Compton Gamma Ray Observatory Research Articles

The GUSBAD Catalog of Gamma‐Ray Bursts

The GUSBAD catalog of gamma-ray bursts (GRBs) is based on archival BATSE DISCLA data covering the full 9.1 years of the Compton Gamma Ray Observatory mission. The catalog contains 2204 GRBs, including 589 bursts not listed in the Current BATSE Burst Catalog. The GUSBAD catalog is uniform in the sense that the detection criteria are the same throughout and that the properties given in the catalog are available for every burst. The detection and the derivation of the properties of the GRBs were carried out automatically. This makes the GUSBAD catalog especially suitable for statistical work and simulations, such as used in the derivation of V/Vmax. We briefly touch upon a potential problem in defining a GRB duration that is physically meaningful.

Distribution of Spectral Lags in Gamma‐Ray Bursts

Using the data acquired in the Time To Spill (TTS) mode for long gamma-ray bursts (GRBs) collected by the Burst and Transient Source Experiment on board the Compton Gamma Ray Observatory (BATSE/CGRO), we have carefully measured spectral lags in time between the low (25-55 keV) and high (110-320 keV) energy bands of individual pulses contained in 64 multi-peak GRBs. We find that the temporal lead by higher-energy gamma-ray photons (i.e., positive lags) is the norm in this selected sample set of long GRBs. While relatively few in number, some pulses of several long GRBs do show negative lags. This distribution of spectral lags in long GRBs is in contrast to that in short GRBs. This apparent difference poses challenges and constraints on the physical mechanism(s) of producing long and short GRBs. The relation between the pulse peak count rates and the spectral lags is also examined. Observationally, there seems to be no clear evidence for systematic spectral lag-luminosity connection for pulses within a given long GRB.

The Burst and Transient Source Experiment (BATSE) Earth Occultation Catalog of Low‐Energy Gamma‐Ray Sources

The Burst and Transient Source Experiment (BATSE), aboard the Compton Gamma Ray Observatory (CGRO), provided a record of the low-energy gamma-ray sky (approx. 20-1000 keV) between 1991 April and 2000 May (9.1 yr). BATSE monitored the high-energy sky using the Earth occultation technique (EOT) for point sources whose emission extended for times on the order of the CGRO orbital period (approx. 92 min) or greater. Using the EOT to extract flux information, a catalog of sources using data from the BATSE Large Area Detectors has been prepared. The first part of the catalog consists of results from the all-sky monitoring of 58 sources, mostly Galactic, with intrinsic variability on timescales of hours to years. For these sources, we have included tables of flux and spectral data, and outburst times for transients. Light curves (or flux histories) have been placed on the World Wide Web. We then performed a deep sampling of these 58 objects, plus a selection of 121 more objects, combining data from the entire 9.1 yr BATSE data set. Source types considered were primarily accreting binaries, but a small number of representative active galaxies, X-ray-emitting stars, and supernova remnants were also included. The sample represents a compilation of sources monitored and/or discovered with BATSE and other high-energy instruments between 1991 and 2000, known sources taken from the HEAO 1 A-4 and Macomb & Gehrels catalogs. The deep sample results include definite detections of 83 objects and possible detections of 36 additional objects. The definite detections spanned three classes of sources: accreting black hole and neutron star binaries, active galaxies, and Supernova remnants. The average fluxes measured for the fourth class, the X-ray emitting stars, were below the confidence limit for definite detection.

A Study of Prompt Emission Mechanisms in Gamma‐Ray Bursts

The principal paradigm for the generation of the nonthermal particles that are responsible for the prompt emission of gamma-ray bursts invokes diffusive shock acceleration at shocks internal to the dynamic ultrarelativistic outflow. This paper explores expectations for burst emission spectra arising from shock acceleration theory in the limit of particles cooling much slower than their acceleration. Parametric fits to burst spectra obtained by the Compton Gamma Ray Observatory (CGRO) are explored for the cases of the synchrotron, inverse Compton, and synchrotron self-Compton (SSC) radiation mechanisms, using a linear combination of thermal and nonthermal electron populations. These fits demand that the preponderance of electrons that are responsible for the prompt emission reside in an intrinsically nonthermal population, strongly contrasting particle distributions obtained from acceleration simulations. This implies a potential conflict for acceleration scenarios in which the nonthermal electrons are drawn directly from a thermal gas, unless radiative efficiencies only become significant at highly superthermal energies. It is also found that the CGRO data preclude effective spectroscopic discrimination between the synchrotron and inverse Compton mechanisms. This situation may be resolved with future missions probing gamma-ray bursts, namely Swift and GLAST. However, the SSC spectrum is characteristically too broad near the νFν peak to viably account for bursts such as GRB 910601, GRB 910814, and GRB 990123. We conclude that the SSC mechanism may be generally incompatible with differential burst spectra steeper than around E-2.5 above the peak, unless the synchrotron component is strongly self-absorbed.

Highly Structured Models for Spectral Analysis in High-Energy Astrophysics

The Chandra X-Ray Observatory, launched by the space shuttle Columbia in July 1999, has taken its place with the Hubble Space Telescope, the Compton Gamma Ray Observatory and the Spitzer Infrared Space Telescope in NASA’s fleet of state of the art space-based Great Observatories. As the world’s premier X-ray observatory, Chandra gives astronomers a powerful tool to investigate black holes, exploding stars and colliding galaxies in the hot turbulent regions of the universe. Chandra uses four pairs of ultra-smooth high-resolution mirrors and efficient X-ray photon counters to produce images at least 30 times sharper than any previous X-ray telescope. Unlocking the information in these images, however, requires subtle statistical analysis; currently popular statistical methods typically involve Gaussian approximations (e.g., minimum χ2 fitting), which are not justifiable for the high-resolution low-count data. In this article, we employ modern Bayesian computational techniques (e.g., expectation–maximization-type algorithms, the Gibbs sampler and Metropolis–Hastings) to fit new highly structured models that account for the Poisson nature of photon counts, background contamination, image blurring due to instrumental constraints, photon absorption, photon pileup and source features such as spectral emission lines and absorption features. This application demonstrates the flexibility and power of modern Bayesian methodology and algorithms to handle highly structured models that are convolved with complex data collection mechanisms involving nonignorable missing data.

GLAST: GeV astronomy in a multiwavelength context

The GLAST Large Area Telescope (LAT), successor to Energetic Gamma-ray Experiment Telescope (EGRET) on the Compton Observatory, will play an important role in multiwavelength studies during the second half of this decade. Operating at energies between 20 MeV and greater than 300 GeV with sensitivity 30 or more times greater than that of EGRET, the LAT will offer good spatial and time resolution over a large (>2 sr) field of view. The LAT will bring insight to the whole range of high-energy γ-ray phenomena, including bursts, active galactic nuclei, pulsars, supernova remnants, diffuse emission and unidentified sources. In essentially all cases, the maximum scientific return will come from coordinated (although not necessarily simultaneous) multiwavelength observations. Particularly with its planned scanning mode of operation, GLAST will have full sky coverage on relatively short time scales. The LAT team looks forward to cooperating with observers at other wavelengths.

Stacking Searches for Gamma‐Ray Emission above 100 MeV from Radio and Seyfert Galaxies

The EGRET telescope on board Compton Gamma Ray Observatory detected more than 60 sources of high-energy gamma radiation associated with active galactic nuclei (AGNs). All but one of those belong to the blazar subclass; the only exception is the nearby radio galaxy Centaurus A. Since there is no obvious reason other than proximity to expect Cen A to be the only nonblazar AGN emitting in high-energy gamma rays, we have utilized the stacking technique to search for emission above 100 MeV from two nonblazar AGN subclasses, radio galaxies and Seyfert galaxies. Maps of gamma-ray counts, exposure, and diffuse background have been created, then co-added in varying numbers based on sorts by redshift, 5 GHz flux density, and optical brightness, and finally tested for gamma-ray emission. No detection significance greater than 2 σ has been found for any subclass, sorting parameter, or number of objects co-added. Monte Carlo simulations have also been performed to validate the technique and estimate the significance of the results.

Gamma-ray line emission from galaxy clusters

We report first results of a study dedicated to a search for γ-ray line emission from rich galaxy clusters, finding traces of emission from the region of Virgo and Coma near the Northern Galactic Pole (NGP). The study is based on the 6 years of COMPTEL data collected during the extended observational programme of the Compton Gamma Ray Observatory (CGRO) monitoring of the Virgo and 3C273 region. We point out the advantages of performing this study using an energy range that contains the strongest excitation γ-ray lines of 16 O at 6.13, 6.9 and 7.1 MeV, as well as lines at 6.175 MeV ( 15 O*) and 6.322 MeV ( 15 N*) of the spallation products of the cosmic ray interactions with 16 O. We discuss a generation mechanism for γ-ray line emission in merging clusters via excitation of the abundant element(s) of the intracluster matter at the merger shock by the subrelativistic protons accelerated to energies of ∼ 10 MeV to ∼ 100 MeV/n in the merger process.

Did EGRET detect distant supernova remnants?

It might be thought that supernova remnants (SNRs) more distant than a few kiloparsec from Earth could not have been detected by the high-energy γ-ray experiment EGRET on board of NASA's Compton Gamma-Ray Observatory. This work analyzes the observational status of this statement in the light of new CO studies of SNRs.

MIRAX: a Brazilian X-ray astronomy satellite mission

We describe the “Monitor e Imageador de Raios-X” (MIRAX), an X-ray astronomy satellite mission proposed by the high-energy astrophysics group at the National Institute for Space Research (INPE) in Brazil to the Brazilian Space Agency. MIRAX is an international collaboration that includes, besides INPE, the University of California San Diego, the University of Tübingen in Germany, the Massachusetts Institute of Technology and the Space Research Organization Netherlands. The payload of MIRAX will consist of two identical hard X-ray cameras (10–200 keV) and one soft X-ray camera (2–28 keV), both with angular resolution of ∼5–7 ′. The basic objective of MIRAX is to carry out continuous broadband imaging spectroscopy observations of a large source sample (∼9 months/yr) in the central Galactic plane region. This will allow the detection, localization, possible identification, and spectral/temporal study of the entire history of transient phenomena to be carried out in one single mission. MIRAX will have sensitivities of ∼ 5 mCrab/day in the 2–10 keV band (∼2 times better than the All Sky Monitor on Rossi X-ray Timing Explorer) and 2.6 mCrab/day in the 10–100 keV band (∼40 times better than the Earth Occultation technique of the Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory). The MIRAX spacecraft will weigh about 200 kg and is expected to be launched in a low-altitude (∼600 km) circular equatorial orbit around 2007/2008.

Pulsar populations and unidentified γ-ray sources

The EGRET telescope on the Compton Gamma-ray Observatory detected over 200 sources and the majority of these are still unidentified. At least three subpopulations of EGRET sources have been associated with the Galaxy: bright sources lying along the Galactic plane, weaker sources spatially correlated with the Gould Belt and a high-latitude, halo population. Many of these sources may be pulsars and there are more than two-dozen radio pulsars in or near EGRET source error boxes, most of them recently discovered in the Parkes Multibeam Survey. We present results from several population synthesis studies of pulsars in the Galaxy, which predict the number of pulsars detected by γ-ray and radio surveys assuming models for the high-energy emission beam and its relation to the radio beam. Future γ-ray pulsar detections by AGILE and GLAST together with the recent large rise in the radio pulsar population will give greatly improved statistics. The relative numbers of radio and γ-ray pulsars detected in the plane and in the Gould Belt populations will provide an important discriminator between models of high-energy emission and will profoundly affect the numbers and birth rate of pulsars in the Galaxy.

Perspectives on MeV astronomy instrumentation

Although remarkable progress has been made in gamma ray astrophysics recently (witness the recent advances in understanding gamma ray bursts) many of the early scientific objectives remain unfulfilled. This is due primarily to the rather modest improvement in sensitivity (several hundred) from the pioneering balloon observations in the 1960s to the Compton Observatory and INTEGRAL missions. Important objectives related to supernovae, novae and the search for line emissions from compact objects will benefit from another factor of 100 improvement in sensitivity. Several instrumental techniques to achieving dramatic improvements will be discussed. These include advanced Compton telescopes using detectors with high spectral and spatial resolution, and concepts for Laue collectors and focusing gamma ray telescopes.

The Gamma‐Ray Blazar PKS 0208−512 from MeV to GeV Energies

We present a comprehensive report on the high-energy properties of the γ-ray blazar PKS 0208-512 as observed with EGRET and the Imaging Compton Telescope (COMPTEL) of the Compton Gamma Ray Observatory (CGRO), spanning the entire nine-year mission (1991-2000). More high-significance detections were recorded with EGRET of PKS 0208-512 (nine at greater than 6 σ confidence) than for any other γ-ray blazar. On timescales of weeks to months, PKS 0208-512 is one of the five most variable of the γ-ray blazars. We find a statistically significant correlation between intensity and spectral index for PKS 0208-512, and this source constitutes the best example of spectral hardening with intensity within the EGRET database of blazar observations. Under the assumption of isotropic Eddington-limited emission, we infer a black hole mass of approximately 6 × 107 M☉ at the nucleus of PKS 0208-512. Gamma-ray transparency arguments, however, clearly support the hypothesis of relativistic beaming in PKS 0208-512, with a minimum relativistic Doppler factor for the jet of δ 3.2 at GeV energies. We have also carried out a comprehensive reanalysis of the COMPTEL data associated with PKS 0208-512 at MeV energies and reexamined the earlier reports of excess 1-3 MeV emission from this source. For individual CGRO viewing periods, we find upper limits only for MeV emission from PKS 0208-512. Our reanalysis of the original COMPTEL data for the period 1993 May-June leads us to conclude that the significance of the original reported detection is marginal at best and that there is no compelling evidence at present for the existence of a distinct flaring state for PKS 0208-512 at MeV energies.

Absolute timing with IBIS, SPI and JEM-X aboard INTEGRAL

We have verified the absolute timing capabilities of the high-energy instruments aboard INTEGRAL, i.e. the imager IBIS, the spectrometer SPI and the X-ray monitor JEM-X. Calibration observations of the Crab, contemporaneous with the Rossi X-ray Timing Explorer (RXTE), have been used to measure the absolute phase of the main pulse of the Crab pulse profile using the same Jodrell Bank radio ephemeris. The three INTEGRAL instruments and RXTE give within the statistical and systematic uncertainties consistent results: The X-ray main pulse is leading the radio pulse by . Also the shapes of the X-ray pulse profiles as measured by the different instruments are fully consistent with each other. In addition, we present the first measurement of the absolute phase of the main pulse at γ-ray energies above 30 MeV using data from the EGRET instrument aboard the Compton Gamma-Ray Observatory: The γ-ray main pulse is leading the radio one by , consistent with the value for the X-ray main pulse. Comparing absolute arrival times at multiple frequencies gives important constraints to models explaining the production of non-thermal emission in magnetospheres of rotation powered neutron stars.

An upper limit of the number and energy of electrons accelerated at an extended coronal shock wave

Type II radio emission is the most direct signature of extended shock waves in the solar corona, and a prime example of electron acceleration at a shock. Yet, the energetics of these electrons is poorly known. On 1996 August 19 and 1997 October 07 type II bursts were observed in association with flares behind the solar limb. While Earth-orbiting X-ray detectors aboard GOES and the Compton Gamma-Ray Observatory did not register emission above background, theUlysses spacecraft, which had the candidate active region in its field of view, saw a major burst during the 1996 event. We use this peculiar configuration to infer the energetics of electrons accelerated in the low corona from the Ulysses observations, and to derive upper limits of the number and energy content of shock-accelerated electrons from the thresholds of the Earth-orbiting detectors. The inferred upper limit is similar to electron numbers estimated for large particle events at 1 AU, but the fluxes and energy fluxes of shock-accelerated electrons are much smaller than those required to produce a conspicuous hard X-ray burst.

Correlated Radio:X‐Ray Emission in the Hard States of Galactic Microquasars

We present results of our study of correlated radio and X-ray emission in two black hole candidates and Galactic microquasars GRS 1915+105 and Cyg X-1 in their steady long-term hard states, along with Cyg X-3 (using data obtained from the Rossi X-Ray Timing Explorer all-sky monitor [RXTE-ASM], Compton Gamma Ray Observatory Burst and Transient Source Experiment [CGRO-BATSE], and Green Bank Interferometer [GBI]). We detect a pivotal behavior in the X-ray spectrum of GRS 1915+105, correlated to the radio emission. Similar to the results obtained for Cyg X-3, the flux of X-rays softer than the pivoting point correlates with the radio emission, while the corresponding harder X-ray flux anticorrelates with both the radio and the softer X-ray emission, in this state. We examine all the previously reported correlations of X-ray properties with the radio emission in Galactic microquasars and argue that these are consistent with a general picture where a spectral pivoting is a common feature in these sources with the shape of the spectrum determining the flux of radio emission, during the hard states. We also detect a general monotonic increase in the radio emission of these sources with the soft X-ray emission spanning about 5 orders of magnitude. We qualitatively explain these findings with a two-component advective flow model where the location of a boundary layer between the thin disk and the Comptonizing region determines the spectral shape and also the amount of outflow.

Gamma‐Ray Spectra and Variability of the Crab Nebula Emission Observed by BATSE

We report ~600 days of BATSE earth occultation observations of the total gamma-ray (30 keV to 1.7 MeV) emission from the Crab Nebula between 1991 May 24 (TJD 8400) and 1994 October 3 (TJD 9628). Light curves from 35-100, 100-200, 200-300, 300-400, 400-700, and 700-1000 keV show that positive fluxes were detected by BATSE in each of these six energy bands at significances of approximately 31, 20, 9.2, 4.5, 2.6, and 1.3 σ, respectively, per day. We also observed significant flux and spectral variations in the 35-300 keV energy region, with timescales of days to weeks. The spectra below 300 keV, averaged over typical Compton Gamma Ray Observatory viewing periods of 6-13 days, can be well described by a broken power law with average indices of ~2.1 and ~2.4, varying around a spectral break at ~100 keV. Above 300 keV, the long-term averaged spectra, averaged over three 400 day periods (TJD 8400-8800, 8800-9200, and 9200-9628, respectively), are well represented by the same power law with index of ~2.34 up to ~670 keV, plus a hard spectral component extending from ~670 keV to ~1.7 MeV, with a spectral index of ~1.75. The latter component could be related to a complex structure observed by COMPTEL in the 0.7-3 MeV range. Above 3 MeV, the extrapolation of the power-law continuum determined by the low-energy BATSE spectrum is consistent with fluxes measured by COMPTEL in the 3-25 MeV range and by EGRET from 30-50 MeV. We interpret these results as synchrotron emission produced by the interaction of particles ejected from the pulsar with the field in different dynamical regions of the nebula system, as observed recently by the Hubble Space Telescope, XMM-Newton, and Chandra.

Neutrinos from individual gamma-ray bursts in the BATSE catalog

We estimate the neutrino emission from individual γ-ray bursts observed by the BATSE detector on the Compton Gamma-Ray Observatory. Neutrinos are produced by photoproduction of pions when protons interact with photons in the region where the kinetic energy of the relativistic fireball is dissipated allowing the acceleration of electrons and protons. We also consider models where neutrinos are predominantly produced on the radiation surrounding the newly formed black hole. From the observed redshift and photon flux of each individual burst, we compute the neutrino flux in a variety of models based on the assumption that equal kinetic energy is dissipated into electrons and protons. Where not measured, the redshift is estimated by other methods. Unlike previous calculations of the universal diffuse neutrino flux produced by all γ-ray bursts, the individual fluxes (compiled at http://www.arcetri.astro.it/~dafne/grb/) can be directly compared with coincident observations by the AMANDA telescope at the South Pole. Because of its large statistics, our predictions are likely to be representative for future observations with larger neutrino telescopes.

GLAST large area telescope — daily survey of high energy sky

GLAST Large Area Telescope was proposed to NASA in 1999 as a follow-up of EGRET on-board Compton Gamma-Ray Observatory by an international collaboration. The proposal has been approved as a part of the GLAST observatory mission in its capability to explore a wide range of astrophysics with 5–40 times higher sensitivity and extended energy coverage (20MeV to 300GeV) than EGRET. The instrument consists of 16 towers of e+e- pair tracker, 16 blocks of segmented electro-magnetic calorimeter, and a set of anti-coicidence plastic scintillator tiles covering the tracker towers. It will have 5–10 times larger on-axis effective area, 6 times wider field-of-view (FOV), and up to 5 times better angular resolution when compared with EGRET. The Large Area Telescope will cover about 40% of the sky above the Earth's horizon in its FOV at any given time and will scan nearly the entire Universe every orbit (∼ 90min): about 20% of Gamma-Ray Bursts will be observed from the onset of the bursts to the initial after-glow phase; all longer-lasting transients and variabilities will be detected daily at the improved sensitivity. The instrument has been prototyped twice between 1995 and 2001, designed almost to the Flight Model by the international collaboration of the US (NASA and DoE), France, Italy, Japan, and Sweden. The first prototype consisted of one tower of e+e- pair trackers, one block of segmented calorimeters and a smaller set of anti-coicidence plastic scintillator tiles (Beam Test Engineering Model, BTEM), which was put into e +, p, and γ beams at SLAC in the winter of 1999–2000. It was subsequently modified for a balloon experiment (Balloon Flight Engineering Model, BFEM) and flown at Palestine, Texas in August 2001. Data collected in the test experiments have been analyzed and compared with predictions of computer simulation codes such as Geant4. These studies have confirmed validity of the basic design, brought up a few issues for further improvement, and gathered data on the cosmic-ray background expected in the orbit. All subsystems of GLAST-LAT are scheduled to be completed and come to SLAC in 2005 for integration. The integrated instrument will go through a set of tests before being integrated onto the spacecraft. The spacecraft will be put into a Low Earth Orbit (altitude ∼ 450–550km) in the fall of 2006 and the LAT will begin collecting data after the commissiong phase. The astronomical gamma-ray data collected in the observation will be processed by the LAT team and archived for public use GLAST Science Support Center at Goddard Space Center in Greenbelt, Maryland.

Testing of Inverse Compton Models for Active Galactic Nuclei with Gamma‐Ray and Radio Observations

We compare the Metsahovi 22 and 37 GHz total flux density variation data of active galactic nuclei with EGRET (on board the Compton Gamma Ray Observatory) gamma-ray observations from Cycles 1-4. Statistical analysis reveals that there is a genuine connection between the high-frequency radio properties of the sources, e.g., the type of the source, the phase of the radio flare, the associated radio variability brightness temperature, and the gamma-ray emission. Typically the most likely gamma-ray emitter is a high optical polarization quasar with a large Doppler boosting factor. The highest levels of gamma-ray emission are observed during the initial (or peak) stages of high-frequency radio flares. BL Lac objects are much weaker gamma-ray emitters, with no clear correlation between radio and gamma variations. We calculate the distance of the gamma-ray-emitting region from the core in quasars and obtain an average value of 4.9 pc, well downstream from the accretion disk or the broad-line region. This indicates that the strongest time-dependent gamma-ray emission of quasars originates in the relativistic jet, within the same shocks that produce the synchrotron flares at radio frequencies induced by the synchrotron self-Compton mechanism. Weaker gamma-ray emission does not seem to be associated with shocks and may be caused by other mechanisms, such as external Compton processes.