Vojtech Simon v Monitoring of the X-ray long- term activity of Aquila X-1 v Astronomical Institute, The Czech Academy of Sciences, Ondrejov, Czech Republic Ondrejov, Czech Republic v Czech Technical University in Prague, FEE, Prague, Czech Republic Talk: 12 th INTEGRAL/BART Workshop, April 2015, Karlovy Vary, Czech Republic

Transient X-ray sources: Transient X-ray sources:  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 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 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 2 The importance of the X-ray monitoring (I) The importance of the X-ray monitoring (I)

Occasional pointing in any spectral band is not enough: 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) 3 We will discuss the activity in X-rays and how monitoring helps. The importance of the X-ray monitoring (II) The importance of the X-ray monitoring (II)

What can we expect from data from X-ray monitors? What can we expect from data from X-ray monitors? 4 Various physical processes produce specific large-amplitude Various physical processes produce specific large-amplitude variations of X-ray luminosity on a timescale of days, weeks, to variations of X-ray luminosity on a timescale of days, weeks, to years and decades. years and decades. The characteristic features (e.g. outbursts) can be investigated The characteristic features (e.g. outbursts) can be investigated even in a single-band X-ray light curve (monitors often work with even in a single-band X-ray light curve (monitors often work with a single band (typically in soft X-rays, a few keV)). a single band (typically in soft X-rays, a few keV)). Even some model predictions are already available: Even some model predictions are already available: 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)

5 Test of stability of monitors with time LMC X-4 4U The selection criterion:  the X-ray activity of such systems is already known  the light curves display smooth profiles  the values of intensities measured by a given monitor are similar to those of Aql X-1 BAT/Swift Simon (2014, New Astronomy) Aql X-1 4U

Donor – thermal radiation Donor – thermal radiation (optical, IR) (optical, IR) Outer disk region – thermal Outer disk region – thermal radiation (UV, optical, IR) radiation (UV, optical, IR) Jets: synchrotron (radio, IR?) Jets: synchrotron (radio, IR?) 6 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)) Close vicinity of the compact object Close vicinity of the compact object Comptonizing cloud (inverse Comptonizing cloud (inverse Compton scattering – hard X-rays) Compton scattering – hard X-rays) Structure of Aql X-1 Donor, Donor, lobe-filling star Mass stream Accretion disk Stream impact onto disk Compact object (neutron star)

Monitoring: to obtain sufficient number of well-mapped outbursts in a given SXT to determine a meaningful ensemble of outbursts Maitra & Bailyn (2008 Maitra & Bailyn (2008) Simultaneous observations of the outburst in the optical and X-ray bands: duration of outburst in various bands and X-ray/optical ratio may differ substantially – mass outflow from the inner disk region? 7 Soft X-ray Optical Optical Optical Aql X-1 – relation between X-ray and optical luminosity

8 Typical X-ray spectra during various phases of outburst Soft state Hard state Maccarone & Coppi (2003)  X-ray spectrum undergoes large variations during outburst.  Continuum by far dominates.  Most emission is radiated in the softest band around the peak of the outburst. of the outburst. Aql X-1 – X-ray spectra

ASM / RXTE ASM / RXTE (one-day means) (one-day means) 1.5 – 12 keV Effect of increase of the monitor’s sensitivity (similar spectral bands): Better-defined features Better-defined features of the intense outbursts of the intense outbursts 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) 9 Vela 5B Vela 5B (10-day means) (10-day means) 3 – 12 keV – 12 keV Priedhorsky & Terrell (1984) Counts / s Aql X-1 – improving the sensitivity of the monitor

 Peak intensity and outburst morphology largely vary for the individual outbursts in a given SXT: monitoring is necessary individual outbursts in a given SXT: monitoring is necessary 10 Aql X-1 = V1333 Aql ASM/RXTE 1.5–12 keV Simon (2002) Bright outburst Faint outburst Aql X-1 – examples of outbursts in X-rays

Fitted linear part of decay Decaying branch (measure of the 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. Residuals of the fit (time-scale 2x larger) Simon(2002) 11 Aql X-1 = V1333 Aql Aql X-1 – properties of outbursts in ASM/RXTE data ( keV)

12 Evolution of the hardness ratios with intensity during one intense outburst:  Hysteresis especially in HR2  The hardest spectrum occurs in the rising phase of the outburst the rising phase of the 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) Simon (2002, 2014) Aql X-1 – evolution of the hardness ratios during an outburst Aql X-1 – evolution of the hardness ratios during an outburst

13 Open circle: peak intensity of an outburst  The more intense the outburst, the larger the outburst, the larger the hardness ratio of the hardness ratio of the peak intensity peak intensity  A very hard state occurs in the start and the end in the start and the end of the outburst of the outburst Simon (2002) Aql X-1 – evolution of the hardness ratios Aql X-1 – evolution of the hardness ratios in an ensemble of outbursts

14 Outbursts are visible in a large range of energies (2 – 50 keV) Profile of the outburst in the 15 – 50 keV band largely differs from that in softer X-rays Aql X-1 – recent activity: MAXI & BAT/Swift data Aql X-1 – recent activity: MAXI & BAT/Swift data BAT/Swift 15 – 50 keV MAXI

MJD – hard state after the end of the main peak of outburst No.1 How to pick up LMXB from several heavily sampled Gaia data points Aql X-1 – definition of the object in MAXI observations 15 Aql X-1 is well defined in all bands of the monitor (no blending)

16 Aql X-1 – evolution of outburst No.1 Aql X-1 – evolution of outburst No.1 Outburst visible in a large range of energies (2 – 50 keV) Profile of the outburst in the 15 – 50 keV band largely differs from that in softer X-rays Strong peak in soft X- rays is absent in the BAT data MAXI BAT/Swift 15 – 50 keV

The hardest spectrum occurs in the rising phase of the outburst MAXIhardness ratios: MAXI hardness ratios: HR1= Flux (4-10 keV) / Flux (2-4 keV) HR2= Flux (10-20 keV) / Flux (4-10 keV) Aql X-1 – evolution of the hardness ratio during outburst Aql X-1 – evolution of the hardness ratio during outburst Evolution of hardness ratio HR1 with intensity HR1 with intensity Time evolution of hardness ratio HR1 ratio HR1 17 Primary peak of intensity of intensity

MJD – early rise of a very bright outburst (No.3) How to pick up LMXB from several heavily sampled Gaia data points Aql X-1 – definition of the object in MAXI observations 18 Aql X-1 is well defined in all bands of the monitor (no blending)

19 Aql X-1 – evolution of outburst No.3 Aql X-1 – evolution of outburst No.3 Outburst visible in a large range of energies (2 – 50 keV) Profile of the outburst in the 15 – 50 keV band largely differs from that in softer X-rays Strong peak in hard X- rays coincides with the rising phase of the soft X-ray outburst MAXI BAT/Swift 15 – 50 keV

The hardest spectrum occurs in the rising phase of the outburst MAXIhardness ratios: MAXI hardness ratios: HR1= Flux (4-10 keV) / Flux (2-4 keV) HR2= Flux (10-20 keV) / Flux (4-10 keV) Aql X-1 – evolution of the hardness ratio during outburst Aql X-1 – evolution of the hardness ratio during outburst 20 Evolution of hardness ratio HR1 with intensity HR1 with intensity Time evolution of hardness ratio HR1 ratio HR1

21 Superposition of two largely different outbursts:  Largely different hardness ratios hardness ratios can occur in the can occur in the peak intensity of peak intensity of the outburst the outburst  Decaying branch can evolve with can evolve with different hardness different hardness ratios ratios Aql X-1 – evolution of the hardness ratios during outburst Aql X-1 – evolution of the hardness ratios during outburst  The hardest state occurs in the beginning of the outburst no matter how large the peak intensity will be

22 Relation between O-C curve and T C : Linear profile of the O-C curve (regardless of the slope): constant T C Parabolic profile of O-C: linear change of T C Aql X-1 – the recurrence time of outbursts Aql X-1 – the recurrence time of outbursts 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. Times of the peaks of the outbursts determined from the observations of different monitors are often in good mutual agreement.

 The profiles in a given X-ray band differ for the individual outbursts  Hysteresis of the hardness ratios with the progress of the outburst is visible is visible  The hardest X-ray spectrum often in the beginning of the outburst  Large structural changes occur during the rise to the peak intensity of the outburst (the evolution in the 15–50 keV band largely differs of the outburst (the evolution in the 15–50 keV band largely differs from the evolution in softer X-ray bands) from the evolution in softer X-ray bands)  The recurrence time of the outbursts is largely variable but the occurrence of the outbursts is NOT chaotic – the profile of the O-C diagram shows only small epoch-to-epoch fluctuations. – The O-C curve does not differ significantly for the individual – The O-C curve does not differ significantly for the individual monitors. monitors. How to pick up LMXB from several heavily sampled Gaia data points Conclusions 23

Acknowledgements: Acknowledgements: This study was supported by grants J and S provided by the Grant Agency of the Czech Republic. This research has made use of the observations provided by the MAXI/ISS team (maxi.riken.jp/top/), the ASM/RXTE teams at MIT and at the RXTE SOF and GOF at NASA's GSFC (Levine et al., 1996, ApJ,469,L33),and public data from BAT/Swift transient (Levine et al., 1996, ApJ,469,L33), and public data from BAT/Swift transient monitor provided by the BAT/Swift team (Krimm et al., 2013, ApJS, 209, 14). I 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 via astro.troja.mff.cuni.cz/ftp/hec/HEC13/. 24