[Paper Review] SkyHopper mission science case I: Identification of high redshift Gamma-Ray Bursts through space-based near-infrared afterglow observations

M. Thomas, Michele Trenti+10 more|arXiv (Cornell University)|May 11, 2022

Gamma-ray bursts and supernovae45 references5 citations

TL;DR

This paper proposes a rapid-response near-infrared nano-satellite mission (SkyHopper) to detect high-redshift gamma-ray burst (GRB) afterglows, using a Monte Carlo simulation framework to model orbital visibility and observing efficiency. It demonstrates that a single satellite can detect 72.5% of Swift GRB triggers within 2 hours, including 1–3 high-redshift (z > 5) GRBs annually—significantly advancing the current total of 23 such GRBs discovered in 24 years.

ABSTRACT

Long-duration gamma-ray burst (GRB) afterglow observations offer cutting-edge opportunities to characterise the star formation history of the Universe back to the epoch of reionisation, and to measure the chemical composition of interstellar and intergalactic gas through absorption spectroscopy. The main barrier to progress is the low efficiency in rapidly and confidently identifying which bursts are high redshift ($z > 5$) candidates before they fade, as this requires low-latency follow-up observations at near-infrared wavelengths (or longer) to determine a reliable photometric redshift estimate. So far this task has been performed by instruments on the ground, but sky visibility and weather constraints limit the number of GRB targets that can be observed and the speed at which follow-up is possible. In this work we develop a Monte Carlo simulation framework to investigate an alternative approach based on the use of a rapid-response near-infrared nano-satellite, capable of simultaneous imaging in four bands from $0.8$ to $1.7\mu$m (a mission concept called SkyHopper). We find that such a nano-satellite is capable of detecting in the H band (1.6 $\mu$m) $72.5\% \pm 3.1\%$ of GRBs concurrently observable with the Swift satellite via its UVOT instrument (and $44.1\% \pm 12.3\%$ of high redshift ($z>5$) GRBs) within 60 minutes of the GRB prompt emission. This corresponds to detecting $\sim 55$ GRB afterglows per year, of which 1-3 have $z > 5$. These rates represent a substantial contribution to the field of high-$z$ GRB science, as only 23 $z > 5$ GRBs have been collectively discovered by the entire astronomical community over the last $\sim 24$ years. Additionally, we find that launching a mini-constellation of 3 near-infrared nano-satellites would increase the detection fraction of afterglows to $\sim 83\%$ and substantially reduce the latency in the photometric redshift determination.

Motivation & Objective

• To address the critical challenge of identifying high-redshift (z > 5) GRBs quickly, which requires near-infrared follow-up but is limited by ground-based weather and visibility constraints.
• To evaluate the feasibility and performance of a space-based near-infrared nano-satellite constellation for rapid follow-up of GRB afterglows.
• To quantify the detection efficiency and photometric redshift reliability of such a mission using simulated GRB triggers from the Swift/UVOT and GROND data.
• To determine the optimal observing strategy and constellation size (1–3 satellites) for maximizing early-time detection and high-redshift GRB yield.

Proposed method

• Developed a Monte Carlo simulation framework combining orbital line-of-sight modeling with random GRB event generation to simulate observing runs over multiple mission cycles.
• Used a reference sample of 88 afterglows observed with the GROND instrument on the MPG/ESO telescope as a benchmark for detection performance.
• Simulated a nano-satellite in a 550 km polar Sun-synchronous orbit, capable of simultaneous imaging in four near-infrared bands (0.8–1.7 µm), with a limiting magnitude of HAB = 20.
• Evaluated two observing strategies: a fixed 1-minute exposure followed by 5-minute exposures, and long continuous exposures, to determine optimal timing for early detection.
• Assessed performance of single and multi-satellite constellations (up to 10 satellites) by calculating detection fractions and latency distributions for afterglow visibility.
• Computed detection probabilities for high-redshift GRBs (z > 5) and compared results across different constellation sizes and observing strategies.

Experimental results

Research questions

• RQ1 Can a near-infrared nano-satellite achieve timely detection of GRB afterglows within 60 minutes of prompt emission, enabling reliable photometric redshift estimation?
• RQ2 What is the expected detection rate of high-redshift (z > 5) GRBs using a single nano-satellite, and how does it compare to the current observational record?
• RQ3 How does the performance of a nano-satellite constellation scale with the number of satellites, and what is the optimal constellation size for maximizing detection efficiency?
• RQ4 Which observing strategy—initial short exposure followed by longer exposures—yields the highest probability of early detection compared to continuous long exposures?
• RQ5 What is the impact of reduced telecommand latency on detection performance, and does launching more than three satellites provide diminishing returns?

Key findings

• A single near-infrared nano-satellite in a 550 km Sun-synchronous orbit can detect 72.5% ± 3.1% of Swift GRB triggers within 2 hours of prompt emission, corresponding to approximately 55 GRB afterglows per year.
• The same mission can detect 1–3 high-redshift GRBs (z > 5) annually, representing a substantial increase over the current total of 23 such GRBs discovered in the past 24 years.
• A 3-satellite constellation increases the detection fraction to 83% and boosts high-redshift GRB detection to 58%, significantly improving early-time coverage and reliability.
• The optimal observing strategy is a 1-minute initial exposure followed by 5-minute exposures, which maximizes early detection probability (30% within 10 minutes) and outperforms long continuous exposures due to orbital visibility variability.
• A 3-satellite constellation achieves performance comparable to a 10-satellite constellation in terms of timely detection (under 11 minutes) when telecommand latency is zero, but offers only ~10% higher total detection rate (91% vs. 83%).
• Launching more than three nano-satellites yields diminishing returns compared to reducing telecommand latency or increasing telescope aperture, suggesting 2–3 satellites is the optimal constellation size.