A Surprising Periodicity Detected during a Super-outburst of V844 Herculis by TESS

Abstract

We identify a previously undetected periodicity at a frequency of 49.08 ± 0.01 days−1 (period of 29.34 ± 0.01 minutes) during a super-outburst of V844 Her observed by TESS. V844 Her is an SU UMa type cataclysmic variable with an orbital period of 78.69 minutes, near the period minimum. The frequency of this new signal is constant in contrast to the superhump oscillations commonly seen in SU UMa outbursts. We searched without success for oscillations during quiescence using MDM, TESS, and XMM-Newton data. The lack of a periodic signal in the XMM light curve and the relatively low X-ray luminosity of V844 Her suggest that it is not a typical IP. We consider the possibility that the 29-minute signal is the result of super-Nyquist sampling of a dwarf nova oscillation with a period near the 2-minute cadence of the TESS data. Our analysis of archival AAVSO photometry from a 2006 super-outburst supports the existence of a 29-minute oscillation, although a published study of an earlier super-outburst did not detect the signal. We compare the X-ray properties of V844 Her with short orbital period intermediate polars (IP), V1025 Cen and DW Cnc. We conclude that the new signal is a real photometric oscillation coming from the V844 Her system and that it is unlikely to be an aliased high-frequency oscillation. The steady frequency of the new signal suggests that its origin is related to an asynchronously rotating white dwarf in V844 Her, although the precise mechanism producing the flux variations remains unclear.