2025MNRAS.538.2186P

In this study, we present a detailed analysis of public archival soft X-ray data on the classical nova V339 Delphini (Nova Del 2013) during its outburst, obtained using the Chandra High-Resolution Camera Spectrometer and Low Energy Transmission Grating, as well as XMM-Newton in 2013. The observations, spanning from days 85.2 to 112.0 after the optical maximum, capture the nova during its luminous supersoft X-ray source phase. The spectra reveal numerous absorption features with blue-shifted velocities ranging from $\sim$724 to $\sim$1474 km s$^{-1}$, with the majority of lines blue-shifted by approximately 1200 km s$^{-1}$. We confirm the presence of a short-period modulation of the X-ray flux with a period of approximately 54 s, as well as the drift of this period, which was detected on days 97.0 and 112.0 during the outburst with both XMM-Newton and Chandra. This period modulation is transient in nature, with significant variations in amplitude and pulse profile over time-scales of a few thousand seconds, likely due to temporary obscuration events that affect the emission from the central hot source. The pulse profiles exhibit substantial deviations from a pure sinusoidal shape, which may be related to the period drift. Additionally, the modulation amplitude shows a possible anticorrelation with the count rates on day 97.0, likely also caused by temporary obscuration events influencing the central source's emission.