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NASA NTRS1997

Lateral Shock of the R Aquarii Jet

The R Aqr jet was observed with the VLA B-configuration at two epochs separated by approximately 13.2 yr. Comparison of the resulting 6 cm continuum images show that the radio jet has undergone a lateral counterclockwise rotation of approximately 6 deg-12 deg on the plane of the sky. The model of jet parcels on independent trajectories is difficult to reconcile with these observations and leads us to consider a path-oriented jet (i.e., younger parcels follow the same path as older parcels). Comparison of the most recent radio image with a nearly contemporaneous HST/FOC ultraviolet image at approximately 2330 Angstroms suggests that the ultraviolet emission lies along the leading side of the rotating radio jet. In conjunction with a proper motion analysis of the jet material that yields empirical space-velocity and resulting acceleration-magnitude relationships as a function of distance from the central source, we evaluate the observational results in terms of a schematic model in which the jet emission consists of plane-parallel isothermal shocks along the leading edge of rotation. In such a radiating shock, the ultraviolet-emitting region is consistent with the adiabatic region in the form of a high-temperature, low-density sheath that surrounds the cooled postshock radio-emitting region. Within the context of the schematic model, we obtain the temperatures, densities, and pressures within the preshock, adiabatic, and postshock regions as a function of distance from the central source; the physical parameters so derived compare favorably to previously published estimates. We obtain a total jet mass of 3.1 x 10(exp -5) solar mass and an age of approximately 115 yr. We evaluate the model in the context of its density-boundary condition, its applicability to an episodic or quasi-continuous jet, and angular momentum considerations.

Hollis, J. M.↗
NASA NTRS1997

Multifrequency Studies

The present report concerns the following projects 'Multifrequency observations of the BL Lacertae object MK 421"; "EUVE and multifrequency observations of the quasars 3C 273 and 3C 279"; "EUVE observations of the symbiotic star RAquarii"; and "Morphology of the local HI void and ionization of He in the local ISM". The common theme of the above projects is UV observations and analysis of a wide range of astrophysical classes or objects, from the diffuse interstellar medium, to symbiotic stars (R Aqr), to active galactic nuclei (AGNs). Understanding the local (primarily neutral hydrogen or HI) interstellar medium is of great importance to detecting nearby EUV sources. Specifically, the ionization of helium is strongly influencing the extinction in the extreme ultraviolet (EUV) and the optical depth can vary almost by an order of magnitude depending on the degree of ionization of various species including neutral helium Vel ) and singly ionized helium (Hell). The local interstellar medium (ISM) contains a giant void. Understanding the local ISM and the extent of the HI void engulfing the Sun is crucial to our understanding of the EUV emission from nearby strongly EUV-emitting stars like R Aqr, as well as subtracting local absorption effects from absorption effects in distant sources themselves such as quasars, BL Lacertae objects and AGNs in general. A compilation of known properties of the local ISM with appropriate references has been performed. R Aqr is a symbiotic stellar system comprised of a mass-losing approx.1 - 2 solar mass Mira- like long period variable with a 387 day period and a approx. solar mass,) hot companion/accretion disk that is believed to give rise to the symmetrical jet seen at UV, optical radio wavelengths. Recent observations of R Aqr have been conducted at continuum wavelengths with the Very Large Array (VLA) at 7 mm that have resolved the stellar components in RAqr and a geometrical distance of approx. 200pc has been obtained, confirming that R Aqr is the nearest symbiotic star system to the Sun. The status of the local ISM is, therefore, of great importance in attempting to understand the UV properties of the hot companion in R Aqr and its prominent jet. Here a multifrequency approach is required from both ground observations (e.g. with the VLA) as well as from space-borne observations (with the International Ultraviolet Explorer-IUE; and the Hubble Space Telescope-HST). The R Aqr jet has been observed with the VLA B-configuration at two epochs separated by approx. 13.2 yr. Comparison of the resulting 6 cm continuum images show that the radio jet has undergone a lateral counterclockwise rotation of approx. 6 deg - 12 deg on the plane of the sky. Comparison of the most recent radio image with a nearly contemporaneous HSTIFOC ultraviolet image at approx 2330 A suggests that the UV emission lies along the leading edge of the rotating radio jet.

Kafatos, Menas↗
NASA NTRS1994

Multiwavelength Search for Correlated Time Variations in the Brightest EGRET Blazars

We have found that the emission in the blazar jets is highly stratified, as evidenced by time delays between the peaks of the flares at different wavebands. This fits well with the predictions of the jet models. Not all of the blazars have yet been observed long enough with the VLBA to determine the motions of their components. However, for those 39 blazars for which this analysis is possible as of August 1997, the apparent superluminal speeds are very high, ranging from about 10 to 35c (for a Hubble constant of 65 km/s Mpc. If this trend holds for the remainder of the sample (to be determined after the results of observations in July 1997 are analyzed along with those from previous epochs), the implication is that the EGRET-detected blazars have stronger relativistic beaming than does the general radio-bright blazar population.

Marscher, Alan P.↗
NASA NTRS1997

Cluster Observations for Combined X-Ray and Sunyaev-Zel'dovich Estimates of Peculiar Velocities and Distances

Measurements of the peculiar velocities of galaxy clusters with respect to the Hubble flow allow the determination of the gravitational field from all matter in the universe, not just the visible component. The Sunyaev-Zel'dovich (SZ) effect (the inverse-Compton scattering of cosmic microwave background photons by the hot gas in clusters of galaxies) allows these velocities to be measured without the use of empirical distance indicators. Additionally, because the magnitude of the SZ effect is independent of redshift, the technique can be used to measure velocities out to the epoch of cluster formation. The SZ technique requires a determination of the temperature of the hot cluster gas from X-ray observations, and measurements of the SZ effect at millimeter wavelengths to separate the contribution from the thermal motions within the gas from that of the cluster peculiax velocity. We have constructed a bolometric receiver, the Sunyaev-Zel'dovich Infrared Experiment, specifically to make measurements of the SZ effect at millimeter wavelengths in order to apply the SZ technique to peculiar velocity measurements. This receiver has already been used to set limits to the peculiar velocities of two galaxy clusters at z approx. 0.2. As a test of the SZ technique, the double cluster pair Abell 222 and 223 was selected for observation. Measurements of the redshifts of the two components suggest that, if the clusters are gravitationally bound, they should exhibit a relative velocity of 10OO km/ s, well above the expected precision of 200 km/ s (set by astrophysical confusion) that is expected from the SZ method. The temperature can be measured from ASCA data which we obtained for this cluster pair. However, in order to ensure that the temperature estimate from the ASCA data was not dominated by cooling flows within the cluster, we requested ROSAT HRI observations of this cluster pair. Analysis of the X-ray properties of the cluster pair is continuing by combining the ROSAT observations funded under this proposal, archival PSPC data and our own PI ASCA data. The imaging data shows that neither cluster has a strong cooling flow, although A223 is the more compact of the two components. A223 also has a sub-cluster projected 1 Mpc to the North. The observed properties of this cluster pair are strikingly similar to those of the well-studied pair of A399 and A401 which are widely believed to be interacting and are in the initial stages of merging. This strengthens our selection of this cluster pair as a test for cluster peculiar motions. Combining these X-ray observations with SUZIE measurements of the SZ effect will provide important constraints on cluster merger models. This part of the program has been heavily delayed by bad weather at Mauna Kea, however SZ observations were obtained in November 1997 as part of a separately funded effort. These data are currently being analyzed.

Lange, A. E.↗
NASA NTRS1998

GPS Monitoring of Surface Change During and Following the Fortuitous Occurrence of the M(sub w) = 7.3 Landers Earthquake in our Network

Accomplishments: (1) Continues GPS monitoring of surface change during and following the fortuitous occurrence of the M(sub w) = 7.3 Landers earthquake in our network, in order to characterize earthquake dynamics and accelerated activity of related faults as far as 100's of kilometers along strike. (2) Integrates the geodetic constraints into consistent kinematic descriptions of the deformation field that can in turn be used to characterize the processes that drive geodynamics, including seismic cycle dynamics. In 1991, we installed and occupied a high precision GPS geodetic network to measure transform-related deformation that is partitioned from the Pacific - North America plate boundary northeastward through the Mojave Desert, via the Eastern California shear zone to the Walker Lane. The onset of the M(sub w) = 7.3 June 28, 1992, Landers, California, earthquake sequence within this network poses unique opportunities for continued monitoring of regional surface deformation related to the culmination of a major seismic cycle, characterization of the dynamic behavior of continental lithosphere during the seismic sequence, and post-seismic transient deformation. During the last year, we have reprocessed all three previous epochs for which JPL fiducial free point positioning products available and are queued for the remaining needed products, completed two field campaigns monitoring approx. 20 sites (October 1995 and September 1996), begun modeling by development of a finite element mesh based on network station locations, and developed manuscripts dealing with both the Landers-related transient deformation at the latitude of Lone Pine and the velocity field of the whole experiment. We are currently deploying a 1997 observation campaign (June 1997). We use GPS geodetic studies to characterize deformation in the Mojave Desert region and related structural domains to the north, and geophysical modeling of lithospheric behavior. The modeling is constrained by our existing and continued GPS measurements, which will provide much needed data on far-field strain accumulation across the region and on the deformational response of continental lithosphere during and following a large earthquake, forming the basis for kinematic and dynamic modeling of secular and seismic-cycle deformation. GPS geodesy affords both regional coverage and high precision that uniquely bear on these problems.

Miller, M. Meghan↗
NASA NTRS1998

Exploring the Unknown: Selected Documents in the History of the US Civilian Space Program: Using Space - Volume 3

One of the most important developments of the twentieth century has been the movement of humanity into space with machines and people. The underpinnings of that movement -why it took the shape it did; which individuals and organizations were involved; what factors drove a particular choice of scientific objectives and technologies to be used; and the political, economic, managerial, and international contexts in which the events of the space age unfolded- are all important ingredients of this epoch transition from an earthbound to spacefaring people. This desire to understand the development of spaceflight in the United States sparked this documentary history series. 'Exploring the Unknown' is a multi-volume series containing a selection of key documents in history of the U.S. civil space program. This current volume, Volume III, focusing on the use of space for practical applications, prints 112 key documents on the history of satellite communications, remote sensing of earth, and space as an investment in economic growth, edited for ease of use. Each is introduced by a headnote providing context, bibliographical information, and background information necessary to understanding the document.

Logsdon, John M.↗
NASA NTRS1998

On the Correlation Between Maximum Amplitude and Smoothed Monthly Mean Sunspot Number during the Rise of the Cycle (from t = 0-48 months Past Sunspot Minimum)

During the rise from sunspot minimum to maximum, the observed value of smoothed monthly mean sunspot number at maximum RM is found to correlate with increasing strength against the current value of smoothed monthly mean sunspot number R(t), where t is the elapsed time in months from minimum. On the basis of the modern era sunspot cycles (i.e., cycles 10-22), the inferred linear correlation is found to be statistically important (i.e., at the 95-percent level of confidence) from about 11 mo past minimum and statistically very important (i.e.. at the 99-percent level of confidence) from about 15 mo past minimum; ignoring cycle 19, the largest cycle of the modern era, the inferred linear correlation is found to be statistically important from cycle onset. On the basis of R(t), estimates of RM can be gauged usually to within about +/- 30 percent during the first 2 yr and to within about +/- 20 percent (or better) after the first 2 yr of a cycle's onset. For cycle 23, because controversy exists regarding the placement of its minimum (i.e., its onset), being either May 1996 or perhaps August 1996 (or shortly thereafter), estimates of its RM are divergent, being lower (more like a mean size cycle) when using the earlier epoch of minimum and higher (above average in size) when using the later-occurring minimum. For smoothed monthly mean sunspot number through October 1997 (t = 17 or 14 mo, respectively), having a provisional value of 32.0. the earlier minimum date projects an RM of 110.3 +/- 33.1, while the later minimum date projects one of 137.2 +/- 41.2. The projection is slowly decreasing in size using the earlier onset date, while it is slowly increasing in size using the later onset date.

Wilson, Robert M.↗
NASA NTRS1997

The Nature and Evolutionary History of GRO J1744-28

GRO J1744-28 is the first known X-ray source to display bursts, periodic pulsations, and quasi-periodic oscillations. This source may thus provide crucial clues that will lead to an understanding of the differences in the nature of the X-ray variability from various accreting neutron stars. The orbital period is 11.8 days, and the measured mass function of 1.31 x 10(exp -4) solar mass is one of the smallest among all known binaries. If we assume that the donor star is a low-mass giant transferring matter through the inner Lagrange point, then we can show that its mass is lower than approximately 0.7 solar mass and probably closer to 0.25 solar mass. Higher mass, but unevolved, donor stars are shown to be implausible. We also demonstrate that the current He core mass of the donor star lies in the range of 0.20-0.25 solar mass. Thus, this system is most likely in the final stages of losing its hydrogen-rich envelope, with only a small amount of mass remaining in the envelope. If this picture is correct, then GRO J1744-28 may well represent the closest observational link that we have between the low-mass X-ray binaries and recycled binary pulsars in wide orbits. We have carried out a series of binary evolution calculations and explored, both systematically and via a novel Monte Carlo approach, the range of initial system parameters and input physics that can lead to the binary parameters of the present-day GRO J1744-28 system. The input parameters include both the initial total mass and the core mass of the donor star, the neutron-star mass, the strength of the magnetic braking, the mass-capture fraction, and the specifics of the core mass/radius relation for giants. Through these evolution calculations, we compute probability distributions for the current binary system parameters (i.e., the total mass, core mass, radius, luminosity, and K-band magnitude of the donor star, the neutron star mass, the orbital inclination angle, and the semimajor axis of the binary). Our calculations yield the following values for the GRO J1744-28 system parameters (with 95% confidence limits in parentheses): donor star mass: 0.24 solar mass (0.2-0.7 solar mass); He core mass of the donor star: 0.22 solar mass (0.20-0.25 solar mass); neutron-star mass: 1.7 solar mass (1.39-1.96 solar mass); orbital inclination angle: 18deg (7deg-22deg); semi- major axis: 64 lt-s (60-67 lt-s); radius of the donor star: 6.2 solar radius(6-9 solar radius); luminosity of donor star: 23 solar luminosity (15-49 solar luminosity), and long-term mass transfer rate at the current epoch: 5 x 10(exp -10)solar mass/yr (2 x 10(exp -10) to 5 x 10(exp -9)solar mass/yr). We deduce that the magnetic field of the underlying neutron star lies in the range of approximately 1.8 x 10(exp 11)G to approximately 7 x 10(exp 11)G, with a most probable value of 2.7 x 10(exp 11)G. This is evidently sufficiently strong to funnel the accretion flow onto the magnetic polar caps and suppress the thermonuclear flashes that would otherwise give rise to the type 1 X-ray bursts observed in most X-ray bursters. We present a simple paradigm for magnetic accreting neutron stars where X-ray pulsars, GRO J1744-28, the Rapid Burster, and the type 1 X-ray bursters may form a continuum of possible behaviors among accreting neutron stars, with the strength of the neutron-star magnetic field serving as a crucial parameter that determines the mode of X-ray variability from a given object.

Rappaport, S.↗
NASA NTRS1996

Really Cool Stars at the Galactic Center

New and existing K-band spectra for 19 Galactic center late-type stars have been analyzed along with representative spectra of disk and bulge M giants and supergiants. Absorption strengths for strong atomic and molecular features have been measured. The Galactic center stars generally exhibit stronger absorption features centered near Na I (2.206 microns) and Ca I (2.264 microns) than representative disk M stars at the same CO absorption strength. Based on the absolute K - band magnitudes and CO and H2O absorption strengths for the Galactic center stars and known M supergiants and asymptotic giant branch (AGB) stars, we conclude that only IRS 7 must be a supergiant. Two other bright stars in our Galactic center sample are likely supergiant as well. The remaining bright, cool stars in the Galactic center that we have observed are most consistent with being intermediate mass/age AGB stars. We identify four of the Galactic center stars as long period variables based on their K-band spectral properties and associated photometric variability. Estimates of initial masses and ages for the GC stars suggest multiple epochs of star formation have occurred in the Galactic center over the last 7-100 Myr. 0 1996 American Astronomical Society.

Blum, R. D.↗
NASA NTRS1999

Virtual Petaflop Simulation: Parallel Potential Solvers and New Integrators for Gravitational Systems

"The orbit of any one planet depends on the combined motion of all the planets, not to mention the actions of all these on each other. To consider simultaneously all these causes of motion and to define these motions by exact laws allowing of convenient calculation exceeds, unless I am mistaken, the forces of the entire human intellect" -Isaac Newton 1687. Epochal surveys are throwing down the gauntlet for cosmological simulation. We describe three keys to meeting the challenge of N-body simulation: adaptive potential solvers, adaptive integrators and volume renormalization. With these techniques and a dedicated Teraflop facility, simulation can stay even with observation of the Universe. We also describe some problems in the formation and stability of planetary systems. Here, the challenge is to perform accurate integrations that retain Hamiltonian properties for 10(exp 13) timesteps.

Lake, George↗
NASA NTRS1998

This New Ocean: A History of Project Mercury

When Congress created the National Aeronautics and Space Administration (NASA) in 1958, it charged NASA with the responsibility "to contribute materially to . . . the expansion of human knowledge of phenomena in the atmosphere and space" and "provide for the widest practicable and appropriate dissemination of information concerning its activities and the results thereof." NASA wisely interpreted this mandate to include responsibility for documenting the epochal progress of which it is the focus. The result has been the development of a historical program by NASA as unprecedented as the task of extending man's mobility beyond his planet. This volume is not only NASA's accounting of its obligation to disseminate information to our current generation of Americans. It also fulfills, as do all of NASA's future-oriented scientific-technological activities, the further obligation to document the present as the heritage of the future. The wide-ranging NASA history program includes chronicles of day-to-day space activities; specialized studies of particular fields within space science and technology; accounts of NASA's efforts in organization and management, where its innovations, while less known to the public than its more spectacular space shots, have also been of great significance; narratives of the growth and expansion of the space centers throughout the country, which represent in microcosm many aspects of NASA's total effort; program histories, tracing the successes- and failures- of the various projects that mark man's progress into the Space Age; and a history of NASA itself, incorporating in general terms the major problems and challenges, and the responses thereto, of our entire civilian space effort. The volume presented here is a program history, the first in a series telling of NASA's pioneering steps into the Space Age. It deals with the first American manned-spaceflight program: Project Mercury. Although some academicians might protest that this is "official" history, it is official only in the fact that it has been prepared and published with the support and cooperation of NASA. It is not "official" history in the sense of presenting a point of view supposedly that of NASA officialdom-if anyone could determine what the "point of view" of such a complex organism might be. Certainly, the authors were allowed to pursue their task with the fullest freedom and in accordance with the highest scholarly standards of the history profession.

Swenson, Loyd S., Jr.↗
NASA NTRS1996

Really Cool Stars at the Galactic Center

New and existing K-band spectra for 19 Galactic center late-type stars have been analyzed along with representative spectra of disk and bulge M giants and supergiants. Absorption strengths for strong atomic and molecular features have been measured. The Galactic center stars generally exhibit stronger absorption features centered near Na I (2.206 microns) and Ca I (2.264 microns) than representative disk M stars at the same CO absorption strength. Based on the absolute K-band magnitudes and CO and H2O absorption strengths for the Galactic center stars and known M supergiants and asymptotic giant branch (AGB) stars, we conclude that only IRS 7 must be a supergiant. Two other bright stars in our Galactic center sample are likely supergiants as well. The remaining bright, cool stars in the Galactic center that we have observed are most consistent with being intermediate mass/age AGB stars. We identify four of the Galactic center stars as long period variables based on their K-band spectral properties and associated photometric variability. Estimates of initial masses and ages for the GC stars suggest multiple epochs of star formation have occurred in the Galactic center over the last 7-100 Myr.

Blum, R. D.↗
NASA NTRS1998

A 2 THz Heterodyne Array Receiver for SOFIA

We proposed to perform a comprehensive design study of a 16-element heterodyne array receiver for SOFIA. The array was designed to utilize hot-electron bolometers in an efficient, low-cost waveguide mount to achieve low noise performance between approx. 1500 and 2400 GHz. Due to the prevailing physical conditions in the interstellar medium, this frequency range is one of the richest in the FIR portion of the spectrum. An array designed for this wavelength range will make excellent use of the telescope and the available atmospheric transmission, and will provide a new perspective on stellar, chemical, and galaxy evolution in the present as well as past epochs. A few of the most important molecular and atomic species which the instrument will sample are CII, OI, CO, OH, NII, and CH. The system used the most sensitive detectors available in an efficient optical system. The local oscillator was a compact CO2 pumped far-infrared laser currently under development for SOFIA. The backend spectrometer was an array acousto-optic spectrometer (aAOS). The spectrometer utilizes proven hardware and technologies to provide broadband performance (greater than or equal to 1 GHz per AOS channel) and high spectral resolution (1 MHz) with the maximum sensitivity and minimum complexity and cost. The proposed instrument would be the fastest and most sensitive heterodyne receiver ever to operate in the 1.5 - 2.4 THz band. One of the key technologies developed for the proposed instrument is the laser micromachining of waveguide structures. These structures provide both the optical link between the instrument and the telescope (via an array of efficient feedhorns) and the impedance transformation between the detectors and free space. With the assistance of funds provided from this grant, we were able to fabricate and test the world's first laser micromachined feedhorns. Figure 1 is a photograph of the 2 THz double feedhorn structure designed and constructed under the auspices of this grant. The quality of the waveguide structure is far better than that obtainable using any other fabrication technique. Figure 2 is the beam pattern obtained from it. The beam parameters are an excellent match to what is expected from theory. The success of this experiment demonstrates the viability of using laser micromachined components in the development of high performance, large format array receivers. Figure 3 is an illustration of the focal plane array concept we developed lot- SOFIA base upon this work. Un- fortunately, our instrument proposal was not selected as a first generation SOFIA instrument. However, we have continued our development efforts and will propose to build a 2nd generation instrument based on the same design concepts. Our work under NAG 2-1057 was very rewarding and we appreciate the opportunity provided by NASA to pursue it. The research led directly to publishing 2 papers (listed below) and the award of an NSF grant to our group to construct a laser micromachining system on the University of Arizona campus.

Walker, Christopher K.↗
NASA NTRS1998

HST Spatially Resolved Spectra of the Accretion Disc and Gas Stream of the Nova-Like Variable UX Ursae Majoris

Time-resolved eclipse spectroscopy of the nova-like variable UX UMa obtained with the Hubble Space Telescope/Faint Object Spectrograph (HST/FOS) on 1994 August and November is analysed with eclipse mapping techniques to produce spatially resolved spectra of its accretion disk and gas stream as a function of distance from the disk centre. The inner accretion disk is characterized by a blue continuum filled with absorption bands and lines, which cross over to emission with increasing disk radius, similar to that reported at optical wavelengths. The comparison of spatially resolved spectra at different azimuths reveals a significant asymmetry in the disk emission at ultraviolet (UV) wavelengths, with the disk side closest to the secondary star showing pronounced absorption by an 'iron curtain' and a Balmer jump in absorption. These results suggest the existence of an absorbing ring of cold gas whose density and/or vertical scale increase with disk radius. The spectrum of the infalling gas stream is noticeably different from the disc spectrum at the same radius suggesting that gas overflows through the impact point at the disk rim and continues along the stream trajectory, producing distinct emission down to 0.1 R(sub LI). The spectrum of the uneclipsed light shows prominent emission lines of Lyalpha, N v lambda1241, SiIV Lambda 1400, C IV Lambda 1550, HeII Lambda 1640, and MgII Lambda 2800, and a UV continuum rising towards longer wavelengths. The Balmer jump appears clearly in emission indicating that the uneclipsed light has an important contribution from optically thin gas. The lines and optically thin continuum emission are most probably emitted in a vertically extended disk chromosphere + wind. The radial temperature profiles of the continuum maps are well described by a steady-state disc model in the inner and intermediate disk regions (R greater than or equal to 0.3R(sub LI) ). There is evidence of an increase in the mass accretion rate from August to November (from V = 10 (exp -8.3 +/-0.1) to 10(exp -8.1 +/- 0.1 solar mass yr(exp -1)), in accordance with the observed increase in brightness. Since the UX UMA disc seems to be in a high mass accretion, high-viscosity regime in both epochs, this result suggests that the mass transfer rate of UX UMA varies substantially (approximately equal to 50 per cent) on time-scales of a few months. It is suggested that the reason for the discrepancies between the prediction of the standard disk model and observations is not an inadequate treatment of radiative transfer in the disc atmosphere, but rather the presence of addition important sources of light in the system besides the accretion disk (e.g., optically thin contiuum emission from the disk wind and possible absorption by circumstellar cool gas).

Baptista, Raymundo↗
NASA NTRS1998

A 2 THz Heterodyne Array Receiver for SOFIA: Summary of Research

We proposed to perform a comprehensive design study of a 16-element heterodyne array receiver for SOFIA. The array was designed to utilize hot-electron bolometers in an efficient, low-cost waveguide mount to achieve low noise performance between approximately 1500 and 2400 GHz. Due to the prevailing physical conditions in the interstellar medium, this frequency range is one of the richest in the Far-Infra Red (FIR) portion of the spectrum. An array designed for this wavelength range will make excellent use of the telescope and the available atmospheric transmission, and will provide a new perspective on stellar, chemical, and galactic evolution in the present as well as past epochs. A few of the most important molecular and atomic species which the instrument will sample are CII, OI, CO, OH, NII, and CH. The system used the most sensitive detectors available in an efficient optical system. The local oscillator was a compact CO2 pumped far-infrared laser currently under development for SOFIA. The backend spectrometer was an array acousto-optic spectrometer (aAOS). The spectrometer utilizes proven hardware and technologies to provide broadband performance (> 1 GHz per AOS channel) and high spectral resolution (1 MHz) with the maximum sensitivity and minimum complexity and cost. The proposed instrument would be the fastest and most sensitive heterodyne receiver ever to operate in the 1.5 - 2.4 THz band. One of the key technologies developed for the proposed instrument is the laser micromachining of waveguide structures. These structures provide both the optical link between the instrument and the telescope (via an array of efficient feedhorns) and the impedance transformation between the detectors and free space. With the assistance of funds provided from this grant, we were able to fabricate and test the world's first laser micromachined feedhorns. The quality of the waveguide structure is far better than that obtainable using any other fabrication technique. The beam parameters are an excellent match to what is expected from theory. The success of this experiment demonstrates the viability of using laser micromachined components in the development of high performance, large format array receivers. We have continued our development efforts and will propose to build a 2nd generation instrument based on the same design concepts.

Walker, Christopher K.↗
NASA NTRS1999

Transient Infall Events in the Disk of AB Aurigae: The Beta Pictoris Phenomenon at 2-4 Mega Years

Surveys of Herbig Ae/Be stars have indicated that infalling circumstellar gas similar to that routinely detected toward beta Pic is preferentially observed close to the disk midplane. Recent millimeter interferometry of the historically prototypical Herbig Ae star, AB Aur, indicates that this star is viewed slightly above the disk midplane (i = 76 deg). The wealth of archival spectroscopic data for this star makes it an ideal test of the geometry for infalling or accreting gas in these stars. We present a review of the extant IUE and Hubble Space Telescope GHRS spectral data for AB Aur and find, in contrast to previous studies of this star, which focused on Mg II, that infalling gas is detected at all epochs with covering factors similar to those reported for HD 100546 and that an episode of enhanced infall was serendipitously detected during the 1990 monitoring run.

Grady, C. A.↗
NASA NTRS1997

IAUC 6530: GRO J1744-28; C/1995 01

We have recorded a series of outbursts starting on 1996 Dec. 2 that are consistent with the position of GRO J1744-28, the bursting pulsar. The bursts are detected between 20 and 100 keV, and on Dec. 2 they came in variable rates ranging from every 300 to every 500 s, with a preliminary total number estimated between 80 and 1 00 events. After Dec. 3, the burst rate dropped to 15-20 bursts/day, where it has remained. The current average error radius of the burst locations is about 4 deg. The burst fluence is currently about 2 x 10E-7 erg cmE-2. This indicates that the source is in outburst again. Although the source location is currently very close to the sun, we encourage observations in other wavelengths whenever possible. Persistent, hard x-ray emission from GRO J1744-28 (IAUC 6272, 6284, 6285) is currently being detected through both pulsation and earth occultation. Pulsations with a period near 0.467 s became detectable on Dec. 17, rising to a 20- to 40-keV-rms-pulsed flux of 1.3(1) x 10E-9 erg cmE-1 sE-1 (170 mCrab) by 1997 Jan. 3. On Jan. 1.0, the intrinsic pulse frequency was 2.1408983(4) Hz, and the spin-up rate was 2.8(5) x l0E-12 HzsE-1. This assumes the following circular orbit parameters: P = 11.83665(14) days; epoch of longitude 90 deg = JD 2450126.9977(4) TDB; a sin i = 2.6371(5) light-s (determined using data from the 1995 Dec.-l996 Apr. outburst). Earth-occultation monitoring of flux from GRO J1744-28 shows its intensity increasing by a factor of about two during 1996 Dec. 25-1997 Jan. 6. Although source confusion prevents a precise flux estimate, subtracting a 25-percent background for unresolved galactic-center sources yields a persistent flux for Jan. 3-6 in the energy bands 20-30, 30-40, and 40-50 keV of 590, 470, and 290 mcrab, respectively."

Kouveliotou, Chryssa↗
NASA NTRS1998

The X-Ray, Optical and Infrared Counterpart to GRB 980703

We report on our X-ray, optical and infrared follow-up observations of GRB980703. A previously unknown X-ray source in the GRB error box displays a powerlaw decline with index alpha= -1.0,5(exp +0.24 sub -0.29), fairly consistent with the average optical/infrared decay index alpha = -1.61 plus or minus 0.12. We invoke host galaxy extinction to match the observed spectral slope with the slope expected from 'fireball' models. The hypothesis that between 1998 July 4.4 and 8.4 UT no spectral breaks are present in the infrared to X-ray spectral range is in good agreement with the observations. For these two epochs we obtain an extinction of A(sub v) = 1.50 +/- 0.11 (July 4.4) and A(sub v) = 0.85 +/- 0.26 (July 8.4). From the X-ray data we estimate the optical extinction to be A(sub v) = 20.2(+12.3/-7.3), inconsistent with the former value. Optical spectra we took confirm the redshift of z=0.966 found by Djorgovski et al. We compare the afterglow of GRB 980703 with that of GRB 970508 and find that the fraction of the energy in the magnetic field is much lower in the case of GRB 980703.

Vreeswijk, P. M.↗