arXiv subjects

J. Wilms

Publications and source records attributed to J. Wilms.

At least 289 records · Page 16Linked to original sources

arXiv2001

Quasi-Periodic Oscillation in Seyfert galaxies: Significance levels. The Case of Mrk 766

We discuss methods to compute significance levels for the existence of quasi-periodic oscillations (QPOs) in Active Galactic Nuclei (AGN) which take the red-noise character of the X-ray lightcurves of these objects into account. Applying epoch folding and periodogram analysis to the XMM-Newton observation of the Seyfert galaxy Mrk 766, a possible QPO at a timescale of 4200s has been reported. Our computation of the significance of this QPO, however, shows that the 4200s peak is not significant at the 95% level. We conclude that the 4200s feature is an artifact of the red-noise process and not the result of a physical process within the Active Galactic Nuclei.

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arXiv2001

The optical long `period' of LMC X-3

We have studied the long term variability of LMC X-3 in optical lightcurves spanning six years, in order to search for optical signatures which could confirm/refute the suggestion that the `modulation' is the result of accretion rate variability rather than accretion disc precession. We find that there is no stable period in the optical lightcurves, that the optical and X-ray lightcurves are correlated, with an X-ray lag and that there is no modulation in the optical colours. We argue that these effects agree with the variable mass accretion rate model proposed by Wilms et al. (2001).

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arXiv2001

RXTE monitoring of Centaurus A

We report on the analysis from ~110 ks of X-ray observations of Centaurus A carried out with the Proportional Counter Array (PCA) and the High Energy X-ray Timing Experiment (HEXTE) instruments on Rossi X-ray Timing Explorer (RXTE) during three monitoring campaigns over the last 4 years (10 ks in 1996, 74 ks in 1998, and 25 ks in 2000). The joint PCA/HEXTE X-ray spectrum can be well described by a heavily absorbed power law with photon index 1.8 and a narrow iron line due to fluorescence of cold matter. The measured column depth decreased by about 30% between 1996 and 2000, while the detected 2-10 keV continuum flux remained constant between 1996 and 1998, but increased by 60% in 2000. Since in all three observations the iron line flux did not vary, a corresponding decrease in equivalent width was noted. No appreciable evidence for a reflection continuum in the spectrum was detected. We present the interpretation of the iron line strength through Monte Carlo computations of various geometries. No significant temporal variability was found in Cen A at time scales from days to tens of minutes.

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arXiv2001

Cygnus X-1 from RXTE: Monitoring the short term variability

We present temporal and spectral results from monitoring Cygnus X-1 with the Rossi X-ray Timing Explorer (RXTE) in 1998 and 1999. We concentrate on the long term evolution of the hard state timing properties, comparing it to the 1996 soft state evolution. This leads to the following results: 1. the hard and soft state time lag spectra are very similar, 2. during state transitions, the lags in the 1-10 Hz range increase by more than an order of magnitude, 3. in the hard state itself, flaring events can be seen --- the temporal and spectral evolution during the flare of 1998 July identifies it as a ``failed state transition''. During (failed) state transitions, the time lag spectra and the power spectra change predominantly in the 1-10 Hz range. We suggest that this additional variability is produced in ejected coronal material disrupting the synchrotron radiation emitting outflows present in the hard state.

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arXiv2001

Discovery of a Cyclotron Resonant Scattering Feature in the RXTE Spectrum of 4U 0352+309 (X Per)

We have discovered a ~29 keV Cyclotron Resonant Scattering Feature (CRSF) in the X-Ray spectrum of 4U 0352+309 (X Per) using observations taken with the Rossi X-Ray Timing Explorer. 4U 0352+309 is a persistent low luminosity (L_x = 4.2x10^34 ergs/s) X-ray pulsar, with a 837 s period and which accretes material from the Be star X Per. The X-Ray spectrum, unusual when compared to brighter accreting pulsars, may be due to the low mass accretion rate and could be typical of the new class of persistent low luminosity Be/X-Ray binary pulsars. We attempted spectral fits with continuum models used historically for 4U 0352+309, and found that all were improved by the addition of a CRSF at ~29 keV. The model that best fit the observations is a combination of a 1.45+/-0.02 keV blackbody with a 5.4x10^8 cm^2 area, and a power-law with a 1.83+/-0.03 photon index modified by the CRSF. In these fits the CRSF energy is 28.6+1.5-1.7 keV, implying a magnetic field strength of 2.5(1+z)x10^12 G in the scattering region (where z is the gravitational redshift). Phase resolved analysis shows that the blackbody and cyclotron line energies are consistent with being constant through the pulse.

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arXiv2000

The 35-day cycle in Her X-1 as observational appearance of freely precessing neutron star and forcedly precessing accretion disk

A careful analysis of X-ray light curves and pulse profiles of Her X-1 obtained over more than 20 years strongly evidences for free precession of a magnetized neutron star with rotational axis inclined to the orbital plane as a central clock underlying the observed 35-day period. Strong asymmetric X-ray illumination of the optical star atmosphere leads to the formation of gaseous streams coming out of the orbital plane and forming a tilted accretion disk around the neutron star. Such a disk precesses due to tidal forces and dynamical action of gaseous streams from the secondary companion. The locking of these torques with neutron star precession makes the net disk precession period to be very close to that of the neutron star free precession.

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arXiv2000

On the Absorption of X-rays in the Interstellar Medium

We present an improved model for the absorption of X-rays in the ISM intended for use with data from future X-ray missions with larger effective areas and increased energy resolution such as Chandra and XMM, in the energy range above 100eV. Compared to previous work, our formalism includes recent updates to the photoionization cross section and revised abundances of the interstellar medium, as well as a treatment of interstellar grains and the H2molecule. We review the theoretical and observational motivations behind these updates and provide a subroutine for the X-ray spectral analysis program XSPEC that incorporates our model.

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arXiv2000

Discovery of Recurring Soft to Hard State Transitions in LMC X-3

We present the analysis of an RXTE monitoring campaign of the canonical soft state black hole candidates LMC X-1 and LMC X-3. In contrast to LMC X-1, which does not exhibit any periodic spectral changes, we find that LMC X-3 exhibits stron spectral variability on time scales of days to weeks. For typical RXTE ASM count rates, the luminosity variations of LMC X-3 are due to changes of the phenomenological disk blackbody temperature. During episodes of especially low luminosity (ASM count rates < 0.6 counts/sec), kT strongly decreases and the power law significantly hardens to a photon index of ~1.8. These changes are consistent with state changes of LMC X-3 from the soft state to the canonical hard state of galactic black hole candidates. We argue that the long term variability of LMC X-3 might be due to a wind-driven limit cycle such as discussed by Shields et al. (1986)

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arXiv2000

A Good Long Look at the Black Hole Candidates LMC X-1 and LMC X-3

We present results from 170ksec long RXTE observations of LMC X-1 and LMC X-3, taken in 1996 December, where their spectra can be described by a disc black body plus an additional soft (Gamma~2.8) high-energy power-law (detected up to 50keV in LMC X-3). These observations, as well as archival ASCA observations, constrain any narrow Fe line present in the spectra to have an equivalent width <90eV, broad lines (~150eV EW, σ~ 1keV) are permitted. We also study the variability of LMC X-1. Its X-ray power spectral density (PSD) is approximately f^{-1} between 10^{-3} and 0.3Hz with a rms variability of ~7%. Above 5keV the PSD shows evidence of a break at f > 0.2Hz, possibly indicating an outer disc radius of ~1000GM/c^2 in this likely wind-fed system. Furthermore, the coherence function between variability in the > 5keV band and variablity in the lower energy bands is extremely low. We discuss the implications of these observations for the mechanisms.

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arXiv2000

The 1999 Hercules X-1 Anomalous Low State

A failed main-on in the 35d cycle of Her X-1 was observed with the Rossi X-Ray Timing Explorer (RXTE) on 1999 April 26. Exceptions to the normal 35d cycle have been seen only twice before; in 1983 and again 1993. We present timing and spectral results of this latest Anomalous Low State (ALS) along with comparisons to the main-on and normal low states. Pulsations were observed in the 3-18 keV band with a fractional RMS variation of (0.037+-0.003). Spectral analysis indicates that the ALS spectrum has the same shape as the main-on but is modified by heavy absorption and scattering. We find that 70% of the observed emission has passed through a cold absorber (N_H=5.0x10^{23}cm^{-2}). This partially absorbing spectral fit can be applied to the normal low state with similar results. We find that the ALS observations may be interpreted as a decrease in inclination of the accretion disk causing the central X-Ray source to be obscured over the entire 35d cycle.

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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

Multifrequency Observations of the Virgo Blazars 3C 273 and 3C 279 in CGRO Cycle 8

We report first observational results of multifrequency campaigns on the prominent Virgo blazars 3C 273 and 3C 279 which were carried out in January and February 1999. Both blazars are detected from radio to gamma-ray energies. We present the measured X- to gamma-ray spectra of both sources, and for 3C 279 we compare the 1999 broad-band (radio to gamma-ray) spectrum to measured previous ones.

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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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arXiv2000

On the inability of Comptonization to produce the broad X-ray iron lines observed in Seyfert nuclei

It has recently been suggested that Compton downscattering may give rise to the broad iron lines seen in the X-ray spectra of Seyfert 1 galaxies. This challenges the standard model in which these lines originate from the innermost regions of the black hole accretion disk with Doppler shifts and gravitational redshifts giving rise to the broadened line profile. Here, we apply observational constraints to the Compton downscattering model for MCG-6-30-15 and NGC3516, the two best cases to date of Seyfert galaxies with relativistically broad lines. We show that the continuum source in MCG-6-30-15 required by the constrained model violates the black body limit. In the case of NGC3516, only a very small region of parameter space is compatible with the constraints. Hence, we conclude that the Comptonization model is not a viable one for the broad line seen in these two objects. The accretion disk model remains the best interpretation of these data.

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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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