Investigation of a likely orbital periodicity of Nova Hercules 2021 in X-rays and γ-rays

We report a detection of a ∼0.153 d period in the classical nova V1674 Her using the NICER observations taken within a month since the outburst (i.e. ∼MJD 59405). The X-ray period is consistent with the orbital period previously found in the optical band, strongly suggesting the NICER signal as the X-ray orbital periodicity of the system. A seemingly double-humped profile was obtained by folding the detrended X-ray light curve with the period after removing the rotational X-ray pulsations of the nova. The profile may be caused by occultation by the companion or the accretion disc, possibly indicating a high inclination of the system. The γ-ray emission of V1674 Her with a significance level 5 σ was detected by Fermi-LAT close to its optical peak and the emission faded away within 1 d, which is the shortest duration known for a γ-ray nova. Folded on 0.153 d, a marginal γ-ray variability can be also seen in the LAT light curve, but without the double-hump feature observed in X-rays. If the γ-ray modulation is real, its origin is probably different from that observed in the X-ray and optical bands.