Detection of a SU UMa-type Dwarf Nova with Long-lived Stage A Superhumps near the Lower Edge of the Period Gap

Abstract

CXOGBS J174633.4–262314 (CX645), originally identified in the Galactic Bulge Survey, has been proposed as either a cataclysmic variable (CV) or a quiescent low-mass X-ray binary candidate. We present the first analysis of high-precision TESS photometry covering a ∼20 day superoutburst-like event, independently confirmed by ATLAS observations, and accompanied by prominent positive superhumps. We obtain the first photometric determination of the orbital period, Porb = 2.049(14) hr. Stage A and Stage B analyses yield consistent mass ratios of qA = 0.213(5) and qB = 0.196(33), giving q ≈ 0.205. Despite this relatively large mass ratio, Stage A is constrained to last ≥22 cycles based on a conservative estimate, suggesting a relatively slow eccentric-mode growth compared with systems of similar mass ratios. The superhump amplitude, eclipse depth, and eclipse FWHM modulate coherently at the disk-precession frequency (fprec ≈ 0.658 day−1) but reach their extrema at mutually offset precession phases, suggesting that a simple geometric interpretation based solely on a precessing eccentric disk is unlikely to fully explain the observed variations and that additional phase-dependent modulation may contribute. The eclipse depth versus the out-of-eclipse flux relation may trace a hysteresis-like loop consistent with disk expansion, and the precession signal persists into quiescence, consistent with the thermal–tidal decoupling scenario. Combined with its Gaia color–magnitude position, these results identify CX645 for the first time as an SU UMa-type dwarf nova located near the lower edge of the CV period gap, with behavior broadly consistent with the thermal–tidal instability (TTI) model.