[Paper Review] Gamma-ray sources at high latitudes (Extragalactic gamma-ray sources)

D. F. Torres|arXiv (Cornell University)|Aug 5, 2003

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This paper reviews high-latitude extragalactic gamma-ray sources, focusing on Active Galactic Nuclei (AGNs) as the dominant population in the EGRET catalog. It analyzes statistical properties, variability, and multiwavelength correlations, concluding that unidentified high-latitude sources are likely AGNs—particularly blazars—based on flux and spectral similarities, though alternative candidates like molecular clouds or dark matter halos remain debated.

ABSTRACT

This Chapter provides a review of $γ$-ray sources lying at high Galactic latitudes. Their statistical properties and variability status, as well as studies involving cross correlations with lower frequency catalogs and multiwavelength observations, are summarized. The case for active galactic nuclei is analyzed with special emphasis, since they represent the largest population of high energy sources known to date. Other potential $γ$-ray emitters (including nearby starburst galaxies, normal galaxies, molecular clouds in the Galactic halo, galaxy clusters, and radio galaxies) that may appear in the next generation of $γ$-ray catalogs and, perhaps, that might have been already observed by EGRET as unidentified detections, are discussed.

Motivation & Objective

• To characterize the statistical properties and variability of high-latitude gamma-ray sources detected by EGRET.
• To evaluate the likelihood that unidentified high-latitude sources are AGNs, particularly blazars, based on flux, spectral index, and log N–log S distributions.
• To assess alternative explanations for unidentified sources, including starburst galaxies, radio galaxies, galaxy clusters, and cold molecular clouds.
• To test the hypothesis that unidentified sources may be associated with cosmic rays or baryonic dark matter clouds via cross-correlation with microwave and lower-energy catalogs.
• To evaluate the role of multiwavelength and simultaneous observations in resolving source identities and advancing gamma-ray astrophysics.

Proposed method

• Analysis of the Third EGRET Catalog (3EG) using flux and spectral index distributions for sources at |b| > 10°.
• Use of log N–log S plots to compare the distribution of unidentified high-latitude sources with known AGNs, distinguishing between peak and average fluxes.
• Application of unification models to interpret AGN classes, particularly blazars, as viewed along different line-of-sight angles.
• Stacking analysis of radio galaxies to derive upper limits on gamma-ray flux, using data from EGRET and theoretical models.
• Modeling of gamma-ray emission from cold molecular clouds via p-p interactions with cosmic rays in the Galactic halo.
• Cross-correlation of EGRET sources with ultra-high-energy cosmic ray arrival directions and WMAP microwave data to test for associations with dark matter or baryonic clouds.

Experimental results

Research questions

• RQ1 Are the unidentified high-latitude gamma-ray sources in the EGRET catalog predominantly AGNs, especially blazars?
• RQ2 How do the spectral and flux distributions of unidentified high-latitude sources compare to those of known AGNs?
• RQ3 Can cold molecular clouds in the Galactic halo explain the population of faint, soft, and extended unidentified gamma-ray sources?
• RQ4 Is there evidence for a population of baryonic dark matter clouds emitting gamma rays via p-p interactions, and do they predict detectable microwave counterparts?
• RQ5 What is the significance of positional coincidences between EGRET sources and ultra-high-energy cosmic ray arrival directions?

Key findings

• Unidentified high-latitude sources are fainter, softer (spectral index 2.40 ± 0.04), and have a steeper log N–log S distribution than low-latitude sources (spectral index 2.18 ± 0.04), indicating a distinct population.
• The similarity between the peak-flux distribution of unidentified high-latitude sources and AGNs suggests that many unidentified sources may be AGNs, particularly blazars.
• The Gould Belt, with its enhanced supernova rate, is a plausible origin for some of the faint, soft high-latitude sources via cosmic-ray interactions in remnants.
• Modeling of cold molecular clouds as gamma-ray emitters predicts strong microwave emission, but WMAP data show no such bright, unassociated microwave sources, ruling out this explanation for most sources.
• No significant positional correlation was found between 33 ultra-high-energy cosmic rays and EGRET blazar candidates, with a probability of statistical fluctuation exceeding 2σ, rejecting a physical association.
• M87 is a likely candidate for future gamma-ray detection by GLAST, with an EGRET upper limit of 2.8×10⁻⁸ photons cm⁻² s⁻¹ above 100 MeV and theoretical flux predictions near this threshold.