Dave Axon Memorial Meeting
Dave
D
Measuring
the Spin of Accreting Supermassive Black Holes in AGN
Martin
1
Ward ,
1
Done ,
1
Collinson ,
2
Jin
3
Middleton
Chris
James
C.
and Matt
1Durham University, 2IHEP China, 3University of Amsterdam
Measuring the spin of a black hole is important for two reasons. Firstly, it is a necessary
ingredient for our understanding of black hole physics. Secondly, it contains an imprint of the history of black hole
growth over cosmic time. Recent results from continuum fitting studies (Done et al. 2013), to determine the spin of
supermassive black holes (SMBHs) in active galactic nuclei (AGN), are discussed. These results are in conflict with
previous estimates based on analysis of the iron emission line profile, which often predicts very high spins. For example
Fabian et al. (2009) adopted this latter method in their study of 1H 0707–495. Here we mention the advantages and
disadvantages of these two studies, and the wider implications for black hole growth.
Continuum Fitting – model of Done et al. (2013)
Black hole spin – Theory
Figure 1 (left): Unified model of AGN
Three properties characterize black
holes: mass, spin and charge (which is
negligible in an astrophysical setting)
Spin yields information about the BH
accretion history and possibly also jet
power, growth of the host galaxy and
gravitational wave signature.
Done et al. (2013) have applied a continuum fitting method to the
Narrow Line Seyfert 1 (NLS1), PG 1244+026. This involves fitting a
physically motivated, energy conserving model to the SED. This model
can be used to constrain the spin, as shown below. Problems are that
the peak of the SED is unobservable, and the BH mass must be known.
Spin is difficult to measure. It only affects
emission from close to the last stable orbit,
Rlso:
Zero-spin BH: Rlso = 6 Rg
Max-spin BH: Rlso = 1.24 Rg ,
where Rg is the gravitational radius. We
expect a high-spin BH to have a much
hotter, faster rotating component of its
accretion disc which should be observable.
Figure 4 (left panel): SED theoretical model. The effect of the different AGN
components are highlighted by the different colours.
(Right panel): The resultant SED model for PG 1244+026. Here, the colours refer
to different spin states.
Figure 2 (right) compares high spin AGN (top)
with low spin AGN (bottom).
Iron profiles – Fabian et al. (2009)
L-line
K-line
The data in Fig. 4 show that the highest spins would produce a very
hot inner disc, and are so are strongly ruled out by our model.
Relativistic beaming
Cosmic Implications
Figure 5: Simulations of galaxy
collisions can aid our understanding of the BH growth history.
Flux
Gravitational
redshift
Doppler
broadening
Energy (keV)
Figure 3 (left panel): The relativistically broadened iron lines observed by Fabian
et al. in 1H 0707–495. (Right panel): The assumed broadening mechanisms.
The profile of the X-ray iron lines in 1H 0707–495 implies that the
emission originates from the innermost region of the accretion disc. This
suggests a small radius of innermost stable orbit, which is hence
indicative of a high BH spin. One problem is that the amount of
reflection required at soft and hard energies can be different.
Summary
•SED continuum fitting provides a new method of estimating the spin
of SMBH in AGN
• When applied to the NLS1 PG 1244+026, this method strongly ruled
out the possibility of a high-spin
•Measuring X-ray iron line profiles can also yield spin estimates , but in
the case of a different NLS1 other researches find a high spin. As yet
we are unable to apply both techniques to the same AGN
•The amount of spin can be used as a means of tracking the mode of
accretion by which black holes grow
The results of these spin
studies have implications for
the process by which the BH
has accreted matter over
cosmic time.
A low spin is expected to result from chaotic accretion, where matter
falls onto the BH from random directions. However prolonged
unidirectional accretion will add net angular momentum to the BH,
resulting in a high spin. Simulations exist for both either modes, but
they are mutually exclusive in terms of the resulting spin.
References
•Done, C., et al., 2013, MNRAS (submitted)
•Fabian, A., et al., 2009, Natur, 459, 540
Figures
•Figure 1: Pierre Auger Observatory [http://www.auger.org/news/PRagn/about_AGN.html]
•Figure 2: Jet Propulsion Laboratory [http://www.jpl.nasa.gov/spaceimages/wallpaper.php?id=PIA16696]
•Figure 3 (left): Adapted from Fig. 1 in Fabian et al (2009)
•Figure 3 (right): Adapted from Chris Done (personal communication, 2013)
•Figure 4 (left): Fig. 5 in Done et al. (2010)
•Figure 4 (right): Fig. 4 in Done et al. (2013)
•Figure 5: University of Chicago [http://kicp.uchicago.edu/research/highlights/highlight_2006-03-21.html]