<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>spin0,I would be tickled pink if they could determine that with absolute certainty. Detecting a shadow on the accretion disk? Is that even possible? I've never heard that as an option for direct detection. Sounds interesting, but my intuition (whatever that's worth) tells me there's something fundementally wrong with that. Could you elaborate on that or point me in the direction of a source that could? <br /> Posted by derekmcd</DIV></p><p>Here's an interesting paper touching the very subject. They suggest a method for observing the shadow of the event horizon and spin of a stellar mass BH by using accretion disk's X-ray spectra and light curve with an eclipsing binary (BH and a star). They elaborate on how companion star's atmosphere might affect observations. (contains illustrative simulated images) </p><p><em>ABSTRACT<br />We calculate eclipsing light curves for accretion flows around a rotating black hole taking into account the atmospheric effects of the companion star. <strong>In the cases of no atmospheric effects, the light curves contain the information of the black hole spin because most of X-ray photons around 1 keV usually comes from the blue-shifted part of the accretion flow near the black-hole shadow and the size and the position of the black-hole shadow depend on the spin. In these cases, when most of the emission comes from the vicinity of the event horizon, the light curves becomes asymmetric at ingress and egress.</strong> We next investigate the atmospheric absorption and scattering effects of the companion stars. By using the solar-type atmospheric model, we have taken into account the atmospheric effects of the companion star, such as the photoionization by HI and HeI. We found that the eclipsing light curves observed at 1 keV possibly contain the information of the black-hole spin. However, in our atmospheric model, the effects of the atmosphere are much larger than the effects of the black-hole spin. Therefore, even in the case that the light-curves contain the information of the black hole spin, it may be difficult to extract the information of the black hole spin if we do not have the realistic atmospheric profiles, such as, the temperature, the number densities for several elements. Even in such cases, the light curve asymmetries due to the rotation of the accretion disc exist. Only when we have the reliable atmospheric model, in principle, the information of the strong-gravity regions, such as, the black hole spin, can be obtained from the eclipsing light curves.</em></p><p>
Eclipsing light curves for accretion flows around a rotating black hole and atmospheric effects of the companion star<br />(Rohta Takahashi, Ken-Ya Watarai - 20 Apr 2007)</p><p> </p><p>But I don't know if Chandra's angular and spectroscopic resolution is accurate enough for using such method nor wheter the suggested method actually works.
<br /> </p><p> </p><p>NASA mentioned also ground based observations - could it mean VLBI?</p><p>Here's a short and interesting paper about VLBI, angular resolution and event horizons:</p><p>
Towards the Event Horizon - High Resolution VLBI Imaging of Nuclei of Active Galaxies</p><p>(T.P. Krichbaum, D.A. Graham, A. Witzel, J.A. Zensus, A. Greve, M. Grewing, M. Bremer, S. Doeleman, R.B. Phillips, A.E.E. Rogers, H. Fagg, P. Strittmatter, L. Ziurys - To appear in the conference proceedings "Exploring the Cosmic Frontier: Astrophysical Instruments for the 21st Century", held in Berlin, Germany, May 17 - 21, 2004, ESO Astrophysical Symposia Series, in press. - Submitted on 5 Jul 2006)</p><p><strong><em>"Thus one can hope that within less than a decade from now, the detailed imaging of the ‘event horizon’ of SMBHs and a better understanding of the coupling between ‘central engine’ and jet will become possible."</em></strong></p><p> </p><p>Anyway my guesses about an observed event horizon are just that - pure guesses based on the few clues of the media advisor combined with some wishful thinking. But it would be so cool!
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