D. Yentis

The University of Edinburgh, Edinburgh, Scotland, United Kingdom

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Publications (104)379.92 Total impact

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    ABSTRACT: We report the first high-resolution (R = 4000) spectroscopic observation of the binary DA white dwarf Feige 24 in the extreme-ultraviolet band 220-250 Å. A stellar atmosphere model assuming a homogeneous element distribution yields a best fit to the data that excludes a significant abundance of photospheric helium. The upper limit on the photospheric helium abundance is 2.5 × 10–6 (90% confidence), equivalent to a lower limit of 1.2 × 10–13 M ☉ on the overlying layer of hydrogen. An ionized interstellar He component (3.9 × 1017 cm–2) is clearly present along the line of sight, which implies an He ionization fraction of 0.72, considerably higher than is typical of the local interstellar medium. However, some of this material may be associated with circumstellar gas, which has been detected by analysis of the C IV absorption line doublet in a Hubble Space Telescope/Space Telescope Imaging Spectrograph spectrum.
    The Astrophysical Journal 03/2011; 730(2):115. DOI:10.1088/0004-637X/730/2/115 · 6.28 Impact Factor
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    ABSTRACT: We report the first high-resolution (R = 4000) spectroscopic observation of the binary DA white dwarf Feige 24 in the extreme-ultraviolet band 220–250A˚. A stellar atmosphere model assuming a homogeneous element distribution yields a best fit to the data that excludes a significant abundance of photospheric helium, a surprising result contrary to predictions by common envelope evolution theory. The upper limit on the photospheric helium abundance is 2.5×10−6 (90% confidence), and the lower limit on the overlying layer of hydrogen is 1.2×10−13 M⊙. An ionized He component (3.9×1017 cm−2) is clearly present along the line-of-sight, which if completely interstellar implies a He ionization fraction of 0.7, considerably higher than is typical of the local interstellar medium.
    11/2010; DOI:10.1063/1.3527881
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    ABSTRACT: We report a high-resolution (R = 3000-4000) spectroscopic observation of the DA white dwarf G191-B2B in the extreme-ultraviolet band 220-245 Å. A low-density, ionized He component is clearly present along the line of sight, which if completely interstellar implies a He ionization fraction considerably higher than is typical of the local interstellar medium. However, some of this material may be associated with circumstellar gas, which has been detected by analysis of the C IV absorption-line doublet in a Hubble Space Telescope/Space Telescope Imaging Spectrograph spectrum. A stellar atmosphere model assuming a uniform element distribution yields a best fit to the data that includes a significant abundance of photospheric He. The 99% confidence contour for the fit parameters excludes solutions in which photospheric He is absent, but this result needs to be tested using models allowing abundance gradients.
    The Astrophysical Journal 12/2008; 565(1):L47. DOI:10.1086/339054 · 6.28 Impact Factor
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    ABSTRACT: This paper reports X-ray spectral observations of a relatively nearby (z = 0.048) BL Lac object 1ES 1959+65, which is a potential TeV emitter. The observations include 31 short pointings made by the Unconventional Stellar Aspect (USA) experiment on board the Advanced Research and Global Observation Satellite (ARGOS), and 17 pointings by the Proportional Counter Array (PCA) on board the Rossi X-Ray Timing Explorer (RXTE). Most of these observations were spaced by less than 1 day. 1ES 1959+65 was detected by the ARGOS USA detector in the range 1-16 keV, and by the PCA in the 2-16 keV range but at different times. During the closely spaced RXTE observations beginning on 2000 July 28, the ending of one flare and the start of another are visible, associated with spectral changes, where the photon index Γ ranges between ~1.4 and ~1.7, and the spectrum is harder when the source is brighter. This implies that 1ES 1959+65 is an X-ray-selected BL Lac type (XBL) blazar, with the X-ray emission likely to originate via the synchrotron process. The USA observations reveal another flare that peaked on 2000 November 14 and doubled the flux within a few days, again associated with spectral changes of the same form. The spectral variability correlated with the flux and timing characteristics of this object that are similar to those of other nearby BL Lac objects and suggest relativistic beaming with a Doppler factor δ ≥ 1.6 and magnetic fields on the order of a few milligauss. We also suggest that the steady component of the X-ray emission—present in this object as well as in other XBLs—may be due to the large-scale relativistic jet (such as measured by Chandra in many radio-loud active galactic nuclei) but pointing very closely to our line of sight.
    The Astrophysical Journal 12/2008; 571(2):763. DOI:10.1086/340065 · 6.28 Impact Factor
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    ABSTRACT: Using the USA Experiment on the Advanced Research and Global Observation Satellite and the Rossi X-Ray Timing Explorer, we have extensively observed the X-ray transient XTE J1118+480 during its recent outburst in 2000 April-June. We present detailed monitoring of the evolution of a low-frequency quasi-periodic oscillation (QPO) that drifts from 0.07 to 0.15 Hz during the outburst. We examine possible correlations of the QPO frequency with the flux and spectral characteristics of the source, and we compare this QPO with low-frequency QPOs observed in other black hole candidates.
    The Astrophysical Journal 12/2008; 544(1):L45. DOI:10.1086/317287 · 6.28 Impact Factor
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    ABSTRACT: The Joint astrophysical Plasmadynamic EXperiment (J-PEX) is a high-resolution extreme ultraviolet (EUV) spectrometer (220-245 Å) used for the study of white dwarf atmospheres. Significant improvements have been achieved in both the normal-incidence gratings and the focal-plane detector since its first successful sounding rocket flight in 2001. The spherical laminar gratings have been replaced by paraboloidal gratings. The substrates of the new gratings have measured slope errors less than 0.35 arcsec. The gratings were recorded holographically and the rulings transferred into the silica substrates by ion etching. This procedure was followed by polymer overcoat to reduce the blaze angle of the groove profile. The detector uses microchannel plates with 6 mum pores and a cross-strip anode, providing 17.9 mum resolution in the dispersion direction. The detector employs a KBr photocathode with a projected efficiency of 0.24 at 256 Å. Using ray tracing simulations, we predict the resolving power expected from the spectrometer during upcoming EUV calibrations with a He II discharge source.
    Proceedings of SPIE - The International Society for Optical Engineering 01/2006; DOI:10.1117/12.670064 · 0.20 Impact Factor
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    ABSTRACT: We have continued our detailed analysis of the high-resolution (R= 4000) spectroscopic observation of the DA white dwarf G191-B2B, obtained by the Joint Astrophysical Plasmadynamic Experiment (J-PEX) normal incidence sounding rocket-borne telescope, comparing the observed data with theoretical predictions for both homogeneous and stratified atmosphere structures. We find that the former models give the best agreement over the narrow waveband covered by J-PEX, in conflict with what is expected from previous studies of the lower resolution but broader wavelength coverage Extreme Ultraviolet Explorer spectra. We discuss the possible limitations of the atomic data and our understanding of the stellar atmospheres that might give rise to this inconsistency. In our earlier study, we obtained an unusually high ionization fraction for the ionized He ii present along the line of sight to the star. In the present paper, we obtain a better fit when we assume, as suggested by Space Telescope Imaging Spectrograph results, that this He ii resides in two separate components. When one of these is assigned to the local interstellar cloud, the implied He ionization fraction is consistent with measurements along other lines of sight. However, the resolving power and signal-to-noise available from the instrument configuration used in this first successful J-PEX flight are not sufficient to clearly identify and prove the existence of the two components.
    Monthly Notices of the Royal Astronomical Society 08/2005; 362(4):1273 - 1278. DOI:10.1111/j.1365-2966.2005.09394.x · 5.23 Impact Factor
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    ABSTRACT: We discuss a search for aperiodic millisecond variability in the integrated 1 to 25 keV X-ray region of Cyg X-1. We have examined HEAO A-1 archival data and RXTE guest observer data for indications of excess power above the Poisson noise #oor using the relative integral power analysis and the Fourier transform method. We present results that are in disagreement with the results of Meekins et al. #1984#. We attribute the discrepancy to an instrumental e#ect for which Meekins et al. #1984# did not apply a correction. With the correction we see no evidence for excess power above 25 Hz. Our analysis of RXTE data is in agreement with previously published results of di#erent data sets and shows no sign of excess power above 30 Hz. Submitted to Astrophysical Journal Work supported by Department of Energy contract DE#AC03#76SF00515. C. Chaput ,E.Bloom , L. Cominsky 1;2 , G. Godfrey , P. Hertz ,J.Scargle , G. Shabad , H. Wen , K. Wood and D. Yentis Received ; accepted Stanford Linear Accelerator Center Sonoma State University Naval Research Laboratory NASA Ames Research Center # 2# ABSTRACT We have conducted a search for aperiodic millisecond variability in the integrated 1to25 keV X-ray region of Cyg X-1. We have examined HEAO A-1 archival data and Rossi X-ray Timing Explorer #RXTE# guest observer data for evidence of excess power above the Poisson noise #oor using the relative integral power analysis and the Fourier transform method. Our results are in disagreement with the results of Meekins et al.
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    ABSTRACT: APEX is a proposed mission for a Small Explorer (SMEX) satellite. The instrument is a suite of 8 near-normal incidence EUV spectrometers and is the outgrowth of 17 years of research at NRL on multilayer coatings and holographic ion-etched diffraction gratings. A prototype spectrometer has been flown successfully on a sounding rocket. We have examined different multilayer and gratings designs and produced a configuration optimized for the proposed science. APEX will achieve a peak effective area of at least 30-50 cm2 in the range 90-275 Å with resolution ~10,000, significant improvements on Chandra and EUVE.
    Proceedings of SPIE - The International Society for Optical Engineering 01/2004; DOI:10.1117/12.508553 · 0.20 Impact Factor
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    ABSTRACT: APEX is a proposed mission for a Small Explorer (SMEX) satellite. APEX will investigate the density, temperature, composition, magnetic field, structure, and dynamics of hot astrophysical plasmas (log T = ~5-7), which emit the bulk of their radiation at EUV wavelengths and produce critical spectral diagnostics not found at other wavelengths. APEX addresses basic questions of stellar evolution and galactic structure through high-resolution spectroscopy of white dwarf stars, cataclysmic variables, the local interstellar medium, and stellar coronae. Thus APEX complements the Chandra, Newton-XMM, FUSE, and CHIPS missions. The instrument is a suite of 8 near-normal incidence spectrometers (~90-275 Angstroms, resolving power ~10,000, effective area 30-50 cm2) each of which employs a multilayer-coated ion-etched blazed diffraction grating and a microchannel plate detector of high quantum efficiency and high spatial resolution. The instrument is mounted on a 3-axis stabilized commercial spacecraft bus with a precision pointing system. The spacecraft is launched by a Taurus vehicle, and payload size and weight fit comfortably within limits for the 2210 fairing. Of order 100 targets will be observed over the baseline mission of 2 years. These are selected carefully to maximize scientific return, and all were detected in the EUVE and the ROSAT WFC surveys.
    Proceedings of SPIE - The International Society for Optical Engineering 12/2003; DOI:10.1117/12.508548 · 0.20 Impact Factor
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    ABSTRACT: Type I X-ray bursts are believed to be caused by unstable burning on the surface of a neutron star in a binary system. In this paper we examine the Unconventional Stellar Aspect (USA) experiment archive of binary neutron star and Black Hole Candidate (BHC) data for evidence of X-ray bursts. We scanned 927.5 ks of neutron star data and found 19 Type I X-ray bursts, yielding an average burst rate λ = 2.05*E-2 bursts/ks, or 48.8 ks between bursts. We scanned 972.3 ks of BHC data and found no evidence for X-ray bursts, yielding a 95% confidence limit of <= 3.1*E-3 bursts/ks, or >325 ks between bursts, on the rate of X-ray bursts in BHCs. We discuss how this provides evidence against the presence of a surface on the compact object in these systems. Work at SLAC was supported by Department of Energy contract DE-AC 03-76-SFO0515. Basic research in X-ray astronomy at the Naval Research Laboratory is supported by ONR/NRL. This work was also supported by the NASA Applied Information Systems Research Program.
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    The Astrophysical Journal Supplement Series 02/2003; 144(2):299-300. DOI:10.1086/344693 · 14.14 Impact Factor
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    ABSTRACT: EUVE and the ROSAT WFC have left a tremendous legacy in astrophysics at EUV wavelengths. More recently, Chandra and XMM-Newton have demonstrated at X-ray wavelengths the power of high-resolution astronomical spectroscopy, which allows the identification of weak emission lines, the measurement of Doppler shifts and line profiles, and the detection of narrow absorption features. This leads to a complete understanding of the density, temperature, abundance, magnetic, and dynamic structure of astrophysical plasmas. However, the termination of the EUVE mission has left a gaping hole in spectral coverage at crucial EUV wavelengths ( 100-300 Angstroms), where hot (105-108 K) plasmas radiate most strongly and produce critical spectral diagnostics. CHIPS will fill this hole only partially as it is optimized for diffuse emission and has only moderate resolution (R 150). For discrete sources, we have successfully flown a follow-on instrument to the EUVE spectrometer (Aeff 1 cm2, R 400), the high-resolution spectrometer J-PEX (Aeff 3 cm2, R 3000). In this paper we build on the J-PEX prototype and present a strawman design for an orbiting spectroscopic observatory, APEX, a SMEX-class instrument containing a suite of spectrometers that together achieve both high effective area (>20 cm2) and high spectral resolution ( 10000) over the range 100-300 Angstroms. We discuss in depth the optical design, the wavelength coverage, and the effective area. This work is supported by the Office of Naval Research and NRL, and by NASA Space Astrophysics and Research Analysis grants
    Proceedings of SPIE - The International Society for Optical Engineering 01/2003; DOI:10.1117/12.459808 · 0.20 Impact Factor
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    ABSTRACT: 4U 1630-472 is a transient low mass x-ray binary (LMXB). The lack of type I X-ray bursts in addition to characteristics in the X-ray timing and spectrum classify the compact object as a black hole candidate (BHC). This particular transient undergoes outbursts every 550 - 690 days. We analyzed archival data of this source's 1999 outburst obtained from the RXTE and USA X-ray satellites. The behavior of the disk temperature throughout the outburst is discussed along with appearance of a low frequency, ~ 1 Hz, QPO during the onset of the outburst.
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    ABSTRACT: We report on the successful sounding rocket flight of the high resolution (R=3000-4000) J-PEX EUV spectrometer. J-PEX is a novel normal incidence instrument, which combines the focusing and dispersive elements of the spectrometer into a single optical element, a multilayer-coated grating. The high spectral resolution achieved has had to be matched by unprecedented high spatial resolution in the imaging microchannel plate detector used to record the data. We illustrate the performance of the complete instrument through an analysis of the 220-245A spectrum of the white dwarf G191-B2B obtained with a 300 second exposure. The high resolution allows us to detect a low-density ionized helium component along the line of sight to the star and individual absorption lines from heavier elements in the photosphere.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2002; DOI:10.1117/12.459810 · 0.20 Impact Factor
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    ABSTRACT: A field of 1.013 sr in the ROSAT All-Sky Survey (RASS), centered on the south Galactic pole (SGP), has been searched in a systematic, objective manner for clusters of galaxies. The procedure relied on a correlation of the X-ray positions and properties of ROSAT sources in the field with the distribution of galaxies in the COSMOS digitized database, which was obtained from scanning the plates of the UK Schmidt IIIa-J optical survey of the southern sky. The study used the second ROSAT survey database ( RASS-2) and included several optical observing campaigns to measure cluster redshifts. The search, which is a precursor to the larger REFLEX survey encompassing the whole southern sky, reached the detection limits of both the RASS and the COSMOS data and yielded a catalog of 186 clusters in which the lowest flux is 1.5 x 10(-12) ergs cm(-2) s(-1) in the 0.1-2.4 keV band. Of these 157 have measured redshifts. Using a flux limit of 3.0 x 10(-12) ergs cm(-2) s(- 1) a complete subset of 112 clusters was obtained, of which 110 have measured redshifts. The spatial distribution of the X-ray clusters out to a redshift of 0.15 shows an extension of the Local Supercluster to the Pisces-Cetus supercluster (z < &SIM;0.07), and an orthogonal structure at higher redshift (0.07 < z < 0.15). This result is consistent with large-scale structure suggested by optical surveys.
    The Astrophysical Journal Supplement Series 06/2002; 140(2). DOI:10.1086/324519 · 14.14 Impact Factor
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    ABSTRACT: A field of 1.013sr in the ROSAT All-Sky Survey (RASS), centered on the south Galactic pole (SGP), has been searched in a systematic, objective manner for clusters of galaxies. The procedure relied on a correlation of the X-ray positions and properties of ROSAT sources in the field with the distribution of galaxies in the COSMOS digitized database, which was obtained from scanning the plates of the UK Schmidt IIIa-J optical survey of the southern sky. The study used the second ROSAT survey database (RASS-2) and included several optical observing campaigns to measure cluster redshifts. The search, which is a precursor to the larger REFLEX survey encompassing the whole southern sky, reached the detection limits of both the RASS and the COSMOS data and yielded a catalog of 186 clusters in which the lowest flux is 1.5x10-12ergs/cm2/s in the 0.1-2.4keV band. Of these 157 have measured redshifts. Using a flux limit of 3.0x10-12ergs/cm2/s a complete subset of 112 clusters was obtained, of which 110 have measured redshifts. The spatial distribution of the X-ray clusters out to a redshift of 0.15 shows an extension of the Local Supercluster to the Pisces-Cetus supercluster (z<0.07), and an orthogonal structure at higher redshift (0.07
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    ABSTRACT: This paper reports X-ray spectral observations of a relatively nearby z = 0.048 BL Lacertae (BL Lac) object 1ES1959+650, which is a potential TeV emitter. The observations include 31 short pointings made by the Unconventional Stellar Aspect (USA) Experiment on board the Advanced Research and Global Observation Satellite (ARGOS), and 17 pointings by the PCA on board the Rossi X-ray Timing Explorer (RXTE). Most of these observations were spaced by less than 1 day. \es was detected by the ARGOS USA detector in the range 1-16 keV, and by the PCA in the 2-16 keV range but at different times. During the closely spaced RXTE observations beginning on 2000 July 28, an ending of one flare and a start of another are visible, associated with spectral changes, where the photon index Gamma ranges between ~ 1.4 and 1.7, and the spectrum is harder when the source is brighter. This implies that 1ES1959 is an XBL-type blazar, with the X-ray emission likely to originate via the synchrotron process. The USA observations reveal another flare that peaked on 2000 November 14 and doubled the flux within a few days, again associated with spectral changes of the same form. The spectral variability correlated with the flux and timing characteristics of this object that are similar to those of other nearby BL Lacs, and suggest relativistic beaming with a Doppler factor delta > 1.6 and magnetic fields of the order of a few mG. We also suggest that the steady component of the X-ray emission -- present in this object as well as in other XBLs -- may be due to the large-scale relativistic jet (such as measured by Chandra in many radio-loud AGN), but pointing very closely to our line of sight.
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    01/2002; 264:38.
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    01/2002; 264:184.

Publication Stats

984 Citations
379.92 Total Impact Points

Institutions

  • 2002
    • The University of Edinburgh
      • Institute for Astronomy (IfA)
      Edinburgh, Scotland, United Kingdom
  • 1986
    • St. John's College
      Saint John, Indiana, United States
  • 1980
    • University of Birmingham
      Birmingham, England, United Kingdom