Gary Lee Case

When A Standard Candle Flickers

The Crab Nebula is the only hard X-ray source in the sky that is both bright enough and steady enough to be easily used as a standard candle. As a result, it has been used as a normalization standard by most X-ray/gamma-ray telescopes. Although small-scale variations in the nebula are well known, since the start of science operations of the Fermi Gamma-ray Burst Monitor (GBM) in 2008 August, a ~7% (70 mCrab) decline has been observed in the overall Crab Nebula flux in the 15-50 keV band, measured with the Earth occultation technique. This decline is independently confirmed in the ~15-50 keV band with three other instruments: the Swift Burst Alert Telescope (Swift/BAT), the Rossi X-ray Timing Explorer Proportional Counter Array (RXTE/PCA), and the Imager on-Board the INTEGRAL Satellite (IBIS). A similar decline is also observed in the ~3-15 keV data from the RXTE/PCA and in the 50-100 keV band with GBM, Swift/BAT, and INTEGRAL/IBIS. The pulsed flux measured with RXTE/PCA since 1999 is consistent with the pulsar spin-down, indicating that the observed changes are nebular. Correlated variations in the Crab Nebula flux on a ~3 year timescale are also seen independently with the PCA, BAT, and IBIS from 2005 to 2008, with a flux minimum in 2007 April. As of 2010 August, the current flux has declined below the 2007 minimum.

X-RAY AND OPTICAL OBSERVATIONS OF A 0535+26

We present recent contemporaneous X-ray and optical observations of the Be/X-ray binary system A 0535+26 with the Fermi/Gamma-ray Burst Monitor (GBM) and several ground-based observatories. These new observations are put into the context of the rich historical data (since similar to 1978) and discussed in terms of the neutron-star-Be-disk interaction. The Be circumstellar disk was exceptionally large just before the 2009 December giant outburst, which may explain the origin of the unusual recent X-ray activity of this source. We found a peculiar evolution of the pulse profile during this giant outburst, with the two main components evolving in opposite ways with energy. A hard 30-70 mHz X-ray quasi-periodic oscillation was detected with GBM during this 2009 December giant outburst. It becomes stronger with increasing energy and disappears at energies below 25 keV. In the long term a strong optical/X-ray correlation was found for this system, however in the medium term the H alpha equivalent width and the V-band brightness showed an anti-correlation after similar to 2002 August. Each giant X-ray outburst occurred during a decline phase of the optical brightness, while the H alpha showed a strong emission. In late 2010 and before the 2011 February outburst, rapid V/R variations are observed in the strength of the two peaks of the H alpha line. These had a period of similar to 25 days and we suggest the presence of a global one-armed oscillation to explain this scenario. A general pattern might be inferred, where the disk becomes weaker and shows V/R variability beginning similar to 6 months following a giant outburst.

THREE YEARS OF FERMI GBM EARTH OCCULTATION MONITORING: OBSERVATIONS OF HARD X-RAY/SOFT GAMMA-RAY SOURCES

The Gamma-ray Burst Monitor (GBM) on board Fermi has been providing continuous data to the astronomical community since 2008 August 12. In this paper, we present the results of the analysis of the first three years of these continuous data using the Earth occultation technique to monitor a catalog of 209 sources. From this catalog, we detect 99 sources, including 40 low-mass X-ray binary/neutron star systems, 31 high-mass X-ray binary/neutron star systems, 12 black hole binaries, 12 active galaxies, and 2 other sources, plus the Crab Nebula, and the Sun. Nine of these sources are detected in the 100-300 keV band, including seven black hole binaries, the active galaxy Cen A, and the Crab. The Crab and Cyg X-1 are also detected in the 300-500 keV band. GBM provides complementary data to other sky-monitors below 100 keV and is the only all-sky monitor above 100 keV. Up-to-date light curves for all of the catalog sources can be found online.

FIRST RESULTS FROM FERMI GAMMA-RAY BURST MONITOR EARTH OCCULTATION MONITORING: OBSERVATIONS OF SOFT GAMMA-RAY SOURCES ABOVE 100 keV

The NaI and BGO detectors on the Gamma-ray Burst Monitor (GBM) on Fermi are now being used for long-term monitoring of the hard X-ray/low-energy gamma-ray sky. Using the Earth occultation technique as demonstrated previously by the BATSE instrument on the Compton Gamma-Ray Observatory, GBM can be used to produce multiband light curves and spectra for known sources and transient outbursts in the 8 keV to 1 MeV energy range with its NaI detectors and up to 40 MeV with its BGO detectors. Over 85% of the sky is viewed every orbit, and the precession of the Fermi orbit allows the entire sky to be viewed every ~26 days with sensitivity exceeding that of BATSE at energies below ~25 keV and above ~1.5 MeV. We briefly describe the technique and present preliminary results using the NaI detectors after the first two years of observations at energies above 100 keV. Eight sources are detected with a significance greater than 7σ: the Crab, Cyg X-1, SWIFT J1753.5–0127, 1E 1740-29, Cen A, GRS 1915+105, and the transient sources XTE J1752–223 and GX 339-4. Two of the sources, the Crab and Cyg X-1, have also been detected above 300 keV.

TETRA observation of gamma-rays at ground level associated with nearby thunderstorms.

[1] Terrestrial gamma-ray flashes (TGFs)-very short, intense bursts of electrons, positrons, and energetic photons originating from terrestrial thunderstorms-have been detected with satellite instruments. TGF and Energetic Thunderstorm Rooftop Array (TETRA), an array of NaI(Tl) scintillators at Louisiana State University, has now been used to detect similar bursts of 50 keV to over 2 MeV gamma-rays at ground level. After 2.6 years of observation, 24 events with durations 0.02-4.2 ms have been detected associated with nearby lightning, three of them coincident events observed by detectors separated by ∼1000 m. Nine of the events occurred within 6 ms and 5 km of negative polarity cloud-to-ground lightning strokes with measured currents in excess of 20 kA. The events reported here constitute the first catalog of TGFs observed at ground level in close proximity to the acceleration site.

Compton scattered transition radiation from very high energy particles

Abstract X-ray transition radiation can be used to measure the Lorentz factor of relativistic particles. At energies approaching γ = E / mc 2 =10 5 , transition radiation detectors can be optimized by using thick (∼5–10 mil) foils with large (∼5–10 mm) spacings. This implies X-ray energies ≳100 keV and the use of scintillators as the X-ray detectors. Compton scattering of the X-rays out of the particle beam then becomes an important effect. We discuss the design of very high energy detectors, the use of metal radiator foils rather than the standard plastic foils, inorganic scintillators for detecting Compton scattered transition radiation, and the application to the ACCESS cosmic ray experiment.

CASTER: a concept for a Black Hole Finder Probe based on the use of new scintillator technologies

The primary scientific mission of the Black Hole Finder Probe (BHFP), part of the NASA Beyond Einstein program, is to survey the local Universe for black holes over a wide range of mass and accretion rate. One approach to such a survey is a hard X-ray coded-aperture imaging mission operating in the 10-600 keV energy band, a spectral range that is considered to be especially useful in the detection of black hole sources. The development of new inorganic scintillator materials provides improved performance (for example, with regards to energy resolution and timing) that is well suited to the BHFP science requirements. Detection planes formed with these materials coupled with a new generation of readout devices represent a major advancement in the performance capabilities of scintillator-based gamma cameras. Here, we discuss the Coded Aperture Survey Telescope for Energetic Radiation (CASTER), a concept that represents a BHFP based on the use of the latest scintillator technology.

Angular resolution obtained with a LaBr3-based rotational modulator

A Rotational Modulator (RM) gamma ray imager, consisting of a single grid of lead slats rotating above an array of detectors with diameter equal to the slat spacing, has the capability of providing angular resolution significantly better than the geometric resolution (i.e., the ratio of detector diameter to mask/detector separation). The sensitivity, weight, and angular resolution are comparable to that of a coded aperture device, but with significantly less complexity. As the grid rotates, the transmission from a source is modulated on each detector between 0 and 100%. The count profile is cross-correlated with precalculated modulation profiles to produce an approximate source image. Deconvolution of this image with the known imager response can accurately resolve point sources and complex emissions. The appropriate deconvolution technique can achieve angular resolution better than the basic geometrical resolution of the instrument. A prototype RM developed at Louisiana State University features high sensitivity and energy resolution, functional angular resolution of 15, and a simple readout system. The detector array consists of 19 1.5 × 1 thick cerium-doped lanthanum bromide (LaBr3:Ce) crystals. LaBr3 produces significantly more light than other common scintillators, offering < 3% FWHM energy resolution at 662 keV. A grid spaced ~1.2 m from the detection plane with slat width 1.5 offers a 13.8° field of view. We present our reconstruction technique, deconvolution algorithms, and simulated and experimental imaging results.

Lanthanum bromide-based rotational modulation gamma ray imager

A time modulation imaging device uses a periodic structure which translates or rotates above one or few position-insensitive detectors. One common design, a Rotational Modulation Collimator (RMC) uses a bi-grid collimator which rotates above a single detector, and is able to attain very good angular resolution. The two grids cost sensitivity and weight, however, making the RMC unattractive for certain applications. A Rotational Modulator (RM) consists of a single grid of transparent and opaque slats of width a, above an array of several detectors with diameter d, subject to the constraint a = d. The sensitivity, weight, and angular resolution can be comparable to that of a coded aperture device. As the grid rotates, the transmission from a source is modulated on each detector between 0 and 100%. This count profile is cross-correlated with pre-calculated modulation profiles to produce an initial source image. Further processing of the image with a “cleaning” technique that incorporates information from the point-spread function can accurately resolve point sources. In an RMC imager recently constructed at LSU, LaBr3:Ce detectors are used, which produce significantly more light than other common scintillators, offering ≪ 3% FWHM energy resolution at 662 keV. The instrument features high sensitivity and energy resolution, angular resolution of 0.8° (1σ), and a simple readout system. The detector array consists of 19 1:5″×1″ thick LaBr3:Ce detectors in a concentric circular layout. A grid spaced ∼1.2 m from the detection plane with slat width 1:5″ offers a field of view radius of 6:9°. We present our reconstruction technique, cleaning algorithms, and imaging results for the RM prototype.

A High Sensitivity Gamma Ray Imager (HiSGRI) Based on Wavelength-Shifting Fiber Readout of LaBr3 Scintillators

Cerium-doped lanthanum bromide offers increased light yield and peak emission at shorter wavelengths compared to standard inorganic scintillators such as NaI or CsI. As a result, lanthanum bromide scintillators can be used as the basis of large scale hard X-ray/gamma ray imaging systems. By coupling a Ce:LaBr3 scintillator with orthogonal layers of wavelength-shifting fibers viewed by multianode photomultiplier tubes to provide x-y position together with an Anger camera array of large area photomultiplier tubes to measure energy, a detector can be constructed with millimeter-scale position resolution and energy resolution only somewhat less than that obtained with bulk scintillators. We present measurements of the light yields and position resolution achievable with such a system.