2025RAA....25b5022O

Novae are found to have GeV to TeV γ-ray emission, which reveals the shock acceleration from the white dwarfs. Recently, V1405 Cas was reported to radiate suspicious γ-ray by Fermi-LAT with low significance (4.1σ) after the optical maximum. Radio observations reveal that it is one of the five brightest novae surrounded by low-density ionized gas columns. Here we report a continuous search for GeV γ-ray from Fermi-LAT. No γ-ray was found. For V1405 Cas the flux level is lower than other well-studied Fermi novae, and the γ-ray maximum appears at t₀ + 145 days. γ-ray of V1405 Cas is used to search potential γ-ray periodicity. No γ-ray periodicity was found during the time of observation. By comparing multi-wavelength data, the γ-ray upper limit to optical flux ratio with a value at around 10^–4 is obtained to constrain the shock acceleration. Long-term analysis from Swift-XRT gets X-ray spectral in the post-shock phase, which indicates that V1405 Cas became a super-soft source. The best-fit black body temperature at the super soft state is 0.11-0.19 keV.