The stunted outbursts of UU Aquarii are likely mass-transfer events

TL;DR

This study analyzes a stunted outburst in UU Aqr, revealing that it is likely caused by enhanced mass-transfer events rather than disc instability, with evidence from eclipse mapping and disc viscosity measurements.

Contribution

It provides the first eclipse mapping analysis of a stunted outburst in UU Aqr, linking it to mass-transfer events instead of disc instability.

Findings

Disc asymmetries indicate a precessing elliptical disc.

Disc viscosity alpha_{hot} is estimated at 0.2.

Outburst initiation involves increased disc wind and delayed disc response.

Abstract

We report a time-lapse eclipse mapping analysis of B-band time-series of the nova-like variable UU Aqr along a typical stunted outburst in 2002 August. Disc asymmetries rotating in the prograde sense in the eclipse maps are interpreted as a precessing elliptical disc with enhanced emission at periastron. From the disc expansion velocity a disc viscosity alpha_{hot}= 0.2 is inferred. The outburst starts with a 10-fold increase in uneclipsed light, probably arising in an enhanced disc wind; the disc response is delayed by 2 d. The results are inconsistent with the disc instability model and suggest that the stunted outburst of UU Aqr are the response of its viscous accretion disc to enhanced mass-transfer events.