Contribution of Unresolved Pulsar Wind Nebulae and TeV Halos to the Ultra-high-energy Galactic Diffuse Gamma-Ray Emission

Core Concepts

Unresolved pulsar wind nebulae and TeV halos contribute only a limited fraction to the ultra-high-energy Galactic diffuse gamma-ray emission, suggesting a significant portion of the emission originates from other sources, potentially hadronic interactions.

Abstract

Bibliographic Information:

Kaci, S., Giacinti, G., & Semikoz, D. (2024). On the Contribution of Unresolved Pulsars to the Ultra-high-energy Galactic Diffuse Gamma-Ray Emission. arXiv preprint arXiv:2407.20186.

Research Objective:

This study aims to estimate the contribution of unresolved pulsar wind nebulae (PWNe) and TeV halos to the ultra-high-energy (UHE) Galactic diffuse gamma-ray emission measured by the Large High Altitude Air Shower Observatory (LHAASO).

Methodology:

The researchers employed a data-driven approach using information from the Australia Telescope National Facility (ATNF) and LHAASO catalogs. They generated a synthetic population of pulsars based on the statistical properties of observed pulsars, including their spatial distribution, age, spindown power, and gamma-ray emission characteristics. They then simulated the gamma-ray flux from these unresolved sources and compared it to the LHAASO measurements.

Key Findings:

• The contribution of unresolved PWNe and TeV halos to the UHE diffuse gamma-ray flux in the outer Galaxy (125° ≤ l < 235° and |b| ≤ 5°) is always below 18% ± 2%.
• In the inner Galaxy (15° ≤ l < 125° and |b| ≤ 5°), the contribution is energy-dependent and varies with the assumed minimum spindown power of gamma-ray emitting pulsars.
• The maximum contribution is estimated to be less than 38% ± 10% at 20 TeV and decreases to less than 21% ± 6% above 100 TeV.

Main Conclusions:

The study concludes that unresolved PWNe and TeV halos cannot solely account for the observed UHE Galactic diffuse gamma-ray emission, particularly above a few tens of TeV. This suggests a significant contribution from other sources, potentially hadronic interactions of cosmic rays with the interstellar medium.

Significance:

This research provides valuable constraints on the origin of the UHE Galactic diffuse gamma-ray emission, which is crucial for understanding cosmic-ray propagation and the sources of PeV cosmic rays.

Limitations and Future Research:

The study acknowledges the limitations of relying solely on data-driven approaches and the uncertainties associated with the assumed pulsar properties. Future research could explore the contribution of other potential sources, such as supernova remnants and microquasars, to the diffuse gamma-ray background.

Stats

The contribution of unresolved pulsars to the diffuse flux in the outer Galaxy is always smaller than 18% ± 2%.
In the inner Galaxy, the contribution of unresolved pulsars is limited to ~38% ± 10% of the flux reported by LHAASO around 20 TeV.
Above 100 TeV, the contribution of unresolved pulsars to the diffuse flux is smaller than ~21% ± 6%.
The weakest pulsar (in terms of spindown power) detected by LHAASO has a spindown of ~2.7 × 10^34 erg s−1.

Quotes

"We conclude that the UHE Galactic diffuse gamma-ray emission cannot be dominated by unresolved pulsar sources above a few tens of TeV."

Key Insights Distilled From

On the Contribution of Unresolved Pulsars to the Ultra-high-energy Galactic Diffuse Gamma-Ray Emission

by Samy Kaci, G... at arxiv.org 11-04-2024

https://arxiv.org/pdf/2407.20186.pdf

Deeper Inquiries

How might future gamma-ray observatories with improved sensitivity and resolution impact the study and identification of unresolved sources contributing to the diffuse gamma-ray background?

Future gamma-ray observatories with enhanced sensitivity and resolution hold immense potential to revolutionize our understanding of the diffuse gamma-ray background and its contributing sources, particularly the elusive unresolved ones. Here's how:

Resolving the Faint End: Improved sensitivity will enable the detection of significantly fainter sources compared to current instruments. This is crucial for uncovering the numerous faint, unresolved sources like pulsar wind nebulae (PWNe) and TeV halos that may collectively contribute a substantial fraction of the diffuse emission. By pushing the detection limits lower, these observatories can unveil a hidden population of sources previously masked by their faintness.

Sharper Vision: Enhanced angular resolution will allow for a more precise separation of individual sources, even in crowded regions like the Galactic plane. This will be instrumental in disentangling the contributions of various source populations, including pulsars, supernova remnants (SNRs), and perhaps even dark matter annihilation or decay products. By resolving sources that currently appear blended together, a clearer picture of the diffuse background's composition will emerge.

Unveiling Morphology: Higher resolution will also facilitate the study of the morphology of extended sources like PWNe and TeV halos. This can provide valuable insights into the energetics and evolution of these objects, further constraining their contribution to the diffuse emission. By mapping the spatial distribution of gamma-ray emission within these sources, we can better understand their underlying physical processes.

Spectral Fingerprinting: Improved spectral measurements, particularly at higher energies, will be crucial for distinguishing between different emission mechanisms. For instance, hadronic and leptonic processes produce distinct spectral features in gamma rays. By obtaining more precise spectral data, we can better differentiate between these scenarios and determine the dominant origin of the diffuse emission.

Multi-Messenger Synergy: Future gamma-ray observatories will likely operate in conjunction with other multi-messenger facilities, such as neutrino telescopes and gravitational wave detectors. This synergy will be invaluable for identifying the sources of the diffuse emission. For example, the simultaneous detection of neutrinos and gamma rays from a particular region of the sky would strongly suggest a hadronic origin, potentially linked to cosmic-ray acceleration in SNRs or other powerful astrophysical objects.
In summary, future gamma-ray observatories with their superior capabilities will usher in a new era in the study of the diffuse gamma-ray background. By resolving fainter sources, providing sharper images, and enabling more precise spectral measurements, they will help us unravel the mysteries surrounding the origin and composition of this enigmatic emission, ultimately deepening our understanding of the high-energy universe.

Could the observed excess in diffuse gamma-ray emission reported by LHAASO be attributed to systematic uncertainties in the measurements or background estimation rather than unresolved sources?

While the excess diffuse gamma-ray emission observed by LHAASO is intriguing and suggestive of significant contributions from unresolved sources, it's crucial to carefully consider potential systematic uncertainties in the measurements or background estimation before concluding that the excess is entirely astrophysical. Here are some factors that could potentially contribute to the observed excess:

Background Modeling: Accurately modeling and subtracting the diffuse gamma-ray background, which arises from interactions of cosmic rays with interstellar gas and radiation fields, is a complex task. Uncertainties in the distribution and properties of interstellar matter, as well as the cosmic-ray spectrum and propagation, can introduce systematic errors in the background estimation. An underestimation of the background could lead to an apparent excess in the measured diffuse flux.

Instrument Calibration: Precise calibration of the instrument's energy response and effective area is crucial for accurately measuring the gamma-ray flux. Any systematic uncertainties in the calibration could propagate into the final flux measurements, potentially resulting in an overestimation of the diffuse emission.

Point Source Subtraction: LHAASO, like other gamma-ray observatories, has a finite angular resolution. This means that faint or nearby point sources might not be individually resolved and could potentially contribute to the diffuse emission. Incomplete or inaccurate subtraction of these unresolved point sources could lead to an overestimation of the truly diffuse component.

Systematic Effects in PSF: The point spread function (PSF) describes the instrument's response to a point source and is crucial for analyzing extended emission. Uncertainties or systematic effects in the PSF, particularly at high energies, could affect the deconvolution process used to separate point sources from diffuse emission, potentially leading to an overestimation of the diffuse flux.

Energy-Dependent Effects: The magnitude of systematic uncertainties could vary with energy. For instance, background modeling might be more challenging at higher energies where the gamma-ray sky is less well-studied. Similarly, instrument calibration uncertainties could be more pronounced at the extremes of the energy range.
It's important to note that the LHAASO collaboration has carefully considered and addressed many potential systematic uncertainties in their analysis. However, it's always prudent to remain cautious and explore all possible explanations for unexpected observations. Further scrutiny of the data, improved background models, and independent measurements from other instruments will be crucial for confirming and understanding the origin of the LHAASO excess.

If a significant portion of the UHE diffuse gamma-ray emission originates from hadronic interactions, what implications does this have for our understanding of the distribution and properties of cosmic rays in the Galaxy?

If a substantial fraction of the ultra-high-energy (UHE) diffuse gamma-ray emission is indeed produced through hadronic interactions, it would have profound implications for our understanding of cosmic rays and their behavior in the Galaxy. Here's why:

Cosmic-Ray Distribution: Hadronic gamma rays are produced when cosmic rays, primarily protons and heavier nuclei, collide with interstellar gas, primarily hydrogen. The intensity and spatial distribution of these gamma rays would therefore trace the distribution of cosmic rays throughout the Galaxy. A strong hadronic component in the diffuse emission would suggest a more widespread and perhaps less confined distribution of cosmic rays than previously thought.

Cosmic-Ray Spectrum: The energy spectrum of the diffuse gamma rays carries information about the energy spectrum of the parent cosmic rays. By studying the spectral shape and features of the hadronic gamma-ray emission, we can infer properties of the cosmic-ray spectrum, particularly at high energies where direct measurements are challenging.

Cosmic-Ray Propagation: The propagation of cosmic rays through the Galaxy is a complex process influenced by magnetic fields and interactions with interstellar matter. The spatial and spectral characteristics of the diffuse gamma-ray emission can provide valuable constraints on cosmic-ray propagation models. For instance, the presence or absence of spectral features could indicate energy-dependent diffusion or interactions with different interstellar environments.

Sources of Cosmic Rays: Identifying the sources responsible for accelerating cosmic rays to ultra-high energies is a major open question in astroparticle physics. The spatial correlation of hadronic gamma-ray emission with potential cosmic-ray accelerators, such as supernova remnants (SNRs) or pulsar wind nebulae (PWNe), could provide clues about the origin of these energetic particles.

Interstellar Medium Properties: The intensity of hadronic gamma-ray emission depends not only on the cosmic-ray flux but also on the density and distribution of target material in the interstellar medium (ISM). By studying the spatial variations and spectral features of the emission, we can gain insights into the properties of the ISM, such as the distribution of molecular clouds, the strength of magnetic fields, and the presence of cosmic-ray-driven shocks.
In conclusion, a significant hadronic contribution to the UHE diffuse gamma-ray emission would provide a wealth of information about cosmic rays, their sources, and their interactions with the interstellar environment. It would offer a unique window into the non-thermal universe and deepen our understanding of the most energetic particles in our Galaxy.