* Astronomy

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RE: IGR J17091-3624

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Title: Why is IGR J17091-3624 so faint? Constraints on distance, mass, and spin from 'phase-resolved' spectroscopy of the 'heartbeat' oscillations
Authors: Anjali Rao, S. V. Vadawale

IGR J17091--3624 is a transient X-ray source and is believed to be a Galactic black hole candidate. Recently, it has received a considerable attention due to the detection of peculiar variability patterns known as 'heartbeats', which are quasi-periodic mini-outbursts repeated over timescales ranging between 5 and 70 s. So far, such variability patterns have been observed only in GRS 1915+105 and these are classified as rho- and nu-variability classes. Here, we present the results of `phase-resolved' spectroscopy of the 'heartbeat' oscillations of IGR J17091-3624 using data from simultaneous observations made by RXTE and XMM-Newton. We find that the 0.7--35 keV spectra can be fitted with a 'canonical' model for black hole sources consisting of only two components---a multi-temperature disk black body and a power law (or its equivalent). We attempt to constrain the system parameters of the source by simultaneously fitting spectra during different phases of the burst profile while tying the system parameters across the phases. The results indicate that the source is a high inclination binary (i>53°). Further, the observed low flux from the source can be explained only if the black hole spin is very low, along with constraints on the black hole mass (<5 solar masses) and the distance (>20 kpc). For higher inclination angles, which is favoured by the data, the black hole spin is required to be negative. Thus, low or retrograde spin could be the reason for the low luminosity of the source.

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Title: The peculiar 2011 outburst of the black hole candidate IGR J17091-3624, a GRS 1915+105 like source?
Authors: Fiamma Capitanio, Melania Del Santo, Enrico Bozzo, Carlo Ferrigno, Giovanni De Cesare, Adamantia Paizis

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 January 2011. 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 to the high inclination angle of the system as well.

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Title: Evidence of two unique variability classes from IGR J17091-3624
Authors: Mayukh Pahari (TIFR, India), Sudip Bhattacharyya (TIFR, India), J. S. Yadav (TIFR, India), S. K. Pandey (PRSU, India)

IGR J17091-3624 is the second black hole X-ray binary after GRS 1915+105, which showed large and distinct variabilities. The study of these variability classes can be useful to understand the accretion-ejection mechanisms of accreting black holes, and hence to probe the strong gravity regime. We report the discovery of two new variability classes (C1 and C2) from IGR J17091-3624 from the 2011 outburst Rossi X-ray Timing Explorer data. These unique classes will be useful to have complete details about the source, and to learn new aspects about variabilities. For examples, the C1 class shows that the intensity and period of oscillations, energy spectrum and power spectrum can clearly evolve in tens of seconds. Moreover, in such a small time scale, soft-lag becomes hard-lag. The C2 class shows that the variability and the nonvariability can occur at similar energy spectrum, and a soft state is not required for variability to happen.

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IGR J17091-3624: NASA'S Chandra Finds Fastest Wind From Stellar-Mass Black Hole

This artist's impression shows a binary system containing a stellar-mass black hole called IGR J17091-3624, or IGR J17091 for short. The strong gravity of the black hole, on the left, is pulling gas away from a companion star on the right. This gas forms a disk of hot gas around the black hole, and the wind is driven off this disk.
New observations with NASA's Chandra X-ray Observatory have clocked the fastest wind ever seen blowing off a disk around this stellar-mass black hole. Stellar-mass black holes are born when extremely massive stars collapse and typically weigh between five and 10 times the mass of the Sun.
The record-breaking wind is moving about twenty million miles per hour, or about three percent the speed of light. This is nearly ten times faster than had ever been seen from a stellar-mass black hole, and matches some of the fastest winds generated by supermassive black holes, objects millions or billions of times more massive.

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Title: Discovery of high-frequency quasi-periodic oscillations in the black-hole candidate IGR J17091-3624
Authors: Diego Altamirano, Tomaso Belloni

We report the discovery of 8.5 sigma high-frequency quasi-periodic oscillations (HFQPOs) at 66 Hz in the RXTE data of the black hole candidate IGR J17091-3624, a system whose X-ray properties are very similar to those of microquasar GRS 1915+105. The centroid frequency of the strongest peak is ~66 Hz, its quality factor above 5 and its rms is between 4 and 10%. We found a possible additional peak at 164 Hz when selecting a subset of data; however, at 4.5 sigma level we consider this detection marginal. These QPOs have hard spectrum and are stronger in observations performed between September and October 2011, during which IGR J17091-3624 displayed for the first time light curves which resemble those of the gamma variability class in GRS 1915+105. We find that the 66 Hz QPO is also present in previous observations (4.5 sigma), but only when averaging ~235 ksec of relatively high count rate data. The fact that the HFQPOs frequency in IGR J17091-3624 matches surprisingly well that seen in GRS 1915+105 raises questions on the mass scaling of QPOs frequency in these two systems. We discuss some possible interpretations, however, they all strongly depend on the distance and mass of IGR J17091-3624, both completely unconstrained today.

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NASA's RXTE Detects 'Heartbeat' of Smallest Black Hole Candidate

An international team of astronomers has identified a candidate for the smallest-known black hole using data from NASA's Rossi X-ray Timing Explorer (RXTE). The evidence comes from a specific type of X-ray pattern, nicknamed a "heartbeat" because of its resemblance to an electrocardiogram. The pattern until now has been recorded in only one other black hole system.
Named IGR J17091-3624 after the astronomical coordinates of its sky position, the binary system combines a normal star with a black hole that may weigh less than three times the sun's mass. That is near the theoretical mass boundary where black holes become possible.
Gas from the normal star streams toward the black hole and forms a disk around it. Friction within the disk heats the gas to millions of degrees, which is hot enough to emit X-rays. Cyclical variations in the intensity of the X-rays observed reflect processes taking place within the gas disk. Scientists think that the most rapid changes occur near the black hole's event horizon, the point beyond which nothing, not even light, can escape.
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Title: The faint "heartbeats" of IGR J17091-3624: an exceptional black-hole candidate
Authors: D. Altamirano, T. Belloni, M. Linares, M. van der Klis, R. Wijnands, P. A. Curran, M. Kalamkar, H. Stiele, S. Motta, T. Munoz-Darias, P. Casella, H. Krimm

We report on the first 180 days of RXTE observations of the outburst of the black hole candidate IGR J17091-3624. This source exhibits a broad variety of complex light curve patterns including periods of strong flares alternating with quiet intervals. Similar patterns in the X-ray light curves have been seen in the (up to now) unique black hole system GRS 1915+105. In the context of the variability classes defined by Belloni et al. (2000) for GRS 1915+105, we find that IGR J17091-3624 shows the \nu,
ho, \alpha, \lambda, \beta and \mu classes as well as quiet periods which resemble the \chi class, all occurring at 2-60 keV count rate levels which can be 10-50 times lower than observed in GRS 1915+105. The so-called
ho class "heartbeats" occur as fast as every few seconds and as slow as ~100 seconds, 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 all models requiring near Eddington luminosities for GRS 1915+105-like variability fail, or IGR J17091-3624 lies at a distance well in excess of 20 kpc or, it harbours one of the least massive black holes known (< 3 M_sun).

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