One-zone Model Research Articles

Correlation between optical and γ-ray flux variations in BL Lacs

ABSTRACT We report here results of the analysis of correlated flux variations between the optical and GeV γ-ray bands in three bright BL Lac objects, namely AO 0235+164, OJ 287, and PKS 2155−304. This was based on the analysis of about 10 yr of data from the Fermi Gamma-ray Space Telescope covering the period between 2008 August 8 and 2018 August 8 along with optical data covering the same period. For all the sources, during the flares analysed in this work, the optical and γ-ray flux variations are found to be closely correlated. From broad-band spectral energy distribution modelling of different epochs in these sources using the one-zone leptonic emission model, we found that the optical–ultraviolet emission is dominated by synchrotron emission from the jet. The γ-ray emission in the low synchrotron peaked sources AO 0235+164 and OJ 287 is found to be well fitted with external Compton (EC) component, while the γ-ray emission in the high synchrotron peaked source PKS 2155−304 is well fitted with synchrotron self-Compton component. Further, we note that the γ-ray emission during the high-flux state of AO 0235+164 (epochs A and B) requires seed photons from both the dusty torus and broad-line region, while the γ-ray emission in OJ 287 and during epochs C and D of AO 0235+164 can be modelled by EC scattering of infrared photons from the torus.

Correlations between γ-ray luminosity and magnetization of the jet as well as relativistic electron injection power: cases for Mrk 421, 3C 454.3 and 3C 279

ABSTRACTBy fitting high-quality and simultaneous multiwavelength spectral energy distributions at multiple epochs with a one-zone leptonic jet model, we study the jet properties of three famous blazars: Mrk 421, 3C 454.3 and 3C 279. In the jet model, the emitting electron energy distributions are calculated by solving the kinetic equations of electron injection, escape, adiabatic and radiative energy losses. To explore multidimensional parameter space systematically, we employ a Markov chain Monte Carlo fitting technique. The properties of the emission regions we have derived here are consistent with those in previous studies, for example, the particle-dominated and low-magnetization jet. The new finding is that there is a tight correlation between γ-ray luminosity and electron injection power and an anticorrelation between γ-ray luminosity and the jet magnetization parameter. The results suggest that the same energy-dissipative mechanism (such as a shock) could be operating in the jets of different types of blazars, and that the origin of γ-ray flares is associated with the particle acceleration process.

Unveiling the broad-band spectral and temporal properties of PKS 0903-57 during its brightest flare

ABSTRACT We carried a detailed spectral and temporal study of blazar PKS 0903-57 using the Fermi-LAT and Swift-XRT/UVOT observations, during its brightest flaring period MJD 58931–58970. During this period, the maximum daily averaged γ-ray flux ($\rm F_{0.1-500\, GeV}$) of $\rm 9.42\times 10^{-6}\, ph\, cm^{-2}\, s^{-1}$ is observed on MJD 58951.5, the highest γ-ray flux detected from PKS 0903-57 till now. Several high-energy (HE) photons ($\gt 10$ GeV) consistent with the source location at high probability (>99 per cent) are detected, and the γ-ray light curve in the active state shows multiple substructures with asymmetric profile. In order to understand the possible physical scenario responsible for the flux enhancement, we carried a detailed broad-band spectral study of PKS 0903-57 by choosing different flux states from its active period. Neglecting the multiband variability in each of the selected time intervals, we could reproduce their averaged broad-band SEDs with a one-zone leptonic model whose parameters were derived with a χ2-fit. We found that the broad-band SED during different flux states can be reproduced by the synchrotron, synchrotron-self-Compton (SSC), and external-Compton (EC) processes. The seed photons for EC process from BLR or IR torus provide acceptable fits to the GeV spectrum in all the flux states; however, the detection of HE photons together with the equipartition condition suggests that the EC/IR process is a more likely scenario. Further, a detailed comparison between the fit parameters shows that the flux enhancement from quiescent-state to the flaring-state is mostly related to increase in the bulk Lorentz factor of the emission region and change in the break energy of the source spectrum.

The impact of (n,γ) reaction rate uncertainties of unstable isotopes on the i-process nucleosynthesis of the elements from Ba to W

ABSTRACT The abundances of neutron (n)-capture elements in the carbon-enhanced metal-poor (CEMP)-r/s stars agree with predictions of intermediate n-density nucleosynthesis, at Nn ∼ 1013–1015 cm−3, in rapidly accreting white dwarfs (RAWDs). We have performed Monte Carlo simulations of this intermediate-process (i-process) nucleosynthesis to determine the impact of (n,γ) reaction rate uncertainties of 164 unstable isotopes, from 131I to 189Hf, on the predicted abundances of 18 elements from Ba to W. The impact study is based on two representative one-zone models with constant values of Nn = 3.16 × 1014 and 3.16 × 1013 cm−3 and on a multizone model based on a realistic stellar evolution simulation of He-shell convection entraining H in a RAWD model with [Fe/H] = −2.6. For each of the selected elements, we have identified up to two (n,γ) reactions having the strongest correlations between their rate variations constrained by Hauser–Feshbach computations and the predicted abundances, with the Pearson product–moment correlation coefficients |rP| > 0.15. We find that the discrepancies between the predicted and observed abundances of Ba and Pr in the CEMP-i star CS 31062−050 are significantly diminished if the rate of 137Cs(n,γ)138Cs is reduced and the rates of 141Ba(n,γ)142Ba or 141La(n,γ)142La increased. The uncertainties of temperature-dependent β-decay rates of the same unstable isotopes have a negligible effect on the predicted abundances. One-zone Monte Carlo simulations can be used instead of computationally time-consuming multizone Monte Carlo simulations in reaction rate uncertainty studies if they use comparable values of Nn. We discuss the key challenges that RAWD simulations of i process for CEMP-i stars meet by contrasting them with recently published low-Z asymptotic giant branch (AGB) i process.

X-Ray-to-radio Offset Inference from Low-count X-Ray Jets

Abstract Observations of positional offsets between the location of X-ray and radio features in many resolved, extragalactic jets indicates that the emitting regions are not cospatial, an important piece of evidence in the debate over the origin of the X-ray emission on kiloparsec scales. The existing literature is nearly exclusively focused on jets with sufficiently deep Chandra observations to yield accurate positions for X-ray features, but most of the known X-ray jets are detected with tens of counts or fewer, making detailed morphological comparisons difficult. Here we report the detection of X-ray-to-radio positional offsets in 15 extragalactic jets from an analysis of 22 sources with low-count Chandra observations, where we utilized the Low-count Image Reconstruction Algorithm. This algorithm has allowed us to account for effects such as Poisson background fluctuations and nearby point sources which have previously made the detection of offsets difficult in shallow observations. Using this method, we find that in 55% of knots with detectable offsets, the X-rays peak upstream of the radio, questioning the applicability of one-zone models, including the IC/CMB model for explaining the X-ray emission. We also report the nondetection of two previously claimed X-ray jets. Many, but not all, of our sources follow a loose trend of increasing offset between the X-ray and radio emission, as well as a decreasing X-ray-to-radio flux ratio along the jet.

Modeling of time-resolved LIBS spectra obtained in Martian atmospheric conditions with a stationary plasma approach

In this study, we investigate the characteristics of the LIBS plasma in Martian atmospheric conditions using stationary modeling of the LIBS plasma. LIBS spectra are simulated from a one-dimensional model of the plasma divided into two zones along the line of sight. The simulations are based on local thermal equilibrium and carried out using radiative transfer. The simulated spectra are fitted first to synthetic LIBS spectra to test the implementation and in a second step to time-resolved LIBS data of a carbonate sample obtained in experimentally simulated Martian atmospheric conditions. From the fits to the synthetic LIBS spectra, we confirm that plasmas with spatial gradients in the temperature and densities can be well described in a two-zone plasma model. From the fits to the measured data this observation also holds true. We find that the most of the emission lines are well described in the two-zone model and that the two-zone model is a significant improvement from the one-zone model. Furthermore, we obtain typical values of the plasma properties in the two-zone approximation from 500 ns to 1250 ns after plasma initiation and derive the effect of self-absorption, which is predicted to decrease the strongest pixel intensities by a factor of almost two orders of magnitudes.

Detection of the hard X-ray non-thermal emission from Kepler’s supernova remnant

Abstract We report the first robust detection of the hard X-ray emission in the 15–30 keV band from Kepler’s supernova remnant with the silicon PIN-type semiconductor detector of the hard X-ray detector (HXD-PIN) onboard Suzaku. The detection significance is 7.17σ for the emission from Kepler’s entire X-ray emitting region. The energy spectrum is found to be well reproduced by a single power-law function with a photon index of $3.13^{+1.85+0.69}_{-1.52-0.36}$, where the first and second errors represent $90\%$-statistical and systematic errors, respectively. The X-ray flux is determined to be $2.75_{-0.77-0.82}^{+0.78+0.81}\times 10^{-12}\:$erg s−1 cm−2 in the 15–30 keV band. The wider-band X-ray spectrum in the 3–30 keV band, where the soft X-ray Suzaku/XIS spectrum is combined, shows that the non-thermal component does not have a significant X-ray roll-off structure. We find that the broad-band energy spectrum from the radio band, X-ray data of this work, and TeV upper limits can be reproduced with the one-zone leptonic model with a roll-off energy of νroll = 1.0 × 1017 Hz and magnetic field strength of B > 40 μG. Application of the diagnostic method using indices in the soft and hard X-ray band to the data indicates that the maximum energy of the accelerated electrons in Kepler’s SNR is limited by the age of the remnant. The indication is consistent with the results of the one-zone leptonic modeling.

MAGIC Observations of the Nearby Short Gamma-Ray Burst GRB 160821B *

Abstract The coincident detection of GW170817 in gravitational waves and electromagnetic radiation spanning the radio to MeV gamma-ray bands provided the first direct evidence that short gamma-ray bursts (GRBs) can originate from binary neutron star (BNS) mergers. On the other hand, the properties of short GRBs in high-energy gamma-rays are still poorly constrained, with only ∼20 events detected in the GeV band, and none in the TeV band. GRB 160821B is one of the nearest short GRBs known at z = 0.162. Recent analyses of the multiwavelength observational data of its afterglow emission revealed an optical-infrared kilonova component, characteristic of heavy-element nucleosynthesis in a BNS merger. Aiming to better clarify the nature of short GRBs, this burst was automatically followed up with the MAGIC telescopes, starting from 24 s after the burst trigger. Evidence of a gamma-ray signal is found above ∼0.5 TeV at a significance of ∼ 3σ during observations that lasted until 4 hr after the burst. Assuming that the observed excess events correspond to gamma-ray emission from GRB 160821B, in conjunction with data at other wavelengths, we investigate its origin in the framework of GRB afterglow models. The simplest interpretation with one-zone models of synchrotron-self-Compton emission from the external forward shock has difficulty accounting for the putative TeV flux. Alternative scenarios are discussed where the TeV emission can be relatively enhanced. The role of future GeV–TeV observations of short GRBs in advancing our understanding of BNS mergers and related topics is briefly addressed.

Forecasting of a boosted locomotive gas diesel engine parameters with one- and two-stage charging systems

Conversion of locomotive engines for operation on natural gas lowers considerably expenses for fuel and reduces exhaust emissions which makes it possible to omit large and expensive aftertreatment systems. The permanent need to raise the engine power requires a considerable increase of the boost pressure. This can be realized by using a high pressure turbocharger or a two-stage charging system. In the research, parameters of a high boosted D200 6-cylinder locomotive engine having D/S=200/280 mm are forecasted using a one-zone model developed in MADI. An analysis was carried out to explain why the 1st stage compressor of the two-stage charging system should be specially profiled to have its map tilted to the right. Calculations were performed for the gas diesel engine having a break mean effective pressure (BMEP) 2.7 MPa with one and two-stage charging systems. In both cases, close fuel efficiency was obtained, though for the two-stage charging system, the boost air pressure was higher. The engine with one turbocharger had no reserves for further power augmentation while the two-stage charging system enabled to increase the boost air pressure further. Therefore, parameters of the engine having a higher BMEP 3.2 MPa were calculated. In that case, not to exceed the peak combustion pressure, a retarded fuel injection was used which resulted in fuel efficiency drop by approximately 1.5%.

Photohadronic modelling of the 2010 gamma-ray flare from Mrk 421

ABSTRACT Blazars are a subclass of active galactic nuclei (AGNs) that have a relativistic jet with a small viewing angle towards the observer. Recent results based on hadronic scenarios have motivated an ongoing discussion of how a blazar can produce high energy neutrinos during a flaring state and which scenario can successfully describe the observed gamma-ray behaviour. Markarian 421 is one of the closest and brightest objects in the extragalactic gamma-ray sky and showed flaring activity over a 14-days period in 2010 March. In this work, we describe the performed analysis of Fermi-LAT data from the source focused on the MeV range (100 MeV–1 GeV), and study the possibility of a contribution coming from the pγ interactions between protons and MeV SSC target photons to fit the very high energy (VHE) gamma-ray emission. The fit results were compared with two leptonic models (one-zone and two-zone) using the Akaike Information Criteria (AIC) test, which evaluates goodness-of-fit alongside the simplicity of the model. In all cases, the photohadronic model was favoured as a better fit description in comparison to the one-zone leptonic model, and with respect to the two-zone model in the majority of cases. Our results show the potential of a photohadronic contribution to a lepto-hadronic origin of gamma-ray flux of blazars. Future gamma-ray observations above tens of TeV and below 100 MeV in energy will be crucial to test and discriminate between models.

A Multiwavelength Study of Distant Blazar PKS 0537-286

We report the results of broadband observations of distant blazar PKS 0537-286 (z = 3.1) using data spanning more than ten years from the Fermi Large Area Telescope together with Swift UVOT/XRT archival data taken between 2005 and 2017. In the γ -ray band, the peak flux above 100 MeV, F$_{γ}$ = (6 23 ± 0 56) ∙ 10$^{-7}$ photon cm$^{-2}$ s$^{-1}$ observed on MJD 57874 within one week, corresponds to L$_{γ}$ = 2 46 ∙ 10$^{49}$ erg s$^{-1}$ isotropic γ -ray luminosity. The Swift XRT data analyses show that the X-ray emission is characterized by a significantly hard photon index, Γ$_{X-ray}$ ≤ 1.3 , and an X-ray flux of 4 ∙ 10$^{-12}$ erg cm$^{-2}$ s$^{-1}$, which is almost constant over twelve years. The spectral energy distribution is modeled within one-zone leptonic models assuming the emission region is within the broad-line region. The observed X-ray and γ -ray data are modeled as inverse Compton scattering of (i) only synchrotron photons and (ii) synchrotron and external photons on the electron population that produces the radioto-optical emission. The modeling shows that the nonthermal electrons in the jet of PKS 0537-286 have a hard power-law index (<1.9) and that the jet should be particle dominated with a luminosity within 10$^{45}$-10$^{46}$ erg s$^{-1}$.

Studies of extragalactic background light with TeV BL Lacertae objects

ABSTRACT Very high energy (VHE; E ≥ 100 GeV) gamma-rays from cosmological distances are attenuated by the extragalactic background light (EBL) in the infrared to ultraviolet bands. By contrasting measured versus intrinsic emission,we can derive the EBL photon density. However, we do not know the intrinsic spectra and the EBL separately, only their combined effect. Here we first present a flexible model-dependent optical depth method to study the EBL by fitting the emission spectra of TeV BL Lacertae objects (BL Lacs) via a one-zone leptonic synchrotron self-Compton model (SSC). We have little information about electron energy distributions (EEDs) in the jet, which is critically important to build spectral energy distributions (SEDs) in the SSC scenario. Based on current particle acceleration models, we use two types of EEDs to fit the observed spectra: a power-law log-parabola (PLLP) EED and a broken power-law (BPL) EED. We find that the upper limit of the EBL density is about 30 n W m−2 sr−1, which is similar to the published measurement. Furthermore, we propose an unprecedented method to test the radiation mechanisms involved in TeV objects, by simply comparing the reduced EBL density with the limit obtained by galaxy counts. We demonstrate that for some BL Lacs, at least, the one-zone SSC model should be reconsidered.

Suppression of H2-cooling in protogalaxies aided by trapped Lyα cooling radiation

ABSTRACT We study the thermal evolution of UV-irradiated atomic cooling haloes using high-resolution three-dimensional hydrodynamic simulations. We consider the effect of H− photodetachment by Lyα cooling radiation trapped in the optically-thick cores of three such haloes, a process that has not been included in previous simulations. Because H− is a precursor of molecular hydrogen, its destruction can diminish the H2 abundance and cooling. We find that the critical UV flux for suppressing H2-cooling is decreased by ∼15–50 per cent in our fiducial models. Previous one-zone modelling found a larger effect, with Jcrit reduced by a factor of a few; we show that adopting a constant halo mass to determine the trapped Lyα energy density, as is done in the one-zone models, yields a larger reduction in Jcrit, consistent with their findings. Our results nevertheless suggest that Lyα radiation may have an important effect on the thermal evolution of UV-irradiated haloes, and therefore on the potential for massive black hole formation.

Constraining models of the pulsar wind nebula in SNR G0.9+0.1 via simulation of its detection properties using the Cherenkov Telescope Array

ABSTRACT SNR G0.9+0.1 is a well-known source in the direction of the Galactic Centre composed by a Supernova Remnant (SNR) and a Pulsar Wind Nebula (PWN) in the core. We investigate the potential of the future Cherenkov Telescope Array (CTA), simulating observations of SNR G0.9 + 0.1. We studied the spatial and spectral properties of this source and estimated the systematic errors of these measurements. The source will be resolved if the very high-energy emission region is bigger than ∼0.65′. It will also be possible to distinguish between different spectral models and calculate the cutoff energy. The systematic errors are dominated by the Instrument Response Function instrumental uncertainties, especially at low energies. We computed the evolution of a young PWN inside an SNR using a one-zone time-dependent leptonic model. We applied the model to the simulated CTA data and found that it will be possible to accurately measure the cutoff energy of the γ-ray spectrum. Fitting of the multiwavelength spectrum will allow us to constrain also the magnetization of the PWN. Conversely, a pure power-law spectrum would rule out this model. Finally, we checked the impact of the spectral shape and the energy density of the Inter-Stellar Radiation Fields on the estimate of the parameters of the PWN, finding that they are not significantly affected.

Neutron Star Mergers as the Main Source of r-process: Natal Kicks and Inside-out Evolution to the Rescue

Abstract Binary neutron star mergers (BNSMs) are currently the most promising source of r-process thanks to the detection of GW170817. The estimated occurring frequency and the amount of mass ejected per merger indicate that BNSMs by themselves can account for all the r-process enrichment in the Galaxy. However, the decreasing trend of [Eu/Fe] versus [Fe/H] of disk stars for [Fe/H] ≳ −1 in the solar neighborhood is inconsistent with the flat trend expected from BNSMs with a standard delay time distribution (DTD) ∝ t −1. This has led to the suggestion that either additional sources or modification to the DTD of BNSMs is required to match the observations. We investigate the effects of natal kicks received during the birth of neutron star binaries on the chemical evolution of r-process element Eu in the Milky Way by combining the results from the galactic dynamics code galpy with a one-zone Galactic chemical evolution model omega. We show that when key inputs from simulations of the inside-out disk evolution are combined with natal kicks, BNSMs can naturally reproduce the observed decreasing trend of [Eu/Fe] with [Fe/H] in the solar neighborhood without the need for modification to the DTD or additional r-process sources.

Development and Validation of a One-zone Model for Fire-induced Pressure Prediction in Airtight Houses

Development and Validation of a One-zone Model for Fire-induced Pressure Prediction in Airtight Houses

Reverberation of pulsar wind nebulae (I): impact of the medium properties and other parameters upon the extent of the compression

ABSTRACT The standard approach to the long-term evolution of pulsar wind nebulae (PWNe) is based on one-zone models treating the nebula as a uniform system. In particular for the late phase of evolved systems, many of the generally used prescriptions are based on educated guesses for which a proper assessment lacks. Using an advanced radiative code, we evaluate the systematic impact of various parameters, like the properties of the supernova ejecta, of the inner pulsar, as well of the ambient medium, upon the extent of the reverberation phase of PWNe. We investigate how different prescriptions shift the starting time of the reverberation phase, how this affects the amount of the compression, and how much of this can be ascribable to the radiation processes. Some critical aspects are the description of the reverse shock evolution, the efficiency by which at later times material from the ejecta accretes on to the swept-up shell around the PWN, and finally the density, velocity, and pressure profiles in the surrounding supernova remnant. We have explicitly treated the cases of the Crab Nebula, and of J1834.9−0846, taken to be representatives of the more and the less energetic pulsars, respectively. Especially for the latter object, the prediction of large compression factors is confirmed, even larger in the presence of radiative losses, also confirming our former prediction of periods of superefficiency during the reverberation phase of some PWNe.

Sensitivity studies on the thermal beta+ decay and thermal neutrino decay rates in the one-zone X-ray burst model

ABSTRACT We investigated how the thermal beta+ decay rates and their corresponding neutrino energy loss rates affect the properties of X-ray bursts. Previous studies carried out showed how the beta+ decay rates alone affect the properties of X-ray bursts. Here, we performed the sensitivity studies on neutrino energy loss rates and their impact on X-ray bursts. Furthermore, all the experimental beta+ decay rates related to X-ray bursts were measured. However, the temperature of X-ray bursts can be as high as 1.5 GK. Thus, at such a high temperature, the nuclei can be in excited states due to thermal effects. Experiments cannot directly measure the thermal beta decay rates and thermal neutrino loss rates. We performed sensitivity tests on the thermal beta+ decay rates and their thermal neutrino losses in one-zone X-ray burst models in different ignition conditions. We discovered that beta decays due to thermal effects do affect the light curves in some ignition conditions. The thermal beta decay rates of 68Se also have major effects on the final abundances in most circumstances. We further found that a neutrino energy loss rate of 64Ga has noticeable effects on the light curves of X-ray bursts in general.

Correlation between optical and γ-ray flux variations in bright flat spectrum radio quasars

ABSTRACT Blazars are known to show flux variations over a range of energies from low-energy radio to high-energy γ-rays. Cross-correlation analysis of the optical and γ-ray light curves in blazars shows that flux variations are generally correlated in both bands, however, there are exceptions. We explored this optical–GeV connection in four flat spectrum radio quasars by a systematic investigation of their long-term optical and γ-ray light curves. On analysis of the four sources, namely 3C 273, 3C 279, PKS 1510−089, and CTA 102, we noticed different behaviours between the optical and GeV flux variations. We found instances when (i) the optical and GeV flux variations are closely correlated, (ii) there are optical flares without γ-ray counterparts, and (iii) γ-ray flares without optical counterparts. To understand these diverse behaviours, we carried out broad-band spectral energy distribution (SED) modelling of the sources at different epochs using a one-zone leptonic emission model. The optical–UV emission is found to be dominated by emission from the accretion disc in the sources PKS 1510−089, CTA 102, and 3C 273, while in 3C 279, the synchrotron radiation from the jet dominates the optical–UV emission. Our SED analysis indicates that (i) correlated optical and γ-ray flux variations are caused by changes in the bulk Lorentz factor (Γ), (ii) γ-ray flares without optical counterparts are due to increase in Γ and/or the electron energy density, and (iii) an optical flare without γ-ray counterpart is due to increase in the magnetic field strength.

The impact of starbursts on element abundance ratios

ABSTRACT We investigate the impact of bursts in star formation on the predictions of one-zone chemical evolution models, adopting oxygen (O), iron (Fe), and strontium (Sr), as representative α, iron-peak, and s-process elements, respectively. To this end, we develop and make use of the Versatile Integrator for Chemical Evolution (VICE), a python package designed to handle flexible user-specified evolutionary parameters. Starbursts driven by a temporary boost of gas accretion rate create loops in [O/Fe]–[Fe/H] evolutionary tracks and a peak in the stellar [O/Fe] distribution at intermediate values. Bursts driven by a temporary boost of star formation efficiency have similar effects, and they also produce a population of α-deficient stars during the depressed star formation phase following the burst. This α-deficient population is more prominent if the outflow rate is tied to a time-averaged star formation rate (SFR) instead of the instantaneous SFR. Theoretical models of Sr production predict a strong metallicity dependence of supernova and asymptotic giant branch star yields, though comparison to data suggests an additional, nearly metallicity-independent source. Evolution of [Sr/Fe] and [Sr/O] during a starburst is complex because of this metallicity dependence and the multiple time-scales at play. Moderate amplitude (10–20 per cent) sinusoidal oscillations in SFR produce loops in [O/Fe]–[Fe/H] tracks and multiple peaks in [O/Fe] distributions, a potential source of intrinsic scatter in observed sequences. We investigate the impact of a factor ∼2 enhancement of Galactic star formation ∼2 Gyr ago, as suggested by some recent observations. VICE is publicly available at &amp;lt;http://pypi.org/project/vice/&amp;gt;.