Search

One of the scientific objectives of NASA’s Fermi Gamma‐ray Space Telescope is the study of Gamma‐Ray Bursts (GRBs). The Fermi Gamma‐Ray Burst Monitor (GBM) was designed to detect and localize bursts for the Fermi mission. By means of an array of 12 NaI(Tl) (8 keV to 1 MeV) and two BGO (0.2 to 40 MeV) scintillation detectors, GBM extends the energy range (20 MeV to >300 GeV ) of Fermi’s main instrument, the Large Area Telescope (LAT), into the traditional range of current GRB databases. The physical detector response of the GBM instrument to GRBs is determined with the help of Monte Carlo simulations, which are supported and verified by on‐ground individual detector calibration measurements. We present the principal instrument properties, which have been determined as a function of energy and angle, including the channel‐energy relation, the energy resolution and the effective area.

The Fermi Gamma‐ray Space Telescope launched on June 11, 2008 carries two experiments onboard—the Large Area Telescope (LAT) and the Gamma‐ray Burst Monitor (GBM). The primary mission of the GBM instrument is to support the LAT in observing γ‐ray bursts (GRBs) by providing low‐energy measurements with high temporal and spectral resolution as well as rapid burst locations over a large field‐of‐view (⩾8 sr). The GBM will complement the LAT measurements by observing GRBs in the energy range 8 keV to 40 MeV, the region of the spectral turnover in most GRBs. The GBM detector signals are processed by the onboard digital processing unit (DPU). We describe some of the hardware features of the DPU and its expected limitations during intense triggers.

We present the observations of GRB090510 performed by the Fermi Gamma-Ray Space Telescope and the Swift observatory. This is a bright, short burst that shows an extended emission detected in the GeV range. Furthermore, its optical emission initially rises, a feature so far observed only in long bursts, while the X-ray flux shows an initial shallow decrease, followed by a steeper decay. This exceptional behavior enables us to investigate the physical properties of the GRB outflow, poorly known in short bursts. We discuss internal shock and external shock models for the broadband energy emission of this object., Comment: Comments: Submitted to ApJ Letters. Contact Authors: Massimiliano De Pasquale (mdp@mssl.ucl.ac.uk), Mathew Page (mjp@mssl.ucl.ac.uk), Kenji Toma (toma@astro.psu.edu), Veronique Pelassa (pelassa@lpta.in2p3.fr). Minor change in the authorlist

The GLAST Burst Monitor (GBM) is designed to provide wide field of view observations of gamma‐ray bursts and other fast transient sources in the energy range 10 keV to 30 MeV. The GBM is composed of several unshielded and uncollimated scintillation detectors (twelve NaI and two BGO) that are widely dispersed about the GLAST spacecraft. As a result, reconstructing source locations, energy spectra, and temporal properties from GBM data requires detailed knowledge of the detectors' response to both direct radiation as well as that scattered from the spacecraft and Earth's atmosphere. This full GBM instrument response will be captured in the form of a response function database that is derived from computer modeling and simulation. The simulation system is based on the GEANT4 Monte Carlo radiation transport simulation toolset.

The region of the LMC containing SN 1987A was observed during a balloon flight of a hard-X-ray telescope on April 8-10, 1988. Significant continuum emission was detected in the 45-400-keV range and attributed to SN 1987A. Compared to an observation with the same instrument in October 1987, the source intensity decreased by about 50 percent, while the spectral shape remained qualitatively the same. This may represent the first clear indication that the hard-X-ray emission is entering the declining phase expected as the ejecta become optically thin to the Co-56 gamma rays.

The GLAST Burst Monitor (GBM) is designed to provide wide field of view observations of gamma‐ray bursts and other fast transient sources in the energy range 10 keV to 30 MeV. The GBM is composed of several unshielded and uncollimated scintillation detectors (twelve NaI and two BGO) that are widely dispersed about the GLAST spacecraft. As a result, reconstructing source locations, energy spectra, and temporal properties from GBM data requires detailed knowledge of the detectors’ response to both direct radiation as well as that scattered from the spacecraft and Earth’s atmosphere. This full GBM instrument response will be captured in the form of a response function database that is derived from computer modeling and simulation. The simulation system is based on the GEANT4 Monte Carlo radiation transport simulation toolset, and is being extensively validated against calibrated experimental GBM data. We discuss the architecture of the GBM simulation and modeling system and describe how its products will be used for analysis of observed GBM data. Companion papers describe the status of validating the system.

The GLAST Burst Monitor (GBM) comprises 12 NaI and 2 BGO detectors dispersed about the GLAST spacecraft. The GBM instrument simulation software must generate an accurate response function database for all detectors in their flight configuration to optimize the mission science return. Before science analysis codes use the response database, we must confirm that our simulation codes and models can reproduce laboratory observations. To validate the simulation effort, Monte Carlo results are compared to calibrated laboratory measurements collected with a variety of radiation sources.

The GLAST Burst Monitor (GBM) consists of 12 NaI detectors and 2 BGO detectors mounted on two sides of the spacecraft. This provides detection capability for energies between 10 keV and 30 MeV, and sensitivity to all directions in the sky. Extensive measurements using radioactive sources have been performed on the isolated detectors. Simulations of these measurements using the GEANT4 Monte Carlo radiation transport simulations toolset have also been preformed. For gamma‐ray burst the spacecraft itself will have a significant impact on the detector response, therefore the final stage of validations involves measuring the detector response to radioactive sources after the spacecraft is fully assembled, and validating the results using simulation. Details of these simulations will be given in this paper.

We report on how the the GLAST Burst Monitor (GBM) Flight Software will detect gamma‐ray rate increases, a process known as “triggering”, and on how the Flight Software will locate and classify the causes of the triggers.

One of the scientific goals of the Large‐Area Telescope (LAT) on GLAST is the study of gamma‐ray bursts (GRBs) in the energy range from ∼20 MeV to ∼300 GeV. In order to extend the energy measurement towards lower energies a secondary instrument, the GLAST Burst Monitor (GBM), will measure GRBs from ∼10 keV to ∼30 MeV and will therefore allow the investigation of the relation between the keV and the MeV‐GeV emission from GRBs over more than six energy decades. These unprecedented measurements will furthermore permit the exploration of the unknown aspects of the high‐energy burst emission. The status of the GBM project approximately one year before launch is reported here.

The selection of the GLAST burst monitor (GBM) by NASA will allow the investigation of the relation between the keV and the MeV-GeV emission from gamma-ray bursts. The GBM consists of 12 NaI and 2 BGO crystals allowing a continuous measurement of the energy spectra of gamma-ray bursts from ~5 keV to \~30 MeV. One feature of the GBM is its high time resolution for time-resolved gamma-ray spectroscopy. Moreover the arrangement of the NaI crystals allows a rapid on-board location (, 4 pages with 2 figures

The MEGA mission would enable a sensitive all-sky survey of the medium-energy ?-ray sky (0.3-50 MeV). This mission will bridge the huge sensitivity gap between the COMPTEL and OSSE experiments on the Compton Gamma Ray Observatory, the SPI and IBIS instruments on INTEGRAL and the visionary ACT mission. It will, among other things, serve to compile a much larger catalog of sources in this energy range, perform far deeper searches for supernovae, better measure the galactic continuum emission as well as identify the components of the cosmic diffuse emission. The large field of view will allow MEGA to continuously monitor the sky for transient and variable sources. It will accomplish these goals with a stack of Si-strip detector (SSD) planes surrounded by a dense high-Z calorimeter. At lower photon energies (below ~30 MeV), the design is sensitive to Compton interactions, with the SSD system serving as a scattering medium that also detects and measures the Compton recoil energy deposit. If the energy of the recoil electron is sufficiently high (> 2 MeV), the track of the recoil electron can also be defined. At higher photon energies (above ~10 MeV), the design is sensitive to pair production events, with the SSD system measuring the tracks of the electron and positron. We will discuss the various types of event signatures in detail and describe the advantages of this design over previous Compton telescope designs. Effective area, sensitivity and resolving power estimates are also presented along with simulations of expected scientific results and beam calibration results from the prototype instrument.

One of the scientific objectives of the GLAST mission is the study of gamma-ray bursts (GRBs) which will be measured by the Large-Area Telescope, the main instrument of GLAST, in the energy range from ~20 MeV to ~300 GeV. In order to extend the energy measurement towards lower energies a secondary instrument, the GLAST Burst Monitor (GBM) will measure GRBs from ~10 keV to ~25 MeV and will thus allow the investigation of the relation between the keV and the MeV-GeV emission from GRBs. The GBM consists of 12 circular NaI crystal discs and 2 cylindrical BGO crystals. The NaI crystals are optimized for gamma radiation from ~10 keV to ~1 MeV and the BGO crystals from ~150 keV to ~25 MeV. The NaI crystals are oriented in such a way that the measured relative counting rates allow a rapid determination of the position of a gamma-ray burst within a wide FoV of ~8.6 sr. This position will be communicated within seconds to the LAT which may then be reoriented to observe the long-lasting high-energy gamma-ray emission from GRBs. This will allow the exploration of the unknown aspects of the high-energy burst emission and their connection with the well-known low-energy emission. Another important feature of the GBM is its high time resolution of ~10 microseconds for time-resolved gamma-ray spectroscopy.

Only two different types of gamma-ray transient sources are presently known: over one thousand Gamma-Ray Bursters (GRBs) and only three Soft Gamma-Ray repeaters (SGRs). The latter are distinguished by their propensity for recurrent burst behaviour, in contrast to the nonrepeating GRB sources. Recurrent emission from one of the repeaters, SGR1900 + 14, has been detected earlier by the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory. Here we report renewed burst activity from SGR1806 - 20, the most prolific of the three known SGRs. This detection of reactivation of this source has been rapidly followed by identification of an X-ray counterpart, which also coincides with a compact radio source now identified as a plerionic (pulsar-powered) supernova remnant. In combination, these results are leading to a convergence of ideas about the nature of SGRs, which can now be firmly identified as neutron stars. That BATSE has detected no new sources in its two and a half years of operation indicates that SGRs are rare in our Galaxy.

The highly transient nature of cosmic low-energy gamma-ray sources offers unique insight into the astrophysics of extreme processes. Sensitive, long-term, all-sky monitoring is required to make sense of the variable emission. Instrumentation for such observations must have the combination of large effective collection area, wide field-of-view and good angular resolution. Recent advances in position-sensitive detectors and associated electronics make it possible to consider a multi-scatter Compton telescope, combined with a coded mask, as an all-sky monitor in the energy range /spl sim/10 keV to a few MeV. We present simulations of various configurations of such an instrument and show that it can provide a substantial improvement over other sky monitor strategies in this energy regime.

The study of highly transient low-energy gamma-ray sources such as active galaxies, X-ray binaries, soft gamma-ray repeaters and gamma-ray bursts offers unique insight into the astrophysics of extreme physical processes. Sensitive all-sky monitoring of low-energy gamma rays is therefore highly desirable. Development of next-generation instrumentation is technically challenging since a combination of large effective collection area, wide field-of-view and good angular resolution is required. Recent advances in position-sensitive detectors and associated electronics have made it possible to consider a multi-scatter Compton telescope as an all-sky monitor for energies in the range /spl sim/30 keV to a few MeV. We have performed simulations of various configurations of such an instrument and find that it can provide a substantial improvement over current sky monitors in this energy regime.

Unsupervised pattern recognition algorithms support the existence of three gamma-ray burst classes; Class I (long, large fluence bursts of intermediate spectral hardness), Class II (short, small fluence, hard bursts), and Class III (soft bursts of intermediate durations and fluences). The algorithms surprisingly assign larger membership to Class III than to either of the other two classes. A known systematic bias has been previously used to explain the existence of Class III in terms of Class I; this bias allows the fluences and durations of some bursts to be underestimated (Hakkila et al., ApJ 538, 165, 2000). We show that this bias primarily affects only the longest bursts and cannot explain the bulk of the Class III properties. We resolve the question of Class III existence by demonstrating how samples obtained using standard trigger mechanisms fail to preserve the duration characteristics of small peak flux bursts. Sample incompleteness is thus primarily responsible for the existence of Class III. In order to avoid this incompleteness, we show how a new dual timescale peak flux can be defined in terms of peak flux and fluence. The dual timescale peak flux preserves the duration distribution of faint bursts and correlates better with spectral hardness (and presumably redshift) than either peak flux or fluence. The techniques presented here are generic and have applicability to the studies of other transient events. The results also indicate that pattern recognition algorithms are sensitive to sample completeness; this can influence the study of large astronomical databases such as those found in a Virtual Observatory., 29 pages, 6 figures, 3 tables, Accepted for publication in The Astrophysical Journal

Three short very soft gamma-ray (SGR) transient events from a location consistent with that of the SGR 1900 + 14, first described by Mazets et al. (1979), were detected by the Burst and Transient Source Experiment. The results of observations of the temporal and spectral properties of the SGR 1900 + 14 suggest that the SGR phase lasts at least 13 years, lending support to the suggestion by Kouveliotou et al. (1987) and Fishman et al. (1989) that SGRs are related to galactic (possibly population I) objects, perhaps neutron stars.

Time-resolved spectroscopy is performed on eight bright, long gamma-ray bursts (GRBs) dominated by single emission pulses that were observed with the {\it Fermi Gamma-ray Space Telescope}. Fitting the prompt radiation of GRBs by empirical spectral forms such as the Band function leads to ambiguous conclusions about the physical model for the prompt radiation. Moreover, the Band function is often inadequate to fit the data. The GRB spectrum is therefore modeled with two emission components consisting of optically thin nonthermal synchrotron radiation from relativistic electrons and, when significant, thermal emission from a jet photosphere, which is represented by a blackbody spectrum. To produce an acceptable fit, the addition of a blackbody component is required in 5 out of the 8 cases. We also find that the low-energy spectral index \alpha is consistent with a synchrotron component with \alpha = -0.81\pm 0.1. This value lies between the limiting values of \alpha = -2/3 and \alpha = -3/2 for electrons in the slow and fast-cooling regimes, respectively, suggesting ongoing acceleration at the emission site. The blackbody component can be more significant when using a physical synchrotron model instead of the Band function, illustrating that the Band function does not serve as a good proxy for a nonthermal synchrotron emission component. The temperature and characteristic emission-region size of the blackbody component are found to, respectively, decrease and increase as power laws with time during the prompt phase. In addition, we find that the blackbody and nonthermal components have separate temporal behaviors.

We have used observations of Cygnus X-1 from the Compton Gamma-Ray Observatory (CGRO) and BeppoSAX to study the variation in the MeV gamma-ray emission between the hard and soft spectral states, using spectra that cover the energy range 20 keV up to 10 MeV. These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux decreases during the soft state, the flux at energies above 1 MeV increases, resulting in a significantly harder gamma-ray spectrum at energies above 1 MeV. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft gamma-ray energies. These data extend this picture, for the first time, to energies above 1 MeV. We have used two different hybrid thermal/non-thermal Comptonization models to fit broad band spectral data obtained in both the hard and soft spectral states. These fits provide a quantitative estimate of the electron distribution and allow us to probe the physical changes that take place during transitions between the low and high X-ray states. We find that there is a significant increase (by a factor of ~4) in the bolometric luminosity as the source moves from the hard state to the soft state. Furthermore, the presence of a non-thermal tail in the Comptonizing electron distribution provides significant constraints on the magnetic field in the source region., 19 pages, 9 figures, accepted for publication in ApJ. Final version includes expanded discussion section

The distribution of GRB durations is bimodal, but there is little additional evidence to support the division of GRBs into short and long classes. Based on simple hardness ratios, several studies have shown a tendency for longer GRBs to have softer energy spectra. Using a database of standard model fits to BATSE GRBs, we compare the distributions of spectral parameters for short and long bursts. Our preliminary results show that the average spectral break energy differs discontinuously between short and long burst classes, but within each class shows only a weak dependence on burst duration., 3 pages, 1 postscript figure. To appear in the proceedings of the October 2000 Rome Workshop "Gamma-Ray Bursts in the Afterglow Era"

We have used observations from CGRO to study the variation in the MeV emission of Cygnus X-1 between its low and high X-ray states. These data provide a measurement of the spectral variability above 1 MeV. The high state MeV spectrum is found to be much harder than that of the low state MeV spectrum. In particular, the power-law emission seen at hard X-ray energies in the high state spectrum (with a photon spectral index of 2.6) is found to extend out to at least 5 MeV, with no evidence for any cutoff. Here we present the data and describe our efforts to model both the low state and high state spectra using a hybrid thermal/nonthermal model in which the emission results from the Comptonization of an electron population that consists of both a thermal and nonthermal component., 5 pages, submitted to the proceedings of the GAMMA 2001 High Energy Astrophysics Symposium (Baltimore, April 4-6, 2001)

An analysis of 153 gamma-ray bursts detected by the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory is reported. The number versus intensity distribution does not follow the -3/2 power law expected for a spatially extended homogeneous distribution of sources, but at the same time the angular distribution is isotropic within statistical limits. Taken together these results are consistent with the spatial distribution of any known population of galactic objects, but may be consistent with the bursts being at cosmological distances.

FiberGLAST is one of several instrument concepts being developed for possible inclusion as the primary Gamma-ray Large Area Space Telescope (GLAST) instrument. The predicted FiberGLAST effective area is more than 12,000 cm2 for energies between 30 MeV and 300 GeV, with a field of view that is essentially flat from 0°–80°. The detector will achieve a sensitivity more than 10 times that of EGRET. We present results of simulations that illustrate the sensitivity of FiberGLAST for the detection of gamma-ray bursts.

The FiberGLAST scintillating fiber telescope is a large-area instrument concept for NASA's GLAST program. The detector is designed for high-energy gamma-ray astronomy, and uses plastic scintillating fibers to combine a photon pair tracking telescope and a calorimeter into a single instrument. A small prototype detector has been tested with high energy photons at the Thomas Jefferson National Accelerator Facility. We report on the result of this beam test, including scintillating fiber performance, photon track reconstruction, angular resolution, and detector efficiency.

FiberGLAST is a scintillating fiber gamma-ray detector designed for the GLAST mission. The system described below provides superior effective area and field of view for modest cost and risk. An overview of the FiberGLAST instrument is presented, as well as a more detailed description of the principle elements of the primary detector volume. The triggering and readout electronics are described, and Monte Carlo Simulations of the instrument performance are presented.

A scintillating fiber detector is currently being studied for the NASA Gamma-Ray Large Area Space Telescope (GLAST) mission. This detector utilizes modules composed of a thin converter sheet followed by an x, y plane of scintillating fibers to examine the shower of particles created by high energy gamma-rays interacting in the converter material. The detector is composed of a tracker with 90 such modular planes and a calorimeter with 36 planes. The two major component of this detector are the scintillating fibers and their associated photodetectors. Here we present current status of development and test result of both of these. The Hamamatsu R5900-00-M64 multianode photomultiplier tube (MAPMT) is the baseline readout device. A characterization of this device has been performed including noise, cross- talk, gain variation, vibration, and thermal/vacuum test. A prototype fiber/MAPMT system has been tested at the Center for Advanced Microstructures and Devices at Louisiana State University with a photon beam and preliminary results are presented.

We present the results of a 2.5-year multiwavelength monitoring programme of Cygnus X-1, making use of hard and soft X-ray data, optical spectroscopy, UBVJHK photometry and radio data. In particular we confirm that the 5.6-day orbital period is apparent in all wavebands and note the existence of a wavelength-dependence to the modulation, in the sense that higher energies reach minimum first. We also find a strong modulation at a period of 142 +/- 7 days, which we suggest is due to precession and/or radiative warping of the accretion disc. Strong modulation of the hard and soft X-ray flux at this long period may not be compatible with simple models of an optically thin accretion flow and corona in the low state. We present the basic components required for more detailed future modelling of the system - including a partially optically thick jet, quasi-continuous in the low state, the base of which acts as the Comptonising corona. In addition, we find that there are a number of flares which appear to be correlated in at least two wavebands and generally in more. We choose two of these flares to study in further detail and find that the hard and soft X-rays are well-correlated in the first and that the soft X-rays and radio are correlated in the second. In general, the optical and infrared show similar behaviour to each other but are not correlated with the X-rays or radio., Accepted for publication in MNRAS, 2 figures in colour

Average cosmological invariant parameters (ACIPs) are calculated for six groups of BATSE cosmic gamma-ray bursts selected by their peak fluxes on the 1.024 s time scale. The ACIPs represent the average temporal and spectral properties of these events equally in the observer frame of reference and in the co-moving frames of outbursting emitters. The parameters are determined separately for rise fronts and for back slopes of bursts, defined as the time profiles before and after the main peaks, respectively. The ACIPs for the rise fronts are found to be different for different intensity groups, while the ACIPs for the back slopes show no significant dependence on intensity. We conclude that emitters of bursts manifest standard average properties only during the back slopes of bursts., Comment: 18 pages, 3 figures. Accepted to ApJ (It is scheduled for the September 20, 1999 issue, Vol. 523)

The spectral evolution of the brightest BATSE GRBs is investigated in terms of the superposition of high- and low-energy emission components. Spectra and time histories of the components are derived and studied. The low-energy components of different GRBs have similar spectral properties and the high-energy components have similar properties as well. However, there is qualitative difference between the low and the high energy components. We also show that the apparently unlike lightcurves of the components of a given burst can be traced to the time history of a single hidden process that we interpret as the activity of the central engine. We conclude that the success of the two component model is evidence in favour of an internal shock model.

Three type I X-ray bursts and one burst candidate from 4U 1608-52 were observed with RXTE on March 22, 1996. Between the two strongest bursts, an enhanced noise component below 0.01 Hz was observed in the X-ray light curve with RXTE/PCA. It is probably caused by the subsequent time-dependent nuclear burning after the main X-ray burst. This supports the idea that there is a correlation between the Very-Low-Frequency-Noise (VLFN) in Low-Mass X-ray Binaries (LMXBs) and the nuclear burning. Assuming that the time scale of the variations is the time scale for one fire to ignite another, our result suggests the burning front was propagating at less than 3.0×104 cm s−1 on the neutron star surface.

Recent BeppoSAX and RXTE observations have drawn renewed attention to X-ray afterglow from gamma-ray bursts. The low-energy sensitivity of the BATSE Spectroscopy Detectors (SDs) may be exploited to detect GRB afterglow at energies below 20 keV. Application of the Earth occultation technique to SD data allows us to search for 6–20 keV emission on time scales from tens of minutes to many days. We present preliminary results of such a search using a sample of sufficiently well-located BATSE bursts.

The synchrotron shock model (SSM) for gamma-ray burst emission makes a testable prediction: The observed low-energy power-law photon number spectral index cannot exceed −2/3 (where the photon model is defined with a positive index). We have collected spectral fit parameters for well over 100 bursts observed by the Burst And Transient Source Experiment on board the Compton Gamma-Ray Observatory. Using this database, we find evidence that the SSM has crossed over the line of death.

A large data-base of four channel burst spectral data with 64ms time resolution has been developed and analyses are presented here demonstrating that the behaviors of the high- and low-energy emission in bursts are distinctly different. Analyses of the spectral evolution across bursts and the peaks within bursts show that burst low-energy spectra are less variable and exhibit more coherent hard-to-soft spectral evolution than the higher energy spectra.

GRS1915+105 is an extraordinary X-ray transient which exhibits superluminal radio jets. In this paper, ASCA observations of the GRS1915+105 conducted from 1994 to 1997 are reported. Observations are carried out on the following dates each for ∼ 20 ksec exposure; Sep 27 1994, April 20 1995, Oct 23 1996 and Apr 25 1997.

We have identified two types of bursts with luminosities that differ by about an order of magnitude. The more luminous HE bursts are identified by strong high-energy flux (E>300 keV). The dimmer NHE bursts are identified by a marked lack of high-energy flux. The spectral properties that are used to identify these two burst populations are also influenced by redshifting in the cosmological scenario. The observed burst spectral and intensity data are compared to both a cosmological model with a monoluminous burst population and a cosmological model containing two burst populations with distinctly different luminosities. These comparisons are used to characterize the luminosity difference between HE and NHE bursts more accurately as well as to evaluate the consistency between the redshifting effects and the shape of the intensity distribution for the two models.

We have developed an automatic search procedure to identify low-energy spectral features in GRBs. We have searched 133,000 spectra from 117 bright bursts and have identified 12 candidate features with significances ranging from our threshold of P = 5E-5 to P = 1E-7. Several of the candidates have been examined in detail, including some with data from more than one BATSE spectroscopy detector. The evidence for spectral features appears good; however, the features have not conclusively been shown to be narrow lines., To appear in Gamma-Ray Bursts: 4th Huntsville Symposium, ed. C. Meegan, R. Preece and T. Koshut

The hard X-ray emission from Cygnus X-1 has been monitored continually by BATSE since the launch of CGRO in April 1991. We present the hard X-ray intensity and spectral history of the source covering a period of more than five years. Power spectral analysis shows a significant peak at the binary orbital period. The 20-100 keV orbital light curve is roughly sinusoidal with a minimum near superior conjunction of the X-ray source and an rms modulation fraction of approximately 1.7%. No longer-term periodicities are evident in the power spectrum. We compare our results with other observations and discuss the implications for models of the source geometry., 5 pages (aipproc LaTeX), 4 PostScript figures, to appear in Proceedings of the Fourth Compton Symposium, ed. C. D. Dermer, M. S. Strickman, and J. D. Kurfess, AIP Conf. Proc. 410 (New York: AIP), in press

The flux and spectrum of the black hole candidate GX339-4 has been monitored by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-ray Observatory (CGRO) since the observatory became operational in May 1991. Between the summer of 1991 and the fall of 1996, eight outbursts from GX339-4 were observed. The history of these outbursts is one of declining fluence or total energy release, as well as a shortening of the time between outbursts. A rough linear correlation exists between the fluence emitted during an outburst and the time elapsed between the end of the previous outburst and the beginning of the current one. The peak flux is also roughly linearly correlated with outburst fluence. The lightcurves of the earlier, more intense, outbursts (except for the second one) can be modeled by a fast exponential (time constant ~ 10 days) followed by a slower exponential (~ 100 days) on the rise and a fast exponential decay (~ 5 days) on the fall. The later, weaker, outbursts are modeled with a single rising time constant (~ 20 days) and a longer decay on the fall (~ 50 days). An exponential model gives a marginally better fit than a power law to the rise/decay profiles. GX339-4 is a unique source in having more frequent outbursts than other low mass x-ray binary black hole candidates. These observations can be used to constrain models of the behavior of the accretion disk surrounding the compact object., Accepted for Publication in the Astrophysical Journal Letters, AASTEX

Aquila X-1 is one of the most active of the soft x-ray transients (SXTs). Soft x-ray outbursts can produce emission close to the Eddington limit in the 1–10 keV band and are typically observed about once a year. Between becoming operational in the spring of 1991, and the beginning of 1996, BATSE observed at least three episodes of hard x-ray emission lasting 50 days or longer, during which the 20–100 keV flux exceeded 50 mCrab [1]. XTE-ASM has observed two outbursts in the 2–10 keV range since becoming operational in early 1996. One of these outbursts was also observed by BATSE. The cause of the hard x-ray emission, and its relation to the soft x-ray emission, is still unknown.

Between the summer of 1991 and the fall of 1996, BATSE observed eight outbursts from the black hole candidate GX339-4. They occurred in a declining sequence with two types of outbursts. A rough linear correlation exists between the fluence emitted during an outburst and the time elapsed between the end of the previous outburst and the beginning of the current one. The light curves of the earlier, more intense, outbursts (except for the second one) can be modeled by a fast exponential (time constant ∼10 days) followed by a slower exponential (∼100 days) on the rise and a fast exponential decay (∼5 days) on the fall. The later, weaker, outbursts are modeled with a single rising time constant (∼20 days) and a longer decay on the fall (∼50 days). These observations can be used to constrain models of the behavior of the accretion disk surrounding the compact object.

Strong oscillations near 1000 Hz have been discovered in a number of low mass x-ray binaries with NASA’s Rossi X-Ray Timing Explorer. Their behavior should ultimately lead us to a better understanding of the accretion dynamics and the fundamental properties of the neutron star itself. Here we summarize the known properties of fast QPOs, focusing on results from the source 4U 0614+091. We highlight some of the aspects yet to be explored.

Analyzing data from the BATSE instrument on the Compton Gamma Ray Observatory, we have found evidence of two hard X-ray outbursts from the ultra-soft transient and black hole candidate 4U 1630-47. Both outbursts are nearly simultaneous with observations at lower energies by other instruments. In July 1994, the source reached a hard X-ray (20–100 keV) flux of ∼100 mCrab, with a possible brief flare of ∼200 mCrab. BATSE occultation images confirm that this emission originated from 4U 1630-47. This outburst corresponds to an archival detection of the source by ASCA, as reported by Parmar et al. [9]. In June 1996, during the extended outburst observed by RXTE [6], the 20–100 keV flux also reached ∼100 mCrab. The origin of this emission was verified by BATSE images as well. Kuulkers et al. [5] present RXTE/ASM data from the same period of time. We compare the BATSE data with the ASCA and RXTE/ASM results in order to push our knowledge of this source’s spectral characteristics into the hard X-ray range.

We present a global comparison of long term observations of the hard X-ray (20–100 keV), soft X-ray (1.5–12 keV), infrared (1–2 μm) and radio (2.25, 8.3 and 15 GHz) bands for the unusual X-ray binary Cygnus X-3. Data were obtained in the hard X-ray band from CGRO/BATSE, in the soft X-ray band from RXTE/ASM, in the radio band from the Green Bank Interferometer and Ryle Telescope and in the infrared band from various ground based observatories. Radio flares, quenched radio states and quiescent radio emission can all be associated with changes in the hard and soft X-ray intensity. The injection of plasma into the radio jet is directly related to changes in the hard and soft X-ray emission. The infrared observations are examined in the context of these findings.

We are studying a gamma-ray burst mission concept called burst arcsecond imaging and spectroscopy (BASIS) as part of NASA's new mission concepts for astrophysics program. The scientific objectives are to accurately locate bursts, determine their distance scale, and measure the physical characteristics of the emission region. Arcsecond burst positions (angular resolution approximately 30 arcsec, source positions approximately 3 arcsec) will be obtained for approximately 100 bursts per year using the 10 - 100 keV emission. This will allow the first deep, unconfused counterpart searches at other wavelengths. The key technological breakthrough that makes such measurements possible is the development of CdZnTe room-temperature semiconductor detectors with fine (approximately 100 micron) spatial resolution. Fine spectroscopy will be obtained between 0.2 and 150 keV. The 0.2 keV threshold will allow the first measurements of absorption in our galaxy and possible host galaxies, constraining the distance scale and host environment.

Gamma-ray bursts remain one of the outstanding unsolved mysteries of astronomy. The next generation of instruments will address specific aspects of the gamma-ray burst problem and attempt to answer fundamental questions such as the distance scale. However, missing from the crop of planned or proposed instruments is one which combines high sensitivity and a large field of view, so that detailed studies can be performed on a large sample of weak bursts. Such a combination is difficult to obtain at a reasonable cost with the techniques currently used. We describe a novel application of the Compton telescope technique to the energy range 50 - 300 keV which can, in principle, provide the required capabilities using position sensitive semiconductor detectors.

We report on the on-going project to search for serendipitous time-correlated optical photographic observations of γ-ray bursts (GRBs). The regular photography at 12 observatories is used to look for plates which have been exposed simultaneously with a GRB detected by BATSE and contain its position. The results of this search for GRBs detected during the previous 4 yrs of BATSE operation are presented.

We are studying a gamma-ray burst mission concept called the Burst ArcSecond Imaging and Spectroscopy (BASIS) as part of NASA’s New Mission Concepts for Astrophysics program. The scientific objectives are to accurately locate bursts, determine their distance scale, and measure the physical characteristics of the emission region. Arcsecond burst positions (angular resolution ∼30 arcsec, source positions ∼3 arcsec for >10−6 erg/cm2 bursts) would be obtained for ∼100 bursts per year using the 10–200 keV emission. This would allow the first deep, unconfused counterpart searches at other wavelengths. The key technological breakthrough that makes such measurements possible is the development of CdZnTe room-temperature semiconductor detectors with fine (∼100 micron) spatial resolution. Fine spectroscopy would be obtained between 0.2 and 200 keV. The 0.2 keV threshold would allow the first measurements of absorption in our Galaxy and possible host galaxies, constraining the distance scale and host environment.