Harvesting BAT-GUANO with NITRATES (Non-Imaging Transient Reconstruction And TEmporal Search): Detecting and localizing the faintest GRBs with a likelihood framework

Abstract

The detection of the gravitational wave counterpart GRB 170817A, underluminous compared to the cosmological GRB population by a factor of 10,000, motivates significant effort in detecting and localizing a dim, nearby, and slightly off-axis population of short GRBs. Swift/BAT is the most sensitive GRB detector in operation, and the only one that regularly localizes GRBs to arcminute precision, critical to rapid followup studies. However, the utility of BAT in targeted sub-threshold searches had been historically curtailed by the unavailability of the necessary raw data for analysis. The new availability of time-tagged event (TTE) data from the GUANO system (arXiv:2005.01751), motivates renewed focus on developing sensitive targeted search analysis techniques to maximally exploit these data. While computationally cheap, we show that the typical coded-mask deconvolution imaging is limited in its sensitivity due to several factors. We formalize a maximum likelihood framework for the analysis of BAT data wherein signals are forward modelled through the full instrument response, and -- coupled with the development of new response models -- demonstrate its superior sensitivity to typical imaging via archival comparisons, injection campaigns, and, after implementing as a targeted search, a large number of low-latency GRB discoveries and confirmed arcminute localizations to date. We also demonstrate independent localization of some out-of-FOV GRBs for the first time. NITRATES's increased sensitivity boosts the discovery rate of GRB 170817A-like events in BAT by a factor of at least $3-4$x, along with enabling joint analyses and searches with other GRB, GW, neutrino, and FRB instruments. We provide public access to the response functions and search pipeline code.

arXiv commentAccepted for publication in ApJ. v2 as updated after useful referee feedback. Clarified notation, definition of the test statistic, new figures, better readability. The codebase and instrument responses can be found at https://github.com/Swift-BAT/NITRATES

References in corpus (26)

• The NumPy array: a structure for efficient numerical computation
• GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral
• The Astropy Project: Building an inclusive, open-science project and status of the v2.0 core package
• Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A
• GWTC-2: Compact Binary Coalescences Observed by LIGO and Virgo During the First Half of the Third Observing Run
• The Burst Alert Telescope (BAT) on the Swift MIDEX Mission
• An Ordinary Short Gamma-Ray Burst with Extraordinary Implications: Fermi-GBM Detection of GRB 170817A
• Short-Duration Gamma-Ray Bursts
• INTEGRAL Detection of the First Prompt Gamma-Ray Signal Coincident with the Gravitational Wave Event GW170817
• The 70 Month Swift-BAT All-Sky Hard X-Ray Survey
• The Swift/BAT Hard X-ray Transient Monitor
• Gamma-Ray Observations of a Giant Flare from The Magnetar SGR 1806-20
• A Photometric Redshift of z ~ 9.4 for GRB 090429B
• A very luminous magnetar-powered supernova associated with an ultra-long gamma-ray burst
• Cosmic X-ray background and Earth albedo Spectra with Swift/BAT
• The Distance to NGC 4993: The Host Galaxy of the Gravitational-wave Event GW170817
• A Precise Distance to the Host Galaxy of the Binary Neutron Star Merger GW170817 Using Surface Brightness Fluctuations
• Gamma-ray burst progenitors
• Rapid Spectral Variability of a Giant Flare from a Magnetar in NGC 253
• The Astrophysical Multimessenger Observatory Network (AMON): Performance and Science Program
• Gamma-ray Urgent Archiver for Novel Opportunities (GUANO): Swift/BAT event data dumps on demand to enable sensitive sub-threshold GRB searches
• The sensitivity of coded mask telescopes
• BAT X-ray Survey - I: Methodology and X-ray Identification
• Analysis of Sub-threshold Short Gamma-ray Bursts in Fermi GBM Data
• The First Short GRB Millimeter Afterglow: The Wide-Angled Jet of the Extremely Energetic SGRB 211106A
• Coded Aperture Imaging in High-Energy Astrophysics

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