Low-frequency Lag Research Articles

X-ray time lags in PG 1211+143

We investigate the X-ray time lags of a recent ~630ks XMM-Newton observation of PG 1211+143. We find well-correlated variations across the XMM-Newton EPIC bandpass, with the first detection of a hard lag in this source with a mean time delay of up to ~3ks at the lowest frequencies. We find that the energy-dependence of the low-frequency hard lag scales approximately linearly with log(E) when averaged over all orbits, consistent with the propagating fluctuations model. However, we find that the low-frequency lag behaviour becomes more complex on timescales longer than a single orbit, suggestive of additional modes of variability. We also detect a high-frequency soft lag at ~10^{-4}Hz with the magnitude of the delay peaking at <0.8ks, consistent with previous observations, which we discuss in terms of small-scale reverberation.

The Growth of Health Spending in the USA: 1776 to 2026

This review utilizes a number of historical and contemporary sources to trace the growth of national health expenditures in the United States from 1776 to 2026, supporting four empirical generalizations: 1) Peak rates of growth occurred during the decade 1960-1970 as the modern national health system took shape. This surge in spending was fostered by large public investments in workforce training, buildings, equipment and research and began prior to the implementation of Medicare and Medicaid. 2) Growth was significantly more rapid from 1975-1995 than from 1995-2015 for reasons that are not fully clear. 3) The health share of GDP was almost constant near 4% from 1930-1955 despite major advances in technology, organization and financing. 4) There are lags between macroeconomic fluctuations and corresponding changes in health expenditure. Examination of annual data can estimate business cycle lags on the order of 3 to 6 years with confidence. However, there may also be low-frequency lags over multiple decades that cannot yet be clearly delineated with the available data. Modern medical care in 2015 is still recognizably close in form to the 1970 version, much more so than medical practice in 1970 was to that in 1930 or 1945. Highly organized and regulated, with academic medical centers at the core, there is “almost” universal insurance coverage for more than 80% of the population that subsidizes a safety net for the remainder. This patchwork of public and private insurance has cracked and frayed as the health share of GDP expanded toward 20% of GDP making the current system appear unsustainable and portending major revisions within the next twenty years. Issues regarding measurement of national health expenditures, market definition, temporal dynamics, and decomposition are discussed. Appendices provide information on data sources and modeling.

Suzaku observations of Mrk 335: confronting partial covering and relativistic reflection

We report on the deepest X-ray observation of the narrow-line Seyfert 1 galaxy Mrk 335 in the low-flux state obtained with Suzaku. The data are compared to a 2006 high-flux Suzaku observation when the source was ~10-times brighter. Describing the two flux levels self-consistently with partial covering models would require extreme circumstances, as the source would be subject to negligible absorption during the bright state and ninety-five per cent covering with near Compton-thick material when dim. Blurred reflection from an accretion disc around a nearly maximum spinning black hole (a>0.91, with preference for a spin parameter as high as ~ 0.995) appears more likely and is consistent with the long-term and rapid variability. Measurements of the emissivity profile and spectral modelling indicate the high-flux Suzaku observation of Mrk 335 is consistent with continuum-dominated, jet-like emission (i.e. beamed away from the disc). It can be argued that the ejecta must be confined to within ~25 rg if it does not escape the system. During the low-flux state the corona becomes compact and only extends to about 5 rg from the black hole, and the spectrum becomes reflection-dominated. The low-frequency lags measured at both epochs are comparable indicating that the accretion mechanism is not changing between the two flux levels. Various techniques to study the spectral variability (e.g. principal component analysis, fractional variability, difference spectra, and hardness ratio analysis) indicate that the low-state variability is dominated by changes in the power law flux and photon index, but that changes in the ionisation state of the reflector are also required. Most notably, the ionisation parameter becomes inversely correlated with the reflected flux after a long-duration flare-like event during the observation.

Simultaneous NuSTAR and XMM–Newton 0.5–80舁keV spectroscopy of the narrow-line Seyfert 1 galaxy SWIFT J2127.4+5654

We present a broad band spectral analysis of the joint XMM-Newton and NuSTAR observational campaign of the Narrow Line Seyfert 1 SWIFT J2127.4+5654, consisting of 300 ks performed during three XMM-Newton orbits. We detect a relativistic broadened iron K$\alpha$ line originating from the innermost regions of the accretion disc surrounding the central black hole, from which we infer an intermediate spin of $a$=$0.58^{+0.11}_{-0.17}$. The intrinsic spectrum is steep ($\Gamma=2.08\pm0.01$) as commonly found in Narrow Line Seyfert 1 galaxies, while the cutoff energy (E$_{\rm c}=108^{+11}_{-10}$ keV) falls within the range observed in Broad Line Seyfert 1 Galaxies. We measure a low-frequency lag that increases steadily with energy, while at high frequencies, there is a clear lag following the shape of the broad Fe K emission line. Interestingly, the observed Fe K lag in SWIFT J2127.4+5654 is not as broad as in other sources that have maximally spinning black holes. The lag amplitude suggests a continuum-to-reprocessor distance of about $ 10-20\ r_{\mathrm{g}}$. These timing results independently support an intermediate black hole spin and a compact corona.

Discovery of high-frequency iron K lags in Ark 564 and Mrk 335

We use archival XMM–Newton observations of Ark 564 and Mrk 335 to calculate the frequency-dependent time lags for these two well-studied sources. We discover high-frequency Fe K lags in both sources, indicating that the red wing of the line precedes the rest-frame energy by roughly 100 and 150 s for Ark 564 and Mrk 335, respectively. Including these two new sources, Fe K reverberation lags have been observed in seven Seyfert galaxies. We examine the low-frequency lag-energy spectrum, which is smooth, and shows no feature of reverberation, as would be expected if the low-frequency lags were produced by distant reflection off circumnuclear material. The clear differences in the low- and high-frequency lag-energy spectra indicate that the lags are produced by two distinct physical processes. Finally, we find that the amplitude of the Fe K lag scales with black hole mass for these seven sources, consistent with a relativistic reflection model where the lag is the light travel delay associated with reflection of continuum photons off the inner disc.

Modeling Wide-Frequency-Range Pilot Equalization for Control of Aircraft Pitch Dynamics

In continuous manual control tasks, human controllers adapt their control strategy to the dynamics of the controlled element. This compensation for the controlled-element dynamics is performed around the pilot–vehicle system crossover frequency, in order to obtain satisfactory performance of the combined pilot–vehicle system, but is also seen to extend to frequencies well above crossover. For a controlled element representing the linearized pitch dynamics of a small conventional jet aircraft, an extension to the models for pilot equalization described in the literature was found to be needed for the modeling of the adopted pilot equalization dynamics over a wide frequency range. Measured pilot describing functions revealed that pilots use a combination of low-frequency lag and high-frequency lead equalization to compensate for the characteristics of these typical aircraft pitch dynamics around the short-period mode. An additional high-frequency lead term in the pilot equalization transfer function was found to allow for the modeling of these adopted equalization dynamics over a wide frequency range, thereby also yielding a significant increase in the percentage of measured control inputs that is explained by the pilot model. Furthermore, for this controlled element the extended model for the equalization dynamics was found to be important for the interpretation of the changes in pilot control behavior that occur due to the presence of physical motion feedback.

Sudden changes in spectrum of an echo cause a breakdown of the precedence effect

The effect of changing the frequency components of an echo relative to the sound source was examined in a two-choice discrimination task. Subjects sat in an anechoic chamber and discriminated the direction of the lag noise burst within a lead-lag pair presented over loudspeakers. The leading noise burst was broadband, and the lagging burst was either high- or low-pass filtered. On some conditions, this test burst pair was preceded by a conditioning train of burst pairs, which also had a broadband lead and either a high- or low-frequency lag. When the frequency content of the echo was held constant across the conditioning train and test burst pair, echo suppression that was built up during the repeating train was maintained for the test burst pair, shown by the subjects' poor performance in detecting the location of the lagging burst. By comparison, subjects had little difficulty in localizing the lagging burst when the frequency content of the echo changed between the conditioning train and the test burst, indicating that any buildup of suppression during the train was broken when the lagging burst's spectrum shifted. The data are consistent with an interpretation in which echo suppression is temporarily broken when listeners' built-up expectations about room acoustics are violated.

Manual control of single-loop systems: Part II

Abstract The c. 1959 mathematical model for human operator control dynamics has been validated and extended to produce a practically complete mathematical description of manual control dynamics for single-loop systems. This model is essential to the analytical design of closed-loop man-machine systems, and it facilitates understanding of the human as a control device. An extensive number of selected experiments using 9 subjects, 4 forms of plant dynamics of general applicability, and 3 principal forcing functions, yielded definitive describing function data over a frequency range of two decades including system crossover. Models were constructed at three levels of detail: 1) a crossover model which is easily and usefully applied; 2) an extended crossover model which accounts more adequately for low frequency lags and plant dynamics; and 3) a precision model which provides a description so detailed that inferences can be drawn about neuromuscular functions. The resulting adaptive, optimalizing c. 1965 human operator mathematical model is presented, with a detailed summary of its adjustments for proper application.

Time lag in photoelectric cells

§ 1. This work continues that of Campbell and Stoodley and of Campbell. § 2. Frequency-response curves are compared with oscillograph traces of the current through the cell when light is suddenly thrown on or off it. A theory of the comparison is given; it is shown that the comparison provides a test of the assumption of independence, i.e. the assumption that time lag arises wholly in processes following a single instantaneous light-impulse, and not from the interaction of succeeding impulses. The experiments are wholly adverse to the assumption; low-frequency time lag must arise in great part from such interaction. The evidence on which the assumption was based previously is re-examined; it is found to be exceptional and not typical, and therefore quite inconclusive. § 3. Attempts to determine directly how time lag is affected by the size of the cells, other things being as far as possible equal, failed because it was evident that factors other than those contemplated were being varied accidentally. But measurements on a single long cell with several anodes indicated that low-frequency time lag is not due directly to the finite velocity of the ions. Incidentally the positive ion current was found - as is to be expected - to show greater time lag than the whole current. § 4. The experiments on cells cooled in liquid air, described in paper II, were continued; the previous results were confirmed; temperature has an effect separate from that on the density of the gas. § 5. The very small time lag of argon-filled cells with sensitized potassium cathodes was found to be due to hydrogen evolved from the cathode. When this is removed the time lag increases greatly. Further investigation of this remarkable result is deferred. § 6. In a brief discussion it is pointed out that the distinction previously made between low-frequency and high-frequency time lag must be maintained; the latter is undoubtedly due to the finite velocity of the ions, the former is not. For the low-frequency lag metastable states are still the most plausible explanation; but there are difficulties in the failure of the assumption of independence and in the effect of hydrogen. No alternative theory is proposed at present; more facts are required.