arXiv subjects

Alexander Lange

Publications and source records attributed to Alexander Lange.

17 recordsLinked to original sources

arXiv2026

Chandra X-ray Observations of the Pulsar Wind Nebula within CTA 1

We present deep Chandra observations of the pulsar wind nebula (PWN) powered by PSR J0007+7303 in the composite supernova remnant CTA 1. The merged ACIS image shows a $\sim20''$ jet extending south of the pulsar and bending toward the southwest, a faint counter-jet to the north, and a compact torus oriented approximately perpendicular to the jet axis. Using an archival observation from 2003 we perform relative astrometry over a $\sim20$ yr baseline and constrain the pulsar's transverse velocity to $\lesssim 200~\mathrm{km~s^{-1}}$ at the distance of 1.4 kpc at 95% confidence. Spatially resolved spectroscopy shows hard spectra for the jet and torus (photon indicies $\Gamma \approx 1.2-1.4$) and a softer spectrum for the extended nebula ($\Gamma = 1.85 \pm 0.11$), indicating minimal radiative cooling in the compact regions. Modeling of the torus, associated with the termination shock, as an inclined circle yields a viewing angle $\zeta \approx 50^\circ$. The outer gap and two-pole caustic pulsar emission models then imply a moderate magnetic inclination ($\alpha \sim 20^\circ$-$70^\circ$). Broadband spectral energy distribution (SED) modeling from radio to PeV $\gamma$-rays for a one-zone leptonic scenario yields a low magnetic field ($B \approx 1.4$-$3.2~\mu\mathrm{G}$) and a high electron cutoff energy ($E_{\rm cut} \sim 0.2$-$0.3~\mathrm{PeV}$), indicating that the magnetic field decreases rapidly outside of the compact nebula. These results establish CTA 1 as a young, low X-ray efficiency PWN with a hard injection spectrum capable of accelerating particles to PeV energies.

astro-ph.HE↗

arXiv2026

A Dataset for Automatic Vocal Mode Classification

The Complete Vocal Technique (CVT) is a school of singing developed in the past decades by Cathrin Sadolin et al.. CVT groups the use of the voice into so called vocal modes, namely Neutral, Curbing, Overdrive and Edge. Knowledge of the desired vocal mode can be helpful for singing students. Automatic classification of vocal modes can thus be important for technology-assisted singing teaching. Previously, automatic classification of vocal modes has been attempted without major success, potentially due to a lack of data. Therefore, we recorded a novel vocal mode dataset consisting of sustained vowels recorded from four singers, three of which professional singers with more than five years of CVT-experience. The dataset covers the entire vocal range of the subjects, totaling 3,752 unique samples. By using four microphones, thereby offering a natural data augmentation, the dataset consists of more than 13,000 samples combined. An annotation was created using three CVT-experienced annotators, each providing an individual annotation. The merged annotation as well as the three individual annotations come with the published dataset. Additionally, we provide some baseline classification results. The best balanced accuracy across a 5-fold cross validation of 81.3\,\% was achieved with a ResNet18. The dataset can be downloaded under https://zenodo.org/records/14276415.

cs.SD↗

arXiv2025

Super-Eddington Accretion Geometry: a Remarkable Stability of the Hidden Ultraluminous X-Ray Source Cygnus X-3

We report on the average and orbital phase-resolved polarization of Cyg X-3 in the hard state during the 2023 Imaging X-ray Polarimetry Explorer (IXPE) observational campaign. We find the polarization degree of $ 21.2 \pm 0.4 \% $ and polarization angle of $ 92.2 \pm 0.5^\circ $, well compatible with the first hard-state IXPE observation in 2022. As the observed polarization depends on both the accretion geometry and the X-ray emission mechanism, which we attribute to reflection from the optically thick envelope surrounding the central source, our result indicates that both are very stable on year-long timescale. We discuss time- and energy-dependent polarization properties and their implications for the geometry and stability of the accretion funnel.

astro-ph.HE↗

arXiv2025

The nature of UHE source 1LHAASO J1740+0948u and its connection to PSR J1740+1000

We present multi-wavelength analysis of 1LHAASO J1740+0948u and its surroundings including the pulsar wind nebula of middle-aged pulsar PSR J1740+1000. Although a dozen X-ray sources are found within the UHE emission site, careful analysis shows that they are unlikely to produce the observed UHE emission. The most likely particle accelerator is pulsar J1740+1000 which if offset by 13' north of the UHE source but appears to be connected to it by an extended feature seen in X-rays. For a plausible pulsar distance of 1.2 kpc, 1LHAASO J1740+0948u must be located about 5 pc away which requires rapid transport of electrons along the feature to avoid radiative losses. This poses several challenges for standard pulsar theory. Firstly, being produced $\lesssim$ 10 kyrs ago, particles must have been accelerated to the energy corresponding to a large fraction of the pulsar's full potential drop across the polar cap. Secondly, due to the lack of TeV emission extension toward the pulsar, particles must be accumulating in the UHE region. In this context, we discuss two possible scenarios: a tail filled with pulsar wind and confined by the bow-shock due to the fast pulsar's motion and an ISM filament filled by the most energetic pulsar wind particles escaping from the apex of the bow-shock.

astro-ph.HE↗

arXiv2025

FermiPhased: A tool for phase-resolved likelihood analysis of Fermi-LAT data

The Fermi Large Area Telescope has enabled detailed studies of high-energy astrophysical sources. To support analysis, we present FermiPhased, a flexible, open-source tool for phase-resolved studies of pulsars, binaries, and other periodically variable sources. Built on the Fermipy framework, FermiPhased offers three modes: standard, adaptive (fixed counts), and joint phase-resolved analysis, enabling users to flexibly bin data based on phase, count statistics, or jointly fit different epochs of interest. FermiPhased is optimized for parallel execution and use on computing clusters. It enables parallelized extraction of phase-resolved fluxes, spectra, and intermediary data products, with tutorials and documentation available on GitHub.

astro-ph.HE↗

arXiv2025

The Vela pulsar and its pulsar wind nebula Vela-X using 13 years of Fermi-LAT Observations

We present results of more than 13 years of Fermi-LAT data analysis for the Vela pulsar from 60 MeV to 100 GeV and its pulsar wind nebula (PWN), Vela-X, for E > 1 GeV in the off-pulse phases. We find the Vela-X PWN can be best characterized using two extended components: a large radial Gaussian accompanied by an off-set, compact radial disk, both with a similar spectral index, \Gamma \sim 2.3. The common spectral properties support a common PWN origin, but a supernova remnant component is plausible for the compact radial disk. With an updated Vela-X model, the phase resolved spectral properties of the Vela pulsar are explored through a phase-resolved analysis. The phase-resolved spectral properties of the pulsar are presented, such as the SED peak energy E$_p$, the width of the SED at its peak, d$_p$, and the asymptotic (low-energy) spectral index, $\Gamma_0$, are presented. The best-fit spectral models for each LAT pulse peak (Peak 1 and Peak 2) are extrapolated to UV energies and compared to archival, phase-resolved spectra at UV, X-ray, soft \gamma-ray and TeV energies. We also discuss the physical implications of our modeling and the data comparisons.

astro-ph.HE↗

arXiv2025

Orbital Phase-resolved Analysis of X-ray and Gamma-ray Observations of the High-Mass Gamma-ray Binary 4FGL J1405.1-6119

We present the results of multi-wavelength observations of the High-Mass Gamma-Ray Binary 4FGL J1405.1-6119. A pair of joint XMM-Newton and NuSTAR observations taken in 2019 (sampling the gamma-ray maximum and X-ray maximum) characterize the emission of soft and hard X-rays. We find variability of the hydrogen column density along our line of sight, $N_{\rm H}$, and photon index, $\Gamma$, and find no evidence of pulsations in X-rays. We also refine a new best-fit orbital period to $P=13.7157\pm0.0014$ days, the first orbital phase-resolved analysis based on nearly 16 years of Fermi--LAT observations of 4FGL J1405.1-6119 and the evolution of the spectral shape as a function of orbital phase. Finally, the X-ray and $\gamma$-ray spectra for the phases sampled in the new X-ray observations can be interpreted in the framework of the intrabinary shock model, previously applied to High-Mass Gamma-Ray binaries such as LS 5039.

astro-ph.HE↗

arXiv2024

Sharp Periodic Flares and Long-Term Variability in the High-Mass X-ray Binary XTE J1829-098 from RXTE PCA, Swift BAT and MAXI Observations

XTE J1829-098 is a transient X-ray pulsar with a period of ~7.8 s. It is a candidate Be star system, although the evidence for this is not yet definitive. We investigated the twenty-year long X-ray light curve using the Rossi X-ray Timing Explorer Proportional Counter Array (PCA), Neil Gehrels Swift Observatory Burst Alert Telescope (BAT), and the Monitor of All-sky X-ray Image (MAXI). We find that all three light curves are clearly modulated on the ~244 day orbital period previously reported from PCA monitoring observations, with outbursts confined to a narrow phase range. The light curves also show that XTE J1829-098 was in an inactive state between approximately December 2008 and April 2018 and no strong outbursts occurred. Such behavior is typical of Be X-ray binary systems, with the absence of outbursts likely related to the dissipation of the Be star's decretion disk. The mean outburst shapes can be approximated with a triangular profile and, from a joint fit of this to all three light curves, we refine the orbital period to 243.95 +/- 0.04 days. The mean outburst profile does not show any asymmetry and has a total phase duration of 0.140 +/- 0.007. However, the PCA light curve shows that there is considerable cycle-to-cycle variability of the individual outbursts. We compare the properties of XTE J1829-098 with other sources that show short phase-duration outbursts, in particular GS 1843-02 (2S 1845-024) which has a very similar orbital period, but longer pulse period, and whose orbit is known to be highly eccentric.

astro-ph.HE↗

arXiv2023

NuSTAR observation of the Vela pulsar and its nebula

We present the analysis of 200-ks NuSTAR observation of the Vela pulsar and the pulsar wind nebula (PWN). The phase-resolved spectra corresponding to two main peaks in the folded pulse profile differ significantly. The spectrum of Peak 1 is significantly harder than that of Peak 2 in qualitative agreement with the earlier RXTE results. However, for both spectra, the values of power-law (PL) fit photon indices are noticeably larger than the previously reported values. The harder (Peak 1) spectrum has a photon index of $1.06\pm0.16$ which is close to those measured for the bright inner jets of the PWN. We used the off-pulse interval to remove the emission from the pulsar and measure the compact pulsar wind nebula (PWN) spectrum in hard X-rays. We also measured the spectrum from the south-western (SW) region of the PWN which is resolved by NuSTAR from the compact PWN. For both regions, we fit the NuSTAR spectra by themselves and together with the Chandra X-ray Observatory spectra. We found that the PWN spectrum (for both regions) requires a more complex model than a simple PL. The fits to compact PWN spectrum favor exponentially cutoff PL model, with the cutoff energy of about 50 keV, over the broken PL model. The observed synchrotron photon energies imply electrons accelerated to about 150 TeV.

astro-ph.HE↗

arXiv2023

Chandra X-ray Observations of PSR J1849-0001, its Pulsar Wind Nebula, and the TeV Source HESS J1849-000

We obtained a 108 ks Chandra X-ray Observatory (CXO) observation of PSR J1849-0001 and its pulsar wind nebula (PWN), coincident with the TeV source HESS J1849-000. By analyzing the new and archival CXO data, we resolved the pulsar from the PWN, explored the PWN morphology on arcsecond and arcminute scales, and measured the spectra of different regions of the PWN. Both the pulsar and the compact inner PWN spectra are hard with power-law photon indices of $1.20 \pm 0.07$ and $1.49 \pm 0.20$, respectively. The jet-dominiated PWN has a relatively low luminosity, lack of gamma-ray pulsations, relatively hard and nonthermal spectrum of the pulsar, and sine-like pulse profile, which indicates a relatively small angle between the pulsar's spin and magnetic dipole axis. In this respect, it shares similar properties with a few other so-called MeV pulsars. Although the joint X-ray and TeV spectral energy distribution can be roughly described by a single-zone model, the obtained magnetic field value is unrealistically low. A more realistic scenario is the presence of a relic PWN, no longer emitting synchrotron X-rays but still radiating in TeV via inverse-Compton upscattering. We calso serendipitously detected surprisingly bright X-ray emission from a very wide binary whose components should not be interacting.

astro-ph.HE↗

arXiv2023

A lack of 9-s periodicity in the follow-up NuSTAR observation of LS 5039

The Nuclear Spectroscopic Array (NuSTAR) observed the gamma-ray binary LS 5039 for a second time in order to check for the presence of a periodic signal candidate found in the data from the previous NuSTAR observation. We do not detect the candidate signal in the vicinity of its previously reported frequency, assuming the same orbital ephemeris as in our previous paper. This implies that the previously reported periodic signal candidate was a noise fluctuation. We also perform a comparison of the lightcurves from the two NuSTAR observations and the joint spectral fitting. Our spectral analysis confirms the phase-dependence found from a single NuSTAR observation at a higher significance level.

astro-ph.HE↗

arXiv2017

A mathematical framework for predicting lifestyles of viral pathogens

Despite being similar in structure, functioning, and size viral pathogens enjoy very different mostly well-defined ways of life. They occupy their hosts for a few days (influenza), for a few weeks (measles), or even lifelong (HCV), which manifests in acute or chronic infections. The various transmission routes (airborne, via direct contact, etc.), degrees of infectiousness (referring to the load required for transmission), antigenic variation/immune escape and virulence define further pathogenic lifestyles. To survive pathogens must infect new hosts; the success determines their fitness. Infection happens with a certain likelihood during contact of hosts, where contact can also be mediated by vectors. Besides structural aspects of the host-contact network, three parameters/concepts appear to be key: the contact rate and the infectiousness during contact, which encode the mode of transmission, and third the immunity of susceptible hosts. From here, what can be concluded about the evolutionary strategies of viral pathogens? This is the biological question addressed in this paper. The answer extends earlier results (Lange & Ferguson 2009, PLoS Comput Biol 5 (10): e1000536) and makes explicit connection to another basic work on the evolution of pathogens (Grenfell et al. 2004, Science 303: 327-332). A mathematical framework is presented that models intra- and inter-host dynamics in a minimalistic but unified fashion covering a broad spectrum of viral pathogens, including those that cause flu-like infections, childhood diseases, and sexually transmitted infections. These pathogens turn out as local maxima of the fitness landscape. The models involve differential- and integral equations, agent-based simulation, networks, and probability.

q-bio.PE↗

arXiv2015

Different ways of looking at the force between two nano crystals

The potential of mean force (PMF) between two nano crystals (NCs) represents an effective interaction potential that can be used to study the assembly of NCs to various superstructures. For a given temperature, the effective interaction is obtained best from molecular dynamics simulations. Based on a density functional approach, this study proposes three methods of predicting the PMF for any temperature based on a single molecular dynamics simulation for one temperature. The three methods construct the PMF by considering the ligands as an ideal gas, as hard-sphere chains, or as Lennard-Jones interaction sites. To apply this methodology, the density of the interaction centers must be extracted from the simulation data. For the ideal gas model, a straightforward sampling procedure with a fixed lattice in space leads to free energies that are too large in order to consistently explain the simulation data for different temperatures. Naive sampling does not account for the small momenta added to the NCs when coupled to a thermostat. A method is proposed that corrects for the unphysical steps during the simulation. The ideal gas contribution computed for the corrected density is significantly smaller than the one obtained from naive sampling and can thus explain the temperature dependence of the PMF correctly. For the hard-sphere chain model, where a weighted density is used, the correction of the particle density is not essential. However, the PMF calculated based on the corrected density confirms our approach. All three models predict PMF curves in very good agreement with simulation results, but they differ in the number of input parameters and the computational effort. Based on the modeling results, we predict the existence of an additional attractive force at small distances of the NCs - a depletion force.

cond-mat.mes-hall↗

arXiv2014

Reconstruction of disease transmission rates: applications to measles, dengue, and influenza

Transmission rates are key in understanding the spread of infectious diseases. Using the framework of compartmental models, we introduce a simple method that enables us to reconstruct time series of transmission rates directly from incidence or disease-related mortality data. The reconstruction exploits differential equations, which model the time evolution of infective stages and strains. Being sensitive to initial values, the method produces asymptotically correct solutions. The computations are fast, with time complexity being quadratic. We apply the reconstruction to data of measles (England and Wales, 1948-67), dengue (Thailand, 1982-99), and influenza (U.S., 1910-27). The Measles example offers comparison with earlier work. Here we re-investigate reporting corrections, include and exclude demographic information. The dengue example deals with the failure of vector-control measures in reducing dengue hemorrhagic fever (DHF) in Thailand. Two competing mechanisms have been held responsible: strain interaction and demographic transitions. Our reconstruction reveals that both explanations are possible, showing that the increase in DHF cases is consistent with decreasing transmission rates resulting from reduced vector counts. The flu example focuses on the 1918/19 pandemic, examining the transmission rate evolution for an invading strain. Our analysis indicates that the pandemic strain could have circulated in the population for many months before the pandemic was initiated by an event of highly increased transmission.

q-bio.QM↗

arXiv2013

Computation of the Ramsey Numbers $R(C_4,K_9)$ and $R(C_4,K_{10})$

The Ramsey number $R(C_4,K_m)$ is the smallest $n$ such that any graph on $n$ vertices contains a cycle of length four or an independent set of order $m$. With the help of computer algorithms we obtain the exact values of the Ramsey numbers $R(C_4,K_9)=30$ and $R(C_4,K_{10})=36$. New bounds for the next two open cases are also presented.

math.CO↗

arXiv2013

Use of MAX-CUT for Ramsey Arrowing of Triangles

In 1967, Erdős and Hajnal asked the question: Does there exist a $K_4$-free graph that is not the union of two triangle-free graphs? Finding such a graph involves solving a special case of the classical Ramsey arrowing operation. Folkman proved the existence of these graphs in 1970, and they are now called Folkman graphs. Erdős offered \$100 for deciding if one exists with less than $10^{10}$ vertices. This problem remained open until 1988 when Spencer, in a seminal paper using probabilistic techniques, proved the existence of a Folkman graph of order $3\times 10^9$ (after an erratum), without explicitly constructing it. In 2008, Dudek and Rödl developed a strategy to construct new Folkman graphs by approximating the maximum cut of a related graph, and used it to improve the upper bound to 941. We improve this bound first to 860 using their approximation technique and then further to 786 with the MAX-CUT semidefinite programming relaxation as used in the Goemans-Williamson algorithm.

math.CO↗

arXiv2004

The Epstein-Glaser approach to pQFT: graphs and Hopf algebras

The paper aims at investigating perturbative quantum field theory (pQFT) in the approach of Epstein and Glaser (EG) and, in particular, its formulation in the language of graphs and Hopf algebras (HAs). Various HAs are encountered, each one associated with a special combination of physical concepts such as normalization, localization, pseudo-unitarity, causality and an associated regularization, and renormalization. The algebraic structures, representing the perturbative expansion of the S-matrix, are imposed on the operator-valued distributions which are equipped with appropriate graph indices. Translation invariance ensures the algebras to be analytically well-defined and graded total symmetry allows to formulate bialgebras. The algebraic results are given embedded in the physical framework, which covers the two recent EG versions by Fredenhagen and Scharf that differ with respect to the concrete recursive implementation of causality. Besides, the ultraviolet divergences occuring in Feynman's representation are mathematically reasoned. As a final result, the change of the renormalization scheme in the EG framework is modeled via a HA which can be seen as the EG-analog of Kreimer's HA.

hep-th↗