The dwarf nova EX Draconis: a short review

Abstract

EX Draconis (EX Dra) is a long period dwarf nova showing ~2 mag outburst which lasts for ~7 d and recur on a timescale of (20-30) d. Its deep eclipses allows one to trace the changes in surface brightness and radius of its accretion disk along the outburst cycle and to perform critical tests of the predictions of the thermal-viscous disk instability (DI) and the mass transfer outburst (MTO) models proposed to explain dwarf nova outbursts. The results of four critical tests are in clear contradiction with DI while in good agreement with MTO expectations. Furthermore, the observed variations in brightness and outer disk radius throughout EX Dra outbursts are well described by the response of a high-viscosity (alpha = 3-4) accretion disk to events in which the mass transfer rate increases by factors of ~30 for ~7 d, in line with MTO expectations. We further argue that the old expectation of accretion disk theory, alpha <= 1, seems unjustified and contradicts the values derived from dwarf nova outburst decline timescales if they are driven by MTO.

Figures (3)

• Figure 1: Time-lapse eclipse mapping of the dwarf nova EX Dra along its outburst cycle. Left-hand panels: Data (green dots) and model (solid line) light curves in (a) quiescent, (b) early rise, (c) late rise, (d) outburst maximum, (e) early decline, and (f) late decline stages. Vertical dotted lines mark ingress/egress phases of disk center. Middle panels: eclipse maps in a logarithmic grayscale; dark regions are brighter. Dotted lines show the primary Roche lobe and the ballistic trajectory of the gas from the secondary star; crosses mark the center of the disk. The secondary star is to the right of each panel; the stars and the accretion disk gas rotate counter-clockwise. The numbers in parenthesis indicate the time (in days) elapsed since outburst onset. Right-hand panels: azimuthally-averaged radial brightness temperature distributions for the eclipse maps in the middle panels. Dashed lines show the 1-$\sigma$ limit on the average temperature for a given radius. A dotted vertical line depicts the radial position of the BS in quiescence; vertical ticks mark the position of the outer edge of the disk (in red) and the radial position at which the disk temperature falls below 11000 K (blue). Steady-state disc models for mass accretion rates of $\log$ Ṁ$= -8.0$ and $-9.0 \;M_\odot\;$yr$^{-1}$ are plotted as dotted lines for comparison. Numbers in parenthesis list the integrated disk luminosity. From bc01.
• Figure 2: Top: Comparison of the vertical scaleheights of the disk and gas stream for EX Dra. Radial runs of the disk scaleheight are shown for DI (blue) and MTO (red, solid line for $\alpha=4$ and dashed line for $\alpha=3$) disk models. The vertical scaleheight of the gas stream is shown for $T_2= 3800\pm 200$ K; dashed lines depict the corresponding 1-$\sigma$ limits. A vertical dotted line marks the outer disk radius at early rise, $R_d=0.6\,R_\mathrm{L1}$bc01. Bottom: Disk and gas stream midplane densities as a function of mass transfer rate for a disk radius of $R_\mathrm{d}= 0.6\,R_{L1}$. The red (green) lines show the MTO disk midplane densities for $\alpha=4$ ($\alpha=3$), while blue lines show the range of possible DI disk midplane densities. Black lines show the gas stream midplane densities for $T_2= 3800\pm 200$ K. Vertical dotted lines mark the inferred mass transfer rates in quiescence (in red for $\alpha=4$ and in green for $\alpha=3$) and in outburst.
• Figure 3: Top: Average $V$- (blue points) and $R$-band (red points) EX Dra outburst light curves with respect to the time since outburst onset, together with corresponding best-fit MTO model light curves for gray atmosphere local emission. The input parameters for this model are listed together with the $\chi^2$ value of the fit. Bottom: Changes in disk radius as a function of time from outburst onset. The disk radius measurements by bc01 are shown as points with error bars in comparison to the MTO model outer disk radius (solid red line) and the stream penetration radius (dashed red line). Black dashed and dotted lines depict the circularization radius and the radius of shortest stream distance from the WD, respectively.