OAO/MITSuME Photometry of Dwarf Novae: I SU Ursae Majoris

TL;DR

This study presents multi-band photometry of SU Ursae Majoris, revealing complex quiescent states, characteristic normal outbursts, and pre-outburst reddening, suggesting a complicated thermal cycle and new insights into accretion disk behavior.

Contribution

It provides detailed multi-band observations of SU UMa, discovering pre-outburst reddening and complex thermal cycles, offering new perspectives on dwarf nova outburst mechanisms.

Findings

Quiescence divided into three types based on magnitude and color.

Normal outbursts are outside-in type with 11-21 day intervals.

Pre-outburst reddening of $g'-R_c$ detected about 3 days prior to outbursts.

Abstract

We report on simultaneous $g'$, $R_{\rm c}$ and $I_{\rm c}$ photometry of SU Ursae Majoris during 2011 December - 2012 February using OAO/MITSuME. Our photometry revealed that quiescence is divided into three types based on the magnitude and color. Quiescent light curves showed complicated profiles with various amplitudes and time scales. Although no superoutbursts were observed during our run, five normal outbursts occurred with intervals of 11 - 21 d. The shapes of the normal outbursts were characteristic of the outside-in type. During the rising phase of a normal outburst, the light curve showed periodic modulations with a period of ${\sim}$ 0.048111(354) d, but the origin of this peirod was unclear. We examined daily averaged color-color diagram and found that two cycles exist. This implies that the thermal limit cycle in SU UMa is complicated. We newly discovered that $g'-R_{\rm c}$ becomes red about 3 days prior to an outburst. Although the working mechanism on this reddening is unclear, we propose two possibilities: one is that the inner portion of the accretion disk is filled by matter and obscures the central white dwarf, and the other is that the stagnation effect works in the outer region of the accretion disk.