Disc reflection in low-mass X-ray binaries
Wang, Yanan
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Publication date: 2018
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Wang, Y. (2018). Disc reflection in low-mass X-ray binaries. Rijksuniversiteit Groningen.
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Thesis summary and future prospects
6.1 Conclusions chapter by chapter
In this thesis, I focus on the X-ray spectral and timing analysis of accreting neutron stars (NSs) and galactic black-hole candidates (BHCs). Particularly, I studied the effects of reflection off the accretion disc in these systems. I summarize the main findings of each chapter below:
• Chapter 2 - The XMM-NEWTON spectra of the 2012 outburst of the black-hole candidate 4U 1630–47 revisited
In this chapter we reanalysed two XMM-NEWTONobservations of 4U 1630-47 during the 2012 outburst. Using these data, Díaz Trigo et al. (2014) claimed the existence of three relativistically Doppler-shifted emission lines arising from baryonic matter in the jet of this source. We found that if one allows for a non-solar abundances of S and Fe in the interstellar medium (ISM) to-wards the source, the Doppler-shifted emission lines are no longer required. Compared to the model used in Díaz Trigo et al. (2014), our model has fewer free parameters and works well for the same data with the most recent calibra-tion files whereas theirs does not. Furthermore, the value of the Fe in the ISM that we found is consistent with more accurate measurements obtained with spectrometers on board XMM-NEWTON.
Furthermore, we also analysed other four XMM-NEWTONobservations of 4U 1630–47 during the same outburst and I found four absorption lines, asso-ciated with FeXXV, FeXXVIand NiXXVIIIor FeXXVα, and two absorption edges, associated with Fe XXV and Fe XXVI. The equivalent width of the emission and absorption lines in the five observations is anti-correlated with
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the 2–10 keV unabsorbed flux (see Fig. 2.5), which indicates a change in the ionisation fraction of the disc.
This work demonstrated how important it is to consider the elemental abundance in the ISM correctly in spectral analysis.
• Chapter 3 - The reflection spectrum of the low-mass X-ray binary 4U 1636–53
Triggered by the issue of the high-inclination obtained by Pandel, Kaaret & Corbel (2008) and Sanna et al. (2013) for the neutron star 4U 1636–53, we studied three NUSTAR observations of this source to explore the structure of the inner flow. We applied several models which, as in previous papers, yield an inclination angle as high as ∼ 88◦, e.g. the relativistic iron line modelKYRLINE
and the most recent self-consistent relativistic reflection modelRELXILL. On the other hand, the model,RELXILLLP, in which a lamppost geometry of the corona is assumed, gives a reasonable inclination of ∼ 56◦, consistent with the
fact that this source does not show any eclipse or dips in the light curve. Besides that, we explored the variation of the continuum and the reflection as a function of the source state and found that most of the spectral parameters are correlated with the source state.
This work highlights the importance of understanding the geometry of the corona in an accreting system.
• Chapter 4 - The reflection component in the average and heartbeat spectra of
IGR J17091–3624
In the fourth chapter, we studied the spectral evolution of the black-hole candi-date IGR J17091–3624 during the 2016 outburst using the average spectrum of each observation. Participially, we found an observation displaying heartbeat variability and thus we carried out phase-resolved spectroscopy of this obser-vation. One interesting discovery of this work is that on the quasi-periodic oscillation time scale, ∼40–80 s, the reflected emission of the phase-resolved spectra varies independently from that of the direct emission, whereas on the days time scale, ∼ 200 days, these two quantities are strongly correlated. Additionally, even though the quality of the data is not good enough to directly constrain the spin parameter of this source, assuming that the inner radius of the disc both in the average and the phase-resolved spectra is located at the radius of the innermost stable circular orbit, with 90% confidence the spin parameter of the black hole in this system is −0.13 ≤ a∗≤ 0.27.
The phase-resolved spectroscopy improved our understanding of
IGR J17091–3624, which suggests more timing analysis should be applied together with spectral analysis.
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• Chapter 5 - Study of the X-ray properties of the neutron-star binary 4U 1728–
34 from the soft to hard state
We apply several reflection models with different assumptions to study the neu-tron star 4U 1728–34 with XMM-NEWTON. Similar to the result of chapter 3, the changes of the inner radius of the accretion disc are not consistent with the source state. All the reflection models yield a very high iron abundance, up to 10 times solar, which is different from the value derived from the NUS-TAR data reported by Mondal et al. (2017). We also explored the illuminating source to irradiate the disc in this system and found that both the corona and the boundaty layer contributed to the reflection spectrum.
This work makes apparent that XMM-NEWTONdata alone are not suffi-cient to conduct the reflection research.
6.2 Future prospects
Spectroscopy is a widely used tool to probe the physics and geometry of X-ray bina-ries. However, if applied to the average of a full observation, the possible effect of variability in a source is ignored. Spectroscopy together with timing analysis increase the chance to conduct more complete understanding on these systems.
Reverberation lags
A corona is assumed as the primary source illuminating the disc in these systems, which results in a reflection spectrum. Therefore, when the corona changes, both the Comptonized emission and the reflection spectrum change. A soft delay (soft photons lagging hard photons) has been observed, likely due to the extra light-travel path taken by the reflected photons; this process is called reverberation (Uttley et al. 2014). Due to the compactness of the emitting region, if the delay is simply related to a light-travel time, reverberation can be used to measure the inner disc radius which is related to the black-hole mass and spin.
In chapter 3, I found that a lamppost geometry of the corona of the neutron star 4U 1636–53, changed the disc inclination angle from the previous value of 88◦ to
56◦, which resolved a major controversy and indicates the importance of
understand-ing coronal geometry. A reverberation-related study may offer a further confirmation of this conclusion. In the coming years I intend to apply spectral-timing techniques to analyse NICER and NuSTAR data, as well as the data from other satellites; the combination of spectroscopy and reverberation offers a powerful tool to explore the innermost regions in accreting systems. These investigations have the potential to transform our understanding of the inner regions of these systems, to probe accretion under strong field gravity regime and explore theories of gravity beyond Einstein’s
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General Relativity.
Updating or developing new codes
In the current scenario, a point-like corona has been assumed to make the situation computable and minimize the number of free parameters. This assumption seems to fit most data properly. However, the next generation of the detectors will bring much more complex data both in timing and spectral domain. A more realistic and more complex geometry of a compact and extended corona is probably required, e.g. a sphere, a slab or a cylinder. I will start my work by expanding the model to consider a corona that is symmetric arrangement of point-like sources, like the ones as far considered; by defining global properties of the corona (e.g. density of electron temperature) as the function of that in a point-like one to reduce the amount of the free parameters and then run Monte Carlo simulation to test the results.
The launch of the first X-ray polarimetry mission, IXPE by NASA in 2021, will bring a new perspective of X-ray astronomy. Polarimetry, combining spectroscopic (and timing) analysis, is another promising method to infer the origin and the geometry of the corona. I am also interested in applying and exploring this new method in the future.
