Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userAn XSPEC model to explore spectral features from black-hole sources - II. The relativistic iron line in the lamp-post geometry
124
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
In X-ray spectra of several active galactic nuclei and Galactic black hole binaries a broad relativistically smeared iron line is observed. This feature arises by fluorescence when the accretion disc is illuminated by hot corona above it. Due to central location of the corona the illumination and thus also the line emission decrease with radius. It was reported in the literature that this decrease is very steep in some of the sources, suggesting a highly compact corona. We revisit the lamp-post setup in which the corona is positioned on the axis above the rotating black hole and investigate to what extent the steep emissivity can be explained by this scenario. We show the contributions of the relativistic effects to the disc illumination by the primary source - energy shift, light bending and aberration. The lamp-post radial illumination pattern is compared to the widely used radial broken power-law emissivity profile. We find that very steep emissivities require the primary illuminating source to be positioned very near the black hole horizon and/or the spectral power-law index of the primary emission to be very high. The broken power-law approximation of the illumination can be safely used when the primary source is located at larger heights. However, for low heights the lamp-post illumination considerably differs from this approximation. We also show the variations of the iron line local flux over the disc due to the flux dependence on incident and emission angles. The former depends mainly on the height of the primary source while the latter depends on the inclination angle of the observer. Thus the strength of the line varies substantially across the disc. This effect may contribute to the observed steeper emissivity.
rate research
Read More
We report on a new general relativistic computational model enhancing, in various respects, the capability of presently available tools for fitting spectra of X-ray sources. The new model is intended for spectral analysis of black-hole accretion discs. Our approach is flexible enough to allow easy modifications of intrinsic emissivity profiles. Axial symmetry is not assumed, although it can be imposed in order to reduce computational cost of data fitting. The main current application of our code is within the XSPEC data-fitting package, however, its applicability goes beyond that: the code can be compiled in a stand-alone mode, capable of examining time-variable spectral features and doing polarimetry of sources in the strong-gravity regime. Basic features of our approach are described in a separate paper (Dovciak, Karas & Yaqoob 2004). Here we illustrate some of its applications in more detail. We concentrate ourselves on various aspects of line emission and Compton reflection, including the current implementation of the lamp-post model as an example of a more complicated form of intrinsic emissivity.
[Abridged] Context. The high-energy radiation from black-hole binaries may be due to the reprocessing of a lamp located on the black hole axis, emitting X-rays. The observed spectrum is made of 3 components: the direct spectrum; the thermal bump; and the reflected spectrum made of the Compton hump and the iron-line complex. Aims. We aim at computing accurately the complete reprocessed spectrum (thermal bump + reflected) of black-hole binaries over the entire X-ray band. We also determine the strength of the direct component. Our choice of parameters is adapted to a source showing an important thermal component. Methods. We compute in full GR the illumination of a thin disk by a lamp along the rotation axis. We use the ATM21 radiative transfer code to compute the spectrum emitted along the disk. We ray trace this local spectrum to determine the reprocessed spectrum as observed at infinity. We discuss the dependence of the local and ray-traced spectra on the emission angle and spin. Results. We show the importance of the angle dependence of the total disk specific intensity spectrum emitted by the illuminated atmosphere when the thermal disk emission if fully taken into account. High spin implies high temperature in the inner regions, so the emitted thermal disk spectrum covers the iron-line complex. As a result we locally observe absorption lines produced in the hot disk atmosphere. Absorption lines are narrow and disappear after ray tracing the local spectrum. Conclusions. Our results mainly highlight the importance of considering the angle dependence of the local spectrum when computing reprocessed spectra, as was already found in a recent study. The main new result of our work is to show the importance of computing the thermal bump of the spectrum, as this feature can change considerably the observed iron-line complex.
The broad iron spectral features are often seen in X-ray spectra of Active Galactic Nuclei (AGN) and black-hole binaries (BHB). These features may be explained either by the relativistic disc reflection scenario or the partial covering scenario: It is hardly possible to determine which model is valid from time-averaged spectral analysis. Thus, X-ray spectral variability has been investigated to constrain spectral models. To that end, it is crucial to study iron structure of BHBs in detail at short time-scales, which is, for the first time, made possible with the Parallel-sum clocking (P-sum) mode of XIS detectors on board Suzaku. This observational mode has a time-resolution of 7.8~ms as well as a CCD energy-resolution. We have carried out systematic calibration of the P-sum mode, and investigated spectral variability of the BHB GRS 1915+105. Consequently, we found that the spectral variability of GRS 1915+105 does not show iron features at sub-seconds. This is totally different from variability of AGN such as 1H0707--495, where the variation amplitude significantly drops at the iron K-energy band. This difference can be naturally explained in the framework of the partial covering scenario.
Ultra-Luminous X-ray sources (ULXs) are accreting black holes for which their X-ray properties have been seen to be different to the case of stellar-mass black hole binaries. For most of the cases their intrinsic energy spectra are well described by a cold accretion disc (thermal) plus a curved high-energy emission components. The mass of the black hole (BH) derived from the thermal disc component is usually in the range of 100-1000 solar masses, which have led to the idea that this might represent strong evidence of the Intermediate Mass Black Holes (IMBH), proposed to exist by theoretical studies but with no firm detection (as a class) so far. Recent theoretical and observational developments are leading towards the idea that these sources are instead stellar-mass BHs accreting at an unusual super-Eddington regime. In this paper we briefly describe the model SLIMULX that can be used in XSPEC for the fit of thermal spectra of slim discs around stellar mass black holes in the super-Eddington regime. This model consistently takes all relativistic effects into account. We present the obtained results from the fit of the X-ray spectra from NGC 5408 X-1.
We perform the analysis of the iron K_alpha lines detected in three sources representing of three types of accreting compact sources: cataclysmic variable (CV) GK Per, neutron star (NS) Serpens X-1 and black hole (BH) GX 339-4. We find, using data from Epic-PN Camera on-board XMM-Newton observatory,that the iron K_alpha emission line in GK Per has a noticeable red-skewed profile. We compare the GK Per asymmetric line with the red-skewed lines observed by XMM-Newton in Serpens X-1 and GX 339-4. The observation of the K_alpha emission with red-skewed features in CV GK Per cannot be related to the redshift effects of General Relativity (GR). Therefore, if the mechanism of the K_alpha-line formation is the same in CVs, NSs and BHs then it is evident that the GR effects would be ruled out as a cause of red skewness of K_alpha line. The line reprocessing in an outflowing wind has been recently suggested an alternative model for a broad red-shifted iron line formation. In the framework of the outflow scenario the red-skewed iron line is formed in the strong extended wind due to its illumination by the radiation emanating from the innermost part of the accreting material. In this Paper we demonstrate that the asymmetric shapes of the lines detected from these CV, NS and BH sources are well described with the wind (outflow) model. While this fact is hard to reconcile with the relativistic models, it is consistent with the outflowing gas washing out high frequency modulations of the radiation presumably originated in the innermost part of the source.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
