On the signatures of gravitational redshift: the onset of relativistic emission lines

Free access article

Issue		A&A Volume 457, Number 2, October II 2006


Page(s)		485 - 492
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20065615

A&A 457, 485-492 (2006)
DOI: 10.1051/0004-6361:20065615

On the signatures of gravitational redshift: the onset of relativistic emission lines

A. Müller¹ and M. Wold²

¹ Max-Planck-Institut für Extraterrestrische Physik, PO box 1312, 85741 Garching, Germany
e-mail: amueller@mpe.mpg.de
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany

(Received 16 May 2006 / Accepted 9 June 2006)

Abstract
Aims.We quantify the effect of gravitational redshift on emission lines to explore the transition region from the Newtonian to the Einsteinian regime. With the emitting region closer to the Kerr black hole, lines are successively subjected to a stronger gravitationally induced shift and distortion. Simulated lines are compared to broad, optical emission lines observed in Mrk 110.
Methods.We simulate relativistic emission line profiles by using Kerr ray tracing techniques. Emitting regions are assumed to be thin equatorial rings in stationary Keplerian rotation.

The emission lines are characterised by a generalized Doppler factor or redshift associated with the line core.
Results.With decreasing distance from the black hole, the gravitational redshift starts to smoothly deviate from the Newtonian Doppler factor:

shifts of the line cores reveal an effect at levels of 0.0015 to 60% at gravitational radii ranging from 10⁵ to 2. This corresponds to fully relativistic Doppler factors of 0.999985 to 0.4048.

The intrinsic line shape distortion by strong gravity i.e. very asymmetric lines occur at radii smaller than roughly ten gravitational radii.
Conclusions.Due to the asymptotical flatness of black hole space-time, GR effects are ubiquitous and their onset can be tested observationally with sufficient spectral resolution. With a resolving power of ${\sim}100\,000$ , yielding a resolution of $\approx$ 0.1 Å for optical and near-infrared broad emission lines like H $\beta$ , HeII and Pa $\alpha$ , the gravitational redshift can be probed out to approximately 75 000 gravitational radii. In general, gravitational redshift is an important indicator of black hole mass and disk inclination as recently demonstrated by observations of optical lines in Mrk 110. Comparing our simulated lines with this observations, we independently confirm an inclination angle of 30 degrees for the accretion disk. Redshift deviations induced by black hole spin can be probed only very close to the black hole e.g. with X-ray iron lines.

Key words: black hole physics -- relativity -- line: profiles -- galaxies: active -- Galaxy: nucleus -- galaxies: Seyfert

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.