Galactic Gamma-Ray Diffuse Emission at TeV energies with HAWC Data

Abstract

The Galactic gamma-ray diffuse emission is produced by the interaction of cosmic rays with ambient gas and electromagnetic radiation fields in the interstellar medium (ISM). Studying this radiation helps reconstruct the particle transport mechanisms and the particle distribution in the Galaxy. In this work, we analyze the TeV diffuse emission in a chosen region of the Galactic plane using data collected with the High Altitude Water Cherenkov (HAWC) detector. The energy and spatial distributions of the diffuse Galactic gamma-ray radiation have been studied after subtracting extended and point sources detected with greater than 5 sigma significance from the region map. The spectral feature of the aforementioned emission is compatible with the gamma-ray emissivity obtained from the locally measured proton and heavier nuclei spectrum, convolved with the gas distribution in the Galaxy.

The Galactic TeV sky: sources or diffuse emission?

“…the spatially dependent exposure. To estimate the amount of unresolved sources in the diffuse measurement reported in [5], we follow a population synthesis approach as described in [8]: Based on four of the models for the population of Galactic sources described there, namely two azimuthally-symmetric distributions following the SNR and PWN distribution, mSNR…”

“…and mPWN, and two models exhibiting spiral arms, one following the matter density, mSp4, and one the free electron density, mFE, (for properties and references of the models see [8]) we use the HAWC exposure to derive longitude and latitude profiles of the sources that are detectable und the ones that are unresolved in the HAWC diffuse emission measurement [5]. For this purpose we use the HAWC sensitivity as presented in the 3HWC [1] as a function of declination and translate it using the quoted observation time of 1347 days in [5] to a two-dimensional exposure in longitude and latitude. As the models of [8] only quote fluxes at 1 TeV, we convert this to a 7 TeV flux assuming the average spectral index of the HGPS, 2.3.…”

“…The different techniques grant for different instrument characteristics, namely a large field of view and duty cycle at the expense of a reduced angular resolution for ASPDs on the one hand and a small field of view of few degrees and very limited duty cycle endowed with an angular resolution of smaller than 0.1 • for IACTs on the other hand. The Galactic sky has been observed with both techniques, reporting the diffuse emission to take up a fraction of 72.7% for |𝑏| < 2 • (76.1% for |𝑏| < 4 • ) in the HAWC measurement [5] and 25% in the H.E.S.S. measurement [6], shown in Fig.…”