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

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.

Figures (5)

• Figure 1: Top panel; $O-C$ diagram of superhump maxima. $C= {\rm BJD~} 2460871.87867 + 0.05885 E$. The solid straight lines correspond to the periods of the stage-A superhumps. The curved line indicates the parabolic fit to the $O-C$ during the stage-B superhumps. Middle panel; Amplitude of superhumps in magnitude scale. The amplitudes after the rapid decline (after BJD 2460902.2; blue diamonds) are multiplied by 0.1 for visualisation purposes. Bottom panel; Overall optical light curve in outburst. The grey dots represent the data from VSNET and SAAO, binned in 0.1 d. The open circles, filled squares, and open squares represent the data of ASASSN $g$ band, ATLAS $c$ band, and ATLAS $o$ band, respectively. The V-shaped markers indicate the upper limits. The horizontal dashed line represents the brightness of the counterpart in the Gaia EDR3 ($G$=21.00(2) mag).
• Figure 2: Zoomed light curves around the outburst rise (top) and the dip (bottom). Symbols are the same as Figure \ref{['fig:longterm']}. The typical errors are smaller than the marker size. The solid line in the top panel represents the rise timescale of 1.6 d mag$^{-1}$.
• Figure 3: Results of PDM analysis (top) and phase-folded profiles (bottom) of stage-A (left) and stage-B (right) superhumps. The superhump phase is arbitrary. The obtained periods are 0.059387(5) and 0.058864(3) d, respectively.
• Figure 4: Orbital periods versus mass ratios of CVs around the period minimum. The shaded region indicates the possible ranges of those of ASASSN-25dc. Black points represent the samples in kat22updatedSHAmethod. The solid and dashed lines represent the standard and optimal evolutionary path of CVs in kni11CVdonor. Some notable systems beyond the period minimum ($P_\text{orb} > 0.06$ d and $q < 0.06$) are labelled with text. Their abbreviations are as follows; AS16dt = ASASSN-16dt, AS16js = ASASSN-16js, AS16kr = ASASSN-16kr, J0522 = CRTS J052209.7$-$350530, J1057 = SDSS J105754.25$+$275947.5, J1222 = SSS J122221.7$-$311523, J1842 = PNV J18422792$+$4837425 = V529 Dra.
• Figure A5: The outburst light curves of V516 Lyr observed by Kepler (gray circles). They are normalized at the outburst maximum and its epoch. The blue-dashed and red-solid lines represent the smoothed ones of inside-out and outside-in outbursts.