The Erratic Nature Of The Black Hole GRS J191511.6+105644 (V1487 Aquilae)
Since its discovery in 1994, V1487 Aquilae (also known as GRS 1915+105) has been the subject of intense study. Using the ISAAC instrument on the VLT’s 8.2-metre ANTU telescope at the ESO Paranal Observatory and NASA’s Chandra X-ray Observatory, countless scores of astronomers peered into a remote area of the Milky Way to probe the binary system GRS 1915+105: a system which can be conclusively described as a black hole X-ray binary with a low mass optical counterpart (Castro-Tirado, Brandt & Lund 1992; Mirabel et al. 1994; Belloni et al. 1997). Massing in at 14, this is the heaviest known stellar-sized black hole in our Galaxy.
A few objects, for example SS433, within our own Galaxy look very much like miniature versions of the most energetic quasi-stellar objects (QSO’s), observed to emanate from he centres of an interesting class of galaxies known as active galaxies (AGNs) via processes of accretion onto a central black hole. The only difference between the QSO class and these smaller versions, known as microquasars, is rather obviously their size of the black hole onto which material is being accreted (Fender et al. 1999).
Close up, microquasars are binary stellar systems in our Galaxy in which a more or less normal star, such as our Sun, orbits an intensely gravitationally compact object, such as a neutron star or a black hole (Sunyaev et al. 1996). Those microquasars also show energetic outflows and signs of accretion of matter onto the compact object familiar with QSOs (Abell & Margon 1979; Bregman et al. 1981; Begelman et al. 2006). Not unexpectedly, it appears that the most enigmatic of these systems are the ones that contain a black hole.
GRS 1915+105, located in the Aquilae constellation, is one of a handful of such microquasars known in our Galaxy that contains a black hole compact source. First discovered in 1994 by the GRANAT X-ray satellite, GRS 1915+105 was revealed to have an interesting, if highly erratic, series of highly luminous outbursts in the X-ray wave bands.
The variable X-ray radiation has been interpreted an infall of matter onto a spinning Kerr black hole from the inner region of a surrounding accretion disk (Greiner, McCaughrean & Cuby 2001), providing evidence for the erratic and sporadic nature of GRS 1915+105. This enigmatic source was also observed to eject clouds of hot gas at velocities very close to the speed of light. GRS 1915+105 is thus a prototype microquasar and has become a main target for the study of accretion onto a black hole of stellar mass (Fender et al. 1999). This variability has allowed a precise determination of the mass of the compact object in GRS 1915+105: 14.19±0.13.
Knowing the mass of the black hole in GRS 1915+105 now poses challenges to several fields in astrophysics. First of all, it is not easy to understand how such a massive black hole can be formed in a binary stellar system. It is well known that the most massive stars lose significant fractions of their mass through violent stellar winds at the end of their lives. Interaction among the two stars in a binary system can further increase the mass loss by the massive star. Therefore, how any star can retain enough mass to eventually end up forming a black hole as heavy as 14?
Secondly, the spin of the black hole object in GRS 1915+105 is suspected to provide the unusually high accretion disc temperature. If a black hole spins in the same direction as the matter orbiting it within the disc then it can spread inwards, which results in a much hotter disc.
Two X-ray binaries are known to be very hot, GRS 1915+105 and Nova Scorpii (Zhang et al. 1994; Shahbaz et al. 1999; Mason et al. 2010), and it was therefore believed that these two contain black holes that must spin rapidly. A completely different line of evidence for black hole rotation comes from the quasi-periodic oscillations often seen in X-ray binaries. Those oscillations are generally interpreted as due to effects of the spinning black hole on the surrounding accretion disk, although the exact mechanism is a matter of debate.
However, the new mass determination for the black hole in GRS 1915+105 indicates that the picture may not be as simple as that. In fact, if GRS 1915+105 and Nova Scorpii both have rapidly spinning black holes, none of the current theories for the quasi-periodic oscillations seem to work. And so, as is often the case in science, new information also brings new puzzles.
- Castro-Tirado, A.J. et al. (1994) Discovery & Observations By Watch Of The X-Ray Transient GRS 1915+105. The Astrophysical Journal: Supplement Series, 92 (2) pp. 469-472.
- Mirabel I. F., Rodríguez L. F. (1994) A Superluminal Source In The Galaxy. Nature, 371 pp.46-48.
- Fender, R.P. et al. (1999) e-MERLIN Observations Of Relativistic Ejections From GRS 1915+105. Monthly Notices Royal Astronomical Society, 304 (1) pp.865-876.
- Vierdayanti, K.; Mineshige, S.; Ueda, Y. (2010) Probing The Peculiar Behavior Of GRS 1915+105 At Near-Eddington Luminosity. Japan Astronomical Society Publications, 62 (2) pp.239–253.
Suggested Further Reading:
- Seward, F.D.; Charles, P.A. (2010) Exploring The X-Ray Universe. Cambridge University Press,2nd Edition. Cambridge, United Kingdom.