120263601
A Time-differencing Technique for Detecting Radio-quiet Gamma-Ray Pulsars
Detecting periodicity from a γ-ray pulsar is exceedingly difficult without prior knowledge of its pulsation frequency and frequency derivative. The low fluxes attainable in the gamma-ray band mandate very long exposures, making the direct application of Fourier analysis methods computationally prohibitive. Accumulation of phase shifts induced by significant frequency derivatives requires that the large Fourier transforms be repeated over many trials, and occasional "glitches" in the pulsar rotation compound the difficulties. By analyzing the differences of photon arrival times rather than the time series itself, we show that we can maintain good sensitivity while greatly reducing the effects of frequency derivatives and glitches. To demonstrate its power, we use simulated data to compare our time-differencing algorithm with two Fourier methods used in previous searches for radio-quiet γ-ray pulsars.
122398457
Fermi Gamma‐ray Space Telescope, Introduction and Highlights from the First Eight Months
The Fermi Gamma‐ray Space Telescope, formerly named GLAST, is a mission in low‐Earth orbit to observe gamma rays from the cosmos in the broad energy range from 20 MeV to >300 GeV, with supporting observations of gamma‐ray bursts from 8 keV to 30 MeV. The telescope far surpasses previous generations in its ability to detect and localize faint gamma‐ray sources, as well as its ability to see 20% of the sky at any instant and scan the entire sky on a timescale of a few hours. With its recent launch on 11 June 2008, Fermi has opened a new and exciting window on a wide variety of exotic astrophysical objects, including pulsars, black holes, active galactic nuclei, and gamma‐ray bursts, and it will enable new research on such topics as the origin and circulation of cosmic rays and searches for hypothetical new phenomena such as annihilation of supersymmetric dark matter.
15057663
Linac Coherent Light Source Undulator RF BPM System
The Linac Coherent Light Source (LCLS) will be the world's first x-ray free-electron laser (FEL) when it becomes operational in 2009. The LCLS is currently in the construction phase. The beam position monitor (BPM) system planned for the LCLS undulator will incorporate a high-resolution X-band cavity BPM system described in this paper. The BPM system will provide high-resolution measurements of the electron beam trajectory on a pulse-to-pulse basis and over many shots. The X-band cavity BPM size, simple fabrication, and high resolution make it an ideal choice for LCLS beam position detection. We will discuss the system specifications, design, and prototype test results.
110121135
FEEDBACK SYSTEMS IN THE SLC
Two classes of computer-controlled feedback have been implemented to stabilize parameters in subsystems of the SLC: (1) ''slow'' (time scales approx. minutes) feedback, and (2) ''fast'', i.e., pulse-to-pulse, feedback. The slow loops run in a single FEEDBACK process in the SLC host VAX, which acquires signals and sets control parameters via communication with the database and the network of normal SLC microprocessors. Slow loops exist to stabilize beam energy and energy spread, beam position and angle, and timing of kicker magnets, and to compensate for changes in the phase length of the rf drive line. The fast loops run in dedicated microprocessors, and may sample and/or feedback on particular parameters as often as every pulse of the SLC beam. The first implementations of fast feedback are to control transverse beam blow-up and to stabilize the energy and energy spread of bunches going into the SLC arcs. The overall architecture of the feedback software and the operator interface for controlling loops are discussed.
118623913
A Blind Search for Pulsars in EGRET Data using a Time Differencing Technique
Most of the sources in the galactic plane detected by the EGRET instrument on board the Compton Gamma‐Ray Observatory still remain unidentified. The energy spectra of some of the sources suggest that the gamma‐ray emission originates from pulsars. Detecting pulsed gamma‐ray emission without prior knowledge of the pulsar spin parameters is exceedingly difficult. Standard pulsar blind searches rely on the execution of long FFTs, which use a large amount of computational power. We present a pulsar blind search, based on our Time Differencing Technique, that can be carried out more economically, in this example on a standard office PC. To prove the concept of this search method we conducted a blind periodicity search on all 271 sources from the 3rd EGRET catalog.
15325639
A Fresh Look at Diffuse Gamma-ray Emission from the Inner Galaxy
The Energetic Gamma-Ray Experiment Telescope (EGRET) experiment onboard the Compton Gamma-Ray Observatory (CGRO) has provided the most precise measurements of the -ray sky to date. The EGRET measurements of the diffuse emission across the sky show an excess above 1 GeV. This "GeV excess" has been a topic of great debate and interest since its original discovery by Hunter et al. in 1997. We have modified the GLAST simulation and reconstruction software to model the EGRET instrument. This detailed modeling has allowed us to explore the parameters of the EGRET instrument, in both its beam-test configuration and in-orbit on CGRO, in greater detail than has previously been published. We have found that the GeV excess is significantly increased when previously unaccounted for instrumental effects are considered. We will present a new measurement of diffuse -ray emission in the inner Galaxy.
15686414
Mass Segregation in Young LMC Clusters
We present the detailed analysis of Hubble Space Telescope observations of the spatial distributions of different stellar species in two young compact star clusters in the Large Magellanic Cloud (LMC), NGC 1805 and NGC 1818. Based on a comparison of the characteristic relaxation times in their cores and at their half-mass radii with the observed degree of mass segregation, it is most likely that significant primordial mass segregation was present in both clusters, particularly in NGC 1805. Both clusters were likely formed with very similar initial mass functions (IMFs). In fact, we provide strong support for the universality of the IMF in LMC clusters for stellar masses m >= 0.8 M_sun.
17629793
Limits on Reddening and Gas-to-Dust Ratios for Seven Intermediate Redshift Damped Lyα Absorbers from Diffuse Interstellar Bands
We present equivalent width measurements and limits of six diffuse interstellar bands (DIBs, λ4428, λ5705, λ5780, λ5797, λ6284, and λ6613) in seven damped Lyα absorbers (DLAs) over the redshift range 0.091 ≤ z ≤ 0.524, sampling 20.3 ≤ log N(H I) ≤ 21.7. DIBs were detected in only one of the seven DLAs, that which has the highest reddening and metallicity. Based upon the Galactic DIB-N(H I) relation, the λ6284 DIB equivalent width upper limits in four of the seven DLAs are a factor of 4-10 times below the λ6284 DIB equivalent widths observed in the Galaxy, but are not inconsistent with those present in the Magellanic Clouds. Assuming the Galactic DIB-E(B – V) relation, we determine reddening upper limits for the DLAs in our sample. Based upon the E(B – V) limits, the gas-to-dust ratios, N(H I)/E(B – V), of the four aforementioned DLAs are at least ~5 times higher than that of the Galactic interstellar medium. The ratios of two other DLAs are at least a factor of a few times higher. The best constraints on reddening derive from the upper limits for the λ5780 and λ6284 DIBs, which yield E(B – V) ≤ 0.08 for four of the seven DLAs. Our results suggest that, in DLAs, quantities related to dust, such as reddening and metallicity, appear to have a greater impact on DIB strengths than does H I gas abundance; the organic molecules, likely responsible for DIBs in DLA-selected sight lines, are underabundant relative to sight lines in the Galaxy of similarly high N(H I). With regard to the study of astrobiology, this could have implications for the abundance of organic molecules in redshifted galaxies. However, since DLAs are observed to have low reddening, selection bias likely plays a role in the apparent underabundance of DIBs in DLAs.
17372933
MAGE - a GEANT4-based Monte Carlo framework for low-background experiments
A Monte Carlo framework, MaGe, has been developed based on the Geant4 simulation toolkit. Its purpose is to simulate physics processes in low-energy and low-background radiation detectors, specifically for the Majorana and Gerda $^{76}$Ge neutrinoless double-beta decay experiments. This jointly-developed tool is also used to verify the simulation of physics processes relevant to other low-background experiments in Geant4. The MaGe framework contains simulations of prototype experiments and test stands, and is easily extended to incorporate new geometries and configurations while still using the same verified physics processes, tunings, and code framework. This reduces duplication of efforts and improves the robustness of and confidence in the simulation output.
59367290
The Metal-rich Globular Cluster NGC 6553: Observations with WFPC2, STIS, and NICMOS
We present a Hubble Space Telescope study of the metal-rich globular cluster NGC 6553 using WFPC2, NICMOS, and STIS. Our primary motivation is to calibrate the STIS broadband LP magnitude against V555 and I814 magnitudes for stars of known metallicity and absolute (visual) magnitude, for application to our study of LMC globular clusters. NGC 6553 has been shown in earlier studies to have a very unusual color-magnitude diagram, so we also use our data to investigate the reddening, distance, luminosity function, and structure of this cluster. We deduce a higher metallicity and smaller distance modulus than did some previous studies, but we emphasize that very large patchy extinction on small angular scales prohibits accurate determination of the parameters of this cluster. The horizontal branch of NGC 6553 in (V, V-I) is tilted at an angle close to that of the reddening vector. We show that extinction does not, however, explain the tilt, which is presumably a metallicity effect. The color-magnitude diagram shows an apparent second turnoff some 1.5 mag fainter than that of the cluster. We show that this is most likely the background Galactic bulge: however, in that case, the color-magnitude diagram of NGC 6553 is not a good match to that of the field bulge population. The cluster is probably more metal-rich than is the mean field bulge star.
118172758
Triggering at Isabelle; the first 200 nanoseconds
At full luminosity of ISABELLE and with an anticipated pp total cross section of 60 mb, there will be, on the average, one interaction every 70 ns. At the other end of the spectrum, it will probably take about 10 ms to store a complete event onto magnetic tape. In between these two extremes comes a triggering scheme which must select the one event out of 14,285,714 that is of interest to the experimenter. In a typical experiment, a certain small number of signals will come into an electronics trailer on cables that are as short as possible and have propagation velocities as high as possible. With these signals a fast decision will be made as to whether the rest of the information coming in on the slower, longer cables should be further processed or not. The typical length of time for this decision is 200 ns. Some of the problems arising during that first 200 ns are considered, and some examples of attempts to solve those problems are given.