Extreme gravity in neutron-star systems with XEUS Mariano Méndez Kapteyn Astronomical Institute, University of Groningen, The Netherlands
Probing strong gravitational fields Adapted from Kramer et al. (2004) & Psaltis (2004)
NS and strong gravitational fields 1. Kilohertz Quasi-Periodic Oscillations (khz QPOs) as basic General Relativistic (GR) frequencies. 2. khz QPOs and the Innermost Stable Circular Orbit (ISCO). 3. Surface (absorption) lines. 4. Emission lines from the inner accretion disc.
Strong gravitational field: khz QPOs width ν 1 ν 2 amplitude Central freq. van der Klis (1997)
1. The 3 basic GR frequencies ν z : vertical ν θ : azimuthal ν θ κ: Periastron precession κ: radial ν z ν θ : Lense-Thirring
2. The ISCO As the inner edge of the disc moves inwards, driven by mass accretion rate, the azimuthal frequency at that radial position in the disc increases. Maximum orbital frequency is the Keplerian frequency at the ISCO.
2. The ISCO 1200 28 Oct 1996 21 March 1997 10 Sept 1997 9 Feb 1997 Quality factor = frequency / width 1000 Freq (Hz) 800 600 400 1600 2000 2400 2800 3200 4U 1820 30 Intensity (c/s) 600 700 800 900 Frequency (Hz) 4U 1636 53 Zhang et al. (1998); Barret et al. (2006)
2. The ISCO Quality factor = frequency / width 600 700 800 900 Frequency (Hz) 4U 1636 53 4U 1636 53 Barret et al. (2006)
khz QPOs: Complications 1. No unique identification of QPOs with the basic GR (or other) frequencies Possible solution: Simultaneous timing and spectroscopy (see below). 2. Identification of the ISCO requires that: From pair of QPOs, the one at higher frequencies is the Keplerian (vertical) GR frequency (most, but not all, models propose this), and Keplerian (vertical) frequency is the highest frequency in the disc. True for test particles; super- Keplerian frequencies may appear in real discs.
khz QPOs: Complications 3. ISCO complications: High amplitude above ~10 15 kev Modulated flux not from the disc, but due to inverse Compton. Oscillator Modulator Accretion disc No clean test-particle dynamics. Frequency ceiling and drop of Quality factor due (in part) to changes in the modulator?
3. Surface (absorption) lines Scalar-Tensor extensions to Einstein s theory Extra scalar field described in terms of the parameter β. From binary-pulsar timing β 8. Dedeo & Psaltis (2003)
Gravitational redshift
khz QPOs: Complications (revisited) 1. No unique identification of QPOs with the basic GR (or other) frequencies Possible solution using spectroscopy Surface spectral lines may help resolve this. Lines are broadened by: longitudinal and transverse Doppler shifts, special relativistic beaming, gravitational redshifts, light-bending, frame-dragging (Lense- Thirring). Bhattacharyya et al. (2006)
4. Emission lines from the inner disc Suzaku XMM-Newton Serpens X-1 Serpens X-1 Cackett et al. (2008); Bhattacharyya & Strohmayer (2007)
Emission lines: Complications 4U 1636 53 XMM-Newton/Epic-PN σ Counts cm -2 s -1 kev -1 100 10 1 0.1 5 0-5 1 2 5 10 Energy (kev) 100 10 1 0.1 5 0-5 1 2 5 10 Energy (kev) Left: Right: Continuum emission + Line from 6 r g (Continuum emission + Line from 12 r g ) photo-ionized absorption
Instrument requirements: Timing 1. Kilohertz Quasi-Periodic Oscillations (khz QPOs) as basic General Relativistic (GR) frequencies. 2. khz QPOs and the Innermost Stable Circular Orbit (ISCO). HTRS: Band pass: 1 25keV Amplitude increases with energy Effective area: At least 4 RXTE (/4 m 2 ) Spectral resol.: ~20% Energy-dependent time lags Max. count rate: 10 6 c/s Bright sources Time resol.: τ.10 μs
Instrument requirements: Spectroscopy 3. Surface (absorption) lines. 4. Emission lines from the inner accretion disc. WFI + NFI: Band pass: 0.3 25 kev Redshifted Oxygen; continuum around Fe K Effective area: /5 m 2 Absorption lines in individual X-ray bursts (~10 s) Spectral resol.: 2 3 ev at 1 kev Resolve line profile (eg. L-T) Max. count rate: 10 5 counts/s during 1 5 s X-ray bursts; Defocusing?