6 High-Frequency Variability in Neutron-Star Low-Mass X-ray Binaries
321
unattainable in laboratories on Earth. The amount and quality of the data collected in
almost 15 years with the RXTE satellite are unique and, for the moment, unrivalled
by data that have or can be collected by any other satellite in the past or currently
in operation. The rich phenomenology of the kHz QPOs discussed in this chapter
triggered scores of new ideas about accretion, and will guide this area of research for
years to come. The next generation of X-ray satellites with enough time resolution
to observe the millisecond time scales in accreting LMXBs with enough collecting
area, and the capability to deal with very bright objects, will soon be a reality. The
complementary technical capabilities of Athena and eXTP will give a new impulse
to this topic in the coming decade. If we were asked to anticipate now what the
most fruitful areas of research will be when those missions start providing data,
we would dare to say that the most exciting results will come from the correlated
study of the properties of the kHz QPOs and the broad iron line (Sect. 6.9) over
the shortest possible time scales, and the prospect of finding signatures of the
innermost stable circular orbit around a neutron star (Sect. 6.4). The analysis tools
that are needed to achieve these goals have to address simultaneously the spectral
(physical parameters that describe the emission) and timing (rms amplitudes, lags
and coherence function) properties of variable signals over the shortest time scales
allowed by the data. As we showed in this Chapter, initiatives are already being
taken in that direction. This, and the efforts to try and understand the data, will keep
us busy for at least two more decades.
Acknowledgments This work is part of the research programme Athena with project number
184.034.002, which is (partly) financed by the Dutch Research Council (NWO). TMB acknowledges financial contribution from the agreement ASI-INAF n.2017-14-H.0. This research has made
use of data and/or software provided by the High Energy Astrophysics Science Archive Research
Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC.
This research made use of NASA’s Astrophysics Data System.
References
1. M.A. Abramowicz, W. Klu´ zniak, A precise determination of black hole spin in GRO J165540. Astron. Astrophys. 374, L19–L20 (2001)
2. M.A. Abramowicz, T. Bulik, M. Bursa, W. Klu´ zniak, Evidence for a 2:3 resonance in Sco
X-1 kHz QPOs. Astron. Astrophys. 404, L21–L24 (2003)
3. P.C. Agrawal, A broad spectral band Indian Astronomy satellite ‘Astrosat’. Adv. Space Res.
38(12), 2989–2994 (2006)
4. M. Ali Alpar, QPO frequency derivative-frequency correlation indicates non-Keplerian
boundary layer with a maximum in rotation rate. Mon. Not. R. Astron. Soc. 462(1), L98–
L100 (2016)
5. M.A. Alpar, D. Psaltis, The highest dynamical frequency in the inner region of an accretion
disc. Mon. Not. R. Astron. Soc. 391(3), 1472–1476 (2008)
6. M.A. Alpar, J. Shaham, Is GX5 - 1 a millisecond pulsar? Nature 316(6025), 239–241 (1985)
7. D. Altamirano, M. van der Klis, M. Méndez, S. Migliari, P.G. Jonker, A. Tiengo, W. Zhang,
The island state of the atoll source 4U 1820-30. Astrophys. J. 633(1), 358–366 (2005)
321
unattainable in laboratories on Earth. The amount and quality of the data collected in
almost 15 years with the RXTE satellite are unique and, for the moment, unrivalled
by data that have or can be collected by any other satellite in the past or currently
in operation. The rich phenomenology of the kHz QPOs discussed in this chapter
triggered scores of new ideas about accretion, and will guide this area of research for
years to come. The next generation of X-ray satellites with enough time resolution
to observe the millisecond time scales in accreting LMXBs with enough collecting
area, and the capability to deal with very bright objects, will soon be a reality. The
complementary technical capabilities of Athena and eXTP will give a new impulse
to this topic in the coming decade. If we were asked to anticipate now what the
most fruitful areas of research will be when those missions start providing data,
we would dare to say that the most exciting results will come from the correlated
study of the properties of the kHz QPOs and the broad iron line (Sect. 6.9) over
the shortest possible time scales, and the prospect of finding signatures of the
innermost stable circular orbit around a neutron star (Sect. 6.4). The analysis tools
that are needed to achieve these goals have to address simultaneously the spectral
(physical parameters that describe the emission) and timing (rms amplitudes, lags
and coherence function) properties of variable signals over the shortest time scales
allowed by the data. As we showed in this Chapter, initiatives are already being
taken in that direction. This, and the efforts to try and understand the data, will keep
us busy for at least two more decades.
Acknowledgments This work is part of the research programme Athena with project number
184.034.002, which is (partly) financed by the Dutch Research Council (NWO). TMB acknowledges financial contribution from the agreement ASI-INAF n.2017-14-H.0. This research has made
use of data and/or software provided by the High Energy Astrophysics Science Archive Research
Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC.
This research made use of NASA’s Astrophysics Data System.
References
1. M.A. Abramowicz, W. Klu´ zniak, A precise determination of black hole spin in GRO J165540. Astron. Astrophys. 374, L19–L20 (2001)
2. M.A. Abramowicz, T. Bulik, M. Bursa, W. Klu´ zniak, Evidence for a 2:3 resonance in Sco
X-1 kHz QPOs. Astron. Astrophys. 404, L21–L24 (2003)
3. P.C. Agrawal, A broad spectral band Indian Astronomy satellite ‘Astrosat’. Adv. Space Res.
38(12), 2989–2994 (2006)
4. M. Ali Alpar, QPO frequency derivative-frequency correlation indicates non-Keplerian
boundary layer with a maximum in rotation rate. Mon. Not. R. Astron. Soc. 462(1), L98–
L100 (2016)
5. M.A. Alpar, D. Psaltis, The highest dynamical frequency in the inner region of an accretion
disc. Mon. Not. R. Astron. Soc. 391(3), 1472–1476 (2008)
6. M.A. Alpar, J. Shaham, Is GX5 - 1 a millisecond pulsar? Nature 316(6025), 239–241 (1985)
7. D. Altamirano, M. van der Klis, M. Méndez, S. Migliari, P.G. Jonker, A. Tiengo, W. Zhang,
The island state of the atoll source 4U 1820-30. Astrophys. J. 633(1), 358–366 (2005)