Multiwavelength observations of the black hole candidate 1E 1740.7-2942

Observations of the Galactic center region black hole candidate 1E 1740.7-2942 have been carried out using the Caltech Gamma-Ray Imaging Payload (GRIP), the Röntgensatellit (ROSAT) and the Very Large Array (VLA). These multiwavelength observations have helped to establish the association between a bright emitter of hard X-rays and soft γ-rays, the compact core of a double radio jet source, and the X-ray source, 1E 1740.7-2942. They have also provided information on the X-ray and hard X-ray spectrum.

The Galactic center region was observed by GRIP during balloon flights from Alice Springs, NT, Australia on 1988 April 12 and 1989 April 3. These observations revealed that 1E 1740.7-2942 was the strongest source of hard X-rays within ~10° of the Galactic center. The source spectrum from each flight is well fit by a single power law in the energy range 35-200 keV. The best-fit photon indices and 100 keV normalizations are: γ = (2.05 ± 0.15) and K_(100) = (8.5 ± 0.5) x 10^(-5) cm^(-2) s^(-1) keV^(-1) and γ = (2.2 ± 0.3) and K_(100) = (7.0 ± 0.7) x 10^(-5) cm^(-2) s^(-1) keV^(-1) for the 1988 and 1989 observations respectively. No flux above 200 keV was detected during either observation. These values are consistent with a constant spectrum and indicate that 1E 1740.7-2942 was in its normal hard X-ray emission state. A search on one hour time scales showed no evidence for variability.

The ROSAT HRI observed 1E 1740.7-2942 during the period 1991 March 20-24. An improved source location has been derived from this observation. The best fit coordinates (J2000) are: Right Ascension = 17^h43^m54^s.9, Declination = -29°44'45".3, with a 90% confidence error circle of radius 8".5. The PSPC observation was split between periods from 1992 September 28- October 4 and 1993 March 23-28. A thermal bremsstrahlung model fit to the data yields a column density of N_H = 1.12^(+1.51)_(0.18) x cm^(-2) , consistent with earlier X- ray measurements.

We observed the region of the Einstein IPC error circle for 1E 1740.7-2942 with the VLA at 1.5 and 4.9 GHz on 1989 March 2. The 4.9 GHz observation revealed two sources. Source 'A', which is the core of a double aligned radio jet source (Mirabel et al. 1992), lies within our ROSAT error circle, further strengthening its identification with 1E 1740.7-2942.

On the thermal budget of Pahoehoe lava flows

In this thesis I investigate some aspects of the thermal budget of pahoehoe lava flows. This is done with a combination of general field observations, quantitative modeling, and specific field experiments. The results of this work apply to pahoehoe flows in general, even though the vast bulk of the work has been conducted on the lavas formed by the Pu'u 'O'o - Kupaianaha eruption of Kilauea Volcano on Hawai'i. The field observations rely heavily on discussions with the staff of the United States Geological Survey's Hawaiian Volcano Observatory (HVO), under whom I labored repeatedly in 1991-1993 for a period totaling about 10 months.

The quantitative models I have constructed are based on the physical processes observed by others and myself to be active on pahoehoe lava flows. By building up these models from the basic physical principles involved, this work avoids many of the pitfalls of earlier attempts to fit field observations with "intuitively appropriate" mathematical expressions. Unlike many earlier works, my model results can be analyzed in terms of the interactions between the different physical processes. I constructed models to: (1) describe the initial cooling of small pahoehoe flow lobes and (2) understand the thermal budget of lava tubes.

The field experiments were designed either to validate model results or to constrain key input parameters. In support of the cooling model for pahoehoe flow lobes, attempts were made to measure: (1) the cooling within the flow lobes, (2) the amount of heat transported away from the lava by wind, and (3) the growth of the crust on the lobes. Field data collected by Jones [1992], Hon et al. [1994b], and Denlinger [Keszthelyi and Denlinger, in prep.] were also particularly useful in constraining my cooling model for flow lobes. Most of the field observations I have used to constrain the thermal budget of lava tubes were collected by HVO (geological and geophysical monitoring) and the Jet Propulsion Laboratory (airborne infrared imagery [Realmuto et al., 1992]). I was able to assist HVO for part of their lava tube monitoring program and also to collect helicopterborne and ground-based IR video in collaboration with JPL [Keszthelyi et al., 1993].

The most significant results of this work are (1) the quantitative demonstration that the emplacement of pahoehoe and 'a'a flows are the fundamentally different, (2) confirmation that even the longest lava flows observed in our Solar System could have formed as low effusion rate, tube-fed pahoehoe flows, and (3) the recognition that the atmosphere plays a very important role throughout the cooling of history of pahoehoe lava flows. In addition to answering specific questions about the thermal budget of tube-fed pahoehoe lava flows, this thesis has led to some additional, more general, insights into the emplacement of these lava flows. This general understanding of the tube-fed pahoehoe lava flow as a system has suggested foci for future research in this part of physical volcanology.

Analysis of temporal structure in spike trains of visual cortical area MT

The temporal structure of neuronal spike trains in the visual cortex can provide detailed information about the stimulus and about the neuronal implementation of visual processing. Spike trains recorded from the macaque motion area MT in previous studies (Newsome et al., 1989a; Britten et al., 1992; Zohary et al., 1994) are analyzed here in the context of the dynamic random dot stimulus which was used to evoke them. If the stimulus is incoherent, the spike trains can be highly modulated and precisely locked in time to the stimulus. In contrast, the coherent motion stimulus creates little or no temporal modulation and allows us to study patterns in the spike train that may be intrinsic to the cortical circuitry in area MT. Long gaps in the spike train evoked by the preferred direction motion stimulus are found, and they appear to be symmetrical to bursts in the response to the anti-preferred direction of motion. A novel cross-correlation technique is used to establish that the gaps are correlated between pairs of neurons. Temporal modulation is also found in psychophysical experiments using a modified stimulus. A model is made that can account for the temporal modulation in terms of the computational theory of biological image motion processing. A frequency domain analysis of the stimulus reveals that it contains a repeated power spectrum that may account for psychophysical and electrophysiological observations.

Some neurons tend to fire bursts of action potentials while others avoid burst firing. Using numerical and analytical models of spike trains as Poisson processes with the addition of refractory periods and bursting, we are able to account for peaks in the power spectrum near 40 Hz without assuming the existence of an underlying oscillatory signal. A preliminary examination of the local field potential reveals that stimulus-locked oscillation appears briefly at the beginning of the trial.