arXiv subjects

D. Barret

Publications and source records attributed to D. Barret.

143 records · Page 8Linked to original sources

arXiv2000

ASCA Observations of GX 354-0 and KS 1731-260

We report on ASCA observations of the low mass X-ray binaries GX 354-0 and KS 1731-260. The spectrum of GX 354-0 is best described as a power-law or a Comptonized spectrum with tau ~ 5 and kT ~ 8 keV and a residual at ~6.5 keV. The residual may be a disk reflection or a Compton broadened Gaussian line from the hot inner ADAF-like coronal region. The absorption column density to the source is 2.9e22 cm^-2. No soft thermal component was detected. The spectrum from KS 1731-260 is softer and it is best fit with a two component model with a column density of 1.1e22 cm^-2. The likely interpretation is emission from a Comptonizing cloud with an optical depth tau>12 and either a neutron star or a disk blackbody emission. We discuss the likely location of the Comptonizing cloud for both sources within the context of several proposed emission models.

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arXiv2000

Time Lags in Low Mass X-ray Binaries

Using RXTE/PCA data, we have studied the time lag (TL) properties of a sample of four accreting neutron stars (NSs), namely 1E1724-3045, GS1826-238, 4U1705-44 and 4U1728-34. The aim of the study is to identify the spectral and timing state(s) in which TLs are detected. Along this work, we have discovered TLs between the 7-40 keV hard and 2-7 keV soft photons from 4U1728-34 with amplitudes similar to those seen in 4U1705-44 (i.e. 2 ms at 5 Hz). We show that the TLs are only seen in the low states of those sources, but that within the so-called ``island'' spectral state, some sources display TLs whereas some do not. On the other hand, we have found that TLs are detected when the associated Power Density Spectrum (PDS) shows excess power at high frequencies (above 1 Hz).

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arXiv2000

Low and High Frequency Quasi-Periodic Oscillations in 4U1915-05

The X-ray burster and dipper 4U1915-05 (also known as XB1916-053) was observed 19 times for a total exposure of roughly 140 ks between 1996 February and October with the proportional counter array (PCA) aboard Rossi X-ray Timing Explorer (RXTE). We present here the detailed timing study of its X-ray emission. The source was observed in both a high intensity/soft spectral and a low intensity/hard spectral state. The 2-20 keV luminosities are about 1.4x10^37 and 3.2x10^36 ergs/s in the two regimes respectively (assuming a source distance of 9.3 kpc). We confirm that 4U1915-05 is an atoll source based on its spectral behaviour and its aperiodic variability in the frequency range 4x10^{-3}-128 Hz. 4U1915-05 displays red noise below 1 Hz, but is especially remarkable for its quasi-periodic variability detected for the first time at several timescales. We detect low frequency quasi-periodic oscillations (LFQPOs) between 5 and 80 Hz, as well as high frequency QPOs (HFQPOs) between 200 and 1300 Hz. Both LFQPOs and HFQPOs frequencies positively correlate with the mass accretion rate inferred from the position of the source on the color-color diagram. In a narrow range of luminosity, we also detect twin simultaneous HFQPOs separated by 348 +/- 12 Hz.

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arXiv2000

RXTE Studies of Long-Term X-ray Spectral Variations in 4U 1820-30

We present the results of detailed spectral studies of the ultra-compact low mass X-ray binary (LMXB) 4U 1820-30 carried out with the Rossi X-ray Timing Explorer (RXTE) during 1996-7. 4U 1820-30 is an ``atoll'' source X-ray burster (XRB) located in the globular cluster NGC 6624. It is known to have an 11 minute binary period and a ~176 day modulation in its 2--12 keV flux. Observations were made with the PCA and HEXTE instruments on RXTE at roughly one-month intervals to sample this long-term period and study flux-related spectral changes. There are clear correlations between our fitted spectral parameters and both the broad-band (2--50 keV) flux and the position in the color-color diagram, as described by the parameter S_a introduced by Mendez et al. (1999). In addition, we find a strong correlation between the position in the color-color diagram and the frequencies of the kilohertz quasi-periodic oscillations (kHz QPOs) reported by Zhang et al. (1998). This lends further support to the notion that evidence for the last stable orbit in the accretion disk of 4U 1820-30 has been observed. For a model consisting of Comptonization of cool photons by hot electrons plus an additional blackbody component, we report an abrupt change in the spectral parameters at the same accretion rate at which the kHz QPOs disappear. For a model consisting of a multicolor disk blackbody plus a cut-off power law, we find that the inner disk radius reaches a minimum at the same accretion rate at which the kHz QPO frequency saturates, as expected if the disk reaches the last stable orbit. Both models face theoretical and observational problems when interpreted physically for this system.

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arXiv2000

RXTE Studies of X-ray Spectral Variations with Accretion Rate in 4U 1915-05

We present the results of detailed spectral studies of the ultra-compact low mass X-ray binary (LMXB) 4U 1915-05 carried out with the Rossi X-ray Timing Explorer (RXTE) during 1996. 4U 1915-05 is an X-ray burster (XRB) known to exhibit a ~199-day modulation in its 2--12 keV flux. Observations were performed with the PCA and HEXTE instruments on RXTE at roughly one-month intervals to sample this long-term period and study accretion rate-related spectral changes. We obtain good fits with a model consisting of a blackbody and an exponentially cut-off power law. The spectral parameters are strongly correlated with both the broad-band (2--50 keV) luminosity and the position in the color-color diagram, with the source moving from a low hard state to a high soft state as the accretion rate increases. The blackbody component appears to drive the spectral evolution. Our results are consistent with a geometry in which the soft component arises from an optically thick boundary layer and the hard component from an extended Comptonizing corona. Comparing our results with those of a similar study of the brighter source 4U 1820-30 (Bloser et al. 2000), we find that the two ultra-compact LMXBs occupy similar spectral states even though the transitions occur at very different total luminosities.

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arXiv1999

XEUS - The X-ray Evolving Universe Spectroscopy Mission

XEUS is under study by ESA as part of the Horizon 2000+ program to utilize the International Space Station (ISS) for astronomical applications. XEUS will be a long-term X-ray observatory with an initial mirror area of 6m2 at 1 keV that will be grown to 30m2 following a visit to the ISS. The 1 keV spatial resolution is expected to be 2-5'' HEW. XEUS will consist of separate detector and mirror spacecraft aligned by active control to provide a focal length of 50m. A new detector spacecraft, complete with the next generation of instruments, will also be added after visiting the ISS. The limiting sensitivity will then be ~4 10-18 erg/cm2/s - around 250 times better than XMM, allowing XEUS to study the properties of the hot baryons and dark matter at high redshift.

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arXiv1999

Hard X-ray Emission From Low Mass X-ray Binaries

We report on Rossi X-ray Timing Explorer observations of four type I X-ray bursters; namely 1E1724-3045, GS1826-238, SLX1735-269 and KS1731-260. The first three were in a low state (LS) whereas KS1731-260 was in a high state. The LS sources have very similar power spectra, displaying high frequency noise up to \~200 Hz. For KS1731-260, its power spectrum is dominated by noise at frequencies below 20 Hz. In addition a quasi-periodic oscillation at 1200 Hz is detected. The 1-200 keV LS spectra are all consistent with resulting from thermal Comptonization with an electron temperature (kTe) around 25-30 keV. For KS1731-260, the spectrum is also dominated by Comptonization, but with kTe around 3 keV and no significant hard X-ray emission. With the exception of GS1826-238, there is an underlying soft component. For all sources, we have detected an iron line at 6.4 keV. A reflection component is present in the spectra of GS1826-238 and SLX1735-269. We suggest a model in which the region of main energy release, where hard X-rays are produced would be an optically thin boundary layer merged with an Advection Dominated Accretion Flow (ADAF). The soft component observed probably represents the unscattered emission from the accretion disk of variable inner radius. When the accretion rate increases, the inner disk radius shrinks, the strength of the reflected component and associated iron line increase, and the Comptonization region cools off in response to an increased cooling flux from the accretion disk and from the reprocessed/reflected component. Finally, in the light of these observations, we discuss extensively the various criteria recently proposed to distinguish between non-quiescent accreting black holes and neutron stars.

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arXiv1999

Low and high frequency Quasi-Periodic Oscillations in 4U1915-05

The burster and dipper Low Mass X-ray Binary (LMXB) 4U1915-05 (also known as XB1916-053) was observed by the Rossi X-ray Timing Explorer (RXTE) 19 times for a total exposure of 140 ks between 1996 February and October. Here we report on the discovery of Low Frequency (10-40 Hz) Quasi-Periodic Oscillations (LFQPOs) from 4U1915-05. The properties of the LFQPOs are related to the presence of the High Frequency QPOs (HFQPOs) detected simultaneously. We have observed a correlation between the LFQPO frequency and source count rate, as well as a correlation (linear) between the LFQPOs and HFQPOs. Both results cannot be explained by a Beat Frequency Model (BFM). They are also hardly compatible with predictions from the inner disk precession model.

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arXiv1998

RXTE Broad Band X-ray Spectrum of the Burster 1E1724-3045

The X-ray burster 1E1724-3045 located in the globular cluster Terzan 2 is known as one of the persistent (though variable) hard X-ray sources as shown by the SIGMA observations of the Galactic Center region. 1E1724-3045 was observed with the PCA and HEXTE experiments onboard the RXTE on November 1996 for about 100 ksec. The broad band spectral capability and sensitivity of RXTE enables us to study simultaneously the X-ray (2-20 keV) and hard X-ray components (E>20 keV) of this source. During the observation, this ``Atoll'' source was in its ``Island'' state characterized by a hard Comptomized spectrum with an electron temperature kTe~29 keV, an optical depth of 2.9 (spherical geometry) and a temperature for the ``seed'' photons of 1.2 keV. Below 5 keV, there is a soft excess which we fit with a blackbody of kTbb=0.7 keV. The Comptonization temperature is significantly lower those observed for black holes candidates in their low luminosity states (kTe >~ 50 keV). Finally, our observation allows us to associate the presence of an hard tail with a low luminosity X-ray state (1-20 keV luminosity of 10^37 erg/s at 7.7 kpc).

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arXiv1998

Discovery of High Frequency Quasi-Periodic Oscillations in 4u1915-05

The type I X-ray burster and dipper 4U1915-05 (also known as XB1916-053) was monitored by the Rossi X-ray Timing Explorer between February and October, 1996. The source was observed in various spectral states; the highest luminosity state (LX ~ 1.5 10^37 ergs/s, 10 kpc, 1-20 keV) is associated with a soft spectrum, whereas for the lower luminosity state (down to ~5 10^36 ergs/s) the spectrum is significantly harder. Using the high time resolution data provided by the Proportional Counter Array (PCA), we have discovered High-Frequency Quasi-Periodic Oscillations (HFQPOs) in the persistent X-ray emission of 4U1915-05 while its luminosity was ~8 10^36 ergs/s. The QPO frequency ranges from 600 Hz up to ~1000 Hz, with typical Full Width at Half Maximum (FWHM) ~50-100 Hz and Root Mean Squared (RMS) values 15%. In addition, by using the ``shift and add'' technique, we have detected a twin HFQPO (5.5 sigma level) separated from the main peak by ~355 Hz. 4U1915-05 is the eighth Atoll source displaying simultaneous twin HFQPOs. Based on current knowledge of HFQPO sources, our observations suggest that 4U1915-05 might contain a 2.8 (or 5.6) millisecond rotating neutron star.

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arXiv1998

An ASCA X-ray Observation of 1E1724-3045 in the globular cluster Terzan 2

The bright persistent X-ray source and type I X-ray burster 1E1724-3045 located in the globular cluster Terzan 2 was observed by ASCA for about 10 ksec on September 24-25th, 1995 while it was in its hard state, with a luminosity of \~6x10^36 erg/s in the 0.5-10 keV band (d=7.7 kpc). The ASCA spectrum is hard, and reveals the presence of a soft component below 2-3 keV. When combined with non simultaneous RXTE/HEXTE data, we show that the ASCA spectrum can be adequately fitted by a Comptonization model for which the electron temperature is ~30 keV, the optical depth is ~3 for a spherical scattering cloud. The soft component carries off about 13%, and 35% of the 0.1-200 keV luminosity for a blackbody and disk blackbody fit respectively. It is more likely to arise from the accretion disk, whereas the hard Comptonized component is generated in a hot boundary layer interior to the disk. The column density measured towards the source (NH=10^22 cm^-2) is consistent with the value expected from the optical reddening of the cluster. No emission lines were detected, and an upper limit of 30 eV for the equivalent width of a 6.4 keV line (sigma=0.1 keV) has been derived.

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arXiv1998

Determination of the hot intracluster gas temperature from submillimeter measurements

Measurements of the Sunyaev-Zeldovich (hereafter SZ) distorsion of the cosmic microwave background can give interesting physical informations on clusters of galaxies, provided that the electronic temperature of the gas is known. Previous attempts to do so have used the electronic temperature determination obtained from the X-ray spectra. However, if the intergalactic gas is not homogeneous, the X-ray emission will trace the denser component, and the temperature determination may not be relevant for the lower density gas which is dominating the SZ measurements. Moreover, the X-ray brightness decreases very rapidly with the distance, which is not the case for the SZ effect. Distant clusters might be detected from SZ measurements, whereas they are inaccessible to X-ray observations. For these reasons, we have investigated the possibility to derive the electronic temperature of the gas from the SZ measurements in the submillimeter range (lambda ~ 300-600 micro-m). We show that given the sensitivities of the future submillimeter space missions Planck Surveyor and FIRST, the electronic temperature of massive clusters (Y_center=3*10^{-4}) can be determined with an accuracy ranging from 1 to 4 keV depending on its distance and the data avalaible.

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arXiv1998

RXTE Observation of the X-ray burster 1E1724-3045. I : Timing study of the persistent X-ray emission with the PCA

We report on the RXTE/PCA observation of the X-ray burster 1E1724-3045 located in the globular cluster Terzan 2. The persistent X-ray emission shows a high level of noise variability, the so-called High Frequency Noise (HFN) with a fractional Root Mean Squared (RMS) of about 25% in the 2E-3 - 40 Hz range. The strong HFN together with the hardness of its X-ray spectrum suggest that 1E1724-3045 is an ``Atoll'' source which was in its ``Island'' state during the observation. The Fourier Power Density Spectra (PDS) can be modeled in terms of the sum of two ``shot noise'' components for which the shots have a single-side exponential shape. The characteristic shot decay timescales inferred from the best fitting of the PDS are ~680 and 16 msec respectively. The two components contribute similarly to the total RMS (about 15%). The PDS contains also a third component: a broad and asymmetric peaked noise feature centered at 0.8 Hz. This Quasi Periodic Oscillation-like (QPO) feature contributes at the level of ~ 10% to the total RMS. The integrated RMS of all three components shows a positive correlation with energy up to at least 40 keV. We also show that 1E1724-3045 has striking timing similarities with the black hole candidate GRO J0422+32 (Nova Persei 1992). Our observation demonstrates that a low frequency QPO simultaneously with a high level of RMS is not a timing signature unique to black holes. This extends the growing list of similarities between Atoll sources and black hole systems.

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arXiv1997

Kilo-hertz QPO in Island State of 4U1608-52 as Observed with RXTE/PCA

We report RXTE/PCA observations of 4U 1608-52 on March 15, 18 and 22 immediately after the outburst in early 1996. The persistent count rates ranged from 190 to 450 cps (1-60 keV). During this period of time, 4U 1608-52 was in the island state. We detected QPO features in the power density spectra (PDS) at 567-800 Hz on March 15 and 22, with source fractional root-mean-square (rms) amplitude of 13%-17% and widths of 78-180 Hz. The average rms amplitude of these QPO features is positively correlated with the energy. Our results imply that the neutron star spin frequency is possibly between 300 Hz and 365 Hz.

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arXiv1997

Kilo-hertz QPO and X-ray Bursts in 4U 1608-52 in Low Intensity State

We present the results from RXTE/PCA observations of 4U 1608-52 in its island state on March 15, 18 and 22 of 1996. Three type I X-ray bursts were detected in one RXTE orbit on March 22. We observed QPO features peaking at 567-800 Hz on March 15 and 22, with source fractional rms amplitude of 13%-17% and widths of 78-180 Hz in the power density spectra averaged over each spacecraft orbit. The rms amplitudes of these QPOs are positively correlated with the photon energy. The three X-ray bursts, with burst intervals of 16 and 8 minutes, have a duration of 16s. The blackbody emission region of the smallest X-ray burst among the three suggest it was a local nuclear burning. We also discuss a type I X-ray burst candidate in the observation.

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arXiv1996

Evidence from quasi-periodic oscillations for a millisecond pulsar in the low mass x-ray binary 4U 0614+091

We have detected quasi-periodic oscillations (QPOs) near 1 kHz from the low mass X-ray binary 4U 0614+091 in observations with XTE. The observations span several months and sample the source over a large range of X-ray luminosity. In every interval QPOs are present above 400 Hz with fractional RMS amplitudes from 3 to 12%. At high count rates, two high frequency QPOs are detected simultaneously. The difference of their frequency centroids is consistent with 323 Hz in all observations. During one interval a third signal is detected at 328 +/- 2 Hz. This suggests the system has a stable `clock' which is most likely the neutron star with spin period 3.1 msec. Thus, our observations and those of another neutron star system by Strohmayer et al. (1996) provide the first evidence for millisecond pulsars within low-mass X-ray binary systems and reveal the `missing-link' between millisecond radiopulsars and the late stages of binary evolution in low mass X-ray binaries (Alpar et al. 1982). We suggest that the kinematics of the magnetospheric beat-frequency model (Alpar and Shaham 1985) applies to these QPOs. In this interpretation the high frequency signal is associated with the Keplerian frequency of the inner accretion disk and the lower frequency `beat' signal arises from the differential rotation frequency of the inner disk and the spinning neutron star. Assuming the high frequency QPO is a Keplerian orbital frequency for the accretion disk, we find a maximum mass of 1.9 solar masses and a maximum radius of 17 km for the neutron star.

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arXiv1996

Anticorrelated Hard/Soft X-Ray Emission from the X-Ray Burster 4U 0614+091

We have detected transient X-ray activity from the X-ray burster 4U~0614+091 simultaneously with BATSE/CGRO (20-100 keV) and ASM/RXTE (1-12 keV). The peak fluxes reach approximately 40 mCrab in both instruments over a period of about 20 days. The variable emission shows a clear anticorrelation of the hard X-ray flux with the soft X-ray count rate. The observed anticorrelation is another clear counterexample to the notion that only black hole binaries exhibit such correlations. The individual spectra during this period can be fit by power laws with photon indices 2.2+-0.3 (ASM) and 2.7+-0.4 (BATSE), while the combined spectra can be described by a single power law with index 2.09+-0.08. BATSE and the ASM/RXTE are a good combination for monitoring X-ray sources over a wide energy band.

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