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ABSTRACT: We present the results of Rossi X-Ray Timing Explorer (RXTE) observations of GS 1354–644 during a modest outburst in 1997-1998. The source is one of a handful of black hole X-ray transients that are confirmed to be recurrent in X-rays. A 1987 outburst of the same source observed by Ginga was much brighter and showed a high/soft spectral state. In contrast, the 1997-1998 outburst showed a low/hard spectral state. Both states are typical of black hole binaries. The RXTE All-Sky Monitor observed an outburst duration of 150-200 days. PCA and HEXTE observations covered ~70 days near the maximum of the light curve and during the flux decline. Throughout the observations, the spectrum can be approximated by Compton upscattering of soft photons by energetic electrons. The hot electron cloud has a temperature kT ~ 30 keV and optical depth τ ~ 4-5. To fit the data well, an additional iron fluorescent line and reflection component are required, which indicates the presence of optically thick cool material, most probably in the outer part of the accretion disk. Dramatic fast variability was observed and has been analyzed in the context of a shot noise model. The spectrum appeared to be softest at the peaks of the shot noise variability. The shape of the power spectrum was typical for black hole systems in a low/hard state. We note a qualitative difference in the shape of the dependence of fractional variability on energy when we compare systems with black holes and with neutron stars. Since it is difficult to discriminate these systems on spectral grounds, at least in their low/hard states, this new difference might be important.
The Astrophysical Journal 12/2008; 530(2):955. · 6.02 Impact Factor
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ABSTRACT: We present a spectral analysis of XMM-Newton data of ten dwarf novae, nine of which were observed during a quiescent state. The X-ray spectra indicate the presence of a hot, optically thin plasma with a temperature distribution consistent with a simple, isobaric cooling flow. The likely origin of the X-ray emission is cooling plasma in the boundary layer settling onto the white dwarf. Using a cooling flow model, we derive the temperatures, accretion rates, rotational velocities, and elemental abundances of the X-ray emitting gas. We discuss the implications of our findings for the structure of the boundary layer. A comparison of X-ray and ultraviolet luminosities finds no evidence of underluminous boundary layers in any of the quiescent dwarf novae. The X-ray spectrum of EI UMa differs significantly from those of the other objects, showing a strong fluorescent Fe K-alpha line and a strong O VII line triplet. Based on the observational evidence, we argue that EI UMa is most likely an intermediate polar and not as previously thought a dwarf nova. Comment: Accepted for publication in ApJ, 37 pages, 8 figures
03/2005;
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ABSTRACT: Fast X-ray transients (FXTs) with timescales from seconds to hours have been seen by numerous space instruments. We have assembled archival data from Ariel-5, HEAO-1 (A-1 and A-2), WATCH, ROSAT, and Einstein to produce a global fluence-frequency relationship for these events. Fitting the log N-log S distribution over several orders of magnitude to simple power law we find a slope of -1.0. The sources of FXTs are undoubtedly heterogeneous, the -1 power law is an approximate result of the summation of these multiple sources. Two major contributions come from gamma-ray bursts and stellar flares. Extrapolating from the BATSE catalog of GRBs, we find that the fraction of X-ray flashes that can be the X-ray counterparts of gamma-ray bursts is a function of fluence. Certainly most FXTs are not counterparts of standard gamma-ray bursts. The fraction of FXTs from non-GRB sources, such as magnetic stars, is greatest for the faintest FXTs. Our understanding of the FXT phenomenon remains limited and would greatly benefit from a large, homogeneous data set, which requires a wide-field, sensitive instrument. Comment: 36 pages, 8 figures
04/2003;
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Nature 04/2003; 422(6929):277. · 36.28 Impact Factor
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ABSTRACT: We present an analysis of the RXTE observations of 4U 1630-47 during its outburst of 1998. The light curve and the spectral evolution of the outburst were distinctly different from the outbursts of the same source in 1996 and in 1999. Special emphasis of our analysis was on the observations taken during the initial rise of the flux and during the maximum of the outburst. The maximum of the outburst was divided into three plateaus, with almost constant flux within each plateau, and fast jumps between them. The spectral and timing parameters are stable for each individual plateau, but distinctly different between the plateaus. The variability detected on the first plateau is of special interest. During these observations the source exhibits quasi-regular modulations with period of ~10 - 20 s. Our analysis revealed significant differences in spectral and temporal behavior of the source at high and low fluxes during this period of time. The source behavior can be generally explained in the framework of the two-phase model of the accretion flow, involving a hot inner comptonization region and surrounding optically thick disk. The variability and spectral evolution of the source were similar to what was observed earlier for other X-ray Novae. We show that 4U 1630-47 has a variety of properties which are typical for Galactic black hole binaries, both transient and persistent. We argue that this system may be an intermediate case between different groups of black hole candidates. Comment: 14 pages, 12 figures, submitted to MNRAS
Monthly Notices of the Royal Astronomical Society 04/2001; 322(2):309. · 4.90 Impact Factor
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ABSTRACT: Fast X-ray Transients (FXTs) are short duration X-ray sources that have timescales of minutes to hours. They have been observed by many instruments, from HEAO-1 to BeppoSAX, however, they are still unexplained. Due to their wide range of observational characteristics, it is suspected that FXTs are a heterogeneous collection of objects involving different emission mechanisms. The relationship between FXTs and gamma-ray bursts (GRBs) is particularly intriguing. The results of BeppoSAX observations confirm that some but not all fast X-ray transients are counterparts of gamma-ray bursts. A fraction of “classical” FXT are believed to be counterparts of GRBs, based largely on time coincidences with GRB events. Other FXTs might be “gamma-ray silent” GRBs. We discuss the statistics and distribution of FXTs, and compare their characteristics with GRB prompt counterparts and afterglows. © 2000 American Institute of Physics.
AIP Conference Proceedings. 09/2000; 526(1):245-249.
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ABSTRACT: Using ROSAT PSPC data, we have performed several tests aimed at understanding the origin of the soft X-ray spectral component detected from the bulge of M31. We find that a significant soft component in the spectrum of the bulge is spatially correlated with the unresolved X-ray emission near the core of M31, which is probably a hot interstellar medium or perhaps a population of multiple faint sources. For the first time, we extracted the spectrum of this unresolved emission, by removing point sources dominating the integral spectrum of the bulge, and found it to be responsible for the most of soft excess. A soft spectral component is not at all needed to fit the point source spectrum that remains after subtracting the unresolved emission. The integral spectra of bright point sources, both inside and outside of the M31 bulge, can be fitted with a single power-law in the ROSAT band. Our analysis rules out the previous suggestion that all bulge emission in M31 may be generated by low mass X-ray binaries (Irwin & Bregman, 1999). Comment: 11 pages incl. 2 figures, 2 tables, accepted to ApJL
06/2000;
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ABSTRACT: We present results of observations of several X-ray transients with RXTE in 1996–1998, namely, GRS 1739−278, XTE J1748−288, GS 1354−64. 2S1803−245 and XTE J0421+560 (CI Cam). We studied light curves and spectra of their outbursts and compared them with observations of other X-ray transients. We discuss fits of high state spectra with BMC model, and similarities and differences between black holes and neutron stars in their low state. Special attention is paid to CI Cam as possible legate for new class of X-ray transients. © 2000 American Institute of Physics.
AIP Conference Proceedings. 04/2000; 510(1):143-147.
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ABSTRACT: We present the results of Rossi X-ray Timing Explorer observations of GS 1354-644 during a modest outburst in 1997-1998. The source is one of a handful of black hole X-ray transients that are confirmed to be recurrent in X-rays. A 1987 outburst of the same source observed by Ginga was much brighter, and showed a high/soft spectral state. In contrast the 1997-1998 outburst showed a low/hard spectral state. Both states are typical for black hole binaries. The RXTE All Sky Monitor observed an outburst duration of 150 to 200 days. PCA and HEXTE observations covered ~70 days near the maximum of the light curve and during the flux decline. Throughout the observations, the spectrum can be approximated by Compton upscattering of soft photons by energetic electrons. The hot electron cloud has a temperature kT ~30 keV and optical depth tau~4--5. To fit the data well an additional iron fluorescent line and reflection component are required, which indicates the presence of optically thick cool material, most probably in the outer part of the accretion disk. Dramatic fast variability was observed, and has been analyzed in the context of a shot noise model. The spectrum appeared to be softest at the peaks of the shot-noise variability. The shape of the power spectrum was typical for black hole systems in a low/hard state. We note a qualitative difference in the shape of the dependence of fractional variability on energy, when we compare systems with black holes and with neutron stars. Since it is difficult to discriminate these systems on spectral grounds, at least in their low/hard states, this new difference might be important. Comment: 12 pages, 9 figures, accepted for publication in ApJ (Feb. 2000, v.530), uses emulateapj.sty
05/1999;
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ABSTRACT: The effort and expense required to build and maintain an optical-quality telescope increases dramatically with the size of the telescope aperture, 1 and this is especially so in space. But scenarios have been proposed for deploying (for instance, inflatable) telescopes with very large but considerably less than optical-quality apertures. Our interest is in ameliorating the effects of the low quality aperture in order to exploit the raw size of the aperture to obtain high resolution images. We describe an algorithm for generating an adaptive binary mask to correct the time-varying aberrations of very large apertures which are many (possibly hundreds of) wavelengths out of figure. The technique is limited to monochromatic imagery, though the wavelength at which observations are taken can be readily changed on the fly, and for earth-pointing applications, the limited light-gathering power imposed by the monochromatic filter is not a problem. The mask itself can be placed at the exit pup...
02/1998;
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ABSTRACT: With the aim of isolating universal characteristics of gamma-ray burst variability, we compare time histories for 20 bright bursts detected by the Burst and Transient Source Experiment (Fishman et al., 1992). Using an autocorrelation function method, we find that the durations of individual structures within a burst, as well as the burst as a whole, become shorter with increasing energy in most events. We introduce the skewness function, a measure of temporal asymmetry. We find that most bursts exhibit a net skewness, in the sense that the intensity rises more rapidly than it falls. Over short time scales, bursts exhibit no preferred asymmetry. Taken together, these properties suggest that the overall time structure of a burst is due to an explosive phenomenon in which the evolution is initially energetic and rapid and is later slower during a 'cooling' period. We cannot rule out the possibility that short time-scale variability is caused by radiation beams sweeping past the observer.
06/1993;
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ABSTRACT: We demonstrate that high-Z material can be detected and located in three dimensions using radiographs formed by cosmic-ray muons. Detection of high-Z material hidden inside large volume of ordinary cargo is an important and timely task given the danger associated with illegal transport of uranium and heavier elements. Existing radiography techniques are inefficient for shielded material, often expensive and involve radiation hazards, real and perceived. We recently demonstrated that radiographs can be formed using cosmic-ray muons K. N. Borozdin et al., Nature London 422, 277 2003. Here, we show that compact, high-Z objects can be detected and located in three dimensions with muon radiography. The natural flux of cosmic-ray muons P. K. F. Grieder, Cosmic Rays at Earth Elsevier, New York, 2001, approximately 10 000 m 2 min 1 , can form useful images in 1 min, using large-area muon detectors like those used in high-energy physics. © 2003 American Institute of Physics.
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ABSTRACT: We present a case study of strategic technology planning in a large, mission-oriented division of Los Alamos National Laboratory. The Division, Nonproliferation and International Security, was strongly oriented around its eponymous mission. The challenge for our planning process was one of scale: identifying a manageable handful of science and technology thrusts that would drive that mission forward. Based on a detailed analysis of future missions and technology possibilities, we identified three strategic directions: (1) computer-enabled understanding, (2) networked and intelligent sensors, and (3) physics applied to sensors. The lessons we learned apply to strategic planning in other diverse R&D organizations.
Journal of Engineering and Technology Management 23(3):168-181. · 1.03 Impact Factor
