The Faint "Heartbeats" of IGR J17091-3624: An Exceptional Black Hole Candidate

Abstract

We report on the first 180 days of Rossi X-Ray Timing Explorer observations of the outburst of the black holecandidate IGR J17091–3624. This source exhibits a broad variety of complex light curve patterns including periodsof strong flares alternating with quiet intervals. Similar patterns in the X-ray light curves have been seen in the (upto now) unique black hole system GRS 1915+105. In the context of the variability classes defined by Belloni et al.for GRS 1915+105, we find that IGR J17091–3624 shows the ?, ?, a, ?, ß, and µ classes as well as quiet periodswhich resemble the ? class, all occurring at 2–60 keV count rate levels which can be 10–50 times lower thanobserved in GRS 1915+105. The so-called ? class “heartbeats” occur as fast as every few seconds and as slow as~100 s, tracing a loop in the hardness–intensity diagram which resembles that previously seen in GRS 1915+105.However, while GRS 1915+105 traverses this loop clockwise, IGR J17091–3624 does so in the opposite sense.We briefly discuss our findings in the context of the models proposed for GRS 1915+105 and find that either allmodels requiring near Eddington luminosities for GRS 1915+105-like variability fail, or IGR J17091–3624 lies ata distance well in excess of 20 kpc, or it harbors one of the least massive black holes known (<3M).