Thomas A. McGlynn

An Intense Gamma-Ray Flare of PKS 1622–297

We report the observation by the Compton Gamma Ray Observatory of a spectacular flare of radio source PKS 1622-297. A peak flux of (17 ± 3) × 10-6 cm-2 s-1 (E > 100 MeV) was observed. The corresponding isotropic luminosity is 2.9 × 1049 ergs s-1. We find that PKS 1622-297 exhibits γ-ray intraday variability. A flux increase by a factor of at least 3.6 was observed to occur in less than 7.1 hr (with 99% confidence). Assuming an exponential rise, the corresponding doubling time is less than 3.8 hr. A significant flux decrease by a factor of ~2 in 9.7 hr was also observed. Without beaming, the rapid flux change and large isotropic luminosity are inconsistent with the Elliot-Shapiro condition (assuming that gas accretion is the immediate source of power for the γ-rays). This inconsistency suggests that the γ-ray emission is beamed. A minimum Doppler factor of 8.1 is implied by the observed lack of pair-production opacity (assuming X-rays are emitted cospatially with the γ-rays). Simultaneous observation by EGRET and OSSE finds a spectrum adequately fitted by a power law with photon index of -1.9. Although the significance is not sufficient to establish this beyond doubt, the high-energy γ-ray spectrum appears to evolve from hard to soft as a flare progresses.

On the nondetection of extrasolar comets

This paper reports on calculations of the expected rate for the discovery of comets with significantly hyperbolic orbits, including corrections for the observability of these comets. Using current standard models for the formation of comets, a significant number of such comets should have been observed. This lack of detection of extrasolar comets is becoming an embarassment to the theories of solar system and cometary formation and may drive the parameters of these models. 22 refs.

Automated Classification of ROSAT Sources Using Heterogeneous Multiwavelength Source Catalogs

We describe an online system for automated classification of X-ray sources, ClassX, and we present preliminary results of classification of the three major catalogs of ROSAT sources, ROSAT All-Sky Survey (RASS) Bright Source Catalog, RASS Faint Source Catalog, and WGACAT, into six class categories: stars, white dwarfs, X-ray binaries, galaxies, active galactic nuclei, and clusters of galaxies. ClassX is based on a machine-learning technology. It represents a system of classifiers, each classifier consisting of a considerable number of oblique decision trees. These trees are built as the classifier is trained to recognize various classes of objects using a training sample of sources of known object types. Each source is characterized by a preselected set of parameters, or attributes; the same set is then used as the classifier conducts classification of sources of unknown identity. The ClassX pipeline features an automatic search for X-ray source counterparts among heterogeneous data sets in online data archives using Virtual Observatory protocols; it retrieves from those archives all the attributes required by the selected classifier and inputs them to the classifier. The user input to ClassX is typically a file with target coordinates, optionally complemented with target IDs. The output contains the class name, attributes, and class probabilities for all classified targets. We discuss ways to characterize and assess the classifier quality and performance, and we present the respective validation procedures. On the basis of both internal validation and external verification, we conclude that the ClassX classifiers yield reasonable and reliable classifications for ROSAT sources and have the potential to broaden class representation significantly for rare object types.

A Gamma-Ray Flare in NRAO 190

We describe observations of the quasi-stellar object (QSO) NRAO 190 during a gamma-ray flare from 1994 August 9 to 1994 August 29. This QSO was serendipitously detected by the EGRET instrument on the Compton Gamma Ray Observatory in a gamma-ray flare with a luminosity at least 10 times that of its quiescent state. Optical, radio, and microwave data were obtained during or near the gamma-ray observations. The historical behavior of this object places it in the category of bright, flat-spectrum radio sources with strong optical variability that appear to form the largest class of non-Galactic high-energy gamma-ray sources. During the gamma-ray flare the source is observed with l(E > 100 MeV) = 8.4 ± 1.2 × 10-7 photons s-1 cm-2. A single power-law model gives a best-fit photon index of γ = -1.83 ± 0.14. Little evidence for major radio variability is seen during the flare or immediately afterward, although there is some increase in the 10-100 GHz flux over the next several months. There may be a slight hardening of the radio spectrum. In the optical region there are significant fluctuations on timescales of 1 day or less, although the overall optical luminosity is within the range of previous measurements. Optical observations a few weeks after the gamma-ray observations show a drop of about 60% and reduced variability. A contemporary optical spectrum shows that the source may be slightly harder than seen in a previously published spectrum. Radio monitoring of the source over the year subsequent to the flare has shown a very substantial drop in the flux at many frequencies.

Angular momentum and stripping in tidal interactions

A variety of simulations is used here to study the effects of tidal torques on individual particles in interacting stellar systems. The torquing of particles into prograde orbits is shown to compensate for the preferential stripping of prograde orbits, indicating that satellite systems need not show a net retrograde rotation. Tidal remnants should show only moderate degrees of rotation, and prograde rotation may be nearly as common as retrograde. The tidal acceleration along individual orbits is shown to depend primarily on simple three-body interactions. The particle stripping probability is determined as a function of the various model orbital parameters, and prograde torquing as a function of the orbital parameters. Preferential stripping of prograde orbits is seen most strongly for particles in circular orbits, where the satellite is in an elliptical orbit. 36 refs.

A high energy gamma ray survey of clusters of galaxies

Clusters of galaxies may emit high‐energy gamma rays due to a number of emission mechanisms including cosmic ray interactions with the intracluster medium and the annihilation of dark matter. We report on a search of measurements from the EGRET instrument aboard the COMPTON Observatory for gamma‐ray emission from clusters of galaxies. This cluster survey includes a preliminary analysis of data from both pointed observations and the EGRET all‐sky survey. Current upper limits are given and the cosmological implications of the observations are briefly discussed.

Providing comprehensive and consistent access to astronomical observatory archive data: the NASA archive model

Since the turn of the millennium a constant concern of astronomical archives have begun providing data to the public through standardized protocols unifying data from disparate physical sources and wavebands across the electromagnetic spectrum into an astronomical virtual observatory (VO). In October 2014, NASA began support for the NASA Astronomical Virtual Observatories (NAVO) program to coordinate the efforts of NASA astronomy archives in providing data to users through implementation of protocols agreed within the International Virtual Observatory Alliance (IVOA). A major goal of the NAVO collaboration has been to step back from a piecemeal implementation of IVOA standards and define what the appropriate presence for the US and NASA astronomy archives in the VO should be. This includes evaluating what optional capabilities in the standards need to be supported, the specific versions of standards that should be used, and returning feedback to the IVOA, to support modifications as needed. We discuss a standard archive model developed by the NAVO for data archive presence in the virtual observatory built upon a consistent framework of standards defined by the IVOA. Our standard model provides for discovery of resources through the VO registries, access to observation and object data, downloads of image and spectral data and general access to archival datasets. It defines specific protocol versions, minimum capabilities, and all dependencies. The model will evolve as the capabilities of the virtual observatory and needs of the community change.

Building archives in the virtual observatory era

Broad support for Virtual Observatory (VO) standards by astronomical archives is critical for the success of the VO as a research platform. Indeed, a number of effective data discovery, visualization, and integration tools have been created which rely on this broad support. Thus, to an archive, the motivation for supporting VO standards is strong. However, we are now seeing a growing trend among archive developers towards leveraging VO standards and technologies not just to provide interoperability with the VO, but also to support an archives internal needs and the needs of the archives primary user base. We examine the motivation for choosing VO technologies for implementing an archives functionality and list several current examples, including from the Hubble Legacy Archive, NASA HEASARC, NOAO, and NRAO. We will also speculate on the effect that VO will have on some of the ambitious observatory projects planned for the near future.

Hera: the HEASARC web-based data analysis environment

Hera is a new experiment at the HEASARC (High Energy Astrophysics Science Archive Research Center) at the NASA Goddard Space Flight Center to provide a complete data analysis environment over the Internet for archival researchers. This new facility complements the existing Browse database search facility that is available on the Web. With Hera, users can search the HEASARC data archives with a Web browser and save any selected data set to their Hera disk space area. This only takes a few seconds compared to the many minutes or hours that it could take to down load large data sets to the users local machine. The user can then immediately log into one of the available Hera server machines and begin analyzing the data without having to install any local software except for a very small Hera client application program that runs on the users local machine. Hera is currently most useful for expert users who are already familiar with analyzing high energy data sets with the HEASARC software. In the future we intend to make Hera more useful for the novice scientific user by providing more on-line help features to guide the user through the data analysis process.

Classifying the high energy universe with ClassX

Building an automated classifier for high-energy sources provides an opportunity to prototype approaches to building the Virtual Observatory with a substantial immediate scientific return. The ClassX collaboration is combining existing data resources with trainable classifiers to build a tool that classifies lists of objects presented to it. In our first year the collaboration has concentrated on developing pipeline software that finds and combines information of interest and in exploring the issues that will be needed for successful classification. ClassX must deal with many key VO issues: automating access to remote data resources, combining heterogeneous data and dealing with large data volumes. While the VO must attempt to deal with these problems in a generic way, the clear science goals of ClassX allow us to act as a pathfinder exploring particular approaches to addressing these issues.