Publications (18)6.02 Total impact
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Article: A Possible Faint Near-Infrared Counterpart to the Anomalous X-Ray Pulsar 1E 2259+586
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ABSTRACT: We present near-infrared and optical observations of the field of the anomalous X-ray pulsar 1E 2259+586 taken with the Keck telescope. We derive a subarcsecond Chandra position and tie it to our optical reference frame using other stars in the field. We find a very faint source, Ks = 21.7 ± 0.2 mag, with a position coincident with the Chandra position. We argue that this is the counterpart. In the J, I, and R bands, we derive (2 σ) limits of 23.8, 25.6, and 26.4 mag, respectively. As with 4U 0142+61, for which a counterpart has previously been found, our results are inconsistent with models in which the source is powered by accretion from a disk but may be consistent with the magnetar model.The Astrophysical Journal 12/2008; 563(1):L49. · 6.02 Impact Factor -
Article: Chandra Observations of the Anomalous X-ray Pulsars 1E 2259+586 and 4U 0142+61
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ABSTRACT: We present X-ray imaging, timing, and phase resolved spectroscopy of the anomalous X-ray pulsars (AXPs) 1E 2259+586 and 4U 0142+61 using the Chandra X-ray Observatory (CXO). The pulsar spectra are well described by a power law plus blackbody model; we find no evidence for spectral features between 0.5-7.0 keV. We derive a new, precise X-ray position for the sources and determine their spin periods. Phase resolved X-ray spectra of 1E 2259+586 show no significant variation as a function of pulse phase. However, phase resolved X-ray spectroscopy of 4U 0142+61 shows evidence for evolution in phase in either Gamma, or kTBB or some combination thereof as a function of pulse phase. We have detected emission beyond 4 arcsec from the central source and extending beyond 100 arcsec, likely due to dust scattering in the interstellar medium and, for 1E 2259+586, possibly due to the supernova remnant G109.1-1.0.06/2003; -1:207-214. -
Article: Properties of Ultra-Luminous X-ray Sources in the Chandra Archive of Galaxies
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ABSTRACT: We report the spatial, spectral, and temporal properties of Ultra-Luminous X-ray sources (ULXs) as obtained from the Chandra Advanced CCD Imaging Spectrometer archive of external galaxies. We show how these characteristics are used to help distinguish among proposed classes of ULXs (supernovae, intermediate-mass black holes, beamed sources, etc.) and to help establish the origin and history of ULXs based on properties of their host galaxies. Support for this research was provided in part by NASA/Chandra grant AR2-3008X04/2003; 35:716. -
Article: Luminosity functions of X-ray binaries and the dynamical history of their host galaxies
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ABSTRACT: The X-ray sources observed in nearby galaxies show brightness distributions (log(N(>S))--log(S) curves) which can be described as single or broken power laws. Single power-law distributions are often found for sources in galaxies with vigorous ongoing star-formation activity, while broken power laws are found in elliptical galaxies and in bulges of spiral galaxies. The luminosity break can be caused by a population of X-ray binaries which contain a neutron star accreting at the Eddington limit or by aging of the X-ray binary population. We show that a simple birth-death model can reproduce single and broken power-law log(N(>S))--log(S) curves. We have found that power-law log(N(>S))--log(S) curves are a consequence of smooth continuous formation of X-ray binaries and a luminosity break is the signature of a starburst episode in the recent past. The luminosity break is robust and is determined by the lifespans of the X-ray binaries relative to the look-back time to the starburst epoch. The model successfully explains the different forms of log(N(>S))--log(S) curves for the disk and bulge sources in the spiral galaxy M81.03/2003; -
Article: On the Nature of the Eclipsing Bright X-ray Source in the Circinus Galaxy Field
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ABSTRACT: The X-ray spectrum and light curve of the bright source CG X-1 in the field of the Circinus galaxy are re-examined. Previous analyses have concluded that the source is an accreting black hole of about 50 solar masses although it was noted that the light curve resembles that of an AM Her-type system. Here we show that the light curve and orbital dynamics constrain the mass of the compact object to less than 30 solar masses and the mass of the companion to less than 1 solar mass. Combining the mass constraints with the observed X-ray flux, we show that an accreting object must either radiate anisotropically or strongly violate the Eddington limit. If the emission is beamed, then the companion star, which intercepts this flux during eclipse, will be driven out of thermal equilibrium and evaporate within approx. 103 yr. We find, therefore, that the observations are most consistent with the interpretation of CG X-1 as a bright, long-period, AM Her system in the Milky Way.02/2003; -
Article: The Precise Location of the Soft Gamma Repeater SGR 1627-41 with Chandra
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ABSTRACT: We report the precise localization of the Soft Gamma Repeater SGR 1627-41 with the Chandra X-ray Observatory. The best position for SGR 1627-41 was determined to be RA=16:35:51.844, DEC=-47:35:23.31 (J2000) with an accuracy of 0.6 arcsec. We present the results of our search for an IR counterpart to SGR 1627-41 and compare our results to the existing detections and limits of other magnetar infrared and optical observations in the literature. We also present new observations of SGR 1806-20 obtained during the recent reactivation of the source. In addition, we have determined a precise location for archival Chandra observations and reanalyzed archival IR data in the search for a counterpart.02/2003; -
Article: The Peculiar X-ray Transient IGR 16358-4726
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ABSTRACT: The new transient IGR 16358-4726 was discovered on 2003 March 19 with INTEGRAL. We detected the source serendipitously during our 2003 March 24 observation of SGR 1627 - 4lwith the Chandra X-ray observatory at the 1.7 x 10(exp -l0) ergs/s sq cm flux level ( 2-10 keV) with a very high absorption column (N_H = 3.3 x 10(exp 23)/sq cm and a hard power law spectrum of index 0.5(1). We discovered a very strong flux modulation with a period of 5880(50) s and peak-to-peak pulse fraction of 70(6)% (2-10 keV), clearly visible in the X-ray data. The nature of IGR 16358-4726 remains unresolved. The only neutron star systems known with similar spin periods are low luminosity persistent wind-fed pulsars; if this is a spin period, this transient is a new kind of object. If this is an orbital period, then the system could be a compact Low Mass X-ray Binary (LMXB).02/2003; -
Article: Chandra X-Ray Observatory observations of the globular cluster M28 and its millisecond pulsar B1821-24
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ABSTRACT: We report here the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). 46 X-ray sources are detected, of which 12 lie within one core radius of the center. We show that the apparently extended X-ray core emission seen with the ROSAT HRI is due to the superposition of multiple discrete sources for which we determine the X-ray luminosity function down to a limit of about 6xE30 erg/s. For the first time the unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821--24 is measured and found to be best described by a power law with photon index ~ 1.2. Marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum is found, which could be interpreted as cyclotron emission from a corona above the pulsar's polar cap if the the magnetic field is strongly different from a centered dipole. The unabsorbed pulsar flux in the 0.5--8.0 keV band is ~3.5xE-13 ergs/s/cm^2. Spectral analysis of the 5 brightest unidentified sources is presented. Based on the spectral parameters of the brightest of these sources, we suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrum with a hydrogen neutron star atmosphere model yields the effective temperature T_eff^\infty = 90^{+30}_{-10} eV and the radius R_NS^\infty = 14.5^{+6.9}_{-3.8} km. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core of M28. Using the Chandra-derived positions, we also report on the result of searching archival Hubble Space Telescope data for possible optical counterparts. Comment: Accepted for publication in ApJ; 22 pages, 8 figures, 5 tables11/2002; -
Article: Observations of the Jovian System with the Chandra X-Ray Observatory
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ABSTRACT: The {\sl Chandra X-ray Observatory) observed the Jovian system on 25-26 Nov 1999 with the Advanced CCD Imaging Spectrometer (ACIS), in support of the Galileo flyby of Io, and on 18 Dec 2000 with the imaging array of the High Resolution Camera (HRC-I), in support of the Cassini flyby of Jupiter. These sensitive, very high spatial-resolution X-ray observations have revealed that Jupiter's northern x-ray aurora originates at a spot fixed in a coordinate system rotating with the planet at latitude (60--70 deg north) and longitude (160--180 deg System III). Contrary to previous expectations, this location is poleward of the main FUV auroral oval and the foot of the Io Flux Tube, and is apparently connected magnetically to a region of the outer magnetosphere beyond $\sim$30 Jupiter radii. The northern auroral x-ray emission varies with a period $\sim$45 minute and has a an average power of $\sim$1 GW. The earlier view that Jupiter's x-ray aurora resulted from the precipitation of heavy ions from the outer edge of the lo Plasma Torus is now in doubt. Jupiter's disk also emits x-rays with a power of $\sim$2 GW, perhaps resulting from reprocessing of solar x-rays in its atmosphere. These observations reveal for the first time x-ray emission from the Io Plasma Torus, with a power of $\sim$0.1 Gw. The origin of this emission is not currently understood, although bremmstrahlung from non-thermal electrons may play a significant role. Finally, we report the discovery of very faint ($\sim$1--2 MW) soft x-ray emission from the Galilean satellites Io, Europa, and probably Ganymede, most likely as a result of bombardment of their surfaces by energetic ($ greater than $10 keV) H, O, and S ions from the region of the Io Plasma Torus.02/2002; -
Article: A possible faint near-infrared counterpart to the AXP 1E~2259+58.6
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ABSTRACT: We present near-infrared and optical observations of the field of the Anomalous X-ray Pulsar 1E 2259+58.6 taken with the Keck telescope. We derive a subarcsecond Chandra position and tie it to our optical reference frame using other stars in the field. We find a very faint source, Ks = 21.7\pm0.2 mag, with a position coincident with the Chandra position. We argue that this is the counterpart. In the J, I, and R bands, we derive (2 sigma) limits of 23.8, 25.6 and 26.4 mag, respectively. As with 4U 0142+61, for which a counterpart has previously been found, our results are inconsistent with models in which the source is powered by accretion from a disk, but may be consistent with the magnetar model. Comment: 12 pages, 2 figures, accepted for ApJ Letters A manuscript with full resolution figures can be found at http://www.phys.uu.nl/~hulleman/e2259paper2.ps.gz10/2001; -
Article: Measurements with the Chandra X-Ray Observatory's flight contamination monitor
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ABSTRACT: NASA's Chandra X-ray Observatory includes a Flight Contamination Monitor (FCM), a system of 16 radioactive calibration sources mounted to the inside of the Observatory's forward contamination cover. The purpose of the FCM is to verify the ground-to-orbit transfer of the Chandra flux scale, through comparison of data acquired during the ground calibration with those obtained in orbit, immediately prior to opening the Observatory's sun-shade door. Here we report results of these measurements, which place limits on the change in mirror--detector system response and, hence, on any accumulation of molecular contamination on the mirrors' iridium-coated surfaces.05/2000; -
Article: Modeling Chandra Space Environment
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ABSTRACT: This paper describes the development of an environmental risk-mitigation tool for the Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer (ACIS). Because exposure to 100-200 keV protons appears to have degraded the front-illuminated CCD's charge transfer inefficiency (CTI), an accurate tool for predicting encounters with magnetospheric regions rich in these particles is required. We implement standard models to predict bow-shock, magnetopause, and plasma-sheet boundaries. Using these models and solar-wind databases compiled from IMP-8 and ACE measurements, we then calculate the probability that Chandra is located in one of these regions, along with predicted particle flux, to arrive at appropriate safing times for the ACIS detector. Finally, we validate this tool by comparing the model's boundary-crossing and proton flux predictions with measurements from Chandra's on-board particle detector and with data from other spacecraft operating in the Earth's magnetosphere.03/2000; -
Article: Measurements with the Chandra Flight Contamination Monitor
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ABSTRACT: NASA's Chandra X-ray Observatory includes a Flight Contamination Monitor (FCM), a system of 16 radioactive calibration sources mounted to the inside of the Observatory's forward contamination cover. The purpose of the FCM is to verify the ground-to-orbit transfer of the Chandra flux scale, through comparison of data acquired during the ground calibration with those obtained in orbit, immediately prior to opening the Observatory's sun-shade door. Here we report results of these measurements, which place limits on the change in mirror-detector system response and, hence, on any accumulation of molecular contamination on the mirrors' iridium-coated surfaces.02/2000; -
Article: Progress in Using Continuum radiation for AXAF Calibration
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ABSTRACT: X-ray calibration of the AXAF observatory at MSFC's X-Ray Calibration Facility (XRCF) made novel use of the x-ray continuum from a conventional electron-impact source. Taking advantage of the good spectral resolution of solid-state detectors, continuum measurements proved advantageous in calibrating the effective area of AXAF's High-Resolution Mirror Assembly (HRMA) and in verifying its alignment to the XRCF's optical axis. Further verification of AXAF response models will be presented.02/1998; -
Article: The X-ray Polarimeter Experiment (XPE)
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ABSTRACT: Polarimetric studies will provide a new probe of cosmic x-ray sources, supplying important clues to source geometries and emission mechanisms. However, at the present time there is only one measurement of x-ray polarization from a cosmic source, the OSO-8 detection of 19% linear polarization from the Crab Nebula. We propose a new low cost x-ray polarimeter experiment (XPE), ideally sized for small satellites such as NASA's Small Explorer. With peak sensitivity near 15 keV, the XPE complements the Stellar X-Ray Polarimeter (SXRP, with peak sensitvity near 8 keV) on the international Spectrum-X-Gamma mission. The x-ray polarimetric sensitivity of XPE is enhanced by a novel geometry for the beryllium scattering block, and by the use of a low-background position-sensitive proportional counter. The entire experiment is characterized by simplicity and reliability. With its higher energy response, XPE probes deep into the observed x-ray sources. Because it is dedicated solely to x-ray polarimetry, XPE will provide observations of a larger number of bright cosmic x-ray sources. XPE will respond to targets of opportunity such as the bursting pulsar and the galactic superluminal jet x-ray sources. The target list will include pulsing and non-pulsing accreting bright x-ray binaries, an x-ray bright radio pulsar (the Crab), and the AGN Cen A. Pulsing sources in particular, with their tera-gauss magnetic fields, should produce strongly polarized x-ray emission. With a 10-20 keV sensitivity of 1% for a 100,000 s observation of Her X-1, and 0.5% for a 25,000 s observation of GRS1915+105 in its high state, XPE will permit pulse phase spectro-polarimetric studies of pulsing sources, thus constraining the x-ray beam and magnetic field geometry, and probe the stucture and dynamics of cosmic jets. We describe details of the XPE design, and present a sample observing plan with expected sensitivities.04/1997; 29:790. -
Article: Uses of continuum radiation in the AXAF calibration
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ABSTRACT: X-ray calibration of the Advanced X-ray Astrophysics Facility (AXAF) observatory at the MSFC X-Ray Calibration Facility (XRCF) made novel use of the x-ray continuum from a conventional electron-impact source. Taking advantage of the good spectral resolution of solid-state detectors, continuum measurements proved advantageous in calibrating the effective area of AXAF's High-Resolution Mirror Assembly (HRMA) and in verifying its alignment to the XRCF's optical axis.02/1997; -
Article: Statistical Uncertainties in Temperature Diagnostics for Hot Coronal Plasma Using the ASCA SIS
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ABSTRACT: Statistical uncertainties in determining the temperatures of hot (0.5 keV to 10 keV) coronal plasmas are investigated. The statistical precision of various spectral temperature diagnostics is established by analyzing synthetic ASCA Solid-state Imaging Spectrometer (SIS) CCD spectra. The diagnostics considered are the ratio of hydrogen-like to helium-like line complexes of $Z\ge14$ elements, line-free portions of the continuum, and the entire spectrum. While fits to the entire spectrum yield the highest statistical precision, it is argued that fits to the line-free continuum are less susceptible to atomic data uncertainties but lead to a modest increase in statistical uncertainty over full spectral fits. Temperatures deduced from line ratios can have similar accuracy but only over a narrow range of temperatures. Convenient estimates of statistical accuracies for the various temperature diagnostics are provided which may be used in planning ASCA SIS observations. Comment: postscript file of 8 pages+3 figures; 4 files tarred, compressed and uuencoded. To appear in the Astrophysical Journal Letters; contents copyright 1994 American Astronomical Society08/1994; -
Article: Chandra X-ray observations of the globular cluster M28 and optical HST identification of the X-ray sources
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ABSTRACT: We present here the results of Chandra X-Ray Observatory observations of NGC 6626 (M 28), and a first search for optical counterparts by using archival HST data. 46 sources have been detected within the field-of-view of Chandra, 12 of which lie within 1 core radius from the cluster center. We measured for the first time the unconfused X-Ray spectrum of the ms pulsar B1821-24 and present spectral identification for other 5 bright unidentified sources. The brightest one shows spectral parameters suggesting that it is a transiently accreting neutron star in a low-mass X-ray binary in quiescent phase. The apparently extended core emission seen by ROSAT is due to superposition of multiple source for which the X-ray luminosity function is determined. Using Chandra derived position, we also present a preliminary report on the search for the optical counterparts of these sources by using archival HST data.Memorie della Societa Astronomica Italiana. 75:50.
