Some isolated neutron stars show harmonically spaced absorption features in their thermal soft X-ray spectra. The interpretation of the features as a cyclotron line and its harmonics has been suggested, but the usual explanation of the harmonics as caused by relativistic effects fails because the relativistic corrections are extremely small in this case. We suggest that the features correspond to the peaks in the energy dependence of the free-free opacity in a quantizing magnetic field, known as quantum oscillations. The peaks arise when the transitions to new Landau levels become allowed with increasing the photon energy; they are strongly enhanced by the square-root singularities in the phase-space density of quantum states in the case when the free (non-quantized) motion is effectively one-dimensional. To explore observable properties of these quantum oscillations, we calculate models of hydrogen neutron star atmospheres with B \sim 10^{10} - 10^{11} G (i.e., electron cyclotron energy E_{c,e} = 0.1 - 1 keV) and T_{eff} = 1 - 3 MK. Such conditions are thought to be typical for the so-called central compact objects in supernova remnants, such as 1E 1207.4-5209 in PKS 1209-51/52. We show that observable features at the electron cyclotron harmonics form at moderately large values of the quantization parameter, b_{eff} = E_{c,e}/kT_{eff} = 0.5 - 20. The equivalent widths of the features can reach 100 - 200 eV; they grow with increasing b_{eff} and are lower for higher harmonics.
Cyclotron resonant scattering features are an essential tool for studying the magnetic field of neutron stars. The fundamental line provides a measure of the field strength, while the harmonic lines provide information about the structure and configuration of the magnetic field. Until now only a handful of sources are known to display more than one cyclotron line and only two of them have shown a series of harmonics. The aim of this work is to see the first harmonic cyclotron line, confirming the fundamental line at around 22 keV, thus increasing the number of sources with detected harmonic cy…
Cyclotron resonance scattering features (CRSFs) in the X-ray spectrum of an accreting neutron star are modified differently by accretion mounds sustained by magnetic and thermocompositional gradients. It is shown that one can discriminate, in principle, between mounds of different physical origins by studying how the line energy, width, and depth of a CRSF depend on the orientation of the neutron star, accreted mass, surface temperature distribution, and equation of state. CRSF signatures including gravitational light bending are computed for both phase-resolved and phase-averaged spectra on t…
We report on changes of the cyclotron resonance energies of the recurrent transient pulsar, X0331+53 (V0332+53). The whole RXTE data acquired in the 2004-2005 outburst were utilized. The 3-80 keV source luminosity varied between 1.7x10^36 and 3.5x10^38 ers/s, assuming a distance of 7 kpc. We confirmed that the fundamental cyclotron resonance energy changed from ~22 to ~27 keV in a clear anti-correlation to the source luminosity, and without any hysteresis effects between the rising and declining phases of the outburst. In contrast, the second harmonic energy changed from ~49 to ~54 keV, implyi…
With exactly forty years since the discovery of the first cyclotron line in Her X-1, there have been remarkable advancements in the field related to study of the physics of accreting neutron stars -- cyclotron lines have been a major torchbearer in this regard, from being the only direct estimator of the magnetic field strength, a tracer of accretion geometry and an indicator of the emission beam in these systems. The main flurry of activities have centred around studying the harmonic separations, luminosity dependence, pulse phase dependence and more recently the shapes of the line and the tr…
In this paper we make a phase dependent study of the effect of the distortion of local magnetic field due to confinement of accreted matter in X-ray pulsars on the cyclotron spectra emitted from the hotspot . We have numerically solved the Grad-Shafranov equation for axisymmetric static MHD equilibria of matter confined at the polar cap of neutron stars. From our solution we model the cyclotron spectra that will be emitted from the region, using a simple prescription and integrating over the entire mound. Radiative transfer through the accretion column overlying the mound may significantly mod…
We report on observations of the Be/X-ray binary system SwiftJ1626.6-5156 performed with NuSTAR during a short outburst in March 2021, following its detection of by the MAXI monitor and Spektrum-Roentgen-Gamma (SRG) observatory. Our analysis of the broadband X-ray spectrum of the source confirms the presence of two absorption-like features at energies ~9 and ~17 keV previously reported in literature and interpreted as the fundamental cyclotron resonance scattering feature (CRSF) and its first harmonic (based on RXTE data). The better sensitivity and energy resolution of NuSTAR, combined with t…
Accretion onto magnetic neutron stars results in X-ray spectra that often exhibit a cyclotron resonance scattering feature (CRSF) and, sometimes, higher harmonics of it. Two places are suspect for the formation of a CRSF: the surface of the neutron star and the radiative shock in the accretion column. Here we explore the first possibility: reflection at the neutron-star surface of the continuum produced at the radiative shock. It has been proposed that for high-luminosity sources, as the luminosity increases, the height of the radiative shock increases, thus a larger polar area is illuminated,…
Context: Accretion-powered X-ray pulsars show highly energy-dependent and complex pulse-profile morphologies. Significant deviations from the average pulse profile can appear, in particular close to the cyclotron line energies. These deviations can be described as energy-dependent phase lags, that is, as energy-dependent shifts of main features in the pulse profile. Aims: Using a numerical study we explore the effect of cyclotron resonant scattering on observable, energy-resolved pulse profiles. Methods: We generated the observable emission as a function of spin phase, using Monte Carlo simula…
Cyclotron lines, also called cyclotron resonant scattering features (CRSF) are spectral features, generally appearing in absorption, in the X-ray spectra of objects containing highly magnetized neutron stars, allowing the direct measurement of the magnetic field strength in these objects. Cyclotron features are thought to be due to resonant scattering of photons by electrons in the strong magnetic fields. The main content of this contribution focusses on electron cyclotron lines as found in accreting X-ray binary pulsars (XRBP) with magnetic fields on the order of several 1012 Gauss. Also, pos…
The study of variation of measured cyclotron lines is of fundamental importance to understand the physics of the accretion process in magnetized neutron star systems. We investigate the magnetic field formation, evolution and distribution for several High- Mass X-ray Binaries (HMXBs). We focus our attention on the cyclotron lines that have been detected in HMXB classes in their X-ray spectra. As has been correctly pointed out, several sources show variation in cyclotron lines, this can result due to the effect of accretion dynamics, and hence that would reflect the magnetic field characteristi…
Some of X-ray dim isolated neutron stars (XDINS) and central compact objects in supernova remnants (CCO) show absorption features in their thermal soft X-ray spectra. It has been hypothesized that these features could be due to the periodic peaks in free-free absorption opacities, caused by either Landau quantization of electron motion in magnetic fields B<10^{11} G or analogous quantization of ion motion in magnetic fields B>10^{13} G. Here, I review the physics behind cyclotron quantum harmonics in free-free photoabsorption, discuss different approximations for their calculation, and explain…
The evolution of neutron star (NS) magnetic field (B-field) has long been an important topic, which is still not yet settled down. Here, we analyze the NS B-fields inferred by the cyclotron resonance scattering features (CRSFs) for the high mass X-ray binaries (HMXBs) and by the magnetic dipole model for the spin-down pulsars. We find that the B-fields of both the 32 NSHMXBs and 28 young pulsars with the supernova remnants follow the log-normal distributions, with the average values of 3.4 * 10^12 G and 4.1 * 10^12 G respectively, which are further verified to come from the same continuous dis…
We report the broad-band spectral properties of the X-ray pulsar Cep X-4 by using a Suzaku observation in 2014 July. The 0.8-70 keV spectrum was found to be well described by three continuum models - Negative and Positive power-law with Exponential cutoff (NPEX), high energy cutoff power-law and CompTT models. Additional components such as a cyclotron line at $\sim$28 keV and two Gaussian components for iron lines at 6.4 and 6.9 keV were required in the spectral fitting. Apart from these, an additional absorption feature at $\sim$45 keV was clearly detected in residuals obtained from the spect…
Resonant cyclotron scattering (RCS) in pulsar magnetospheres is considered. The photon diffusion equation (Kompaneets equation) for RCS is derived. The photon system is modeled three dimensionally. Numerical calculations show that there exist not only up scattering but also down scattering of RCS, depending on the parameter space. RCS's possible applications to the spectra energy distributions of magnetar candidates and radio quiet isolated neutron stars (INSs) are point out. The optical/UV excess of INSs may caused by the down scattering of RCS. The calculations for RX J1856.5-3754 and RX J07…
Context: The magnetic field is a crucial ingredient of neutron stars. It governs the physics of accretion and of the resulting high-energy emission in accreting pulsars. Studies of the cyclotron resonant scattering features (CRSFs) seen as absorption lines in the X-ray spectra of the pulsars permit direct measuremets of the field strength. Aims: From an analysis of a number of pointed observations with different instruments, the energy of CRSF, Ecyc, has recently been found to decay in Her X-1, which is one of the best-studied accreting pulsars. We present our analysis of a homogeneous and alm…
Cyclotron line scattering features are detected in a few tens of X-ray pulsars (XRPs) and used as direct indicators of a strong magnetic field at the surface of accreting neutron stars (NSs). In a few cases, cyclotron lines are known to be variable with accretion luminosity of XRPs. It is accepted that the observed variations of cyclotron line scattering features are related to variations of geometry and dynamics of accretion flow above the magnetic poles of a NS. A positive correlation between the line centroid energy and luminosity is typical for sub-critical XRPs, where the accretion result…
All the neutron star (NS) atmosphere models published so far have been calculated in the "cold plasma approximation", which neglects the relativistic effects in the radiative processes, such as cyclotron emission/absorption at harmonics of cyclotron frequency. Here we present new NS atmosphere models which include such effects. We calculate a set of models for effective temperatures T_eff =1-3 MK and magnetic fields B \sim 10^{10}-10^{11} G, typical for the so-called central compact objects (CCOs) in supernova remnants, for which the electron cyclotron energy E_{c,e} and its first harmonics ar…
We present a broadband spectral analysis of accreting neutron stars using data from XIS and PIN onboard \emph{Suzaku}. From spectral fits of these sources with a single continuum model including a powerlaw and high energy cut-off, cyclotron lines (where required), we studied the correlation between various spectral parameters. Among 39 sources we studied, 16 are those where the existence of a cyclotron line is known in literature, and 29 need a cutoff energy. Among these 29 sources, 18 have cutoff energy bunched in a range of 3-10 keV while for 11 sources, it spreads over 12-25 keV. This bi-mo…
The accreting compact objects in most of ultraluminous X-ray sources (ULXs) are likely to be neutron stars rather than black holes as suggested by the recent detection of periodic pulsations from some of these sources located in neighboring galaxies and one ULX that has hitherto been discovered in our own galaxy. As a member of the ULX family, NGC 300 ULX1 is a new pulsating ULX (PULX) spinning up at substantially high rates compared with other PULXs. In this paper, the strength of the magnetic field on the surface of the neutron star is inferred from the energy of the cyclotron absorption lin…
The high-mass X-ray binary and accreting X-ray pulsar IGR J16393-4643 was observed by NuSTAR in the 3-79 keV energy band for a net exposure time of 50 ks. We present the results of this observation which enabled the discovery of a cyclotron resonant scattering feature with a centroid energy of 29.3(+1.1/-1.3) keV. This allowed us to measure the magnetic field strength of the neutron star for the first time: B = (2.5+/-0.1)e12 G. The known pulsation period is now observed at 904.0+/-0.1 s. Since 2006, the neutron star has undergone a long-term spin-up trend at a rate of P' = -2e-8 s/s (-0.6 s p…
An overview of the results of observations for the transient X-ray pulsar 4U 0115+63, a member of a binary system with a Be star, since its discovery to the present day (~40 years) based on data from more than dozen observatories and instruments is presented. A overall light curve and the history of change in the spin frequency of the neutron star over the entire history of its observations, which also includes the results of recent measurements made by the INTEGRAL observatory during the 2004, 2008, and 2011 outbursts, are provided. The source's energy spectra have also been constructed from …
We present NuSTAR spectral and timing studies of the Supergiant Fast X-ray Transient (SFXT) IGR J17544-2619. The spectrum is well-described by a ~1 keV blackbody and a hard continuum component, as expected from an accreting X-ray pulsar. We detect a cyclotron line at 17 keV, confirming that the compact object in IGR J17544-2619 is indeed a neutron star. This is the first measurement of the magnetic field in a SFXT. The inferred magnetic field strength, B = (1.45 +/- 0.03) * 10^12 G * (1+z) is typical of neutron stars in X-ray binaries, and rules out a magnetar nature for the compact object. We…
Using recent compilations of detailed X-ray observations and spectral models of exceptional quality, we record the electron cyclotron resonance absorption (ECRA) features that have been detected in 45 pulsating high-mass X-ray binaries (HMXBs) and ultraluminous X-ray (ULX) sources harboring neutron stars, although seven of these detections are still questionable and another 21 are single and/or not independently confirmed. From the comprehensive catalogs of Jaisawal \& Naik and Staubert et al. and from several additional recent observations, we produce two lists of HMXB ECRA sources: a list of…
We analyse all available X-ray observations of X1822-371 made with XMM-Newton, Chandra, Suzaku and INTEGRAL satellites. The observations were not simultaneous. The Suzaku and INTEGRAL broad band energy coverage allows us to constrain the spectral shape of the continuum emission well. We use the model already proposed for this source, consisting of a Comptonised component absorbed by interstellar matter and partially absorbed by local neutral matter, and we added a Gaussian feature in absorption at $\sim 0.7$ keV. This addition significantly improves the fit and flattens the residuals between 0…
We have studied shock in magnetized accretion flow/funnel flow in case of neutron star with bremsstrahlung cooling and cyclotron cooling. All accretion solutions terminate with a shock close to the neutron star surface, but at some region of the parameter space, it also harbours a second shock away from the star surface. We have found that cyclotron cooling is necessary for correct accretion solutions which match the surface boundary conditions.
We report the discovery of absorption features in the X-ray spectrum of the transient X-ray pulsar GROJ2058+42. The features are detected around $\sim10$, $\sim20$ and $\sim30$ keV in both NuSTAR observations carried out during the source type II outburst in spring 2019. The most intriguing property is that the deficit of photons around these energies is registered only in the narrow phase interval covering around 10% of the pulsar spin period. We interpret these absorption lines as a cyclotron resonant scattering line (fundamental) and two higher harmonics. The measured energy allow us to est…
The elusive cyclotron line in 4U 1901+03: hidden, yet present
INAF - IASF-Palermo（意大利国家天体物理研究所帕勒莫天文台）；Department of Astronomy, University of Geneva（日内瓦大学天文学系）；Dr. Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics, Friedrich-Alexander Universität Erlangen-Nürnberg（弗里德里希-亚历山大埃尔朗根-纽伦堡大学卡尔·雷梅斯天文台和埃尔朗根高能粒子物理中心）；Dipartimento di Fisica e Chimica Emilio Segrè, Università di Palermo（巴勒莫大学埃米利奥·塞格雷物理与化学系）；Department of Physics, National and Kapodistrian University of Athens（雅典国立卡波蒂斯坦大学物理系）；European Space Agency (ESA), European Space Astronomy Centre (ESAC)（欧洲空间局欧洲空间天文中心）；INAF - Osservatorio Astronomico di Roma（意大利国家天体物理研究所罗马天文台）
Cyclotron resonance scattering features or cyclotron absorption lines are unique features observed in the hard X-ray spectra of accretion powered X-ray pulsars with magnetic field of the order of 10$^{12}$ G. Detection of these features enables us for the direct estimation of strength of the magnetic field close to the neutron star surface. Corresponding to magnetic field of $\sim$10$^{12}$ G, the fundamental cyclotron lines are expected in 10-100 keV energy range with harmonics expected at multiples of fundamental line energy. However, we detected first harmonic of cyclotron line at less than…
Resonant absorption cyclotron features are a key diagnostic tool to directly measure the strength of the magnetic field of accreting neutron stars. However, typical values for cyclotron features lie in the high-energy part of the spectrum between 20 keV and 50 keV, where detection is often damped by the low statistics from single pointed observations. We show that long-term monitoring campaign performed with Swift-BAT of persistently, but faint, accreting high-mass X-ray binaries is able to reveal in their spectra the presence of cyclotron features. We extracted the average Swift-BAT 15-150 ke…
The recent discovery of periodic pulsations from several members of the ultraluminous X-ray source (ULX) family in nearby galaxies as well as in our own galaxy unveiled the nature of the accreting compact object. Neutron stars rather than black holes are currently believed to power a substantial number of ULXs whether or not pulsations are observed. The detection of cyclotron absorption lines in the X-ray spectrum of a ULX provides an alternative way to identify the compact object as a neutron star. Among the non-pulsating ULXs, the presence of a cyclotron resonance scattering feature (CRSF) i…
We report results of a spectral and timing analysis of the X-ray pulsar XTE J1829-098 using data obtained with the NuSTAR observatory during an outburst in August 2018. A strong absorption feature was detected at the energy of $E_{cyc}\simeq 15$ keV in the source spectrum. This feature was interpreted as a cyclotron resonance scattering line corresponding to the magnetic field strength of the neutron star surface $B\simeq1.7\times10^{12}$ G. The pulse phase-resolved spectroscopy shows that the cyclotron line is significantly detected at all phases of the pulse and its energy and other paramete…
A candidate proton cyclotron feature in the ultraluminous X-ray source NGC 4656 ULX-1
Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata（拉普拉塔国立大学天文与地球物理科学学院）；Instituto de Astrofísica de Canarias (IAC)（加那利天体物理学研究所）；Departamento de Astrofísica, Universidad de La Laguna（拉古纳大学天体物理学系）；Space Telescope Science Institute（太空望远镜科学研究所）；Instituto Argentino de Radioastronomía (CCT La Plata, CONICET（阿根廷射电天文学研究所）；CICPBA（意大利国家天体物理研究所卡利亚里天文台）；UNLP)（空间科学研究所）；INAF, Osservatorio Astronomico di Cagliari（加泰罗尼亚空间研究学院）；Institute of Space Sciences (ICE, CSIC)；Institut d’Estudis Espacials de Catalunya (IEEC)
We present a new Monte-Carlo radiative transfer code, which we have used to model the cyclotron line features in the environment of a variable magnetic field and plasma density. The code accepts an input continuum and performs only the line transfer by including the three cyclotron resonant processes (cyclotron absorption, cyclotron emission, cyclotron scattering). Subsequently, the effects of gravitational red-shift and light bending on the emergent spectra are computed. We have applied our code to predict the observable spectra from three different emission geometries; 1) an optically thin s…
Fingerprints of thermal Comptonization in accreting neutron stars. Plasma-vacuum interplay in cyclotron lines and polarisation
Dr. Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics, Friedrich-Alexander Universität Erlangen-Nürnberg（弗里德里希-亚历山大埃尔朗根-纽伦堡大学卡尔·雷梅斯天文台和埃尔朗根天体粒子物理中心）；INAF - IASF Palermo（意大利国家天体物理研究所巴勒莫分院）；Dipartimento di Fisica e Chimica Emilio Segrè, Università degli Studi di Palermo（巴勒莫大学埃米利奥·塞格雷物理与化学系）；Department of Astronomy, University of Geneva（日内瓦大学天文学系）；INAF, Osservatorio Astronomico di Brera（意大利国家天体物理研究所布里拉天文台）；School of Physics and Astronomy, University of Southampton（南安普顿大学物理与天文学学院）
We study the behaviour of the cyclotron resonant scattering feature (CRSF) of the high mass X-ray binary Vela X-1 using the long-term hard X-ray monitoring performed by the Burst Alert Telescope (BAT) on board Swift. High statistics, intensity selected spectra were built along 11 years of BAT survey. While the fundamental line is not revealed, the second harmonic of the CRSF can be clearly detected in all the spectra, at an energy varying between $\sim 53$ keV and $\sim 58$ keV, directly correlated with the luminosity. We have further investigated the evolution of the CRSF in time, by studying…
Swift J1626.6-5156 is a Be/X-ray binary that was in outburst from December 2005 until November 2008. We have examined RXTE/PCA and HEXTE spectra of three long observations of this source taken early in its outburst, when the PCA 2-20 keV count rate was >70 counts/s/PCU, as well as several combined observations from different stages of the outburst. The spectra are best fit with an absorbed cutoff power law with a ~6.4 keV iron emission line and a Gaussian optical depth absorption line at ~10 keV. We present strong evidence that this absorption-like feature is a cyclotron resonance scattering f…
Constraints on cyclotron features and accretion regime in the high-mass X-ray binary 4U 1700-37 from NuSTAR
Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata（拉普拉塔国立大学天体物理与地球物理科学学院）；Instituto Argentino de Radioastronomía (CCT La Plata, CONICET（阿根廷射电天文研究所）；CICPBA（加那利群岛天体物理研究所）；UNLP)（拉古纳大学天体物理学系）；Instituto de Astrofísica de Canarias (IAC)（哈恩大学物理系）；Departamento de Astrofísica, Universidad de La Laguna（哈恩大学机械与矿业工程系）；Departamento de Física (EPS), Universidad de Jaén；Departamento de Ingeniería Mecánica y Minera (EPSJ), Universidad de Jaén
We present a spectral and timing study of the High Mass X-ray Binary transient source 4U 1700-37 using NuSTAR and ASTROSAT/LAXPC. The source is observed in two different flux states. A combined spectral analysis of NuSTAR's FPMA and FPMB shows the possible hint of a cyclotron line feature at $\sim$16 keV. The line feature is consistently present in different continuum models with at least 3$σ$ confidence level. We do not detect the presence of a previously reported ~39 keV cyclotron line in the combined spectra. A $\sim$16 keV cyclotron feature would suggest that the compact object is a neutro…
Cyclotron resonance scattering features are observed in the spectra of some X-ray pulsars and show significant changes in the line energy with the pulsar luminosity. In a case of bright sources, the line centroid energy is anti-correlated with the luminosity. Such a behaviour is often associated with the onset and growth of the accretion column, which is believed to be the origin of the observed emission and the cyclotron lines. However, this scenario inevitably implies large gradient of the magnetic field strength within the line-forming region, and it makes the formation of the observed line…
We report the results of an analysis of data obtained with the INTEGRAL, Swift and RXTE observatories during the 2010 April and September outbursts of the X-ray pulsar RX J0440.9+4431. The temporal and spectral properties of the pulsar in a wide energy band (0.6-120 keV) were studied for the first time. We discovered a ~32 keV cyclotron resonant scattering feature in the source spectrum, that allowed us to estimate the magnetic field strength of the neutron star as B~3.2 x 10^12 G. The estimate of the magnetic field strength was confirmed by a comprehensive analysis of the noise power spectrum…
Long-term hard X-ray monitoring observations on high mass X-ray binary Vela X-1 from 2003 -- 2011 have been performed by INTEGRAL. I systematically analyzed the average hard X-ray spectra of Vela X-1 from 3 -- 200 keV, with main aims to detect cyclotron resonant scattering features and study their variation patterns with accreting luminosities and orbital phases. The cyclotron scattering lines of Vela X-1 at $\sim 22 - 27$ keV and $49 - 57$ keV are confirmed in the average spectra of Vela X-1. But in the flare states with hard X-ray luminosity higher than $\sim 5\times 10^{36}$ erg s$^{-1}$ (3…
We report the results from an archival XMM-Newton observation of the Supergiant Fast X-ray Transient (SFXT) IGR J18483-0311 during its apastron passage. The measured 0.5--10 keV luminosity state (1.3$\times10^{33}$ erg s$^{-1}$) is the lowest ever reported in the literature, it is best fitted by an absorbed black body model yielding parameters consistent with previous measurements. In addition, we find evidence of an emission line feature at $\sim$3.3 keV in the 0.5--10 keV EPIC-pn source spectrum. We show that its physical explanation in terms of atomic emission line appears unlikely and conv…
We systematically analyzed the spectra of X-ray binary pulsars observed with GINGA (Mihara 1995). A new model NPEX (Negative and Positive power-laws EXponential) was introduced t o represent the pulsar continuum. Combining the NPEX continuum with the CYAB factor (cyclotron resonance scattering model), we successfully fit the whole-band spectra of all the pulsars. A possible physical meaning of the NPEX model is the Comptonized spectra. By using the smooth and concave NPEX model, the cyclotron structures were detected from 12 pulsars, about a half of the 23 sources, including new discoveries fr…
We present a detailed study of the high mass X-ray binary Vela X-1, using observations performed by Insight-HXMT in 2019 and 2020, concentrating on timing analysis and spectral studies including pulse phase-resolved spectroscopy. The cyclotron line energy is found to be at ~21-27 keV and 43-50 keV for the fundamental and first harmonic, respectively. We present the evolution of spectral parameters and find that two line centroid energy ratio E2/E1 evolved from ~2 before MJD 58900 to ~1.7 after that. The harmonic cyclotron line energy has no relation to the luminosity but the fundamental line e…
We have numerically solved the Grad-Shafranov equation for axisymmetric static MHD equilibria of matter confined at the polar cap of neutron stars. From the equilibrium solutions we explore the stability of the accretion mounds using the PLUTO MHD code. We find that pressure driven modes disrupt the equilibria beyond a threshold mound mass, forming dynamic structures, as matter spreads over the neutron star surface. Our results show that local variation of magnetic field will significantly affect the shape and nature of the cyclotron features observed in the spectra of High Mass X-ray Binaries…
We present the results of our spectral and timing analysis of the emission from the transient X-ray pulsar XTE J1946+274 based on the simultaneous NuSTAR and Swift/XRT observations in the broad energy range 0.3-79 keV carried out in June 2018 during a bright outburst. Our spectral analysis has confirmed the presence of a cyclotron absorption line at an energy $\sim38$ keV in both averaged and phase-resolved spectra of the source. Phase-resolved spectroscopy has also allowed the variation in spectral parameters with neutron star rotation phase, whose period is $\simeq15.755$ s, to be studied. T…
We report on an outburst of the high mass X-ray binary 4U 0115+63 with a pulse period of 3.6s in 2008 March/April as observed with RXTE and INTEGRAL. During the outburst the neutron star's luminosity varied by a factor of 10 in the 3--50\,keV band. In agreement with earlier work we find evidence for five cyclotron resonance scattering features at ~10.7, 21.8, 35.5, 46.7, and 59.7keV. Previous work had found an anti-correlation between the fundamental cyclotron line energy and the X-ray flux. We show that this apparent anti-correlation is probably due to the unphysical interplay of parameters o…
It has been recently discovered that the transition of X-ray pulsars to the low luminosity state ($L\lesssim10^{35}\,{\rm erg\,s^{-1}}$) is accompanied by a dramatic spectral change. Namely, the typical power-law-like spectrum with high energy cutoff transforms into a two-component structure with a possible cyclotron absorption feature on top of it. It was proposed that these spectral characteristics can be explained qualitatively by the emission of cyclotron photons in the atmosphere of the neutron star caused by collisional excitation of electrons to upper Landau levels and further comptoniz…
We study the timing and spectral properties of the low-magnetic field, transient magnetar SWIFT J1822.3-1606 as it approached quiescence. We coherently phase-connect the observations over a time-span of ~500 days since the discovery of SWIFT J1822.3-1606 following the Swift-BAT trigger on 2011 July 14, and carried out a detailed pulse phase spectroscopy along the outburst decay. We follow the spectral evolution of different pulse phase intervals and find a phase and energy-variable spectral feature, which we interpret as proton cyclotron resonant scattering of soft photon from currents circula…
We report on the analysis of NuSTAR observations of the Be-transient X-ray pulsar V 0332+53 during the giant outburst in 2015 and another minor outburst in 2016. We confirm the cyclotron-line energy-luminosity correlation previously reported in the source and the line energy decrease during the giant outburst. Based on 2016 observations, we find that a year later the line energy has increased again essentially reaching the pre-outburst values. We discuss this behaviour and conclude that it is likely caused by a change of the emission region geometry rather than previously suggested accretion-i…
We present NuSTAR observations of Vela X-1, a persistent, yet highly variable, neutron star high-mass X-ray binary (HMXB). Two observations were taken at similar orbital phases but separated by nearly a year. They show very different 3-79 keV flux levels as well as strong variability during each observation, covering almost one order of magnitude in flux. These observations allow, for the first time ever, investigations on kilo-second time-scales of how the centroid energies of cyclotron resonant scattering features (CRSFs) depend on flux for a persistent HMXB. We find that the line energy of …
Recapitulation of the resonance condition for the fundamental and higher electron cyclotron harmonics in the Electron Cyclotron Maser Instability (ECMI) enables radiation below and confirms the possibility of radiation in a narrow band above harmonics $n>1$. Near $n=1$ resonance on the confined lower X-mode branch, amplification is supported by the decrease of phase and group speeds. Confined slow large-amplitude quasi-electrostatic X-modes nonlinearly modulate the plasma to form cavitons until self-trapped inside them at further increasing wavenumber. They undergo wave-wave interaction, enabl…
Cyclotron resonance scattering features observed in the spectra of some X-ray pulsars show significant changes of the line centroid energy with the pulsar luminosity. Whereas for bright sources above the so called critical luminosity these variations are established to be connected with the appearance of the high accretion column above the neutron star surface, at low, sub-critical luminosities the nature of the variations (but with the opposite sign) has not been discussed widely. We argue here that the cyclotron line is formed when the radiation from a hotspot propagates through the plasma f…
Cyclotron resonance scattering features observed in the spectra of some X-ray pulsars show significant changes of the line energy with the pulsar luminosity. At high luminosities, these variations are often associated with the onset and growth of the accretion column, which is believed to be the origin of the observed emission and of the cyclotron lines. However, this scenario inevitably implies large gradient of the magnetic field strength within the line-forming region, which makes the formation of the observed line-like features problematic. Moreover, the observed variation of the cyclotron…
