Search

We utilize the joint capabilities of IBIS and SPI to perform a state-resolved 20--2000\,keV analysis of the microquasar Cygnus X-1. In both LHS and HSS, the spectral analysis reveals the presence of a high-energy tail above 400\,keV in addition to a standard, Compton-like continuum in the 20--400\,keV range. We study the polarisation properties of the hard X-ray radiation, as well as of the polarisation of this high-energy tail. Polarisation is detected in the LHS in agreement with previous work. We find potential variations of the LHS polarisation with time and no polarisation detection in the HSS., 4 pages, 2 figures, 12th INTEGRAL Conference

Using all the observations from Rossi X-ray Timing Explorer for Z source GX 349+2, we systematically carry out cross-correlation analysis between its soft and hard X-ray light curves. During the observations from January 9 to January 29, 1998, GX 349+2 traced out the most extensive Z track on its hardness-intensity diagram, making a comprehensive study of cross-correlation on the track. The positive correlations and positively correlated time lags are detected throughout the Z track. Outside the Z track, anti-correlations and anti-correlated time lags are found, but the anti-correlated time lags are much longer than the positively correlated time lags, which might indicate different mechanisms for producing the two types of time lags. We argue that neither the short-term time lag models nor the truncated accretion disk model can account for the long-term time lags in neutron star low mass X-ray binaries (NS-LMXBs). We suggest that the extended accretion disk corona model could be an alternative model to explain the long-term time lags detected in NS-LMXBs., 24 pages, 5 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society

The improvement on the Imaging Air Cherenkov Technique (IACT) led to the discovery of a new type of sources that can emit at very high energies: the gamma-ray binaries. Only six systems are part of this exclusive class. We summarize the latest results from the observations performed with the MAGIC telescopes on different systems as the gamma-ray binary LS I +61$^{\circ}$ 303 and the microquasars SS433, V404 Cygni and Cygnus X-1, which are considered potential VHE gamma-ray emitters. The binary system LS I +61$^{\circ}$ 303 has been observed by MAGIC in a long-term monitoring campaign. We show the newest results of our search for super-orbital variability also in context of contemporaneous optical observations. Besides, we will present the results of the only super-critical accretor known in our galaxy: SS 433. We will introduce the VHE results achieved with MAGIC after 100 hours of observations on the microquasar Cygnus X-1 and report on the microquasar V404 Cyg, which has been observed with MAGIC after it went through a series of exceptional X-ray outbursts in June 2015., Proceedings of High-Energy Phenomena in Relativistic Outflows (HEPRO) VI conference

We present the evolution of the phase lags associated with the type-C QPO in GX 339--4 during the rising phase of the 2006/2007 outburst. We find that the phase lags at the QPO frequency are always positive (hard), and show very different behavior between QPOs with frequencies below and above $\sim1.7$ Hz: when the QPO frequency is below $\sim1.7$ Hz, the phase lags increase both with QPO frequency and energy, while when the QPO frequency is above $\sim1.7$ Hz, the phase lags remain more or less constant. When the QPO frequency is higher than $\sim1.7$ Hz, a broad feature is always present in the lag-energy spectra at around 6.5 keV, suggesting that the reflection component may have a significant contribution to the phase lags. Below $\sim1.7$ Hz, the QPO rms first decreases with energy and then turns to almost flat, while above $\sim1.7$ Hz, the QPO rms increases with energy. During the transition from the low-hard state to the hard-intermediate state, the second harmonic and subharmonic of this QPO appear in the power density spectra. The second-harmonic and subharmonic phase lags show very similar evolution with their centroid frequencies. However, the energy dependence of the second-harmonic and subharmonic phase lags are quite different. Our results suggest that, at different phases of the outburst, different mechanisms may be responsible for the phase lags of the QPO. We briefly discuss the possible scenarios for producing the lags., 15 pages, 12 figures, accepted for publication in ApJ

Timing analysis provides information about the dynamics of matter accreting on to neutron stars and black holes, and hence is crucial for studying the physics of the accretion flow around these objects. It is difficult, however, to associate the different variability components with each of the spectral components of the accretion flow. We apply several new methods to two Rossi X-ray Timing Explorer observations of the black hole binary GRS 1915+105 during its heartbeat state to explore the origin of the X-ray variability and the interactions of the accretion-flow components. We offer a promising window into the disc--corona interaction through analysing the formation regions of the disc aperiodic variabilities with different time-scales via comparing the corresponding transition energies of the amplitude-ratio spectra. In a previous paper, we analysed the Fourier power density as a function of energy and frequency to study the origin of the aperiodic variability, and combined that analysis with the phase lag as a function of frequency to derive a picture of the disc--corona interaction in this source. We here, for the first time, investigate the phase lag as a function of energy and frequency, and display some interesting details of the disc--corona interaction. Besides, the results from the shape of amplitude-ratio spectrum and from several other aspects suggest that the quasi-periodic oscillation originates from the corona., Comment: 12 pages, 7 figures

Sensitive polarization measurements in X-ray may address a wealth of astrophysicalphenomena, which so far remain beyond our understanding throughavailable X-ray spectroscopic, imaging, and timing studies. Though scientificpotential of X-ray polarimetry was realized long ago, there has not been anysignificant advancement in this field for the last four decades since the birth ofX-ray astronomy. The only successfulpolarization measurement in X-rays datesback to 1976, when a Bragg polarimeter onboard OSO-8 measured polarizationof Crab nebula. Primary reason behind the lack in progress is its extreme photonhungry nature, which results in poor sensitivity of the polarimeters. Recently, in the last decade or so, with the advancement in detection technology, X-ray polarimetry may see a significant progress in nearfuture, especiallyin soft X-rays with the invention of photoelectron tracking polarimeters. Thoughphotoelectric polarimeters are expected to provide sensitive polarization measurements of celestial X-ray sources,they are sensitive only in soft X-rays, wherethe radiation from the sources is dominated by thermal radiation and thereforeexpected to be less polarized. On the other hand, in hard X-rays, sources are ex-pected to be highly polarized due to the dominance of nonthermal emission overits thermal counterpart. Moreover, polarization measurements in hard X-rayspromises to address few interesting scientific issues regarding geometry of coronafor black hole sources, emission mechanism responsible for the higher energy peakin the blazars, accretion geometry close to the magnetic poles in accreting neutronstar systems and acceleration mechanism in solar flares. Compton polarimetersprovide better sensitivity than photoelectric polarimeters in hard X-rays with abroad energy band of operation. Recently, with the development of hard X-rayfocusing optics e.g. NuSTAR, Astro-H, it is now possible to conceive Compton polarimeters at the focal plane of such hard X-ray telescopes, which mayprovide sensitive polarization measurements due to fluxconcentration in hardX-rays with a very low background. On the other hand, such a configurationensures implementation of an optimized geometry close to an ideal one for theCompton polarimeters. In this context, we initiated the development of a focal plane Compton polarimeter, consisting of a plastic scatterer surrounded bya cylindrical array of CsI(Tl) scintillators. Geant−4 simulations of the plannedconfiguration estimates 1% MDP for a 100 mCrab source in 1 million seconds ofexposure. Sensitivity of the instrument is found to be critically dependent on thelower energy detection limit of the plastic scatterer; lower the threshold, betteris the sensitivity. In the actual experiment, the plastic is readout by a photomultiplier tube procured from Saint-Gobain. We carried out extensive experimentsto characterize the plastic especially for lower energy depositions. The CsI(Tl)scintillators are readout by Si photomultipliers (SiPM). SiPMs are small in sizeand robust and therefore provide the compactness necessary for the designingof focal plane detectors. Each of the CsI(Tl)-SiPM systems was characterizedprecisely to estimate their energy threshold and detection probability along thelength of the scintillators away from SiPM. Finally, we integrated the Comptonpolarimeter and tested its response to polarized and unpolarized radiation andcompared the experimental results with Geant−4 simulation. Despite the growing realization of the scientific values of X-ray polarimetryand the efforts in developing sensitive X-ray polarimeters, there has not been asingle dedicated X-ray polarimetry mission planned in near future. In this scenario, it is equally important to attempt polarization measurements from theexisting or planned instruments which are not meant for X-ray polarization measurements but could be sensitive to it. There have been several attempts in pastin retrieving polarization information from few of such spectroscopic instrumentslike RHESSI, INTEGRAL-IBIS, INTEGRAL-SPI. Cadmium Zinc Telluride Imager (CZTI) onboard Astrosat, India’s first astronomical mission, is one of suchinstruments which is expected to provide sensitive polarization measurements forbright X-ray sources. CZTI consists of 64 CZT detector modules, each of whichis 5 mm thick and 4 cm × 4 cm in size. Each CZT module is subdivided into 256pixelswith pixel pitch of 2.5 mm. Due to its pixelation nature and significantCompton scattering efficiency at energies beyond 100 keV, CZTI can work as asensitive Compton polarimeter in hard X-rays. DetailedGeant−4 simulationsand polarization experiments with the flight configuration of CZTI show thatCZTI will have significant polarization measurement capability for bright sourcesin hard X-rays. CZTI is primarilya spectroscopic instrument with coded mask imaging. Toproperly utilize the spectroscopic capabilities of CZT detectors, it is importantto generate accurate response matrix for CZTI, which in turn requiresprecisemodelling of the CZT lines shapes for monoenergetic X-ray interaction. CZTdetectors show an extended lower energy tail of an otherwise Gaussian line shapedue to low mobility and lifetime of thecharge carriers. On the other hand, interpixel charge sharing may also contribute to the lower energy tail making the lineshape more complicated. We have developed a model to predict the line shapesfrom CZTI modules taking into account the mobility and lifetime of the chargecarriers and charge sharing fractions. The model predicts the line shape quitewell and can be used to generate pixel-wise response matrix for CZTI.

In this paper we present the results of searches for periodic neutrino emission from a catalog of binary systems. Such modulation, observed in the photon flux, would be caused by the geometry of these systems. In the analysis, the period is fixed by these photon observations, while the phase and duration of the neutrino emission are treated as free parameters to be fit with the data. If the emission occurs during ~20% or less of the total period, this analysis achieves better sensitivity than a time-integrated analysis. We use the IceCube data taken from May 31, 2007 to April 5, 2008 with its 22-string configuration, and from April 5, 2008 to May 20, 2009 with its 40-string configuration. No evidence for neutrino emission is found, with the strongest excess occurring for Cygnus X-3 at 2.1 sigma significance after accounting for trials. Neutrino flux upper limits for both periodic and time-integrated emission are provided., Comment: 16 Pages, 2 figures

We observed the new X-ray transient and black hole candidate XTEJ1652-453 simultaneously with XMM-Newton and the Rossi X-ray Timing Explorer (RXTE). The observation was done during the decay of the 2009 outburst, when XTEJ1652-453 was in the hard-intermediate state. The spectrum shows a strong and broad Fe emission line with an equivalent width of similar to 450 eV. The profile is consistent with that of a line being produced by reflection off the accretion disc, broadened by relativistic effects close to the black hole. The best-fitting inner radius of the accretion disc is similar to 4 gravitational radii. Assuming that the accretion disc is truncated at the radius of the innermost stable circular orbit, the black hole in XTEJ1652-453 has a spin parameter of similar to 0.5. The power spectrum of the RXTE observation has an additional variability component above 50 Hz, which is typical for the hard-intermediate state. No coherent quasi-periodic oscillations at low frequency are apparent in the power spectrum, which may imply that we view the system at a rather low inclination angle.

We observed the new X-ray transient and black hole candidate XTEJ1652-453 simultaneously with XMM-Newton and the Rossi X-ray Timing Explorer (RXTE). The observation was done during the decay of the 2009 outburst, when XTEJ1652-453 was in the hard-intermediate state. The spectrum shows a strong and broad Fe emission line with an equivalent width of similar to 450 eV. The profile is consistent with that of a line being produced by reflection off the accretion disc, broadened by relativistic effects close to the black hole. The best-fitting inner radius of the accretion disc is similar to 4 gravitational radii. Assuming that the accretion disc is truncated at the radius of the innermost stable circular orbit, the black hole in XTEJ1652-453 has a spin parameter of similar to 0.5. The power spectrum of the RXTE observation has an additional variability component above 50 Hz, which is typical for the hard-intermediate state. No coherent quasi-periodic oscillations at low frequency are apparent in the power spectrum, which may imply that we view the system at a rather low inclination angle.

Cygnus X-1 was the first X-ray source widely accepted to be a black hole candidate and remains among the most studied astronomical objects in its class. The detection of non-thermal radio, hard X-rays and gamma rays reveals the fact that this kind of objects are capable of accelerating particles up to very high energies. In order to explain the electromagnetic emission from Cygnus X-1 in the low-hard state we present a model of a black hole corona with both relativistic lepton and hadron content. We characterize the corona as a two-temperature hot plasma plus a mixed non-thermal population in which energetic particles interact with magnetic, photon and matter fields. Our calculations include the radiation emitted by secondary particles (pions, muons and electron/positron pairs). Finally, we take into account the effects of photon absorption. We compare the results obtained from our model with data of Cygnus X-1 obtained by the COMPTEL instrument., 6 pages, 10 figures, presented as a poster in HEPRO II, Buenos Aires, Argentina, October 26-30 2009 / accepted for publication in Int. Jour. Mod. Phys. D

Allá por los años 60, los astrónomos no contábamos con telescopios espaciales suficientemente potentes como para detectar la emisión en rayos X proveniente del espacio. En su lugar, debían utilizarse cohetes que sólo conseguían estar por encima de nuestra atmósfera por algunos minutos, lo cual es necesario pues la atmósfera absorbe gran parte (sino toda) la emisión en rayos X que llega desde el espacio, volviendo luego a la superficie Terrestre. En una de estas misiones fue que identificaron por primera vez a la binaria de rayos X llamada Cygnus X-1. Más tarde, en diciembre de 1970, se lanzó al espacio el primer satélite cuyo propósito era investigar la emisión en rayos X: Uhuru (que en Swahili significa “libertad”). Este satélite confirmó la existencia de la fuente Cygnus X-1 y midió la posición de la fuente junto a otras características interesantes de la misma.

Context. Cygnus X-1 is the candidate with the highest probability of containing a black hole among the X-ray binary systems in the Galaxy. It is also by far the most often studied of these objects. Recently, the International Gamma-Ray Astrophysics Laboratory Imager on board the Integral satellite (INTEGRAL/IBIS) detected strong polarization in the high-energy radiation of this source, between 400 keV and 2 MeV. This radiation has been attributed to a jet launched by the black hole. Aims. We consider whether the corona around the black hole might be the site of production of the polarized emission instead of the jet. Methods. We studied self-consistently the injection of nonthermal particles in the hot, magnetized plasma around the black hole. Results. We show that both the high-energy spectrum and polarization of Cygnus X-1 in the low-hard state can originate in the corona, without needing to invoke a jet. We estimate the degree of polarization in the intermediate state, where there is no jet, to provide a tool to test our model. Conclusions. Contrary to the commonly accepted view, the jet might not be the source of the MeV polarized tail in the spectrum of Cygnus X-1., Facultad de Ciencias Astronómicas y Geofísicas, Instituto Argentino de Radioastronomía

Black holes are now commonplace, among the stars, in Galactic centers, and perhaps other places. But within living memory, their very existence was doubted by many, and few chose to look for them. Zeldovich and Guseinov were first, followed by Trimble and Thorne, using a method that would have identified HDE 226868 as a plausible candidate, if it had been in the 1968 catalogue of spectroscopic binaries. That it was not arose from an unhappy accident in the observing program of Daniel M. Popper long before the discovery of X-ray binaries and the identification of Cygnus X-1 with that hot, massive star and its collapsed companion., Comment: Key Words: Black holes, collapsed stars, Cygnus X-1, HDE 226868, spectroscopic binaries Journal: The Zeldovich Universe Genesis and Growth of the Cosmic Web IAU Symposium 308, 23-28 June 2014, Tallinn, Estonia) http://iau.maido.ee/proceedings

We present a spectral analysis of the black hole candidate and X-ray transient source Swift J1753.5 0127 making use of simultaneous observations of XMM-Newton and Rossi X-ray Timing Explorer (RXTE) in 2006, when the source was in outburst. The aim of this paper is to test whether a thermal component due to the accretion disc is present in the X-ray spectrum. We fit the data with a range of spectral models, and we find that for all of these models the fits to the X-ray energy spectra significantly require the addition of the disc black-body component. We also find a broad iron emission line at around 6.5 keV, most likely due to iron in the accretion disc. Our results confirm the existence of a cool inner disc extending near or close to the innermost circular orbit (ISCO).We further discovered broad emission lines of NVII and OVIII at ~ 0.52 keV and 0.65 keV, respectively in the RGS spectrum of Swift J1753.5-0127., Comment: 17 pages, 6 figures, 4 tables, Accepted 2013 February 19 for publication in Monthly Notices of the Royal Astronomical Society Main Journal

We present our monitoring campaign of the outburst of the black-hole candidate Swift J1753.5-0127, observed with the Rossi X-ray Timing Explorer and the Swift satellites. After ~4.5 years since its discovery, the source had a transition to the hard intermediate state. We performed spectral and timing studies of the transition showing that, unlike the majority of the transient black holes, the system did not go to the soft states but it returned to the hard state after a few months. During this transition Swift J1753.5-0127 features properties which are similar to those displayed by the black hole Cygnus X-1. We compared Swift J1753.5-0127 to one dynamically confirmed black hole and two neutron stars showing that its power spectra are in agreement with the binary hosting a black hole. We also suggest that the prolonged period at low flux that followed the initial flare is reminiscent of that observed in other X-ray binaries, as well as in cataclysmic variables., Accepted for publication in MNRAS

We measured the phase-lag spectrum of the high-frequency quasi-periodic oscillations (QPO) in the black hole systems (at QPO frequencies) GRS 1915+105 (35 Hz and 67 Hz), GRO J1655-40 (300 Hz and 450 Hz), XTE J1550-564 (180 Hz and 280 Hz), and IGR J17091-3624 (67 Hz). The lag spectra of the 67-Hz QPO in, respectively, GRS 1915+105 and IGR J17091-3624, and the 450-Hz QPO in GRO J1655-40 are hard (hard photons lag the soft ones) and consistent with each other, with the hard lags increasing with energy. On the contrary, the lags of the 35-Hz QPO in GRS 1915+105 are soft, with the lags becoming softer as the energy increases; the lag spectrum of the 35-Hz QPO is inconsistent with that of the 67-Hz QPO. The lags of the 300-Hz QPO in GRO J1655-40, and the 180-Hz and the 280-Hz QPO in XTE J1550-564 are independent of energy, consistent with each other and with being zero or slightly positive (hard lags). For GRO J1655-40 the lag spectrum of the 300-Hz QPO differs significantly from that of the 450-Hz QPOs. The similarity of the lag spectra of the 180-Hz and 280-Hz QPO in XTE J1550-564 suggests that these two are the same QPO seen at a different frequency in different observations. The lag spectrum of the 67-Hz QPO in GRS 1915+105 is significantly different from that of the $2.7 \times 10^{-4}$ Hz QPO in the narrow-line Seyfert 1 galaxy RE J1034+396, which disproves the suggestion that the two QPOs are the same physical phenomenon with their frequencies scaled only by the black-hole mass. The lag spectrum of the QPO in RE J1034+396 is similar to that of the 35-Hz QPO in GRS 1915+105, although identifying these two QPOs as being the same physical feature remains problematic. We compare our results with those for the lags of the kilohertz QPOs in neutron-star systems and the broadband noise component in active galactic nuclei, and discuss possible scenarios for producing the lags in these systems., Comment: 10 pages, 5 figures, accepted by MNRAS

Показано, что анализ сигналов полного потока рентгеновской активности микроквазаров, фиксируемых обзорной камерой ASM (All-Sky Monitor) орбитальной обсерватории RXTE (Rossi X-Ray Timing Explorer), позволяет не только установить некоторые механизмы аккреции вещества в двойных звездных системах, но и провести их классификацию в зависимости от проявления эффектов статистической памяти. В качестве примера с помощью частотных зависимостей корреляторов и их производных, вычисляемых на основе формализма функций памяти, выявляются особенности рентгеновского излучения двойной системы Cygnus X-1. Выделены три класса астрофизических объектов в зависимости от соотношения временных масштабов релаксации и существования статистической памяти.

Показано, что анализ сигналов полного потока рентгеновской активности микроквазаров, фиксируемых обзорной камерой ASM (All-Sky Monitor) орбитальной обсерватории RXTE (Rossi X-Ray Timing Explorer), позволяет не только установить некоторые механизмы аккреции вещества в двойных звездных системах, но и провести их классификацию в зависимости от проявления эффектов статистической памяти. В качестве примера с помощью частотных зависимостей корреляторов и их производных, вычисляемых на основе формализма функций памяти, выявляются особенности рентгеновского излучения двойной системы Cygnus X-1. Выделены три класса астрофизических объектов в зависимости от соотношения временных масштабов релаксации и существования статистической памяти.

Показано, что анализ сигналов полного потока рентгеновской активности микроквазаров, фиксируемых обзорной камерой ASM (All-Sky Monitor) орбитальной обсерватории RXTE (Rossi X-Ray Timing Explorer), позволяет не только установить некоторые механизмы аккреции вещества в двойных звездных системах, но и провести их классификацию в зависимости от проявления эффектов статистической памяти. В качестве примера с помощью частотных зависимостей корреляторов и их производных, вычисляемых на основе формализма функций памяти, выявляются особенности рентгеновского излучения двойной системы Cygnus X-1. Выделены три класса астрофизических объектов в зависимости от соотношения временных масштабов релаксации и существования статистической памяти.

We observed the new X-ray transient and black hole candidate XTEJ1652-453 simultaneously with XMM-Newton and the Rossi X-ray Timing Explorer (RXTE). The observation was done during the decay of the 2009 outburst, when XTEJ1652-453 was in the hard-intermediate state. The spectrum shows a strong and broad Fe emission line with an equivalent width of similar to 450 eV. The profile is consistent with that of a line being produced by reflection off the accretion disc, broadened by relativistic effects close to the black hole. The best-fitting inner radius of the accretion disc is similar to 4 gravitational radii. Assuming that the accretion disc is truncated at the radius of the innermost stable circular orbit, the black hole in XTEJ1652-453 has a spin parameter of similar to 0.5. The power spectrum of the RXTE observation has an additional variability component above 50 Hz, which is typical for the hard-intermediate state. No coherent quasi-periodic oscillations at low frequency are apparent in the power spectrum, which may imply that we view the system at a rather low inclination angle.

Cygnus X-1 is the archetypal black hole (BH) binary system in our Galaxy. We report the main results of an extensive search for transient gamma-ray emission from Cygnus X-1 carried out in the energy range 100 MeV - 3 GeV by the AGILE satellite, during the period 2007 July - 2009 October. The total exposure time is about 300 days, during which the source was in the "hard" X-ray spectral state. We divided the observing intervals in 2 or 4 week periods, and searched for transient and persistent emission. We report an episode of significant transient gamma-ray emission detected on 2009, October 16 in a position compatible with Cygnus X-1 optical position. This episode, occurred during a hard spectral state of Cygnus X-1, shows that a 1-2 day time variable emission above 100 MeV can be produced during hard spectral states, having important theoretical implications for current Comptonization models for Cygnus X-1 and other microquasars. Except for this one short timescale episode, no significant gamma-ray emission was detected by AGILE. By integrating all available data we obtain a 2$\sigma$ upper limit for the total integrated flux of $F_{\gamma,U.L.} = 3 \times 10^{-8} \rm ph cm^{-2} s^{-1}$ in the energy range 100 MeV - 3 GeV. We then clearly establish the existence of a spectral cutoff in the energy range 1-100 MeV that applies to the typical hard state outside the flaring period and that confirms the historically known spectral cutoff above 1 MeV., Comment: Accepted for publication by ApJ on the 9th of Feb 2010, 5 pages, 3 figures

We present an analysis of the interactions of a distribution of relativistic protons under the typical conditions of the corona surrounding an accreting black hole. The model is applied to a concrete binary system with a high-mass donor star: the classical source Cygnus X-1, Facultad de Ciencias Astronómicas y Geofísicas

The INTEGRAL satellite observed the binary black hole system Cygnus X-1 on 2003 June 7–11. The source was detected up to 800 keV, its spectrum beeing compatible with a power law of photon index 2.3. In the framework of the accreting black hole phenomenology, the source was in an unusual state, probably the so-called “intermediate state”. Using the results of the analysis performed on these INTEGRAL data, we present here the spectral properties of our observations over the whole energy range covered by the high-energy instruments on board INTEGRAL . We then compare them with those obtained from earlier observations of Cygnus X-1 performed during the INTEGRAL performance and verification phase in the fall of 2002. At that time, the source was found in the low/hard state, indicating that Cygnus X-1 underwent a transition towards a softer state in 2003 June. We discuss the spectral parameters obtained from a Comptonization model.

We studied the energy and frequency dependence of the Fourier time lags and intrinsic coherence of the kilohertz quasi-periodic oscillations (kHz QPOs) in the neutron-star low-mass X-ray binaries 4U 1608-52 and 4U 1636-53, using a large data set obtained with the Rossi X-ray Timing Explorer. We confirmed that, in both sources, the time lags of the lower kHz QPO are soft and their magnitude increases with energy. We also found that: (i) In 4U 1636-53, the soft lags of the lower kHz QPO remain constant at similar to 30 mu s in the QPO frequency range 500-850 Hz, and decrease to similar to 10 mu s when the QPO frequency increases further. In 4U 1608-52, the soft lags of the lower kHz QPO remain constant at 40 mu s up to 800 Hz, the highest frequency reached by this QPO in our data. (ii) In both sources, the time lags of the upper kHz QPO are hard, independent of energy or frequency and inconsistent with the soft lags of the lower kHz QPO. (iii) In both sources the intrinsic coherence of the lower kHz QPO remains constant at similar to 0.6 between 5 and 12 keV, and drops to zero above that energy. The intrinsic coherence of the upper kHz QPO is consistent with being zero across the full energy range. (iv) In 4U 1636-53, the intrinsic coherence of the lower kHz QPO increases from similar to 0 at similar to 600 Hz to similar to 1, and it decreases to similar to 0.5 at 920 Hz; in 4U 1608-52, the intrinsic coherence is consistent with the same trend. (v) In both sources 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 between 700 and 900 Hz where the intrinsic coherence marginally increases. 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.