V4140 Sgr: a short period dwarf nova with a peculiar behavior

Abstract

We report on time series of CCD photometry of V4140 Sgr between 1991 and 2001. The analysis reveals that the object was in the decline from an outburst in 1992 and again in outburst in 2001. The historical light curve collected by amateur astronomers shows low amplitude (1-2 mag) outbursts 5-10 days long, recurrent every 80-90 days, confirming its dwarf nova nature. We derive an outburst decline time scale of 1.2 days mag$^{-1}$. Eclipse mapping techniques were applied to data both in quiescence and in outburst to derive accretion disc surface brightness maps. A distance of $d = 170 \pm 30$ pc is obtained from a method similar to that used to constrain the distance to open clusters. From this distance, disc radial brightness temperature distributions are determined. The temperatures in the quiescent disc vary from $6 000$ K in the inner regions to $3 000$ K near the outer disc and are flatter than the $T \propto r^{-{3/4}}$ law for optically thick steady-state disc. The outburst occurs mainly with a significant increase in brightness of the intermediate and outer disc regions. The disc temperatures remain below the critical effective temperature $T_{\rm crit}$ at all disc radii during outburst. The radial temperature distributions in quiescence and in outburst are significantly different from those of other dwarf novae of similar orbital period. These results cannot be explained within the framework of the disc instability model. We suggest that the small amplitude outbursts of V4140 Sgr are caused by bursts of enhanced mass transfer rate from the secondary star.