James B. Dove

Low‐Luminosity States of the Black Hole Candidate GX 339−4. I. ASCA and Simultaneous Radio/RXTE Observations

We discuss a series of observations of the black hole candidate GX 339(4 in low-luminosity, spec- trally hard states. We present spectral analysis of three separate archival Advanced Satellite for Cosmol- ogy and Astrophysics (ASCA) data sets and eight separate Rossi X-Ray T iming Explorer (RXT E) data sets. Three of the RXT E observations were strictly simultaneous with 843 MHz and 8.3¨9.1 GHz radio observations. All of these observations have (3¨9 keV) —ux ergs s~1 cm~2. The ASCA data show (10~9 evidence for an B6.4 keV Fe line with equivalent width B40 eV, as well as evidence for a soft excess that is well modeled by a power law plus a multicolor blackbody spectrum with peak temperature B150¨200 eV. The RXT E data sets also show evidence of an Fe line with equivalent widths B20¨140 eV. Re—ection models show a hardening of the RXT E spectra with decreasing X-ray —ux; however, these models do not exhibit evidence of a correlation between the photon index of the incident power law —ux and the solid angle subtended by the re—ector. ii Sphere)disk ˇˇ Comptonization models and advection- dominated accretion —ow (ADAF) models also provide reasonable descriptions of the RXT E data. The former models yield coronal temperatures in the range 20¨50 keV and optical depths of qB 3. The model —ts to the X-ray data, however, do not simultaneously explain the observed radio properties. The most likely source of the radio —ux is synchrotron emission from an extended out—ow of size greater than O(107GM/c2). Subject headings: binaries: closeblack hole physicsradiation mechanisms: nonthermal ¨ stars: individual (GX 339(4) ¨ X-rays: stars

Low‐Luminosity States of the Black Hole Candidate GX 339−4. II. Timing Analysis

Here we present timing analysis of a set of eight Rossi X-Ray Timing Explorer (RXTE) observations of the black hole candidate GX 339-4 that were taken during its hard/low state. On long timescales, the RXTE All Sky Monitor data reveal evidence of a 240 day periodicity, comparable to timescales expected from warped, precessing accretion disks. On short timescales all observations save one show evidence of a persistent fQPO≈0.3 Hz quasi-periodic oscillation. The broadband (10-3 to 102 Hz) power appears to be dominated by two independent processes that can be modeled as very broad Lorentzian fits with Q1. The coherence function between soft and hard photon variability shows that if these are truly independent processes, then they are individually coherent, but they are incoherent with one another. This is evidenced by the fact that the coherence function between the hard and soft variability is near unity between 5 × 10−3 to 10 Hz but shows evidence of a dip at f≈1 Hz. This is the region of overlap between the broad Lorentzian fits to the power spectral density. As in Cyg X-1, the coherence also drops dramatically at frequencies 10 Hz. Also as in Cyg X-1, the hard photon variability is seen to lag the soft photon variability with the lag time increasing with decreasing Fourier frequency. The magnitude of this time lag appears to be positively correlated with the flux of GX 339-4. We discuss all of these observations in light of current theoretical models of both black hole spectra and temporal variability.

THREE-DIMENSIONAL MORPHOLOGY OF A CORONAL PROMINENCE CAVITY

We present a three-dimensional density model of coronal prominence cavities, and a morphological fit that has been tightly constrained by a uniquely well-observed cavity. Observations were obtained as part of an International Heliophysical Year campaign by instruments from a variety of space- and ground-based observatories, spanning wavelengths from radio to soft X-ray to integrated white light. From these data it is clear that the prominence cavity is the limb manifestation of a longitudinally extended polar-crown filament channel, and that the cavity is a region of low density relative to the surrounding corona. As a first step toward quantifying density and temperature from campaign spectroscopic data, we establish the three-dimensional morphology of the cavity. This is critical for taking line-of-sight projection effects into account, since cavities are not localized in the plane of the sky and the corona is optically thin. We have augmented a global coronal streamer model to include a tunnel-like cavity with elliptical cross-section and a Gaussian variation of height along the tunnel length. We have developed a semi-automated routine that fits ellipses to cross-sections of the cavity as it rotates past the solar limb, and have applied it to Extreme Ultraviolet Imager observations from the two Solar Terrestrial Relations Observatory spacecraft. This defines the morphological parameters of our model, from which we reproduce forward-modeled cavity observables. We find that cavity morphology and orientation, in combination with the viewpoints of the observing spacecraft, explain the observed variation in cavity visibility for the east versus west limbs.

Discovery of recurring soft-to-hard state transitions in LMC X-3

ABSTRA C T We present the analysis of the approximately three-year long Rossi X-ray Timing Explorerd (RXTE) campaign for monitoring the canonical soft state black-hole candidates LMC X-1 and LMC X-3. In agreement with previous observations, we find that the spectra of both sources can be well-described by the sum of a multi-temperature disc blackbody and a power law. In contrast to LMC X-1, which does not exhibit any periodic spectral changes, we find that LMC X-3 exhibits strong spectral variability on time-scales of days to weeks. The variability pattern observed with the RXTE All Sky Monitor reveals that the variability is more complicated than the 99- or 198-d periodicity discussed by Cowley et al. For typical ASM count rates, the luminosity variations of LMC X-3 are due to changes of the phenomenological disc blackbody temperature, kTin, between , 1t o,1.2 keV. During episodes of especially low luminosity (ASM count rates &0.6 counts s 21 ; four such periods are discussed here), kTin strongly decreases until the disc component is undetectable, and the power law significantly hardens to a photon index of G , 1:8: These changes are consistent with state changes of LMC X-3 from the soft state to the canonical hard state of galactic black holes. We argue that the long-term variability of LMC X-3 might be owing to a winddriven limit cycle, such as that discussed by Shields et al.

Self-consistent Thermal Accretion Disk Corona Models for Compact Objects. I. Properties of the Corona and the Spectrum of Escaping Radiation

We present the properties of accretion disk corona (ADC) models in which the radiation field, the temperature, and the total opacity of the corona are determined self-consistently. We use a nonlinear Monte Carlo code to perform the calculations. As an example, we discuss models in which the corona is situated above and below a cold accretion disk with a plane-parallel (slab) geometry, similar to the model of Haardt & Maraschi. By Comptonizing the soft radiation emitted by the accretion disk, the corona is responsible for producing the high-energy component of the escaping radiation. Our models include the reprocessing of radiation in the accretion disk. Here the photons either are Compton-reflected or photoabsorbed, giving rise to fluorescent line emission and thermal emission. The self-consistent coronal temperature is determined by balancing heating (due to viscous energy dissipation) with Compton cooling, determined using the fully relativistic, angle-dependent cross sections. The total opacity is found by balancing pair productions with annihilations. We find that, for a disk temperature kTBB 200 eV, these coronae are unable to have a self-consistent temperature higher than ~140 keV if the total optical depth is 0.2, regardless of the compactness parameter of the corona and the seed opacity. This limitation corresponds to the angle-averaged spectrum of escaping radiation having a photon index 1.8 within the 5-30 keV band. Finally, all models that have reprocessing features also predict a large thermal excess at lower energies. These constraints make explaining the X-ray spectra of persistent black hole candidates with ADC models very problematic.

A good long look at the black hole candidates LMC X-1 and LMC X-3

ABSTRA C T LMC X-1 and LMC X-3 are the only known persistent stellar-mass black-hole candidates that have almost always shown spectra that are dominated by a soft, thermal component. We present here results from 170-ks-long Rossi X-ray Timing Explorer (RXTE) observations of these objects, taken in 1996 December, where their spectra can be described by a disc blackbody plus an additional softOG , 2:8U high-energy power law (detected up to energies of 50 keV in LMC X-3). These observations, as well as archival Advanced Satellite for Cosmology and Astrophysics (ASCA) observations, constrain any narrow Fe line present in the spectra to have an equivalent width &90 eV. Stronger, broad lines (<150 eV EW, s < 1 keVU are permitted. We also study the variability of LMC X-1. Its X-ray power spectral density (PSD) is approximately proportional to f 21 between 10 23 and 0.3 Hz with a rootmean-square (rms) variability of <7 per cent. At energies .5 keV, the PSD shows evidence of a break at f . 0: 2H z; possibly indicating an outer disc radius of &1000 GM/c 2 in this likely wind-fed system. Furthermore, the coherence function g 2 O fU; a measure of the degree of linear correlation between variability in the .5 keV band and variability in the lower energy bands, is extremely low (&50 per cent). We discuss the implications of these observations for the mechanisms that might be producing the soft and hard X-rays in these systems.

A Ring of Polarized Light: Evidence for Twisted Coronal Magnetism in Cavities

Coronal prominence cavities may be manifestations of twisted or sheared magnetic fields capable of storing the energy required to drive solar eruptions. The Coronal Multi-Channel Polarimeter (CoMP), recently installed at Mauna Loa Solar Observatory, can measure polarimetric signatures of current-carrying magnetohydrodynamic (MHD) systems. For the first time, this instrument offers the capability of daily full-Sun observations of the forbidden lines of Fe XIII with high enough spatial resolution and throughput to measure polarimetric signatures of current-carrying MHD systems. By forward-calculating CoMP observables from analytic MHD models of spheromak-type magnetic flux ropes, we show that a predicted observable for such flux ropes oriented along the line of sight is a bright ring of linear polarization surrounding a region where the linear polarization strength is relatively depleted. We present CoMP observations of a coronal cavity possessing such a polarization ring.

Polarimetric Properties of Flux Ropes and Sheared Arcades in Coronal Prominence Cavities

The coronal magnetic field is the primary driver of solar dynamic events. Linear and circular polarization signals of certain infrared coronal emission lines contain information about the magnetic field, and to access this information either a forward or an inversion method must be used. We study three coronal magnetic configurations that are applicable to polar-crown filament cavities by doing forward calculations to produce synthetic polarization data. We analyze these forward data to determine the distinguishing characteristics of each model. We conclude that it is possible to distinguish between cylindrical flux ropes, spheromak flux ropes, and sheared arcades using coronal polarization measurements. If one of these models is found to be consistent with observational measurements, it will mean positive identification of the magnetic morphology that surrounds certain quiescent filaments, which will lead to a better understanding of how they form and why they erupt.

FORWARD: A Toolset for Multiwavelength Coronal Magnetometry

Determining the 3D coronal magnetic field is a critical, but extremely difficult problem to solve. Since different types of multiwavelength coronal data probe different aspects of the coronal magnetic field, ideally these data should be used together to validate and constrain specifications of that field. Such a task requires the ability to create observable quantities at a range of wavelengths from a distribution of magnetic field and associated plasma -- i.e., to perform forward calculations. In this paper we describe the capabilities of the FORWARD SolarSoft IDL package, a uniquely comprehensive toolset for coronal magnetometry. FORWARD is a community resource that may be used both to synthesize a broad range of coronal observables, and to access and compare synthetic observables to existing data. It enables forward fitting of specific observations, and helps to build intuition into how the physical properties of coronal magnetic structures translate to observable properties. FORWARD can also be used to generate synthetic test beds from MHD simulations in order to facilitate the development of coronal magnetometric inversion methods, and to prepare for the analysis of future large solar telescope data.

A model for the vertical distribution of stars in disk galaxies

We investigate the contribution of multiple velocity dispersion components to the vertical (z) distribution of stars in a disk galaxy by self-consistently solving the Poisson and collisionless Botzmann equations. To reduce significantly the number of free parameters, we incorporate a history of galaxian star formation which produces stellar populations with evolving velocity dispersions. This also allows us to evaluate contributions to the width of the disk from direrent epochs of stellar creation. In general, our predicted vertical brightness distribution is thinner than a standard isothermal sech 2 (z) function if the latter is fitted to the high-z wings of the density distribution, as additional stellar components in our model have smaller vertical velocity dispersions than the dominant, high-z one