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Register a new userFirst observation of a transition between parallel tracks in the kHz QPO frequency vs. intensity diagram
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Authors
Mariano Mendez
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Contrary to theoretical expectations, observations with the Rossi X-ray Timing Explorer (RXTE) show that in X-ray binaries timing properties are not uniquely correlated with X-ray luminosity. For instance, although the frequencies of the kilohertz quasi-periodic oscillations (kHz QPOs) correlate with X-ray flux on short (~few hours) time scales, on time scales longer than a day the QPO appears at more or less the same frequency, whereas the luminosity may be a factor of a few different. The result is a set of almost parallel tracks in a QPO frequency vs. X-ray flux plot. Despite the parallel tracks are a common phenomenon among kHz QPO sources, until now, after five years of observations with RXTE, not a single transition between two of these tracks had been seen. Here I present the first detection of such a transition, in 4U 1636-53.
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We analyzed the recently published kHz QPO data in the neutron star low-mass X-ray binaries (LMXBs), in order to investigate the different correlations of the twin peak kilohertz quasi-eriodic oscillations (kHz QPOs) in bright Z sources and in the less luminous Atoll sources. We find that a power-law relation $ osim t^{b}$ between the upper and the lower kHz QPOs with different indices: $bsimeq$1.5 for the Atoll source 4U 1728-34 and $bsimeq$1.9 for the Z source Sco X-1. The implications of our results for the theoretical models for kHz QPOs are discussed.
We collect the data of twin kilohertz quasi-periodic oscillations (kHz QPOs) published before 2012 from 26 neutron star (NS) low-mass X-ray binary (LMXB) sources, then we analyze the centroid frequency ( u) distribution of twin kHz QPOs (lower frequency u_1 and upper frequency u_2) both for Atoll and Z sources. For the data without shift-and-add, we find that Atoll and Z sources show the different distributions of u_1, u_2 and u_2/ u_1, but the same distribution of Delta u (difference of twin kHz QPOs), which indicates that twin kHz QPOs may share the common properties of LXMBs and have the same physical origins. The distribution of Delta u is quite different from constant value, so is u_2/ u_1 from constant ratio. The weighted mean values and maxima of u_1 and u_2 in Atoll sources are slightly higher than those in Z sources. We also find that shift-and-add technique can reconstruct the distribution of u_1 and Delta u. The K-S test results of u_1 and Delta u between Atoll and Z sources from data with shift-and-add are quite different from those without it, and we think that this may be caused by the selection biases of the sample. We also study the properties of the quality factor (Q) and the root-mean-squared (rms) amplitude of 4U 0614+09 with the data from the two observational methods, but the errors are too big to make a robust conclusion. The NS spin frequency ( u_s) distribution of 28 NS-LMXBs show a bigger mean value (about 408Hz) than that (about 281 Hz) of the radio binary millisecond pulsars (MSPs), which may be due to the lack of the spin detections from Z sources (systematically lower than 281 Hz). Furthermore, on the relations between the kHz QPOs and NS spin frequency u_s, we find the approximate correlations of the mean values of Delta u with NS spin and its half, respectively.
{We investigate the coherence of the twin kilohertz quasi-periodic oscillations (kHz QPOs) in the low-mass X-ray binary (LMXB) theoretically. The profile of upper kHz QPO, interpreted as Keplerian frequency, is ascribed to the radial extent of the kHz QPO emission region, associated with the transitional layer at the magnetosphere-disk boundary, which corresponds to the coherence of upper kHz QPO. The theoretical model for Q-factor of upper kHz QPO is applied to the observational data of five Atoll and five Z sources, and the consistence is implied.
We discovered kHz QPOs in 80 archived RXTE observations from the peculiar low-mass X-ray binary (LMXB) Circinus X-1. In 11 cases these appear in pairs in the frequency range of ~230 Hz to ~500 Hz for the upper kHz QPO and ~56 Hz to ~225 Hz for the lower kHz QPO. Their correlation with each other, which is similar to that of frequencies of kHz QPO pairs in other LMXBs containing a neutron star, and their variation by a factor two confirm that the central object is a neutron star. These are the lowest frequencies of kHz QPO pairs discovered so far and extend the above correlation over a frequency range of factor four. In this new frequency range the frequency difference of the two kHz QPOs increases monotonically by more than ~170 Hz with increasing kHz QPO frequency, challenging theoretical models.
X-ray reverberation lags have recently been discovered in both active galactic nuclei (AGN) and black hole X-ray binaries. A recent study of the neutron star low-mass X-ray binary 4U 1608-52 has also shown significant lags, whose properties hint at a reverberation origin. Here, we adapt general relativistic ray tracing impulse response functions used to model X-ray reverberation in AGN for neutron star low-mass X-ray binaries. Assuming relativistic reflection forms the broad iron line and associated reflection continuum, we use reflection fits to the energy spectrum along with the impulse response functions to calculate the expected lags as a function of energy over the range of observed kHz QPO frequencies in 4U 1608-52. We find that the lag energy spectrum is expected to increase with increasing energy above 8 keV, while the observed lags in 4U 1608-52 show the opposite behavior. This demonstrates that the lags in the lower kHz QPO of 4U 1608-52 are not solely due to reverberation. We do note, however, that the models appear to be more consistent with the much flatter lag energy spectrum observed in the upper kHz QPO of several neutron star low-mass X-ray binaries, suggesting that lower and upper kHz QPOs may have different origins.
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