119167053
Detection of Satellite Remnants in the Galactic Halo with Gaia - II. A modified Great Circle Cell Method
We propose an extension of the GC3 streamer finding method of Johnston et al. (1996) that can be applied to the future Gaia database. The original method looks for streamers along great circles in the sky, our extension adds the kinematical restriction that velocity vectors should also be constrained to lie along these great circles, as seen by a Galactocentric observer. We show how to use these combined criteria starting from heliocentric observables. We test it by using the mock Gaia catalogue of Brown et al. (2005), which includes a realistic Galactic background and observational errors, but with the addition of detailed star formation histories for the simulated satellites. We investigate its success rate as a function of initial satellite luminosity, star formation history and orbit. We find that the inclusion of the kinematical restriction vastly enhances the contrast between a streamer and the background, even in the presence of observational errors, provided we use only data with good astrometric quality (fractional errors of 30 per cent or better). The global nature of the method diminishes the erasing effect of phase mixing and permits the recovery of merger events of reasonable dynamical age. Satellites with a star formation history different to that of the Galactic background are also better isolated. We find that satellites in the range of 10 8 10 9 ;
118384535
Digging supplementary buried channels: investigating the notch architecture within the CCD pixels on ESA's Gaia satellite
The European Space Agency (ESA) Gaia satellite has 106 CCD image sensors which will suffer from increased charge transfer inefficiency (CT I) as a result of radiation damage. To aid the mitigation at low signal levels, the CCD design includes Supplementary Buried Channels (SBCs, otherwise known as ‘notches’) within each C CD column. We present the largest published sample of Gaia CCD SBC Full Well Capacity (FWC) laboratory measurements and simulations based on 13 devices. We find that Gaia CCDs manufactured post-2004 have SBCs with FWCs in the upper half of each CCD that are systematically smaller by two orders of magnitude (650 electrons) compared to those manufactured pre-2004 (thousands of electrons). Gaia’s faint star ( 13 6 G 6 20 mag) astrometric performance predictions by Prod’homme et al. and Holl et al. use pre-2004 SBC FWCs as inputs to their simulations. However, all the CCDs already integrated onto the satellite for the 2013 launch are post2004. SBC FWC measurements are not available for one of our fiv e post-2004 CCDs but the fact it meets Gaia’s image location requirements suggests it has SBC FWCs simi lar to pre2004. It is too late to measure the SBC FWCs onboard the satellite and it is not possible to theoretically predict them. Gaia’s faint star astrometric performance predictions depend o n knowledge of the onboard SBC FWCs but as these are currently unavailable, it is not known how representative of the whole focal plane the current predictions are. Therefore, we suggest Gaia’s initial in-orbit calibrations should include measureme nt of the onboard SBC FWCs. We present a potential method to do this. Faint star astrometric performance predictions based on onboard SBC FWCs at the start of the mission would allow satellite operating conditions or CTI software mitigation to be further optimised to improve the scientific return of Gaia.
55115943
On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright O stars in the sky as predicted from their spectral type with the absolute magnitude calculated from their apparent magnitude and the Hipparcos parallax. We find that many stars appear to be much fainter than expected, up to five magnitudes. We find no evidence for a correlation between magnitude differences and the stellar rotational velocity as suggested for OB stars by Lamers et al. (1997, AA Howarth & Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) and find that they are consistent with the Hipparcos measurements. Although O stars conform nicely to the simulation, we notice that some B stars in the sample of Lamers et al. have a magnitude difference larger than expected.
119117067
ELSA and the frontiers of astrometry
ELSA stands for the ambitious goal of “European Leadership in Space Astrometry”. In this closing contribution I will examine how the ELSA network has advanced this goal. I also look ahead to the time when the Gaia data will be published and consider what needs to be done to maintain European leadership.
11339076
Learning about Galactic structure with Gaia astrometry
The Gaia mission is reviewed together with the expected contents of the final catalogue. It is then argued that the ultimate goal of Galactic structure studies with Gaia astrometry should be to build a dynamical model of our galaxy which is capable of explaining the contents of the Gaia catalogue. This will be possible only by comparing predicted catalogue data to Gaia’s actual measurements. To complement this approach the Gaia catalogue should be used to recalibrate photometric distance and abundance indicators across the HR‐diagram in order to overcome the lack of precise parallax data at the faint end of the astrometric survey. Using complementary photometric and spectroscopic data from other surveys will be essential in this respect.
1110969
OB Associations, Open Clusters, and the Luminosity Calibration of the Nearer Stars
In the context of the luminosity calibration of the nearer stars I discuss the Hipparcos results on distances to nearby OB associations and open clusters. The shortcomings and assumptions in the analyses used to derive these results are pointed out and for the open clusters a comparison is made with results obtained from main sequence fitting. I conclude that given the considerable uncertainties in the latter technique there is no convincing evidence that the Hipparcos based distances to open clusters beyond the Hyades should not be trusted.
117895476
Stellar Imager (SI): developing and testing a predictive dynamo model for the Sun by imaging other stars
The Stellar Imager mission concept is a space-based UV/Optical interferometer designed to resolve surface magnetic activity and subsurface structure and flows of a population of Sun-like stars, in order to accelerate the development and validation of a predictive dynamo model for the Sun and enable accurate long-term forecasting of solar/stellar magnetic activity.
8789191
Study of the acoustic signature of UHE neutrino interactions in water and ice
The production of acoustic signals from the interactions of ultra-high energy (UHE) cosmic ray neutrinos in water and ice has been studied. A new computationally fast and efficient method of deriving the signal is presented. This method allows the implementation of up to date parameterisations of acoustic attenuation in seawater and ice that now includes the effects of complex attenuation, where appropriate. The methods presented here have been used to compute and study the properties of the acoustic signals which would be expected from such interactions. A matrix method of parameterising the signals, which includes the expected fluctuations, is also presented. These methods are used to generate the expected signals that would be detected in acoustic UHE neutrino telescopes.
119263159
Evaluating the Gamma-Ray Evidence for Self-Annihilating Dark Matter from the Virgo Cluster
Based on three years of Fermi Large Area Telescope (LAT) gamma-ray data of the Virgo cluster, evidence for an extended emission associated with dark matter pair annihilation in the b b-bar channel has been reported by Han et al. [1]. After an in depth spatial and temporal analysis, we argue that the tentative evidence for a gamma-ray excess from the Virgo cluster is mainly due to the appearance of a population of previously unresolved gamma-ray point sources in the region of interest. These point sources are not part of the LAT second source catalogue (2FGL), but are found to be above the standard detection significance threshold when three or more years of LAT data is included.
119251012
High energy gamma-ray properties of the FR I radio galaxy NGC 1275
We report on our study of the high-energy $\gamma-$ray emission from the FR I radio galaxy NGC 1275, based on two years of observations with the Fermi-LAT detector. Previous Fermi studies of NGC 1275 had found evidence for spectral and flux variability on monthly timescales during the first year of Fermi-LAT observations. This variability is also seen in the larger two year data set, during which we observe a large $\gamma-$ray flare (June-August 2010). The increased photon statistics from this large flare have allowed the discovery of flux variability from NGC 1275 on the timescales of days. The largest flux variation observed during this flare being a factor of $\sim 3$ from one day to the next and a resultant $e$-folding risetime of $1.51\pm0.2$ days. The two year averaged $E>$100 MeV $\gamma-$ray spectrum is adequately described by a power-law spectrum, with a photon index, $\Gamma$, of $2.09 \pm 0.02$, and a resultant integrated flux of $F_{\gamma}=(2.2\pm0.1) \times 10^{-7}$ ph cm$^{-2}$s$^{-1}$. While no hysteresis was observed in the photon index$-$flux ($\Gamma_{\gamma}$ vs F$_{\gamma}$) parameter space, there was obvious `harder-when-brighter' behaviour observed during the large $\gamma-$ray flare. Furthermore, during this large flare, NGC 1275 appeared to migrate from the FR I radio galaxy to the BL Lac object region of the photon index$-$luminosity ($\Gamma_{\gamma}$ vs L$_{\gamma}$) paramater space. In this paper we present details of our Fermi-LAT analysis of NGC 1275, including a brief discussion on its implications for $\gamma-$ray blazar sources.
35625352
Status of the nuclear shell model
In this review we concentrate on recent results of shell model calculations and related experimental data obtained for the sd-shell nuclei. Model spaces and effective interactions, Gamow-Teller, magnetic dipole, electric quadrupole, and higher multipole matrix elements, and electron scattering form factors are discussed. (AIP)