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Register a new userEvolution of spectral parameters during a pre-eclipse dip of Her X-1
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Authors
B. Stelzer
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We report on a pre-eclipse dip of the X-ray binary Her X-1 observed by the Rossi X-ray Timing Explorer (RXTE) in July 1996. We present the evolution of the spectral parameters in the 3-18 keV range with a temporal resolution of 16 s and show that the varying flux of the spectrum can be interpreted solely by a time varying column density. We also find that the lightcurve is characterized by symmetric substructures with recurrence time of a few minutes that can be successfully modeled by Gaussian profiles.
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We present a broad-band X-ray timing study of the variations in pulse behavior with superorbital cycle in the low-mass X-ray binary Her X-1. This source shows a 35-day superorbital modulation in X-ray flux that is likely caused by occultation by a warped, precessing accretion disk. Our data set consists of four joint XMM-Newton and NuSTAR observations of Her X-1 which sample a complete superorbital cycle. We focus our analysis on the first and fourth observations, which occur during the bright main-on phase, because these observations have strongly detected pulsations. We added an archival XMM-Newton observation during the short-on phase of the superorbital cycle since our observations at that phase are lower in signal to noise. We find that the energy-resolved pulse profiles show the same shape at similar superorbital phases and the profiles are consistent with expectations from a precessing disk. We demonstrate that a simple precessing accretion disk model is sufficient to reproduce the observed pulse profiles. The results of this model suggest that the similarities in the observed pulse profiles are due to reprocessing by a precessing disk that has returned to its original precession phase. We determine that the broad-band spectrum is well fit by an absorbed power law with a soft blackbody component, and show that the spectral continuum also exhibits dependence on the superorbital cycle. We also present a brief analysis of the energy resolved light curves of a pre-eclipse dip, which shows soft X-ray absorption and hard X-ray variability during the dip.
Results of a 1999 July 8-10 BeppoSAX observation during an anomalous low-state of Her X-1 are presented. The standard on-state power-law and blackbody continuum model is excluded at high confidence unless partial covering is included. This gives a power-law photon index of 0.63 +/- 0.02 and implies that 0.28 +/- 0.03 of the flux undergoes additional absorption of (27 +/- 7) 10^22 atom/cm2. 11% of the observed 0.1-10 keV flux is from the 0.068 +/- 0.015 keV blackbody. 1.237747(2) s pulses with a semi-amplitude of 2.1 +/- 0.8% are detected at >99.5% confidence and confirmed by RXTE measurements. This implies that Her X-1 underwent substantial spin-down close to the start of the anomalous low-state. The spectral and temporal changes are similar to those recently reported from 4U1626-67. These may result from a strongly warped disk that produces a spin-down torque. The X-ray source is then mostly viewed through the inner regions of the accretion disk. A similar mechanism could be responsible for the Her X-1 anomalous low-states. Shadowing by such an unusually warped disk could produce observable effects in the optical and UV emission from the companion star.
We present a first analysis of a high resolution X-ray spectrum of the ionized stellar wind of Vela X-1 during eclipse. The data were obtained with the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray Observatory. The spectrum is resolved into emission lines with fluxes between 0.02 and 1.04x10^4 ph/cm^2/s. We identify lines from a variety of charge states, including fluorescence lines from cold material, a warm photoionized wind. We can exclude signatures from collisionally ionized plasmas. For the first time we identify fluorescence lines from L-shell ions from lower Z elements. We also detect radiative recombination continua from a kT = 10 eV (1.2 x 10^5 K) photoionized optically thin gas. The fluorescence line fluxes infer the existence of optically thick and clumped matter within or outside the warm photoionized plasma.
eROSITA (extended ROentgen Survey with an Imaging Telescope Array) instrument onboard the Russian-German `Spectrum-Roentgen-Gamma (SRG) mission observed the Her X-1/HZ Her binary system in multiple scans over the source during the first and second SRG all-sky surveys. Both observations occurred during a low state of the X-ray source when the outer parts of the accretion disk blocked the neutron star from view. The orbital modulation of the X-ray flux was detected during the low states. We argue that the detected X-ray radiation results from scattering of the emission of the central source by three distinct regions: (a) an optically thin hot corona with temperature $sim (2-4) times 10^6$ K above the irradiated hemisphere of the optical star; (b) an optically thin hot halo above the accretion disk; and (c) the optically thick cold atmosphere of the optical star. The latter region effectively scatters photons with energies above $5-6$ keV.
We present spin-resolved X-ray data of the neutron star binary Her X-1. We find evidence that the Iron line at 6.4 keV originates from the same location as the blackbody X-ray component. The line width and energy varies over both the spin period and the 35 day precession period. We also find that the correlation between the soft and hard X-ray light curves varies over the 35 day period.
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