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Register a new userGamma-Ray Burst Triangulation with a Near-Earth Network
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Kevin C. Hurley
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We study the characteristics of Near-Earth-Networks (NENs) of gamma-ray burst (GRB) detectors, with the objective of defining a network with all-sky, full-time localization capability for multi-messenger astrophysics. We show that a minimum network consisting of 9 identical spacecraft in two orbits with different inclinations provides a good combination of sky coverage with several-degree localization accuracy with detector areas of 100 cm$^2$. In order to achieve this, careful attention must be paid to systematics. This includes accurate photon timing ($sim$ 0.1 ms), good energy resolution ($sim$ 10%), and reduction of Earth albedo, which are all within current capabilities. Such a network can be scaled in both the number and size of detectors to produce increased accuracy. We introduce a new method of localization which does not rely on on-board trigger systems or on the cross-correlation of time histories, but rather, in ground processing, tests positions over the entire sky and assigns probabilities to them to detect and localize events. We demonstrate its capabilities with simulations. If the NEN spacecraft can downlink at least several hundred time- and energy-tagged events per second, and the data can be ground-processed as they are received, it can in principle derive GRB positions in near-real time over the entire sky.
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Context. Gamma-ray bursts can be located via arrival time signal triangulation using gamma-ray detectors in orbit throughout the solar system. The classical approach based on cross-correlations of binned light curves ignores the Poisson nature of the time-series data, and is unable to model the full complexity of the problem. Aims. To present a statistically proper and robust GRB timing/triangulation algorithm as a modern update to the original procedures used for the Interplanetary Network (IPN). Methods. A hierarchical Bayesian forward model for the unknown temporal signal evolution is learned via random Fourier features (RFF) and fitted to each detectors time-series data with time-differences that correspond to GRBs position on the sky via the appropriate Poisson likelihood. Results. Our novel method can robustly estimate the position of a GRB as verified via simulations. The uncertainties generated by the method are robust and in many cases more precise compared to the classical method. Thus, we have a method that can become a valuable tool for gravitational wave follow-up. All software and analysis scripts are made publicly available here (https://github.com/grburgess/nazgul) for the purpose of replication.
Gamma-ray Bursts (GRBs) are the most powerful transients in the Universe, over-shining for a few seconds all other $gamma$-ray sky sources. Their emission is produced within narrowly collimated relativistic jets launched after the core-collapse of massive stars or the merger of compact binaries. THESEUS will open a new window for the use of GRBs as cosmological tools by securing a statistically significant sample of high-$z$ GRBs, as well as by providing a large number of GRBs at low-intermediate redshifts extending the current samples to low luminosities. The wide energy band and unprecedented sensitivity of the Soft X-ray Imager (SXI) and X-Gamma rays Imaging Spectrometer (XGIS) instruments provide us a new route to unveil the nature of the prompt emission. For the first time, a full characterisation of the prompt emission spectrum from 0.3 keV to 10 MeV with unprecedented large count statistics will be possible revealing the signatures of synchrotron emission. SXI spectra, extending down to 0.3 keV, will constrain the local metal absorption and, for the brightest events, the progenitors ejecta composition. Investigation of the nature of the internal energy dissipation mechanisms will be obtained through the systematic study with XGIS of the sub-second variability unexplored so far over such a wide energy range. THESEUS will follow the spectral evolution of the prompt emission down to the soft X-ray band during the early steep decay and through the plateau phase with the unique ability of extending above 10 keV the spectral study of these early afterglow emission phases.
The detection of astrophysical Gamma-Ray Bursts (GRBs) has always been intertwined with the challenge of identifying the direction of the source. Accurate angular localization of better than a degree has been achieved to date only with heavy instruments on large satellites, and a limited field of view. The recent discovery of the association of GRBs with neutron star mergers gives new motivation for observing the entire $gamma$-ray sky at once with high sensitivity and accurate directional capability. We present a novel $gamma$-ray detector concept, which utilizes the mutual occultation between many small scintillators to reconstruct the GRB direction. We built an instrument with 90 (9,mm)$^3$ csi~scintillator cubes attached to silicon photomultipliers. Our laboratory prototype tested with a 60,keV source demonstrates an angular accuracy of a few degrees for $sim$25 ph,cm$^{-2}$ bursts. Simulations of realistic GRBs and background show that the achievable angular localization accuracy with a similar instrument occupying $1$l volume is $<2^circ$. The proposed concept can be easily scaled to fit into small satellites, as well as large missions.
Using the Gamma Ray Burst Monitor (GBM) on-board Fermi, we are monitoring the hard X-ray/soft gamma ray sky using the Earth occultation technique. Each time a source in our catalog enters or exits occultation by the Earth, we measure its flux using the change in count rates due to the occultation. Currently we are using CTIME data with 8 energy channels spanning 8 keV to 1 MeV for the GBM NaI detectors and spanning 150 keV to 40 MeV for the GBM BGO detectors. Our preliminary catalog consists of galactic X-ray binaries, the Crab Nebula, and active galactic nuclei. In addition, to Earth occultations, we have observed numerous occultations with Fermis solar panels. We will present early results. Regularly updated results can be found on our website http://gammaray.nsstc.nasa.gov/gbm/science/occultation
The Gamma-Ray Burst Monitor (GBM) will significantly augment the science return from the Fermi Observatory in the study of Gamma-Ray Bursts (GRBs). The primary objective of GBM is to extend the energy range over which bursts are observed downward from the energy range of the Large Area Telescope (LAT) on Fermi into the hard X-ray range where extensive previous data exist. A secondary objective is to compute burst locations on-board to allow re-orientiong the spacecraft so that the LAT can observe delayed emission from bright bursts. GBM uses an array of twelve sodium iodide scintillators and two bismuth germanate scintillators to detect gamma rays from ~8 keV to ~40 MeV over the full unocculted sky. The on-board trigger threshold is ~0.7 photons/cm2/s (50-300 keV, 1 s peak). GBM generates on-board triggers for ~250 GRBs per year.
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