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Register a new usernazgul: A statistical approach to gamma-ray burst localization. Triangulation via non-stationary time-series models
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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.
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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.
The Fermi Gamma-ray Burst Monitor (GBM) has detected over 1400 Gamma-Ray Bursts (GRBs) since it began science operations in July, 2008. We use a subset of over 300 GRBs localized by instruments such as Swift, the Fermi Large Area Telescope, INTEGRAL, and MAXI, or through triangulations from the InterPlanetary Network (IPN), to analyze the accuracy of GBM GRB localizations. We find that the reported statistical uncertainties on GBM localizations, which can be as small as 1 degree, underestimate the distance of the GBM positions to the true GRB locations and we attribute this to systematic uncertainties. The distribution of systematic uncertainties is well represented (68% confidence level) by a 3.7 degree Gaussian with a non-Gaussian tail that contains about 10% of GBM-detected GRBs and extends to approximately 14 degrees. A more complex model suggests that there is a dependence of the systematic uncertainty on the position of the GRB in spacecraft coordinates, with GRBs in the quadrants on the Y-axis better localized than those on the X-axis.
We describe Glowbug, a gamma-ray telescope for bursts and other transients in the 30 keV to 2 MeV band. It was recently selected for funding by the NASA Astrophysics Research and Analysis program, with an expected launch in the early 2020s. Similar in concept to the Fermi Gamma Burst Monitor (GBM) and with similar sensitivity, Glowbug will join and enhance future networks of burst telescopes to increase sky coverage to short Gamma-Ray Bursts (SGRBs) from binary neutron star (BNS) mergers, including possible SGRBs from NS-black hole mergers. With the recent discovery of the SGRB coincident with the gravitational wave transient GW170817, we know such events occur with reasonable frequency. Expanded sky coverage in gamma rays is essential, as more detections of gravitational waves are expected with the improved sensitivity of the upgraded ground-based interferometers in the coming years.
It has been established that Gamma-Ray Bursts (GRB) can produce Very High Energy radiation (E > 100 GeV), opening a new window on the investigation of particle acceleration and radiation properties in the most energetic domain. We expect that next-generation instruments, such as the Cherenkov Telescope Array (CTA), will mark a huge improvement in their observation. However, constraints on the target visibility and the limited duty cycle of Imaging Atmospheric Cherenkov Telescopes (IACT) reduce their ability to react promptly to transient events and to characterise their general properties. Here we show that an instrument based on the Extensive Air Shower (EAS) array concept, proposed by the Southern Wide Field-of-view Gamma-ray Observatory (SWGO) Collaboration, has promising possibilities to detect and track VHE emission from GRBs. Observations made by the Fermi Large Area Telescope (Fermi-LAT) identified some events with a distinct spectral component, extending above $1,$GeV or even $10,$GeV, which can represent a substantial fraction of the emitted energy and also arise in early stages of the process. Using models based on these properties, we estimate the possibilities that a wide field of view and large effective area ground-based monitoring facility has to probe VHE emission from GRBs. We show that the ability to monitor VHE transients with a nearly continuous scanning of the sky grants an opportunity to access simultaneous electromagnetic counterparts to Multi-Messenger triggers up to cosmological scales, in a way that is not available to IACTs.
GRBAlpha is a 1U CubeSat mission with an expected launch date in the first half of 2021. It carries a 75 x 75 x 5 mm CsI(Tl) scintillator, read out by a dual-channel multi-pixel photon counter (MPPC) setup, to detect gamma-ray bursts (GRBs). The GRB detector is an in-orbit demonstration for the detector system on the Cubesats Applied for MEasuring and LOcalising Transients (CAMELOT) mission. While GRBAlpha provides 1/8th of the expected effective area of CAMELOT, the comparison of the observed light curves with other existing GRB monitoring satellites will allow us to validate the core idea of CAMELOT, i.e. the feasibility of timing-based localization.
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