Simulations of the Fe Kα Energy Spectra from Gravitationally Microlensed Quasars
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Abstract The analysis of the Chandra X-ray observations of the gravitationally lensed quasar RX J1131−1231 revealed the detection of multiple and energy-variable spectral peaks. The spectral variability is thought to result from the microlensing of the Fe K α emission, selectively amplifying the emission from certain regions of the accretion disk with certain effective frequency shifts of the Fe K α line emission. In this paper, we combine detailed simulations of the emission of Fe K α photons from the accretion disk of a Kerr black hole with calculations of the effect of gravitational microlensing on the observed energy spectra. The simulations show that microlensing can indeed produce multiply peaked energy spectra. We explore the dependence of the spectral characteristics on black hole spin, accretion disk inclination, corona height, and microlensing amplification factor and show that the measurements can be used to constrain these parameters. We find that the range of observed spectral peak energies of QSO RX J1131−1231 can only be reproduced for black hole inclinations exceeding 70° and for lamppost corona heights of less than 30 gravitational radii above the black hole. We conclude by emphasizing the scientific potential of studies of the microlensed Fe K α quasar emission and the need for more detailed modeling that explores how the results change for more realistic accretion disk and corona geometries and microlensing magnification patterns. A full analysis should furthermore model the signal-to-noise ratio of the observations and the resulting detection biases.Keywords:
Gravitational microlensing
Black hole (networking)
Corona (planetary geology)
Numerical simulations and theoretical studies of the gravitational microlensing effect of a population of small bodies distributed along the line of sight to a compact light source such as a quasar indicate that caustic crossing effects will be present given a sufficiently large optical depth to lensing. These events will produce characteristic patterns in the quasar light curve. In this paper we use a large sample of quasar light curves to search for features with the properties of caustic crossing events. Two good candidates are presented which it is argued are not easily explained as intrinsic variation of the quasars. The relation between these events and microlensing features seen in multiple quasar systems are discussed, as well as the implications for the dark matter problem of a population of microlensing bodies.
Gravitational microlensing
Caustic (mathematics)
Line-of-sight
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A small fraction of all quasars are strongly lensed and multiply imaged, with usually a galaxy acting as the main lens. Some, or maybe all of these quasars are also affected by microlensing, the effect of stellar mass objects in the lensing galaxy. Usually only the photometric aspects of microlensing are considered: the apparent magnitudes of the quasar images vary independently because the relative motion between source, lens and observer leads to uncorrelated magnification changes as a function of time. However, stellar microlensing on quasars has yet another effect, which was first explored by Lewis & Ibata (1998): the position of the quasar – i.e. the center-of-light of the many microimages – can shift by tens of microarcseconds due to the relatively sudden (dis-)appearance of a pair of microimages when a caustic is being crossed. Here we explore quantitatively the astrometric effects of microlensing on quasars for different values of the lensing parameters κ and γ (surface mass density and external shear) covering most of the known multiple quasar systems. We show examples of microlens-induced quasar motion (i.e. astrometric changes) and the corresponding light curves for different quasar sizes. We evaluate statistically the occurrence of large “jumps” in angular position and their correlation with apparent brightness fluctuations. We also show statistical relations between positional offsets and time from random starting points. As the amplitude of the astrometric offset depends on the source size, astrometric microlensing signatures of quasars – combined with the photometric variations – will provide very good constraints on the sizes of quasars as a function of wavelength. We predict that such signatures will be detectable for realistic microlensing scenarios with near future technology in the infrared/optical (Keck-Interferometry, VLTI, SIM, GAIA). Such detections will show that not even high redshift quasars define a “fixed” coordinate system.
Gravitational microlensing
Proper motion
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According to models of gravitational microlensing, the brightness of a quasar image will vary depending on the size of the quasar among other quantities. When observing a quasar through different filters this can lead to the determination of the quasar size in the different wavelengths relative to each other. Observations of the quadruple quasar Q2237 + 031 indicate that at least one of the images is affected by microlensing. Furthermore, the brightness variation of the continuum in this image is found to be larger than the emission line variability. This result agrees with standard quasar models predicting that the emission line region is much larger than the central engine coupled with microlensing models predicting that a larger source size produces smaller brightness variations
Gravitational microlensing
Bracken
Line (geometry)
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Quasar microlensing analyses implicitly generate a model of the variability of the source quasar. The implied source variability may be unrealistic yet its likelihood is generally not evaluated. We used the damped random walk (DRW) model for quasar variability to evaluate the likelihood of the source variability and applied the revised algorithm to a microlensing analysis of the lensed quasar RX J1131-1231. We compared the estimates of the source quasar disk and average lens galaxy stellar mass with and without applying the DRW likelihoods for the source variability model and found no significant effect on the estimated physical parameters. The most likely explanation is that unreliastic source light curve models are generally associated with poor microlensing fits that already make a negligible contribution to the probability distributions of the derived parameters.
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Abstract In this paper results from a monitoring programme of a large sample of quasars comprising regular yearly observations over a period of 23 years are presented. Structure functions of the light curves are calculated and compared with predictions for models of quasar variability of current interest. These include recently published models of varibility from accretion disk instability, variability from starbursts or supernovae, and variations caused by the microlensing effect of compact bodies along the line of sight. The analysis favours the accretion disk model for low luminosity AGN, but suggests that the variations of more luminous quasars are dominated by microlensing.
Gravitational microlensing
Line-of-sight
Accretion disc
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We present the results of the first long-term (2.2 years) spectroscopic monitoring of a gravitationally lensed quasar, namely the Einstein Cross QSO 2237+0305.We spatially deconvolve deep VLT/FORS1 spectra to accurately separate the spectrum of the lensing galaxy from the spectra of the quasar images.Accurate cross-calibration of the observations at 31 epochs from October 2004 to December 2006 is carried out using foreground stars observed simultaneously with the quasar.The quasar spectra are further decomposed into a continuum component and several broad emission lines.
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Microlensing in strong-lens systems with quasars provides a unique detailed view of the internal structure of active black holes to the scale of nano-arcseconds. The case is made for the potential of this approach to revolutionize our understanding of quasar physics and its role in broader questions, with a coherent community strategy.
Gravitational microlensing
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We present the results of a monitoring campaign of the double quasar SBS 1520+530 at Maidanak observatory from April 2003 to August 2004. We obtained light curves in V and R filters that show small-amplitude mag intrinsic variations of the quasar on time scales of about 100 days. The data set is consistent with the previously determined time delay of days by Burud et al. (2002, A&A, 391, 481). We find that the time delay corrected magnitude difference between the quasar images is now larger by mag than during the observations by Burud et al. (2002). This confirms the presence of gravitational microlensing variations in this system.
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Abstract Microlens-induced variability in multiple quasars can be used to study two cosmological issues of great interest, the size and brightness profile of quasars on one hand, and the distribution of compact (dark) matter along the line of sight on the other. Here a summary is given of recent theoretical progress as well as observational evidence for quasar microlensing, plus a discussion of desired observations and required theoretical studies.
Gravitational microlensing
Line-of-sight
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Selective amplification of the line and continuum source by microlensing in a lensed quasar can lead to changes of continuum spectral slopes and line shapes in the spectra of the quasar components. Comparing the spectra of different components of the lensed quasar and the spectra of an image observed in different epochs one can infer the presence of millilensing, microlensing and intrinsic variability. Especially, microlensing can be used for investigation of the unresolved broad line (BLR) and continuum emitting region structure in active galactic nuclei (AGN). Therefore the spectroscopic monitoring of selected lensed quasars with 3D spectroscopy open new possibility for investigation of the BLR structure in AGN. Here we discuss observational effects that may be present during the BLR microlensing in the spectra of lensed QSOs.
Gravitational microlensing
QSOS
Line-of-sight
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