X-ray spectral analysis of the low-luminosity active galactic nucleus NGC 7213 using long XMM-Newton observations

We present the X-ray spectral results from the longest X-ray Multi-mirror Mission–Newton observation, 133 ks, of the low-luminosity active galactic nucleus NGC 7213. The hardness-ratio analysis of the X-ray light curves discloses a rather constant X-ray spectral shape, at least for the observed exposure time, enabling us to perform X-ray spectral studies using the total observed spectrum. Apart from a neutral Fe K? emission line, we also detect narrow emission lines from the ionized iron species, Fe?xxv and Fe?xxvi. Our analysis suggests that the neutral Fe K? originates from a Compton-thin reflector, while the gas responsible for the high-ionization lines is collisionally excited. The overall spectrum, in the 0.3–10?keV energy band, registered by the European Photon Imaging Camera can be modelled by a power-law component (with a slope of ? ? 1.9) plus two thermal components at 0.36 and 8.84?keV. The low-energy thermal component is entirely consistent with the X-ray spectral data obtained by the Reflection Grating Spectrometer between 0.35–1.8?keV

Do the visual conditions at the point of escape affect European sea bass escape behavior?

European sea bass (Dicentrarchus labrax), an important species for the Mediterranean aquaculture industry, has been reported to escape from sea cage installations. Fish escapes are caused mainly by operational and technical failures that eventually result into a creation of a tear. Escapees may interact with wild stocks through interbreeding, transfer of pathogens and competition for food. The aim of this study was to examine at which extent the presence of a visible obstacle close to a tear on the net have an influence on sea bass propensity to escape. Fish were initially confined into small sea cages, with a tear at one side. The escape behavior was tested under experimental conditions. It is clearly demonstrated that sea bass was able to locate a tear on the net pen, immediately after its appearance. Crossings occurred in all cages, in singles or in a series of up to seven individuals. The presence of an obstacle close to the net tear altered the escape behavior of D. labrax resulting in a delay that eventually reduced the escape rate. Concluding, it is highly recommended that sea bass cages should be kept internally the culture array. Furthermore, the placement of artificial obstacles close to the sea cages could be an efficient practice that mitigates the escape risk after severe environmental conditions

The ‘harder when brighter’ X-ray behaviour of the low-luminosity active galactic nucleus NGC 7213

We present the first robust evidence of an anticorrelation between the X-ray photon index, Γ, and the X-ray luminosity in a single low-luminosity active galactic nucleus (LLAGN), NGC 7213. Until today, such anticorrelation trends have been seen only in large samples of LLAGN that span a wide range of X-ray fluxes, although the opposite behaviour (i.e. a positive correlation between Γ and X-ray luminosity) has been extensively studied for individual X-ray bright active galactic nuclei. For NGC 7213, we use the long-term X-ray monitoring data of the Rossi X-ray Timing Explorer (RXTE), regularly obtained on average every two days from 2006 March to 2009 December. Based on our X-ray data, we derive the Γ versus flux and the hardness ratio versus flux relations, indicating clearly that NGC 7213 follows a ‘harder when brighter’ spectral behaviour. Additionally, by analysing radio and optical data, and combining data from the literature, we form the most complete spectral energy distribution (SED) of the source across the electromagnetic spectrum yielding a bolometric luminosity of 1.7 × 1043 erg s-1. Phenomenologically, the SED of NGC 7213 is similar to that of a low-ionization nuclear emission-line region. The robust anticorrelation trend that we find between Γ and X-ray luminosity together with the low accretion rate of the source, 0.14 per cent that of the Eddington limit, makes NGC 7213 the first LLAGN exhibiting a similar spectral behaviour with that of black hole X-ray binaries in the ‘hard state’

Rapid X-ray Variability of Seyfert 1 Galaxies

The rapid and seemingly random fluctuations in X-ray luminosity of Seyfert galaxies provided early support for the standard model in which Seyferts are powered by a supermassive black hole fed from an accretion disc. However, since EXOSAT there has been little opportunity to advance our understanding of the most rapid X-ray variability. Observations with XMM-Newton have changed this. We discuss some recent results obtained from XMM-Newton observations of Seyfert 1 galaxies. Particular attention will be given to the remarkable similarity found between the timing properties of Seyferts and black hole X-ray binaries, including the power spectrum and the cross spectrum (time delays and coherence), and their implications for the physical processes at work in Seyferts.Comment: To appear in From X-ray Binaries to Quasars: Black Hole Accretion on All Mass Scales, ed. T. J. Maccarone, R. P. Fender, and L. C. Ho (Dordrecht: Kluwer

The WEBT BL Lacertae Campaign 2001 and its extension : Optical light curves and colour analysis 1994–2002

BL Lacertae has been the target of four observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration. In this paper we present UBVRI light curves obtained by theWEBT from 1994 to 2002, including the last, extended BL Lac 2001 campaign. A total of about 7500 optical observations performed by 31 telescopes from Japan to Mexico have been collected, to be added to the ∼15 600 observations of the BL Lac Campaign 2000. All these data allow one to follow the source optical emission behaviour with unprecedented detail. The analysis of the colour indices reveals that the flux variability can be interpreted in terms of two components: longer-term variations occurring on a fewday time scale appear as mildly-chromatic events, while a strong bluer-when-brighter chromatism characterizes very fast (intraday) flares. By decoupling the two components, we quantify the degree of chromatism inferring that longer-term flux changes imply moving along a ∼0.1 bluerwhen- brighter slope in the B − R versus R plane; a steeper slope of ∼0.4 would distinguish the shorter-term variations. This means that, when considering the long-term trend, the B-band flux level is related to the R-band one according to a power law of index ∼1.1. Doppler factor variations on a “convex” spectrum could be the mechanism accounting for both the long-term variations and their slight chromatism.Reig Torres, Pablo, [email protected]

Multifrequency variability of the blazar AO 0235+164 the WEBT campaign in 2004-2005 and long-term SED analysis

A huge multiwavelength campaign targeting the blazar AO 0235+164 was organized by the Whole Earth Blazar Telescope (WEBT) in 2003-2005 to study the variability properties of the source. Monitoring observations were carried out at cm and mm wavelengths, and in the near-IR and optical bands, while three pointings by the XMM-Newton satellite provided information on the X-ray and UV emission. We present the data acquired during the second observing season, 2004-2005, by 27 radio-to-optical telescopes. They reveal an increased near-IR and optical activity with respect to the previous season. Increased variability is also found at the higher radio frequencies, down to 15 GHz, but not at the lower ones. The radio (and optical) outburst predicted to peak around February-March 2004 on the basis of the previously observed 5-6 yr quasi-periodicity did not occur. The analysis of the optical light curves reveals now a longer characteristic time scale of 8 yr, which is also present in the radio data. The spectral energy distributions corresponding to the XMM-Newton observations performed during the WEBT campaign are compared with those pertaining to previous pointings of X-ray satellites. Bright, soft X-ray spectra can be described in terms of an extra component, which appears also when the source is faint through a hard UV spectrum and a curvature of the X-ray spectrum. Finally, there might be a correlation between the X-ray and optical bright states with a long time delay of about 5 yr, which would require a geometrical interpretation

Modelling the UV/optical continuum time-lags in AGN

International audienceThermal reverberation in accretion discs of active galactic nuclei is thought to be the reason of the continuum UV/optical time lags seen in these sources. Recently, we studied thermal reverberation of a standard Novikov-Thorne accretion disc illuminated by an X–ray point-like source, and we derived an analytic prescription for the time lags as function of wavelength. In this work, we use this analytic function to fit the time-lags spectra of seven Seyferts, that have been intensively monitored, in many wave-bands, in the last few years. We find that thermal reverberation can explain the observed UV/optical time lags in all these sources. Contrary to previous claims, the magnitude of the observed UV/optical time-lags is exactly as expected in the case of a standard accretion disc in the lamp-post geometry, given the black hole mass and the accretion rate estimates for the objects we study. We derive estimates of the disc accretion rates and corona height for a non-spinning and a maximally spinning black hole scenarios. We also find that the modelling of the continuum optical/UV time-lags can be used to estimate the black hole spin, when combined with additional information. We also find that the model under-predicts the observed X–ray to UV time-lags, but this difference is probably due to the broad X-ray auto-correlation function of these sources

Measuring the broad-band power spectra of active galactic nuclei with RXTE

We develop a Monte Carlo technique to test models for the true power spectra of intermittently sampled light curves against the noisy, observed power spectra, and produce a reliable estimate of the goodness of fit of the given model. We apply this technique to constrain the broad-band power spectra of a sample of four Seyfert galaxies monitored by the Rossi X-ray Timing Explorer (RXTE ) over three years. We show that the power spectra of three of the AGN in our sample (MCG-6-30-15, NGC 5506 and NGC 3516) flatten significantly towards low frequencies, while the power spectrum of NGC 5548 shows no evidence of flattening. We fit two models for the flattening: a 'knee' model, analogous to the low-frequency break seen in the power spectra of BHXRBs in the low state (where the power-spectral slope flattens to α =0), and a 'high-frequency break' model (where the power-spectral slope flattens to α =1), analogous to the high-frequency break seen in the high- and low-state power spectra of the classic BHXRB Cyg X-1. Both models provide good fits to the power spectra of all four AGN. For both models, the characteristic frequency for flattening is significantly higher in MCG-6-30-15 than in NGC 3516 (by a factor of ~10), although both sources have similar X-ray luminosities, suggesting that MCG-6-30-15 has a lower black hole mass and is accreting at a higher rate than NGC 3516. Assuming linear scaling of characteristic frequencies with black hole mass, the high accretion rate implied for MCG-6-30-15 favours the high-frequency break model for this source, and further suggests that MCG-6-30-15, and possibly NGC 5506, may be analogues of Cyg X-1 in the high state. Comparison of our model fits with naive fits, where the model is fitted directly to the observed power spectra (with errors estimated from the data), shows that Monte Carlo fitting is essential for reliably constraining the broad-band power spectra of AGN light curves obtained to date