Mid-infrared emission from dust around quiescent low-mass X-ray binaries

We report the discovery of excess 4.5 and 8 mu m emission from three quiescent black hole low-mass X-ray binaries, A0620-00, GS 2023+338, and XTE J1118+480, and the lack of similar excess emission from Cen X-4. The mid-infrared emission from GS 2023+338 probably originates in the accretion disk. However, the excess emission from A0620-00 and XTE J1118+480 is brighter and peaks at longer wavelengths, and thus most likely originates from circumbinary dust that is heated by the light of the secondary star. For these two sources, we find that the inner edges of the dust distributions lie near 1.7 times the binary separations, which are the minimum radii at which circumbinary disks would be stable against tidal disruption. The excesses are weak at 24 mu m, which implies that the dust does not extend beyond about 3 times the binary separations. The total masses of circumbinary material are between and g. The material could be the remains of fallback disks produced in supernovae, or material from the companions injected into circumbinary orbits during mass transfer.