🔭 astrophysics

Search for Gamma-ray emission from Abell 119 galaxy cluster using INTEGRAL/ISGRI, COMPTEL, and DAMPE data

This study presents a comprehensive multi-wavelength search for non-thermal gamma-ray emission from the Abell 119 galaxy cluster using INTEGRAL/ISGRI, COMPTEL, and DAMPE data, finding no statistically significant detection and establishing upper limits that suggest the previously reported GeV excess remains unconfirmed.

Original authors: Siddhant Manna, Shantanu Desai

Published 2026-03-02

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CC BY 4.0

Original authors: Siddhant Manna, Shantanu Desai

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe is a giant, noisy party. Most of the time, the noise comes from the "hot gas" in galaxy clusters—think of it as the loud music and chatter of the crowd. But sometimes, scientists suspect there might be a secret, high-energy DJ playing a different kind of music: gamma rays. These are the most energetic particles in the universe, created by violent events like colliding galaxies or particles moving at near-light speed.

The galaxy cluster Abell 119 is like a VIP section of this cosmic party. It's massive, nearby, and recently, some scientists thought they heard a faint, mysterious beat coming from it—a "GeV excess" (a bump in the energy data) that suggested this secret DJ was indeed playing.

The Mission: The Cosmic Detective Squad
Authors Siddhant Manna and Shantanu Desai decided to play the role of cosmic detectives. They wanted to see if this "mystery beat" was real or just a trick of the light. To do this, they didn't just use one pair of ears; they brought in a whole team of specialized listening devices, each tuned to a different part of the sound spectrum.

They used data from three different "ears" (telescopes) spanning a massive range of energy:

INTEGRAL/ISGRI: The "Hard X-ray" ear. It listens to the lower, punchier frequencies (30–100 keV).
COMPTEL: The "MeV" ear. It listens to the middle frequencies (0.75–30 MeV), bridging the gap between X-rays and high-energy gamma rays.
DAMPE: The "GeV-TeV" ear. It listens to the highest, most energetic frequencies (3 GeV–1 TeV).

The Investigation: Searching for the Ghost Signal
The scientists looked at decades of archived data, essentially rewinding the tape of the universe to see if they could catch Abell 119 in the act.

The Hard X-ray Check (INTEGRAL): They looked at the cluster with the ISGRI camera. Imagine shining a flashlight into a dark room to find a specific object. They scanned the area and found 28 other bright "objects" (like other galaxies or black holes), but when they pointed their light directly at Abell 119? Nothing. Just background noise. The signal they were looking for was quieter than the static of the universe itself.
The MeV Check (COMPTEL): Next, they tuned into the middle frequencies. It's like trying to hear a whisper in a storm. They analyzed the data with extreme precision, looking for any pattern that didn't fit the background. Again, silence. The data looked exactly like what you'd expect if there were no special gamma-ray source there.
The High-Energy Check (DAMPE): Finally, they checked the highest energies. This is like looking for a specific high-pitched squeak in a rock concert. Even with the powerful DAMPE telescope, they found no evidence of the extra energy that the previous studies claimed to see.

The Verdict: The "Ghost" is Likely a Hallucination
The previous studies that claimed to see a signal were like hearing a rumor that a ghost was in the house. This new study is the team of experts coming in with thermal imaging cameras, motion detectors, and sound recorders. They checked every corner, every frequency, and every angle.

The Result: They found no ghost.

While they couldn't completely rule out that a tiny amount of signal exists (it might be too faint for current tools to hear), they proved that if there is a signal, it is far weaker than previously thought. The "mystery beat" from Abell 119 is likely just a statistical fluke—a case of the universe playing a trick on the scientists' ears.

Why Does This Matter?
Think of it like trying to solve a puzzle. If you think a piece is there, but you look everywhere and it's not, you have to rethink the picture.

For Physics: Galaxy clusters are supposed to be factories for high-energy particles. If we can't find them, it tells us our theories about how these factories work might need a tune-up.
For the Future: This study tells us that to find the real "secret DJ" of the universe, we need better, more sensitive instruments (like the next generation of telescopes). We can't find the signal with our current "ears," so we need to build better ones.

In a Nutshell:
The authors took a massive, multi-tool approach to investigate a suspected cosmic mystery. After listening to the universe across seven decades of energy, they concluded that Abell 119 is likely just a quiet, normal galaxy cluster, and the "gamma-ray ghost" reported by others was probably just a false alarm.

1. Problem Statement

The detection of non-thermal gamma-ray emission from galaxy clusters remains a major open question in high-energy astrophysics. Such emission would provide critical insights into the population of cosmic rays (specifically protons) within the intracluster medium (ICM) and the dynamics of cluster mergers.

Context: Recent studies using Fermi-LAT data have reported tentative gamma-ray excesses from the nearby merging cluster Abell 119 ( $z=0.044$ ). Harale and Paul (2025) reported a $\sim4\sigma$ excess, and Li and Han (2026) reported a moderate excess ( $TS \approx 20.7$ ) in the GeV range.
The Gap: These detections have not reached the canonical $5\sigma$ significance threshold. Crucially, there has been no coordinated, multi-band analysis to test the spectral consistency of these signals from hard X-rays (keV) through MeV to GeV–TeV energies. Without confirmation in complementary energy bands, the origin of the signal (hadronic cosmic rays vs. point-source contamination) remains uncertain.
Objective: To perform a comprehensive, broadband search for non-thermal emission from Abell 119 using archival data from INTEGRAL/ISGRI (hard X-rays), COMPTEL (MeV gamma-rays), and DAMPE (GeV–TeV gamma-rays) to either confirm the Fermi-LAT excess or place robust upper limits.

2. Methodology

The authors conducted an independent analysis of archival data spanning over seven decades in photon energy ( $\sim30$ keV to $1$ TeV). Each instrument was analyzed using its standard data reduction pipelines and statistical frameworks.

Statistical Framework:
- Detection significance was quantified using the Test Statistic (TS), defined as $TS = -2 \ln(L_0/L_1)$ , where $L_0$ and $L_1$ are the maximum likelihoods for the background-only and source-included hypotheses, respectively.
- A $5\sigma$ detection threshold corresponds to $TS \approx 25$ .
- In the absence of significant detection, $95\%$ confidence-level (CL) or $3\sigma$ flux upper limits were derived.
Instrument-Specific Analysis:
- INTEGRAL/ISGRI (30–100 keV):
  - Analyzed 38 Science Windows (ScWs) within a $5^\circ$ radius of Abell 119.
  - Used OSA v11.2 software to generate sky mosaics in four logarithmic energy bands (30–41, 41–54.5, 54.5–73.5, 73.5–99.5 keV).
  - Employed a model-independent, image-based approach to derive upper limits, avoiding systematic uncertainties associated with spectral fitting of coded-mask data.
- COMPTEL (0.75–30 MeV):
  - Re-analyzed archival data from the Compton Gamma Ray Observatory (CGRO) covering four viewing periods.
  - Utilized the GammaLib/ctools framework.
  - Performed maximum-likelihood spectral analysis using two binning strategies: a fine 16-bin scheme and a coarse 4-bin scheme.
  - Modeled Abell 119 as a point source with a power-law spectrum ( $\Gamma$ ) and included standard instrumental background components.
- DAMPE (3 GeV–1 TeV):
  - Analyzed data from Jan 2016 to Dec 2023 ( $\sim1.92 \times 10^8$ s livetime).
  - Used a $10^\circ$ Region of Interest (ROI) centered on the cluster.
  - Applied High-Energy Trigger (HET) selection criteria and excluded South Atlantic Anomaly passages.
  - Performed binned maximum-likelihood analysis with a power-law source model ( $\Gamma=2$ ) against Galactic and isotropic diffuse backgrounds.

3. Key Results

The study found no statistically significant emission from Abell 119 in any of the analyzed energy bands.

Hard X-rays (INTEGRAL/ISGRI):
- No sources were detected at the cluster position. The nearest detected source was $\sim4.8^\circ$ away, far outside the ISGRI Point Spread Function (PSF).
- Maximum significance at the cluster center was $1.21\sigma$ (in the 30–41 keV band).
- Result: Derived a $3\sigma$ upper limit on the integrated flux:
  $F_{30-100 \text{ keV}} \lesssim 8.5 \times 10^{-11} \text{ erg cm}^{-2} \text{ s}^{-1}$
- This is consistent with, though less stringent than, previous Swift/BAT limits, confirming no hard X-ray excess above thermal emission.
MeV Gamma-rays (COMPTEL):
- Likelihood fits yielded $TS \approx 0$ across all energy bins and binning schemes.
- Result: Derived $95\%$ CL upper limits ranging from $\sim9 \times 10^{-11}$ to $\sim8 \times 10^{-10} \text{ erg cm}^{-2} \text{ s}^{-1}$ across the 0.75–30 MeV band. The most stringent limit was in the 0.75–1.89 MeV band ( $< 8.87 \times 10^{-11} \text{ erg cm}^{-2} \text{ s}^{-1}$ ).
GeV–TeV Gamma-rays (DAMPE):
- The broadband fit resulted in $TS \simeq 0$ .
- Result: Derived bin-by-bin $95\%$ CL upper limits spanning $\sim10^{-12}$ to $\sim10^{-10} \text{ erg cm}^{-2} \text{ s}^{-1}$ from 3 GeV to 1 TeV.
- The limits in the 3–10 GeV range do not strictly rule out the flux levels claimed by Fermi-LAT studies but confirm that any emission lies near the sensitivity floor of current instruments.
Combined Spectral Energy Distribution (SED):
- The authors constructed a combined SED spanning seven decades in energy.
- The plot clearly shows that the tentative Fermi-LAT GeV excess (reported by Harale & Paul and Li & Han) is not supported by corresponding emission in the keV or MeV bands, nor is it confirmed by DAMPE in the high-energy tail.

4. Key Contributions

First Multi-Band Constraints: This is the first study to provide a coordinated, broadband analysis of Abell 119 from hard X-rays (keV) to TeV energies, filling a critical gap in the spectral coverage of this cluster.
Reanalysis of COMPTEL: Provided the first MeV constraints on Abell 119 using re-analyzed COMPTEL data, a regime previously unexplored for this specific target.
DAMPE Constraints: Established the first GeV–TeV upper limits for Abell 119 using the DAMPE detector, offering an independent check on Fermi-LAT results.
Spectral Consistency Test: Demonstrated that the tentative GeV excess lacks spectral continuity with lower-energy non-thermal emission, casting doubt on a hadronic origin for the Fermi-LAT signal.

5. Significance and Conclusion

Tentative Nature of GeV Excess: The absence of confirmation across keV–MeV–TeV bands suggests that the reported GeV excess from Abell 119 is likely not a robust detection of cluster-wide non-thermal emission. The signal may be due to statistical fluctuations or unresolved point sources (e.g., radio galaxies identified in the PMN catalog within the error region).
Constraints on Cosmic Rays: The derived upper limits place stringent constraints on the cosmic-ray proton-to-thermal energy fraction in Abell 119, challenging models that predict high levels of hadronic activity in merging clusters.
Future Outlook: The authors conclude that definitive conclusions regarding non-thermal emission in Abell 119 require next-generation MeV observatories (e.g., COSI, AMEGO-X) and improved GeV–TeV facilities (e.g., CTA) to reach the necessary sensitivity to distinguish between a genuine cluster signal and background contamination.

1. Problem Statement

2. Methodology

3. Key Results

4. Key Contributions

5. Significance and Conclusion

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