The Likely Counterpart to $\gamma$-Ray Excess from The Northwest Region of Arp 220

TL;DR

This study reanalyzed extit{Fermi}-LAT data to identify the gamma-ray counterpart of the excess in Arp 220's northwest region, finding a likely association with the blazar CRATES J153246+234400 and confirming the excess's variability.

Contribution

The paper provides an improved analysis of gamma-ray excess in Arp 220, identifying a likely counterpart and characterizing its variability and spectral properties.

Findings

Photon flux approximately three times higher than previous estimates.

Significance level increased to about 8.15 sigma.

CRATES J153246+234400 is likely the gamma-ray counterpart.

Abstract

The unknown $\gamma$-ray excess in the northwest region of Arp 220 was revisited by analyzing $\sim$11.8 years of the \textit{Fermi} Large Area Telescope (\textit{Fermi}-LAT) data in this study. We found that its photon flux was approximately three times higher than that of the previous study in the 0.2-100 GeV band, and the corresponding significance level$\sim8.15\sigma$ was approximately four times higher than before. The light curves of 15 and 45 time bins from the whole time all showed two active periods, and the variability of the second period was more significant than that of the first period. The spectral indices from the two active periods were not statistically different and were close to the range of $\gamma$-ray flat-spectrum radio quasars observed by \textit{Fermi}-LAT. Because the position of CRATES J153246+234400 was consistent with the best-fit position of our analysis, we suggest that CRATES J153246+234400 is more likely a $\gamma$-ray counterpart for the variational region. For Arp 220, there was no significant variability in the $\gamma$-ray emission.