arXiv subjects

Peter A. Becker

Publications and source records attributed to Peter A. Becker.

At least 19 recordsLinked to original sources

arXiv2026

Pulse profiles of accreting neutron stars - A review of analysis methods, observations, and theoretical models

X-ray pulsars are highly magnetized ($B\sim 10^{12}$ G) neutron stars accreting from a donor star. Their characteristic X-ray emission arises from accreted material decelerated from relativistic velocities near the magnetic poles of the neutron star. As our line of sight onto the magnetic poles changes with the rotation of the neutron star, the X-rays are periodically modulated, resulting in X-ray pulsations. The shape of the pulse profiles depends on the physics of the interaction between the bright X-rays from the magnetic poles with the infalling matter, the location of the magnetic poles on the neutron star with respect to its spin axis, and on the properties of the space-time around the neutron star. In this review we give a pedagogical introduction to the accretion mechanisms operating in the various types of accreting neutron star systems and the observational techniques used to characterize the pulse profiles. We summarize how the pulse profiles depend on X-ray luminosity and energy and discuss the attempts to connect theoretically these observables with the physical accretion mechanisms. We conclude with an outline of future observational needs and further developments for theoretical models of magnetic accretion.

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arXiv2026

A New Relativistic Model for Spectral Formation in Accretion-Powered X-ray Pulsars: Pulse Profiles and Phase-Averaged Spectra

We develop a new analytical model describing the radiative and dynamical structure of an accretion-powered X-ray pulsar, including relativistic effects and a detailed representation of the rotational and magnetic geometry of the neutron star and the two accretion columns. The model provides for the first time a simultaneous calculation of both the phase-averaged spectrum and the pulse profile for an accretion-powered X-ray pulsar. The X-ray continuum spectrum is calculated using the analytical model of Becker & Wolff (2022), which assumes a conical accretion column geometry. The trajectory of the radiation escaping from the two columns is tracked through the curved spacetime using the Schwarzschild metric. The angular distribution of the radiation escaping from the surfaces of the columns (the beaming pattern) is represented using a set of "laser-like" emission directions, with associated amplitudes, called weight coefficients, that each contribute "sub-profiles" to the observed pulse profile. The sub-profiles provide basis functions that are used to fit the observed pulse profile. This yields a set of weight coefficients that determine the beaming pattern of the emission from the accretion column. We use the new model to analyze NuSTAR data for Her X-1, allowing the determination of the temperature, accretion rate, and magnetic field strength, as well as the rotational inclination angle and the latitudes of the two magnetic poles. The method also yields the beaming pattern of the emission, hence providing for the first time a self-consistent phenomenological description of the physical and radiative structures of the two accretion columns.

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arXiv2026

Flux variability of the "10 keV feature" of 4U 0115+63

X-ray spectra of accretion-powered X-ray pulsars can often be described using a power-law continuum with a high-energy cutoff, which might be further modified by additional spectral components. The Be X-ray binary system 4U 0115+63 is well known for having one of the highest numbers of detected harmonics of its cyclotron resonant scattering features (CRSFs), a pronounced spectral component known as the ''10 keV feature,'' and quasiperiodic oscillations (QPOs) with a period of about 500 s during outbursts. The changes in count rate by a factor of two during the approximately 500 s QPOs allow us to probe the variation in the spectral components with flux. We study the ''10 keV feature'' in emission, aiming to disentangle it from the broadband continuum and CRSFs and investigate its origin. We focus on the flux-dependent behavior of the CRSF and its harmonics, and particularly the contribution of the ''10 keV feature,'' as seen in the flux-resolved analysis of two NuSTAR observations of the 2015 outburst. Comparing the flux-resolved spectra of a given observation with the respective total dataset revealed a distinct change in overall spectral shape at the position of the ''10 keV feature'' but no comparable deviation at the energies of the harmonic CRSFs. The change associated with the ''10 keV feature'' does not seem to involve its centroid energy, which remains constant within a given observation. We find indications for an anticorrelation between the continuum flux and the ratio of the ''10 keV feature'' flux to the continuum flux within each observation. The analysis strengthens previous claims that the ''10 keV feature'' shows some independence from the remaining features. This result supports the interpretation that the ''10 keV feature'' has a different formation mechanism than the continuum emission, although its origin lies within the same physical environment.

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arXiv2025

Energy-resolved pulse profile changes in V 0332+53: Indications of wings in the cyclotron absorption line profile

We aim to investigate the energy-resolved pulse profile changes of the accreting X-ray pulsar V 0332+53, focusing in the cyclotron line energy range, using the full set of available NuSTAR observations. We applied a tailored pipeline to study the energy dependence of the pulse profiles and to build the pulsed fraction spectra (PFS) for the different observations. We studied the profile changes also using cross-correlation and lag spectra. We re-analysed the energy spectra to search for links between the local features observed in the PFS and spectral emission components associated with the shape of the fundamental cyclotron line. In the PFS data, with sufficiently high statistics, we observe a consistent behaviour around the cyclotron line energy. Specifically, two Gaussian-shaped features appear symmetrically on either side of the putative cyclotron line. These features exhibit minimal variation with source luminosity, and their peak positions consistently remain on the left and right of the cyclotron line energy. Associated with the cyclotron line-forming region, we interpret them as evidence for the resonant cyclotron absorption line wings, as predicted by theoretical models of how the cyclotron line profile should appear along the observer's line of sight. A phase-resolved analysis of the pulse in the energy bands surrounding these features enables us to determine both the spectral shape and the intensity of the photons responsible for these peaks in the PFS. Assuming these features correspond to a spectral component, we used their shapes as priors for the corresponding emission components, finding a statistically satisfactory description of the spectra. To explain these results, we propose that our line of sight is close to the direction of the spin axis, while the magnetic axis is likely orthogonal to it.

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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.

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arXiv2024

Spectral evolution of RX J0440.9+4431 during the 2022-2023 giant outburst observed with Insight-HXMT

In 2022-2023, the X-ray pulsar RX J0440.9+4431 underwent a Type II giant outburst, reaching a peak luminosity L_x ~ 4*10^{37} erg/s. In this work, we utilize Insight-HXMT data to analyze the spectral evolution of RX J0440.9+4431 during the giant outburst. By analysing the variation of the X-ray spectrum during the outburst using standard phenomenological models, we find that as the luminosity approaches the critical luminosity, the spectrum became flatter, with the photon enhancement predominantly concentrated around ~ 2 keV and 20-40 keV. The same behavior has also been noted in Type II outbursts from other sources. While the phenomenological models provide good fits to the spectrum, this approach is sometimes difficult to translate into direct insight into the details of the fundamental accretion physics. Hence we have also analyzed spectra obtained during high and low phases of the outburst using a new, recently-developed physics-based theoretical model, which allows us to study the variations of physical parameters such as temperature, density, and magnetic field strength during the outburst. Application of the theoretical model reveals that the observed spectrum is dominated by Comptonized bremsstrahlung emission emitted from the column walls in both the high and low states. We show that the spectral flattening observed at high luminosities results from a decrease in the electron temperature, combined with a compactification of the emission zone, which reduces the efficiency of bulk Comptonization. We also demonstrate that when the source is at maximum luminosity, the spectrum tends to harden around the peak of the pulse profile, and we discuss possible theoretical explanations for this behavior. We argue that the totality of the behavior in this source can be explained if the accretion column is in a quasi-critical state when at the maximum luminosity observed during the outburst.

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arXiv2024

Theoretical Analysis of the RX J0209.6-7427 X-ray Spectrum During Giant Outburst

We model the spectral formation occurring in the binary X-ray pulsar RX~J0209.6-7427 during the 2019 super-Eddington outburst. Using a theoretical model previously developed by the authors, we are able to produce spectra that closely resemble the phase-averaged X-ray spectra observed using NuSTAR and Insight-HXMT during low and high luminosity states of the outburst, respectively. The theoretical model simulates the accretion of fully ionized gas in a dipole magnetic field, and includes a complete description of the radiation hydrodynamics, matter distribution, and spectral formation. Type II X-ray outbursts provide an opportunity to study accretion over a large range of luminosities for the same neutron star. The analysis performed here represents the first time both the outburst low and high states of an accretion-powered X-ray pulsar are modeled using a physics-based model rather than standard phenomenological fitting with arbitrary mathematical functions. We find the outer polar cap radius remains constant and the column is more fully-filled with increasing luminosity, Comptonized bremsstrahlung dominates the formation of the phase-averaged X-ray spectrum, and a negative correlation exists between cyclotron centroid energy and luminosity, as expected. The super-Eddington nature of the outburst is rendered possible due to the low scattering cross section for photons propagating parallel to the magnetic field. We also find emission through the column top dominates in both the low and high states, implying the pulse profiles should have a roughly sinusoidal shape, which agrees with observed properties of ultra-luminous X-ray pulsars.

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arXiv2022

An Analytical Fourier-Transformation Model for the Production of Hard and Soft X-Ray Time Lags in AGNs: Application to 1H 0707-495

The variability of the X-ray emission from active galactic nuclei is often characterized using time lags observed between soft and hard energy bands in the detector. The time lags are usually computed using the complex cross spectrum, which is based on the Fourier transforms of the hard and soft time series data. It has been noted that some active galactic nuclei display soft X-ray time lags, in addition to the more ubiquitous hard lags. Hard time lags are thought to be produced via propagating fluctuations, spatial reverberation, or via the thermal Comptonization of soft seed photons injected into a hot electron cloud. The physical origin of the soft lags has been a subject of debate over the last decade. Currently, the reverberation interpretation is recognized as a leading theory. In this paper, we explore the alternative possibility that the soft X-ray time lags result partially from the thermal and bulk Comptonization of monochromatic seed photons, which in the case of the narrow-line Seyfert 1 galaxy 1H 0707-495, may correlate with fluorescence of iron L-line emission. In our model, the seed photons are injected into a hot, quasi-spherical corona in the inner region of the accretion flow. We develop an exact, time-dependent analytical model for the thermal and bulk Comptonization of the seed photons based on a Fourier-transformed radiation transport equation, and we demonstrate that the model successfully reproduces both the hard and soft time lags observed from 1H 0707-495.

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arXiv2022

A Generalized Analytical Model For Thermal And Bulk Comptonization In Accretion-Powered X-Ray Pulsars

We develop a new theoretical model describing the formation of the radiation spectrum in accretion-powered X-ray pulsars as a result of bulk and thermal Comptonization of photons in the accretion column. The new model extends the previous model developed by the authors in four ways: (1) we utilize a conical rather than cylindrical geometry; (2) the radiation components emitted from the column wall and the column top are computed separately; (3) the model allows for a non-zero impact velocity at the stellar surface; and (4) the velocity profile of the gas merges with Newtonian free-fall far from the star. We show that these extensions allow the new model to simulate sources over a wide range of accretion rates. The model is based on a rigorous mathematical approach in which we obtain an exact series solution for the Green's function describing the reprocessing of monochromatic seed photons. Emergent spectra are then computed by convolving the Green's function with bremsstrahlung, cyclotron, and blackbody photon sources. The range of the new model is demonstrated via applications to the high-luminosity source Her X-1, and the low-luminosity source X Per. The new model suggests that the observed increase in spectral hardness associated with increasing luminosity in Her X-1 may be due to a decrease in the surface impact velocity, which increases the $P$d$V$ work done on the radiation field by the gas.

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arXiv2021

Fitting strategies of accretion column models and application to the broadband spectrum of Cen X-3

Due to the complexity of modeling the radiative transfer inside the accretion columns of neutron star binaries, their X-ray spectra are still commonly described with phenomenological models, for example, a cutoff power law. While the behavior of these models is well understood and they allow for a comparison of different sources and studying source behavior, the extent to which the underlying physics can be derived from the model parameters is very limited. During recent years, several physically motivated spectral models have been developed to overcome these limitations. Their application, however, is generally computationally much more expensive and they require a high number of parameters which are difficult to constrain. Previous works have presented an analytical solution to the radiative transfer equation inside the accretion column assuming a velocity profile that is linear in the optical depth. An implementation of this solution that is both fast and accurate enough to be fitted to observed spectra is available as a model in XSPEC. The main difficulty of this implementation is that some solutions violate energy conservation and therefore have to be rejected by the user. We propose a novel fitting strategy that ensures energy conservation during the $χ^2$-minimization which simplifies the application of the model considerably. We demonstrate this approach as well a study of possible parameter degeneracies with a comprehensive Markov-chain Monte Carlo analysis of the complete parameter space for a combined NuSTAR and Swift/XRT dataset of Cen X-3. The derived accretion-flow structure features a small column radius of $\sim$63 m and a spectrum dominated by bulk-Comptonization of bremsstrahlung seed photons, in agreement with previous studies.

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arXiv2020

A two-fluid model for black-hole accretion flows: particle acceleration and disc structure

Hot, tenuous advection-dominated accretion flows around black holes are ideal sites for the Fermi acceleration of relativistic particles at standing shock waves in the accretion disc. Previous work has demonstrated that the shock-acceleration process can be efficient enough to power the observed, strong outflows in radio-loud active galaxies such as M87. However, the dynamical effect (back-reaction) on the flow, exerted by the pressure of the relativistic particles, has not been previously considered, and this effect can have a significant influence on the disc structure. We reexamine the problem by developing a new, two-fluid model for the structure of the accretion disc that includes the dynamical effect of the relativistic particle pressure, combined with the pressure of the background (thermal) gas. The new model is analogous to the two-fluid model of cosmic ray acceleration in supernova-driven shock waves. As part of the model, we also develop a new set of shock jump conditions, which are solved along with the hydrodynamic conservation equations to determine the structure of the accretion disc. The solutions include the formation of a mildly relativistic outflow (jet) at the shock radius, driven by the relativistic particles accelerated in the disc. One of our main conclusions is that in the context of the new two-fluid accretion model, global smooth (shock-free) solutions do not exist, and the disc must always contain a standing shock wave, at least in the inviscid case considered here.

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arXiv2020

A two-fluid model for black-hole accretion flows: Particle acceleration, outflows, and TeV emission

The multi-wavelength spectrum observed from M87 extends from radio wavelengths up to TeV gamma-ray energies. The radio through GeV components have been interpreted successfully using SSC models based on misaligned blazar jets, but the origin of the intense TeV emission detected during flares in 2004, 2005, and 2010 remains puzzling. It has been previously suggested that the TeV flares are produced when a relativistic proton jet originating in the core of M87 collides with a molecular cloud (or stellar atmosphere) located less than one parsec from the central black hole. We explore this scenario in detail here using a self-consistent model for the acceleration of relativistic protons in a shocked, two-fluid ADAF accretion disc. The relativistic protons accelerated in the disc escape to power the observed jet outflows. The distribution function for the jet protons is used to compute the TeV emission produced when the jet collides with a cloud or stellar atmosphere. The simulated broadband radiation spectrum includes radio, X-ray, and GeV components generated via synchrotron, as well as TeV emission generated via the production and decay of muons, positrons, and electrons. The self-consistency of the model is verified by computing the relativistic particle pressure using the distribution function, and comparing it with the relativistic particle pressure obtained from the hydrodynamical model. We demonstrate that the model is able to reproduce the multi-wavelength spectrum from M87 observed by VERITAS and HESS during the high-energy flares in 2004, 2005, and 2010.

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arXiv2019

Analyzing the December 2013 Orphan Gamma-Ray Flare From 3C 279

Multiwavelength monitoring of the blazar 3C 279 observed a very bright, 12-hour, orphan gamma-ray flare on 20 Dec 2013 with a uniquely hard Fermi-LAT spectrum and high Compton dominance. We work with a one-zone, leptonic model with both first- and second-order Fermi acceleration, which now reproduces the unique flaring behavior. We present a simplified analytic electron energy distribution to provide intuition about how particle acceleration shapes multi-wavelength blazar jet emission spectra. The contributions of individual processes in relativistic jets is fundamental to understanding the particle energy budget in the formation and propagation of astrophysical jets. We show that first- and second-order Fermi acceleration are sufficient to explain the flare, and that magnetic reconnection is not needed. Our analysis suggests that the flare is initiated by an increase in the particle energies due to shock acceleration, which also increases the stochastic acceleration. The higher energy particle preferentially occupy the outer jet, along the sheath, which decreases the apparent magnetic field and synchrotron radiation, while increasing electron exposure to the broad line region photon fields, driving up the external Compton emission.

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arXiv2019

The giant outburst of 4U 0115+634 in 2011 with Suzaku and RXTE

We present an analysis of X-ray spectra of the high mass X-ray binary 4U 0115+634 as observed with Suzaku and RXTE in 2011 July, during the fading phase of a giant X-ray outburst. We used a continuum model consisting of an absorbed cutoff power-law and an ad-hoc Gaussian emission feature centered around 8.5 keV, which we discuss to be due to cyclotron emission. Our results are consistent with a fundamental cyclotron absorption line centered at ${\sim}10.2$ keV for all observed flux ranges. At the same time we rule out significant influence of the 8.5 kev Gaussian on the CRSF parameters, which are not consistent with the cyclotron line energies and depths of previously reported flux-dependent descriptions. We also show that some continuum models can lead to artificial line-like residuals in the analyzed spectra, which are then misinterpreted as unphysically strong cyclotron lines. Specifically, our results do not support the existence of a previously claimed additional cyclotron feature at ${\sim}15$ keV. Apart from these features, we find for the first time evidence for a He-like Fe XXV emission line at ${\sim}6.7$ keV and weak H-like Fe XXVI emission close to ${\sim}7.0$ keV.

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arXiv2019

Electron Acceleration In Blazars: Application to the 3C 279 Flare on 2013 December 20

The broadband spectrum from the 2013 December 20 $γ$-ray flare from 3C~279 is analyzed with our previously-developed one-zone blazar jet model. We are able to reproduce two SEDs, a quiescent and flaring state, the latter of which had an unusual SED, with hard $γ$-ray spectrum, high Compton dominance, and short duration. Our model suggests that there is insufficient energy for a comparable X-ray flare to have occurred simultaneously, which is an important constraint given the lack of X-ray data. We show that first- and second-order Fermi acceleration are sufficient to explain the flare, and that magnetic reconnection is not needed. The model includes particle acceleration, escape, and adiabatic and radiative energy losses, including the full Compton cross-section, and emission from the synchrotron, synchrotron self-Compton, and external Compton processes. We provide a simple analytic approximation to the electron distribution solution to the transport equation that may be useful for simplified modeling in the future.

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arXiv2019

The Physics of Accretion Onto Highly Magnetized Neutron Stars

Studying the physical processes occurring in the region just above the magnetic poles of strongly magnetized, accreting binary neutron stars is essential to our understanding of stellar and binary system evolution. Perhaps more importantly, it provides us with a natural laboratory for studying the physics of high temperature and high density plasmas exposed to extreme radiation, gravitational, and magnetic fields. Observations over the past decade have shed new light on the manner in which plasma falling at velocities near the speed of light onto a neutron star surface is halted. Recent advances in modeling these processes have resulted in direct measurement of the magnetic fields and plasma properties. On the other hand, numerous physical processes have been identified that challenge our current picture of how the accretion process onto neutron stars works. Observation and theory are our essential tools in this regime because the extreme conditions cannot be duplicated on Earth. This white paper gives an overview of the current theory, the outstanding theoretical and observational challenges, and the importance of addressing them in contemporary astrophysics research.

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arXiv2019

Time-Dependent Electron Acceleration in Pulsar Wind Termination Shocks: Application to the 2007 September Crab Nebula Gamma-Ray Flare

In 2007 September, the Crab Nebula exhibited a bright gamma-ray flare in the GeV energy range that was detected by AGILE. The observed emission at >160 MeV indicates that the radiating electrons had energies above the classical synchrotron radiation-reaction limit, thus presenting a serious challenge to classical models for electron acceleration in astrophysical environments. In this paper, we apply our recently developed time-dependent self-similar analytical model describing electrostatic acceleration in the explosive reconnection region around the pulsar wind termination shock to the 2007 September flare. This event was unique in that it displayed both long-duration "wave" and short-duration "sub-flare" features. The unusual temporal variation makes this flare an especially interesting test for our model. We demonstrate that our model can reproduce the time-dependent gamma-ray spectrum for this event, as well as the associated gamma-ray light curve, obtained by integrating the spectrum for photon energies >100 MeV. This establishes that our time-dependent electrostatic acceleration model can explain both wave and sub-flare transients, which lends further support to the theoretical framework we have developed. We also further examine the validity of the self-similar electric and magnetic field evolution implied by our model. We conclude that strong electrostatic acceleration driven by shock-induced magnetic reconnection is able to power the Crab Nebula gamma-ray flares by energizing the electrons on sub-Larmor timescales.

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arXiv2017

Time-Dependent Electron Acceleration in Pulsar-Wind Termination Shocks: Application to the 2011 April Crab Nebula Gamma-Ray Flare

The $γ$-ray flares from the Crab nebula observed by {\it AGILE} and {\it Fermi}-LAT between 2007-2013 reached GeV photon energies and lasted several days. The strongest emission, observed during the 2011 April "super-flare," exceeded the quiescent level by more than an order of magnitude. These observations challenge the standard models for particle acceleration in pulsar wind nebulae, because the radiating electrons have energies exceeding the classical radiation-reaction limit for synchrotron. Particle-in-cell simulations have suggested that the classical synchrotron limit can be exceeded if the electrons also experience electrostatic acceleration due to shock-driven magnetic reconnection. In this paper, we revisit the problem using an analytic approach based on solving a fully time-dependent electron transport equation describing the electrostatic acceleration, synchrotron losses, and escape experienced by electrons in a magnetically confined plasma "blob" as it encounters and passes through the pulsar-wind termination shock. We show that our model can reproduce the $γ$-ray spectra observed during the rising and decaying phases of each of the two sub-flare components of the 2011 April super-flare. We integrate the spectrum for photon energies $\ge 100\,$MeV to obtain the light curve for the event, which agrees with the observations. We find that strong electrostatic acceleration occurs on both sides of the termination shock, driven by magnetic reconnection. We also find that the dominant mode of particle escape changes from diffusive escape to advective escape as the blob passes through the shock.

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