The peculiar 2011 outburst of the black hole candidate IGR J17091-3624, a GRS 1915+105-like source?

We report on the long-term monitoring campaign of the black hole candidate IGR J17091-3624 performed with INTEGRAL and Swift during the peculiar outburst started on 2011 January. We have studied the two-month spectral evolution of the source in detail. Unlike the previous outbursts, the initial transition from the hard to the soft state in 2011 was not followed by the standard spectral evolution expected for a transient black hole binary. IGR J17091-3624 showed pseudo-periodic flare-like events in the light curve, closely resembling those observed from GRS 1915+105. We find evidence that these phenomena are due to the same physical instability process ascribed to GRS 1915+105. Finally, we speculate that the faintness of IGR J17091-3624 could be not only due to the high distance of the source but also due to the high inclination angle of the system.