AGN evolution, clustering and the X-ray background

We combine optical, X-ray (Einstein and ROSAT) and infrared (IRAS) data to investigate the evolution and spatial distribution of AGN with particular emphasis on the implications for the origin of the diffuse X-ray background. First, we derive the IRAS Seyfert luminosity function to test the continuity of properties between the Seyfert and the QSO population. The QSO luminosity function evolved back to z ~ 0, agrees well with the Seyfert luminosity function. In particular, the similarity of the faint parts of the two luminosity functions, suggests that the optical luminosity function is not severely affected by incompleteness due to reddening. We analyze the clustering properties of the IRAS Seyfert sample in order to probe the AGN clustering evolution. We detect clear clustering (5σ) at scales 100 deg(^-2). Spectroscopic follow up observations show that most of these sources are QSOs. The identified QSOs contribute ~ 30% at 1 keV and therefore this is the lower limit of the AGN contribution to the X-ray background. No other class of sources contributing substantially to the X-ray background has been yet identified

Searching for cluster substructure using APM and ROSAT data

We present a detailed study of the morphological features of 22 rich galaxy clusters. Our sample is constructed from a cross-correlation of optical (Abell+APM) data with X-ray (0.1 - 2.4) keV ROSAT pointed observations. We systematically compare cluster images and morphological parameters in an attempt to reliably identify possible substructure in both optical and the X-ray images. To this end, we compute various moments of the optical and X-ray surface-brightness distribution such as the ellipticities, center-of-mass shifts and ellipsoidal orientations. We assess the significance of our results using Monte Carlo simulations. We find significant correlations between the optical and X-ray morphological parameters, indicating that in both parts of the spectrum it is possible to identify correctly the dynamical state of a cluster. Most of our clusters (17/22) have a good 1-to-1 correspondence between the optical and the X-ray images and about 10 appear to have strong indications of substructure. This corresponds to a minimum percentage of order 45 per cent which is in very good accordance with other similar analyses. Finally, 5 out of 22 systems seem to have distinct subclumps in the optical which are not verified in the X-ray images, and thus are suspect of being due to optical projection effects. These results will serve as a useful guide in interpreting subsequent analyses of large optical cluster catalogues.Comment: 15 pages, including 9 figures, MNRAS in press, revised versio

Board Structure and Bank Performance: Evidence for the Greek Banking Industry during Crisis Period

The recent financial crisis has heightened the research interest worldwide in the relationship between various corporate governance mechanisms and firm performance. Nevertheless, few published papers focus on investigating this nexus for the case of the banking industry. This study is the first that empirically assesses the impact of board structure on bank performance for the case of Greek banks using a variety of econometric methodologies. Exhaustive empirical findings are presented based on a sample of thirteen Greek banks and for a period of severe sovereign debt crisis (2008-2014). Empirical findings support an inverted U-shaped relation between board size and bank performance and between the proportion of independent board members and performance of Greek banks. All empirical findings are generated after we control for M&A activity, bank size and capital adequacy of each bank. Overall, our results document the positive contribution of the implemented corporate governance regulatory framework on the Greek bank value. Keywords: Corporate Governance, Bank Performance, Board Structure. JEL Classifications: C52, G34, G2

Corporate governance mechanisms and bank performance: evidence from the Greek banks during crisis period

This paper is the first research attempt that investigates the impact of a large number of corporate governance mechanisms on the performance of Greek banks,employing widely accepted in the literature of corporate governance econometric models. Results indicate that system GMM models are more suitable methodological tools than pooledOLS and fixed effects models to address well-known econometric problems, such as endogeneity, simultaneity and unobserved heterogeneity of individual banks. The findings, as derived from the application of GMM models, imply that increasing the board size and the number of independent directors can both have positive impact on the performance of Greek banks, but only up to a certain point. Thus, bank efficiency will increase as board size and the proportion of independent directors grow up to a point where these relationships hit a maximum from which bank performance decreases. Our multi-model estimations failed to trace any significant contribution of the number of female and foreign directors on the performance of Greek banks. Finally, the dual appointment of a CEO as Chairman appears to affect negatively two out of four proxies of bank performance. Overall, the results provide support for the positive impact of corporate governance mechanisms on the performance of Greek banks. The significance of these findings increases, considering that the period under study (2008-2014) is marked by high market volatility and uncertainty due to the well-known debt crisis that plagues Greece since the beginning of 2008

Variable X-ray Absorption in the Seyfert 2 Galaxy Mrk 348

We present RXTE monitoring observations of the Seyfert 2 galaxy Mrk 348 spanning a 6 month period. The time-averaged spectrum in the 3-20 keV band shows many features characteristic of a Compton-thin Seyfert 2 galaxy, namely a hard underlying power-law continuum (photon index = 1.8) with heavy soft X-ray absorption (N_h ~ 10^23 cm^-2) plus measureable iron line emission (equivalent width ~ 100 eV) and, at high energy, evidence for a reflection component (R < 1). During the first half of the monitoring period the X-ray continuum flux from Mrk 348 remained relatively steady. However this was followed by a significant brightening of the source (by roughly a factor of 4) with the fastest change corresponding to a doubling of its X-ray flux on a timescale of about 20 days. The flux increase was accompanied by a marked softening of X-ray spectrum most likely attributable to a factor 3 decline in the intrinsic line-of-sight column density. In contrast the iron line and the reflection components showed no evidence of variability. These observations suggest a scenario in which the central X-ray source is surrounded by a patchy distribution of absorbing material located within about a light-week of the nucleus of Mrk 348. The random movement of individual clouds within the absorbing screen, across our line of sight, produces substantial temporal variations in the measured column density on timescales of weeks to months and gives rise to the observed X-ray spectral variability. However, as viewed from the nucleus the global coverage and typical thickness of the cloud layer remains relatively constant.Comment: 19 pages, 3 figures Accepted for publication in the Astrophysical Journa

The properties of supermassive black holes and their host galaxies for type 1 and 2 active galactic nuclei in the eFEDS and COSMOS fields

In this study, our primary objective is to compare the properties of supermassive black holes (SMBHs) and their host galaxies between type 1 and type 2 active galactic nuclei (AGNs). In our analysis, we use X-ray detected sources in two fields, namely the eFEDS and the COSMOS-Legacy. To classify the X-ray sources, we performed a spectral energy distribution (SED) fitting analysis, using the CIGALE code. The robustness of our analysis was paramount so, to ensure this, we imposed stringent selection criteria. Thus, only sources with extensive photometric data across the optical, near- and mid-infrared part of the spectrum and reliable host galaxy properties and classifications were included. The final sample consists of 3312 AGNs, of which 3049 are classified as type 1 and 263 as type 2. The sources span a redshift range of 0.5  2 and log,[LX,2−10 keV(erg s−1)] > 44. Similar patterns emerge when we categorize AGNs based on their X-ray obscuration levels (NH). However, in this case, the observed differences are pronounced only for low-to-intermediate LX AGNs and are also sensitive to the NH threshold applied for the AGN classification. These comprehensive findings enhance our understanding of the intricate relationships governing AGN types and their host galaxy properties across diverse cosmic epochs and luminosity regimes.This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement no. 101004168, the XMM2ATHENA project. This research has made use of TOPCAT version 4.8 (Taylor 2005) and Astropy (Astropy Collaboration 2022).Peer reviewe

Determinants of share repurchases: a quantile regression approach

This study utilizes quantile regressions to investigate the effect of the determinants of share repurchases on firms at different points of the share repurchases distribution. Empirical results from a large panel of NYSE repurchasing firms, document an asymmetric effect of several determinants on share repurchases in terms of size, significance and direction. Excess capital, stock options and growth opportunities are significant throughout the distribution and their impact increases at successive quantiles while ownership concentration and leverage exhibit sign reversals between lower and upper quantiles. These differing effects are attributed to highly heterogeneous firm characteristics across quantiles