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We report on the first 180 days of RXTE observations of the outburst of theblack hole candidate IGR J17091-3624. This source exhibits a broad variety ofcomplex light curve patterns including periods of strong flares alternatingwith quiet intervals. Similar patterns in the X-ray light curves have been seenin the (up to now) unique black hole system GRS 1915+105. In the context of thevariability classes defined by Belloni et al. (2000) for GRS 1915+105, we findthat IGR J17091-3624 shows the \nu, \rho, \alpha, \lambda, \beta and \muclasses as well as quiet periods which resemble the \chi class, all occurringat 2-60 keV count rate levels which can be 10-50 times lower than observed inGRS 1915+105. The so-called \rho class "heartbeats" occur as fast as every fewseconds and as slow as ~100 seconds, tracing a loop in the hardness-intensitydiagram which resembles that previously seen in GRS 1915+105. However, whileGRS 1915+105 traverses this loop clockwise, IGR J17091-3624 does so in theopposite sense. We briefly discuss our findings in the context of the modelsproposed for GRS 1915+105 and find that either all models requiring nearEddington luminosities for GRS 1915+105-like variability fail, or IGRJ17091-3624 lies at a distance well in excess of 20 kpc or, it harbors one ofthe least massive black holes known (< 3 M_sun).

The faint "heartbeats" of IGR J17091-3624: an exceptional black-hole candidate