Concise estimate of the expected number of detections for stellar-mass binary black holes by eLISA

We study prospects for detecting extragalactic binary black holes similar to GW150914 by evolved Laser Interferometer Space Antenna (eLISA). We find that the majority of detected binary black holes will not merge within reasonable observation periods of eLISA in any configuration. While long-arm detectors are highly desired for promoting multiband gravitational-wave astronomy by increasing the detections of merging binaries, the number of total detections can be increased also by improving the acceleration noise. A monochromatic approximation works well to derive semiquantitative features of observational prospects for non-merging binaries with clearly indicating the parameter dependence. Our estimate also suggests that the number of galaxies in the error volume is so small that the host galaxy may be determined uniquely with high confidence.