Detection of the microquasar V404 Cygni at gamma-rays revisited: short flaring events in quiescence

Abstract

The microquasar V404 Cygni (also known as GS 2023+338) was previously reported to have weak GeV gamma-ray emission in sub-day time periods during its 2015 outburst. In order to provide more detailed information at the high energy range for this black hole binary system, we conduct analysis to the data obtained with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (Fermi). Both LAT database and source catalog used are the latest. In addition to the previously reported detection at the peak of the 2015 outburst, we find possible detection ($\sim 4\sigma$) of the source during 3-day time period of 2015 Aug. 17--19 (at the end of the 2015 outburst) and one convincing detection ($\simeq 7\sigma$) in 2016 Aug. 23--25. The latter high-significance detection shows that the gamma-ray emission of the source is soft with photon index $\Gamma \sim 2.9$. As gamma-ray emission from microquasars is considered to be associated with their jet activity, we discuss the results by comparing with those well studied cases, namely Cyg X-3 and Cyg X-1. The detection helps identify V404 Cygni as a microquasar with detectable gamma-ray emission in its quiescent state, and adds interesting features to the microquasar group, or in a more general context to X-ray binaries with jets.

9 pages, 5 figures

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