J. Eric Grove

Spectral Transitions in Cygnus X-1 and Other Black Hole X-Ray Binaries

We show that the model proposed by Esin, McClintock, & Narayan for the low state, intermediate state, and high state of the black hole soft X-ray transient, Nova Muscae 1991, is consistent with the spectral evolution of the black hole X-ray binary, Cyg X-1, during the hard-to-soft state transition observed in 1996. We also apply the model to the outbursts of two other black hole X-ray transients, GRO J0422+32 and GRO J1719-24.

OSSE and RXTE Observations of GRS 1915+105: Evidence for Nonthermal Comptonization

GRS 1915+105 was observed by the Oriented Scintillation Spectroscopy Experiment (OSSE) aboard the Compton Gamma Ray Observatory nine times in 1995-2000, and eight of those observations were simultaneous with those by the Rossi X-Ray Timing Explorer (RXTE). We present an analysis of all of the OSSE data and of two RXTE-OSSE spectra with the lowest and highest X-ray fluxes. The OSSE data show a power-law-like spectrum extending up to 600 keV without any break. We interpret this emission as strong evidence for the presence of nonthermal electrons in the source. The broadband spectra cannot be described by either thermal or bulk-motion Comptonization, whereas they are well described by Comptonization in hybrid thermal/nonthermal plasmas.

X-ray and γ-ray spectra and variability of the black hole candidate GX 339—4

We analyse five observations of the X-ray binary GX 339‐4 by the soft γ -ray OSSE detector on board CGRO simultaneous with either Ginga or RXTE observations. The source was bright during four of them, with the luminosity of L ∼ 10 37 erg s −1 and the spectrum typical for hard states of accreting black holes, and it was in an off state during the fifth one, with L ∼ 10 35 erg s −1 . Our broad-band spectral fits show the mean electron energy of electrons in the Comptonizing plasma decreasing with increasing luminosity within the hard (bright) state. For the observation with the best statistics at soft γ -rays, ∼1/4 of energy in the Comptonizing plasma is probably carried by non-thermal electrons. Then, considering the efficiency of Comptonized hybrid synchrotron emission allows us to obtain an upper limit on the strength of the magnetic field in the X-ray source. Furthermore, this synchrotron emission is capable of producing the optical spectrum observed in an optically-high state of GX 339‐4. In the off state, the hard X-ray spectrum is consistent with being dominated by bremsstrahlung. The unusually strong Fe Kα line observed by the RXTE/PCA during that state is found not to be intrinsic to the source but to originate mostly in the Galactic diffuse emission.

Limits on Nuclear Gamma-Ray Emission from Orion

The discovery of γ-ray line emission in the 3-7 MeV range from the Orion complex was recently reported. The observed Compton Telescope (COMPTEL) spectrum suggested that the emission results from the de-excitation of excited states of 12C and 16O. We report on a search for these lines using the Oriented Scintillation Spectroscopy Experiment (OSSE) on the Compton Gamma Ray Observatory (CGRO) during a 5 week observation in 1995 from April to June. The OSSE detectors were pointed midway between the Orion A and B radio sources in three different viewing configurations. We find no compelling evidence for line emission near 4.4 or 6.1 MeV. The sensitivity of the OSSE measurements is dependent on the widths of the reported C and O lines and on the source location and spatial extent. A point source at the flux level reported by COMPTEL and located on-axis would have been detected by OSSE at ~7 σ and ~5 σ levels of confidence for narrow and broad lines, respectively. A spatially distributed source of the same strength with a distribution following the intense CO emission localized around Orion A and Orion B would have been detected by OSSE at ~3.5 σ and ~2.5 σ, respectively. Thus, these OSSE observations require that any γ-ray line source must be even more extended to be consistent with the reported COMPTEL intensity.

The calorimeter of the Fermi Large Area Telescope

The Large Area Telescope (LAT), the primary instrument on the Fermi Gamma-ray Space Telescope, has been making revolutionary observations of the high-energy (20 MeV - 300 GeV) gamma-ray sky since its launch in June 2008. The LAT calorimeter is a modular array of 1536 CsI(Tl) crystals supported within 16 carbon fiber structures and read out at each crystal end with silicon PIN photodiodes to provide both energy and position information. The hodoscopic crystal stack allows imaging of electromagnetic showers and cosmic rays for improved energy measurement and background rejection. Signals from the array of photodiodes are processed by custom ASICs and commercial ADCs. We describe the calorimeter design and the primary factors that led those design choices.

Characterizing the source properties of terrestrial gamma-ray flashes

Monte Carlo simulations are used to determine source properties of terrestrial gamma-ray flashes (TGFs) as a function of atmospheric column depth and beaming geometry. The total mass per unit area traversed by all the runaway electrons (i.e., the total grammage) during a TGF, Ξ, is introduced, defined to be the total distance traveled by all the runaway electrons along the electric field lines multiplied by the local air mass density along their paths. It is shown that key properties of TGFs may be directly calculated from Ξ and its time derivative, including the gamma-ray emission rate, the current moment, and the optical power of the TGF. For the calculations presented in this paper, a standard TGF gamma-ray fluence, F0 = 0.1 cm-2 above 100 keV for a spacecraft altitude of 500 km, and a standard total grammage, Ξ0 = 1018 g/cm2, are introduced, and results are presented in terms of these values. In particular, the current moments caused by the runaway electrons and their accompanying ionization are found for a standard TGF fluence, as a function of source altitude and beaming geometry, allowing a direct comparison between the gamma rays measured in low-Earth orbit and the VLF-LF radio frequency emissions recorded on the ground. Such comparisons should help test and constrain TGF models and help identify the roles of lightning leaders and streamers in the production of TGFs.

Development of the strontium iodide coded aperture (SICA) instrument

The work reports on the development of a Strontium Iodide Coded Aperture (SICA) instrument for use in space-based astrophysics, solar physics, and high-energy atmospheric physics. The Naval Research Laboratory is developing a prototype coded aperture imager that will consist of an 8 x 8 array of SrI2:Eu detectors, each read out by a silicon photomultiplier. The array would be used to demonstrate SrI2:Eu detector performance for space-based missions. Europium-doped strontium iodide (SrI2:Eu) detectors have recently become available, and the material is a strong candidate to replace existing detector technology currently used for space-based gamma-ray astrophysics research. The detectors have a typical energy resolution of 3.2% at 662 keV, a significant improvement over the 6.5% energy resolution of thallium-doped sodium iodide. With a density of 4.59 g/cm and a Zeff of 49, SrI2:Eu has a high efficiency for MeV gamma-ray detection. Coupling this with recent improvements in silicon photomultiplier technology (i.e., no bulky photomultiplier tubes) creates high-density, large-area, low-power detector arrays with good energy resolution. Also, the energy resolution of SrI2:Eu makes it ideal for use as the back plane of a Compton telescope.