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Vojtech Simon

Monitoring of low-mass binary X-ray sources. v. Vojtech Simon. v. Astronomical Institute, Academy of Sciences 251 65 Ondrejov, Czech Republic. v. Talk: AXRO, Dec 10–13, 2012. The importance of the long-term coverage (I). Transient X-ray sources:

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Vojtech Simon

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

  2. The importance of the long-term coverage (I) • 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: • transitions between the high/low states (and fluctuations in the high • state) are usually fast (~days) and unpredictable • Superorbital X-ray variations: • timescale of weeks and months • stability of the cycle depends on the parameters of the system 2

  3. The importance of the long-term coverage (II) • 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. We will discuss the activity in various X-ray bands and how monitoring helps. 3

  4. Typical structure of low-mass X-ray binaries(LMXBs) • Donor – thermal radiation • (optical, IR) • Comptonizing cloud around the • compact object (inverse Compton • scattering – hard X-rays) • 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 (NS, BH) Accretion disk 4

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

  6. What can we expect from data from X-ray monitors? • Various physical processes produce specific large-amplitude variations of X-ray • luminosity on a timescale of days, weeks, to years and decades. • 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 • single band (typically in soft X-rays, a few keV)). • Even some model predictions are already available • 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 • distinguish between various processes influencing the luminosity. Properties of the basic outburst light curves in soft X-rays Dependence of the outburst profile on irradiation of the disk Simulated cycle-length of the outbursts Mass accretion rate (g/s) Mass accretion rate (g/s) (model: Dubus et al. 2001) 6

  7. ASM/RXTE - monitor for soft X-rays • 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 cm2 • 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

  8. Typical X-ray spectrum of a low-mass X-ray binary in outburst (a high state) Simon (2012) • The highest intensity in • the soft X-ray band • Steep decrease of intensity • with growing energy – it is • better to construct monitors • observing in soft X-ray • band KS 1731-260 ASCA spectrum Smooth lines: fits by HEC13 Narita et al. (2001) • 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 • influence mostly the softest band 8

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

  10. BAT/Swift – monitor for very hard X-rays • Mission: NASA Swift • Aperture: Coded mask • Detecting area: 5200 cm2 • 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

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

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

  13. Properties of the ensemble of outbursts in soft X-rays (1.5–12 keV) Aql X-1 = V1333 Aql ASM/RXTE data (1.5 – 12 keV) 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. Evolution of hardness ratios with intensity during a set of outbursts. Big circle: peak of a given outburst ASM/RXTEhardness ratios: HR1 = Flux (3-5 keV) / Flux (1.5-3 keV) HR2 = Flux (5-12 keV) / Flux (3-5 keV) Simon (2002) 13

  14. Aql X-1 = V1333 Aql Relation between the O-C curve and TC: Linear profile of the O-C curve – constant TC Parabolic profile ofO-C– linear change of TC • Variations of the recurrence time TC • (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 Imax and TC 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. Peak intensity of outburst Evolution of residuals of the mean recurrence time TC of outbursts Simon (2002, 2010) 14

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

  16. GX 339 – 4 Black hole transient ASM/RXTE (1.5-12 keV) Faint very hard outburst 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 Tang et al. (2011) 16

  17. Z and atoll source A borderline source between transient and persistent neutron star systems XTE J1701-462 A very long (~600 d) outburst of the neutron star SXT 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) 17

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

  19. XTE J1701-462 Primary peak of soft X-ray luminosity Plateau of soft X-ray luminosity Final decay of luminosity • 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 19

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

  21. XTE J1701-462 • 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 • determined from the HEC13 fit. • The intervals of +/-25 days from the conjunction with the Sun are marked. 21

  22. Time evolution of cycle XTE J1701-462 Weighted wavelet Z-transform of the X-ray light curve during the outburst Segment A Segment B WWZ-transform of the HEC13 fit to the modulation in the ASM data WWZ (method of Foster 1996) Residuals of the HEC13 fit to the ASM data averaging through the modulation. 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) 22

  23. 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). XTE J1701-462 Maxima of intensity (closed circles) Minima of intensity (open circles) Distance between dot-dashed lines: the length of the cycle used in the ephemeris (21 days). 23

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

  25. XTE J1701-462 Time evolution of cycle Segment A Segment B 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 25

  26. Cygnus X-2 Z source: persistent neutron star system ASM/RXTE (1.5-12 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 BAT/Swift (15-50 keV) 26

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

  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 28

  29. 4U 1820-30 Atoll source: NS persistent source ASM/RXTE (1.5-12 keV) BAT/Swift (15-50 keV) Superorbital modulation probably due to Mazeh & Shaham mechanism (Mazeh & Shaham 1979;Chou & Grindlay2001) State transitions occur during some episodes of minimum of soft X-ray luminosity 29

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

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

  32. ASM/RXTEhardness ratios: HR1= Flux (3-5 keV) / Flux (1.5-3 keV) HR2= Flux (5-12 keV) / Flux (3-5 keV) 4U 1820-30 Simon (2003) • Evolution of the hardness ratioswith 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

  33. 4U 1820-30 Hardness ratio vs. intensity • Dependence of HR1 and HR2 on the • 1.5-12 keV intensity • Smooth lines: HEC13 fits to the whole • data set. • Empty circles: the arithmetic means • for three episodes of deepbrief low • states (BLS) superimposed on the • 172 day cycle • Individual data of BLS – dark (blue) • points. Simon (2003) 33

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

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

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

  37. 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

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