Chandra detections of two quiescent black hole X-ray transients

Using the Chandra X-Ray Observatory, we have detected the black hole transients V4641 Sgr and XTE J1859+226 in their low-luminosity quiescent states. The 0.3-8 keV luminosities are (4.0(-2.4)(+3.3))x10(31) (d/7 kpc)(2) ergs s(-1) and (4.2(-2.2)(+4.8))x10(31) (d/11 kpc)(2) ergs s(-1) for V4641 Sgr and XTE J1859+226, respectively. With the addition of these two systems, 14 out of the 15 transients with confirmed black holes (via compact object mass measurements) now have measured quiescent luminosities or sensitive upper limits. The only exception is GRS 1915+105, which has not been in quiescence since its discovery in 1992. The luminosities for V4641 Sgr and XTE J1859+226 are consistent with the median luminosity of 2x10(31) ergs s(-1) for the systems with previous detections. Our analysis suggests that the quiescent X-ray spectrum of V4641 Sgr is harder than for the other systems in this group, but because of the low statistical quality of the spectrum, it is not clear if V4641 Sgr is intrinsically hard or if the column density is higher than the interstellar value. Focusing on V4641 Sgr, we compare our results to theoretical models for X-ray emission from black holes in quiescence. Also, we obtain precise X-ray positions for V4641 Sgr and XTE J1859+226 via cross-correlation of the X-ray sources detected near our targets with IR sources in the Two Micron All Sky Survey catalog.