Fast Time Variability Research Articles

Very low-frequency oscillations from the 11 Hz pulsar in Terzan 5: frame dragging back on the table.

ABSTRACT We present a re-analysis of 47 Rossi X-ray Timing Explorer observations of the 11 Hz accreting pulsar IGR J17480-2446 in Terzan 5 during its 2010 outburst. We studied the fast-time variability properties of the source and searched for quasi-periodic oscillations (QPOs) in a large-frequency range. General relativity predicts that frame dragging occurs in the vicinity of a spinning compact object and induces the precession of matter orbiting said object. The relativistic precession model predicts that this frame dragging can be observed as QPOs with a characteristic frequency in the light curves of accreting compact objects. Such QPOs have historically been classified as horizontal branch oscillations in neutron star (NS) systems, and for a NS spinning at 11 Hz, these oscillations are expected at frequencies below 1 Hz. However, previous studies of IGR J17480-2446 have classified QPOs at 35–50 Hz as horizontal branch oscillations, thus casting doubts on the frame-dragging nature of such QPOs. Here, we report the detection of seven very low-frequency QPOs, previously undetected, with centroid frequencies below 0.3 Hz, and which can be ascribed to frame dragging. We also discuss the possible nature of the QPOs detected at 35–50 Hz in this alternative scenario.

Rapid spectral variability of a giant flare from a magnetar in NGC253.

Magnetars are neutron stars with extremely strong magnetic fields (1013 to 1015 gauss)1,2, which episodically emit X-ray bursts approximately 100 milliseconds long and with energies of 1040 to 1041 erg. Occasionally, they also produce extremely bright and energetic giant flares, which begin with a short (roughly 0.2 seconds), intense flash, followed by fainter, longer-lasting emission that is modulated by the spin period of the magnetar3,4 (typically 2 to 12 seconds). Over the past 40 years, only three such flares have been observed in our local group of galaxies3-6, and in all cases the extreme intensity of the flares caused the detectors to saturate. It has been proposed that extragalactic giant flares are probably a subset7-11 of short γ-ray bursts, given that the sensitivity of current instrumentation prevents us from detecting the pulsating tail, whereas the initial bright flash is readily observable out to distances of around 10 to 20 million parsecs. Here we report X-ray and γ-ray observations of the γ-ray burst GRB200415A, which has a rapid onset, very fast time variability, flat spectra and substantial sub-millisecond spectral evolution. These attributes match well with those expected for a giant flare from an extragalactic magnetar12, given that GRB200415A is directionally associated13 with the galaxy NGC253 (roughly 3.5 million parsecs away). The detection of three-megaelectronvolt photons provides evidence for the relativistic motion of the emitting plasma. Radiation from such rapidly moving gas around a rotating magnetar may have generated the rapid spectral evolution that we observe.

Mass and spin measurements for the neutron star 4U1608−52 through the relativistic precession model

We present a systematic analysis of the complete set of observations of the neutron star low-mass X-ray binary 4U1608-52 obtained by the Rossi X-ray Timing Explorer's Proportional Counter Array. We study the spectral and fast-time variability properties of the source in order to determine the mass and spin of the neutron star via the relativistic precession model, and find 24 observations containing usable sets of the necessary three quasi-periodic oscillations (triplets) with which to accomplish this task, along with a further 7 observations containing two of the three quasi-periodic oscillations each. We calculate the spin and mass of the source for each of the triplets, and find that they give physically realistic estimates clustering in the spin range $0.19 < a < 0.35$ and mass range $2.15 < M/\textrm{M}_{\odot} < 2.6$. Neutron stars present environments for studying matter under the most extreme conditions of pressure and density; as their equation of state is not yet known, accurate measurements of their mass and spin will eventually allow for the discrimination between various models. We discuss the implications of our findings in the context of equation of state predictions, physically allowed spin ranges, emission proximity to the innermost stable circular orbit and possible model inaccuracies.

Modeling and control of hypersonic vehicle dynamic under centroid shift

Due to the huge flight scale and the fast speed of hypersonic vehicle, the system must be of strong nonlinearity, coupling, and fast time variability, which give rise to the huge challenge for the design of controller. The good control performance must be based on the elaborately designed controller, which is established in the carefully designed center of mass. Once the center of mass moves unexpectedly, it is bound to affect the ability of existing flight controller to maintain the stability of hypersonic vehicle, resulting in serious consequences, even loss of control. Based on Newton’s laws and Varignon’s theory, a mathematical model for hypersonic vehicle with centroid shift is built up to research the influence of centroid on the motion of hypersonic vehicle. The zero-input response tests are conducted from the different aircraft body axes of the coordinate. Simulation results show that such influence is coupling, abrupt, irregular, and time-variant. In order to inhibit the bad influence of unexpected centroid shift, terminal sliding mode controller combined with radial basis function neural networks and just terminal sliding mode controller alone are adopted to handle such problems in view of robust control itself and auxiliary compensation. Simulation results show that such influence can be inhibited and compensated in a certain region, and the further research is still needed.

Fault-tolerant anti-windup control for hypersonic vehicles in reentry based on ISMDO

Due to the extreme large flight scale of Hypersonic Vehicle (HSV), the system inevitably possesses strong nonlinearity, coupling, fast time-variability and is also sensitive to disturbance and fault. The method of external anti-windup system combined with the terminal sliding mode control law (TSMC) is presented for the nonlinear control problem under the restriction of control surfaces for HSV. It can realize the compensation for the control surface saturation and let the HSV smoothly track the command signals. Then, the improved sliding mode disturbance observer (ISMDO) is proposed to estimate unknown parameters and strong external disturbance as well as the unknown actuator fault. This method does not need the information of disturbance and the fault bounds and has fewer learning parameters, which makes it suitable for the real-time control. Finally, the simulation test of attitude control for the reentry HSV is conducted, and the results show the effectiveness and robustness of the proposed scheme.

Multi-messenger Light Curves from Gamma-Ray Bursts in the Internal Shock Model

Abstract Gamma-ray bursts (GRBs) are promising as sources of neutrinos and cosmic rays. In the internal shock scenario, blobs of plasma emitted from a central engine collide within a relativistic jet and form shocks, leading to particle acceleration and emission. Motivated by present experimental constraints and sensitivities, we improve the predictions of particle emission by investigating time-dependent effects from multiple shocks. We produce synthetic light curves with different variability timescales that stem from properties of the central engine. For individual GRBs, qualitative conclusions about model parameters, neutrino production efficiency, and delays in high-energy gamma-rays can be deduced from inspection of the gamma-ray light curves. GRBs with fast time variability without additional prominent pulse structure tend to be efficient neutrino emitters, whereas GRBs with fast variability modulated by a broad pulse structure can be inefficient neutrino emitters and produce delayed high-energy gamma-ray signals. Our results can be applied to quantitative tests of the GRB origin of ultra-high-energy cosmic rays, and have the potential to impact current and future multi-messenger searches.

Sliding mode decoupling control of a generic hypersonic vehicle based on parametric commands

A sliding mode decoupling attitude controller based on parametric commands is proposed for a generic hypersonic vehicle (GHV). This vehicle model has fast time variability and strong coupling, is highly nonlinear, and includes uncertain parameters. The design of the controller takes these features into account. First, for the purpose of decoupling, the inner loop of the controller is designed using the dynamic inversion (DI) method. Input/output linearization is achieved using full-state feedback to globally linearize the nonlinear dynamics of selected controlled outputs. Second, to improve the robustness of the attitude control system, the sliding mode control (SMC) method is used to design the outer loop of the controller. Although the DI and SMC methods result in decoupling and robustness, there exists serious inconsistency between the commands of the attitude angles and the commands of the first-order differential of the attitude angles. To solve this problem and achieve a trade-off between dynamic response speed and attitude-tracking precision, we propose a parametric method for calculating the commands of the first-order differential of the attitude angles. Finally, simulation studies are conducted for the trimmed cruise conditions of 33.5 km altitude and Mach 15, and the responses of the vehicle to the step commands of pitch angle, yaw angle, and rolling angle are examined. The simulation studies demonstrate that the proposed controller is robust with respect to the uncertain parameters and atmospheric disturbance and meets the performance requirements of the GHV with acceptable control inputs.

On the area of accretion curtains from fast aperiodic time variability of the intermediate polar EX Hya

We present results of a study of the fast timing variability of the magnetic cataclysmic variable (mCV) EX Hya. It was previously shown that one may expect the rapid flux variability of mCVs to be smeared out at timescales shorter than the cooling time of hot plasma in the post shock region of the accretion curtain near the WD surface. Estimates of the cooling time and the mass accretion rate, thus provide us with a tool to measure the density of the post-shock plasma and the cross-sectional area of the accretion funnel at the WD surface. We have probed the high frequencies in the aperiodic noise of one of the brightest mCV EX Hya with the help of optical telescopes, namely SALT and the SAAO 1.9m telescope. We place upper limits on the plasma cooling timescale $\tau<$0.3 sec, on the fractional area of the accretion curtain footprint $f<1.6\times10^{-4}$, and a lower limit on the specific mass accretion rate $\dot{M}/A \gtrsim $3 g/sec/cm$^{-2}$. We show that measurements of accretion column footprints via eclipse mapping highly overestimate their areas. We deduce a value of $\Delta r/r \lesssim 10^{-3}$ as an upper limit to the penetration depth of the accretion disc plasma at the boundary of the magnetosphere.

Black hole spin measurements through the relativistic precession model: XTE J1550-564

Abstract We present a systematic analysis of the complete set of observations of the black hole (BH) binary XTE J1550-564 obtained by the Rossi X-ray Timing Explorer. We study the fast time variability properties of the source and determine the spin of the BH through the relativistic precession model. Similarly to what is observed in the BH binary GRO J1655-40, the frequencies of the QPOs and broad-band noise components match the general relativistic frequencies of particle motion close to the compact object predicted by the relativistic precession model. The combination of two simultaneously observed quasi-periodic oscillation (QPO) frequencies together with the dynamical BH mass from optical/infrared observations yields a spin equal to a = 0.34 ± 0.01, consistent with previous determinations from X-ray spectroscopy. Based on the derived BH parameters, the low-frequency QPO emission radii vary from ∼30 gravitational radii (Rg) to the innermost stable orbit for this spin (∼5 Rg), where they sharply disappear as observed for the case of GRO J1655-40.

Precise mass and spin measurements for a stellar-mass black hole through X-ray timing: the case of GRO J1655−40

We present a systematic analysis of the fast time variability properties of the transient black hole binary GRO J1655-40, based on the complete set of RossiXTE observations. We demonstrate that the frequencies of the quasi-periodic oscillations and of the broad band noise components and their variations match accurately the strong field general relativistic frequencies of particle motion in the close vicinity of the innermost stable circular orbit, as predicted by the relativistic precession model. We obtain high precision measurements of the black hole mass (M = (5.31 +/- 0.07) M solar masses, consistent with the value from optical/NIR observations) and spin (a/M = 0.290 +/- 0.003), through the sole use of X-ray timing.

TIME SERIES ANALYSIS OF GAMMA-RAY BLAZARS AND IMPLICATIONS FOR THE CENTRAL BLACK-HOLE MASS

Radiation from the blazar class of of active galactic nuclei (AGN) exhibits fast time variability which is usually ascribed to instabilities in the emission region near the central supermassive black hole. The variability time scale is generally faster in higher energy region, and data recently provided by the {\it Fermi} Gamma-ray Space Telescope in the GeV energy band enable a detailed study of the temporal behavior of AGN. Due to its wide field-of-view in the scanning mode, most sky regions are observed for several hours per day and daily light curves of many AGN have been accumulated for more than 4 years. In this paper we investigate the time variability of 15 well-detected AGNs by studying the normalized power spectrum density (NPSD) of their light curves in the GeV energy band. One source, 3C 454.3, shows a specific time scale of $6.8\times10^5$ s, and this value suggests, assuming the internal shock model, a mass for the central black hole of $(10^8$--$10^{10})M_\odot$ which is consistent with other estimates. It also indicates the typical time interval of ejected blobs is $(7$--$70)$ times the light crossing time of the Schwarzschild radius.

SNR Estimation Algorithm Based on Pilot Symbols for DFT-Spread OFDM Systems over Underwater Acoustic Channels

Abstract Effective and low complexity methods for signal to noise ratio (SNR) estimation remain a challenge to underwater acoustic communication due to the ocean environment characteristic which can be summarized as simultaneously severe frequency selectivity and fast-time variability. In this study, a novel SNR estimation method was proposed by using only one training symbols in the pilots and applied to DFT-Spread OFDM transmission system over underwater acoustic channels. The method will be effective against frequency selectivity and fast time varying of underwater acoustic channels. Simulations showed technically feasibility of our proposal. Further, a systematic investigation in the shallow water near Xiamen demonstrated that single training symbol based SNR estimator would be applicable for DFT-Spread OFDM system over underwater acoustic channels.

The evolution of the timing properties of the black-hole transient GX 339–4 during its 2002/2003 outburst

We present the results of the timing and color analysis of more than two hundred RXTE/PCA observations of the bright black-hole transient GX 339-4 obtained during its 2002/2003 outburst. The color-intensity evolution of the system, coupled to the properties of its fast time variability, allow the identification of four separate states. Depending on the state, strong noise is detected, together with a variety of quasi-periodic oscillations at frequencies from 0.2 to 8 Hz. We present a characterization of the timing parameters of these states and compare them to what has been observed in other systems. These results, together with those obtained from energy spectra, point towards a common evolution of black-hole transients through their outbursts.

The X-ray fast-time variability of Sco X-2 (GX 349+2) with RXTE

Sco X-2 (GX 349+2) is a low-mass X-ray binary and Z source. We have analysed 156 ks of Rossi X-ray Timing Explorer data, obtained in 1998 January, on this source. We investigated the fast-time variability as a function of position on the Z track. During these observations, Sco X-2 traced out the most extensive Z track ever reported from this object, making this the most comprehensive study thus far. We found the broad peaked flaring branch noise that is typical of Sco X-2, with a centroid frequency in the range 3.3--5.8 Hz. We also discovered low frequency noise, and a new peaked noise feature, with centroid frequencies in the range 5.4--7.6 Hz and 11--54 Hz, respectively. We discuss the phenomenology of these features, their relationship with the power spectral components found in other low-mass X-ray binaries, and the implications for current models. In particular, the low frequency noise we observed was strongest at intermediate energies, in contrast to the low frequency noise seen in other Z sources. We also detected very low frequency noise, and have calculated complex cross spectra between intensity and hardness. We found that the very low frequency noise is not entirely due to motion along the Z track.

On the X-ray fast-time variability of Sco X-2 (GX 349+2)

We have analysed archived Ginga data on the Z source Sco X-2 (GX 349+2). We present the first detailed investigation of its X-ray fast-time variability, as a function of position in the Z track. During the two-day observation over the period 5-7 March 1989, the source was in the so-called flaring branch, and the lower part of the so-called normal branch. We found broad peaked noise with a centroid frequency and width of ~4-7 Hz and ~6-12 Hz respectively. The peaked noise was strongest in the lower flaring branch, with a maximum fractional rms amplitude of ~3% . We conclude that it is not a manifestation of atoll source high frequency noise, as had been suggested, and compare it with the power spectral features seen in other Z sources. We find that the peaked noise is markedly different to the quasi-periodic oscillations found in the normal and flaring branches of Sco X-1; however it bears some resemblance to that seen in the flaring branch of Cyg X-2 at low overall intensities.

On the Nature of XTE J0421+560/CI Camelopardalis

We present the results of an analysis of RXTE, BATSE, and optical/infrared data of the 1998 outburst of the X-ray transient system XTE J0421+560 (CI Cam). The X-ray outburst shows a very fast decay (initial e-folding time ~0.5 days, slowing down to ~2.3 days). The X-ray spectrum in the 2-25 keV band is complex, softening considerably during decay and with strongly variable intrinsic absorption. A strong iron emission line is observed. No fast time variability is detected (<0.5% rms in the 1-4096 Hz band at the outburst peak). The analysis of the optical/IR data suggests that the secondary is a B[e] star surrounded by cool dust, and places the system at a distance of 2 kpc. At this distance, the peak at 2-25 keV luminosity is ~4 × 1037 ergs s-1. We compare the properties of this peculiar system with those of the Be/NS LMC transient A 0538-66 and suggest that CI Cam is of similar nature. The presence of strong radio emission during outburst indicates that the compact object is likely to be a black hole or a weakly magnetized neutron star.

Miniature EPSPs and sensory encoding in the primary afferents of the vestibular lagena of the toadfish, Opsanus tau.

The synaptic activity transmitted from vestibular hair cells of the lagena to primary afferent neurons was recorded in vitro using sharp, intracellular microelectrodes. At rest, the activity was composed of miniature excitatory postsynaptic potentials (mEPSPs) at frequencies from 5 to 20/s and action potentials (APs) at frequencies betwen 0 and 10/s. mEPSPs recorded from a single fiber displayed a large variability. For mEPSPs not triggering APs, amplitudes exhibited an average coefficient of variance (CV) of 0.323 and rise times an average CV of 0.516. APs were only triggered by mEPSPs with larger amplitudes (estimated 4-6 mV) and/or steeper maximum rate of rise (10.9 mV/ms, +/- 3.7 SD, n=4 experiments) compared to (3.50 mV/ms, +/-0.07 SD, n=6 experiments) for nontriggering mEPSPs. The smallest mEPSPs showed a fast rise time (0.99 ms between 10% and 90% of peak amplitude) and limited variability across fibers (CV:0.18) confirming that they were not attenuated signals, but rather represented single-transmitter discharges (TDs). The mEPSP amplitude and rise-time relationship suggests that many mEPSPs represented several, rather than a single pulse of secretion of TDs. According to the estimated overall TD frequency, the coincidence of TDs contributing to the same mEPSP were not statistically independent, indicating a positive interaction between TDs that is reminiscent of the way subminiature signals group to form miniature signals at the neuromuscular junction. Depending on the duration and intensity of efferent stimulation, a complete block of AP initiation occurred either immediately or after a delay of a few seconds. Efferent stimulation did not significantly change AP threshold level, but abruptly decreased mEPSP frequency to a near-complete block that followed the block of APs. Maximum mEPSP rate of rise decreased during, and recovered progressively after, efferent stimulation. After termination of efferent stimulation, mEPSP amplitude did not recover instantly and for a few seconds the amplitude distribution of synaptic events showed fewer large-amplitude events than during the control period. This confirms that mEPSP amplitude and rate of rise properties, which are critical for triggering afferent APs, are modified by efferent activity. The depression of afferent AP firing during efferent stimulation corresponded to a decrease in mEPSP frequency and, to a lesser extent, a decrease in mEPSP amplitude and rate of rise, suggesting, a decrease in the level of interaction among TDs contibuting to a mEPSP.

Correlation between Energy Spectral States and Fast Time Variability and Further Evidence for the Marginally Stable Orbit in 4U 1820−30

We report the results from a long-term monitoring data set obtained with the Rossi X-Ray Timing Explorer (RXTE) from the bright globular cluster binary source 4U 1820-30. During this monitoring, the source flux varied from 2.9?10 -->?9 to 8.5?10 -->?9 ergs cm-2 s-1 in the 2-10 keV band, which correspond to, respectively, count rates of 1600 and 4500 counts s-1 in the nominal 2-60 keV RXTE proportional counter array band. Its energy spectral state was observed to encompass both the banana branches and the island state. Kilohertz quasi-periodic oscillations (kHz QPOs) were observed to exist in the upper banana and lower banana branches, as well as in the island state. In particular, we report that the kHz QPOs were present only when the soft color (defined as the flux ratio of the 3.9-5.0 keV band to the 1.7-3.9 keV band) was below 0.78 without regard to the overall count rate and without regard to whether the source is in any one of the three branches or states. Their centroid frequency was well correlated with the overall count rate below 2500 counts s-1. When the count rate was above 2500 counts s-1, the QPO frequency was consistent with a constant independent of the overall count rate. We take this as further evidence that the inner edge of the accretion had reached the marginally stable orbit, and therefore the constant 1060?20 Hz is the marginally stable orbit frequency.

Fast Sinusoidal Oscillations in the Radio Flux of GRS 1915+105

We report the observation of events with fast (≤1 hr) time variability in Very Large Array observations of the Galactic superluminal source GRS 1915+105. The database consists of observations made at 67 epochs over 3 yr. We have detected fast variations on three occasions. Here we report the most remarkable of the events, observed in 1995 September 6. Over a period of approximately 2 hr, the 3.6 cm flux density of GRS 1915+105 showed several clear sinusoidal oscillations with period of 30.2 ± 0.4 minutes, going from ~40 to ~65 mJy. We discuss possible scenarios for the production of this fast sinusoidal variation in the radio flux density and propose that the variability could be the result of a hot spot orbiting the collapsed companion of the binary system.

X-Ray Observations of NGC 2110 with Ginga and ASCA

We present the Ginga and ASCA results for the narrow emission line galaxy NGC 2110. Using Ginga data we found a fast time-variability of a few hours, superimposed on a relatively constant X-ray flux. The X-ray spectrum was well fitted with an absorbed power- law plus iron K emission-line model, in which no significant reflection component w as required. Using ASCA data we confirmed the previous detection of extra soft X-ray emission and a K-shell emission line from “cold” iron in the simple absorbed power-law spectrum. For the first time we detected a K-shell absorption edge of “cold” iron. This edge feature, combined with the Ginga results, indicates that the reflection process is not dominant in NGC 2110. We propose that these iron line and edge features are due to a partial covering material with NH ∼ 1023 cm−2 , which presumably comprises “hidden” broad line region (BLR) clouds.