X-ray Reverberation Research Articles

Ionized X-ray reverberation mapping: testing a light-bending model for 1H0707−495

The narrow-line Seyfert 1 galaxy 1H0707−495 has shown the first clear evidence of X-ray reverberation in which the soft, disc-reflection and Fe-L-dominated 0.3–1 keV band lags behind the hard, continuum-dominated 1–4 keV band by ≈ 30 s. A simple reflection model, however, cannot explain the different amplitudes of variability between the continuum and reflection components in time-resolved spectra. Here we apply a light-bending model (LBM) to 1H0707−495 to determine whether or not it can fully account for both the time-averaged spectrum and observed X-ray variability and time lags. A point-like source is assumed to move vertically from 3.5rg to 8 rg approximately along the rotation axis of the black hole. Photon paths are integrated via a numerical method for each source position to calculate the corresponding direct and ionized reflection spectra from the accretion disc. The source is assumed to move up and down in such a way as to reproduce the 1–4 keV light curve from the XMM–Newton observations. We find that, even though the reproduced time-averaged spectrum generally agrees with the observations, this LBM cannot account for the observed time lags between the continuum and reflection spectrum. Furthermore, timing properties such as coherence are sensitive to the ionization state of the inner disc. We conclude that a more complex model of the source geometry and intrinsic variability are required, and probably one in which fluctuations propagating through the disc are directly associated with the X-ray sources. Our ionized reverberation mapping model is completely general, allowing other scenarios to be investigated in the future. Reverberation signatures are beginning to constrain the true nature of the X-ray source of not only 1H0707−495, but also other active galactic nuclei.

X-ray reverberation close to the black hole in RE J1034+396

In previous work, we discussed the detection of reverberation delays in the NLS1 1H0707-495. The delays originate close to the black hole. Here, we show that RE J1034+396 shows very similar lag properties. At low frequencies (< 1 \times 10-4 Hz), the time lag between energy bands increases with energy separation, similar to that commonly seen in Galactic black holes and other AGN. At higher frequencies (~ 3.5 \times 10-4 Hz), the soft (< 1 keV) and hard (> 3 keV) bands lag behind the intermediate band (1-3 keV). The simplest interpretation is that the intermediate band is dominated by the direct power-law continuum, while the soft and hard bands are dominated by the relativistically-smeared reflected emission. The low frequency delays are present in both available observations. The high frequency lags are only seen in one observation. In the observation where high frequency reverberation delays are observed, the spectrum contains a power-law component and there is a QPO in the light curve. In the other observation, no power-law component is required and no QPO is seen. The lags originate a few gravitational radii from the black hole, and the QPO is associated with the power-law emitting corona.

Negative X-ray reverberation time delays from MCG–6-30-15 and Mrk 766

Abstract We present an X-ray time lag analysis, as a function of Fourier frequency, for MCG–6-30-15 and Mrk 766 using long-term XMM–Newton light curves in the 0.5–1.5 and the 2–4 keV energy bands, together with some physical modelling of the corresponding time lag spectra. Both the time lag spectra of MCG–6-30-15 and Mrk 766 show negative values (i.e. soft band variations lag behind the corresponding hard band variations) at high frequencies, around 10−3 Hz, similar to those previously observed from 1H 0707−495. The remarkable morphological resemblance between the time lag spectra of MCG–6-30-15 and Mrk 766 indicate that the physical processes responsible for the observed soft time delays are very similar in the two sources, favouring a reflection scenario from material situated very nearby to the central black hole.

X-ray reverberation in 1H 0707−495 revisited

The narrow-line Seyfert 1 galaxy 1H0707-495 has previously been identified as showing time lags between flux variations in the soft- (0.3-1 keV) and medium-energy (1-4 keV) X-ray bands that oscillate between positive and negative values as a function of the frequency of the mode of variation. Here we measure and analyse the lags also between a harder X-ray band (4-7.5 keV) and the soft and medium bands, using existing XMM-Newton data, and demonstrate that the entire spectrum of lags, considering both the full energy range, 0.3-7.5 keV, and the full frequency range, 10^-5 < nu < 10^-2 Hz, are inconsistent with previous claims of arising as reverberation associated with the inner accretion disk. Instead we demonstrate that a simple reverberation model, in which scattering or reflection is present in all X-ray bands, explains the full set of lags without requiring any ad hoc explanation for the time lag sign changes. The range of time delays required to explain the observed lags extends up to about 1800 s in the hard band. The results are consistent with reverberation caused by scattering of X-rays passing through an absorbing medium whose opacity decreases with increasing energy and that partially-covers the source. A high covering factor of absorbing and scattering circumnuclear material is inferred.

Broad iron L line and X-ray reverberation in 1H0707-495

A detailed analysis of a long XMM-Newton observation of the Narrow Line Seyfert 1 galaxy 1H0707-495 is presented, including spectral fitting, spectral variability and timing studies. The two main features in the spectrum are the drop at ~ 7 keV and a complex excess below 1 keV. These are well described by two broad, K and L, iron lines. Alternative models based on absorption, although they may fit the high energy drop, cannot account for the 1 keV complexity and the spectrum as a whole. Spectral variability shows that the spectrum is composed of at least two components, which are interpreted as a power-law dominating between 1-4 keV, and a reflection component outside this range. The high count rate at the iron L energies has enabled us to measure a significant soft lag of ~ 30 s between 0.3-1 and 1-4 keV, meaning that the direct hard emission leads the reflected emissions. We interpret the lag as a reverberation signal originating within a few gravitational radii of the black hole.

X-ray reverberation studies of AGN

The broad iron line seen in the X-ray spectra of Active Galactic Nuclei (AGN) is a unique probe of the very near environment of the black hole where strong field general relativistic effects are important. The iron fluorescence line is produced when the X-ray continuum emission, which originates in the accretion disk corona, back-scatters or “reflects” off the disk. The line is sensitive to Doppler, transverse Doppler shifts and special relativistic boosting from the motion of the disk and gravitational redshifting from being deep in the potential well of the black hole. The sudden activation of X-ray flares in the corona (the presence of which are suggested by observation) provides a variable illumination pattern over the disk, and the reflected “echo” from these flares can be used to map out the astrophysics of this region in detail. This is the technique of reverberation mapping.