K. Watanabe

INTEGRAL SPI limits on electron-positron annihilation radiation from the galactic plane

The center of our Galaxy is a known strong source of electron-positron 511 keV annihilation radiation. Thus far, however, there have been no reliable detections of annihilation radiation outside of the central radian of our Galaxy. One of the primary objectives of the INTEGRAL (International Gamma-Ray Astrophysics Laboratory) mission, launched in 2002 October, is the detailed study of this radiation. The Spectrometer on INTEGRAL (SPI) is a high-resolution, coded-aperture gamma-ray telescope with an unprecedented combination of sensitivity, angular resolution, and energy resolution. We report results from the first 10 months of observation. During this period a significant fraction of the observing time was spent in or near the Galactic plane. No positive annihilation flux was detected outside of the central region ( l > 40°) of our Galaxy. In this paper we describe the observations and data analysis methods and give limits on the 511 keV flux.

The Diffuse Gamma-Ray Background from Supernovae

The diffuse extragalactic ?-ray background in the MeV region is believed to be due to photons from radioactivity produced in supernovae throughout the history of galaxies in the universe. In particular, ?-ray line emission from the decay chain 56Ni?56Co?56Fe provides the dominant photon source (Clayton & Silk). Although iron synthesis occurs in all types of supernovae, the contribution to the background is dominated by Type Ia events due to their higher photon escape probabilities. Estimates of the star formation history in the universe suggest a rapid increase by a factor ~10 from the present to a redshift zp ~1.5, beyond which it either remains constant or decreases slowly. Little is known about the cosmological star formation history for redshift exceeding z ~5. We integrate the observed star formation history to determine the cosmic ?-ray background (CGB) from the corresponding supernova rate history. In addition to ?-rays from short-lived radioactivity in Type Ia supernovae (SN Ias) and Type II, Ib, and Ic supernovae (SN IIs, SN Ibs, SN Ics) we also calculate the minor contributions from long-lived radioactivities (26Al, 44Ti, 60Co, and electron-positron pair annihilation). The time-integrated ?-ray spectrum of model W10HMM (Pinto & Woosley) was used as a template for Type II supernovae, and for SN Ias we employ model W7 (Nomoto et al.). Although progenitor evolution for Type Ia supernovae is not yet fully understood, various arguments suggest delays of order 1-2 Gyr between star formation and the production of SN Ias. The effect of this delay on the CGB is discussed. We emphasize the value of ?-ray observations of the CGB in the MeV range as an independent tool for studies of the cosmic star formation history. If the delay between star formation and SN Ia activity exceeds 1 Gyr substantially and/or the peak of the cosmic star formation rate occurs at a redshift much larger than unity, the ?-ray production of SN Ias would be insufficient to explain the observed CGB and a so far undiscovered source population would be implied. Alternatively, the cosmic star formation rate would have to be higher (by a factor 2-3) than commonly assumed, which is in accord with several upward revisions reported in the recent literature.

Constraints on Association of Single-Pulse Gamma-Ray Bursts and Supernovae

We explore the hypothesis, similar to one recently suggested by Bloom and colleagues, that some nearby supernovae are associated with smooth, single-pulse gamma-ray bursts, possibly having no emission above ~300 keV. We examine BATSE bursts with durations longer than 2 s, fitting those which can be visually characterized as single-pulse events with a lognormal pulse model. The fraction of events that can be reliably ascertained to be temporally and spectrally similar to the exemplar, GRB 980425—possibly associated with SN 1998bw—is 4/1573 or 0.25%. This fraction could be as high as 8/1573 (0.5%) if the dimmest bursts are included. Approximately 1.5% of bursts are morphologically similar to GRB 980425 but have emission above ~300 keV. A search of supernova catalogs containing 630 detections during BATSEs lifetime reveals only one burst (GRB 980425) within a 3 month time window and within the total 3 σ BATSE error radius that could be associated with a Type Ib/c supernova. Thus, we find no further evidence to support a single-pulse GRB and SN Ib/c connection. We also find no tendency for any set of single-pulse GRBs to fall near the supergalactic plane, whereas SNe of Type Ib/c do show this tendency—evidence that the two phenomena are not related.

The MeV cosmic gamma-ray background measured with SMM

Given the Solar Maximum Mission (SMM) Gamma-Ray Spectrometer’s (GRS) nine years of exposure and large field of view, its data contain a tremendously significant signal from the isotropic cosmic γ-ray background (CGB) in the energy range 0.3–8.0 MeV. We have extracted this signal by modeling its modulation by the Earth’s motion through the GRS field of view, along with several other background components, such as from the SAA and Earth Albedo γ rays. We can quantify the success of the technique and evaluate possible systematic errors because we have many independent measurements of the CGB, which should be constant in time, and because all known other background components have narrow lines that should not be present in the CGB. We thus obtain the definitive measurement of the CGB in this energy range to date. We compare the CGB spectrum with line emission from iron production in thermonuclear supernovae, and conclude that some other source(s) probably dominate the 1 MeV region.

Studying the High-Energy Gamma-Ray Sky with GLAST

Building on the success of the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory, the Gamma-ray Large Area Space Telescope (GLAST) will make a major step in the study of such subjects as blazars, gamma-ray bursts, the search for dark matter, supernova remnants, pulsars, diffuse radiation, and unidentified high-energy sources. The instrument will be built on new and mature detector technologies such as silicon strip detectors, low-power low-noise LSI, and a multilevel data acquisition system. GLAST is in the research and development phase, and one full tower (of 25 total) is now being built in collaborating institutes. The prototype tower will be tested thoroughly at SLAC in the fall of 1999.

Gamma‐Ray Bursts and Cosmic Radiation Backgrounds

If gamma‐ray bursts trace the cosmic star formation rate to large redshifts, their prompt and delayed emissions provide new tools for early universe cosmology. In addition to probing the intervening matter via absorption lines in the optical band, GRB continua also contribute to the evolving cosmic radiation background. We discuss the contribution of GRBs to the high‐energy background, and the effect pair creation off low‐energy background photons has on their observable TeV spectra.