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Register a new userStudy of the accreting pulsar 4U 0115+634 with a bulk and thermal Comptonization model
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Highly magnetized pulsars accreting matter in a binary system are bright sources in the X-ray band (0.1-100 keV). Despite the early comprehension of the basic emission mechanism, their spectral energy distribution is generally described by phenomenological or simplified models. We propose a study of the spectral emission from the high mass X-ray binary pulsar 4U 0115+634 by means of thermal and bulk Comptonization models based on the physical properties of such objects. For this purpose, we analyze the BeppoSAX data in the energy range 0.7-100 keV of the 1999 giant outburst, 12 days after the maximum. We model the spectral energy distribution of the system using a two-component continuum. At higher energy, above ~7 keV, the emission is due to thermal and bulk Comptonization of the seed photons produced by cyclotron cooling of the accretion column, and at lower energy, the emission is due to thermal Comptonization of a blackbody source in a diffuse halo close to the stellar surface. From the best fit parameters, we argue that the cyclotron emission is produced ~1.7 km above the stellar surface, and escapes from the column near its base, where the absorption features are generated by the interaction with the magnetic field in a surrounding halo. We find that in 4U 0115+634, the observed spectrum is dominated by reprocessed cyclotron radiation, whereas in other bright sources with stronger magnetic fields such as Her X-1, the spectrum is dominated by reprocessed bremsstrahlung.
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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.
Transient X-ray binaries produce major outbursts in which the X-ray flux can increase over the quiescent level by factors as large as $10^7$. The low-mass X-ray binary V 404 Cyg and the high-mass system 4U 0115+634 underwent such major outbursts in June and October 2015, respectively. We present here observations at energies above hundreds of GeV with the VERITAS observatory taken during some of the brightest X-ray activity ever observed from these systems. No gamma-ray emission has been detected by VERITAS in 2.5 hours of observations of the microquasar V 404 Cyg from 2015, June 20-21. The upper flux limits derived from these observations on the gamma-ray flux above 200 GeV of F $< 4.4times 10^{-12}$ cm$^{-2}$ s$^{-1}$ correspond to a tiny fraction (about $10^{-6}$) of the Eddington luminosity of the system, in stark contrast to that seen in the X-ray band. No gamma rays have been detected during observations of 4U 0115+634 in the period of major X-ray activity in October 2015. The flux upper limit derived from our observations is F $< 2.1times 10^{-12}$ cm$^{-2}$ s$^{-1}$ for gamma rays above 300 GeV, setting an upper limit on the ratio of gamma-ray to X-ray luminosity of less than 4%.
ROTSE IIId (The Robotic Optical Transient Experiment) observations of X-ray binary system 4U 0115+634/V635 Cas obtained during 2004 June and 2005 January make possible, for the first time, to study the correlation between optical and type II X-ray outbursts. The X-ray outburst sharply enhanced after periastron passage where the optical brightness was reduced by 0.3 magnitude for a few days. We interpret the sharp reduction of optical brightness as a sign of mass ejection from the outer parts of the disc of the Be star. After this sharp decrease, the optical brightness healed and reached the pre X-ray outburst level. Afterwards, gradual decrease of the optical brightness followed a minimum then a gradual increase started again. Qualitatively, change of optical lightcurve suggests a precession of the Be star disc around a few hundred days. We also investigate the periodic signatures from the archival RXTE-ASM (Rossi X-ray Timing Explorer - All Sky Monitor) light curve covering a time span of $sim 9$ years. We find significant orbital modulation in the ASM light curve during the type I X-ray outburst.
Aims: Spectral and temporal analysis of the NuSTAR observation Galactic Be-XRB Swift J1845.7-0037. during its recent outburst. Methods: For the spectral analysis we use both phenomenological and physics-based models. We employ an often used empirical model to identify the main characteristics of the spectral shape in relation to nominal spectral characteristics of X-ray pulsars. Additionally, we used the latest version of Bulk & Thermal comptonization model (BW), to assess the validity of the spectral components required by the empirical model and to investigate the origin of the hard X-ray emission. We also analyzed the source light-curve, studying the pulse shape at different energy ranges and tracking the spectral evolution with pulse phase by using the model independent hardness ratio (HR). Results: We find that while both the empirical and physical (BW) spectral models can produce good spectral fits, the BW model returns physically plausible best-fit values for the source parameters and does not require any additional spectral components to the non-thermal, accretion column emission. The BW model also yielded an estimation of the neutron star magnetic field placing it in the 10^12G range. Conclusions: Our results, show that the spectral and temporal characteristics of the source emission are consistent with the scattering processes expected for radiation dominated shocks within the accretion column of highly magnetized accreting neutron stars. We further indicate that physically-derived spectral models such as BW, can be used to tentatively infer fundamental source parameters, in the absence of more direct observational signatures.
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 sources energy spectra have also been constructed from the INTEGRAL data obtained during the 2011 outburst for a dynamic range of its luminosities (~10^{37} - 7 x 10^{37} erg/s). We show that apart from the fundamental harmonic of the cyclotron absorption line at energy ~11 keV, its four higher harmonics at energies ~24, 35.6, 48.8, and 60.7 keV are detected in the spectrum. We have performed a detailed analysis of the sources spectra in the 4-28 keV energy band based on all of the available RXTE archival data obtained during bright outbursts in 1995-2011. We have confirmed that modifying the sources continuum model can lead to the disappearance of the observed anticorrelation between the energy of the fundamental harmonic of the cyclotron absorption line and the sources luminosity. Thus, the question about the evolution of the cyclotron absorption line energy with the luminosity of the X-ray pulsar 4U 0115+63 remains open and a physically justified radiation model for X-ray pulsars is needed to answer it.
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