The Compton Spectrometer and Imager

The High Energy Transient Explorer (HETE ) mission is devoted to the study of gamma‐ray bursts (GRBs) using soft X‐ray, medium X‐ray, and gamma‐ray instruments mounted on a compact spacecraft. The HETE satellite was launched into equatorial orbit on 9 October 2000. A science team from France, Japan, Brazil, India, Italy, and the US is responsible for the HETE mission, which was completed for ∼ 1/3 the cost of a NASA Small Explorer (SMEX). The HETE mission is unique in that it is entirely “self‐contained,” insofar as it relies upon dedicated tracking, data acquisition, mission operations, and data analysis facilities run by members of its international Science Team. A powerful feature of HETE is its potential for localizing GRBs within seconds of the trigger with good precision (∼ 10′) using medium energy X‐rays and, for a subset of bright GRBs, improving the localization to ∼ 30″accuracy using low energy X‐rays. Real‐time GRB localizations are transmitted to ground observers within seconds via a dedicated network of 14 automated “Burst Alert Stations,” thereby allowing prompt optical, IR, and radio follow‐up, leading to the identification of counterparts for a large fraction of HETE ‐localized GRBs. HETE is the only satellite that can provide near‐real time localizations of GRBs, and that can localize GRBs that do not have X‐ray, optical, and radio afterglows, during the next two years. These capabilities are the key to allowing HETE to probe further the unique physics that produces the brightest known photon sources in the universe. To date (December 2002), HETE has produced 31 GRB localizations. Localization accuracies are routinely in the 4′– 20′ range; for the five GRBs with SXC localization, accuracies are ∼1–2′. In addition, HETE has detected ∼ 25 bursts from soft gamma repeaters (SGRs), and >600 X‐ray bursts (XRBs).

The Polarimeter for Relativistic Astrophysical X-ray Sources (PRAXyS) is one of three Small Explorer (SMEX) missions selected by NASA for Phase A study, with a launch date in 2020. The PRAXyS Observatory exploits grazing incidence X-ray mirrors and Time Projection Chamber Polarimeters capable of measuring the linear polarization of cosmic X-ray sources in the 2-10 keV band. PRAXyS combines well-characterized instruments with spacecraft rotation to ensure low systematic errors. The PRAXyS payload is developed at the Goddard Space Flight Center with the Johns Hopkins University Applied Physics Laboratory, University of Iowa, and RIKEN (JAXA) collaborating on the Polarimeter Assembly. The LEOStar-2 spacecraft bus is developed by Orbital ATK, which also supplies the extendable optical bench that enables the Observatory to be compatible with a Pegasus class launch vehicle. A nine month primary mission will provide sensitive observations of multiple black hole and neutron star sources, where theory predicts polarization is a strong diagnostic, as well as exploratory observations of other high energy sources. The primary mission data will be released to the community rapidly and a Guest Observer extended mission will be vigorously proposed.

The wide-field infrared explorer (WIRE) is a small explorer (SMEX) mission that will fly in the fall of 1998. The WIRE mission proposes to conduct a 4-month survey of more than 100 degree<SUP>2</SUP> of sky using 12- and 25-micrometers detectors. The instrument requires cryogenic cooling of its focal planes and telescope in order to achieve the required performance sensitivity. In addition, because of the SMEX nature of the experiment, the mass of the cryostat must be less than 60 kg. The most mass efficient system meeting the lifetime requirement was determined to be a dual stage H<SUB>2</SUB>/H<SUB>2</SUB> cryostat. The focal planes are conductively cooled by the primary H<SUB>2</SUB> while the telescope is cooled by the secondary H<SUB>2</SUB>. The secondary H<SUB>2</SUB> also protects the primary H<SUB>2</SUB> by intercepting the parasitic heat loads. This paper describes the design and performance of the H<SUB>2</SUB>/H<SUB>2</SUB> cryostat.

Despite a rich phenomenology, gamma-ray bursts (GRBs) are divided into two classes based on their duration and spectral hardness--the long-soft and the short-hard bursts. The discovery of afterglow emission from long GRBs was a watershed event, pinpointing their origin to star-forming galaxies, and hence the death of massive stars, and indicating an energy release of about 10(51) erg. While theoretical arguments suggest that short GRBs are produced in the coalescence of binary compact objects (neutron stars or black holes), the progenitors, energetics and environments of these events remain elusive despite recent localizations. Here we report the discovery of the first radio afterglow from the short burst GRB 050724, which unambiguously associates it with an elliptical galaxy at a redshift z = 0.257. We show that the burst is powered by the same relativistic fireball mechanism as long GRBs, with the ejecta possibly collimated in jets, but that the total energy release is 10-1,000 times smaller. More importantly, the nature of the host galaxy demonstrates that short GRBs arise from an old (> 1 Gyr) stellar population, strengthening earlier suggestions and providing support for coalescing compact object binaries as the progenitors.

It has long been known that there are two classes of gamma-ray bursts (GRBs), mainly distinguished by their durations. The breakthrough in our understanding of long-duration GRBs (those lasting more than approximately 2 s), which ultimately linked them with energetic type Ic supernovae, came from the discovery of their long-lived X-ray and optical 'afterglows', when precise and rapid localizations of the sources could finally be obtained. X-ray localizations have recently become available for short (duration <2 s) GRBs, which have evaded optical detection for more than 30 years. Here we report the first discovery of transient optical emission (R-band magnitude approximately 23) associated with a short burst: GRB 050709. The optical afterglow was localized with subarcsecond accuracy, and lies in the outskirts of a blue dwarf galaxy. The optical and X-ray afterglow properties 34 h after the GRB are reminiscent of the afterglows of long GRBs, which are attributable to synchrotron emission from ultrarelativistic ejecta. We did not, however, detect a supernova, as found in most nearby long GRB afterglows, which suggests a different origin for the short GRBs.

The gamma-ray burst monitor (GBM) on the Fermi Gamma-Ray Space Telescope, for the first time, detected a short gamma ray burst (SGRB) signal that accompanies a gravitational wave signal GW170817 in 2017. The detection and localization of the gravitational wave and gamma-ray source led all other space-and ground-based observatories to measure its kilonova and afterglow across the electromagnetic spectrum, which started a new era in astronomy, the so-called multi-messenger astronomy. Therefore, localizations of short gamma-ray bursts, as counterparts of verified gravitational waves, are of crucial importance since this will allow observatories to measure the kilonovae and afterglows associated with these explosions. Our results show that an automated network of observatories, such as the Stellar Observations Network Group, can be coupled with an interconnected multi-hop array of CubeSats for transients (IMPACT) to localize SGRBs. IMPACT is a mega-constellation of 80 CubeSats, each of which is equipped with gammaray detectors with ultrahigh temporal resolution to conduct full sky surveys in an energy range of 50-300 keV and downlink the required data promptly for high-accuracy localization of the detected SGRB to a ground station. Additionally, we analyze propagation and transmission delays from receipt of an SGRB signal to ground station offload to consider the effects of constellation design, link, and network parameters such as satellites per plane, data rate, and coding gain from erasure correcting codes among others. IMPACT will provide near-real-time localization of SGRBs with a total delay of 5 s and will enable Stellar Observations Network Group telescopes to join the efforts to pursue multi-messenger astronomy and help decipher the underlying physics of these events.

Short gamma-ray bursts (GRBs) are associated with binary neutron star mergers, which are multimessenger astronomical events that have been observed both in gravitational waves and in the multiband electromagnetic spectrum1. Depending on the masses of the stars in the binary and on details of their largely unknown equation of state, a dynamically evolving and short-lived neutron star may be formed after the merger, existing for approximately 10-300 ms before collapsing to a black hole2,3. Numerical relativity simulations across different groups consistently show broad power spectral features in the 1-5-kHz range in the post-merger gravitational-wave signal4-14, which is inaccessible by current gravitational-wave detectors but could be seen by future third-generation ground-based detectors in the next decade15-17. This implies the possibility of quasiperiodic modulation of the emitted gamma rays in a subset of events in which a neutron star is formed shortly before the final collapse to a black hole18-21. Here we present two such signals identified in the short bursts GRB 910711 and GRB 931101B from archival Burst and Transient Source Experiment (BATSE) data, which are compatible with the predictions from numerical relativity.

Context. Among the large variety of astrophysical sources that we can observe, gamma-ray bursts (GRBs) are the most energetic of the whole Universe. Their emission peaks in the γ-ray band, with a duration from a fraction of a second to a few hundred seconds, and is followed by an afterglow covering the whole electromagnetic spectrum. The definition of a general picture describing the physics behind GRBs has always been a compelling task, but the results obtained so far from observations have revealed a puzzling landscape. The lack of a clear, unique paradigm calls for further observations and additional, independent techniques for this purpose. Polarimetry constitutes a very useful example as it allows us to investigate some features of the source such as the geometry of the emitting region and the magnetic field configuration. Aims. To date, only a handful of bursts detected by space telescopes have been accompanied by ground-based spectro-polarimetric follow-up, and therefore such an analysis of more GRBs is of crucial importance in order to increase the sample of bursts with multi-epoch polarisation analysis. In this work, we present the analysis of the GRB 080928 optical afterglow, with observations performed with the ESO-VLT FORS1 instrument. Methods. Starting from raw data taken in the imaging polarimetry (IPOL) and spectro-polarimetry (PMOS) modes, we performed data reduction, followed by the photometric analysis of IPOL data, taken ~14 and ~40 h after the burst detection, and spectroscopy of PMOS data (t ~ 14.95 h). After computing the reduced Stokes parameters Q/I and U/I, which describe the linear polarisation of the emitted radiation, we obtained the polarisation degree for the three observing epochs. Results. We find that the GRB optical afterglow was not significantly polarised on the first observing night. The polarisation degree (P) grew on the following night to a level of P ~ 4.5%, giving evidence of polarised radiation at a 4σ confidence level. The GRB 080928 light curve is not fully consistent with standard afterglow models, making any comparison with polarimetric models partly inconclusive. The most conservative interpretation is that the GRB emission was characterised by a homogeneous jet and was observed at an angle of 0.6 &lt;θobs/θjet &lt; 0.8. Moreover, the non-zero polarisation degree on the second night suggests the presence of a dominant locally ordered magnetic field in the emitting region.

High-energy astrophysics focuses on understanding the origins of the most energetic radiation and particles observed or theoretically predicted in the universe. This area of research probes the most mysterious and most difficult-to-observe astrophysical phenomena. Understanding these phenomena is worth the effort, however, because they may be key to understanding the origin of the universe and the fundamentals of particle physics. Recent observations of extreme events such as ultrahigh-energy cosmic rays and gamma ray bursts have led to significant progress in determining their sources and modeling the mechanisms that produced them. Relativistic jets accelerated from massive compact objects, such as neutron stars or black holes, have been implicated as the primary culprits for producing high- energy events. Here we present three Reviews of the more rapidly advancing areas of research, ultrahigh-energy cosmic rays, gamma ray bursts, and relativistic jets. The highest energy cosmic rays are thought to be extragalactic. However, their source remains a mystery, as discussed in a Review by Guenter Sigl (p. [73][1]). About 12 events have been detected by large-area, ground-based detectors that sample the secondary particles of the cosmic rays. These observations, combined with theoretical models, suggest that ultrahigh-energy cosmic rays come from the earliest processes in the formation of the universe, nonstandard particle physics, or acceleration of particles by energetic processes such as gamma ray bursts. Gamma ray bursts—which last for only a few seconds and generate about 1053 ergs per second of energy output, greater than any other astrophysical phenomenon—are now being detected at a rate of about one per day. Peter Meszaros (p. [79][2]) reviews our current understanding of these curious events. The Italian-Dutch satellite BeppoSAX in particular has helped to locate gamma ray bursts, and accurate locations make it much easier to observe the afterglows with ground-based telescopes and satellites. These observations have led to a model that favors the collapse of a massive object into a neutron star or black hole, generating jets that create gamma ray bursts. Relativistic jets, collimated flows of plasma that travel at almost the speed of light, may produce gamma ray bursts, ultrahigh-energy cosmic rays, and many other high-energy phenomena. David Meier, Shinji Koide, and Yutaka Uchida (p. [84][3]) review the latest observations of jet structures, mainly from radio and x-ray wavelength observations, and focus on magnetohydrodynamic simulations of relativistic jets that reproduce these structures. A neutron star or black hole, in a binary system or in the act of formation, will accrete mass and angular momentum. Differential rotation of this mass and momentum may twist the magnetic field lines of the system, producing bipolar jets, and simulations can provide the appropriate conditions to accelerate particles to produce ultra high-energy cosmic rays or create gamma ray bursts in the jet stream. Relativistic jets also make up the centerpiece of the first of three News stories. Mark Sincell (p. [66][4]) describes how the jets of recently discovered objects known as microquasars may shed light on the mechanisms behind quasars—galaxy-spanning structures powered by supermassive black holes. Next, Govert Schilling (p. [68][5]) tells of another attempt to link two similar-looking phenomena: faint high- energy blips known as soft gamma repeaters and anomalous x-ray pulsars. Finally, Richard Stone (p. [70][6]) travels to Tatarstan to report on a project intended to rival much larger Western ground- and space-based attempts to detect gravitational waves from binary pulsars Thus, the combination of numerical simulations, observations, and theory is improving our understanding of the origins of high-energy astrophysical phenomena. The future looks bright for resolving some of these great mysteries in the early 21st century. [1]: /lookup/doi/10.1126/science.291.5501.73 [2]: /lookup/doi/10.1126/science.291.5501.79 [3]: /lookup/doi/10.1126/science.291.5501.84 [4]: /lookup/doi/10.1126/science.10.1126/SCIENCE.291.5501.66 [5]: /lookup/doi/10.1126/science.10.1126/SCIENCE.291.5501.68 [6]: /lookup/doi/10.1126/science.10.1126/SCIENCE.291.5501.70

If the weak equivalence principle (WEP) is broken, the measured values of the parametrized post-Newtonian parameter $\gamma$ from photons with left- and right-handed circular polarizations should differ slightly, leading to the arrival-time difference of these two circular components. Thus, the polarization vector of a linearly polarized light may rotate during the propagation. The rotation angle of the polarization vector depends on both the photon energy and the distance of the source. It is believed that if the rotation angle differs by more than $\pi/2$ over an energy range, then the net polarization of the signal would be significantly suppressed and could not be as high as the observed level. Thus, the detection of highly polarized photons implies that the relative rotation angle ($\Delta\Theta$) should not be too large. In this paper, we give a detailed calculation on the evolution of gamma-ray burst (GRB) polarization arising from a possible violation of the WEP, and we find that more than $60\%$ of the initial polarization degree can be conserved even if $\Delta\Theta$ is larger than $\pi/2$. In addition, to tightly constrain the WEP violation, GRBs with harder spectra and polarization observations in a wider energy range seem to be favored. Applying our formulas to the measurements of linear polarization from GRB 110721A and GRB 061122, we obtain strict limits on the differences of the $\gamma$ values as low as $\Delta\gamma<1.3\times10^{-33}$ and $\Delta\gamma<0.8\times10^{-33}$. These provide the most stringent limits to date on a deviation from the WEP, improving at least 6 orders of magnitude over previous bounds.

Electron–positron pairs in physics and astrophysics: From heavy nuclei to black holes

The coalesence of compact binary systems might be the plausible candidate to power short Gamma-Ray Bursts (GRBs), since in this scenario a low mass disk can be formed, and the lifetime of the disk is comparable to the duration of the short GRBs. After merger, a neutrino-dominated accretion flow (NDAF) surrounding a stellar-mass black hole (BH) will be formed and neutrino-antineutrino annihilation above the disk may power short GRBs. The results of the simulation show that the disk mass of the NDAF has a limit after merger events. So the question is: can annihilations of neutrinos from NDAFs owning such disk masses power all the observed short GRBs? Using the current short GRB observational data and fireball model, we estimate the disk mass of the NDAF. The neutrino annihilation luminosity as a function of BH spin and accretion rate is fitted from the global solutions of NDAFs. The results show that the estimation of the disk mass mainly depends on its output energy, jet opening angle, and central BH characteristic of GRBs. And some short GRBs require massive disks, which approach or exceed the limits in simulations. If the precise value of jet opening angle is considered, the real requirements of the disk masses will be much larger than the present results, since we use only the lower limit of them in the calculation. These results reflect that the neutrino annihilation process is deficient to power short GRBs. We suggest that there may exit other magnetohydrodynamic processes or some mechanisms to increase the efficiency of the neutrino emission, and then provide energy to power short GRBs.

It has been argued that the long gamma-ray burst (GRB) of GRB 060614 without an associated supernova (SN) has challenged the current classification and fuel model for long GRBs, and thus a tidal disruption model has been proposed to account for such an event. Since it is difficult to detect SNe for long GRBs at high redshift, the absence of an SN association cannot be regarded as the solid criterion for a new classification of long GRBs similar to GRB 060614, called GRB 060614-type bursts. Fortunately, we now know that there is an obvious periodic substructure observed in the prompt light curve of GRB 060614. We thus use such periodic substructure as a potential criterion to categorize some long GRBs into a new class of bursts, which might have been fueled by an intermediate-mass black hole (IMBH) gulping a star, rather than a massive star collapsing to form a black hole. Therefore, the second criterion to recognize for this new class of bursts is whether they fit the tidal disruption model. From a total of 328 Swift GRBs with accurately measured durations and without SN association, we find 25 GRBs satisfying the criteria for GRB 060614-type bursts: seven of them are with known redshifts and 18 with unknown redshifts. These new bursts are ~6% of the total Swift GRBs, which are clustered into two subclasses: Type I and Type II with considerably different viscous parameters of accretion disks formed by tidally disrupting their different progenitor stars. We suggest that the two different kinds of progenitors are solar-type stars and white dwarfs: the progenitors for four Type I bursts with viscous parameter of around 0.1 are solar-type stars, and the progenitors for 21 Type II bursts with viscous parameter of around 0.3 are white dwarfs. The potential applications of this new class of GRBs as cosmic standard candles are discussed briefly

The mergers of compact binaries with at least one neutron star component are the potential leading sites of the production and ejection of r-process elements. Discoveries of galactic binary pulsars, short gamma-ray bursts, and gravitational-wave detections have all been constraining the rate of these events, while the gravitational wave plus broadband electromagnetic coverage of binary neutron star merger (GW170817) has also placed constraints on the properties (mass and composition) of the merger ejecta. But uncertainties and ambiguities in modeling the optical and infrared emission make it difficult to definitively measure the distribution of heavy isotopes in these mergers. In contrast, gamma rays emitted in the decay of these neutron-rich ejecta may provide a more direct measurement of the yields. We calculate the gamma production in remnants of neutron star mergers, considering two epochs: a kilonova epoch, lasting about two weeks, and a much later epoch of tens and hundreds of thousands of years after the merger. For the kilonova epoch, when the expanding ejecta is still only partially transparent to gamma radiation, we use 3D radiative transport simulations to produce the spectra. We show that the gamma-ray spectra associated with beta- and alpha-decay provide a fingerprint of the ejecta properties and, for a sufficiently nearby remnant, may be detectable, even for old remnants. We compare our gamma spectra with the potential detection limits of next generation detectors, including the Lunar Occultation Explorer (LOX), the All-sky Medium Energy Gamma-ray Observatory (AMEGO), and the Compton Spectrometer and Imager (COSI). We show that fission models can be discriminated via the presence of short-lived fission fragments in the remnant spectra.

This thesis presents a number of studies into Single Photon Emission Imaging (SPEI) that can be grouped into three main sections: Monte Carlo Radiation Transport Modelling, Development of a Hybrid SPEI System and Fundamental Study of Advanced Compton Imaging. The first section of this thesis, Monte Carlo Radiation Transport Modelling, presents an overview of the Monte Carlo radiation transport modelling toolkit Geant4 and a newly derived low energy Compton scattering model, the Monash University Compton scattering model. This Compton scattering model was developed to address the limitation present in the majority of Monte Carlo bound atomic electron Compton scattering models: incorrect determination of the ejected direction of Compton electrons due to the non-zero momentum of the bound atomic electron. A theoretical foundation that ensures the conservation of energy and momentum in the relativistic impulse approximation was utilised to develop energy and directional algorithms for both the scattered photon and ejected Compton electron from first principles. Assessment of this model was undertaken in two steps: comparison with respect to two Compton scattering classes of Geant4 adapted from Ribberfors' work, and experimental comparison with respect to Compton electron kinetic energy spectra obtained from the Compton scattering of 662 keV photons off the K-shell of gold. It was shown that this new Compton scattering model was a viable replacement for the majority of computational models that have been adapted from Ribberfors' work. Additionally, this model was shown to be able to reproduce the Compton scattering triply differential cross-section Compton electron kinetic energy spectra of 662 keV photons K-shell scattering off of gold to within experimental uncertainty. The second section of this thesis, Development of a Hybrid SPEI System, presents the development of a novel hybrid collimated SPEI system: the Pixelated Emission Detector for RadiOisotopes (PEDRO). The PEDRO is a conceptual proof of principle hybrid SPEI system that was designed to explore and quantify the relationship between spatial resolution and sensitivity, inherent in SPEI, over the energy range of 30 keV to 511 keV. This system was originally intended to be constructed from a Compton camera stack located behind a coded mask composed of a mix of pinholes, slats and/or open areas. A total of three studies are presented in this section that outline the development of: 1) a Geant4 application to be used for optimisation of PEDRO with respect to a robust metric, 2) an automated routine for the optimisation of large-area slits in the outer regions of a coded mask for PEDRO which has a central region allocated for pinholes, and 3) a novel experimentally motivated image deblurring technique for multi-plane Gamma cameras such as PEDRO. These three studies illustrated two main points: 1) it may indeed be possible to overcome the trade-off between spatial resolution and sensitivity inherent in SPEI through hybrid Compton-mechanical collimation, and 2) the multi-plane nature of hybrid collimation systems can be capitilised on to reduce the level of image blurring associated with collimator opening geometry and, in turn, improve recovered image quality. Finally, the third section of this thesis, Fundamental Study of Advanced Compton Imaging, presents a preliminary study into the fundamental limits of recoil electron tracking enhanced Compton collimation, or Advanced Compton Imaging, for photon energies below 2 MeV. A custom Geant4 application of an idealised detection system was developed and four different detection materials were tested over a wide range of electron tracking resolutions. Increased image performance and point source convergence was observed with respect to standard Compton imaging regardless of recoil electron tracking accuracy in all tested materials across the investigated energy range. Additionally, the rate of point source convergence with respect to standard Compton imaging was discovered to be maximised for electron tracking accuracies of 45° to 60° Full Width at Tenth Maximum (FWTM) for photons of incident energy greater than 500 keV. Further study with more detailed simulation and image recovery frameworks is required to assess the validity of these observed trends.