Francis E. Marshall

Bright X-ray Flares in Gamma-Ray Burst Afterglows

Gamma-ray burst (GRB) afterglows have provided important clues to the nature of these massive explosive events, providing direct information on the nearby environment and indirect information on the central engine that powers the burst. We report the discovery of two bright x-ray flares in GRB afterglows, including a giant flare comparable in total energy to the burst itself, each peaking minutes after the burst. These strong, rapid x-ray flares imply that the central engines of the bursts have long periods of activity, with strong internal shocks continuing for hundreds of seconds after the gamma-ray emission has ended.

Discovery of X-ray and Extreme Ultraviolet Emission from Comet C/Hyakutake 1996 B2

During its close approach to Earth, comet C/Hyakutake 1996 B2 was observed at extreme ultraviolet and x-ray wavelengths with the Rœntgen X-ray Satellite and Rossi X-ray Timing Explorer. The emission morphology was symmetric with respect to a vector from the comets nucleus toward the sun, but not symmetric around the direction of motion of the comet with respect to interplanetary dust. A slowly varying emission and a large impulsive event that varied on time scales of 1 to 2 hours were observed. An interaction between the comet and the solar wind/solar magnetic field seems to be the most likely mechanism for the observed emission.

Observation of X-ray Lines from a Gamma-Ray Burst (GRB991216): Evidence of Moving Ejecta from the Progenitor

We report on the discovery of two emission features observed in the x-ray spectrum of the afterglow of the gamma-ray burst (GRB) of 16 December 1999 by the Chandra X-ray Observatory. These features are identified with the Ly(alpha) line and the narrow recombination continuum by hydrogenic ions of iron at a redshift z = 1.00 +/- 0.02, providing an unambiguous measurement of the distance of a GRB. Line width and intensity imply that the progenitor of the GRB was a massive star system that ejected, before the GRB event, a quantity of iron approximately 0.01 of the mass of the sun at a velocity approximately 0.1 of the speed of light, probably by a supernova explosion.

RXTE Measurement of the Diffuse X-Ray Emission from the Galactic Ridge: Implications for the Energetics of the Interstellar Medium

The diffuse X-ray emission from the thin disk surrounding the Galactic midplane (the so-called Galactic ridge) was measured with the Rossi X-Ray Timing Explorer proportional counter array in order to determine the spatial extent, spectral nature, and origin of the emission. Spatial examination of the diffuse emission in the central 30? of the plane in Galactic longitude reveals the presence of two components: a thin disk of full width 05 centered roughly on the Galactic midplane and a broad component that can be approximated as a Gaussian distribution with FWHM of about 4?. Assuming an average distance of 16 kpc to the edge of the Galaxy, a scale height of about 70 pc and 500 pc is derived for the thin and broad disk components, respectively. Spectral examination of the emission clearly reveals the presence of a hard power-law tail above 10 keV and an emission line from He-like iron, indicating both thermal and possibly nonthermal origins for the diffuse emission. The averaged spectrum from the ridge in the 3-35 keV band can be modeled with a Raymond-Smith plasma component of temperature ~2-3 keV and a power-law component of photon index ~1.8. Based on this finding, we argue that the temperature of the hot phase of the interstellar medium (ISM) is less than the previously reported values of 5-15 keV. Motivated by the similarities between the characteristics of the thermal component of the Galactic ridge emission in our model and the thermal emission from supernova remnants (SNRs), we discuss the origin of the thermal emission in terms of a population of SNRs residing in the Galactic disk. We find that a supernova explosion rate of less than 5 century-1 is adequate to power the thermal emission from the ridge. The origin of the emission in the hard X-ray band modeled by a power law remains uncertain. Possible contributions from nonthermal bremsstrahlung of cosmic-ray electrons and protons; inverse Compton scattering of energetic electrons from ambient microwave, infrared, and optical photons; nonthermal emission from SNRs; and emission from discrete X-ray sources are discussed. We speculate that bremsstrahlung of accelerated electrons and protons in SNR sites can play a significant role in producing the hard tail of the spectrum. Moreover, their collisional losses can play a major role in the ionization of the ISM.

X-ray emission from comet Hale-Bopp

The discovery of X-ray emission from comets has created a number of questions about the physical mechanism producing the radiation. There are now a variety of explanations for the emission, from thermal bremsstrahlung of electrons off neutrals or dust, to charge exchange induced emission from solar wind ions, to scattering of solar X-rays from attogram dust, to reconnection of solar magnetic field lines. In an effort to understand this new phenomenon, we observed but failed to detect in the X-ray the very dusty and active comet C/Hale-Bopp 1995 O1 over a two year period, September 1996 to December 1997, using the ROSAT HRI imaging photometer at 0.1–2.0 keV and the ASCA SIS imaging spectrometer at 0.5–10.0 keV. The results of our Hale-Bopp non-detections, when combined with spectroscopic imaging 0.08–1.0 keV observations of the comet by EUVE and BeppoSAX, show that the emission has the same spectral shape and strong variability seen in other comets. Comparison of the ROSAT photometry of the comet to our ROSAT database of 8 comets strongly suggests that the overall X-ray faintness of the comet was due to an emission mechanism coupled to gas, and not dust, in the comet’s coma.

Measurement of the Galactic X-Ray/Gamma-Ray Background Radiation: Contribution of Discrete Sources

The Galactic background radiation near the Scutum arm was observed simultaneously with RXTE and OSSE in order to determine the spectral shape and the origin of the emission in the hard X-ray/soft γ-ray band. The spectrum in the 3 keV to 1 MeV band is well modeled by four components: a high-energy continuum dominating above 500 keV that can be characterized by a power law of photon index ~1.6 (an extrapolation from measurements above ~1 MeV), a positron annihilation line at 511 keV and positronium continuum, a variable hard X-ray/soft γ-ray component that dominates between 10 and 200 keV (with a minimum detected flux of ~7.7 × 10-7 photons cm-2 s-1 keV-1 deg-2 at 100 keV averaged over the field of view of OSSE) and that is well modeled by an exponentially cutoff power law of photon index ~0.6 and energy cutoff at ~41 keV, and finally a thermal plasma model of solar abundances and temperature ~2.6 keV that dominates below 10 keV. We estimate that the contribution of bright discrete sources to the minimum flux detected by OSSE was ~46% at 60 keV and ~20% at 100 keV. The remaining unresolved emission may be interpreted either as truly diffuse emission with a hard spectrum (such as that from inverse Compton scattering) or the superposition of discrete sources that have very hard spectra.

GRB 060607A: a gamma-ray burst with bright asynchronous early X-ray and optical emissions

The early optical emission of the moderately high redshift (

A search for X-ray emission from rich clusters, extended halos around clusters, and superclusters

z=3.08

NuSTAR observatory science operations: on-orbit acclimation

) GRB 060607A shows a remarkable broad and strong peak with a rapid rise and a relatively slow power-law decay. It is not coincident with the strong early-time flares seen in the X-ray and gamma-ray energy bands. There is weak evidence for variability superposed on this dominant component in several optical bands that can be related to flares in high energy bands. While for a small number of GRBs, well-sampled optical flares have been observed simultaneously with X-ray and gamma ray pulses, GRB 060607A is one of the few cases where the early optical emission shows no significant evidence for correlation with the prompt emission. In this work we first report in detail the broad band observations of this burst by Swift. Then by applying a simple model for the dynamics and the synchrotron radiation of a relativistic shock, we show that the dominant component of the early emissions in optical wavelengths has the same origin as the tail emission produced after the main gamma ray activity. The most plausible explanation for the peak in the optical light curve seems to be the cooling of the prompt after the main collisions, shifting the characteristic synchrotron frequency to the optical bands. It seems that the cooling process requires a steepening of the electron energy distribution and/or a break in this distribution at high energies. The sharp break in the X-ray light curve at few thousands of seconds after the trigger, is not observed in the IR/optical/UV bands, and therefore can not be a jet break. Either the X-ray break is due to a change in the spectrum of the accelerated electrons or the lack of an optical break is due to the presence of a related delayed response component (Abbreviated).

GRB 060607A: A GRB with Bright Asynchronous Early

The all-sky data base acquired with the HEAO A-2 experiment has been searched for X-ray emission on a variety of metagalactic size scales which had wither been predicted or previously detected. We present results in 0.2--60 keV energy range. The optically richest clusters, including those from which a microwave decrement has been observed, appear to be relatively underluminous in X-rays. Observations of Abell 576 show its luminosity to be less than earlier estimates and, moreover, less than the luminosity predicted from its microwave decrement, unless the intracluster gas is a factor of approx.10 hotter than in typical clusters.Extended halos around clusters were not detected in our data, and weak sources appear to be responsible for the apparent effect around several clusters. Near SC 0627 there are two X-ray sources, and the identification of the dominant source with SC 0627 is probably incorrect.New spectral observations of Abell 401 and 2147, possible superclusters, reveal that they have typical cluster spectra with iron-line emission. Our substantially lower intensities for the 4U supercluster candidates indicate that there is no requirement for supercluster X-ray emission above the contribution of the member clusters.