Unveiling the Milky Way's Hidden Glow: Diffuse Gamma Rays measured by LHAASO

In this episode, we explore the fascinating world of diffuse gamma rays emanating from the Galactic plane. A new study using the Large High Altitude Air Shower Observatory's Water Cherenkov Detector Array (LHAASO-WCDA) has provided unprecedented insights into these mysterious emissions.

What are diffuse gamma rays? These high-energy photons are produced when cosmic rays, energetic particles that constantly bombard our galaxy, interact with interstellar gas and radiation. Studying these gamma rays provides valuable information about the distribution and behavior of cosmic rays, helping us unravel their origins and propagation throughout the Milky Way.

LHAASO-WCDA's groundbreaking observations: The LHAASO-WCDA has detected diffuse gamma-ray emissions across a wide energy range, from 1 TeV to 25 TeV, bridging a crucial gap between previous observations by space-based and ground-based detectors.

Mapping the Galactic plane: The study focused on two distinct regions of the Galactic plane: the inner region (15° < l < 125°, |b| < 5°) and the outer region (125° < l < 235°, |b| < 5°). This wide coverage allowed scientists to create detailed maps of the diffuse gamma-ray emission, revealing intriguing patterns.

Surprising findings: The LHAASO-WCDA measurements show that the diffuse gamma-ray fluxes are significantly higher than predicted by conventional models that consider only interactions between cosmic rays and interstellar gas. This excess suggests the presence of additional, as yet unidentified sources of gamma rays in the Milky Way.

Possible explanations: Several hypotheses have been proposed to explain the unexpected gamma-ray excess.

• One leading contender is the existence of a population of unresolved sources, such as pulsar wind nebulae or pulsar halos, that emit gamma rays at energies below tens of TeV.
• Other possibilities include contributions from young massive star clusters or modifications to our understanding of cosmic ray propagation.

Future directions: Combining LHAASO-WCDA's observations with data from other experiments, such as neutrino detectors, will be crucial to pinpoint the origin of the diffuse gamma-ray excess and further illuminate the mysteries of cosmic rays and their journey through our galaxy.

Reference: Zhen Cao et al. "Measurement of Very-high-energy Diffuse Gamma-ray Emissions from the Galactic Plane with LHAASO-WCDA" (arXiv:2411.16021v1)

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: LHAASO Collaboration