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The Link Between RE J1034+396 & Seyfert I 1H0707-495 Galaxy: X-Ray Reverberations

28/12/2011

RE J1034+396 is a Narrow Line Seyfert 1 galaxy (NLS1, z = 0.042), a sub-class of active galactic nucleus. Seyfert galaxies are characterized by extremely bright nuclei, and spectra which have very bright emission lines. These emission lines exhibit strong Doppler broadening (Capriotti et al. 1982; Matt et al. 1992; Fabian et al. 1995; Molina et al. 2007), as the emitting material is revolving around the black hole with high speeds, emitting photons at varying Doppler shifts. Such broadening implies velocities ranging from 500 to 4000 kms-1, and are believed to originate near an accretion disc surrounding the central black hole (Antonucci 1993).

RE J1034+396 has an unusual multi-wavelength spectrum, with no significant big blue bump in the UV/Optical bands. It is very bright in the EUV and soft X-rays making it one of the brightest objects with a so-called ‘soft excess’ (Puchnarewicz et al. 1995; Pounds et al. 1995). The soft excess below  2 keV is a common feature in the X-ray spectra of many AGN, particularly of the NLS1 class. It is too hot to be thermal blackbody emission from the accretion disc, while the similarity of its shape in objects with different masses argues against an origin in a cool comptonised region existing along with the hot corona emitting at higher energies.

A corona of this type is expected to depend on the seed photons from the disc, which in turn depend on the mass of the black hole (Gierlínski & Done 2004). Atomic processes are more likely to be at work through line-of-sight absorption or reflection. The smoothness of the observed emission rules out the former (Schurch & Done 2008), while the latter works if the emission originates very close to the black hole where strong relativistic effects acts to smear out sharp atomic features (Crummy et al. 2006; Nardini et al. 2011).

One other object, 1H0707-495, also shows a strong soft excess below 1 keV. It was shown recently that because of the high iron abundance, its soft excess is mainly due to a strong iron L emission (Fabian et al. 2009; Zoghbi et al. 2010) produced by partially ionised reflection (Ross & Fabian 2005; George & Fabian 1991). The compactness of the reflecting region and its proximity to the black hole produces strong relativistic broadening that smears out the reflection features. The iron L-line is observed together with the more commonly seen broad K-line (e.g. see Miller 2007 for a recent review).

1H0707-495 also shows a time delay between the hard band (1.0–4.0 keV) dominated by the direct power-law emission, and the soft band (0.3–1.0 keV) dominated by the reflected emission (Fabian et al. 2009; Zoghbi et al. 2010). The soft lag is the first to be detected, and is a confirmation that the soft band is dominated by reprocessed emission. The magnitude of the lag itself (30–50 seconds) is consistent with light travel time at  2 gravitational radii from the black hole, as inferred from the energy spectrum. Detailed energy-dependent lag spectra show that the lag traces the shape of the reflection spectrum, further confirming its interpretation (Zoghbi et al. 2011).

Zoghbi & Fabian (2011) have recently concluded that the lag in RE J1034 extends over a wide frequency range, similar to 1H0707-495. The frequency of the QPO(a narrow frequency band by definition) is smaller than the frequency where the lag is maximum. The accretion disc around the super massive black hole in RE J1034 is reflecting the random stochastic signal of the illuminating source, which is this case happens to have a quasi-periodic oscillation (QPO), but Zoghbi & Fabian (2011) conclude that generally it does not. Zoghbi & Fabian (2011) found that the difference in frequency between the QPO and the lag is possibly an indication that the corona producing the QPO is slightly extended, while the compact reflection from the disc is emitted in a very small region and dominated by light-bending effects.

Zoghbi & Fabian (2011)’s latest timing results confirm the inner reflector model, which also gives a self-consistent spectral model. The X-ray spectra and timing of both RE J1034+396 and 1H0707-495 are dominated by emission and reflection from just a few gravitational radii around the central black hole within both active galaxies.

Journal References:

  • Zoghbi, A.; Uttley, P. & Fabian, A. C. (2011) Understanding Reverberation Lags In 1H0707-495Royal Astronomical Society Monthly Notices, 412 (1): pp. 59-64.
  • Zoghbi, A. & Fabian, A.C. (2011) X-ray Reverberation Close To The Black Hole In The Active Galaxy RE J1034+396Royal Astronomical Society Monthly Notices, 418 (4): pp. 2642-2647.
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