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NASA NTRS1995

Microlensing by the galactic bar

We compute the optical depth and duration distribution of microlensing events towrd Baade's window in a model composed of a Galactic disk and a bar. The bar model is a self-consistent dynamical model built out of individual orbits that has been populated to be consistent with the COBE maps of the Galaxy and kinematic observations of the Galactic bulge. We find that most of the lenses are in the bulge with a line-of-sight distance 6.25 kpc (adopting R(sub 0) = 8 kpc). The microlensing optical depth of a 2 x 10(exp 10) solar mass bar plus a truncated disk is (2.2 +/- 0.45) x 10(exp -6), consistent with the large optical depth (3.2 +/- 1.2) x 10(exp -6) found by Udalski et al. (1994). This model optical depth is enhanced over the predictions of axisymmetric models by Kiraga & Paczynski (1994) by slightly more than a factor of 2, since the bar is elongated along the line of sight. The large Einstein radius and small transverse velocity dispersion also predict a longer event duration in the self-consistent bar model than in the Kiraga-Paczynski model. The event rate and duration distribution also depend on the lower mass cutoff of the lens mass function. With a 0.1 solar mass cutoff, five to seven events (depending on the contribution of disk lenses) with a logarithmic mean duration of 20 days are expected for the Optical Gravitational Lensing Experiment (OGLE) according to our model, while Udalski et al. (1994) observed nine events with durations from 8 to 62 days. On the other hand, if most of the lenses are brown dwarfs, our model predicts too many short-duration events. A Kolmogorov-Smirnov test finds only 7% probability for the model with 0.01 solar mass cutoff to be consistent with current data.

Zhao, Hongsheng↗
NASA NTRS1995

Galactic cosmic-ray mediation of a spherical solar wind flow. 1: The steady state cold gas hydrodynamical approximation

Realistic models of the outer heliosphere should consider that the interstellar cosmic-ray pressure becomes comparable to pressures in the solar wind at distances more than 100 AU from the Sun. The cosmic-ray pressure dynamically affects solar wind flow through deceleration. This effect, which occurs over a scale length of the order of the effective diffusion length at large radial distances, has important implications for cosmic-ray modulation and acceleration. As a first step toward solution of this nonlinear problem, a steady state numerical model was developed for a relatively cold spherical solar wind flow which encounters the confining isotropic pressure of the surrounding Galactic medium. This pressure is assumed to be dominated by energetic particles (Galactic cosmic rays). The system of equations, which are solved self-consistently, includes the relevant hydrodynamical equations for the solar wind flow and the spherical cosmic-ray transport equation. To avoid the closure parameter problem of the two-fluid model, the latter equation is solved for the energy-dependent cosmic-ray distribution function.

Le Roux, J. A.↗
NASA NTRS1995

Discovery and imaging of a Galactic cirrus cloud with the far ultraviolet space telescope

We present new far-ultraviolet (1400-1800 A) data concerning a Galactic cirrus cloud G251.2+73.3 near the north Galactic pole obtained with the space-borne imaging telescope FAUST (Far Ultraviolet Space Telescope). We obtain a good correlation between the far-ultraviolet (FUV) and IRAS 100 micrometers surface brightnesses, their relation being I(sub FUV) = (128 +/- 3) I(sub 100 micrometers) - (264 +/- 9), where the I(sub FUV) flux is given in units of photon/s/sq cm/A/sr and I(sub 100 micrometers) in MJy/sr. Using uvbyH-beta photometry, we get a distance of 120 pc and a visual extinction in the center of the cloud of 0.39 mag corresponding to an extinction of 1.0 mag at 1565 A. We have performed a multiple scattering calculation for the scattered light using the Monte Carlo method. These calculations provide restrictions on the FUV scattering properties of the interstellar dust.

Haikala, Lauri K.↗
NASA NTRS1995

Search for Compton-backscattered annihilation radiation from the galactic center with the OSSE

An emission feature near 170 keV, interpreted as Compton-backscattered 511 keV positron-annihilation radiation, has been reported twice by balloon-borne germanium spectrometers from within approximately 15 deg of the Galactic center (Leventhal, MacCallum, & Stang 1978; Smith et al. 1993). Upper limits on this feature set by HEAO 3 (Mahoney, Ling, & Wheaton 1993) and other instruments indicate that it must be transient. We have searched data from the Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory (CGRO) for this feature, using daily spectral accumulations from all pointings near the Galactic center up to 1993 August, and covering most of the region viewed by the balloon instruments. We find no evidence for backscatter emission. Under the hypothesis that the source is 1E 1740.7-2942, the OSSE data set (186 days) disagrees with the balloon measurements with 99.3% confidence. The average daily 3 sigma OSSE upper limit on bakscatter flux from 1E 1740.7-2942 is 6.8 x 10(exp -4) photons/sq cm/s, compared to the 1.3 x 10(exp -3) photons/sq cm reported by the balloon observations. We also saw no evidence in 186 days for linelike emission from the point source EXS 1737.9-2952 recently discovered by Grindlay, Covault, & Manandhar (1993). This source exhibited bright emission from 83-111 keV, which has been interpreted as doubly backscattered 511 keV radiation. The average daily 3 sigma upper limit from OSSE for this line is 9.8 x 10(exp -4) photons/sq cm/s, or approximately 8% of the reported flux.

Smith, D. M.↗
NASA NTRS1995

An HST study of galactic inerstellar zinc and chromium

We present a survey of interstellar Zn II and Cr II absorption extracted from the Hubble Space Telescope Goddard High Resolution Spectrograph (HST GHRS) data archive. We find clear evidence for an enhanced depletion of Zn from the gas phase with increasing fractional abundance of molecular hydrogen f(H2). Our lower limit to the Galactic interstellar metallicity is approximately 65% of the solar value as determined by the measured Zn abundances in the lowest f(H2) sightlines, (N(Zn)/N(H(sup 0)(sub tot)) = -0.19 +/- 0.04. The correspondingly high depletion of Cr with respect to solar (N(Cr/N(H(sup 0)(sub tot)) = -1.44 +/- 0.26 indicates that there are significant amounts of dust present in these lines of sight. The Galactic abundances of Zn and Cr in the ISM provide a fundamental reference point which is used to understand the metal enrichment and dust formation history of damped Lyman alpha QSO absorption-line systems, normally believed to arise from intervening precursors to modern disk galaxies. Although the spread in Zn abundances is large for both the local ISM and in damped Lyman alpha systems, we still find a substantial difference (factor of 4-10) in metallicity between the two sets. This survey and future observations of more distant objects which probe the full extent of the Milky Way halo provide a more complete picture of the enrichment and depletion characteristics of present-day galaxies.

Roth, Katherine C.↗
NASA NTRS1996

Shielding against galactic cosmic rays

Ions of galactic origin are modified but not attenuated by the presence of shielding materials. Indeed, the number of particles and the absorbed energy behind most shield materials increases as a function of shield thickness. The modification of the galactic cosmic ray composition upon interaction with shielding is the only effective means of providing astronaut protection. This modification is intimately conntected with the shield transport porperties and is a strong function of shield composition. The systematic behavior of the shield properites in terms of microscopic energy absorption events will be discussed. The shield effectiveness is examined with respect to convectional protection practice and in terms of a biological endpoint: the efficiency for reduction of the probability of transformation of shielded C3H1OT1/2 mouse cells. The relative advantage of developing new shielding technologies is discussed in terms of a shield performance as related to biological effect and the resulting uncertainty in estimating astronaut risk.

Schimmerling, W.↗
NASA NTRS1995

KAO and AAT observations of the Galactic Center filaments

We have used the Kuiper Airborne Observatory (KAO) and the Anglo-Australian Telescope (AAT) to investigate the nature of the filamentary radio emission from the Galactic center region. KAO observations of the FIR line and continuum emission from the radio peak G0.095+0.012 and the E2 thermal radio filament northeast of the Galactic center can be produced by numerous nearby stars with T(sub eff) approx. 35,000 K; these can account for both the FIR luminosity and the excitation of the gas. Much of the FIR continuum and most of the strong (Si II) (34.8 micron) line emission are probably produced in the ionized gas of the filament. The FIR (O III) 52 and 88 micron lines imply an electron density of a few hundred; when compared with the radio emission measure, this implies the filament is roughly tubular or somewhat flattened in the plane of the sky. The (O III) and (S III) lines show higher excitation associated with the filament, and suggest that exciting stars may be located within the filaments and/or southeast of the E2 filament. AAT observations in the near infrared (NIR) in fact reveal a nearby cluster of hot stars southeast of the E2 filament. Additional hot stars, not identifiable from their NIR spectra, are likely to be present. These stars and those in the cluster can plausibly produce the observed radio and FIR emission in the region. The morphology of the filament is not explained by existing information however.

Erickson, Edwin F.↗
NASA NTRS1997

Spectral and Temporal Properties of Galactic Black Hole Systems

Kusunose, Mineshige & Yamada (1996; hereafter KMY) extended the model of Kusunose & Mineshige (1995) to the Galactic black hole candidates by considering nonthermal electron injection with gamma(EQ\0(,\s\up2(less than),\s\do-l(_))) 10. The effects of pair escape and advection on the disk structure and general relativistic effects on the emission spectrum were also examined. They found that the energy spectral index (alpha)(sub x) of the power law X-rays is about-0.8 and-2.0 when 1(sub soft)/1 = 0.2 and 2, respectively, where 1(sub soft)/1 is the ratio of the compactness of the injected soft photons to that of the gravitational energy. The power law index was found to be nearly independent of the mass accretion which is consistent with the observed luminosity independence. The model with small 1(sub soft)/1 (less than 1) shows promise for explaining the low state observed in Galactic black hole candidates. Model fits were provided for GX339-4 and Cyg X- 1 data from COMPTEL and OSSE on the Compton Gamma Ray Observatory. The difference in emission spectra between thermal disks and the model of KMY appears only in the energy range greater than 100 keV. Li, Kusunose and Liang (1996) studied stochastic particle acceleration to produce nonthermal particle distributions which then were used in the model of Kusunose & Mineshige (1995) to model the spectrum above 1 Mev from GBHC's. Under certain conditions, stochastic electron acceleration overcame Coulomb and Compton losses resulting in a suprathermal electron population. Good fits were obtained by COMPTEL and OSSE observations of Cyg X-1 and GRO J0422+23. Kusunose & Mineshige (1996a) examined the role of electron-positron pairs in advection-dominated disks. They found that the results for advection-dominated disks without pairs are not qualitatively changed by including pairs. Summaries of work sponsored by this grant are given in Wheeler, Kim, Moscoso, Kusunose & Mineshige (1996) and Kusunose (1996) Work was also done on developing a model for an e(+-) pair wind from the inner disk region of a black hole. The model consists of three zones: a pair production/annihilation zone at the base of the wind, a pair annihilation zone slightly further out from the disk and a wind zone extending to infinity where no annihilation occurs. The model assumes an input X-ray / gamma-ray power-law spectrum as a function of photon energy and angular distribution. Pairs are created in the pair production/annihilation zone via photon-photon, photonparticle and particle-particle collisions. The bulk velocity of the pairs is obtained from the momentum component perpendicular to the disk taking into account the radiation pressure acceleration of the pairs. Energy balance in the pair production/annihilation zone is calculated by assuming that the momentum component parallel to the disk is thermalized and by taking into account heating/cooling via anisotropic thermal Compton scattering along with the cooling processes of bremsstrahlung and synchrotron radiation. The pair density is also calculated. The outer boundary of the pair production/annihilation zone is where the optical depth for photon-photon pair produciton is unity. Beyond this point only pair annihilation will occur in the pair annihilation zone. When the pair density becomes small, pair annihilation will be negligible and the pairs will flow freely to infinity. In this model we have found that the X-ray power-law spectral index, alpha(sub x) is the primary parameter which determines the density, temperature and velocity of the pair production/annihilation zone (Moscoso, Kusunose & Wheeler 1996).

Wheeler, J. Craig↗
NASA NTRS1998

Variability Analysis and the Structure of Active Galactic Nuclei

This five-year Long-Term Space Astrophysics grant provided the support for several major steps in advancing our knowledge of the internal structure of active galactic nuclei. The single largest portion of this program had to do with the development and application of techniques for "reverberation mapping", the use of spectral monitoring of several different bands related by radiation reprocessing to infer the internal geometry of sources. Major steps were taken in this regard, particularly in establishing the distribution in radius of emission line material, and in relating the apparent reprocessing of continuum bands to the underlying structure of the accretion disk. Another major effort built directly upon these results. Once the case for continuum reprocessing was made by the monitoring, it next behooved us to understand the spectral output of AGN as a result of this reprocessing. As a result, our view of continuum production in AGN is now much better focussed on the key problems. A third focus of effort had to do with the nature of X-ray variability in AGN, and what it can tell us about the dynamics of extremely hot material in the immediate outskirts of the supermassive black holes that form the central engines of active galactic nuclei. In addition to these primary efforts, this grant also supported many other, smaller projects. Several of these were demonstrations of how the material spewed out of AGN in relativistic.ets generate the radiation by which we observe them. J Finally, the portion of this study that had to do with continuum production by accretion disks in AGN led naturally to several papers in which new developments were presented having to do with "advection-dominated accretion disks", those disks in which accretion appears to proceed at a substantial rate, but in which radiation processes are weak.

Krolik, Julian H.↗
NASA NTRS1998

A Three-Dimensional Analysis of the Galactic Gamma-Ray Emission Resulting from Cosmic-Ray Interaction with the Interstellar Gas and Radiation Fields

This final report outlines the progress of the contractor's support for the analysis of data under ADP (NRA 96-ADP-08; Proposal No. 167-96 adp). The primary task object was to construct a 3-D model for the distribution of high-energy (20 MeV-30 GeV) gamma-ray emission in the Galactic disk. Under this task the contractor was to utilize data from the EGRET instrument on the Compton Gamma-Ray Observatory, HI and CO surveys, radio-continuum surveys at 408 MHz, 1420 MHz, 5 GHz, and 19 GHz, the COBE Diffuse Infrared Background Experiment (DIRBE) all-sky maps from I to 240 um, and ground-based B,V, J, H and K photometry. The respective contributions to the high-latitude gamma-ray emission from cosmic ray-matter interactions, inverse Compton scattering, and extra-galactic emission were to be determined.

Sodraski, Thomas J.↗
NASA NTRS1999

Interpreting Broad Double-Peaked Emission Lines in Active Galactic Nuclei

The principal objectives of this project were to probe the inner regions of active galactic nuclei and to test general relativity in the strong-field limit. The approach takes advantage of broad atomic line emission observed from material deep in the potential well of an active galactic nucleus which contains key information as to the physics of the system. Line profiles in a wide range of wavebands from optical to X-ray have provided compelling evidence of the existence of a relativistic accretion disk around a supermassive black hole in a number of galaxies. The simplest model posits a geometrically thin disk in Keplerian orbit, with general relativistic effects in evidence. This model is the point of departure for the proposed work. We developed a high-performance numerical code to calculate photon trajectories in a Schwarzschild or Kerr metric and implemented it on parallel supercomputers. This code includes a general purpose ray tracer that calculates line profiles, light curves, and other observable quantities for a wide variety of emitter configurations. The versatility comes from the fact that the ray tracing algorithm does not depend on any symmetries regarding emitter locations. The speed comes from parallel implementation which enables us to sample hitherto unattainable volumes of disk model parameter space. During the period 1 March 1997 through 28 February 1998, two papers, supported in whole or in part by this grant, were published in refereed journals. They are reproduced in their entirety in the next two sections of this report.

Halpern, Jules↗
NASA NTRS1999

Research on the Nature of Accretion in Active Galactic Nuclei

he purpose of this grant was to support theoretical research on the nature of accretion in active galactic nuclei. In the brief time of the award (one year), four papers that appeared in refereed journals were written, as well as two invited reviews in conference proceedings These papers significantly advanced our understanding of the structure of the most important parts of bright accretion disks around accreting black holes, such as active galactic nuclei.

Krolik, J. H.↗
NASA NTRS1998

What is the Nature of Accretion in Active Galactic Nuclei?

The purpose of this grant was to support theoretical research on the nature of accretion in active galactic nuclei. In the brief time of the award, four papers that appeared in refereed journals were written, as well as two invited reviews in conference proceedings. These papers significantly advanced our understanding of the structure of the most important parts of bright accretion disks around accreting black holes, such as active galactic nuclei.

Krolik, Julian H.↗
NASA NTRS1998

X-Ray Emission from the Unidentified Gamma Ray Transient GRO J1838-0415 and X-Ray Localization of Gamma-Ray Transients near the Galactic Plane

The investigation was focused on studying the residual X-ray emission from possible counterparts of enigmatic gamma-ray transients near the Galactic plane. These sources (discovered by our group at Columbia) are highly variable gamma-ray sources with no radio-loud spectrally flat blazar in their approx 1 deg. diameter error box. BSAX Narrow Field Instruments pointed the gamma-ray error box of the prominent non-blazar Energetic Gamma Ray Experiment Telescope (EGRET) source GRO 1838-04 twice on April 17, 1997, and on April 20, 1997. Most of the EGRET error box was imaged and several faint X-ray sources were detected. BSAX Wide Field Camera data were also obtained for a variety of sources near the Galactic plane, including GRO J1838-04.

Tavani, Marco↗
NASA NTRS1999

High Energy Properties of Galactic Superluminals

Intensive searches across the electromagnetic spectrum have shown that transient X-ray binary systems exhibit properties similar to the central engines of AGNs. Having proven that accretion disks are present in both types of systems, it is now the task to determine the accretion physics which brings about the observed phenomena. The galactic superluminal transient sources, which are strongly suspected of containing black holes, are particularly attractive for studying state changes, jets, quasi-periodic behavior, and flaring/outburst behavior on timescales a million times shorter than those of their extragalactic counterparts. The galactic sources GRS 1915+105, GRO J1655-40 and more recently-detected transients such as XTE J1748-288 have demonstrated a rich variety of spectral and temporal behaviors. I review some of these observations from the perspective of high energy astronomy with emphasis on recent data from the Compton Observatory and Rossi X-ray Timing Explorer.

Alan, Harmon B.↗
NASA NTRS1998

Variability Analysis and the Structure of Active Galactic Nuclei

This five-year Long-Term Space Astrophysics grant provided the support for several major steps in advancing our knowledge of the internal structure of active galactic nuclei. The single largest portion of this program had to do with the development and application of techniques for 'reverberation mapping, the use of spectral monitoring of several different bands related by radiation reprocessing to infer the internal geometry of sources. Major steps were taken in d-ds regard, particularly in establishing the distribution in radius of emission line material, and in relating the apparent reprocessing of continuum bands to the underlying structure of the accretion disk. Another major effort built directly upon these results. Once the case for continuum reprocessing was made by the monitoring, it next behooved us to understand the spectral output of AGN as a result of this reprocessing. As a result, our view of continuum production in AGN is now much better focussed on the key problems. A third focus of effort had to do with the nature of X-ray variability in AGN, and what it can tell us about the dynamics of extremely hot material in the immediate outskirts of the supermassive black holes that form the central engines of active galactic nuclei. In addition to these primary efforts, this grant also supported many other, smaller projects. Several of these were demonstrations of how the material spewed out of AGN in relativistic jets generate the radiation by which we observe them. Finally, the portion of this study that had to do with continuum production by accretion disks in AGN led naturally to several papers in which new developments were presented having to do with 'advection-dominated accretion disks', those disks in which accretion appears to proceed at a substantial rate, but in which radiation processes are weak.

Krolik, Julian H.↗
NASA NTRS1997

Mid-Infrared Observations of the Galactic Center

Under this grant, Susan Stolovy completed her thesis work and performed an analysis of the galactic center. For her thesis Susan analyzed observations of the galactic center obtained with the KAO using the KEGS spectrograph, built at Cornell. These observations present a study of the distribution and kinematics of the atomic gas in the inner few parsecs of the Galaxy as traced by the forbidden [SiII] line at 34.814 microns. The integrated [SiII] emission peaks near Sgr A* and extends past the inner edge of the Circumnuclear Disk (CND), passing through a gap in the dense molecular material to the northwest. The [SiII] maps have a spatial resolution of 15" and a spectral resolution of 50 km/s. The spectra, which are characterized by broad linewidths of order 100 km/s, are kinematically consistent with the CND rotation to the southwest but not to the north. The northern extension may be experiencing shocks and is likely to be infalling along the Northern Arm. Observations of high [Sill]/ [OI] and [SiII]/dust continuum ratios support the conjecture that turbulent motions and shocks in the inner few parsecs of the Galaxy are destroying dust grains, thus elevating the abundance of atomic silicon.

Stolovy, Susan↗
NASA NTRS2000

ASCA Observation of an "X-Ray Shadow" in the Galactic Plane

The diffuse X-ray background (DXB) emission near the Galactic plane (l,b about 25.6 deg., 0.78 deg.) has been observed with ASC.4. The observed re-ion is toward a Galactic molecular'cloud which was recently reported to cast a deep X-ray shadow in the 0.5 - 2.0 keV band DXB. The selection of this particular region is intended to provide a constraint on the spatial distribution of the DXB emission along the line of sight: i.e., the molecular cloud is optically thick at < 2 keV and so the bulk of the observed soft X-rays must originate in the foreground of the cloud, which is at about 3 kpc from the Sun. In the 0.8 - 9.0 keV band, atomic emission lines have been detected, and the observed spectrum is primarily from thermal plasmas. Although the detailed nature of the spectrum is complicated, the observed DXB emission appears to originate from the multiple components of hot plasmas including a thermal plasma of T about 10(exp 7) K, which prevails within about 3 kpc from the Sun.

Park, Sangwook↗