1,721,098 research outputs found
Polarization signatures of strong gravity in active galactic nuclei accretion discs
No full textThe effects of strong gravity on the polarization of the Compton reflection from an X-ray-illuminated accretion disc are studied. The gravitational field of a rotating black hole influences Stokes parameters of the radiation along the propagation to a distant observer. Assuming the lamp-post model, the degree and the angle of polarization are examined as functions of the observer's inclination angle, of the height of the primary source and of the inner radius of the disc emitting region. It is shown that polarimetry can provide essential information on the properties of black holes sources, and it is argued that time variation of the polarization angle is a strong signature of general-relativity effects. The expected polarization degree and angle should be detectable by new-generation polarimeters, such as that planned for the Xeus mission
Relativistic features from X-ray illuminated spots in AGN
No full textNarrow spectral features in the 5-6 keV range were recently discovered in the X-ray spectra of a few AGN. We discuss the possibility that these features are redshifted iron lines from orbiting spots on the surface of the accretion disc, and compare numerical models with the observations. If the model is correct, these features can be used to measure the mass of Supermassive Black Holes in Active Galactic Nuclei, a task for future X-ray missions like Constellation-X
Thermal disc emission from a rotating black hole: X-ray polarization signatures
No full textThermal emission from the accretion disc around a black hole can be polarized, due to Thomson scattering in a disc atmosphere. In Newtonian space, the polarization angle must be either parallel or perpendicular to the projection of the disc axis on the sky. As first pointed out by Stark and Connors in 1977, General Relativity effects strongly modify the polarization properties of the thermal radiation as observed at infinity. Among these effects, the rotation of the polarization angle with energy is particularly useful as a diagnostic tool. In this paper, we extend the Stark and Connors calculations by including the spectral hardening factor, several values of the optical depth of the scattering atmosphere and rendering the results to the expected performances of planned X-ray polarimeters. In particular, to assess the perspectives for the next generation of X-ray polarimeters, we consider the expected sensitivity of the detectors on board the planned POLARIX and International X-ray Observatory missions. We assume the two cases of a Schwarzschild and an extreme Kerr black hole with a standard thin disc and a scattering atmosphere. We compute the expected polarization degree and the angle as functions of the energy as they could be measured for different inclinations of the observer, optical thickness of the atmosphere and different values of the black hole spin. We assume the thermal emission dominates the X-ray band. Using the flux level of the microquasar GRS 1915+105 in the thermal state, we calculate the observed polarization
Relativistic spectral features from X-ray-illuminated spots and the measure of the black hole mass in active galactic nuclei
No full textVariation in the primary and reprocessed radiation from an orbiting spot around a black hole
No full textWe study light curves and spectra [equivalent widths (EWs) of the iron line and some other spectral characteristics] which arise by reflection on the surface of an accretion disc, following its illumination by a primary off-axis source-an X-ray 'flare', assumed to be a point-like source just above the accretion disc resulting in a spot with radius Delta r/r less than or similar to 1. We consider General Relativity effects (energy shifts, light bending, time-delays) near a rotating black hole, and we find them all important, including the light bending and delay amplification due to the spot motion. For some sets of parameters, the observed reflected flux exceeds the observed flux from the primary component. We show that the orbit-induced variations in the EW with respect to its mean value can be as high as 30 per cent for an observer's inclination of 30 degrees, and much more at higher inclinations. We calculate the ratio of the reflected flux to the primary flux and the hardness ratio which we find to vary significantly with the spot phase mainly for small orbital radii. This offers the chance to estimate the lower limit of the black hole spin if the flare arises close to the black hole
General relativistic modelling of the negative reverberation X-ray time delays in AGN
No full textWe present the first systematic physical modelling of the time-lag spectra between the soft (0.3–1 keV) and the hard (1.5–4 keV) X-ray energy bands, as a function of Fourier frequency, in a sample of 12 active galactic nuclei which have been observed by XMM–Newton. We concentrate particularly on the negative X-ray time-lags (typically seen above 10-4 Hz), i.e. soft-band variations lag the hard-band variations, and we assume that they are produced by reprocessing and reflection by the accretion disc within a lamp-post X-ray source geometry. We also assume that the response of the accretion disc, in the soft X-ray bands, is adequately described by the response in the neutral Fe Kα line at 6.4 keV for which we use fully general relativistic ray-tracing simulations to determine its time evolution. These response functions, and thus the corresponding time-lag spectra, yield much more realistic results than the commonly used, but erroneous, top-hat models. Additionally, we parametrize the positive part of the time-lag spectra (typically seen below 10-4 Hz) by a power law. We find that the best-fitting black hole (BH) masses, M, agree quite well with those derived by other methods, thus providing us with a new tool for BH mass determination. We find no evidence for any correlation between M and the BH spin parameter, α, the viewing angle, θ, or the height of the X-ray source above the disc, h. Also on average, the X-ray source lies only around 3.7 gravitational radii above the accretion disc and θ is distributed uniformly between 20° and 60°. Finally, there is a tentative indication that the distribution of α may be bimodal above and below 0.62
X-ray spectra and polarization from accreting black holes: polarization from an orbiting spot
No full textThe polarization from a spot orbiting around Schwarzschild and extreme Kerr black holes is studied. We assume different models of local polarization. Firstly, as a toy model we set the local polarization vector either normal to the disc plane, or perpendicular to the toroidal magnetic field. Then we examine the more realistic situation with a spot arising due to the emission from the primary source above the disc. We employ either Rayleigh single scattering or Compton multiple scattering approximations. The time dependence of the degree and angle of polarization during the spot revolution is examined as a function of the observer's inclination angle and black hole angular momentum. The gravitational and Doppler shifts, lensing effect as well as time delays are taken into account
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