Timing and Spectral Properties of Neutron Star Low-Mass X-ray Binaries

Sudip Bhattacharyya Department of Astronomy and Astrophysics

Tata Institute of Fundamental Research

Different types of NS LMXBs

Atoll and Z sources are identified by their tracks in the colour-colour diagrams (CDs) and hardness-intensity diagrams (HIDs).

Soft colour: background –subtracted count rate ratio in two softer energy bands.Hard colour: background-subtracted count rate ratio in two harder energy bands.Typical Rossi X-ray Timing Explorer (RXTE) proportional counter array (PCA) energy bands: 2.21-3.45, 3.45-5.12, 5.12-8.46, 8.46-18.15 (all in keV).

van der Klis (2006)

Different types of NS LMXBs

van der Klis (2006)

Energy spectrum of NS LMXBs

Two emission components: accretion disk and NS surface: each of them can emit blackbody; each of them can be partially or fully covered with Comptonizing corona. No spectral model uniquely fits the data.

NS LBXB XB 1254-690: an example with RXTE PCA data

Model χν2(dof) Model χν

2(dof)

diskbb+comptt 1.95(30) diskbb+powerlaw 1.95(32)diskbb+cutoffpl 1.97(31) diskbb+bknpower 0.52(30)comptt+bbody 1.50(30) powerlaw+bbody 2.24(32)cutoffpl+bbody 1.65(31) bknpower+bbody 2.39(30)diskbb+bbody 2.75(32) comptt+comptt 0.91(28)powerlaw+comptt 0.95(30) cutoffpl+comptt 0.91(29)bknpower+comptt 0.54(28) powerlaw+powerlaw 32.31(32)cutoffpl+powerlaw 2.33(31) bknpower+powerlaw 3.47(30)cutoffpl+cutoffpl 1.54(30) bknpower+cutoffpl 0.53(29)bknpower+bknpower 3.73(28)

Mukherjee and Bhattacharyya (2011)

NS LBXB XB 1254-690: an example with RXTE PCA data

Mukherjee and Bhattacharyya (2011)

Accreting pulsars

: Orbital frequencyr : Radial epicyclic frequency : Vertical epicyclic frequency- r : Periastron precession frequency- : Nodal or `Lense-Thirring’ precession frequency

Kilohertz quasi-periodic oscillations (kHz QPOs)

KHz QPOs often appearin a pair in the power spectrum, in the 200-1200 Hz frequency range.

Fourier Transform

Power spectrum is the Fourier transform of the intensity vs. time curve.

This observationally robust timing feature have been detected from many neutron star LMXBs.

According to almost all the models, the uniquely high kHz QPO frequencies are either the following accretion disk frequencies, or the beating or resonances among them, or with the neutron star spin frequency spin.

Kilohertz quasi-periodic oscillations (kHz QPOs)

Accretion disk frequencies:

So when the correct model is identified, kHz QPOs can be used to measure the neutron star parameters M and J.

Miller et al. (1998)

Such a constraint on the neutron starmass-radius space is possible, if one of the kHz QPO frequencies is , andthe corresponding radial distance r isgreater than or equal to the neutron star and the ISCO radius.

Apart from neutron star parametermeasurement, KHz QPOs can be useful to study the matter flow in strong gravity region, and to test a law of gravitation.For example, the general relativistic `Lense-Thirring’ precision (frequency: - ) has not yet been detected with certainty in any system.

KHz QPO quality factor vs. frequency: a signature of innermost stable circular orbit(ISCO)?

Barret et al.

Spectral origin of kHz QPO: Comptonized component?

Gilfanov et al. (2003)

Z sources:

(1)Horizontal Branch Oscillations (HBO) : 15-60 Hz

(2)Normal Branch Oscillations (NBO) : about 6 Hz

(3)Flaring Branch Oscillations (FBO) : about 10 Hz

Atoll sources:

(1)Upper Banana (UB): < 1 Hz power law noise (VLFN)

(2)Lower Banana (LB): 10 Hz BLN

(3)Lower Left Banana (LLB): Twin kHz QPO

(4)Island State (IS): Dominant BLN, becoming stronger and lower frequency as flux decreases

Power spectra in different states

Correlations among various timing features

Van der Klis (2006)

ASTROSAT (India’s proposed multiwavelength

astronomy space mission)

In its time, only ASTROSAT will have the capability to study kHz QPOs.

First detection and studyof fast timing features in hard X-rays.

ASTROSAT (India’s proposed multiwavelength

astronomy space mission)

In its time, only ASTROSAT will have the capability to study kHz QPOs.

First detection and studyof fast timing features in hard X-rays.

Thank you!