National Aeronautics and Space Administration

GRO J1655-40 is an X-ray nova black-hole-candidate binary discovered during an outburst in 1994. Its orbital period has been measured in the visible as 2.62 days, and it exhibits irregular outburst activity. Dynamical measurements suggest that the compact object is a ~7 solar mass black hole.

Combining RXTE public Target of Opportunity observations of GRO J1655-40 with the longterm ASM light curves, the discovery of short (on the order of minutes) deep (down to ~8% of the out-of-dip intensity) X-ray dips was investigated over the period from 1996 Feb 21 to 1997 Jun 19.

Fig. 1: RXTE ASM lightcurve of GRO J1655-40 of data from individual dwells of 90sec from 1996 Feb 21 (JD 2450135) to 1997 Oct 3 (JD 2450724). Datapoints separated by < 2 days have been connected to guide the eye. Clearly, deep sharp dips can be seen, which are indicated by arrows. Indicated are also the times of the dips observed with the RXTE PCA and the ROSAT HRI. All the observed dips occurred between photometric orbital phases 0.72 and 0.86, and therefore constitutes the first evidence for orbital variations in X-rays for GRO J1655-40.

Fig. 2: PCA dips: The light curve (upper left panel), hardness curve (lower left panel) and hardness-intensity diagram (HID, lower right panel) of the first public Target of Opportunity data of GRO J1655-40, showing the discovery of absorption dips. The hardness is defined as the ratio of the count rates in the 5.0-13.0 keV and 2.0-5.0 keV bands. The time resolution in the left panel is 0.25 sec. T=0 sec corresponds to 1997 Feb 26, 21:22:21 UTC. The data points in the HID (1 sec averages) are given by open circles. The line through these data points corresponds to a simple model describing the X-ray spectral behaviour in the form of absorption.

Figures and captions courtesy of Erik Kuulkers.

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