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SMBH NGC 3783: A Rapidly Spinning Black Hole

20/07/2011

As a result of the seminal works of Penrose (1969) and Blandfor & Znajek (1977), the spin property of a black hole is now universally considered to play an important role in the energy properties of active galactic nuclei. The role of this property is also considered important role in the production of relativistic jets such as those seen in many classes of active galactic nuclei (AGNs), whereby the magnetic extraction of the rotational energy of a spinning black hole is believed to be essential in causing these relativistic jetted outbursts (Brenneman et al. 2011). Indeed, it is this fundamental property of the central super massive black hole (SMBH) that is considered crucial in determining the energy output and directional intensity of such bursts from AGNs i.e. whether the source is radio-quiet or radio-loud; Wilson & Colbert (1995). Although it is worth noting that the accretion rate of course plays a role of equal importance.

However, the importance of black hole spin goes beyond its role as a possible power source (Brenneman et al. 2011). The spin distribution of the SMBH population (and its dependence on SMBH mass) encodes the black hole growth history, much like the spin period of a pulsar can give clues to its age and evolution (Moderski & Sikora 1996; Volonteri et al. 2005; Berti & Volonteri 2008).

In essence, if local SMBHs have obtained most of their mass during prolonged prograde accretion events in a quasar phase of activity, or in major mergers with similar mass SMBHs, it would be expected that a population of rapidly rotating SMBHs (due to the angular momentum accreted from the disks or transferred at merger) (Rezzolla et al. 2008).

N.B. The parameter  defined by Brenneman et al. (2011) is a measure of a black holes spin (where J is the angular momentum associated with the spinning black hole, G is the universal gravitational constant, c is the speed of light and M is the mass of the black hole). A rapidly rotating black hole can be considered to have .

However, if a consideration was made for mergers with much smaller SMBHs (e.g. Hughes & Blandford 2003) or randomly-oriented accretion events of small packets of material (e.g. King & Pringle 2007) have been the dominant growth mechanism, then most of the SMBHs would be expected to be spinning at a much more modest rate.

Examining relativistically-broadened spectral features from the inner edge of the accretion disc around a SMBH allows for a sophisticated probe of the gravitational properties and mechanics of the SMBH (Reynolds & Nowak 2003; Miller 2007), importantly including the spin of the SMBH itself. Such relativistically-broadened spectral features have been observed in both AGN (Fabian et al.1995) and smaller stellar mass sized black hole systems (Miller et al. 2002; Reis et al. 2008).

The technicalities of using such spectral broadening to measure the spin of black hole is extremely sophisticated astronomical technique to say the least. Something that I, as an astrophysical novice, do not yet fully understand. But, interestingly, these techniques are still undergoing further calibrations. However, such is the power of this method, constraints have been put on a number of SMBHs at the centre of AGNs (MCG–6-30-15, Brenneman & Reynolds 2006; SWIFT J2127.4+5654, Miniutti et al. 2009; 1H0707–495, Zoghbi et al. 2010; Mrk 79, Gallo et al. 2011; Mrk 335 & NGC 7469, Patrick et al. 2010).

Using such methods Brenneman et al. (2011), with support from the Suzaku Key Projects program, have intensively studied nearby AGN with the intent of characterising the accretion disc features in the spectra and setting constraints on the SMBH spin.

The first object of focus is the relatively close Seyfert I.5 galaxy known as NGC 3783 (with a redshift of  as measured by Theureau et al. (1998)). An object which possesses a high-column density and what is known as a multi-component warm absorber that has been well-studied by various spectroscopic X-ray observatories (e.g. HETGS; Kaspi et al. 2002; Krongold et al. 2003; Netzer et al. 2003).

Due to the close proximity of this active galaxy, Brenneman et al’s (2011) study has been able to show, via spectral characteristics of the inner edge of SMBH NGC 3783’s accretion disc, that the central SMBH engine is spinning extremely rapidly with  (to a 90% confidence level).

Such a conclusion is a major step in understand the full dynamics of the central engines of most active galactic nuclei, and will provide further research teams with an interesting footing for future AGN studies.

Journal References:

  • Penrose, R. (1969) Gravitational Collapse: The Role Of General RelativityRivista del Nuovo Cimento, Numero Speziale I, 252.
  • Blandfor & Znajek (1977) Electromagnetic Extraction Of Energy From Kerr Black Holes. Monthly Notices Royal Astronomical Society,  179 pp. 433-456.
  • Loar, A. (1991) Line Profiles From A Disk Around A Rotating Black Hole. The Astrophysical Journal, 376 (1) pp.90-94.
  • Moderski, R. & Sikora, M. (1996) On Black Hole Evolution In Active Galactic NucleiMonthly Notices Royal Astronomical Society,  283 (3) pp. 854-864.
  • Brenneman, J.W. et al. (2011) The Spin Of The Supermassive Black Hole In NGC 3783. The Astrophysical Journal, 736 (2) Article I.D.: 103.
Suggested Further Reading:
  • Kerr, R. P. (1963) Gravitational Field Of A Spinning Mass As An Example Of Algebraically Special MetricsPhysical. Review. Lett., 11 (5) pp.237-238.

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