X-ray spectral variability of seven LINER nuclei with XMM-Newton and Chandra data

One of the most important features in active galactic nuclei (AGN) is the variability of their emission. Variability has been discovered at X-ray, UV, and radio frequencies on time scales from hours to years. Among the AGN family and according to theoretical studies, Low-Ionization Nuclear Emission Line Region (LINER) nuclei would be variable objects on long time scales. Our purpose is to investigate spectral X-ray variability in LINERs and to understand the nature of these kinds of objects, as well as their accretion mechanism. Chandra and XMM-Newton public archives were used to compile X-ray spectra of seven LINER nuclei at different epochs with time scales of years. To search for variability we fit all the spectra from the same object with a set of models, in order to identify the parameters responsible for the variability pattern. We also analyzed the light curves in order to search for short time scale (from hours to days) variability. Whenever possible, UV variability was also studied. We found spectral variability in four objects, with variations mostly related to hard energies (2-10 keV). These variations are due to changes in the soft excess, and/or changes in the absorber, and/or intrinsic variations of the source. Another two galaxies seem not to vary. Short time scale variations during individual observations were not found. Our analysis confirms the previously reported anticorrelation between the X-ray spectral index and the Eddington ratio, and also the correlation between the X-ray to UV flux ratio and the Eddington ratio. These results support an Advection Dominated Accretion Flow (ADAF) as the accretion mechanism in LINERs.Comment: 35 pages, 53 figures, recently accepted pape

X-ray spectral variability of Seyfert 2 galaxies

Variability across the electromagnetic spectrum is a property of AGN that can help constraining the physical properties of these galaxies. This is the third of a serie of papers with the aim of studying the X-ray variability of different families of AGN. The main purpose of this work is to investigate the variability pattern in a sample of optically selected type 2 Seyfert galaxies. We use the 26 Seyferts in the Veron-Cetty and Veron catalogue with data available from Chandra and/or XMM-Newton public archives at different epochs, with timescales ranging from a few hours to years. All the spectra of the same source are simultaneously fitted and we let different parameters to vary in the model. Whenever possible, short-term variations and/or long-term UV flux variations are studied. We divide the sample in Compton-thick, Compton-thin, and changing-look candidates. Short-term variability at X-rays is not found. From the 25 analyzed sources, 11 show long-term variations; eight (out of 11) are Compton-thin, one (out of 12) is Compton-thick, and the two changing-look candidates are also variable. The main driver for the X-ray changes is related to the nuclear power (nine cases), while variations at soft energies or related with absorbers at hard X-rays are less common, and in many cases these variations are accompained with variations of the nuclear continuum. At UV frequencies nuclear variations are nor found. We report for the first time two changing-look candidates, MARK273 and NGC7319. A constant reflection component located far away from the nucleus plus a variable nuclear continuum are able to explain most of our results; the Compton-thick candidates are dominated by reflection, which supresses their continuum making them seem fainter, and not showing variations, while the Compton-thin and changing-look candidates show variations.Comment: Accepted for publication in A&

X-ray spectral variability of LINERs selected from the Palomar sample

Variability is a general property of active galactic nuclei (AGN). At X-rays, the way in which these changes occur is not yet clear. In the particular case of low ionisation nuclear emission line region (LINER) nuclei, variations on months/years timescales have been found for some objects, but the main driver of these changes is still an open question. The main purpose of this work is to investigate the X-ray variability in LINERs, including the main driver of such variations, and to search for eventual differences between type 1 and 2 objects. We use the 18 LINERs in the Palomar sample with data retrieved from Chandra and/or XMM-Newton archives corresponding to observations gathered at different epochs. All the spectra for the same object are simultaneously fitted in order to study long term variations. The nature of the variability patterns are studied allowing different parameters to vary during the spectral fit. Whenever possible, short term variations from the analysis of the light curves and UV variability are studied.Comment: 49 pages, accepted. arXiv admin note: text overlap with arXiv:1305.222

Local versus regional processes and the control of community structure

The contribution of local (e.g., competition) and regional (e.g., dispersal) processes in the structure of communities remains an unresolved issue. In general, a tendency to assume local processes to be deterministic and regional to be stochastic dominates, although it is challenged. Fortunately, it can be cast as a testable proposition: if correct, the degree of determinism in the final community structure might indicate which process is more prominent in the control of community structure. However, recent findings have also suggested that stochastic patterns can arise from local processes and that dispersal can homogenize communities, which would make them appear deterministic irrespective of the mechanism involved. To evaluate these competing expectations we conducted an experiment where the initial communities had the same composition and species abundances. We hypothesized that if local processes dominate, then arrays of communities will show divergence of community structures whether connected by dispersal or not (i.e., being fully isolated). Alternatively, if regional processes dominate, the dispersal connected communities should converge while isolated ones should not. We found, however, that both groups of experimental communities showed similar patterns of change - a decline in similarity and a tendency to diverge. This suggests that biological interactions, demographic stochasticity, or both, exert noticeable control over community structure such that they reduce similarity among replicate communities and diversify their final states. We speculate that these mechanisms enhance potential for species additions, particularly in conjunction with factors such as dispersal and the size of the regional species pool

Atomically resolved structure of InAs quantum dots

InAs was grown by molecular-beam epitaxy onto GaAs(001) until quantum dots (QDs) formed. At this point, the growth was interrupted and the uncovered QDs were investigated in situ by scanning tunneling microscopy (STM). Atomically resolved STILI images of the QDs revealed that four dominating bounding facets occur, whose Miller indices were identified to be {137}. The assignment of the facet orientation was based on experiments on planar high Miller index GaAs surfaces. In addition, the latter experiments indicated that (137) facets are thermodynamically stable only up to a certain size. This conclusion is assumed to explain the sharp size distribution of InAs QDs

Step structure on GaAs(113)A studied by scanning tunneling microscopy

The GaAs(113)A surface was prepared by molecular-beam epitaxy and in situ characterized by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). The occurrence of an (8×1) reconstruction as proposed by Wassermeier et al. [Phys. Rev. B 51, 14 721 (1995)] was confirmed. Overview STM images reveal a striking anisotropy in the step structure of this surface. While steps along [332¯] (the 1× direction of the reconstruction) are straight for up to 2000 Å, steps along [11¯0] are extremely rough. In this direction, kinks occur typically after less than 100 Å. The ratio of the respective lateral step densities is 8±4. This anisotropy is explained by applying the electron counting rule (ECR) to one-dimensional islands. While islands along [332¯] fulfil the ECR, it is violated by islands along [11¯0]. Thus, if structures formed additionally perpendicular to step edges along [332¯], they would be energetically unfavorable. Hence, growth occurs mainly by propagation along [332¯]. It is suggested that the determining structural element of GaAs(113)A−(8×1) is the zigzag chain of As dimers

Experimental evidence for a stable GaAs surface near (113)

GaAs surfaces vicinal to (113) with a continuous range of misorientation angles up to 11.5° in all azimuthal directions were created by grinding a spherical depression into (113) oriented samples. Thin homoepitaxial layers were grown onto these samples by molecular beam epitaxy (MBE), and the surfaces were in situ studied by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The surface quality in the depression was verified by reproducing LEED patterns of the (113) and (114) surfaces. A stable GaAs surface was found that is oriented from (113) by 9°±2° towards [11̅0̅]. STM and LEED images of this surface are presented

A statistical analysis of electric self-potential time series associated to two 1993 earthquakes in Mexico

Recent studies related with earthquake prediction involve statistical studies of the ground electric self-potential behavior. Published results about the complexity of this kind of processes encourage us to study the statistical behavior of the ground electric self-potential recorded in Guerrero state, Mexico. This region is characterized by high seismicity. The electric self-potential variations were recorded in the Acapulco station directly from the ground. The sampling period was four seconds and the data were stored from March to December of 1993. Two significant earthquakes (EQs) occurred near this station, 15 May and 24 October whose magnitudes were Mw=6.0 and Mw=6.6 respectively. A preliminary processing was carried out consisting of a moving average of the original time series in order to filter the very high frequencies and to complete short lacks of data and outliers. Then, a visual inspection of the complete filtered signal was performed to search some seismic electric signals (SES), which were ambiguously depicted. Subsequently, a detrending of µ=0 was applied with the windows of 3.3, 6.6 and 10 h. Later, the analysis of the spectral exponent β was made, showing changes during the total period examined, and the most evident changes occurred during the preparation mechanism of the Mw=6.6 EQ. Fifteen days before the 24 October EQ, a Brownian-noise like behavior was displayed (β≈2), having a duration of about two days. In addition a Higuchi fractal method and wavelet analysis were made confirming the presence of the β-anomaly