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Register a new userDisappearing Pulses in Vela X-1
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P. Kretschmar
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We present results from a 20 h RXTE observation of Vela X-1, ncluding a peculiar low state of a few hours duration, during which the pulsation of the X-ray emission ceased, while significant non-pulsed emission remained. This ``quiescent state was preceded by a ``normal state without any unusual signs and followed by a ``high state of several hours of increased activity with strong, flaring pulsations. while there is clear spectral evolution from the normal state to the low state, the spectra of the following high state are surprisingly similar to those of the low state.
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We present results from four observations of the accreting X-ray pulsar Vela X-1 with RXTE in 1996 February. The light curves show strong pulse to pulse variations, while the average pulse profiles are quite stable, similar to previous results. Below 5keV the pulse profiles display a complex, 5-peaked structure with a transition to a simple, double peak above about 15keV. We analyze phase-averaged, phase-resolved, and on-pulse minus off-pulse spectra. The best spectral fits were obtained using continuum models with a smooth high-energy turnover. In contrast, the commonly used power law with exponential cutoff introduced artificial features in the fit residuals. Using a power law with a Fermi-Dirac cutoff modified by photoelectric absorption and an iron line, the best fit spectra are still unacceptable. We interpret large deviations around 25 and 55keV as fundamental and second harmonic cyclotron absorption lines. If this result holds true, the ratio of the line energies seems to be larger than 2. Phase resolved spectra show that the cyclotron lines are strongest on the main pulse while they are barely visible outside the pulses.
The wind-accreting X-ray binary pulsar and cyclotron line source Vela X-1 has been observed extensively during INTEGRAL Core Program observations of the Vela region in June-July and November-December 2003. In the latter set of observations the source showed intense flaring -- see also Staubert et al. (2004), these proceedings. We present early results on time averaged and time resolved spectra, of both epochs of observations. A cyclotron line feature at ~53 keV is clearly detected in the INTEGRAL spectra and its broad shape is resolved in SPI spectra. The remaining issues in the calibration of the instruments do not allow to resolve the question of the disputed line feature at 20-25 keV. During the first main flare the average luminosity increases by a factor of ~10, but the spectral shape remains very similar, except for a moderate softening.
We present pulse phase-resolved X-ray spectra of the high mass X-ray binary Vela X-1 using the Rossi X-ray Timing Explorer. We observed Vela X-1 in 1998 and 2000 with a total observation time of ~90 ksec. We find an absorption feature at 23.3 +1.3 -0.6 kev in the main pulse, that we interpret as the fundamental cyclotron resonant scattering feature (CRSF). The feature is deepest in the rise of the main pulse where it has a width of 7.6 +4.4 -2.2 kev and an optical depth of 0.33 +0.06 -0.13. This CRSF is also clearly detected in the secondary pulse, but it is far less significant or undetected during the pulse minima. We conclude that the well known CRSF at 50.9 +0.6 -0.7 kev, which is clearly visible even in phase-averaged spectra, is the first harmonic and not the fundamental. Thus we infer a magnetic field strength of B=2.6 x 10^12 G.
We investigate the structure of the wind in the neutron star X-ray binary system Vela X-1 by analyzing its flaring behavior. Vela X-1 shows constant flaring, with some flares reaching fluxes of more than 3.0 Crab between 20-60 keV for several 100 seconds, while the average flux is around 250 mCrab. We analyzed all archival INTEGRAL data, calculating the brightness distribution in the 20-60 keV band, which, as we show, closely follows a log-normal distribution. Orbital resolved analysis shows that the structure is strongly variable, explainable by shocks and a fluctuating accretion wake. Analysis of RXTE ASM data suggests a strong orbital change of N_H. Accreted clump masses derived from the INTEGRAL data are on the order of 5 x 10^19 -10^21 g. We show that the lightcurve can be described with a model of multiplicative random numbers. In the course of the simulation we calculate the power spectral density of the system in the 20-100 keV energy band and show that it follows a red-noise power law. We suggest that a mixture of a clumpy wind, shocks, and turbulence can explain the measured mass distribution. As the recently discovered class of supergiant fast X-ray transients (SFXT) seems to show the same parameters for the wind, the link between persistent HMXB like Vela X-1 and SFXT is further strengthened.
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.
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