helps to distinguish between various processes influencing the luminosity. distinguish between various processes influencing the luminosity. Properties of the basic outburst light curves in soft X-rays (model: Dubus et al. 2001) Dependence of the outburst profile on irradiation of profile on irradiation of the disk the disk Simulated cycle-length of the outbursts Mass accretion rate (g/s)

7. Mission: RXTE (Rossi X-Ray Timing Explorer) Three wide-angle shadow cameras equipped with proportional counters, each with 6 x 90 degrees FOV Collecting area: 90 cm 2 Detector: Xenon proportional counter, position-sensitive Energy range: 1.5 – 12 keV in three bands: 1.5 – 3 keV 3 – 5 keV 5 – 12 keV Time resolution: 80% of the sky every 90 min (but one-day means are usually used to increase the sensitivity) Spatial resolution: 3 x 15 arcmin Sensitivity: 30 mCrab (but obs. show ~13 mCrab for one-day means) Operation: January 1996 – 2012 ASM 7 ASM/RXTE - monitor for soft X-rays ASM/RXTE - monitor for soft X-rays

8. Smooth lines: fits by HEC13 Spectral changes can be measured in the ASM/RXTE data (band A, band B, band C) Spectral changes can be measured in the ASM/RXTE data (band A, band B, band C) Absorption of X-rays can be measured even by monitors – it is predicted to Absorption of X-rays can be measured even by monitors – it is predicted to influence mostly the softest band influence mostly the softest band 8 KS 1731-260 Typical X-ray spectrum of a low-mass X-ray binary in outburst (a high state) Typical X-ray spectrum of a low-mass X-ray binary in outburst (a high state) Simon (2012) The highest intensity in The highest intensity in the soft X-ray band the soft X-ray band Steep decrease of intensity Steep decrease of intensity with growing energy – it is with growing energy – it is better to construct monitors better to construct monitors observing in soft X-ray observing in soft X-ray band band Narita et al. (2001) ASCA ASCAspectrum

9. V1500 Cyg – classical nova All-Sky Monitor (ASM/RXTE) Z Cam – dwarf nova View toward the Galactic center Most detected objects are binary systems with mass-accreting neutron star or black hole. 9 Composed view of X-ray sky in soft X-rays (1.5–12 keV)

10. Mission: NASA Swift Aperture: Coded mask Detecting area: 5200 cm 2 Field of view: 1.4 sr (partially-coded) Detection elements: 256 modules of 128 elements Telescope PSF: 17 arcmin Energy range: 15 - 150 keV (15 - 50 keV is used for monitoring of X-ray sources) Operation: since 2004 10 BAT/Swift – monitor for very hard X-rays

11. Most X-ray binary sources concentrate toward the Galactic plane. 11 BAT/Swift monitor Composed view of X-ray sky in very hard X-rays (15–50 keV) Tueller et al. (2010)

12. ASM/RXTE (one-day means) 1.5–12 keV 1996-2008 Outbursts in NS soft X-ray transient Aql X-1 Progress with the recent monitors: monitors: Better-defined features Better-defined features of the intense events of the intense events (e.g. outbursts or high (e.g. outbursts or high states generally) states generally) Minor outbursts can be Minor outbursts can be resolved (important for resolved (important for the assessment of the the assessment of the activity of the object) activity of the object) 12 Vela 5B (10-day means) 3–12 keV 1969-1976 Priedhorsky & Terrell (1984) Counts / s X-ray light curves from monitors: the effect of improving sensitivity

13. Decaying branch (measure of propagation of the so-called cooling front across the disk) – the most stable part of outburst, its slope is independent of the peak luminosity of outburst. ASM/RXTE data (1.5 – 12 keV) (1.5 – 12 keV) Simon (2002) 13 Properties of the ensemble of outbursts in soft X-rays (1.5–12 keV) Aql X-1 = V1333 Aql Aql X-1 = V1333 Aql Evolution of hardness ratios with intensity during a set of outbursts. Big circle: peak of a given outburst ASM/RXTEhardness ratios: ASM/RXTE hardness ratios: HR1 = Flux (3-5 keV) / Flux (1.5-3 keV) HR2 = Flux (5-12 keV) / Flux (3-5 keV)

14. 14 Relation between the O-C curve and T C : Linear profile of the O-C curve – constant T C Parabolic profile of O-C – linear change of T C Variations of the recurrence time T C (cycle-length) of outbursts are large, but not chaotic – occasional jumps are not explicable by the evolutionary processes. This SXT serves as a clear evidence that the variations of I max and T C are reliable only if also the faint outbursts are detected. Times of the peaks of the outbursts determined from the observations of different monitors are often in good mutual agreement. Simon (2002, 2010) Aql X-1 = V1333 Aql Aql X-1 = V1333 Aql Peak intensity of outburst Evolution of residuals of the mean recurrence time T C of outbursts

15. WWZ-transform (method of Foster 1996) Time evolution of the recurrence time 15 Peak intensity of outburst Evolution of the residuals of the mean recurrence time the mean recurrence time Recurrence time of outbursts Investigation of a relation between two parameters easily measurable by the monitor – recurrence time of outbursts, and the peak soft X-ray intensity: T C ~ 136 days, modulated by a cycle-length of about 5.4 years. The most luminous outbursts occur after the time of the longest T C. WWZ-transform can be used even for seach for cycles in so transient events like outbursts Simon(2008) Soft X-ray transient GRS 1747–312

16. 16 GX 339 – 4 Black hole transient ASM/RXTE (1.5-12 keV) BAT/Swift (15-50 keV) Start of outburst Peak of outburst Very large differences between the profiles of the outburst in the soft and the very hard X-rays: Start of the outburst almost simultaneous for both bands State transition close to the peak luminosity in both bands Very fast decay of the BAT luminosity The outburst lasts much longer in the BAT band Faint very hard outburst Tang et al. (2011)

17. 17 XTE J1701-462 A very long (~600 d) outburst of the neutron star SXT ASM/RXTE (1.5-12 keV) BAT/Swift (15-50 keV) Residuals of the fit smoothed by the two-sided moving averages with various filter half-widths Q Number of data in each mean conjunction Z and atoll source A borderline source between transient and persistent neutron star systems

18. 18 Typical X-ray spectrum during outburst XTE J1701-462 ASM/RXTE Ding et al. (2011) PCA/RXTEspectrum BAT/Swift

19. 19 Time evolution of the smoothed residuals of the ASM/RXTE and BAT/Swift measurements during the outburst The largest fluctuations during the primary peak of soft X-ray luminosity XTE J1701-462 Primary peak of soft X-ray luminosity Plateau of soft X-ray luminosity Final decay of luminosity

20. 20 XTE J1701-462 Time evolution of hardness ratio HR = I ASM / I BAT during decline of the outburst HR was determined for the data included in a segment of 30 days Primary peak of soft X-ray luminosity Final decay of luminosity

21. 21 XTE J1701-462 HEC13 fit to the fluctuations in the one-day means in the ASM/RXTE light curve HEC13 fit to the fluctuations in the one-day means in the ASM/RXTE light curve The times of maxima and minima of the cycle in segment A and segment B were The times of maxima and minima of the cycle in segment A and segment B were determined from the HEC13 fit. determined from the HEC13 fit. The intervals of +/-25 days from the conjunction with the Sun are marked. The intervals of +/-25 days from the conjunction with the Sun are marked.

22. 22 Segment A Segment B Time evolution of cycle WWZ (method of Foster 1996) XTE J1701-462 WWZ-transform of the HEC13 fit to the modulation in the ASM data The best cycle-length (in days). Only the segments with the amplitude larger than 15 percents of its peak value. Amplitude of the cycle-length in segments A and B (the horizontal lines - amplitude of 15 percents of its peak value) Weighted wavelet Z-transform of the X-ray light curve during the outburst Residuals of the HEC13 fit to the ASM data averaging through the modulation.

23. 23 Distance between dot-dashed lines: the length of the cycle used in the ephemeris (21 days). XTE J1701-462 O-C diagram for the times of the maxima and minima of the ASM/RXTE intensity in the cycle during the outburst (segments A and B). Maxima of intensity (closed circles) Minima of intensity (open circles)

24. Segment of the ASM light curve of the Segment of the ASM light curve of the outburst with the large-amplitude outburst with the large-amplitude fluctuations fluctuations Cyclic fluctuations are prominent Cyclic fluctuations are prominent in all ASM bands (HEC13 fits with in all ASM bands (HEC13 fits with identical parameters) identical parameters) Vertical lines: times of the minima of Vertical lines: times of the minima of I A in the fit. I A in the fit. ASM hardness ratios determined from ASM hardness ratios determined from the fits to I A, I B, and I C – absorption of the fits to I A, I B, and I C – absorption of X-rays is NOT dominant X-rays is NOT dominant Accompanying intensity variations in Accompanying intensity variations in the BAT data – no cyclic modulation, the BAT data – no cyclic modulation, only rapid fluctuations only rapid fluctuations 24 XTE J1701-462 Search for the nature of the modulation the modulation

25. 25 XTE J1701-462 Time evolution of cycle ASM/RXTE (1.5-12 keV) BAT/Swift (15-50 keV) Segment A Segment B Striking discrepancy between the behavior in the soft and the hard X-ray bands

26. 26 Cygnus X-2 Z source: persistent neutron star system ASM/RXTE (1.5-12 keV) BAT/Swift (15-50 keV) Striking discrepancy between the character of activity in the soft and the hard X-ray bands: Soft band – superorbital variations Hard band – only rapid large-amplitude fluctuations

27. 27 Cygnus X-2 WWZ-transform of the one-day means of the ASM/RXTE intensity Amplitude of this cycle-length The best cycle-length (in days) - only segments with amplitude larger than 20 percents of its peak value WWZ-transform of the HEC13 fit to the ASM curve.

28. 28 Cygnus X-2 ASM/RXTE (1.5-12 keV) BAT/Swift (15-50 keV) Striking discrepancy between the behavior in the soft and the hard X-ray bands

29. 29 4U 1820-30 Atoll source: NS persistent source Superorbital modulation probably due to Mazeh & Shaham mechanism Superorbital modulation probably due to Mazeh & Shaham mechanism (Mazeh & Shaham 1979;Chou & Grindlay (Mazeh & Shaham 1979; Chou & Grindlay 2001) State transitions occur during some episodes of minimum of soft X-ray luminosity ASM/RXTE (1.5-12 keV) BAT/Swift (15-50 keV)

30. 30 4U 1820-30 ASM/RXTE one-day means of intensity in ASM/RXTE one-day means of intensity in the 1.5-12 keV band the 1.5-12 keV band Triangles – times of maxima and minima Triangles – times of maxima and minima of intensity in the 172 d cycle (Mazeh & of intensity in the 172 d cycle (Mazeh & Shaham mechanism) Shaham mechanism) Profile of the cycle is smoothed by the Profile of the cycle is smoothed by the moving averages for Q =17 days moving averages for Q =17 days Vertical bars: prominent and well covered Vertical bars: prominent and well covered episodes of brief low states episodes of brief low states Simon (2003)

31. 31 4U 1820-30 Simon (2003) ASM/RXTE light curve folded with ASM/RXTE light curve folded with the 172 day cycle of activity (the the 172 day cycle of activity (the years 1996-2002) years 1996-2002) Folded data are smoothed by the Folded data are smoothed by the two-sided moving averages for two-sided moving averages for the filter half-widths Q =0.05, 0.06, the filter half-widths Q =0.05, 0.06, 0.08, 0.10, and 0.12 phase. 0.08, 0.10, and 0.12 phase. Skewness of the residuals Number of data Smoothed residuals Smoothed residuals of the fits

32. Evolution of the hardness ratios with the 172 day cycle of activity (Mazeh & Shaham mechanism) Smoothing by the two-sided moving averages for Q =0.05 and 0.08 phase. 32 4U 1820-30 Simon(2003) ASM/RXTEhardness ratios: ASM/RXTE hardness ratios: HR1= Flux (3-5 keV) / Flux (1.5-3 keV) HR2= Flux (5-12 keV) / Flux (3-5 keV)

33. 33 4U 1820-30 Simon (2003) Dependence of HR 1 and HR 2 on the Dependence of HR 1 and HR 2 on the 1.5-12 keV intensity 1.5-12 keV intensity Smooth lines: HEC13 fits to the whole Smooth lines: HEC13 fits to the whole data set. data set. Empty circles: the arithmetic means Empty circles: the arithmetic means for three episodes of deep brief low for three episodes of deep brief low states (BLS) superimposed on the states (BLS) superimposed on the 172 day cycle 172 day cycle Individual data of BLS – dark (blue) Individual data of BLS – dark (blue) points. points. Hardness ratio vs. intensity

34. Dense series of X-ray observations from monitors covering the Dense series of X-ray observations from monitors covering the intervals of at least several years are necessary to investigate intervals of at least several years are necessary to investigate the properties of the long-term activity of binary X-ray sources: the properties of the long-term activity of binary X-ray sources: to resolve the outbursts and/or transitions between high and to resolve the outbursts and/or transitions between high and low states low states to place these events in the context of the long-term activity to place these events in the context of the long-term activity of a given system of a given system to form a representative ensemble of events in to form a representative ensemble of events in (a) a given X-ray binary system, (a) a given X-ray binary system, (b) in a type of X-ray binary systems (b) in a type of X-ray binary systems This is important for our understanding of the physical processes involved in these systems. 34 General conclusions (I)

35. Profiles of features of the long-term activity are measurable by Profiles of features of the long-term activity are measurable by the monitors – a very large variety exists. Search for the the monitors – a very large variety exists. Search for the common features is needed. common features is needed. Search for the accompanying spectral variations (changes of Search for the accompanying spectral variations (changes of hardness ratios are measurable by some monitors – e.g. hardness ratios are measurable by some monitors – e.g. ASM/ RXTE or a combination of simultaneous ASM/ RXTE & ASM/ RXTE or a combination of simultaneous ASM/ RXTE & BAT/ Swift observing). BAT/ Swift observing). We emphasize the very important role of the spectral region of We emphasize the very important role of the spectral region of the X-ray monitor – a very hard X-ray band like the one in the X-ray monitor – a very hard X-ray band like the one in BAT/ Swift sometimes maps quite a different activity (probably BAT/ Swift sometimes maps quite a different activity (probably coming from a different spectral component). coming from a different spectral component). 35 General conclusions (II)

36. Time evolution of the recurrence time T C of outbursts is very little Time evolution of the recurrence time T C of outbursts is very little studied mainly because of the lack of data. Only very few SXTs studied mainly because of the lack of data. Only very few SXTs with quite short T C of less than a year could be investigated so with quite short T C of less than a year could be investigated so far. far. Important: measuring T C and its time evolution can often be Important: measuring T C and its time evolution can often be made even if the individual monitors work with made even if the individual monitors work with different spectral bands – the time of the outburst different spectral bands – the time of the outburst in SXT is often comparable for the soft and the in SXT is often comparable for the soft and the hard X-ray band. Very long time segments can be hard X-ray band. Very long time segments can be thus investigated. thus investigated. 36 General conclusions (III)

37. Acknowledgements: Acknowledgements: This research has made use of the observations provided by the ASM/RXTE team. I also acknowledge the use of public data from the Swift data archive. This study was supported by the grant 205/08/1207 provided by the Grant Agency of the Czech Republic and the project RVO:67985815. I made use of the code developed by Dr. G. Foster and available at http: //www.aavso.org/data/software/wwz.shtml. I thank Prof. P. Harmanec for providing me with the code HEC13. The Fortran source version, compiled version and brief instructions how to use the program can be obtained via http: //astro.troja.mff.cuni.cz/ftp/hec/HEC13/. Some images come from the web pages of HEASARC. 37