TESS detection of periodic brightness variations during the rise of classical nova PGIR22akgylf

Abstract: Classical novae are transient events powered by thermonuclear burning in a layer of hydrogen-rich material accreted by a white dwarf from its binary companion. Most classical novae reach optical maximum within ~1 d, but a rare few rise far more slowly. We probe the envelope structure and ejection mechanism of the slowly-rising nova PGIR22akgylf with TESS photometry spanning 3 to 16 d after the nova discovery, supplemented by ground-based observations that cover its full ~133 d ascent to maximum. We detect a 0.1802 +/-0.0012 d periodic brightness modulation with a peak-to-peak amplitude of ~0.02 mag, identified with PGIR22akgylf via temporal and spatial coincidence. The period is stable over the two weeks of TESS coverage, suggesting an orbital origin. Whether this period corresponds to the full or half orbital period, it implies a dwarf donor companion. At the time of the TESS observations the nova was >~6 mag above quiescence (but still 4 mag below peak), so its light should be dominated by the expanding photosphere. We interpret the periodic signal as arising from the binary orbital motion distorting the nova envelope while its size remains comparable to the binary separation. This interpretation points to common-envelope interaction as a contributor to shell ejection in PGIR22akgylf and demonstrates that the slow-rise phenomenon is not exclusive to thermonuclear eruptions in symbiotic binaries, where the large orbital separation of the giant companion inhibits such interaction.