Five years of SGR 1900+14 observations with BeppoSAX

We present a systematic analysis of all the BeppoSAX data of the soft gamma-ray repeater SGR 1900+14: these observations allowed us to study the long term properties of the source quiescent emission. In the observation carried out before the 1998 giant flare the spectrum in the 0.8-10 keV energy range was harder and there was evidence for a 20-150 keV emission, possibly associated with SGR 1900+14. This possible hard tail, if compared with the recent INTEGRAL detection of SGR 1900+14, has a harder spectrum (power-law photon index ~1.6 versus ~3) and a 20-100 keV flux ~4 times larger. In the last BeppoSAX observation (April 2002), while the source was entering the long quiescent period that lasted until 2006, the 2-10 keV flux was ~25% below the historical level. We also studied in detail the spectral evolution during the 2001 flare afterglow. This was characterized by a softening that can be interpreted in terms of a cooling blackbody-like component.Comment: Accepted for publication on Astronomy and Astrophysics on August 31, 200

Structure and stereochemistry of the base excision repair glycosylase MutY reveal a mechanism similar to retaining glycosidases.

MutY adenine glycosylases prevent DNA mutations by excising adenine from promutagenic 8-oxo-7,8-dihydroguanine (OG):A mismatches. Here, we describe structural features of the MutY active site bound to an azaribose transition state analog which indicate a catalytic role for Tyr126 and approach of the water nucleophile on the same side as the departing adenine base. The idea that Tyr126 participates in catalysis, recently predicted by modeling calculations, is strongly supported by mutagenesis and by seeing close contact between the hydroxyl group of this residue and the azaribose moiety of the transition state analog. NMR analysis of MutY methanolysis products corroborates a mechanism for adenine removal with retention of stereochemistry. Based on these results, we propose a revised mechanism for MutY that involves two nucleophilic displacement steps akin to the mechanisms accepted for 'retaining' O-glycosidases. This new-for-MutY yet familiar mechanism may also be operative in related base excision repair glycosylases and provides a critical framework for analysis of human MutY (MUTYH) variants associated with inherited colorectal cancer

Magnetars' Giant Flares: the case of SGR 1806-20

We first review on the peculiar characteristics of the bursting and flaring activity of the Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars. We then report on the properties of the SGR 1806-20's Giant Flare occurred on 2004 December 27th, with particular interest on the pre and post flare intensity/hardness correlated variability. We show that these findings are consistent with the picture of a twisted internal magnetic field which stresses the star solid crust that finally cracks causing the giant flare (and the observed torsional oscillations). This crustal fracturing is accompanied by a simplification of the external magnetic field with a (partial) untwisting of the magnetosphere.Comment: 6 pages, 2 figures; accepted for publication in the Chinese Journal for Astronomy and Astrophysics (Vulcano conference - 2005

Energy Level Statistics of Quantum Dots

We investigate the charging energy level statistics of disordered interacting electrons in quantum dots by numerical calculations using the Hartree approximation. The aim is to obtain a global picture of the statistics as a function of disorder and interaction strengths. We find Poisson statistics at very strong disorder, Wigner- Dyson statistics for weak disorder and interactions, and a Gaussian intermediate regime. These regimes are as expected from previous studies and fundamental considerations, but we also find interesting and rather broad crossover regimes. In particular, intermediate between the Gaussian and Poisson regimes we find a two-sided exponential distribution for the energy level spacings. In comparing with experiment, we find that this distribution may be realized in some quantum dots.Comment: 21 pages 10 figure

Exciton lifetime in InAs/GaAs quantum dot molecules

The exciton lifetimes $T_1$ in arrays of InAs/GaAs vertically coupled quantum dot pairs have been measured by time-resolved photoluminescence. A considerable reduction of $T_1$ by up to a factor of $\sim$ 2 has been observed as compared to a quantum dots reference, reflecting the inter-dot coherence. Increase of the molecular coupling strength leads to a systematic decrease of $T_1$ with decreasing barrier width, as for wide barriers a fraction of structures shows reduced coupling while for narrow barriers all molecules appear to be well coupled. The coherent excitons in the molecules gain the oscillator strength of the excitons in the two separate quantum dots halving the exciton lifetime. This superradiance effect contributes to the previously observed increase of the homogeneous exciton linewidth, but is weaker than the reduction of $T_2$. This shows that as compared to the quantum dots reference pure dephasing becomes increasingly important for the molecules

INTEGRAL high energy detection of the transient IGR J11321-5311

Context: The transient hard X-ray source IGR J11321-5311 was discovered by INTEGRAL on June 2005, during observations of the Crux spiral arm. To date, this is the only detection of the source to be reported by any X/gamma-ray mission. Aims: To characterize the behaviour and hence the nature of the source through temporal and spectral IBIS analysis. Methods: Detailed spectral and temporal analysis has been performed using standard INTEGRAL software OSA v.5.1. Results: To date, IGR J11321-5311 has been detected only once. It was active for about 3.5 hours, a short and bright flare lasting about 1.5 hours is evident in the IBIS light curve. It reached a peak flux of about 80 mCrab or 2.2x10E-9 erg cmE-2 sE-1 (20--300 keV),corresponding to a peak luminosity of 1.1x10E37 erg sE-1 (assuming a distance of 6.5 kpc). During the outburst, the source was detected with a significance of 18 sigma (20--300 keV) and 8 sigma (100--300 keV). The spectrum of the total outburst activity (17--300 keV) is best fitted by the sum of a power law (Gamma=0.55+/-0.18) plus a black body (kT=1.0{+0.2}_{-0.3} keV), with no evidence for a break up to 300 keV. A spectral analysis at Science Window level revealed an evident hardening of the spectrum through the outburst. The IBIS data were searched for pulsations with no positive result. Conclusions: The X-ray spectral shape and the flaring behaviour favour the hypothesis that IGR J11321-5311 is an Anomalous X-ray Pulsar, though a different nature can not be firmly rejected at the present stage.Comment: accepted for publication in A&A letter, 4 pages, 6 figure

Revisiting the Benefits of Higher Education

The economic returns of higher education (HE) in terms of enhanced earnings are well established. The wider set of ‘non-economic benefits’ in the areas of health, generic skills and citizenship are less widely recognised. In an earlier report, we presented preliminary findings on the wider benefits, drawing on data collected at age 33 from the National Child Development Study (based on a cohort born in 1958). This report updates the earlier conclusions through new findings from a more extensive analysis involving both the 1958 cohort and the more recent 1970 British Cohort Study cohort (based on a cohort born in 1970). The current analysis incorporates more recent data collected in both cohorts in 2000 and at an older age (1958 cohort). We report findings in five major areas. First, we examine the mobility of graduates. To what extent do some parts of the country gain and lose graduates and graduate types of occupation at the expense of others? We then report findings on the ‘marginal returns’ to higher education in four domains of life: health, labour market, citizenship and values, and parenting