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Search Results: 1 - 10 of 167665 matches for " E. Perlman "
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Predictive Mining of Time Series Data in Astronomy
E. Perlman,A. Java
Physics , 2002,
Abstract: We discuss the development of a Java toolbox for astronomical time series data. Rather than using methods conventional in astronomy (e.g., power spectrum and cross-correlation analysis) we employ rule discovery techniques commonly used in analyzing stock-market data. By clustering patterns found within the data, rule discovery allows one to build predictive models, allowing one to forecast when a given event might occur or whether the occurrence of one event will trigger a second. We have tested the toolbox and accompanying display tool on datasets (representing several classes of objects) from the RXTE All Sky Monitor. We use these datasets to illustrate the methods and functionality of the toolbox. We also discuss issues that can come up in data analysis as well as the possible future development of the package.
A Search for Intraday Variability in the Blazar PKS 2005-489
T. A. Rector,E. S. Perlman
Physics , 2003, DOI: 10.1086/375644
Abstract: Rapid, multi-color optical monitoring carried out for a multiwavelength campaign during 22-27 August 2000 is presented for the bright and variable BL Lac object PKS 2005-489. Previous multiwavelength monitoring campaigns of PKS 2005-489 have found complex variability behavior both in flux density and in broadband spectrum, with different modes and timescales from days to hours. During this campaign, PKS 2005-489 was observed to be in an unusual state, bright in the optical but faint in the X-ray, just prior to an X-ray flare that began on 2 September 2000. PKS 2005-489 was also observed to have a flatter optical spectrum than prior epochs. A linear increase in optical flux density was observed during the first 5.2 days of the campaign. However, intraday and color variability were not detected. The large discrepancy between the ratio of X-ray to optical flux density observed during this campaign and that observed in prior epochs indicates a lack of long-term correlation between the two wavebands; and it illustrates that simple particle injection events in the jet are insufficient to explain the long-term variability behavior of this object. Twenty-three in-field standard stars are identified for future optical monitoring programs.
Detection of Exceptional X-Ray Spectral Variability in the TeV BL Lac 1ES 2344+514
P. Giommi,P. Padovani,E. Perlman
Physics , 1999, DOI: 10.1046/j.1365-8711.2000.03353.x
Abstract: We present the results of six BeppoSAX observations of 1ES 2344+514, five of which were taken within a week. 1ES 2344+514, one of the few known TeV BL Lac objects, was detected by the BeppoSAX Narrow Field Instruments between 0.1 to \~50 keV. During the first five closely spaced observations 1ES 2344+514 showed large amplitude luminosity variability, associated with spectacular spectral changes, particularly when compared to the last observation when the source was found to be several times fainter, with a much steeper X-ray spectrum. The energy dependent shape of the lightcurve and the spectral changes imply a large shift (factor of 30 or more in frequency) of the peak of the synchrotron emission. At maximum flux the peak was located at or above 10 keV, making 1ES 2344+514 the second blazar (after MKN501) with the synchrotron peak in the hard X-ray band. The shift, and the corresponding increase in luminosity, might be due to the onset of a second synchrotron component extending from the soft to the hard X-ray band where most of the power is emitted. Rapid variability on a timescale of approximately 5000 seconds has also been detected when the source was brightest.
X-Ray Luminosity and Spectral Variability in the TEV BL Lac 1ES2344+514
P. Giommi,P. Padovani,E. Perlman
Physics , 1998, DOI: 10.1016/S0920-5632(98)00250-3
Abstract: The results of a series of five \sax observations of the TeV BL Lac object 1ES2344+514 are briefly presented. Large amplitude luminosity variability, associated to impressive spectral changes in the hard X-rays, have been found. The shape of the lightcurve depends on energy, with the flare starting and ending in the hard band, but with maximum intensity possibly reached earlier in the soft X-rays. The luminosity and spectral changes may be due to a shift of the peak of the synchrotron emission from the soft X-rays to the hard X-ray band similar to that detected during \sax observations of MKN 501.
Modeling Multi-Echelon Multi-Supplier Repairable Inventory Systems with Backorders  [PDF]
Yael Perlman, Ilya Levner
Journal of Service Science and Management (JSSM) , 2010, DOI: 10.4236/jssm.2010.34050
Abstract: This paper considers an inventory system responsible for repairable equipments located at several operational sites, each in different area. When a failure occurs at the operational site, spare parts are required. We analyze a multiple-supplier inventory system that includes an internal repair shop that offers several modes of repair with different repair times and an external supplier of spare parts. The network model of the problem presented here efficiently solves the problem for deterministic demands that vary over time with backorders taken into account.
Perishable Inventory Management in Healthcare  [PDF]
Yael Perlman, Ilya Levner
Journal of Service Science and Management (JSSM) , 2014, DOI: 10.4236/jssm.2014.71002

This study addresses a problem encountered in a nation-wide, large-scale healthcare supply chain that comprises several hundred medical organizations (hospitals, clinics, pharmacies, etc.) and provides highly advanced medical care to several million people. The medical products in the system are perishable, meaning that they become unusable beyond a certain expiry date. It is necessary to track the ages of units in stock and to plan and control the inventory accordingly. The models developed herein represent a multi-echelon, multi-supplier inventory system and unite together aspects of perishability and outsourcing under deterministic demand for medical products, which include both perishable and deteriorating goods. The objective of the study is to determine the optimal number of products to be purchased from regular and outsource suppliers so as to meet the required demand at the minimum operating cost. The solution is a network-flow model that can be used to determine the trade-off between the quantities of items to be ordered from the two types of suppliers in each time period. In addition, the study analyzes different distribution policies to account for the perishable nature of the products. Further insights are obtained by applying the model to a case study of a real-life healthcare supply chain from which interesting results are drawn.

Mid-ir emission from the nucleus of centaurus a
J. T. Radomski,C. Packham,N. A. Levenson,E. Perlman
Revista mexicana de astronomía y astrofísica , 2007,
Abstract: We present high spatial resolution mid-IR images of the nuclear region of Centaurus A. Images were obtained at 8.8 um, N-band (10.5uem), and 18.3um using the mid-IR in- strument T-ReCS on Gemini South, a pro- genitor to CanariCam on the Gran Telescopio Canarias (GTC).
What Types of Jets does Nature Make: A New Population of Radio Quasars
P. Padovani,E. Perlman,H. Landt,P. Giommi,M. Perri
Physics , 2003, DOI: 10.1086/373899
Abstract: We use statistical results from a large sample of about 500 blazars, based on two surveys, the Deep X-ray Radio Blazar Survey (DXRBS), nearly complete, and the RASS-Green Bank survey (RGB), to provide new constraints on the spectral energy distribution of blazars, particularly flat-spectrum radio quasars (FSRQ). This reassessment is prompted by the discovery of a population of FSRQ with spectral energy distribution similar to that of high-energy peaked BL Lacs. The fraction of these sources is sample dependent, being ~ 10% in DXRBS and ~ 30% in RGB (and reaching ~ 80% for the Einstein Medium Sensitivity Survey). We show that these ``X-ray strong'' radio quasars, which had gone undetected or unnoticed in previous surveys, indeed are the strong-lined counterparts of high-energy peaked BL Lacs and have synchrotron peak frequencies, nu_peak, much higher than ``classical'' FSRQ, typically in the UV band for DXRBS. Some of these objects may be 100 GeV - TeV emitters, as are several known BL Lacs with similar broadband spectra. Our large, deep, and homogeneous DXRBS sample does not show anti-correlations between nu_peak and radio, broad line region, or jet power, as expected in the so-called ``blazar sequence'' scenario. However, the fact that FSRQ do not reach X-ray-to-radio flux ratios and nu_peak values as extreme as BL Lacs and the elusiveness of high nu_peak - high-power blazars suggest that there might be an intrinsic, physical limit to the synchrotron peak frequency that can be reached by strong-lined, powerful blazars. Our findings have important implications for the study of jet formation and physics and its relationship to other properties of active galactic nuclei.
BeppoSAX Observations of Synchrotron X-ray Emission from Radio Quasars
P. Padovani,L. Costamante,G. Ghisellini,P. Giommi,E. Perlman
Physics , 2002, DOI: 10.1086/344406
Abstract: We present new BeppoSAX LECS, MECS, and PDS observations of four flat-spectrum radio quasars (FSRQ) having effective spectral indices alpha_ro and alpha_ox typical of high-energy peaked BL Lacs. Our sources have X-ray-to-radio flux ratios on average ~ 70 times larger than ``classical'' FSRQ and lie at the extreme end of the FSRQ X-ray-to-radio flux ratio distribution. The collected data cover the energy range 0.1 - 10 keV (observer's frame), reaching ~ 100 keV for one object. The BeppoSAX band in one of our sources, RGB J1629+4008, is dominated by synchrotron emission peaking at ~ 2 x 10^16 Hz, as also shown by its steep (energy index alpha_x ~ 1.5) spectrum. This makes this object the FIRST known FSRQ whose X-ray emission is not due to inverse Compton radiation. Two other sources display a flat BeppoSAX spectrum (alpha_x ~ 0.7), with weak indications of steepening at low X-ray energies. The combination of BeppoSAX and ROSAT observations, (non-simultaneous) multifrequency data, and a synchrotron inverse Compton model suggest synchrotron peak frequencies ~ 10^15 Hz, although a better coverage of their spectral energy distributions is needed to provide firmer values. If confirmed, these values would be typical of ``intermediate'' BL Lacs for which the synchrotron and inverse Compton components overlap in the BeppoSAX band. Our sources, although firmly in the radio-loud regime, have powers more typical of high-energy peaked BL Lacs than of FSRQ, and indeed their radio powers put them near the low-luminosity end of the FSRQ luminosity function. We discuss this in terms of an anti-correlation between synchrotron peak frequency and total power, based on physical arguments, and also as possibly due to a selection effect.
Parameter Correlations and Cosmological Properties of BL Lac Objects
P. Giommi,P. Padovani,M. Perri,H. Landt,E. Perlman
Physics , 2002,
Abstract: Using three complete, radio flux limited, blazar samples we compare the LogN-LogS and the preliminary radio luminosity function of the general population of BL Lacs to those of the subclass of high energy synchrotron peaked (HBL) BL Lacs. We also examine recent results on the cosmological evolution in different samples of BL Lacs and we investigate the controversial issue of the correlation between the synchrotron peak frequency and radio luminosity in BL Lacertae objects. We find that the fraction of HBL objects is approximately the same at all observed radio fluxes and luminosities implying that there cannot be any strong correlation between the position of the synchrotron peak and radio luminosity. The amount of cosmological evolution in BL Lacs is confirmed to be low and negative at low radio fluxes, although the large number of objects without redshift prevents a precise estimation. At high radio fluxes the amount of cosmological evolution is zero or slightly positive but this could be induced by a possible contamination with Flat Spectrum Radio Quasars.
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