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NASA NTRS1994

Investigation relative to the Roentgen Satellite (ROSAT)

Reports include: High Resolution Observations of the Central Region of M31; The X-ray Emission of Low-X-ray-Luminosity Early-Type Galaxies: Gas Versus Compact Sources; Interaction Between Cluster Gas and Radio Features of Cygnus A; Hot Gas and Dark Halos in Early-Type Galaxies; A Gravitational Lens in X-rays - 0957+461; How Massive are Early-Type Galaxies?; Three Crab-Like SNR in the Large Magellanic Cloud; and Soft X-ray Emission from Boundary Layers in Cataclysmic Variables. Papers submitted to the Astrophysical Journal are attached.

Elvis, Martin S.↗
NASA NTRS1994

GRO: Studies of high energy pulsars with EGRET

The reported e(sup +/-) annihilation line from the Crab pulsar has three remarkable properties. It is very strong (approximately 10(exp 40)e(sup +/-) annhilations/sec.), red shifted (by about 70 KeV relative to mc(exp 2) = 511 KeV), and very narrow (width approximately equals 10 KeV). The e(sup +/-) annihilation did not occur near the star in polar cap accelerator models so that the redshift, suggestive in its magnitude of a gravitational redshift from annihilation near the stellar surface, did not have an explanation. Therefore, concentration was placed on e(sup +/-) production from outer-magnetosphere accelerator models.

Ruderman, Malvin↗
NASA NTRS1993

Near-simultaneous ROSAT and Ginga observations of the 1991 X-ray transient in Musca

The quasi simultaneous Rosat/Ginga observations of the Musca X-ray transient are reported. During its all sky survey, Rosat observed the Musca 1991 X-ray transient on 24-25 Jan. 1991, two weeks after outbursts, for about 170 seconds. The intensity was found to be nearly 6 Crab in the Rosat band. A combined fit of Rosat and Ginga data from 25 Jan. 1991 with a multitemperature disk blackbody model data plus a power law component results in a maximal temperature of the disk of about kT = 0.96 keV at an absorbing column of N(sub H) = 2.2 x 10(exp 21)/sq cm. A minimum distance to the black hole binary of at least 4 to 5 kpc was derived. Including the accretion disk inclination angle of i = 26 +/- 25 degrees determined from the shape of the positron annihilation line, and the quiescent optical brightness together with the most probable spectral type of the companion, a black hole mass of M = (6 +/- 1.5) of the solar mass and a distance of about (11 +/- 3) kpc, were derived. Additional Rosat observations in Aug. 1991 and Mar. 1992 suggest that the exponential intensity decay until 240 days after the outburst is followed by a steeper decline between 240 and 410.

Greiner, J.↗
NASA NTRS1994

A six year timing solution for PSR B1951 + 32

Six years of pulse time-of-arrival measurements have been collected from observations of the young, fast pulsar PSR B1951 + 32 in the CTB 80 supernova remnant using the Jodrell Bank and Arecibo radio telescopes. The combined data allow for the determination of a phase connected solution between 1987 August 21 and 1993 September 21. This solution provides an accurate rotational ephemeris for high energy x-ray and gamma-ray observations of this source made during the six years of radio timing observations to an accuracy of better than one tenth of a pulse period (4 ms). The pulsar has an activity parameter comparable with the value seen in the Crab pulsar.

Foster, R. S.↗
NASA NTRS1994

Polar cap models of gamma-ray pulsars: Emision from single poles of nearly aligned rotators

We compare a new Monte Carlo simulation of polar cap models for gamma-ray pulsars with observations of sources detected above 10 MeV by the Compton Observatory (CGRO). We find that for models in which the inclination of the magnetic axis is comparable to the angular radius of the polar cap, the radiation from a single cap may exhibit a pusle with either a single broad peak as in PSR 1706-44 and PSR 1055-52, or a doubly peaked profile comparable to those observed from the Crab, Vela and Geminga pulsars. In general, double pulses are seen by observers whose line of sight penetrates into the cap interior and are due to enhanced emission near the rim. For cascades induced by culvature radiation, increased rim emission occurs even when electrons are accelerated over the entire cap, since electrons from the interior escape along magnetic field lines with less curvature and hence emit less radiation. However, we obtain better fits to the duty cycles of observed profiles if we make the empirical assumption that acceleration occurs only near the rim. In either case, the model energy spectra are consistent with most of the observed sources. The beaming factors expected from nearly aligned rotators, based on standard estimates for the cap radius, imply that their luminosities need not be as large as in the case of orthogonal rotators. However, small beam angles are also a difficutly with this model because they imply low detection probablities. In either case the polar cap radius is a critical factor, and in this context we point out that plasma loading of the field lines should make the caps larger than the usual estimates based on pure dipole fields.

Daugherty, Joseph K.↗
NASA NTRS1994

Asymptotic domination of cold relativistic MHD winds by kinetic energy flux

We study the conditions which lead to the conversion of most Poynting flux into kinetic energy flux in cold, relativistic hydromagnetic winds. It is shown that plasma acceleration along a precisely radial flow is extremely inefficient due to the near cancellation of the toroidal magnetic pressure and tension forces. However, if the flux tubes in a flow diverge even slightly faster than radially, the fast magnetosonic point moves inward from infinity to a few times the light cylinder radius. Once the flow becomes supermagnetosonic, further divergence of the flux tubes beyond the fast point can accelerate the flow via the 'magnetic nozzle' effect, thereby further converting Poynting flux to kinetic energy flux. We show that the Grad-Shafranov equation admits a generic family of kinetic energy-dominated asymptotic wind solutions with finite total magnetic flux. The Poynting flux in these solutions vanishes logarithmically with distance. The way in which the flux surfaces are nested within the flow depends only on the ratio of angular velocity to poliodal 4-velocity as a function of magnetic flux. Radial variations in flow structure can be expressed in terms of a pressure boundary condition on the outermost flux surface, provided that no external toriodal field surrounds the flow. For a special case, we show explicitly how the flux surfaces merge gradually to their asymptotes. For flows confined by an external medium of pressure decreasing to zero at infinity we show that, depending on how fast the ambient pressure declines, the final flow state could be either a collimated jet or a wind that fills the entire space. We discuss the astrophysical implications of our results for jets from active galactic nuclei and for free pulsar winds such as that believed to power the Crab Nebula.

Begelman, Mitchell C.↗
NASA NTRS1995

Calibrating the broad band X-ray telescope

This paper describes the calibration of the two solid-state Si(Li) X-ray detectors and the X-ray telescopes that flew as part of Goddard Space Flight Center's Broad Band X-Ray Telescope (BBXRT) experiment on board the space shuttle Columbia in 1990 December. During the 9 day shuttle mission, BBXRT performed approximately 150 observations of 82 celestial sources. The content of the archive is summarized here. Although BBXRT had a relatively short life, it stands as a milestone in X-ray astronomy as being the first instrument to offer moderate spectral resolution over a wide bandpass (0.3-12.0 keV). Among other things, this paper discusses the effective area calibration of the instrument, the flux calibration and flux corrections for off-axis observations, the detector background, and optimal background subtraction techniques. The on-axis effective area calibration for the central detector elements was performed using data from the Crab Nebula, while other carefully selected targets were used for calibration of the outer detector elements. The remaining systematic uncertainties in the effective area calibration for point sources observed both on and off axis are generally less than 5%-10%. The energy scale is known to better than 0.5% at 6.0 keV for both detectors. The results presented here have an impact on the calibration of other medium resolution X-ray experiments such as the CCDs on board ASCA (Astro-D) as well as the Advanced X-Ray Astrophysics Facility detectors.

Weaver, K. A.↗
NASA NTRS1994

The nature of the compact X-ray source in the supernova remnant G27.4+0.0

High-resolution X-ray imaging data obtained with ROSAT is used to constrain the nature of the central compact source in the supernova remnant G27.4+0.0. Diffuse emission is seen from throughout the approximately 4 min diameter radio shell, while the central source remains unresolved at approximately 3 sec. We combine archival data from the Einstein HRI, IPC, and MPC with the ROSAT HRI data to define the X-ray spectra of the diffuse and point-like emission. The bulk of the shell radiation is consistent with that of a approximately 10(exp 7) K plasma, although a higher temperature component is also suggested by the data; coupled with the remnant's size and distance, we derive an age of between 500 and 2500 yr. The point source has a substantially harder spectrum, with a power-law photon index less than or approximately equal to 1. A search for periodic modulation from the point source yields upper limits ranging from 10%-35% for periods between 0.025 and 1000 s, depending on the assumed pulse shape. No aperiodic variability on timescales of from 10(exp 3) to 10(exp 8) s is required, although a factor of approximately 2 change between the Einstein and ROSAT eras is possible. We show that the point source cannot represent thermal emission from the surface of a young neutron star and is unlikely to be explained as nonthermal, Crab-like X-ray pulses or a small synchrotron nebula. The most likely models involve accretion-powered systems -- either a wind-fed neutron star with a massive companion or a low-mass X-ray binary. In all probability, this is the youngest X-ray binary in the Galaxy.

Helfand, David J.↗
NASA NTRS1994

X-ray emission from PSR 0355+54

We have obtained a 20 ks observation of PSR 0355+54 using the ROSAT position sensitive proportional counter (PSPC). The pulsar is detected with a count rate of 4.2(+/- 1.3) x 10(exp -3)/s above the background. While the approximately 70 source counts are insufficient for spectral fitting, we have derived source parameters for specific cases of power law as well as blackbody spectra. For a Crab-like spectrum (photon index alpha = 2) we find L(sub x)(0.1-2.4 keV) = 1.0 x 10(exp 32) ergs/s, somewhat higher than upper limits reported from Einstein observations but consistent with typical L(sub x) versus E-dot values for other pulsars. For blackbody emission, we derive a temperature upper limit of approximately 9.5 x 10(exp 5) K for emission from the entire neutron star surface, which is consistent with standard models for cooling of the neutron star interior given a characteristic age 10(exp 5.75) yr. No evidence is present for modulation at the 156 ms pulsar period, setting a weak upper limit of approximately 75% for the pulsed fraction of the X-ray signal.

Slane, Patrick↗
NASA NTRS1995

The search for MeV gamma-ray pulsars with COMPTEL

The Compton Gamma Ray Observatory (CGRO) completed a full sky survey in November 1993 during which the number of known gamma-ray pulsars more than doubled. During this survey the Compton Telescope (COMPTEL) observed the classical isolated pulsars Crab and Vela and detected PSR 1509-58. Attempts to detect the newly discovered pulsars, Geminga, PSR 1706-44 and PSR 1055-52, in the COMPTEL energy range provide only upper limits. The results of these analyses are presented together with the outcome of a search for further candidate radio pulsars whose ephemerides are given in the Princeton Pulsar Catalogue.

Bennett, K.↗
NASA NTRS1995

Gamma-ray pulsars: Emission zones and viewing geometries

There are now a half-dozen young pulsars detected in high-energy photons by the Compton Gamma-Ray Observatory (CGRO), showing a variety of emission efficiencies and pulse profiles. We present here a calculation of the pattern of high-energy emission on the sky in a model which posits gamma-ray production by charge-depleted gaps in the outer magnetosphere. This model accounts for the radio to gamma-ray pulse offsets of the known pulsars, as well as the shape of the high-energy pulse profiles. We also show that about one-third of emitting young radio pulsars will not be detected due to beaming effects, while approximately 2.5 times the number of radio-selected gamma-ray pulsars will be viewed only high energies. Finally we compute the polarization angle variation and find that the previously misunderstood optical polarization sweep of the Crab pulsar arises naturally in this picture. These results strongly support an outer magnetosphere location for the gamma-ray emission.

Romani, Roger W.↗
NASA NTRS1995

OSSE observations of GX 339-4

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory (CGRO) observed the Galactic black hole candidate GX 339-4 as a target of oppurtunity in 1991 September, in response to the outburst reported by Burst and Transient Source Experiment (BATSE). We report here on energy spectra in the 50 keV-10 MeV range obtained by OSSE. The source was detected from 50 to 400 keV at a level relative to the Crab Nebula of approximately 30%. The observed spectrum was prescribed reasonably well by a power law with an exponential cutoff; a least-squares fit yielded a photon index of 0.88 +/- 0.05 and a cutoff energy of 68 +/- 2 keV. The addition of a Compton reflection component did not significantly improve the overall fit. An optically thin thermal bremsstrahlung spectrum also provides a good fit, and the thermal Comptonization model of Sunyaev & Titarchuk, while deficient in describing the data above approximately 200 keV, cannot formally be ruled out. A pure power law with reflection does not fit the observed spectrum. During a follow-up observation made in 1991 November the intensity of the source below 100 keV had dropped by more than a factor of 40, and it was no longer detected above approximately 100 keV. The energy spectrum during the November observation could be characterized by a power law with a photon index of 2.3 +/- 0.3; the spectrum was fitted equally well with the same exponentially cutoff power-law model applied to the September observation, reduced in intensity by a factor of approximately 40. During the 1991 September observation, the luminosity in the 50-400 keV band was approximately 2 x 10(exp 37) ergs/s (assuming a distance of 4 kpc), no more than a factor of 5 below the soft X-ray luminosity of GX 339-4 observed in its X-ray high state. The luminosity during the 1991 November observation was approximately 5 x 10(exp 35) ergs/s. Extrapolations of both the exponentially cutoff power-law and Sunyaev-Titarchuk models to the approximately 5-20 keV X-ray band yield flux levels very close to that observed by Ginga during an overlapping interval in 1991 September, when GX 339-4 was reported to be in its low state. This may be one of the strongest indications to date of a direct correspondence between the low X-ray state and gamma-ray outbursts of GX 339-4.

Grabelsky, D. A.↗
NASA NTRS1994

Detection of a flaring low-energy gamma-ray source

We report the detection of a flaring gamma-ray source by the University of New Hampshire (UNH) balloon-borne coded aperture gamma-ray telescope (DGT) on 1984 October 2. The source was detected at the significance level of 7.2 sigma over the energy range 160-2000 keV. The intensity in the range (160-200) keV was 1.1 Crab. The best-fit position of the source is given by R.A. = 3h 25.8m and Decl. = 67 deg 653 min and is located in the constellation of Camelopardia. The source was visible within the Field of View (FOV) of the telescope for approximately = 2 hr and exhibited signs of flaring. The derived photon spectrum can be equally fitted by an optically thin bremsstrahlung distribution of kT approximately = 52 keV or a power law of the form, dN(E)/dE = 3.7 x 10(exp -6) (E/400)(exp -4.5) photons/sq cm/keV. We compare its spectral characteristics ad energy output to various types of fast X-ray transients. No measurable flux could be detected from CG 135+1, the COS B source which was in the FOV and therefore, we present 2 sigma upper flux limits on its spectral emission over the energy range 160 keV to 9.3 MeV.

Bhattacharya, Dipen↗
NASA NTRS1994

The pulsed hard X-ray spectrum of PSR B1509-58

Oriented Scintillation Spectrometer Experiment (OSSE) observed the 150 ms X-ray pulsar PSR B1509-58 m the supernova remnant MSH 15-52 for 4 weeks in 1992. The pulsed spectrum from 50 keV to 5 MeV is well represented by a single-power-law photon spectrum of the form (3.14 +/- 0.16) x 10(exp -6) x (E/118.5 keV)(exp -1.68 +/- -0.09) photons cm(exp -2)s(exp -1)keV(exp -1). This is significantly harder than the Crab pulsar spectrum in this energy range. The Ginga soft X-ray spectrum (2-60 keV) reported by Kawai et al. is significantly harder than the observed OSSE spectrum and predicts a flux 2 times higher than we observe in the approximately 55-170 keV energy band. This requires a break to a steeper spectrum somewhere in the intermediate energy range (approximately 20-80 keV). The spectrum must soften again at higher energies or the pulsar would have easily been detected by EGRET, COS B, and SAS 2.

Matz, S.↗
NASA NTRS1996

Crustaceans as a model for microgravity-induced muscle atrophy

Atrophy of skeletal muscles is a serious problem in a microgravity environment. It is hypothesized that the unloading of postural muscles, which no longer must resist gravity force, causes an accelerated breakdown of contractile proteins, resulting in reduction in muscle mass and strength. A crustacean model using the land crab, Gecarcinus lateralis, to assess the effects of spaceflight on protein meatabolism is presented. The model is compared to a developmentally-regulated atrophy in which a premolt reduction in muscle mass allows the withdrawal of the large claws at molt. The biochemical mechanisms underlying protein breakdown involves both Ca2(+) -dependent and multicatalytic proteolytic enzymes. Crustacean claw muscle can be used to determine the interactions between shortening and unloading at the molecular level.

Mykles, D. L.↗
NASA NTRS1997

New COMPTEL results on pulsar studies at MeV energies

The Compton telescope (COMPTEL) onboard the Compton Gamma Ray Observatory (CGRO) detected the pulsar PSR B1951-32 at MeV energies, and found indications of a signal from PSR B0656+14. In the combined spectra from COMPTEL and the energetic gamma ray experiment telescope (EGRET) onboard CGRO, it can be seen that the maximum luminosities of these objects are reached in the COMPTEL energy range. These spectra can be compared with those from four other pulsars observed in MeV energies with COMPTEL. The spectral properties of five of the six pulsars, Vela, PSR B1509-58, PSR B1951-32 and PSR B0656+14, require breaks and bends at MeV energies. The sixth pulsar, the Crab pulsar, approximately follows a power law flux relation from keV to GeV energies. It is concluded that this spectral behavior may play a role in the discrimination between current gamma ray emission models.

Hermsen, W.↗
NASA NTRS1997

High energy survey of supernova remnants with BATSE

The burst and transient source experiment (BATSE) survey of the high energy emission from supernova remnants is reported on. The ability of BATSE to continuously monitor the entire sky in the 20 keV to 2 MeV energy range enables a large group of remnants to by studied at high energies. Preliminary analysis indicates the likely detection of several supernova remnants other than the Crab nebula. Among these are MSH 15-52, Vela, Cas A and possibly HB 9. The techniques employed are discussed together with the status of the survey and its limitations.

McCollough, M. L.↗
NASA NTRS1997

Hubble Images from 1996

Primarily composed of animation, movies, and stills, this video is divided into 12 segments or slugs as the video refers to them. They are: Global Map of Pluto, Images of Pluto, Surface Map of Pluto, Helix Nebula- NGC 7293, Gaseous Knots, Animation of the Formation of the Helix Nebula, Crab Nebula, Jupiter Aurora Movie, Birth of a Quasar, Merging Galaxies, and Spiral Galaxies.

Source record↗