Catching TeV emission from GRB 221009A and alike with LHAASO, LACT and SWGO

This paper estimates the detection rates of TeV-emitting Gamma-Ray Bursts similar to GRB 221009A for current and upcoming ground-based observatories (LHAASO, LACT, and SWGO) under two emission models, predicting annual detection frequencies ranging from 0.03 to 0.4 events depending on the instrument and model assumptions.

The Gist

The Cosmic Fireworks Forecast: Catching the Universe's Brightest Bursts

Imagine the universe as a vast, dark ocean. Occasionally, a massive underwater volcano erupts, sending a blinding flash of light shooting straight up into the sky. In astronomy, these are called Gamma-Ray Bursts (GRBs). They are the most energetic explosions since the Big Bang, outshining entire galaxies for a few seconds.

For decades, we could only see the "visible light" part of these fireworks. But recently, a giant telescope in China called LHAASO spotted something incredible: the "ultra-hot" part of the explosion, with energies so high they were previously thought impossible to detect. This event, named GRB 221009A, was like finding a firework that didn't just burn bright, but burned blue-hot in a way no one expected.

This paper is essentially a forecast for future cosmic fireworks. The authors are asking: "If we build more telescopes, how often will we catch these ultra-powerful explosions?"

Here is the breakdown of their study using simple analogies:

1. The Three Hunters (The Telescopes)

To catch these fleeting bursts, the scientists are looking at three different types of "hunters" (telescopes) on Earth. Think of them as different fishing nets:

• LHAASO (The Wide Net): Located in the mountains of China, this is a massive array of detectors spread over a huge area. It's like a giant fishing net that stays in the water 24/7. It doesn't care if it's cloudy or the moon is bright; it just watches. It's great at spotting where a burst happened, but it's a bit "blurry" (lower resolution).
• LACT (The Sharp-Eyed Binoculars): This is a planned array of telescopes right next to LHAASO. Imagine a team of photographers with high-end cameras. They can only work on clear, dark nights (no clouds, no moon), so they are "off" most of the time. However, when they are on, they can see the fireworks in crystal clear detail.
• SWGO (The Southern Net): This is a future project planned for Chile (the Southern Hemisphere). It's like a second giant net for the other side of the world, ensuring we don't miss anything happening "down south."

2. The Two Guessing Games (The Models)

Since we can't predict exactly how every future explosion will look, the authors used two different "recipes" to simulate what might happen:

• Recipe A (The "Copycat" Model): They assume every future GRB looks exactly like the one LHAASO just saw (GRB 221009A), just scaled up or down in brightness. It's like assuming every time you bake a cake, it tastes exactly the same, just bigger or smaller.
• Recipe B (The "Physics Chef" Model): This is more complex. They use the laws of physics (how electrons bounce around and crash into light) to predict the explosion's behavior. It's like a chef predicting how a cake will taste based on the ingredients and the oven temperature, rather than just copying a previous cake.

3. The Foggy Atmosphere (The EBL Problem)

Here is the tricky part. The universe isn't empty; it's filled with a faint, invisible "fog" of ancient starlight called the Extragalactic Background Light (EBL).

• The Analogy: Imagine trying to see a lighthouse from 100 miles away. The air is thick with mist. The light from the lighthouse hits the mist and gets scattered or absorbed before it reaches your eyes.
• The Result: The higher the energy of the gamma rays (the "hotter" the firework), the more likely they are to get eaten by this cosmic fog. This means we can only see the brightest, closest explosions clearly.

4. The Forecast (The Results)

After running millions of simulations with their "nets" and "recipes," here is what they predict for the next year:

• LHAASO (The Wide Net): Will likely catch a massive, ultra-bright GRB about once every 20 to 25 years. It's a rare catch, but because the net is so big, it's the best at spotting the "big ones" that are nearby.
• LACT (The Sharp Binoculars): Will catch them about once every 16 to 30 years. Because it can only work on clear nights, it misses a lot of opportunities, even though it sees them very clearly.
• SWGO (The Southern Net): This is the star of the show! Because it has a huge net and a lower energy threshold (it can see "cooler" fireworks that the others miss), it might catch a bright GRB 2 to 4 times a year.

5. Why This Matters

The authors conclude that GRB 221009A was a lucky break—a "one-in-a-million" event for the current telescopes. But with the new generation of telescopes (especially SWGO), we are entering a new era.

Instead of waiting decades for a lucky shot, we might soon catch these cosmic fireworks regularly. Every time we catch one, it's like getting a free sample of the universe's most extreme physics lab, helping us understand:

• How black holes are born.
• How particles are accelerated to near-light speed.
• What the "fog" of the universe is made of.

In short: We just found a new way to see the universe's brightest explosions. With bigger, smarter telescopes coming online, we are about to go from "waiting for a shooting star" to "watching a nightly fireworks display."

Technical Summary

Here is a detailed technical summary of the paper "Catching TeV emission from GRB 221009A and alike with LHAASO, LACT and SWGO."

1. Problem Statement

Gamma-Ray Bursts (GRBs) are the most energetic electromagnetic explosions in the universe. While their prompt emission and afterglows have been extensively studied in lower energy bands, the detection of Very-High-Energy (VHE, $\gtrsim 100$ GeV) radiation has historically been hindered by the limited sensitivity of early instruments and severe attenuation from the Extragalactic Background Light (EBL).

The recent breakthrough detection of GRB 221009A by the Large High Altitude Air Shower Observatory (LHAASO), which observed photons up to 13 TeV, suggests that VHE emission may be a common component of bright GRBs. However, the rarity of such nearby, ultra-bright events raises a critical question: What is the expected detection rate of similar GRBs for current and upcoming ground-based observatories? Understanding this rate is essential for planning follow-up strategies and constraining the physical mechanisms of GRB jets.

2. Methodology

The authors estimate the detection rates for three specific observatories: LHAASO (operational), LACT (Large Array of Imaging Atmospheric Cherenkov Telescopes, planned co-located with LHAASO), and SWGO (Southern Wide-field Gamma-ray Observatory, planned).

The methodology involves the following steps:

• Emission Modeling: Two distinct models were adopted to predict the intrinsic VHE spectrum of GRBs similar to GRB 221009A:

  1. Power-Law Model: Assumes all long GRBs share the same intrinsic spectral shape as GRB 221009A (a simple power law with index $\Gamma \approx -2.35$), with luminosities scaled by their isotropic-equivalent energy ($E_{iso}$) in the 1–$10^4$ keV band.
  2. Synchrotron Self-Compton (SSC) Model: A physically motivated model based on the afterglow SSC process. Parameters (electron index $p$, energy fractions $\epsilon_e$ and $\epsilon_B$, and ISM density $n$) were constrained using a Markov Chain Monte Carlo (MCMC) fit to combined LHAASO and AGILE satellite data from GRB 221009A. The initial kinetic energy is scaled based on the GRB's peak luminosity and duration.

• Population Simulation: The study utilized the luminosity and redshift distribution functions derived from Fermi-GBM long GRB samples. Synthetic GRBs were generated with:

  • Redshifts $z \in [0, 2]$ (limited by EBL absorption).
  • Peak luminosities $L_{pk} \in [10^{46}, 10^{54}]$ erg s$^{-1}$.
  • Duration parameters ($T_{90}$) following a log-normal distribution.

• Propagation Effects: The intrinsic spectra were modified to account for:

  • Cosmological Redshift: Shifting the energy spectrum.
  • EBL Absorption: Attenuation via pair production ($\gamma\gamma \to e^+e^-$). The authors tested multiple EBL models (Saldana-Lopez et al., Finke et al., Gilmore et al.) to quantify uncertainty.

• Sensitivity Analysis:

  • Expected signal ($N_{src}$) and background ($N_{bkg}$) event counts were calculated for each instrument using their specific effective areas ($A_{eff}$) and gamma-ray survival fractions ($\epsilon$).
  • Detection significance was evaluated using the Li-Ma test statistic (TS). A TS of 25 corresponds to a $5\sigma$ detection.
  • The analysis incorporated duty cycles (100% for EAS arrays, $\sim18\%$ for IACTs) and Field of View (FoV) fractions.

3. Key Contributions

• Comparative Performance Analysis: The paper provides a rigorous comparison between Extensive Air Shower (EAS) arrays (LHAASO, SWGO) and Imaging Atmospheric Cherenkov Telescopes (IACTs, LACT) specifically for GRB detection. It highlights the trade-off between the high duty cycle/wide FoV of EAS arrays and the superior sensitivity/low energy threshold of IACTs.
• Dual-Model Approach: By employing both a phenomenological power-law model and a physical SSC model, the study brackets the uncertainty in theoretical predictions, offering a robust range of expected detection rates.
• Synergy Assessment: The work explicitly analyzes the potential of co-located instruments (LHAASO and LACT), where LHAASO acts as a trigger for LACT follow-up, maximizing the scientific return of transient events.
• EBL Uncertainty Quantification: The study quantifies how different EBL models affect detection rates, finding variations up to $\approx 24\%$.

4. Key Results

The study derives the annual $5\sigma$ detection rates for GRB 221009A-like events:

| Observatory | Type | $5\sigma$Detection Rate (yr$^{-1}$) |$3\sigma$Detection Rate (yr$^{-1}$) |
| :--- | :--- | :--- | :--- |
| LHAASO (WCDA) | EAS | 0.04 – 0.05 | 0.06 – 0.08 |
| LHAASO (KM2A) | EAS | 0.004 – 0.005 | 0.006 – 0.007 |
| LACT | IACT | 0.03 – 0.06 | 0.04 – 0.09 |
| SWGO | EAS | 0.2 – 0.4 | 0.5 – 0.7 |

Key Findings:

• SWGO Superiority: SWGO is predicted to have the highest detection rate ($\sim 0.2-0.4$ yr$^{-1}$) due to its large effective area, low energy threshold, and high duty cycle.
• LACT Limitations: Despite superior point-source sensitivity, LACT's detection rate is limited by its low duty cycle ($\sim 18\%$) and higher energy threshold, which increases susceptibility to EBL absorption.
• KM2A Rarity: The detection of GRB 221009A by KM2A was a "lucky" event near the instrument's sensitivity limit. The study estimates another similar detection by KM2A would occur only once every $\sim 210-270$ years.
• Model Dependence: The Power-Law model generally predicts higher rates than the SSC model for low-luminosity/low-redshift GRBs, while the SSC model converges or exceeds the Power-Law predictions for high-luminosity bursts due to longer integration times of the afterglow.
• Redshift Sensitivity:
  • WCDA: Detectable up to $z \lesssim 0.5$.
  • KM2A: Detectable up to $z \lesssim 0.2$.
  • LACT: Detectable up to $z \lesssim 0.7$.
  • SWGO: Detectable up to $z \lesssim 1.1$.

5. Significance

This work establishes a realistic expectation for the future of VHE GRB astronomy. It demonstrates that while current IACTs (like LACT) are powerful for detailed follow-up, wide-field EAS arrays (LHAASO and SWGO) are the primary drivers for discovering new VHE GRBs due to their ability to monitor large sky areas continuously.

The results suggest that the next decade will see a transition from rare, serendipitous detections to a statistically significant sample of VHE GRBs. This sample will be crucial for:

1. Probing the physical origin of TeV emission (e.g., SSC vs. external Compton).
2. Constraining jet structures and circumburst environments.
3. Understanding particle acceleration in relativistic shocks.
4. Enabling multi-messenger astronomy by providing triggers for gravitational wave and neutrino observatories.

The paper concludes that the synergy between EAS arrays (for discovery/triggers) and IACTs (for precision follow-up) represents the optimal strategy for unlocking the physics of the most energetic events in the universe.