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Search Results: 1 - 10 of 463626 matches for " Fiona A. Harrison "
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Gamma-Ray Bursts and the Cosmic Star Formation Rate
Mark R. Krumholz,S. E. Thorsett,Fiona A. Harrison
Physics , 1998, DOI: 10.1086/311657
Abstract: We have tested several models of GRB luminosity and redshift distribution functions for compatibility with the BATSE 4B number versus peak flux relation. Our results disagree with recent claims that current GRB observations can be used to strongly constrain the cosmic star formation history. Instead, we find that relaxing the assumption that GRBs are standard candles renders a very broad range of models consistent with the BATSE number-flux relation. We explicitly construct two sample distributions, one tracing the star formation history and one with a constant comoving density. We show that both distributions are compatible with the observed fluxes and redshifts of the bursts GRB970508, GRB971214, and GRB980703, and we discuss the measurements required to distinguish the two models.
Swift observations of 1FGL J1018.6-5856
Hongjun An,Francois Dufour,Victoria M. Kaspi,Fiona A. Harrison
Physics , 2013, DOI: 10.1088/0004-637X/775/2/135
Abstract: We report on X-ray properties of the gamma-ray binary 1FGL J1018.6-5856 using observations obtained with the Swift X-ray telescope. Using 54 observations made between MJD 55575 and 55984, we find that the X-ray flux is modulated at a period of 16.57 +/- 0.11 days, consistent with previous reports based on gamma-ray data. We find that the X-ray maximum at phase 0 previously reported may not be a persistent feature of the source: the dramatic increases at phase 0 were detected only for ~100 days and not thereafter. Rather, the persistent sinusoidal maximum seems to be at phase 0.3--0.4, and is misaligned with the gamma-ray (GeV) peak. We also find evidence that the source's X-ray flux is correlated with the spectral hardness in the 0.5--10 keV band. Such a correlation has also been reported in the gamma-ray binaries LS 5039 and LS I+61d303 and can help us to understand the X-ray emission mechanisms of the sources.
Large Highly-Ionized Nebulae Around Ultra-luminous X-ray Sources
Dae-Sik Moon,Fiona A. Harrison,S. Bradley Cenko,Jamil A. Shariff
Physics , 2011, DOI: 10.1088/2041-8205/731/2/L32
Abstract: We present the results of deep optical spectroscopic observations using the LRIS spectrograph on the Keck I 10-m telescope of three ultra-luminous X-ray sources (ULXs), Ho IX X-1; M81 X-6; and Ho II X-1. Our observations reveal the existence of large (100 - 200 pc diameter) highly-ionized nebulae, identified by diffuse He II (4686 Angstrom) emission, surrounding these sources. Our results are the first to find highly-ionized nebulae of this extent, and the detection in all three objects indicates this may be a common feature of ULXs. In addition to the extended emission, Ho IX X-1 has an unresolved central component containing about one-third of the total He II flux, with a significant velocity dispersion of ~ 370 km/s, suggestive of the existence of a photo-ionized accretion disk or an extremely hot early-type stellar counterpart. Most of the He II emission appears to be surrounded by significantly more extended Hbeta emission, and the intensity ratios between the two lines, which range from 0.12 - 0.33, indicate that photo-ionization is the origin of the He II emission. Sustaining these extended nebulae requires substantial X-ray emission, in the range ~ 10^{39} - 10^{40} ergs/s, comparable to the measured X-ray luminosities of the sources. This favors models where the X-ray emission is isotropic, rather than beamed, which includes the interpretation that ULXs harbor intermediate-mass black holes.
A Comparison of X-ray and Mid-Infrared Selection of Obscured AGN
Megan E. Eckart,Ian D. McGreer,Daniel Stern,Fiona A. Harrison,David J. Helfand
Physics , 2009, DOI: 10.1088/0004-637X/708/1/584
Abstract: We compare the relative merits of AGN selection at X-ray and mid-infrared wavelengths using data from moderately deep fields observed by both Chandra and Spitzer. The X-ray-selected AGN sample and associated optical follow-up are drawn from the SEXSI program. Mid-infrared data in these fields are derived from Spitzer imaging, and mid-infrared AGN selection is accomplished primarily through application of the IRAC color-color AGN `wedge' selection technique. Nearly all X-ray sources in these fields which exhibit clear spectroscopic signatures of AGN activity have mid-infrared colors consistent with IRAC AGN selection. These are predominantly the most luminous X-ray sources. X-ray sources that lack high-ionization and/or broad lines in their optical spectra are far less likely to be selected as AGN by mid-infrared color selection techniques. The fraction of X-ray sources identified as AGN in the mid-infrared increases monotonically as the X-ray luminosity increases. Conversely, only 22% of mid-infrared-selected AGN are detected at X-ray energies in the moderately deep (~100 ks) Chandra data. We hypothesize that the IRAC AGN that lack X-ray detections are predominantly high-luminosity AGN that are obscured and/or lie at high redshift. A stacking analysis of X-ray-undetected sources shows that objects in the mid-infrared AGN selection wedge have average X-ray fluxes in the 2-8 keV band three times higher than sources that fall outside the wedge. Their X-ray spectra are also harder. It is evident from this comparative study that in order to create a complete, unbiased census of supermassive black hole growth and evolution, a combination of sensitive infrared, X-ray and hard X-ray selection is required. We conclude by discussing what samples will be provided by upcoming survey missions such as WISE, eROSITA, and NuSTAR.
The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) Program: I. Characteristics of the Hard X-Ray Sample
Fiona A. Harrison,Megan E. Eckart,Peter H. Mao,David J. Helfand,Daniel Stern
Physics , 2003, DOI: 10.1086/378087
Abstract: The Serendipitous Extragalactic X-ray Source Identification (SEXSI) Program is designed to extend greatly the sample of identified extragalactic hard X-ray 2-10 keV sources at intermediate fluxes ($\sim 10^{-13} - 10^{-15} erg/cm2/s$). SEXSI, which studies sources selected from more than 2 deg$^2$, provides an essential complement to the {\em Chandra} Deep Fields, which reach depths of $5 \times 10^{-16} erg/cm2/s$ (\hardrange) but over a total area of $< 0.2$ deg$^2$. In this paper we describe the characteristics of the survey and our X-ray data analysis methodology. We present the cumulative flux distribution for the X-ray sample of 1034 hard sources, and discuss the distribution of spectral hardness ratios. Our lognlogs in this intermediate flux range connects to those found in the deep fields, and by combining the data sets, we constrain the hard X-ray population over the flux range where the differential number counts change slope, and from which the bulk of the 2 -- 10 keV X-ray background arises. We further investigate the lognlogs distribution separately for soft and hard sources in our sample, finding that while a clear change in slope is seen for the softer sample, the hardest sources are well-described by a single power-law down to the faintest fluxes, consistent with the notion that they lie at lower average redshift.
NuSTAR observations of the young, energetic radio pulsar PSR B1509-58
Ge Chen,Hongjun An,Victoria M. Kaspi,Fiona A. Harrison,Kristin K. Madsen,Daniel Stern
Physics , 2015,
Abstract: We report on Nuclear Spectroscopic Telescope Array (NuSTAR) hard X-ray observations of the young rotation-powered radio pulsar PSR B1509$-$58 in the supernova remnant MSH 15$-$52. We confirm the previously reported curvature in the hard X-ray spectrum, showing that a log parabolic model provides a statistically superior fit to the spectrum compared with the standard power law. The log parabolic model describes the NuSTAR data, as well as previously published gamma-ray data obtained with COMPTEL and AGILE, all together spanning 3 keV through 500 MeV. Our spectral modelling allows us to constrain the peak of the broadband high energy spectrum to be at 2.6$\pm$0.8 MeV, an improvement of nearly an order of magnitude in precision over previous measurements. In addition, we calculate NuSTAR spectra in 26 pulse phase bins and confirm previously reported variations of photon indices with phase. Finally, we measure the pulsed fraction of PSR B1509$-$58 in the hard X-ray energy band for the first time. Using the energy resolved pulsed fraction results, we estimate that the pulsar's off-pulse emission has a photon index value between 1.26 and 1.96. Our results support a model in which the pulsar's lack of GeV emission is due to viewing geometry, with the X-rays originating from synchrotron emission from secondary pairs in the magnetosphere.
A Position Sensitive X-ray Spectrophotometer using Microwave Kinetic Inductance Detectors
Benjamin A. Mazin,Megan E. Eckart,Bruce Bumble,Sunil Golwala,Peter K. Day,Jonas Zmuidzinas,Fiona A. Harrison
Physics , 2006, DOI: 10.1063/1.2390664
Abstract: The surface impedance of a superconductor changes when energy is absorbed and Cooper pairs are broken to produce single electron (quasiparticle) excitations. This change may be sensitively measured using a thin-film resonant circuit called a microwave kinetic inductance detector (MKID). The practical application of MKIDs for photon detection requires a method of efficiently coupling the photon energy to the MKID. We present results on position sensitive X-ray detectors made by using two aluminum MKIDs on either side of a tantalum photon absorber strip. Diffusion constants, recombination times, and energy resolution are reported. MKIDs can easily be scaled into large arrays.
Spectroscopic confirmation of a galaxy cluster associated with 7C1756+6520 at z=1.416
Audrey Galametz,Daniel Stern,S. Adam Stanford,Carlos De Breuck,Joel Vernet,Roger L. Griffith,Fiona A. Harrison
Physics , 2010, DOI: 10.1051/0004-6361/201014356
Abstract: We present spectroscopic follow-up of an overdensity of galaxies photometrically selected to be at 1.4
The polar Catalysmic Variable 1RXS J173006.4+033813
Varun B. Bhalerao,Marten H. van Kerkwijk,Fiona A. Harrison,Mansi M. Kasliwal,S. R. Kulkarni,Vikram R. Rana
Physics , 2010, DOI: 10.1088/0004-637X/721/1/412
Abstract: We report the discovery of 1RXS J173006.4+033813, a polar cataclysmic variable with a period of 120.21 min. The white dwarf primary has a magnetic field of B = 42+6-5 MG, and the secondary is a M3 dwarf. The system shows highly symmetric double peaked photometric modulation in the active state as well as in quiescence. These arise from a combination of cyclotron beaming and ellipsoidal modulation. The projected orbital velocity of the secondary is K2 = 390+-4 km/s. We place an upper limit of 830+-65 pc on the distance.
Properties of Pt Schottky Type Contacts On High-Resistivity CdZnTe Detectors
Aleksey E. Bolotnikov,Steven E. Boggs,C. M. Hubert Chen,Walter R. Cook,Fiona A. Harrison,Stephen M. Schindler
Physics , 2001, DOI: 10.1016/S0168-9002(01)01506-6
Abstract: In this paper we present studies of the I-V characteristics of CdZnTe detectors with Pt contacts fabricated from high-resistivity single crystals grown by the high-pressure Brigman process. We have analyzed the experimental I-V curves using a model that approximates the CZT detector as a system consisting of a reversed Schottky contact in series with the bulk resistance. Least square fits to the experimental data yield 0.78-0.79 eV for the Pt-CZT Schottky barrier height, and <20 V for the voltage required to deplete a 2 mm thick CZT detector. We demonstrate that at high bias the thermionic current over the Schottky barrier, the height of which is reduced due to an interfacial layer between the contact and CZT material, controls the leakage current of the detectors. In many cases the dark current is not determined by the resistivity of the bulk material, but rather the properties of the contacts; namely by the interfacial layer between the contact and CZT material.
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