Ultraviolet and X-ray variability of NGC 4051 over 45 days with XMM-Newton and Swift

We analyse 15 XMM-Newton observations of the Seyfert galaxy NGC 4051 obtained over 45 days to determine the ultraviolet (UV) light curve variability characteristics and search for correlated UV/X-ray emission. The UV light curve shows variability on all time scales, however with lower fractional rms than the 0.2-10 keV X-rays. On days-weeks timescales the fractional variability of the UV is Fvar ~ 8%, and on short (~ hours) timescales Fvar ~ 2%. The within-observation excess variance in 4 of the 15 UV observations was found be much higher than the remaining 11. This was caused by large systematic uncertainties in the count rate masking the intrinsic source variance. For the four "good" observations we fit an unbroken power-law model to the UV power spectra with slope -2.0 +/- 0.5. We compute the UV/X-ray Cross-correlation function for the "good" observations and find a correlation of ~ 0.5 at time lag of ~ 3 ks, where the UV lags the X-rays. We also compute for the first time the UV/X-ray Cross-spectrum in the range 0-28.5 ks, and find a low coherence and an average time lag of ~ 3 ks. Combining the 15 XMM-Newton and the Swift observations we compute the DCF over +/-40 days but are unable to recover a significant correlation. The magnitude and direction of the lag estimate from the 4 "good" observations indicates a scenario where ~ 25 % of the UV variance is caused by thermal reprocessing of the incident X-ray emission.Comment: 11 pages, 8 figures. Accepted for publication in MNRA

The flux-dependent X-ray time lags in NGC 4051

We present an analysis of the X-ray time lags for the highly variable Seyfert 1 galaxy NGC 4051, based on a series of XMM-Newton observations taken in 2009. We investigate the Fourier frequency dependent time lags in the light curves between the 0.3--1.0 keV and 2.0--5.0 keV energy bands as a function of source flux, including simultaneous modelling of the resulting lag-frequency spectra. We find the shape of the lag-frequency spectra to vary significantly and systematically with source flux. We model the lag-frequency spectra using simple transfer functions, and find that two time lag components are required, one in each energy band. The simplest acceptable fits have only the relative contribution of the lagged component in the hard band varying with flux level, which can be associated with changes in the energy spectrum. We discuss the interpretation of these results in terms of the currently popular models for X-ray time lags.Comment: 9 pages, 4 figures. Accepted for publication in MNRA

All-aromatic biphenylene end-capped polyquinoline and polyimide matrix resins

Biphenylene end-capped polyquinoline and polyimide resins afford low void content graphite-reinforced composites with good initial properties. However, with both resins, rapid degradation occurs during oxidative isothermal aging at elevated temperatures. The degradation is not observed during isothermal aging under a nitrogen atmosphere which suggests that the biphenylene end-cap (or the resulting crosslink/chain extension structures) is not particularly thermooxidatively stable. The nature of the thermooxidative instability is currently under investigation

PMR polyimides-review and update

Fiber reinforced PMR polyimides are finding increased acceptance as engineering materials for high performance structural applications. Prepreg materials based on this novel class of highly processable, high temperature resistant polyimides are commercially available and the PMR concept is used by other investigators. The current status of first and second generation PMR polyimides were reviewed. Emphasis is given to the chemistry, processing and applications of the first generation material known as PMR-15

Effects of graphite fiber stability on the properties of PMR polyimide composites

The effect of the stability of graphite fibers on composite properties after exposure in air at 600 F was investigated. Composites were fabricated from PMR-15 and PMR-2 monomer solutions, using HTS-2 and Celion 6000 graphite fibers as the reinforcement. The effect of long-term exposure in air at 600 F on composite weight loss and mechanical properties was determined. These composites exhibited a significantly increased lifetime at that temperature compared to composites fabricated from HTS fiber sold prior to 1975. The effect of the PMR-15 and PMR-II resin compositions on long-term composite performance at 600 F is also discussed