Absorption Dips in the Light Curves of GRO J1655−40 and 4U 1630−47 during Outburst

Using the RXTE PCA we discovered deep dips in the X-ray light curves of theblack-hole candidates GRO J1655-40 and 4U 1630-47 during outburst. Similar kindof dips for GRO J1655-40 were found in 90s measurements of the RXTE ASM duringthe same outburst. The duration of the dips in both sources is in the order ofminutes. The occurrences of the dips observed with the RXTE PCA and ASM in GROJ1655-40 are consistent with the optically determined orbital period, and werefound between photometric orbital phases 0.72 and 0.86. This constitutes thefirst evidence for orbital variations in X-rays for GRO J1655-40. The PCA dataindicate that an absorbing medium is responsible for these dips. The X-rayspectra during the dips can be best described by a heavily absorbed componentand an unabsorbed component. In the case of GRO J1655-40 we are able constrainthe extent of the absorbing medium and the central X-ray source.Comment: 19 pages, including 3 figures. Accepted for publication in Ap