X-ray variability of AGNs in the soft and the hard X-ray bands

We investigate the X-ray variability characteristics of hard X-ray selected AGNs (based on Swift/BAT data) in the soft X-ray band using the RXTE/ASM data. The uncertainties involved in the individual dwell measurements of ASM are critically examined and a method is developed to combine a large number of dwells with appropriate error propagation to derive long duration flux measurements (greater than 10 days). We also provide a general prescription to estimate the errors in variability derived from rms values from unequally spaced data. Though the derived variability for individual sources are not of very high significance, we find that, in general, the soft X-ray variability is higher than those in hard X-rays and the variability strengths decrease with energy for the diverse classes of AGN. We also examine the strength of variability as a function of the break time scale in the power density spectrum (derived from the estimated mass and bolometric luminosity of the sources) and find that the data are consistent with the idea of higher variability at time scales longer than the break time scale.Comment: 17 pages, 15 Postscript figures, 3 tables, accepted for publication in Ap

Evidence for a Truncated Accretion Disc in the Low Luminosity Seyfert Galaxy, NGC 7213?

We present the broad-band 0.6-150 keV Suzaku and Swift BAT spectra of the low luminosity Seyfert galaxy, NGC 7213. The time-averaged continuum emission is well fitted by a single powerlaw of photon index Gamma = 1.75 and from consideration of the Fermi flux limit we constrain the high energy cutoff to be 350 keV < E < 25 MeV. Line emission from both near-neutral iron K_alpha at 6.39 keV and highly ionised iron, from Fe_(xxv) and Fe_(xxvi), is strongly detected in the Suzaku spectrum, further confirming the results of previous observations with Chandra and XMM-Newton. We find the centroid energies for the Fe_(xxv) and Fe_(xxvi) emission to be 6.60 keV and 6.95 keV respectively, with the latter appearing to be resolved in the Suzaku spectrum. We show that the Fe_(xxv) and Fe_(xxvi) emission can result from a highly photo-ionised plasma of column density N_(H) ~ 3 x 10^(23) cm^(-2). A Compton reflection component, e.g., originating from an optically-thick accretion disc or a Compton-thick torus, appears either very weak or absent in this AGN, subtending < 1 sr to the X-ray source, consistent with previous findings. Indeed the absence of either neutral or ionised Compton reflection coupled with the lack of any relativistic Fe K signatures in the spectrum suggests that an inner, optically-thick accretion disc is absent in this source. Instead, the accretion disc could be truncated with the inner regions perhaps replaced by a Compton-thin Radiatively Inefficient Accretion Flow. Thus, the Fe_(xxv) and Fe_(xxvi) emission could both originate in ionised material perhaps at the transition region between the hot, inner flow and the cold, truncated accretion disc on the order of 10^(3) - 10^(4) gravitational radii from the black hole. The origin for the unresolved neutral Fe K_alpha emission is then likely to be further out, perhaps originating in the optical BLR or a Compton-thin pc-scale torus.Comment: 15 pages, 11 figures, accepted for publication by MNRA

Violations of robustness trade-offs

Biological robustness is a principle that may shed light on system-level characteristics of biological systems. One intriguing aspect of the concept of biological robustness is the possible existence of intrinsic trade-offs among robustness, fragility, performance, and so on. At the same time, whether such trade-offs hold regardless of the situation or hold only under specific conditions warrants careful investigation. In this paper, we reassess this concept and argue that biological robustness may hold only when a system is sufficiently optimized and that it may not be conserved when there is room for optimization in its design. Several testable predictions and implications for cell culture experiments are presented

Signatures of the disk-jet coupling in the Broad-line Radio Quasar 4C+74.26

Here we explore the disk-jet connection in the broad-line radio quasar 4C+74.26, utilizing the results of the multiwavelength monitoring of the source. The target is unique in that its radiative output at radio wavelengths is dominated by a moderately-beamed nuclear jet, at optical frequencies by the accretion disk, and in the hard X-ray range by the disk corona. Our analysis reveals a correlation (local and global significance of 96\% and 98\%, respectively) between the optical and radio bands, with the disk lagging behind the jet by $250 \pm 42$ days. We discuss the possible explanation for this, speculating that the observed disk and the jet flux changes are generated by magnetic fluctuations originating within the innermost parts of a truncated disk, and that the lag is related to a delayed radiative response of the disk when compared with the propagation timescale of magnetic perturbations along relativistic outflow. This scenario is supported by the re-analysis of the NuSTAR data, modelled in terms of a relativistic reflection from the disk illuminated by the coronal emission, which returns the inner disk radius $R_{\rm in}/R_{\rm ISCO} =35^{+40}_{-16}$. We discuss the global energetics in the system, arguing that while the accretion proceeds at the Eddington rate, with the accretion-related bolometric luminosity $L_{\rm bol} \sim 9 \times 10^{46}$ erg s$^{-1}$ $\sim 0.2 L_{\rm Edd}$, the jet total kinetic energy $L_\textrm{j} \sim 4 \times 10^{44}$ erg s$^{-1}$, inferred from the dynamical modelling of the giant radio lobes in the source, constitutes only a small fraction of the available accretion power.Comment: 9 pages and 6 figures, ApJ accepte

NGC 2992 in an X-ray high state observed by XMM: Response of the Relativistic Fe Kα\alpha Line to the Continuum

We present the analysis of an XMM observation of the Seyfert galaxy NGC 2992. The source was found in its highest level of X-ray activity yet detected, a factor $\sim 23.5$ higher in 2--10 keV flux than the historical minimum. NGC 2992 is known to exhibit X-ray flaring activity on timescales of days to weeks, and the XMM data provide at least factor of $\sim 3$ better spectral resolution in the Fe K band than any previously measured flaring X-ray state. We find that there is a broad feature in the \sim 5-7 keV band which could be interpreted as a relativistic Fe K$\alpha$ emission line. Its flux appears to have increased in tandem with the 2--10 keV continuum when compared to a previous Suzaku observation when the continuum was a factor of $\sim 8$ lower than that during the XMM observation. The XMM data are consistent with the general picture that increased X-ray activity and corresponding changes in the Fe K$\alpha$ line emission occur in the innermost regions of the putative accretion disk. This behavior contrasts with the behavior of other AGN in which the Fe K$\alpha$ line does not respond to variability in the X-ray.Comment: 30 pages, 6 figures, Accepted to Ap

Gap Renormalization in Dirty Anisotropic Superconductors: Implications for the Order Parameter of the Cuprates

We contrast the effects of non-magnetic impurities on the properties of superconductors having a \dw\ order parameter, and a highly anisotropic s-wave (ASW) gap with the same nodal structure. The non-vanishing, impurity induced, off-diagonal self-energy in the ASW state is shown to gap out the low energy excitations present in the clean system, leading to a qualitatively different impurity response of the single particle density of states compared to the \dw\ state. We discuss how this behaviour can be employed to distinguish one state from the other by an analysis of high-resolution angle-resolved photoemission spectra.Comment: 12 pages, uuencoded Postscrip

Relativistic Iron K Emission and absorption in the Seyfert 1.9 galaxy MCG-5-23-16

We present the results of the simultaneous deep XMM and Chandra observations of the bright Seyfert 1.9 galaxy MCG-5-23-16, which is thought to have one of the best known examples of a relativistically broadened iron K-alpha line. The time averaged spectral analysis shows that the iron K-shell complex is best modeled with an unresolved narrow emission component (FWHM < 5000 km/s, EW ~ 60 eV) plus a broad component. This latter component has FWHM ~ 44000 km/s and EW ~ 50 eV. Its profile is well described by an emission line originating from an accretion disk viewed with an inclination angle ~ 40^\circ and with the emission arising from within a few tens of gravitational radii of the central black hole. The time-resolved spectral analysis of the XMM EPIC-pn spectrum shows that both the narrow and broad components of the Fe K emission line appear to be constant in time within the errors. We detected a narrow sporadic absorption line at 7.7 keV which appears to be variable on a time-scale of 20 ksec. If associated with Fe XXVI Ly-alpha this absorption is indicative of a possibly variable, high ionization, high velocity outflow. The variability of this absorption feature appears to rule out a local (z=0) origin. The analysis of the XMM RGS spectrum reveals that the soft X-ray emission of MCG-5-23-16 is likely dominated by several emission lines superimposed on an unabsorbed scattered power-law continuum. The lack of strong Fe L shell emission together with the detection of a strong forbidden line in the O VII triplet is consistent with a scenario where the soft X-ray emission lines are produced in a plasma photoionized by the nuclear emission.Comment: 45 pages, 12 figures, 4 tables, accepted for publication in Ap

Regularizing Portfolio Optimization

The optimization of large portfolios displays an inherent instability to estimation error. This poses a fundamental problem, because solutions that are not stable under sample fluctuations may look optimal for a given sample, but are, in effect, very far from optimal with respect to the average risk. In this paper, we approach the problem from the point of view of statistical learning theory. The occurrence of the instability is intimately related to over-fitting which can be avoided using known regularization methods. We show how regularized portfolio optimization with the expected shortfall as a risk measure is related to support vector regression. The budget constraint dictates a modification. We present the resulting optimization problem and discuss the solution. The L2 norm of the weight vector is used as a regularizer, which corresponds to a diversification "pressure". This means that diversification, besides counteracting downward fluctuations in some assets by upward fluctuations in others, is also crucial because it improves the stability of the solution. The approach we provide here allows for the simultaneous treatment of optimization and diversification in one framework that enables the investor to trade-off between the two, depending on the size of the available data set

An Evolutionary Optimization Approach to Risk Parity Portfolio Selection

In this paper we present an evolutionary optimization approach to solve the risk parity portfolio selection problem. While there exist convex optimization approaches to solve this problem when long-only portfolios are considered, the optimization problem becomes non-trivial in the long-short case. To solve this problem, we propose a genetic algorithm as well as a local search heuristic. This algorithmic framework is able to compute solutions successfully. Numerical results using real-world data substantiate the practicability of the approach presented in this paper