arXiv subjects

A. J. Young

Publications and source records attributed to A. J. Young.

60 records · Page 4Linked to original sources

arXiv1999

Iron line reverberation mapping with Constellation-X

The broad X-ray iron line seen in the spectra of many AGN is thought to originate from the inner regions of the putative black hole accretion disk, and hence provides a rare probe of that central region. In principle, future high throughput X-ray spectrometers should be able to examine the temporal response of this fluorescent line to flares in the X-ray continuum source (which energizes this emission line) - i.e. iron line reverberation mapping will be possible. It has been previously found that there are robust reverberation signatures of the black hole spin, mass and the X-ray flare location above the accretion disk. Here, we simulate observations of a bright Seyfert nucleus with the proposed NASA mission Constellation-X and demonstrate the feasibility of detecting these reverberation signatures with this mission. Hence, starting with XMM in 1999, and maturing with Constellation-X in c2010, iron line reverberation will open a new window on the innermost regions of AGN.

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arXiv1999

Variation of the broad X-ray iron line in MCG-6-30-15 during a flare

We report results on the broad iron emission line of the Seyfert galaxy MCG-6-30-15, obtained from the second long ASCA observation in 1997. The time-averaged profile of the broad line is very similar to that seen with ASCA in 1994, so confirming the detailed model fit then obtained. A bright flare is seen in the light curve, during which the continuum was soft. At that time the emission line peaks around 5 keV and most of its emission is shifted below 6 keV with no component detected at 6.4 keV (EW<60 eV). This can be interpreted as the result of an extraordinarily large gravitational redshift due to a dominant flare occurring very close to the black hole at a radius of <5m.

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arXiv1999

The optical variability of the narrow line Seyfert 1 galaxy IRAS 13224-3809

We report on a short optical monitoring programme of the narrow-line Seyfert 1 Galaxy IRAS 13224-3809. Previous X-ray observations of this object have shown persistent giant variability. The degree of variability at other wavelengths may then be used to constrain the conditions and emission processes within the nucleus. Optical variability is expected if the electron population responsible for the soft X-ray emission is changing rapidly and Compton-upscattering infrared photons in the nucleus, or if the mechanism responsible for X-ray emission causes all the emission processes to vary together. We find that there is no significant optical variability with a firm upper limit of 2 per cent and conclude that the primary soft X-ray emission region produces little of the observed optical emission. The X-ray and optical emission regions must be physically distinct and any reprocessing of X-rays into the optical waveband occurs some distance from the nucleus. The lack of optical variability indicates that the energy density of infrared radiation in the nucleus is at most equal to that of the ultraviolet radiation since little is upscattered into the optical waveband. The extremely large X-ray variability of IRAS 13224-3809 may be explained by relativistic boosting of more modest variations. Although such boosting enhances X-ray variability over optical variability, this only partially explains the lack of optical variability.

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arXiv1999

X-ray reflection spectra from ionized slabs

X-ray reflection spectra are an important component in the X-ray spectra of many active galactic nuclei and Galactic black hole candidates. It is likely that reflection takes place from highly ionized surfaces of the accretion disc in some cases. This can lead to strong Comptonization of the emergent iron, and other, absorption and emission features. We present such reflection spectra here, computed in a self-consistent manner with the method described by Ross and Fabian. In particular we emphasise the range where the ionization parameter (the flux to density ratio) ξis around and above 10^4. Such spectra may be relevant to the observed spectral features found in black hole candidates such as Cygnus X-1 in the low/hard state.

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arXiv1998

X-ray iron line reverberation from black hole accretion disks

The relativistically broad X-ray iron line seen in many AGN spectra is thought to originate from the central regions of the putative black hole accretion disk. Both the line profile and strength will vary in response to rapid variability of the primary X-ray continuum source. The temporal response of the line contains information on the accretion disk structure, the X-ray source geometry, and the spin of the black hole. Since the X-ray source will have a size comparable to the fluorescing region of the accretion disk, the general reverberation problem is not invertible. However, progress can be made since, empirically, AGN light curves are seen to undergo dramatic short timescale variability which presumably corresponds to the creation of a single new active region within the distributed X-ray source. The iron line response to these individual events can be described using linear transfer theory. We consider the line response to the activation/flaring of a new X-ray emitting region. Most of our detailed calculations are performed for the case of an X-ray source on the symmetry axis and at some height above the disk plane around a Kerr black hole. We also present preliminary calculations for off-axis flares. We suggest ways in which future, high-throughput X-ray observatories such as XMM and the Constellation X-ray Mission may use these reverberation signatures to probe both the mass and spin of AGN black holes, as well as the X-ray source geometry.

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arXiv1998

Iron line profiles including emission from within the innermost stable orbit of a black hole accretion disc

Reynolds & Begelman (1997) have recently proposed a model in which the broad and extremely redshifted iron line seen during a deep minimum of the light curve of the Seyfert 1 galaxy MCG-6-30-15 originates from matter spiralling into a Schwarzschild black hole, contrary to previous claims that the black hole may be spinning rapidly (Iwasawa et al 1996; Dabrowski et al 1997). Here we calculate in detail the X-ray spectrum produced by their model using the full reflected continuum emission, including absorption features. This calculation takes into account the doppler and relativistic effects. For the range of parameters we consider, we find that the spectrum should show a large photoelectric absorption edge of iron, which is not seen in the data. The absorption edge is a consequence of the line emitting matter within the innermost stable orbit being highly ionized, and is largely independent of the parameters chosen for their model. If we restrict our attention to the 3-10 keV band we may effectively remove this absorption edge by fitting a steeper power law, but this results in a significant underprediction of the 0.4-0.5 keV flux. We conclude that the data on MCG-6-30-15 are more consistent with the Kerr than the Schwarzschild model.

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