arXiv subjects

R. Staubert

Publications and source records attributed to R. Staubert.

At least 127 records · Page 7Linked to original sources

arXiv2000

Multiple Cyclotron Lines in the Spectrum of 4U 0115+63

We report phase resolved spectroscopy of the transient accreting pulsar, 4U0115+63. For the first time, more than two cylotron resonance scattering features are detected in the spectrum of an X-ray pulsar. The shape of the fundamental line appears to be complex, and this is in agreement with predictions of Monte-Carlo models. As in other pulsars, the line energies and optical depths are strong functions of pulse phase. One possible model for this is an offset of the dipole of the neutron star magnetic field.

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arXiv2000

RXTE Studies of Cyclotron Lines in Accreting Pulsars

In this paper, we summarize the RXTE measurements of cyclotron resonance scattering features in 8 accreting pulsars. The wide bandpass and modest resolution of the RXTE instruments make them ideal for measuring these generally broad features. In particular, the high energy response of HEXTE provides a window for discovery of new lines not detectable with proportional counters such as Ginga or the PCA alone.

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arXiv2000

Temporal Evolution of X-ray Lags in Cygnus X-1

We present the long term evolution of the frequency-dependent X-ray time lags of the black hole candidate Cygnus X-1 as measured in 1996 and 1998 with the Rossi X-ray Timing Explorer (RXTE). Lag spectra measured during the 1996 June soft state are very similar to those seen during 1996 December and most of 1998 while Cyg~X-1 was in its hard state. During state transitions, however, the shape and magnitude of the X-ray lag is highly variable and tends to be much larger than outside of the state transitions. This behavior is most obvious in the 1--10 Hz band. The increase of the X-ray lag during the state transitions might be related to the formation and destruction of the synchrotron radiation emitting outflows present during the hard state.

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arXiv1999

Disappearing Pulses in Vela X-1

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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arXiv1999

Monitoring the Short-Term Variability of Cyg X-1: Spectra and Timing

We present first results from the spectral and temporal analysis of an RXTE monitoring campaign of the black hole candidate Cygnus X-1 in 1999. The timing properties of this hard state black hole show considerable variability, even though the state does not change. This has previously been noted for the power spectral density, but is probably even more pronounced for the time lags. From an analysis of four monitoring observations of Cyg X-1, separated by 2 weeks from each other, we find that a shortening of the time lags is associated with a hardening of the X-ray spectrum, as well as with a longer characteristic ``shot time scale''. We briefly discuss possible physical/geometrical reasons for this variability of the hard state properties.

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arXiv1999

RXTE Monitoring of LMC X-3: Recurrent Hard States

The black hole candidate LMC X-3 varies by a factor of four in the soft X-rays on a timescale of 200 or 100days. We have monitored LMC X-3 with RXTE in three to four week intervals starting in December 1996, obtaining a large observational database that sheds light on the nature of the long term X-ray variability in this source. In this paper we present the results of this monitoring campaign, focusing on evidence of recurring hard states in this canonical soft state black hole candidate.

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arXiv1999

Discovery of a Third Harmonic Cyclotron Resonance Scattering Feature in the X-ray Spectrum of 4U 0115+63

We have discovered a third harmonic cyclotron resonance scattering feature (CRSF) in observations of the recent outburst of 4U 0115+63 with the Rossi X-ray Timing Explorer (RXTE). The spectrum in a narrow pulse phase range shows CRSFs at 12.40+0.65/-0.35, 21.45+0.25/-0.38, and 33.56+0.70/-0.90 keV. With centroid energy ratios to the fundamental of 1.73+/-0.08 and 2.71+/-0.13, the CRSFs are not harmonically spaced. Strong variability of the continuum and CRSFs with pulse phase indicate a complex emission geometry near the neutron star polar cap. In addition, one RXTE observation, which spanned periastron passage, revealed a strong 2 mHz quasi-periodic oscillation (QPO). This is slower by two orders of magnitude than the beat-frequency QPO expected in this system and slower by a factor of more than 5 compared with other QPOs seen in accreting X-ray pulsars.

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arXiv1998

Evolution of spectral parameters during a pre-eclipse dip of Her X-1

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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arXiv1998

Vela X-1 as seen by RXTE

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.

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arXiv1998

RXTE Observations of Cygnus X-1: Broad-Band Spectra and High-Resolution Timing

We present results from a 20 ksec RXTE observation of the black hole candidate Cyg X-1. We apply self-consistent accretion disk corona models to these hard state data and show that Comptonization in a spherical corona irradiated by soft photons from an exterior cold disk is able to successfully model the spectrum. We also present the power spectrum, the coherence function, and the time lags for lightcurves from four energy bands. By modeling the high-resolution lightcurves with stochastic linear state space models, we show that the rapid hard state variability of Cyg X-1 can be explained with a single timescale.

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arXiv1998

RXTE Observations of LMC X-1 and LMC X-3

Of all known persistent stellar-mass black hole candidates, only LMC X-1 and LMC X-3 consistently show spectra that are dominated by a soft, thermal component. We present results from long (170ksec) Rossi X-ray Timing Explorer (RXTE) observations of LMC X-1 and LMC X-3 made in 1996 December. The spectra can be described by a multicolor disk blackbody plus an additional high-energy power-law. Even though the spectra are very soft (Gamma is about 2.5), RXTE detected a significant signal from LMC X-3 up to energies of 50keV, the hardest energy at which the object was ever detected. Focusing on LMC X-3, we present results from the first year of an ongoing monitoring campaign with RXTE which started in 1997 January. We show that the appearance of the object changes considerably over its ~200d long cycle. This variability can either be explained by periodic changes in the mass transfer rate or by a precessing accretion disk analogous to Her X-1.

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arXiv1998

Hard X-ray observations of the X-ray pulsar A0535+26

We present the results of the observations of the giant bursts from the X-ray pu lsar A0535+26 made by HEXE onboard Mir-Kvant in April 1989, November 1993 and February 1994. The pulse periods were measured, pulse profiles in different energy bands were produced, and their variability was investigated. The power density spectra (PDS) in 2x10^(-3)-1 Hz range is presented, which shape is typical for flicker-noise processes, usually observed in black hole candidates. The noise rms grows with energy from ~20% at 20 keV to ~30% at 80 keV. The source photon spectrum in the 15-200 keV energy range and its variability over the pulse phase are reported. Approximately the shape of the spectrum can be described by the "canonical" model for X-ray pulsars with power-law index g~1.1, cut-off energy E_c~23 keV and folding energy E_f~19 keV. All these parameters are weakly dependent on the luminosity. The most significant deviation from this continuum is observed at ~100 keV in the spectrum of the main pulse maximum. This feature is interpreted as a cyclotron line. Comparison of the HEXE data with the data from BATSE/CGRO (Bildsten et al., 1997) shows that in the high luminosity state (L~10^38 erg/s) the pulsar's pulse profile differs substantially from the pulse profile in the low-luminosity (L~5x10^36 erg/s) state. This difference is explained by the qualitative change of the polar cap structure with formation of the accretion columns.

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arXiv1998

Analyzing short-term X-ray variability of Cygnus X-1 with Linear State Space Models

Cyg X-1 exhibits irregular X-ray variability on all measured timescales. The usually applied shot noise models describe the typical short-term behavior of this source by superposition of randomly occuring shots with a distribution of shot durations. We have reanalyzed EXOSAT ME observations of Cyg X-1 using the more general Linear State Space Models (LSSMs). These models, which explicitly take the observation-noise into account, model the intrinsic system variability with an autoregressive (AR) process. Our fits show that an AR process of first order can reproduce the system variability of Cyg X-1. A possible interpretation is again the superposition of individual shots, but with a single relaxation timescale tau. This parameter was found to be 0.19s.

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arXiv1997

Properties of the X-ray pulsar GX 301-2 in hard X-rays

In 1993-1994 a series of observations of the X-ray pulsar GX 301-2 by HEXE onboard Mir-Kvant was made. A period of pulsations was measured (it varied between 675 and 678 s) and pulse profiles in different energy bands were produced. The measured luminosity in the 20-100 keV energy range changed substantially between 8x10^34 and 7x10^35 d^2 erg/s (d is the distance to the source in kpc). The obtained spectrum is quite satisfactory described by the canonical model for X-ray pulsars with gamma=1.3, E_c~23 keV, E_f~9 keV. It changed weakly between the observations, but was softest at brightness maximum. Significant variations of the spectral hardness over the pulse phase were detected, but the accumulated data are insufficient to quantify variations in spectral parameters. No significant traces of cyclotron lines were found. An interpretation of the pulse profiles as superposition of emissions from two flat polar caps (with inclusion of gravitational lensing) leads to an estimate of the angle between the magnetic axis and axis of rotation of 40-70 deg and an angle between the direction to the observer and the rotation axis of 75-85 deg.

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arXiv1997

UV to X-ray Spectra of Radio-Quiet Quasars. Comparison With Accretion Disk Models

We present UV to soft X-ray spectra of 31 radio-quiet quasars, comprising data from the IUE ULDA database and the ROSAT pointed observation phase. 90 % of the sample members show a soft X-ray excess above an underlying hard X-ray power law spectrum. Particularly for the steep X-ray spectrum (alpha_energy > 1.5), low redshift subsample (17 objects) the X-ray spectral power law index is strongly correlated with the optical to X-ray broad-band spectral index alpha_ox, indicating that the main contribution to the soft X-ray and optical emission is due to the same emission component. We model the UV/soft X-ray spectra in terms of thermal emission from a geometrically thin alpha-accretion disk. The structure and radiation field of the disk is calculated self-consistently and Compton scattering is treated by the Kompaneets equation. All relativistic effects on the disk structure and the emergent disk spectrum are included. Satisfactory spectral fits of the UV and soft X-ray continuum spectra are achieved when additional non-thermal hard X-ray and IR power law emission components are taken into account. The UV and soft X-ray spectra are well described by emission resulting from accretion rates in the range ~0.1 to ~0.3 times the Eddington accretion rate. Low mass/low redshift objects are found to accret at < 0.15 Mdot_Eddington. Correlations of the accretion disk parameters with alpha_ox are discussed.

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arXiv1997

Analyzing X-ray variability by State Space Models

In recent years, autoregressive models have had a profound impact on the description of astronomical time series as the observation of a stochastic process. These methods have advantages compared with common Fourier techniques concerning their inherent stationarity and physical background. If autoregressive models are used, however, it has to be taken into account that real data always contain observational noise often obscuring the intrinsic time series of the object. We apply the technique of a Linear State Space Model which explicitly models the noise of astronomical data and allows to estimate the hidden autoregressive process. As an example, we have analysed a sample of Active Galactic Nuclei (AGN) observed with EXOSAT and found evidence for a relationship between the relaxation timescale and the spectral hardness.

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arXiv1997

Pulse Phase Spectroscopy of A 0535+26 during its 1994 giant outburst observed with OSSE

We present pulse phase spectroscopy of A 0535+26 in the energy range 35-200 keV from OSSE observations of its giant outburst in 1994. We discuss the phase dependence of the continuum parameters and the cyclotron resonance feature (CRF) at 110 keV already found in the phase averaged spectrum. We find that a CRF is required at every phase. The behaviour of the line parameters with phase and the pulse shape indicate that the emission occurs in a pencil-beam geometry.

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arXiv1996

CGRO, Radio and Optical Observations of the Quasar NRAO 140

We report on Compton Gamma Ray Observatory (CGRO), radio and optical observations of the radio-loud, superluminal quasar NRAO 140. The source is not detected (at a significance of greater than 3 sigma) by any of the CGRO instruments OSSE, Comptel and EGRET. Radio observations simultaneous to the OSSE observation and optical monitoring over three-monthly intervals show no signs of extraordinary behaviour when compared to previous observations. We demonstrate that the CGRO non-detections do not require a spectral break at hard X-rays, but can be explained by a steep spectrum in the MeV range comparable to that of 3C273.

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