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Register a new userDiscovery of kilohertz quasi-periodic oscillations and state transitions in the low-mass X-ray binary 1E~1724-3045 (Terzan 2)
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We have studied the rapid X-ray time variability in 99 pointed observations with the Rossi X-ray Timing Explorer (RXTE)s Proportional Counter Array of the low-mass X-ray binary 1E~1724--3045 which includes, for the first time, observations of this source in its island and banana states, confirming the atoll nature of this source. We report the discovery of kilohertz quasi-periodic oscillations (kHz QPOs). Although we have 5 detections of the lower kHz QPO and one detection of the upper kHz QPO, in none of the observations we detect both QPOs simultaneously. By comparing the dependence of the rms amplitude with energy of kHz QPOs in different atoll sources, we conclude that this information cannot be use to unambiguously identify the kilohertz QPOs as was previously thought. We find that Terzan~2 in its different states shows timing behavior similar to that seen in other neutron-star low mass X-ray binaries (LMXBs). We studied the flux transitions observed between February 2004 and October 2005 and conclude that they are due to changes in the accretion rate.
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The recently updated data of the twin kilohertz quasi-periodic oscillations (kHz QPOs) in the neutron star low-mass X-ray binaries are analyzed. The power-law fitting $ u_{1}=a( u_{2}/1000)^{b}$ and linear fitting $ u_{2}=A u_{1}+B$ are applied, individually, to the data points of four Z sources (GX 17+2, GX 340+0, GX 5-1 and Sco X-1) and four Atoll sources (4U 0614+09, 4U 1608-52, 4U 1636-53 and 4U 1728-34). The $chi^{2}$-tests show that the power-law correlation and linear correlation both can fit data well. Moreover, the comparisons between the data and the theoretical models for kHz QPOs are discussed.
I study the behaviour of the maximum rms fractional amplitude, $r_{rm max}$ and the maximum coherence, $Q_{rm max}$, of the kilohertz quasi-periodic oscillations (kHz QPOs) in a dozen low-mass X-ray binaries. I find that: (i) The maximum rms amplitudes of the lower and the upper kHz QPO, $r^{ell}_{rm max}$ and $r^{rm u}_{rm max}$, respectively, decrease more or less exponentially with increasing luminosity of the source; (ii) the maximum coherence of the lower kHz QPO, $Q^{ell}_{rm max}$, first increases and then decreases exponentially with luminosity, at a faster rate than both $r^{ell}_{rm max}$ and $r^{rm u}_{rm max}$; (iii) the maximum coherence of the upper kHz QPO, $Q^{rm u}_{rm max}$, is more or less independent of luminosity; and (iv) $r_{rm max}$ and $Q_{rm max}$ show the opposite behaviour with hardness of the source, consistent with the fact that there is a general anticorrelation between luminosity and spectral hardness in these sources. Both $r_{rm max}$ and $Q_{rm max}$ in the sample of sources, and the rms amplitude and coherence of the kHz QPOs in individual sources show a similar behaviour with hardness. This similarity argues against the interpretation that the drop of coherence and rms amplitude of the lower kHz QPO at high QPO frequencies in individual sources is a signature of the innermost stable circular orbit around a neutron star. I discuss possible interpretations of these results in terms of the modulation mechanisms that may be responsible for the observed variability.
In the past five years observations with the Rossi X-ray Timing Explorer have revealed fast quasi-periodic oscillations in the X-ray flux of about 20 X-ray binaries. Thought to originate close to the surface of a neutron star, these oscillations provide unique information about the strong gravitational field in which they are produced.
We report on the discovery of mHz quasi-periodic oscillations (QPOs) from the high mass X-ray binary (HMXB) IGRJ19140+0951, during a 40 ks XMM-Newton observation performed in 201 5, which caught the source in its faintest state ever observed. At the start of the observation, IGRJ19140+0951 was at a low flux of 2$times$10$^{-12}$~erg~cm$^{-2}$~s$^{-1}$ ( 2-10 keV; L$_{rm X}$=3$times$10$^{33}$~erg~s$^{-1}$ at 3.6 kpc), then its emission rised reaching a flux 10 times higher, in a flare-like activity. The investigation of the pow er spectrum reveals the presence of QPOs, detected only in the second part of the observation, with a strong peak at a frequency of 1.46$pm{0.07}$~mHz, together with higher harm onics. The X-ray spectrum is highly absorbed (N$_{rm H}$=$10^{23}$~cm$^{-2}$), well fitted by a power-law with a photon index in the range 1.2-1.8. The re-analysis of a Chandra archival observation shows a modulation at 0.17+/-0.05mHz, very likely the neutron star spin period (although a QPO cannot be excluded). We discuss the origin of the 1.46 mHz QPO in the framework of both disc-fed and wind-fed HMXBs, favouring the quasi-spherical accretion scenario. The low flux observed by XMM-Newton leads to about three orders of magnit ude the source dynamic range, overlapping with the one observed from Supergiant Fast X-ray Transients (SFXTs). However, since its duty cycle is not as low as in SFXTs, IGRJ19140 +0951 is an intermediate system between persistent supergiant HMXBs and SFXTs, suggesting a smooth transition between these two sub-classes.
(abridged) We studied the energy and frequency dependence of the Fourier time lags and intrinsic coherence of the kHz QPOs in the NS LMXBs 4U 1608-52 and 4U 1636-53 using RXTE data. In both sources we confirmed energy-dependent soft lags of 10-100 mu s for the lower kHz QPO. We also found that the time lags of the upper kHz QPO are independent of energy and inconsistent with the soft lags of the lower kHz QPO. The intrinsic coherence of the lower kHz QPO remains constant at 0.6 from 5 to 12 keV, and then drops to zero, while for the upper kHz QPO the intrinsic coherence is consistent with zero across the full energy range. The intrinsic coherence of the upper kHz QPO is consistent with zero over the full frequency range of the QPO, except in 4U 1636-53 at ~780 Hz where it increases to 0.13. In 4U 1636-53, for the lower kHz QPO the 4-12 keV photons lag the 12-20 keV ones by 25 mu s in the QPO frequency range 500-850 Hz, with the lags decreasing to 15 mu s at higher frequencies. In 4U 1608-52 the soft lags of the lower kHz QPO remain constant at 40 mu s. In 4U 1636-53, for the upper kHz QPO the 12-20 keV photons lag the 4-12 keV ones by 11 +/- 3 mu s, independent of QPO frequency; we found consistent results for the time lags of the upper kHz QPO in 4U 1608-52. The intrinsic coherence of the lower kHz QPO increases from ~0-0.4 at 600 Hz to 1 and 0.6 at 800 Hz in 4U 1636-53 and 4U 1608-52, respectively. In 4U 1636-53 it decreases to 0.5 at 920 Hz, while in 4U 1608-52 we do not have data above 800 Hz. We discuss our results in the context of scenarios in which the soft lags are either due to reflection off the accretion disc or up-/down-scattering in a hot medium close to the neutron star. We finally explore the connection between, on one hand the time lags and the intrinsic coherence of the kHz QPOs, and on the other the QPOs amplitude and quality factor in these two sources.
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