KHz quasi-periodic oscillations from the 2000 and 2010 x-ray transients located in the globular cluster Terzan 5: EXO1745-248 and IGR J17480-2446

Abstract

EXO1745-248 is a transient neutron star low-mass X-ray binary located in the globular cluster Terzan 5. It was in outburst in 2000 and displayed during one Rossi X-ray Timing Explorer observation a highly coherent quasi-periodic oscillation (QPO) at frequencies between 670 and 715Hz. Applying a maximum likelihood method to fit the X-ray power density spectrum, we show that the QPO can be detected on segments as short as T = 48 s. We find that its width is consistent with being constant, while previous analysis based on longer segment duration (200s) found it variable. If the QPO frequency variations in EXO1745-248 follow a random walk (i.e., the contribution of the drift to the measured width increases like ), we derive an intrinsic width of 2.3 Hz. This corresponds to an intrinsic quality factor of Q 297 ± 50 at 691Hz. We also show that Q is consistent with being constant between 2.5 and 25keV. IGR J17480-2446 is another X-ray transient located in Terzan 5. It is a very interesting object showing accretion-powered pulsations and burst oscillations at 11Hz. We report on the properties of its kHz QPOs detected between October 18 and October 23, soon after the source had moved from the so-called Atoll state to the Z state. Its QPOs are typical of persistent Z sources; in the sense that they have low Q factors (30) and low rms amplitudes (5%). The highest frequency (at 870 Hz), if orbital, sets a lower limit on the inner disk radius of 18.5 km and an upper limit to the dipole moment of the magnetic field μ ≤ 5 × 1026 G cm3. © 2012 The American Astronomical Society. All rights reserved.