Cross-spectral modelling of the black hole X-ray binary XTEJ1550-564: challenges to the propagating fluctuations paradigm

Timing properties of black hole X-ray binaries in outburst can be modeled with mass accretion rate fluctuations propagating towards the black hole. Such models predict time lags between energy bands due to propagation delays. First application of a propagating fluctuations model to black hole power spectra showed good agreement with the data. Indeed, hard lags observed from these systems appear to be in agreement with this generic prediction. Our PROPFLUC code allows to simultaneously predict power spectra, time lags, and coherence of the variability as a function of energy. This was successfully applied to Swift data on the black hole MAXIJ1659-152, fitting jointly the power spectra in two energy bands and the cross-spectrum between these two bands. In the current work, we attempt to to model two high signal to noise Rossi X-ray Timing Explorer (RXTE) observations of the black hole XTE J1550-564. We find that neither observation can be adequately explained by the model even when considering, additionally to previous PROPFLUC versions, different propagation speeds of the fluctuations. After extensive exploration of model extensions, we tentatively conclude that the quantitative and qualitative discrepancy between model predictions and data is generic to the propagating fluctuations paradigm. This result encourages further investigation of the fundamental hypotheses of the propagating fluctuations model. We discuss some of these hypotheses with an eye to future works.Comment: 14 pages, 12 figures, accepted for publication in MNRA

Evolution of the hot flow of MAXI J1543-564

We present a spectral and timing analysis of the black hole candidate MAXI J1543-564 during its 2011 outburst. As shown in previous work, the source follows the standard evolution of a black hole outburst. During the rising phase of the outburst we detect an abrupt change in timing behavior associated with the occurrence of a type-B quasi-periodic oscillation (QPO). This QPO and the simultaneously detected radio emission mark the transition between hard and soft intermediate state. We fit power spectra from the rising phase of the outburst using the recently proposed model propfluc. This assumes a truncated disc / hot inner flow geometry, with mass accretion rate fluctuations propagating through a precessing inner flow. We link the propfluc physical parameters to the phenomenological multi-Lorentzian fit parameters. The physical parameter dominating the QPO frequency is the truncation radius, while broad band noise characteristics are also influenced by the radial surface density and emissivity profiles of the flow. In the outburst rise we found that the truncation radius decreases from $r_o \sim 24$ to $10 R_g$, and the surface density increases faster than the mass accretion rate, as previously reported for XTE J1550-564. Two soft intermediate state observations could not be fitted with propfluc, and we suggest that they are coincident with the ejection of material from the inner regions of the flow in a jet or accretion of these regions into the BH horizon, explaining the drop in QPO frequency and suppression of broad band variability preferentially at high energy bands coincident with a radio flare.Comment: 13 pages, 11 figures, 2 table

The aperiodic timing behaviour of the accretion-driven millisecond pulsar SAX J1808.4-3658

We studied the aperiodic X-ray timing behaviour of the accreting millisecond pulsar SAX J1808.4-3658. The source was recently found to be the first accreting millisecond pulsar that shows the kilohertz quasi-periodic oscillations (kilohertz QPOs) that are found in many other X-ray binaries with accreting neutron stars. The high frequency of these signals reflects the short dynamical time scales in the region near the compact object where they originate. We find that in addition to the kilohertz QPOs SAX J1808.4-3658 shows several low frequency timing features, based on which the source can be classified as a so-called atoll source. The frequencies of the variability components of the atoll sources follow a universal scheme of correlations. The correlations in SAX J1808.4-3658 are similar but show a shift in upper kilohertz QPO frequency. This discrepancy is perhaps related to a stronger or differently configured magnetic field.Comment: 4 pages, 3 figures. To appear in the proceedings of the "The Restless High-Energy Universe" (Amsterdam, The Netherlands), 2003, eds. E.P.J. van den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijer

Timing properties and spectral states in Aquila X-1

We have analyzed five X-ray outbursts of the neutron-star soft X-ray transient Aql X-1 and investigated the timing properties of the source in correlation with its spectral states as defined by different positions in the color-color and hardness-intensity diagrams. The hard color and the source count rate serve as the distinguishing parameters giving rise to three spectral states: a low-intensity hard state, an intermediate state and a high-intensity soft state. These states are respectively identified with the extreme island, island and banana states that characterize the atoll sources. The large amount of data analyzed allowed us to perform for the first time a detailed timing analysis of the extreme island state. Differences in the aperiodic variability between the rise and the decay of the X-ray outbursts are found in this state: at the same place in the color-color diagram, during the rise the source exhibits more power at low frequencies (< 1 Hz), whereas during the decay the source is more variable at high frequencies (> 100 Hz). The very-low frequency noise that characterizes the banana-state power spectra below 1 Hz cannot be described in terms of a single power law but a two-component model is required. In two outbursts a new 6-10 Hz QPO has been discovered and tentatively identified with the normal/flaring branch-like oscillation observed only at the highest inferred mass accretion rates. We have compared the spectral and timing properties of Aql X-1 with those of other atoll and Z sources. Our results argue against a unification scheme for these two types of neutron-star X-ray binaries.Comment: 24 pages, 4 tables, 9 figures, accepted for publication in Ap

Precise Measurements of the Kilohertz Quasi-Periodic Oscillations in 4U 1728-34

We have analyzed seventeen observations of the low-mass X-ray binary and atoll source 4U 1728-34, carried out by the Rossi X-ray Timing Explorer in 1996 and 1997. We obtain precise measurements of the frequencies of the two simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in this source. We show that the frequency separation between the two QPO, $\Delta \nu$, is always significantly smaller than the frequency of the nearly-coherent oscillations seen in this source during X-ray bursts, even at the lowest inferred mass accretion rate, when $\Delta \nu$ seems to reach its maximum value. We also find that $\Delta \nu$ decreases significantly, from $349.3 \pm 1.7$ Hz to $278.7 \pm 11.6$ Hz, as the frequency of the lower frequency kHz QPO increases from 615 to 895 Hz. This is the first time that variations of the kHz QPO peak separation are measured in a source which shows nearly-coherent oscillations during bursts.Comment: Accepted for publication in The Astrophysical Journal Letters. Uses AAS LaTex v4.0 (5 pages plus 4 postscript figures

Intrauterine repair of gastroschisis in fetal rabbits

Objective: Infants with gastroschisis (GS) still face severe morbidity. Prenatal closure may prevent gastrointestinal organ damage, but intrauterine GS repair (GSR) has not been established yet. Methods: In New Zealand White rabbits we developed and compared GS versus GSR: creation of GS was achieved by hysterotomy, right-sided laparotomy of the fetus and pressure on the abdominal wall to provoke evisceration. GSR was accomplished by careful reposition of eviscerated organs and a running suture of the fetal abdominal wall. For study purposes, 18 animals were divided equally into 3 groups: GS, GS with GSR after 2 h, and unmanipulated controls (C). Vitality was assessed by echocardiography. After 5 h all animals were sacrificed. Results: GSR inflicted no increased mortality, because all fetuses survived GS or GS with GSR. All fetuses with GS demonstrated significant evisceration of abdominal organs. In contrast, the abdominal wall of the fetuses from GSR was intact. Conclusion:The present animal model demonstrated the technical feasibility and success of an intrauterine repair of GS for the first time. However, further long-term studies (leaving GS and GSR in utero for several days) will be necessary to compare survival rates and intestinal injury, motility or absorption. The clinical application of GSR in utero remains a vision so far. Copyright (C) 2003 S. Karger AG, Basel

On the harmonics of the low-frequency quasi-periodic oscillation in GRS 1915+105

GRS 1915+105 is a widely studied black hole binary, well known because of its extremely fast and complex variability. Flaring periods of high variability alternate with "stable" phases (the plateaux) when the flux is low, the spectra are hard and the timing properties of the source are similar to those of a number of black hole candidates in hard spectral state. In the plateaux the power density spectra are dominated by a low frequency quasi periodic oscillation (LFQPO) superposed onto a band limited noise continuum and accompanied by at least one harmonic. In this paper we focus on three plateaux, presenting the analysis of the power density spectra and in particular of the LFQPO and its harmonics. While plotting the LFQPO and all the harmonics together on a frequency-width plane, we found the presence of a positive trend of broadening when the frequency increases. This trend can shed light in the nature of the harmonic content of the LFQPO and challenges the usual interpretation of these timing features.Comment: 10 pages, 8 figures. Accepted for publication in MNRA

A Phase Lag between Disk and Corona in GRMHD Simulations of Precessing Tilted Accretion Disks

In the course of its evolution, a black hole (BH) accretes gas from a wide range of directions. Given a random accretion event, the typical angular momentum of an accretion disc would be tilted by $\sim$60$^\circ$ relative to the BH spin. Misalignment causes the disc to precess at a rate that increases with BH spin and tilt angle. We present the first general-relativistic magnetohydrodynamic (GRMHD) simulations spanning a full precession period of highly tilted (60$^\circ$), moderately thin ($h/r=0.1$) accretion discs around a rapidly spinning ($a\simeq0.9$) BH. While the disc and jets precess in phase, we find that the corona, sandwiched between the two, lags behind by $\gtrsim 10^{\circ}$. For spectral models of BH accretion, the implication is that hard non-thermal (corona) emission lags behind the softer (disc) emission, thus potentially explaining some properties of the hard energy lags seen in Type-C low frequency quasi-periodic oscillations in X-Ray binaries. While strong jets are unaffected by this disc-corona lag, weak jets stall when encountering the lagging corona at distances $r \sim 100$ black hole radii. This interaction may quench large-scale jet formation.Comment: 5 pages, 4 figures, submitted to MNRAS, see YouTube playlist for 3D renderings: https://www.youtube.com/playlist?list=PLDO1oeU33GwmwOV_Hp9s7572JdU8JPSS