Aperiodic variability of low-mass X-ray binaries at very low frequencies

We have obtained discrete Fourier power spectra of a sample of persistent low-mass neutron-star X-ray binaries using long-term light curves from the All Sky Monitor on board the Rossi X-ray Timing Explorer. Our aim is to investigate their aperiodic variability at frequencies in the range 1 x 10^{-7}-5 x 10^{-6} Hz and compare their properties with those of the black-hole source Cyg X-1. We find that the classification scheme that divides LMXBs into Z and atoll sources blurs at very low frequencies. Based on the long-term (~ years) pattern of variability and the results of power-law fits (P ~ v^{-a}) to the 1 x 10^{-7}-5 x 10^{-6} Hz power density spectra, low-mass neutron-star binaries fall into three categories. Type I includes all Z sources, except Cyg X-2, and the atoll sources GX9+1 and GX13+1. They show relatively flat power spectra (a < 0.9) and low variability (rms < 20%). Type II systems comprise 4U 1636-53, 4U 1735-44 and GX3+1. They are more variable (20% < rms < 30%) and display steeper power spectra (0.9 < a < 1.2) than Type I sources. Type III systems are the most variable (rms > 30%) and exhibit the steepest power spectra (a > 1.2). The sources 4U 1705-44, GX354-0 and 4U 1820-30 belong to this group. GX9+9 and Cyg X-2 appear as intermediate systems in between Type I and II and Type II and III sources, respectively. We speculate that the differences in these systems may be caused by the presence of different types of mass-donor companions. Other factors, like the size of the accretion disc and/or the presence of weak magnetic fields, are also expected to affect their low-frequency X-ray aperiodic varibility.Comment: 9 pages, 6 figures. To be published in A&

CLEAR: Covariant LEAst-square Re-fitting with applications to image restoration

In this paper, we propose a new framework to remove parts of the systematic errors affecting popular restoration algorithms, with a special focus for image processing tasks. Generalizing ideas that emerged for $\ell_1$ regularization, we develop an approach re-fitting the results of standard methods towards the input data. Total variation regularizations and non-local means are special cases of interest. We identify important covariant information that should be preserved by the re-fitting method, and emphasize the importance of preserving the Jacobian (w.r.t. the observed signal) of the original estimator. Then, we provide an approach that has a "twicing" flavor and allows re-fitting the restored signal by adding back a local affine transformation of the residual term. We illustrate the benefits of our method on numerical simulations for image restoration tasks

The Energy-dependent X-ray Timing Characteristics of the Narrow Line Seyfert 1 Mkn 766

We present the energy-dependent power spectral density (PSD) and cross-spectral properties of Mkn 766, obtained from combining data obtained during an XMM-Newton observation spanning six revolutions in 2005 with data obtained from an XMM-Newton long-look in 2001. The PSD shapes and rms-flux relations are found to be consistent between the 2001 and 2005 observations, suggesting the 2005 observation is simply a low-flux extension of the 2001 observation and permitting us to combine the two data sets. The resulting PSD has the highest temporal frequency resolution for any AGN PSD measured to date. Applying a broken power-law model yields break frequencies which increase in temporal frequency with photon energy. Obtaining a good fit when assuming energy-independent break frequencies requires the presence of a Lorentzian at 4.6+/-0.4 * 10^-4 Hz whose strength increases with photon energy, a behavior seen in black hole X-ray binaries. The cross-spectral properties are measured; temporal frequency-dependent soft-to-hard time lags are detected in this object for the first time. Cross-spectral results are consistent with those for other accreting black hole systems. The results are discussed in the context of several variability models, including those based on inwardly-propagating viscosity variations in the accretion disk.Comment: Accepted for publication in The Astrophysical Journal. 18 pages, 9 figures. Uses emulateapj5.st

An Equivalence Relation between Multiresolution Analysis ofL2(R)

AbstractThis paper is concerned with the development of an equivalence relation between two multiresolution analysis ofL2(R). The relation called unitary equivalence is created by the action of a unitary operator in such a way that the multiresolution structure and the decomposition and reconstruction algorithms remain invariant. A characterization in terms of the scaling functions of the multiresolution analysis is given. Distinct equivalence classes of multiresolution analysis are derived. Finally, we prove that B-splines give rise to nonequivalent examples

Aortic regurgitation in athletes: The challenges of echocardiographic interpretation

Background Bicuspid aortic valve (BAV) is the most common congenital cardiac defect and prone to premature degeneration causing aortic regurgitation (AR). The assessment of AR in athletic individuals poses several challenges as the pathological left ventricle (LV) remodelling caused by AR may overlap with the physiological remodelling of intense exercise. The purpose of this study is to highlight these challenges, review the existing literature and discuss how to tackle these conundrums. As a real-world example, we compare the resting transthoracic echocardiographic (TTE) findings in a cohort of individuals with BAV and AR, sub-grouped into “highly active” or “lightly active”. Methods Adult male subjects with an index TTE performed at a tertiary referral centre between 2019 and 2022 were included if the TTE confirmed a BAV and at least moderate AR. Further strict inclusion criteria were applied and parameters of valve disease severity was made in accordance with existing guidelines. Subjects completed a physical activity questionnaire over the telephone, and were classified into either group 1: “highly active” or group 2: “lightly active” based on their answers. Demographics and TTE parameters were compared between the two groups. Results 30 male subjects (mean age 44±13 years) with BAV-AR were included – 17 were highly active, and 13 lightly active. There was no significant difference in age (group 1, 45±12.7yrs vs group 2, 42±17yrs; p=0.49), height (p=0.45), weight (p=0.268) or severity of AR, when quantitative assessment was possible. Group 1 had a significantly higher stroke volume (131±17mls vs 102±13 mls; p=0.027), larger LV volumes, diastolic dimensions and significantly larger bi-atrial and right ventricular size. This LV dilatation in the context of AR and athleticism poses a diagnostic and management conundrum. Despite this, none of these 17 highly active individuals demonstrated any of the traditional criteria used to consider surgery. Conclusion There is significant overlap between the physiological adaptations to exercise and those caused by AR. Multi-modality imaging and stress testing can aid clinicians in diagnostic and management decisions in exercising individuals when there is discordance between AR severity and symptoms

B and I-band optical micro-variability observations of the BL Lac objects S5 2007+777 and 3C371

We have observed S5 2007+777 and 3C371 in the B and I bands for 13 and 8 nights, respectively, during various observing runs in 2001, 2002 and 2004. The observations resulted in almost evenly sampled light curves, 6-9 hours long. We do not detect any flares within the observed light curves, but we do observe small amplitude, significant variations, in both bands, on time scales of hours and days. The average variability amplitude on time scales of minutes/hours is 2.5% and 1-1.5% in the case of S5 2007+777 and 3C371, respectively. The average amplitudes increase to 5-12% and 4-6%, respectively, on time scales of days. We find that the B and I band variations are highly correlated, on both short and long time scales. During the 2004 observations, which resulted in the longest light curves, we observe two well defined flux-decay and rising trends in the light curves of both objects. When the flux decays, we observe significant delays, with the B band flux decaying faster than the flux in the I band. As a result, we also observe significant, flux related spectral variations as well. The flux-spectral relation is rather complicated, with loop-like structures forming during the flux evolution. The presence of spectral variations imply that the observed variability is not caused by geometric effects. On the other hand, our results are fully consistent with the hypothesis that the observed variations are caused by perturbations which affect different regions in the jet of the sources.Comment: Accepted for publication in Astronomy and Astrophysic

A jet model for black-hole X-ray sources

A jet model for Galactic black-hole X-ray binaries will be presented that appears to explain several observational characteristics. In particular, it explains the energy spectrum from radio to hard X-rays, the time-lags as a function of Fourier frequency, the increase of the variability amplitude (QPO and high frequency) with increasing photon energy, and the narrowing of the autocorrelation function with increasing photon energy. On the other hand, there are additional observational constraints that no model has tried to explain yet. It is important that we all try to address these constraints if we are to make any progress in understanding black-hole X-ray source

A Cutoff in the X-ray Fluctuation Power Density Spectrum of the Seyfert 1 Galaxy NGC 3516

During 1997 March-July, XTE observed the bright, strongly variable Seyfert 1 galaxy NGC 3516 once every ~12.8 hr for 4.5 months and nearly continuously (with interruptions due to SAA passage but not Earth occultation) for a 4.2 day period in the middle. These were followed by ongoing monitoring once every ~4.3 days. These data are used to construct the first well-determined X-ray fluctuation power density spectrum (PDS) of an active galaxy to span more than 4 decades of usable temporal frequency. The PDS shows no signs of any strict or quasi-periodicity, but does show a progressive flattening of the power-law slope from -1.74 at short time scales to -0.73 at longer time scales. This is the clearest observation to date of the long-predicted cutoff in the PDS. The characteristic variability time scale corresponding to this cutoff temporal frequency is 1 month. Although it is unclear how this time scale may be interpreted in terms of a physical size or process, there are several promising candidate models. The PDS appears similar to those seen for Galactic black hole candidates such as Cyg X-1, suggesting that these two classes of objects with very different luminosities and putative black hole masses (differing by more than a factor of 10^5) may have similar X-ray generation processes and structures.Comment: 21 pages, incl. 5 figures, AASTe