Detection of two TeV gamma-ray outbursts from NGC 1275 by LHAASO

Kavli Affiliate: Li Xin Li

| First 5 Authors: Zhen Cao, F. Aharonian, Axikegu, Y. X. Bai, Y. W. Bao

| Summary:

The Water Cherenkov Detector Array (WCDA) is one of the components of Large
High Altitude Air Shower Observatory (LHAASO) and can monitor any sources over
two-thirds of the sky for up to 7 hours per day with >98% duty cycle. In this
work, we report the detection of two outbursts of the Fanaroff-Riley I radio
galaxy NGC 1275 that were detected by LHAASO-WCDA between November 2022 and
January 2023 with statistical significance of 5.2~$sigma$ and 8.3~$sigma$.
The observed spectral energy distribution in the range from 500 GeV to 3 TeV is
fitted by a power-law with a best-fit spectral index of $alpha=-3.37pm0.52$
and $-3.35pm0.29$, respectively. The outburst flux above 0.5~TeV was ($4.55pm
4.21)times~10^{-11}~rm cm^{-2}~s^{-1}$ and ($3.45pm 1.78)times~10^{-11}~rm
cm^{-2}~s^{-1}$, corresponding to 60%, 45% of Crab Nebula flux. Variation
analysis reveals the variability time-scale of days at the TeV energy band. A
simple test by one-zone synchrotron self-Compton model reproduces the data in
the gamma-ray band well.

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