Systematic Search for Extended GeV Gamma-Ray Sources in the Inner Galactic Plane Using 14 Years of Fermi-LAT Data

Core Concepts

This research paper presents a systematic search for extended gamma-ray sources in the inner Galactic plane using 14 years of Fermi-LAT data, resulting in the identification of 22 new extended sources and providing insights into particle acceleration and propagation near pulsars and other celestial objects.

Abstract

• Bibliographic Information: Abdollahi, S., Acero, F., Acharyya, A., et al. (2024). Search for Extended GeV Sources in the Inner Galactic Plane. arXiv preprint arXiv:2411.07162v1.
• Research Objective: To systematically search for and characterize extended gamma-ray sources in the inner Galactic plane using 14 years of Fermi-LAT data above 10 GeV.
• Methodology: The study utilized 14 years of Fermi-LAT Pass 8 data (P8R3 event reconstruction) in the 10 GeV–1 TeV energy range. A semi-automated extended source detection pipeline based on the Fermipy python package was employed. The search was conducted at the positions of 99 bright pulsars, 143 TeV sources (from H.E.S.S. and HAWC catalogs), and an evenly spaced grid of 302 positions covering the inner Galactic plane (|l| ≤ 100◦, |b| ≤ 1◦). A maximum-likelihood estimation was used to analyze the data and identify extended sources based on their spatial and spectral properties.
• Key Findings:
  • The search yielded 40 extended sources, forming the Second Fermi Galactic Extended Sources Catalog (2FGES).
  • Most sources exhibit 68% containment radii smaller than 1.0◦ and relatively hard spectra with photon indices below 2.5.
  • 22 new extended sources above 10 GeV were reported, with 13 coinciding with known TeV sources, 6 coinciding with bright pulsars, 6 associated with 4FGL point sources, and 1 having no prior association.
  • 6 to 8 sources are potentially related to pulsars, either as classical pulsar wind nebulae or pulsar halos.
• Main Conclusions:
  • The study significantly expands the catalog of extended GeV sources in the Galactic plane.
  • The findings suggest a rich population of pulsar halos, providing valuable insights into particle acceleration and propagation mechanisms in the vicinity of pulsars.
  • The identified sources warrant further investigation to better understand their nature and astrophysical implications.
• Significance: This research enhances our understanding of the Galactic population of extended gamma-ray sources, particularly pulsar halos, and their role in cosmic-ray acceleration and diffusion. The 2FGES catalog serves as a valuable resource for future multi-wavelength observations and theoretical modeling efforts.
• Limitations and Future Research: The study focuses on the inner Galactic plane and excludes very large extended sources. Future research could extend the search to other regions of the Galaxy and explore alternative data analysis techniques to characterize larger halos. Further investigation of the identified sources with multi-wavelength observations is crucial to confirm their nature and study their properties in detail.

Stats

The analysis used 14 years of Fermi-LAT Pass 8 data.
The energy range for the analysis was from 10 GeV to 1 TeV.
The search covered the inner Galactic plane (|l| ≤ 100◦, |b| ≤ 1◦).
40 extended sources were identified, forming the Second Fermi Galactic Extended Sources Catalog (2FGES).
Most sources have 68% containment radii smaller than 1.0°.
The photon indices of most sources are below 2.5.
22 new extended sources above 10 GeV were reported.
6 to 8 sources are potentially related to pulsars.

Quotes

Key Insights Distilled From

Search for Extended GeV Sources in the Inner Galactic Plane

by S. Abdollahi... at arxiv.org 11-12-2024

https://arxiv.org/pdf/2411.07162.pdf

Deeper Inquiries

How might future gamma-ray telescopes with improved sensitivity and resolution, such as the Cherenkov Telescope Array (CTA), further refine our understanding of extended GeV sources and pulsar halos?

Future gamma-ray telescopes like the Cherenkov Telescope Array (CTA) hold immense potential to revolutionize our understanding of extended GeV sources and pulsar halos, thanks to their significantly improved sensitivity and angular resolution compared to current instruments like Fermi-LAT. Here's how:

Resolving the Morphology of Extended Sources: CTA's superior angular resolution will enable us to probe the fine-scale structure of extended GeV sources, distinguishing between different morphological features that might be blurred together in Fermi-LAT data. This will be crucial for differentiating between various source classes, such as pulsar halos, supernova remnants (SNRs), and star-forming regions, based on their distinct morphologies. For instance, CTA can resolve the characteristic toroidal structures associated with PWNe, differentiating them from the more diffuse and extended morphology expected for pulsar halos.

Unveiling Fainter Halos and Extending the Energy Range: CTA's enhanced sensitivity will allow us to detect fainter and more distant pulsar halos that are currently below Fermi-LAT's detection threshold. This will provide a more complete census of the pulsar halo population, enabling us to study their properties and evolution across a wider range of ages and environments. Moreover, CTA's broader energy coverage extending into the TeV regime will allow us to study the spectral evolution of these halos over a much wider energy range, providing crucial insights into the acceleration and propagation of particles within these systems.

Characterizing the Magnetic Field Environment: By studying the morphology and spectral characteristics of pulsar halos with CTA, we can gain valuable insights into the magnetic field environment surrounding pulsars. The diffusion of high-energy particles in these halos is heavily influenced by the magnetic field structure, and CTA's observations can help us constrain the properties of these fields, such as their strength and turbulence. This information is crucial for understanding the mechanisms responsible for particle confinement in pulsar halos and their contribution to the Galactic cosmic-ray population.

Multi-wavelength and Multi-messenger Studies: CTA's observations, when combined with data from other wavelengths (e.g., radio, X-ray) and multi-messenger observations (e.g., neutrinos), will provide a comprehensive view of pulsar halos and their surrounding environments. This multi-faceted approach will be crucial for disentangling the complex interplay between pulsars, their nebulae, and the interstellar medium, ultimately leading to a deeper understanding of particle acceleration and transport processes in these astrophysical systems.

Could some of the extended GeV sources identified in this study be explained by alternative mechanisms beyond pulsar halos, such as supernova remnants interacting with molecular clouds?

Yes, absolutely. While pulsar halos are a plausible explanation for some of the extended GeV sources identified in the study, alternative mechanisms can also produce similar observational signatures. One such prominent alternative is the interaction of supernova remnants (SNRs) with molecular clouds.
Here's why SNR-molecular cloud interactions can mimic extended GeV sources:

Cosmic-Ray Acceleration and Diffusion: SNRs are known to be powerful cosmic-ray accelerators. When a SNR expands into a dense molecular cloud, the shock waves generated can accelerate particles to very high energies, producing gamma-rays through various processes like proton-proton interactions and inverse Compton scattering. This can lead to extended gamma-ray emission around the SNR, often spatially correlated with the molecular cloud.

Morphological and Spectral Similarities: The gamma-ray emission from SNR-molecular cloud interactions can exhibit extended morphologies and relatively hard spectra, similar to those expected from pulsar halos. This makes it challenging to differentiate between these two source classes based solely on their gamma-ray properties.

Distinguishing Features and Multi-wavelength Observations: However, there are some key distinctions. SNRs often exhibit non-thermal radio and X-ray emission associated with synchrotron radiation from accelerated electrons, which can be used to differentiate them from pulsar halos. Additionally, the presence of dense molecular gas, traceable through molecular line emission, can further support the SNR-molecular cloud interaction scenario.
Therefore, a combination of high-resolution gamma-ray observations, along with multi-wavelength data in radio, X-ray, and infrared bands, is crucial for accurately identifying the origin of extended GeV sources and distinguishing between pulsar halos and other potential sources like SNR-molecular cloud interactions.

What are the implications of these findings for our understanding of the Galactic cosmic-ray population and the contribution of pulsars to the observed positron excess?

The findings of this study, particularly the detection of new extended GeV sources potentially linked to pulsar halos, have significant implications for our understanding of the Galactic cosmic-ray population and the origin of the positron excess observed at Earth:

Pulsars as Efficient Cosmic-Ray Accelerators: The detection of extended gamma-ray emission around pulsars, interpreted as halos, strengthens the case for pulsars being significant contributors to the Galactic cosmic-ray population. The presence of halos suggests that pulsars can efficiently accelerate particles to very high energies and release them into the interstellar medium, potentially shaping the cosmic-ray spectrum and influencing the energetics of the Galaxy.

Constraining Pulsar Halo Properties and Evolution: By studying the properties of these extended GeV sources, such as their morphology, spectrum, and evolution over time, we can gain valuable insights into the processes governing particle acceleration and diffusion within pulsar halos. This information is crucial for understanding the efficiency with which pulsars inject cosmic rays into the Galaxy and their overall contribution to the cosmic-ray sea.

Implications for the Positron Excess: The observed excess of positrons at Earth, above what is expected from standard cosmic-ray models, has been a long-standing puzzle. While pulsars have been proposed as a primary source of these positrons, the efficiency of their transport to Earth has been debated. The detection of extended GeV sources, potentially linked to pulsar halos, provides further evidence for the escape of high-energy particles from pulsar systems. However, the spatial extent and diffusion properties of these halos are crucial for determining whether these escaped positrons can effectively propagate to Earth and contribute to the observed excess.

Future Directions and Open Questions: Further investigation into the nature of these extended GeV sources, particularly through deeper observations with future telescopes like CTA and multi-wavelength studies, will be crucial for:

Determining the fraction of these sources that are indeed pulsar halos.
Characterizing the diffusion properties of particles within these halos.
Quantifying the contribution of pulsars to the Galactic cosmic-ray population and the positron excess.
These findings highlight the importance of studying extended GeV sources as a window into the complex interplay between pulsars, their environments, and the Galactic cosmic-ray population.