Synthetic Direct Impact Light Curves of the Ultracompact AM CVn Binary Systems V407 Vul and HM Cnc

TL;DR

This paper models synthetic light curves of ultracompact AM CVn binary systems V407 Vul and HM Cnc, supporting the direct impact accretion model and revealing the necessity of two X-ray spots to match observations.

Contribution

It provides the first synthetic light curve models based on a temperature distribution for the direct impact model of these systems.

Findings

Synthetic light curves match observed optical and X-ray behaviors.

Two X-ray spots are required to reproduce the light curve shapes.

The model supports the direct impact accretion scenario.

Abstract

The interacting binary white dwarf (AM CVn) systems HM Cnc and V407 have orbital periods of 5.4 min and 9.5 min, respectively. The two systems are characterized by an "on/off" behaviour in the X-ray light curve, and optical light curves that are nearly sinusoidal and which lead the X-ray light curves in phase by about 0.2 in both systems. Of the models that have been proposed to explain the observations, the one that seems to require the least fine tuning is the direct impact model of Marsh & Steeghs (2002). In this model, the white dwarf primary is large enough relative to the semi-major axis that the accretion stream impacts the surface of the primary white dwarf directly without forming an accretion disc. Marsh & Steeghs proposed that in this situation there could be a flow set up around the equator with a decreasing surface temperature the further one measured from the impact point. In this study, we estimate the light curves that might result from such a temperature distribution, and find them to be reasonable approximations to the observations. One unexpected result is that two distinct X-ray spots must exist to match the shape of the X-ray light curves.