Relativistic Iron Line

As shown in the preceding section, there is accumulating evidence for the absence of a material surface in the secure and candidate black-hole XBs. A further crucial step is to find strong relativistic effects; the unique features expected in the direct vicinity of the event horizon of black holes.

In some Seyfert galaxies, the iron K-lines are found to be relativistically broadened: highly asymmetric, extended to low energies (e.g., [19, 52, 77]). This feature strongly suggests that the line is emitted very close to the black hole where the relativistic Doppler effect and the gravitational redshift are quite significant. These effects produce such a broad, asymmetric iron K-line. The relativistically broad iron K-line from a Schwarzschild black hole was first investigated theoretically by Fabian et al. [18], and later generalized to that from a Kerr black hole [31] (see [19,64] for a review).

Similar effects can be expected in the disk of the black-hole XBs. A generally accepted scenario is that the broad iron K-line is the fluorescent line (6.4 keV in the rest frame) generated in the innermost region of an optically-thick disk, which is irradiated by hard X-rays from an overlying corona. In this case, a reflection continuum with the iron K-absorption edge accompanies the iron K-line. The reflection component of XBs has recently been investigated extensively (see e.g., [85]). At high luminosities, the photoionization of the innermost disk may also produce a relativistic broad iron line [19,64].

There have been a considerable number of reports on the detection of relativis-tically broad, asymmetric iron K-line from several black-hole XBs. The results obtained with proportional counters (e.g., on Ginga and RXTE) suffer from marginal energy resolution, and are to be taken with reservation (references in [39]). More recent results with higher energy resolution (ASCA, BeppoSAX, Chandra and XMM-Newton) include those from Cyg X-1 [20,42], XTE J1650-500 [43,47], XTE J1819.3-2525 [44] and GX 339-4 [45]. Most of them were observed in the HS state. Although the result depends on the model of the underlying continuum, particularly of the reflection component, the observed iron K-lines show a very broad redward-skewed profile, as shown for an example in Fig. 16.8. These results are often fitted to a Laor line [31] from a highly-ionized disk, and the authors suggest a Kerr black hole. However, the interpretation of these broad iron K-lines as a result of the genuine relativistic effects is still subject to further studies.

Comparing to the case of neutron-star LMXBs, where the disk may extend close to the neutron star surface comparable to <3Rs, significant relativistic effects are also expected. Asai et al. [4] studied twenty NS-LMXBs of various luminosities with ASCA, and found iron K-lines from about half of them. Some of the lines appear slightly broad, but not as broad and asymmetric as those found in the blackhole XBs. Detections of iron K-lines were also reported from several neutron-star LMXBs in the HS state, e.g., Sco X-1, 4U 1608-52 [75], GX 17+2 [12], GX 349+2 [13]. These lines are not broad and located at <6.7 keV instead of 6.4keV, corresponding to the line from highly-ionized iron. These differences are yet to be understood.

Fig. 16.8 Broad feature in XTE J1650-500 interpreted as a relativistic iron K-line [47]. Ratio of the data to a model: MCD + power-law with a smeared K-absorption edge. The 4-8 keV band is ignored in fitting

Note Added in Proof

Discoveries of relativistic broad iron lines in a few neutron star LMXBs have recently been reported from high-statistics observations with XMM-Newton [87] and Suzaku [88].

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