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The Black Hole Candidate & X-Ray Transient IGR J17091−3624

23/04/2012

Cataclysmic variables (CVs) are a certain class of binary star system. Some CV systems usually contain a main sequence star, similar to our Sun in mass that is accreting gases onto a compact remnant. The remnant is usually a white dwarf, but sometimes a neutron star (NS) or in rarer cases an ultra-compact singularity known colloquially as a black hole (BH). Such rare ultra-compact systems are known to astronomers as X-ray transients. X-ray transients are more often, throughout their evolution, in a dormant state or quiescent state (Kolb & Burderi 1996; Sguera et al. 2005). During this dormancy they usually remain undetected in X-rays. Occasionally such systems go from a quietly quiescent state into a more energetic state [e.g. GRO J1655 – 40 (Hjellming & Rupen 1995); XTE J1810-197 (Ibrahim et al. 2004); GX 339-4 (Belloni et al. 2005); IGR J11215-5952 (Sidoli et al. 2007); H1743-322 (McClintock et al. 2009)], increasing their X-ray luminosities increase by several orders of magnitude bringing them into focus, as it were, in the X-ray wavelengths. This increase in magnitude is believed to be due to accretion instabilities flowing onto the compact object of the binary system (van Paradijs 1996).

One black hole X-ray transient of intense interest recently is IGR J17091-3624. Lying at just under 8.6kpc away, IGR J17091-3624 has been quite unstable in recent years going into 2 outburst phases since the launch of the Swift X-ray Telescope in 2004. The current outburst phase of X-ray transient IGR J17091-3624 (a.k.a SAX J1709.1-3624) was first bought to the attention of the astronomical community back in February 2011 (ATel #3144): The Swift/BAT hard X-ray transient reported renewed activity in the 15-50 keV band from a location consistent with the X-ray binary system and black hole candidate. The source has increased in intensity from its normal average level of 0.0006  (~3 mCrab*) up to 0.0039 +/- 0.0016  (20 mcrab) on 28 Jan 2011 (MJD 55589), rising further to 0.014 /- 0.003  (60 mCrab) by 03 Feb 2011.

N.B.* 1mCrab is a special astronomical unit which is equal to one thousandth of the intensity of the Crab nebula, a supernova remnant and pulsar wind nebula in the constellation of Taurus (Bevis 1791; Fesen et al. 1997).

From such observations it has been theorised that IGR J17091-3624 is a cousin to the black hole binary system GRS 1915+105 (Capitano et al. 2009) and is itself a plausible black hole candidate. This seems to be almost proven, but recent work by King et al. (2012) has driven this theory into almost certainty with their observations of an extreme X-ray accretion disc wind moving at a intense speed of , which matches some of the fastest winds generated by super massive black holes at the centres of many galaxies (e.g. Feruglio et al. 2010; Dotan & Shaviv 2011). Such massive winds are very unexpected for such a small stellar sized black hole.

N.B.† King et al. (2012) assume a preliminary modest mass for IGR J17091-3624 of 10.

One other interesting property of the recent outbursts is that no counterpart radio emission was observed. King et al. (2012) state that this supports prior findings that the radio jet is absent during the outburst period when winds are seen in black hole binaries (BHBs) (e.g. Miller et al. 2006a, 2008; Neilsen & Lee 2010).

Fast X-ray disk winds are not only seen in BHB like IGR J17091−3624, but also in AGN and quasars (e.g. King et al. 2011b; Chartas et al. 2002). The fastest UV winds observed in AGN are pushed to high velocities by a radiated pressure. It remains to be seen whether a common driving mechanism works across the black hole mass scale to drive fast, highly ionized X-ray disc winds. Further observations of IGR J17091−3624 should allow astronomers to answer the remaining questions regarding accretion disc winds, for example, why strogn disc winds stifle the jets and what the mechanism is which can produce such tremendous winds.

Journal References:

  • Pandey, M. et al. (2006) GMRT Observations Of The Field Of INTEGRAL X-Ray SourcesAstronomy & Astrophysics, 446 (2): pp.471-483.
  • Capitano, F. et al. (2009) The Two Integral X-Ray Transients IGR J17091-3624 & IGR J17098-3628: A Multiwavelength Long-Term Campaign. The Astrophysical Journal690 (2): pp. 1621-1632.
  • King, A.L. et al. (2012) An Extreme X-Ray Disk Wind In The Black Hole Candidate IGR J17091-3624. The Astrophysical Journal: Letters, 746 (2): Article I.D. L20.
  • Wijnards, R. et al. (2012) The Enigmatic Black Hole Candidate & X-ray Transient IGR J17091-3624 In Its Quiescent State As Seen With XMM-Newton. Monthly Notices Royal Astronomical Society: Letters, 422 (1): pp.91-95.
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