Article

The spin of the near-extreme Kerr black hole GRS 1915+105

Harvard University, Cambridge, Massachusetts, United States
The Astrophysical Journal (Impact Factor: 5.99). 06/2006; 652(1). DOI: 10.1086/508457
Source: arXiv

ABSTRACT

Based on a spectral analysis of the X-ray continuum that employs a fully
relativistic accretion-disk model, we conclude that the compact primary of the
binary X-ray source GRS 1915+105 is a rapidly-rotating Kerr black hole. We find
a lower limit on the dimensionless spin parameter of a* greater than 0.98. Our
result is robust in the sense that it is independent of the details of the data
analysis and insensitive to the uncertainties in the mass and distance of the
black hole. Furthermore, our accretion-disk model includes an advanced
treatment of spectral hardening. Our data selection relies on a rigorous and
quantitative definition of the thermal state of black hole binaries, which we
used to screen all of the available RXTE and ASCA data for the thermal state of
GRS 1915+105. In addition, we focus on those data for which the accretion disk
luminosity is less than 30% of the Eddington luminosity. We argue that these
low-luminosity data are most appropriate for the thin alpha-disk model that we
employ. We assume that there is zero torque at the inner edge of the disk, as
is likely when the disk is thin, although we show that the presence of a
significant torque does not affect our results. Our model and the model of the
relativistic jets observed for this source constrain the distance and black
hole mass and could thus be tested by determining a VLBA parallax distance and
improving the measurement of the mass function. Finally, we comment on the
significance of our results for relativistic-jet and core-collapse models, and
for the detection of gravitational waves.

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Available from: Rebecca Shafee, Apr 22, 2013
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    • "Besides those in table 1, there are three more with measured Kerr parameters. GRS 1915+105, which has an a 0.98 (McClintock et al., 2006, ; compare to the model prediction for natal spin: a ∼ 0.2); it is likely that the BH in this system accreted 50% of its present mass (which is currently Table 1: Physical parameters for six BH binaries (three LMXBs and three HMXBs) listed in order of increasing a "
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    ABSTRACT: The spin of a number of black holes (BHs) in binary systems has been measured. In the case of BHs found in low-mass X-ray binaries (LMXBs) the observed values are in agreement with some theoretical predictions based on binary stellar evolution. However, using the same evolutionary models, the calculated spins of BHs in high-mass X-ray binaries (HMXBs) fall short compared to the observations. A possible solution to this conundrum is the accretion of high-specific-angular-momentum material after the formation of the BH, although this requires accretion above the Eddington limit. Another suggestion is that the observed high values of the BHs spin could be the result of an asymmetry during Core Collapse (CC). The only available energy to spin up the compact object during CC is its binding energy. A way to convert it to rotational kinetic energy is by using a Standing Accretion Shock Instability (SASI), which can develop during CC and push angular momentum into the central compact object through a spiral mode ($m = 1$). Here we study the CC-SASI scenario and discuss, in the case of LMXBs and HMXBs, the limits for the spin of a stellar-mass BHs. Our results predict a strong dichotomy in the maximum spin of low-mass compact objects and massive BHs found in HMXBs. The maximum spin value ($|a_\star|$) for a compact object near the mass boundary between BHs and NSs is found to be somewhere between 0.27 and 0.38, depending on whether secular or dynamical instabilities limit the efficiency of the spin up process. For more massive BHs, such as those found in HMXBs, the natal spin is substantially smaller and for $M_{\rm BH}\!>\!8~M_\odot$ spin is limited to values $|a_\star|\!\lesssim\!0.05$. Therefore we conclude that the observed high spins of BHs in HMXBs cannot be the result of a CC-SASI spin up.
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    • "In considering non-BPS configurations, it is natural to seek a construction of gravitational solutions describing microstates of extremal, non-BPS black holes such as the extremal Kerr[9]and Myers-Perry[10]solutions. There is strong observational evidence of the existence of near-extremal black holes in our galaxy[11]. There has also been much interest in a recent conjecture that quantum gravity in the near-horizon region of an extremal Kerr black hole may be described by a chiral 2D CFT[12]. "
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    • "Equation (13) essentially rules out the possibility that current BH candidates have no horizon, or at least something that behaves very much like a horizon for the unstable modes. The possibility of an exact Kerr background with δ so large that there is no ergoregion seems to be unlikely, as we know objects that, when the spacetime around them is described by the Kerr solution, would have an accretion disk with inner edge inside the ergosphere [23–25]. "
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