A probable inside-out dwarf nova outburst from the period bouncer candidate ASASSN-25dc

TL;DR

This paper reports detailed observations of a dwarf nova outburst from a candidate period bouncer, revealing unique features that challenge existing models of dwarf nova superoutbursts in low-mass-ratio systems.

Contribution

It provides the first detailed characterization of an inside-out dwarf nova outburst in a system with a very low mass ratio, questioning current theoretical models.

Findings

Long outburst decline timescale of 35.2 days per magnitude

Small superhump amplitude of approximately 0.08 mag

Absence of the 2:1 tidal resonance despite short superhump period

Abstract

We report optical time-resolved photometric observations of a newly-discovered outbursting system, ASASSN-25dc. Its 8-mag amplitude, 40-day duration, 1-mag dip in the outburst plateau, and positive superhumps are characteristic of a dwarf nova superoutburst in a non-magnetic cataclysmic variable. We establish its stage-A and stage-B superhump periods as 0.059387(5) d and 0.058864(3) d, respectively. The negative superhump period derivative ($-$1.4(2)$\times10^{-5}$ cycle$^{-1}$) during the stage-B superhumps and the empirical relation indicate the mass ratio is 0.054(7), below the period bounce range. The long outburst decline timescale (35.2(1) d mag$^{-1}$) and small superhump amplitude ($\simeq$0.08 mag) observed in ASASSN-25dc are also seen in some period bouncer systems, but not seen in systems well before the period bounce. Despite its short superhump period and indicated small mass ratio, we find no evidence of the excitement of the 2:1 tidal resonance. Moreover, its outburst rise timescale (1.62(9) d mag$^{-1}$) is significantly longer than those measured at less than 0.4 d mag$^{-1}$ in other dwarf nova outbursts around the period minimum. Overall, an inside-out dwarf nova outburst from a massive disc in a system with a mass ratio around or even below the period minimum, but lacking the 2:1 tidal resonance, may explain all these observations. However, this challenges the existing models of dwarf nova superoutbursts, which do not predict these outburst properties in low-mass-ratio systems.