The well established negative correlation between the {alpha}_OX_ spectral slope and the optical/UV luminosity, a by product of the relation between X-rays and optical/UV luminosity, is affected by a relatively large dispersion. The main contributions can be variability in the X-ray/UV ratio and/or changes in fundamental physical parameters. We want to quantify the contribution of variability within single sources (intra-source dispersion) and that due to variations of other quantities different from source to source (inter-source dispersion). We use archival data from the XMM-Newton Serendipitous Source Catalog (XMMSSC, Cat. IX/50) and from the XMM-OM Serendipitous Ultra-violet Source Survey (XMMOM-SUSS3, Cat. II/340). We select a sub-sample in order to decrease the dispersion of the relation due to the presence of Radio-Loud and Broad Absorption Line objects, and to absorptions in both X-ray and optical/UV bands. We use the Structure Function (SF) to estimate the contribution of variability to the dispersion. We analyse the dependence of the residuals of the relation on various physical parameters in order to characterise the inter-source dispersion. We find a total dispersion of {sigma}~0.12 and we find that intrinsic variability contributes for 56 percent of the variance of the {alpha}_OX_-L_UV_ relation. If we select only sources with a larger number of observational epochs (>2) the dispersion of the relation decreases by approximately 15 percent. We find weak but significant dependences of the residuals of the relation on black-hole mass and on Eddington ratio, which are also confirmed by a multivariate regression analysis of {alpha}_OX_ as a function of UV luminosity and black-hole mass and/or Eddington ratio. We find a weak positive correlation of both the {alpha}_OX_ index and the residuals of the {alpha}_OX_-LUV relation with inclination indicators, such as the FWHM(H{beta}) and the EW[OIII], suggesting a weak increase of X-ray/UV ratio with the viewing angle. This suggests the development of new viewing angle indicators possibly applicable at higher redshifts. Moreover, our results suggest the possibility of selecting a sample of objects, based on their viewing angle and/or black-hole mass and Eddington ratio, for which the {alpha}_OX_-LUV relation is as tight as possible, in light of the use of the optical/UV-X-ray luminosity relation to build a distance modulus (DM)-z plane and estimate cosmological parameters.
The observed relation between the X-ray radiation from active galactic nuclei, originating in the corona, and the optical/UV radiation from the disk is usually described by the anticorrelation between the UV to X-ray slope {alpha}_ox_ and the UV luminosity. Many factors can affect this relation, including: i) enhanced X-ray emission associated with the jets of radio-loud AGNs, ii) X-ray absorption associated with the UV broad absorption line (BAL) outflows, iii) other X-ray absorption not associated with BALs, iv) intrinsic X-ray weakness, v) UV and X-ray variability, and non-simultaneity of UV and X-ray observations. The separation of these effects provides information about the intrinsic {alpha}_ox_-L_UV_ relation and its dispersion, constraining models of disk-corona coupling. We use simultaneous UV/X-ray observations to remove the influence of non-simultaneous measurements from the {alpha}_ox_-L_UV_ relation.
We analyze archived Chandra and XMM-Newton X-ray observations of 536 Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) quasars (QSOs) at 1.7<=z<=2.7 in order to characterize the relative UV and X-ray spectral properties of QSOs that do not have broad UV absorption lines (BALs). We constrain the fraction of X-ray-weak, non-BAL QSOs and find that such objects are rare; for example, sources underluminous by a factor of 10 comprise <~2% of optically selected SDSS QSOs. X-ray luminosities vary with respect to UV emission by a factor of <~2 over several years for most sources. UV continuum reddening and the presence of narrow-line absorbing systems are not strongly associated with X-ray weakness in our sample. X-ray brightness is significantly correlated with UV emission-line properties, so that relatively X-ray-weak, non-BAL QSOs generally have weaker, blueshifted CIV {lambda}1549 emission and broader CIII] {lambda}1909 lines. The CIV emission-line strength depends on both UV and X-ray luminosity, suggesting that the physical mechanism driving the global Baldwin effect is also associated with X-ray emission.
Swift monitoring of NGC 4151 with an ~6hr sampling over a total of 69days in early 2016 is used to construct light curves covering five bands in the X-rays (0.3-50keV) and six in the ultraviolet (UV)/optical (1900-5500{AA}). The three hardest X-ray bands (>2.5keV) are all strongly correlated with no measurable interband lag, while the two softer bands show lower variability and weaker correlations. The UV/optical bands are significantly correlated with the X-rays, lagging ~3-4days behind the hard X-rays. The variability within the UV/optical bands is also strongly correlated, with the UV appearing to lead the optical by ~0.5-1days. This combination of >~3day lags between the X-rays and UV and <~1day lags within the UV/optical appears to rule out the "lamp-post" reprocessing model in which a hot, X-ray emitting corona directly illuminates the accretion disk, which then reprocesses the energy in the UV/optical. Instead, these results appear consistent with the Gardner & Done (2017MNRAS.470.3591G) picture in which two separate reprocessings occur: first, emission from the corona illuminates an extreme-UV-emitting toroidal component that shields the disk from the corona; this then heats the extreme-UV component, which illuminates the disk and drives its variability.
NGC 2516 has been observed by Chandra several times in order to correct the plate scale of the spacecraft's focal plane instruments. Because of this, Chandra has observed NGC 2516 with all four imaging arrangements available. In addition, NGC 2516 has been observed as part of the High Resolution Camera (HRC) guaranteed time program and is scheduled for return plate scale calibration visits. This makes it the best cluster to study for long-term variability. NGC 2516 is about 140Myr old and less than 400pc away. In our first paper, Harnden et al., 2001ApJ...547L.141H, we discussed the detection of 150 X-ray sources (42% of which are identified as cluster members) in the calibration data taken during the orbital activation phase of the Chandra mission. In our second paper, Damiani et al., 2003, Cat. <J/ApJ/588/1009>, we combined all the extant data sets and detected 284 sources, more than half of which are considered likely cluster members. In this our third paper, we further explore techniques of combining Advanced CCD Imaging Spectrometer (ACIS) and HRC Chandra data for timing analysis. We have been able to combine almost 70ks of observation time, spread over five epochs, to study variability in this cluster on multiple timescales. We find that while stochastic variability rates are about the same for all objects in the sample, the timescale for detecting variability is shorter for late-type stars. Both stochastic and flare variability rates seen in NGC 2516 are similar to those seen in younger clusters IC 348, NGC 1333, and M42.
We analyze data from five Chandra observations of the spiral galaxy NGC 6946 and from three Chandra observations of the irregular/spiral interacting galaxy pair NGC 4485/4490, with an emphasis on investigating the long-term variability exhibited by the source populations. We detect 90 point sources coincident with NGC 6946 down to luminosities of a few times 10^36^ergs/s, and 38 sources coincident with NGC 4485/90 down to a luminosity of ~1x10^37^ergs/s. Twenty-five (15) sources in NGC 6946 (NGC 4485/90) exhibit long-term (i.e., weeks to years) variability in luminosity; 11 (4) are transient candidates. The single ultraluminous X-ray source (ULX) in NGC 6946 and all but one of the eight ULXs in NGC 4485/90 exhibit long-term flux variability. Two of the ULXs in NGC 4485/90 have not been identified before as ultraluminous sources. The widespread variability in both systems is indicative of the populations being dominated by X-ray binaries, and this is supported by the X-ray colors of the sources. The distribution of colors among the sources indicates a large fraction of high-mass X-ray binaries in both systems. The shapes of the X-ray luminosity functions of the galaxies do not change significantly between observations and can be described by power laws with cumulative slopes ~0.6-0.7 (NGC 6946) and ~0.4 (NGC 4485/90).
We analyze the cross-correlation function between the soft and hard X-rays of atoll source 4U 1735-44 with RXTE data, and find anti-correlated soft and hard time lags of about a hecto-second. In the island state, the observations do not show any obvious correlations, and most observations of the banana branch show a positive correlation. However, anti-correlations are detected in the upper banana branch. These results are different from those of Z-sources (Cyg X-2, GX 5-1), where anti-correlations are detected in the horizontal branch and upper normal branch. In this case, the lag timescales of both this atoll and Z-sources are found to be similar, at a magnitude of several tens to hundreds of seconds. As a comparison, it is noted that anti-correlated lags lasting thousands of seconds have been reported from several black hole candidates in their intermediate states. In addition, for an observation containing four segments that show positive or anti-correlation, we analyze the spectral evolution with the hybrid model. In the observation, the anti-correlation is detected at the highest flux. The fitting results show that the Comptonized component is not the lowest at the highest flux, which suggests that the anti-correlation corresponds to the transition between the soft and hard states. Finally, we compare the corresponding results of atoll source 4U 1735-44 with those observed in Z-sources and black hole candidates, and the possible origins of the anti-correlated time lags are discussed.
We have studied the rapid X-ray time variability in 149 pointed observations with the Rossi X-Ray Timing Explorer (RXTE)'s Proportional Counter Array of the atoll source 4U 1636-53 in the banana state and, for the first time with RXTE, in the island state. We compare the frequencies of the variability components of 4U 1636-53 with those in other atoll and Z sources and find that 4U 1636-53 follows the universal scheme of correlations previously found for other atoll sources at (sometimes much) lower luminosities. Our results on the hectohertz QPO suggest that the mechanism that sets its frequency differs from that for the other components, while the amplitude-setting mechanism is common. A previously proposed interpretation of the narrow low-frequency QPO frequencies in different sources in terms of harmonic mode switching is not supported by our data or by previous data on other sources, and the frequency range that this QPO covers is found to be unrelated to spin, angular momentum, or luminosity.
The X-ray variability of the active galactic nuclei (AGN) has been most often investigated with studies of individual, nearby sources, and only a few ensemble analyses have been applied to large samples in wide ranges of luminosity and redshift. We aim to determine the ensemble variability properties of two serendipitously selected AGN samples extracted from the catalogues of XMM-Newton and Swift, with redshift between ~0.2 and ~4.5, and X-ray luminosities, in the 0.5-4.5keV band, between ~10^43^erg/s and ~10^46^erg/s. We used the structure function (SF), which operates in the time domain, and allows for an ensemble analysis even when only a few observations are available for individual sources and the power spectral density (PSD) cannot be derived. The SF is also more appropriate than fractional variability and excess variance, because these parameters are biased by the duration of the monitoring time interval in the rest-frame, and therefore by cosmological time dilation.
We present the results from a detailed X-ray variability analysis of 66 AGN in the Lockman Hole, which have optical spectroscopic identifications. We compare, quantitatively, their variability properties with the properties of local AGN, and we study the "variability - luminosity" relation as a function of redshift, and the "variability - redshift" relation in two luminosity bins. We use archival data from the last 10 XMM-Newton observations of the Lockman Hole field to extract light curves in the rest frame, 2-10keV band. We use the "normalized excess variance" to quantify the variability amplitude. Using the latest results regarding the AGN power spectral shape and its dependence on black hole mass and accretion rate, we are able to compute model "variability - luminosity" curves, which we compare with the relations we observe. When we consider all the sources in our sample, we find that their variability amplitude decreases with increasing redshift and luminosity.
