Engineering topics

Tennant, Allyn F.

Publications and source records attributed to Tennant, Allyn F..

51 records · Page 3

NASA NTRS2002

Chandra X-ray Observations of the Spiral Galaxy M81

A Chandra X-Ray Observatory ACIS-S (Advanced X-ray Astrophysics Facility (Chandra) CCD (Charge Coupled Device) Imaging Spectrometer) imaging observation is used to study the population of X-ray sources in the nearby Sab galaxy M81 (NGC 3031). A total of 177 sources are detected with 124 located within the D25 isophote to a limiting X-ray luminosity of 3e36 ergs/cm(exp 2)/s. Source positions, count rates, luminosities in the 0.3-8.0 keV band, limiting optical magnitudes, and potential counterpart identifications are tabulated. Spectral and timing analysis of the 36 brightest sources are reported including the low luminosity active galactic nucleus, SN 1993J, and the Einstein discovered ultraluminous X-ray source X6.

Swartz, Douglas A.↗

NASA NTRS2002

Chandra Observations of the Anomalous X-ray Pulsar 4U 0142+61

We present X-ray imaging, timing, and phase resolved spectroscopy of the anomalous X-ray pulsar 4U 0142+61 using the Chandra X-ray Observatory. The spectrum is well described by a power law plus blackbody model with Gamma = 3.35(2), kT=0.458(3) keV, and N-H = 0.91(2) x 10(exp 22)/sq cm); we find no significant evidence for spectral features (0.5 - 7.0 keV). Time resolved X-ray spectroscopy shows evidence for evolution in phase in either Gamma, or kT or some combination thereof as a function of pulse phase. We derive a precise X-ray position for the source and determine its spin period, P=8.68866(30) s. 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.

Patel, Sandeep K.↗

NASA NTRS2001

Discovery of X-Ray Emission from the Crab Pulsar at Pulse Minimum

The Chandra X-Ray Observatory observed the Crab pulsar using the Low-Energy Transmission Grating with the High-Resolution Camera. Time-resolved zeroth-order images reveal that the pulsar emits X-rays at all pulse phases. Analysis of the flux at minimum - most likely non-thermal in origin - places an upper limit (T(sub infinity) < 2.1 MK) on the surface temperature of the underlying neutron star. In addition, analysis of the pulse profile establishes that the error in the Chandra-determined absolute time is quite small, -0.2 +/- 0.1 ms.

Tennant, Allyn F.↗

NASA NTRS2001

Properties of the Chandra Sources in M81

The Chandra X-Ray Observatory obtained a 50 ks observation of the central region of M81 using the ACIS-S in imaging mode. The global properties of the 97 X-ray sources detected in the inner 8.3' x 8.3' field of M81 are examined. Roughly half the sources are concentrated within the central bulge. The remainder are distributed throughout the disk, with the brightest disk sources lying preferentially along spiral arms. The average hardness ratios of both bulge and disk sources are consistent with power-law spectra of index Gamma equals approximately 1.6, indicative of a population of X-ray binaries. A group of much softer sources is also present. The background-source-subtracted log N-log S distribution of the disk follows a power law of index approximately -0.5 with no change in slope over three decades in flux. The log N-log S distribution of the bulge follows a similar shape but with a steeper slope above approximately 4 x 10(exp 37) ergs/s. There is unresolved X-ray flux from the bulge with a radial profile similar to that of the bulge sources. This unresolved flux is softer than the average of the bulge sources, and extrapolating the bulge log N-log S distribution toward weaker sources can account for only 20% of the unresolved flux. No strong time variability was observed for any source with the exception of one bright, soft source.

Tennant, Allyn F.↗

NASA NTRS2001

Properties of the Chandra Sources in M81

The Chandra X-ray Observatory obtained a 50-ks observation of the central region of M81 using the ACIS-S in imaging mode. The global properties of the 97 x-ray sources detected in the inner 8'.3 x 8'.3 field of M81 are examined. Roughly half the sources are concentrated within the central bulge. The remainder are distributed throughout the disk with the brightest disk sources lying preferentially along spiral arms. The average hardness ratios of both bulge and disk sources are consistent with power law spectra of index F approximates 1.6 indicative of a population of x-ray binaries. A group of much softer sources are also present. The background source- subtracted log(N)-log(S) distribution of the disk follows a power law of index about -0.5 with no change in slope over three decades in flux. The log(N)-log(S) distribution of the bulge follows a similar shape but with a steeper slope above about 4 x 10(exp 37) ergs/s. There is unresolved x-ray flux from the bulge with a radial profile similar to that of the bulge sources. This unresolved flux is softer than the average of the bulge sources and extrapolating the bulge log(N)-log(S) distribution towards weaker sources can only account for 20% of the unresolved flux. No strong time variability was observed for any source with the exception of one bright, soft source.

Tennant, Allyn F.↗

NASA NTRS2001

Interstellar X-Ray Absorption Spectroscopy of the Crab Pulsar with the LETGS

We study the interstellar X-ray absorption along the line of sight to the Crab Pulsar. The Crab was observed with the Low Energy Transmission Grating Spectrometer on the Chandra X-ray Observatory, and the pulsar, a point source, produces a full resolution spectrum. The continuum spectrum appears smooth, and we compare its parameters with other measurements of the pulsar spectrum. The spectrum clearly shows absorption edges due to interstellar Ne, Fe, and O. The O edge shows spectral structure that is probably due to O bound in molecules or dust. We search for near-edge structure (EXAFS) in the O absorption spectrum. The Fe L absorption spectrum is largely due to a set of unresolved discrete n=2-3 transitions in neutral or near-neutral Fe, and we analyze it using a new set of dedicated atomic structure calculations, which provide absolute cross sections. In addition to being interesting in its own right, the ISM absorption needs to be understood in quantitative detail in order to derive spectroscopic constraints on possible soft thermal radiation from the pulsar.

Paerels, Frits↗

NASA NTRS2001

ROSAT/Chandra Observations of a Bright Transient in M81

We present a 10-year X-ray light curve and the spectra of a peculiar X-ray transient in the spiral galaxy M81. The source was below the detection limit of ROSAT PSPC before 1993, but it brightened substantially in 1993, with luminosities exceeding the Eddington limit of a 1.5 solar mass compact accretor. It then faded and was not firmly detected in the ROSAT HRI and PSPC observations after 1994. The Chandra image obtained in 2000 May, however, shows an X-ray source at its position within the instrumental uncertainties. The Chandra source is coincident with a star-like object in the Digitized-Sky-Survey. A Hubble image suggests that the optical object may be extended. While these three observations could be of the same object, which may be an X-ray binary containing a black-hole candidate, the possibility that the ROSAT and Chandra sources are two different objects in a dense stellar environment cannot be ruled out. The Hubble data suggests that the optical object may be a globular cluster yet to be identified.

Ghosh, Kajal K.↗

NASA NTRS2001

Chandra X-Ray Observations of the Anomalous X-Ray Pulsar 1E 2259+58.6

We present X-ray imaging, timing, and phase resolved spectroscopy of the anomalous X-ray pulsar 1E 2259+58.6 using the Chandra X-ray Observatory. Time resolved X-ray spectra show no significant variation as a function of pulsar spin phase. The phase averaged spectrum is well described by a powerlaw plus blackbody model; we find no evidence for spectral features. We derive a spin period. We have detected a halo beyond the central source, possibly due to dust scattering in the ISM (interstellar matter).

Patel, Sandeep K.↗

NASA NTRS2001

Chandra Observations of the Crab Pulsar as a Function of Pulse Phase

The Chandra X-Ray Observatory was used to observe the Crab Nebula and its pulsar using the LETGS, i.e. the Low-Energy Transmission Grating (LETG) with the High Resolution Camera Spectroscopy detector (HRC-S). Data from the zeroth-order image was utilized to isolate the pulsar from the surrounding nebula and to measure the pulsar emission as a function of pulse phase. HRC timing problems were overcome by developing special techniques to process the data. For the first time, pulsed x-ray emission has been detected at all pulse phases, allowing us to set a new upper limit to the thermal emission from the surface of the neutron star.

Weisskopf, Martin C.↗

NASA NTRS2000

Discovery of Spatial and Spectral Structure in the X-Ray Emission from the Crab Nebula

The Chandra X-Ray Observatory observed the Crab Nebula and pulsar during orbital calibration. Zeroth-order images with the High-Energy Transmission Grating (HETG) readout by the Advanced Charge Coupled Devices (CCD) Imaging Spectrometer spectroscopy array (ACIS-S) show a striking richness of X-ray structure at a resolution comparable to that of the best ground-based visible-light observations. The HETG-ACIS-S images reveal, for the first time, an X-ray inner ring within the X-ray torus, the suggestion of a hollow-tube structure for the torus, and X-ray knots along the inner ring and (perhaps) along the inward extension of the X-ray jet. Although complicated by instrumental effects and the brightness of the Crab Nebula, the spectrometric analysis shows systematic variations of the X-ray spectrum throughout the nebula.

Weisskopf, Martin C.↗

NASA NTRS1999

Discovery of X-Ray Emission from the Crab Pulsar at Pulse Minimum

The Chandra X-ray Observatory observed the Crab Nebula and Pulsar using the Low-Energy Transmission Grating (LETG) with the High-Resolution Camera (HRC). Time-resolved zeroth-order images reveal that the pulsar emits x rays at all pulse phases. Analysis of the flux at minimum -- most likely nonthermal in origin -- places an upper limit (T(sub infinity) < 2.1 MK) on the surface temperature of the underlying neutron star. In addition, analysis of the pulse profile appears to confirm the absolute timing of the Observatory to within about 0.2 ms.

Tennant, Allyn F.↗

NASA NTRS1998

On the Use of Monochromators for the Calibration of AXAF

Data acquired, during the AXAF calibration at MSFC's X-Ray Calibration Facility (XRCF), using monochromators requires special analysis owing to potentially large spatial variations in the monochromators' beams and to higher-order contributions to the spectral content. A description of the monochromators -- the Double-Crystal Monochromator (DCM) and the HIgh-Resolution Erect-Field Spectrometer (HIREFS\TM) reflection-grating monochromator --- is given, followed by a discussion of the spectral and spatial content as monitored by Beam-Normalization Detector (BND) flow proportional counters (FPCs). Emphasis is given to the methodology employed in determining the spatial content through fits to Beam Uniformity (BU) measurements made nonsimultaneously, either during calibration or during pre-calibration source characterization. We present an interpolation scheme which adequately represents the beam properties over a wide range of source conditions.

Swartz, Douglas A.↗

NASA NTRS1998

Simulating AXAF Grating Spectra of Accreting White Dwarfs

We present simulated AXAF spectra of accreting white dwarfs, using parameters appropriate for magnetic cataclysmic variables. The very high spectral resolution that can be obtained with the High-Energy Transmission Grating of AXAF can resolve the keV X-ray emission lines that characterize the temperature, density and velocity profiles of the shock-heated emission regions of these systems. These simulations demonstrate that actual spectra will allow us to place constraints on the white-dwarf mass and the accretion rate of the systems. The high-resolution spectra also allow the measurement of the velocity of the accretion flow in regions close to the white-dwarf surface.

Tennant, Allyn F.↗

NASA NTRS1998

Calibration Results for the AXAF Flux Contamination Monitor

The Flux Contamination Monitor (FCM) on the Advanced X-ray Astrophysics Facility (AXAF) serves the purposes of transfering the absolute flux calibration from the ground calibration at the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center to operation on orbit and of detecting any changes in molecular contamination of the High Resolution Mirror Assembly (HRMA) between ground calibration and the post-launch activation phase. we describe the design, construction, and characterization of the FCM radioactive sources, and their placement on the Forward Contamination Cover (FCC). We present results from FCM measurements with the AXAF focal plane instruments, particularly the AXAF CCD Imaging Spectrometer (ACIS), during the ground calibration phase at XRCF in 1997. Finally, we describe the plans for FCM on-orbit measurements during observatory activation and the subsequent analysis.

Elsner, Ronald F.↗

NASA NTRS1991

The QDP/PLT user's guide

PLT is a high level plotting package. A Programmer can create a default plot suited for the data being displayed. At run times, users can then interact with the plot overriding any or all of these defaults. The user is also provided the capability to fit functions to the displayed data. This ability to display, interact with, and to fit the data make PLT a useful tool in the analysis of data. The Quick and Dandy Plotter (QDP) program will read ASCII text files that contain PLT commands and data. Thus, QDP provides and easy way to use the PLT software QPD files provide a convenient way to exchange data. The QPD/PLT software is written in standard FORTRAN 77 and has been ported to VAX VMS, SUN UNIX, IBM AIX, NeXT NextStep, and MS-DOS systems.

Tennant, Allyn F.↗