Boosting Factor Research Articles

Cosmic reionization by stellar sources: Population II stars

We study the reionization of the Universe by stellar sources using a numerical approach that combines fast 3D radiative transfer calculations with high-resolution hydrodynamical simulations. By supplementing a one-step radiative transfer code specifically designed for following ionization processes with an adaptive ray-tracing algorithm, we are able to speed up the calculations significantly to the point where handling a vast number of sources becomes technically feasible. This allows us to study how dim low-mass sources, excluded in previous investigations owing to computational limitations, affect the morphological evolution of the reionization process. Ionizing fluxes for the sources are derived from intrinsic star formation rates computed in the underlying hydrodynamical simulations. Analysis of numerically converged results for star formation rates and halo mass functions allows us to assess the consequences of not including low-mass objects and enables us to correct for resolution effects. With these corrections, we are able to reduce the effective mass resolution limit for sources to M∼ 4.0 × 107h−1 M⊙ in a 10 h−1 Mpc comoving box. Our calculations reveal that the process by which ionized regions in the intergalactic medium (IGM) percolate is complex and is especially sensitive to the inclusion of dim sources. Moreover, we find that, given the same level of cosmic star formation, the number of ionizing photons required to reionize the Universe is significantly overestimated if sources with masses below ∼109h−1 M⊙ are excluded. This result stems from the fact that low-mass sources preferentially reside in less clumpy environments than their massive counterparts. Consequently, their exclusion has the net effect of concentrating more of the cosmic ionizing radiation in regions which have higher recombination rates. We present the results of our reionization simulation assuming a range of escape fractions for ionizing photons and make statistical comparisons with observational constraints on the neutral fraction of hydrogen at z∼ 6 derived from the z= 6.28 Sloan Digital Sky Survey (SDSS) quasar of Becker and coworkers. We find that, given the amplitude and form of the underlying star formation predictions, an escape fraction near fesc= 0.10–0.20 is most consistent with the observational results. In these models, reionization is expected to have occurred between z∼ 7–8, although the IGM remains fairly opaque until z≃ 6. Our method is also capable of handling the simultaneous reionization of the helium component in the IGM, allowing us to explore the plausibility of the scenario where sources with harder spectra are primarily responsible for reionization. In this case, we find that if the sources responsible for reionizing hydrogen by z∼ 8 had spectra similar to active galactic nuclei, then the helium component of the IGM should have been reionized by z∼ 6. We find that such an early reionization epoch for helium does not necessarily conflict with observational constraints obtained at z≃ 3, but may be challenged by future observations at higher redshifts. The recent WMAP measurements of the electron scattering optical depth (τe= 0.17 ± 0.04 according to the ‘model independent’ analysis of Kogut et al.) appear to be inconsistent with the relatively late onset of reionization by the normal Population II type stars that we consider. In order to simultaneously match the observations from the z= 6.28 SDSS quasar and the optical depth measurement from WMAP with the sources modelled here, we require a boosting factor for the number of ionizing photons released in the fesc= 0.20 model which evolves from unity at z= 6 to ≳50 by z∼ 18. Such a steep enhancement in the stellar production rate of ionizing photons would be consistent with an IMF that becomes more and more top heavy with increasing redshift.

Stressful life events preceding the first onset of psychosis. An explorative study

The aim of the study was to investigate the frequency and types of psychologically stressful lifeevents preceding first episode psychoses. In a defined urban area, all first-episode psychosispatients aged 18–45, without signs of organic brain disorder or heavy abuse, were investigatedand followed-up during 2–3 years. In the majority of cases, significant releasing factors could bedepicted. The need to understand the specific, private meaning of the preceding events isillustrated by case vignettes. A stressful life situation may not be regarded as the "cause" of thepsychosis but as a more or less important boosting factor. This is a complicated interactionbetween biological and psychological vulnerability factors. It seems important that the patientsafter the acute phase are given opportunities to talk about the events and how to manage themin the future.

Optical monitoring of the BL Lac object ON 231 in 1972–1990

The results of 19-year optical monitoring of ON 231 are presented. Photographic B-band observations were obtained during 123 nights from April 1972 to March 1990 as part of the quasar monitoring program carried out the Astronomical Institute of St. Petersburg State University. The new combined B-band light curve for ON 231 (1892–1999) is appreciably improved by our data, enabling us to identify a number of previously undetected high-amplitude events with time scales of weeks to months. On two nights, intraday variability with amplitudes \( \sim 0\mathop .\limits^m 8 - 0\mathop .\limits^m 9\) was observed. A number of conclusions are drawn about variability components with time scales from hours to decades. There are cases when very similar behavior on time scales from several weeks to several months was observed in the presence of substantially different base brightness levels. Both the amplitudes (\( \sim 1\mathop .\limits^m 5\)) and the characteristic shapes and overall durations of these events were the same. The constancy of the amplitudes of these events in magnitudes (i.e., the proportionality of the luminosity in rapid outbursts to the underlying level of the slowly varying component) can be explained in models that associate the variability with changes in the Doppler boosting factor due to variations in the inclination of the relativistic jet to the line of sight, if the slow and rapid components of the optical variability are associated with a steady component of the relativistic jet and shock waves (superluminal components) moving in the jet, respectively. Evidence for such a connection has been found for several superluminal sources. At the same time, there is evidence that in typical IDV events for ON 231, with durations of ∼1.5–2 hr, the amplitudes depend on the base brightness level of the objects, suggesting that the IDV events and less rapid components of the variability have different natures and/or occur in different locations in the source.

The Milliarcsecond‐Scale Jet in the Quasar J1625+4134

We present Very Long Baseline Array (VLBA) observations of the radio source J1625+4134 at 22 and 15 GHz and analyze them in concurrence with other existing VLBI data on this source. The high resolution images at 15 and 22 GHz show a short and bending jet which has about $270\degr$ difference in position angle with the northern jet detected at lower frequencies. The new high resolution data, combined with the data available in the literature, allow us to estimate the spectral index of the components and identify one of the compact components as the VLBI core based on its flat spectrum between 5 and 22 GHz. Relative to this core component, the jet appears to be bi-directional. The proper motion measurement of the component C2 and the estimate of the Doppler boosting factor suggest that the orientation of the jet is close to the line of sight. The projection effect of an intrinsically sharply bending jet within a few mas from the core or the erratic change in the nozzle direction of the jet may account for the uncommon bi-directional structure of the jet in J1625+4134.

The Beaming Pattern and Spectrum of Radiation from Inverse Compton Scattering in Blazars

By including Klein-Nishina effects, we generalize previous calculations of the beaming pattern of photons produced by inverse Compton scattering. For an isotropic distribution of soft photons upscattered by nonthermal electrons with a power-law density distribution n(γ) ∝ γ-p, embedded in a plasma moving with relativistic bulk speed, we show that the observed radiation intensity is proportional to D3+p, where D is the Doppler boosting factor. This agrees with previous computations performed in the Thomson limit, where the observed spectral index is α = (p - 1)/2 and the beaming pattern is D4+2α. Independent of D, Klein-Nishina effects limit the location of the peak energy peakmec2 of the observed spectral energy distribution such that peak ≲ 1/0, where 0 is the energy of the seed photons in units of mec2. Assuming that the seed photons originate in the broad-line region, we demonstrate that the GeV emission of blazars is significantly modified by Klein-Nishina effects, the spectrum being softer than that calculated in the Thomson limit. We further show that the change in the spectral index of the inverse Compton emission across peak can exceed the value of 0.5 predicted by computations performed in the Thomson limit. The model spectra agree with OSSE and COMPTEL limits on this break without invoking the effects of differential absorption at the edge of a gamma-ray photosphere.

Radio Monitoring of OJ 287 and Binary Black Hole Models for Periodic Outbursts

The BL Lac-type active galaxy OJ 287 exhibits a 12 year periodicity with a double-peaked maxima in its optical flux variations. Several models sought to explain this periodicity, the first one firmly established in any active galactic nucleus (AGN), as a result of the orbital motion of a pair of supermassive black holes. In one class of models the orientation of the jets changes in a regular manner, and the optical flaring is due to a consequent increase in the Doppler boosting factor. In another class of models the optical flaring reflects a true increase in luminosity, either due to an enhanced accretion during the pericenter passage or due to a collision between the secondary black hole and the accretion disk of the primary black hole. However, these models have been based solely on the optical data. Here we consider the full radio flux density monitoring data between 8 and 90 GHz from the Michigan, Metsähovi, and Swedish-ESO Submillimeter Telescope AGN monitoring programs. We find that the radio flux density and polarization data, as well as the optical polarization data, indicate that the first of the two optical peaks is a thermal flare occurring in the vicinity of the black hole and the accretion disk, while the second one is a synchrotron flare originating in a shocked region down the jet. None of the proposed binary black hole models for OJ 287 offers satisfactory explanations for these observations. We suggest a new scenario, in which a secondary black hole penetrates the accretion disk of the primary during the pericenter passage, causing a thermal flare visible only in the optical regime. The pericenter passage enhances accretion into the primary black hole, leading to increased jet flow and formation of shocks down the jet. These become visible as standard radio and optical synchrotron flares roughly a year after the pericenter passage and are identified with the second optical peaks. In addition to explaining the radio and the optical data, our model eliminates the need for a strong precession of the binary and for an ultramassive (≥1010 M☉) primary black hole. If our interpretation is correct, the next periodic optical flare, a thermal one, should occur around 2006 September 25. Nonthermal, simultaneous optical and radio flares should follow about a year later.

VSOP brightness temperature in NRAO 530 exceeds the inverse Compton limit

Abstract We present here space-based VLBI observations with VSOP and a southern hemisphere ground array of the gamma-ray blazar NRAO 530 at 1.6 and 5 GHz. The 5 GHz show a single component with a brightness temperature of ∼ 3 × 10 12 K, significantly in excess of the inverse Compton limit and of the equipartition brightness temperature limit (Readhead 1994). This is strong evidence for relativistic motion in a jet and implies model-dependent Doppler boosting factors in the range 6 to 60. We show that a simple homogeneous sphere probably does not model the emission region accurately. We favor instead an inhomogeneous jet model which requires a Doppler boosting factor of 15.

Total Flux Density Variations in Extragalactic Radio Sources. III. Doppler Boosting Factors, Lorentz Factors, and Viewing Angles for Active Galactic Nuclei

We have calculated Doppler boosting factors, Dvar, for a sample of active galactic nuclei (AGNs) using total flux density variation monitoring data at 22 and 37 GHz. We argue that this method is more accurate than the other commonly used methods based on the synchrotron self-Compton X-ray flux or equipartition of energy. We compare our Doppler factors with other results and conclude that even if the average Dvar for a class of sources is very similar to all others, the variability Doppler factors for individual objects are more accurate and reliable. An important application of precise Doppler factors is presented, namely, calculating the Lorentz factors, Γ, and the viewing angles, θ, of relativistic outflows inAGNs. We find that high-polarization quasars have the greatest Doppler boosting, while low-polarizationquasars and BL Lac objects are less boosted. The two groups of quasars show different characteristics because of different combinations of the Lorentz factor and viewing angle, rather than either a different Γ or θ alone.

Space VLBI Observations Show [ITAL]T[/ITAL][TINF][ITAL]b[/ITAL][/TINF] ] 10[TSUP]12[/TSUP] K in the Quasar NRAO 530

We present here space-based VLBI observations with VLBI Space Observatory Program (VSOP) and a southern hemisphere ground array of the gamma-ray blazar NRAO 530 at 1.6 and 5 GHz. The brightness temperature of the core at 1.6 GHz is 5×1011 K. The size is near the minimum observable value in the direction of NRAO 530 due to interstellar scattering. The 5 GHz data show a single component with a brightness temperature of ~3×1012 K, significantly in excess of the inverse Compton limit and of the equipartition brightness temperature limit. This is strong evidence for relativistic motion in a jet requiring model-dependent Doppler boosting factors in the range 6-60. We show that a simple homogeneous sphere probably does not model the emission region accurately. We favor instead an inhomogeneous jet model with a Doppler boosting factor of 15.

Word set probability boosting for improved spontaneous dialog recognition

Based on the observation that the unpredictable nature of conversational speech makes it almost impossible to reliably model sequential word constraints, the notion of word set error criteria is proposed for improved recognition of spontaneous dialogs. The single-pass adaptive boosting (AB) algorithm enables the language model weights to be tuned using the word set error criteria. In the two-pass version of the algorithm, the basic idea is to predict a set of words based on some a priori information, and perform a rescoring pass wherein the probabilities of the words in the predicted word set are amplified or boosted in some manner. An adaptive gradient descent procedure for tuning the word boosting factor is formulated, which enables the boost factors to be incrementally adjusted to maximize the accuracy of the speech recognition system outputs on held-out training data using the word set error criteria. Two novel models which predict the required word sets are presented: (i) utterance triggers, which capture within-utterance long-distance word interdependencies, and (ii) dialog triggers, which capture local temporal dialog-oriented word relations. The proposed trigger and adaptive boosting (TAB) algorithm, and the single-pass adaptive boosting (AB) algorithm are experimentally tested on a subset of the TRAINS-93 spontaneous dialogs and the TRAINS-95 semispontaneous corpus, and the results summarized.

Modeling malaria vaccines I: New uses for old ideas

Starting from a modification of the model of malaria transmission developed for the Garki project, this paper develops a model containing variables relevant to the simulation of malaria vaccination programs. Modifications include (1) integration of maintenance of immunity dependent on boosting and the possibility of loss immunity; (2) introduction of a boosting factor distinct from susceptibility to infection; (3) reinterpretation of the epidemiological compartments of positive immunes and nonimmunes in terms of severity of disease rather than just infection; (4) interpretation of the different stage-specific levels of immunity; (5) discrimination between different susceptibilities for the immune and nonimmune classes; (6) reformulation of the expression for acquisition of immunity to be biologically more acceptable. Simulations using the Garki model, Nedelman's modification of it, and our Basic model compare the similarities and differences in the predictive behavior of the models. Simulations using the Basic model reproduce observed periodic fluctuations of malaria attributed to the interplay of transmission-blocking immunity and loss of immunity in the absence of boosting in areas of unstable malaria transmission.

Multifrequency observation of the optically violent variable quasar 3C 446

Extensive optical and radio monitoring data and seven multifrequency spectra were obtained of the violently variable quasar 3C 446. The monitoring data suggest a correlation between the radio and optical outbursts, with the optical flare preceding the radio activity by 400-600 days. A difference in the statistical behavior of the optical and radio variability indicates that considerable processing occurs to the optical emitting plasma before it becomes radio emitting plasma. Within the radio band, outbursts proceed from high to low frequencies. An outburst in 1983 showed greater and more rapid variation in the optical than in the near-IR region. The 10-100 μm fluxes did not follow the higher frequency variation, suggesting a time delay between these spectral domains. During another time, the X-ray emission varied on a time scale of days and more rapidly than the UV or optical emission. On a time scale of weeks-months, the X-ray fluxes are well correlated with the UV-IR fluxes but not with the radio fluxes. The multifrequency data show that the flat radio spectrum turns over at 3-10 x 10^(11) Hz and the continuum steepens with frequency; ɑ(IR) = 1.1, ɑ(opt-UV) = -2 to -3. The X-ray emission lies an order of magnitude above an extrapolation of the optical-UV spectrum and has a harder spectrum. The power is primarily concentrated in the submillimeter and infrared region. When the source is faint, a blue bump may be present. The flux in the Lycx line is proportional to the UV continuum flux density when the source is bright but is independent of the continuum level when the source is faint. The data suggest that the X-rays are produced by the inverse Compton process from an emitting region (10^(16) cm) smaller than but related to the synchrotron emitting UV-IR region. The characteristic size of the emitting region increases with decreasing frequency from 10^(16) (X-ray region) to 1-3 x 10^(17) cm (far IR-submillimeter region) to 10^(19)-10^(20) cm (radio region). Plasma conditions are best constrained at the frequency when the source becomes transparent, the far IR-submillimeter band, where B ≈ 3-100 G, n ≈ 40-100 cm^(-3); and the Doppler boosting factor δ ≈ 1-5.