We present a simple protocol which allows fully automated discovery of elementary chemical reaction steps using in cooperation double- and single-ended transition-state optimization algorithmsthe ...freezing string and Berny optimization methods, respectively. To demonstrate the utility of the proposed approach, the reactivity of several single-molecule systems of combustion and atmospheric chemistry importance is investigated. The proposed algorithm allowed us to detect without any human intervention not only “known” reaction pathways, manually detected in the previous studies, but also new, previously “unknown”, reaction pathways which involve significant atom rearrangements. We believe that applying such a systematic approach to elementary reaction path finding will greatly accelerate the discovery of new chemistry and will lead to more accurate computer simulations of various chemical processes.
Observations of thermonuclear X-ray bursts from accreting neutron stars (NSs) in low-mass X-ray binary systems can be used to constrain NS masses and radii. Most previous work of this type has set ...these constraints using Planck function fits as a proxy: the models and the data are both fit with diluted blackbody functions to yield normalizations and temperatures that are then compared with each other. For the first time, we here fit atmosphere models of X-ray bursting NSs directly to the observed spectra. We present a hierarchical Bayesian fitting framework that uses current X-ray bursting NS atmosphere models with realistic opacities and relativistic exact Compton scattering kernels as a model for the surface emission. We test our approach against synthetic data and find that for data that are well described by our model, we can obtain robust radius, mass, distance, and composition measurements. We then apply our technique to Rossi X-ray Timing Explorer observations of five hard-state X-ray bursts from 4U 1702−429. Our joint fit to all five bursts shows that the theoretical atmosphere models describe the data well, but there are still some unmodeled features in the spectrum corresponding to a relative error of 1–5% of the energy flux. After marginalizing over this intrinsic scatter, we find that at 68% credibility, the circumferential radius of the NS in 4U 1702−429 is R = 12.4±0.4 km, the gravitational mass is M = 1.9±0.3 M⊙, the distance is 5.1 < D/ kpc < 6.2, and the hydrogen mass fraction is X < 0.09.
This Feature Article presents an overview of the current status of ring polymer molecular dynamics (RPMD) rate theory. We first analyze the RPMD approach and its connection to quantum ...transition-state theory. We then focus on its practical applications to prototypical chemical reactions in the gas phase, which demonstrate how accurate and reliable RPMD is for calculating thermal chemical reaction rate coefficients in multifarious cases. This review serves as an important checkpoint in RPMD rate theory development, which shows that RPMD is shifting from being just one of recent novel ideas to a well-established and validated alternative to conventional techniques for calculating thermal chemical rate coefficients. We also hope it will motivate further applications of RPMD to various chemical reactions.
ABSTRACT
Magnetars are believed to host the strongest magnetic fields in the present universe ($B\gtrsim 10^{14}$ G) and the study of their persistent emission in the X-ray band offers an ...unprecedented opportunity to gain insight into physical processes in the presence of ultra-strong magnetic fields. Up to now, most of our knowledge about magnetar sources came from spectral analysis, which allowed to test the resonant Compton scattering scenario and to probe the structure of the star magnetosphere. On the other hand, radiation emitted from magnetar surface is expected to be strongly polarized and its observed polarization pattern bears the imprint of both scatterings on to magnetospheric charges and quantum electro-dynamics (QED) effects as it propagates in the magnetized vacuum around the star. X-ray polarimeters scheduled to fly in the next years will finally allow to exploit the wealth of information stored in the polarization observables. Here we revisit the problem of assessing the spectro-polarimetric properties of magnetar persistent emission. At variance with previous investigations, proper account for more physical surface emission models is made by considering either a condensed surface or a magnetized atmosphere. Results are used to simulate polarimetric observations with the forthcoming Imaging X-ray Polarimetry Explorer. We find that X-ray polarimetry will allow to detect QED vacuum effects for all the emission models we considered and to discriminate among them.
A ring polymer molecular dynamics (RPMD) method is proposed for the calculation of the dissociative chemisorption rate coefficient on surfaces. The RPMD rate theory is capable of handling quantum ...effects such as the zero-point energy and tunneling in dissociative chemisorption, while it relies on classical trajectories for the simulation. Applications to H2 dissociative chemisorption are demonstrated. For the highly activated process on Ag(111), strong deviations from Arrhenius behavior are found at low temperatures and attributed to tunneling. On Pt(111), where the dissociation has a barrierless pathway, the RPMD rate coefficient is found to agree with the experimentally derived thermal sticking coefficient within a factor of 2 over a large temperature range. Significant quantum effects are also found.
The cooling phase of thermonuclear (type-I) X-ray bursts can be used to constrain neutron star (NS) compactness by comparing the observed cooling tracks of bursts to accurate theoretical atmosphere ...model calculations. By applying the so-called cooling tail method, where the information from the whole cooling track is used, we constrain the mass, radius, and distance for three different NSs in low-mass X-ray binaries 4U 1702−429, 4U 1724−307, and SAX J1810.8−260. Care is taken to use only the hard state bursts where it is thought that the NS surface alone is emitting. We then use a Markov chain Monte Carlo algorithm within a Bayesian framework to obtain a parameterized equation of state (EoS) of cold dense matter from our initial mass and radius constraints. This allows us to set limits on various nuclear parameters and to constrain an empirical pressure-density relationship for the dense matter. Our predicted EoS results in NS a radius between 10.5−12.8 km (95% confidence limits) for a mass of 1.4 M⊙, depending slightly on the assumed composition. Because of systematic errors and uncertainty in the composition, these results should be interpreted as lower limits for the radius.
Aims. We present the results of the monitoring programmes performed with the Swift/XRT telescope and aimed specifically to detect an abrupt decrease of the observed flux associated with a transition ...to the propeller regime in two well-known X-ray pulsars 4U 0115+63 and V 0332+53. Methods. Both sources form binary systems with Be optical companions and undergo so-called giant outbursts every 3−4 years. The current observational campaigns were performed with the Swift/XRT telescope in the soft X-ray band (0.5−10 keV) during the declining phases of the outbursts exhibited by both sources in 2015. Results. The transitions to the propeller regime were detected at the threshold luminosities of (1.4 ± 0.4) × 1036 erg s-1 and (2.0 ± 0.4) × 1036 erg s-1 for 4U 0115+63 and V 0332+53, respectively. Spectra of the sources are shown to be significantly softer during the low state. In both sources, the accretion at rates close to the aforementioned threshold values briefly resumes during the periastron passage following the transition into the propeller regime. The strength of the dipole component of the magnetic field required to inhibit the accretion agrees well with estimates based on the position of the cyclotron lines in their spectra, thus excluding presence of a strong multipole component of the magnetic field in the vicinity of the neutron star.
Some thermonuclear (type I) X-ray bursts at the neutron star surfaces in low-mass X-ray binaries take place during hard persistent states of the systems. Spectral evolution of these bursts is well ...described by the atmosphere model of a passively cooling neutron star when the burst luminosity is high enough. The observed spectral evolution deviates from the model predictions when the burst luminosity drops below a critical value of 20–70% of the maximum luminosity. The amplitude of the deviations and the critical luminosity correlate with the persistent luminosity, which leads us to suggest that these deviations are induced by the additional heating of the accreted particles. We present a method for computation of the neutron star atmosphere models heated by accreted particles assuming that their energy is released via Coulomb interactions with electrons. We computed the temperature structures and the emergent spectra of the atmospheres of various chemical compositions and investigate the dependence of the results on the velocity of accreted particles, their temperature and the penetration angle. We show that the heated atmosphere develops two different regions. The upper one is the hot (20–100 keV) corona-like surface layer cooled by Compton scattering, and the deeper, almost isothermal optically thick region with a temperature of a few keV. The emergent spectra correspondingly have two components: a blackbody with the temperature close to that of the isothermal region and a hard Comptonized component (a power law with an exponential decay). Their relative contribution depends on the ratio of the energy dissipation rate of the accreted particles to the intrinsic flux from the neutron star surface. These spectra deviate strongly from those of undisturbed, passively cooling neutron star atmospheres, with the main differences being the presence of a high-energy tail and a strong excess in the low-energy part of the spectrum. They also lack the iron absorption edge, which is visible in the spectra of undisturbed low-luminosity atmospheres with solar chemical composition. Using the computed spectra, we obtained the dependences of the dilution and color-correction factors as functions of relative luminosities for pure helium and solar abundance atmospheres. We show that the helium model atmosphere heated by accretion corresponding to 5% of the Eddington luminosity describes well the late stages of the X-ray bursts in 4U 1820−30.
Aims. To estimate the compactness of the thermally-emitting isolated neutron star (INS) RX J0720.4−3125, an X-ray spin-phase-resolved spectroscopic study is conducted. In addition, to identify the ...genuine spin-period, an X-ray timing analysis is performed. Methods. The data from all observations of RX J0720.4−3125 conducted by XMM-Newton EPIC-pn with the same instrumental setup in 2000−2012 were reprocessed to form a homogenous dataset of solar barycenter corrected photon arrival times registered from RX J0720.4−3125. A Bayesian method for the search, detection, and estimation of the parameters of an unknown-shaped periodic signal was employed. A number of single- and double-peaked complex models of light curves from pulsating neutron stars were statistically analyzed. The distribution of phases for the registered photons was calculated by folding the arrival times with the derived spin-period and the resulting distribution of phases, which was approximated with a mixed von Mises distribution, and its parameters were estimated by using the expected maximization method. Spin-phase-resolved spectra were extracted, a number of highly magnetized atmosphere models of an INS were used to perform simultaneous fits, and the results were verified via an Markov chain Monte Carlo approach. Results. The phase-folded light curves in different energy bands with high signal-to-noise ratio show high complexity and variations that depend on time and energy. They can be parameterized with a mixed von Mises distribution, meaning with a double-peaked light curve profile that shows a dependence of the estimated parameters, such as the mean directions, concentrations, and proportion upon the energy band, indicating that radiation emerges from at least two emitting areas. Conclusions. We derive a most-likely genuine spin-period of the isolated neutron star RX J0720−3125 that is twice that reported in the literature, 16.78 s instead of 8.39 s. We determine the gravitational redshift of RX J0720.4−3125 to be z = 0.205-0.003+0.006 and estimate the compactness to be (M/M⊙)/(R/ km) = 0.105 ± 0.002.
Context. Central compact objects (CCOs) in supernova remnants are isolated thermally emitting neutron stars (NSs). They are most probably characterized by a magnetic field strength that is roughly ...two orders of magnitude lower than that of most of the radio and accreting pulsars. The thermal emission of CCOs can be modeled to obtain constraints on the physical parameters of the star such as its mass, radius, effective temperature, and chemical composition. Aims. The CCO in HESS J1731−347 is one of the brightest objects in this class. Starting from 2007, it was observed several times with different X-ray satellites. Here we present our analysis of two new XMM-Newton observations of the source performed in 2013 which increase the total exposure time of the data available for spectral analysis by a factor of about five compared to the analyses presented before. Methods. We use our numerical spectral models for carbon and hydrogen atmospheres to fit the spectrum of the CCO. From our fits, we derive constraints on the physical parameters of the emitting star such as its mass, radius, distance, and effective temperature. We also use the new data to derive new upper limits on the source pulsations and to confirm the absence of a long-term flux and spectral variability. Results. The analysis shows that atmosphere models are clearly preferred by the fit over the blackbody spectral function. Under the assumption that the X-ray emission is uniformly produced by the entire star surface (supported by the lack of pulsations), hydrogen atmosphere models lead to uncomfortably large distances of the CCO, above 7–8 kpc. On the other hand, the carbon atmosphere model formally excludes distances above 5–6 kpc and is compatible with the source located in the Scutum-Crux (~3 kpc) or Norma-Cygnus (~4.5 kpc) Galactic spiral arm. We provide and discuss the corresponding confidence contours in the NS mass–radius plane. The measured effective temperature indicates that the NS is exceptionally hot for the estimated age of ~30 kyr. We discuss possible cooling scenarios to explain this property, as well as possible additional constraints on the star mass and radius from cooling theory.
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