arXiv subjects

T. Belloni

Publications and source records attributed to T. Belloni.

139 records · Page 8Linked to original sources

arXiv1998

Soft X-ray emission from intermediate-age open clusters: IC 4651

We present the results of soft X-ray observations of the intermediate-age open cluster IC 4651 performed with the ROSAT PSPC. We detected 25 sources. Two are identified with a giant binary and a blue straggler respectively, both belonging to the cluster, and two with probable main-sequence members. Two other cases have ambiguous identification. Of the five binaries known in the cluster, the one detected in X rays is the only one whose period is short enough to maintain fast rotation and therefore strong stellar activity at a high age. The detected blue straggler is probably binary, suggesting that binarity is the key to producing a high level of X-ray emission. It is the third blue straggler detected in X rays. The remaining sources need to be identified through optical follow-up.

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arXiv1998

Aquila X-1 from outburst to quiescence: the onset of the propeller effect and signs of a turned-on rotation-powered pulsar

We report on the March-April 1997 BeppoSAX observations of Aql X-1, the first to monitor the evolution of the spectral and time variability properties of a neutron star soft X-ray transient from the outburst decay to quiescence. We observed a fast X-ray flux decay, which brought the source luminosity from ~10^36 to ~10^{33} erg/s in less than 10 days. The X-ray spectrum showed a power law high energy tail with photon index Gamma~2 which hardened to Gamma~1-1.5 as the source reached quiescence. These observations, together with the detection by RossiXTE of a periodicity of a few milliseconds during an X-ray burst, likely indicate that the rapid flux decay is caused by the onset of the propeller effect arising from the very fast rotation of the neutron star magnetosphere. The X-ray luminosity and hard spectrum that characterise the quiescent emission can be consistently interpreted as shock emission by a turned-on rotation-powered pulsar.

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arXiv1998

The X-ray emission of the polar BL Hydri

We report on the analysis of the ASCA and BeppoSAX X-ray observations of the polar system BL Hyi. Emission from both poles is apparent in both observations; the emission from the second pole varies from cycle to cycle, indicating non-stationary accretion there. The temperature of the post-shock region is estimated to be about 10 keV. Inclusion of both complex absorption and Compton reflection significantly improves the quality of the fit. No soft X-ray component is observed; the BeppoSAX/LECS upper limit to the soft component is in agreement with theoretical expectations for this low magnetic field system.

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arXiv1998

BeppoSAX observations of the long period polar system V1309 Ori

We present BeppoSAX observations of the peculiar long period polar system V1309 Ori (RXJ0515.6+0105). The source was detected simultaneously at soft and, for the first time, at hard X-rays with the LECS and the MECS detectors. Both, the LECS and the MECS light curves are irregular with a bursting/flaring type behaviour indicating inhomogeneous accretion onto the white dwarf. This peculiar variability, together with an extreme high soft-to-hard X-ray luminosity ratio, indicates that in V1309 Ori accretion occurs predominantly in highly compressed chunks or ``blobs'' of matter. From coordinated ESO optical spectroscopy, we find indications that the magnetic field strength of the white dwarf is < 70 MG, not expected either from the 8 hr orbital period synchronism or from the strong soft-to-hard X-ray ratio suggesting alternative solutions for sustaining synchronism in this system.

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arXiv1997

BeppoSAX observations of the exotic black hole candidate GX 339-4

GX 339-4 has been observed by BeppoSAX twice in spring 1997 as part of a longer monitoring program. The source was close to the highest levels (50 mCrab) of the extended low state (as measured by the XTE ASM during the last 2 years). Its spectrum was quite hard, similar to the Exosat 1984 off state, but 40 times stronger. The source is detected up to more than 120 keV, enabling the possibility to study its high energy spectrum

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arXiv1997

The viscous disk of GRS 1915+105

GRS 1915+105, one of the two known galactic microquasars, shows an extremely complex variability in the X-ray band, comparable to no other X-ray source in the sky. Making use of RXTE/PCA data, we have analyzed the X-ray spectral distribution throughout the variability. We find that all variations can be attributed to the rapid appearance and disappearance of the inner region of an optically-thick accretion disk. Since the time scale for each event is related to the maximum radius of the disappearing region, the difference in time structure is due to the time distribution of such radii. The observed relation between the extent of the missing inner region of the disk and the duration of an event is in remarkable agreement with the expected radius dependence of the viscous time scale of the radiation-dominated region of an accretion disk.

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arXiv1997

The broad-band X-ray spectrum of RE 0751+14 (PQ Gem)

RE 0751+14 is a member of the class of Soft Intermediate Polars. Unlike classical Intermediate Polars, they are characterized by the presence of a soft component in their X-ray spectra, in addition to the hard component typical of these systems. The broad-band coverage of the Narrow-Field instruments on board BeppoSAX is ideal for the study of such objects. Here we report the preliminary results of the analysis of a BeppoSAX pointing to RE 0751+14. Both components are clearly detected, and their temperature determined (~50 eV and ~40 keV), enabling the possibility of studying them simultaneously.

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arXiv1997

Thermal-viscous instabilities in the accretion disk of GRS 1915+105

We use data obtained with the PCA onboard RXTE to analyze the spectral variations of the black hole candidate and superluminal source GRS 1915+105. Our results suggest that despite the complicated structure of the X-ray light curve, all spectral changes observed in this source can be explained as a sudden disappearing of the inner part of the accretion disk around the compact object, followed by a slower re-filling of the emptied region. The duration of an event and the size of the disappearing region fit remarkably well the expected radius dependence of the viscous time scale for the radiation-pressure dominated region of an accretion disk.

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arXiv1997

A unified model for the spectral variability in GRS 1915+105

We have analyzed the spectral variations of the superluminal black-hole X-ray binary GRS 1915+105 by using data obtained with the PCA on the Rossi XTE. We find that, despite the marked differences in the structure and the time scale of variability, all spectral changes can be attributed to the rapid disappearing of the inner region of an accretion disk, followed by a slower re-filling of the emptied region. The time scale for each event is determined by the extent of the missing part of the disk. The observed relation between the duration of an event and the radius of the disappearing region matches remarkably well the expected radius dependence of the viscous time scale for the radiation-pressure dominated region of an accretion disk.

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arXiv1997

An unstable central disk in the superluminal black-hole X-ray binary GRS 1915+105

We have analyzed the X-ray spectra of the microquasar GRS 1915+105, as observed with the PCA on the Rossi XTE, during periods of stable weak emission, outbursts and rapid flaring. We find that the complicated X-ray intensity curve of this source can be described by the rapid removal and replenishment of matter forming the inner part of an optically thick accretion disk, probably caused by a thermal-viscous instability analogous to that operating in dwarf novae, but here driven by the Lightman-Eardley instability. We find that the mass accretion rate in quiescence is 10^-6 solar masses per year. Only a small fraction of the energy liberated by accretion is emitted as radiation. We suggest that most of this energy is advected into the black hole in the high-viscosity state of the outburst cycle.

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arXiv1996

ASCA observations of the galactic bulge hard x-ray source GRS 1758--258

GRS 1758-258 is one of the few persistent hard X-ray emitters (E>100 keV) in the Galaxy. Using the ASCA satellite, we have obtained the first detailed data on GRS 1758-258 in the 1-10 keV range, where previous observations were affected by confusion problems caused by the nearby strong source GX5-1. The spectrum is well described by a power law with photon index 1.7 without strong Fe emission lines. A prominent soft excess, as observed with ROSAT when the hard X-ray flux was in a lower intensity state, was not detected. However, the presence of a soft spectral component, accounting for at most 5% of the 0.1-300 keV flux, cannot be excluded. The accurate measurement of interstellar absorption (N_H=(1.5+-0.1) x 10^22 cm -2) corresponds to an optical extinction which definitely excludes the presence of a massive companion.

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arXiv1995

Identification of the soft X-ray excess in Cygnus X-1 with disc emission

We present results of a study of the soft X-ray excess in the black hole candidate Cygnus X-1 made with the {\it Rosat PSPC}, using observations taken during persistent emission at orbital phases close to 0.5. The soft excess can be well fitted as a blackbody with temperature $\rm {kT_{bb}}$ = $\rm {0.13\pm 0.02}$ keV. $\rm {kT_{bb}}$ did not vary appreciably with intensity of the source. By assuming that the distance of the source is its lower limit of 2.5 kpc, a luminosity of the soft excess of $\rm {4.7\cdot 10^{36}}$ erg $\rm {s^{-1}}$ was obtained. From this, disc temperatures were calculated as a function of radius, assuming the compact object to be a 10 $\rm{M_{\sun}}$ black hole, in particular, the temperature at 7 Schwarzschild radii expected to be highly representative of the total disc emission. This was found to be 0.13 keV, in very good agreement with the spectral fitting result. This good agreement strongly supports the identification of the soft excess with emission from the disc around a black hole.

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arXiv1995

X-ray emission from the PSR B1259--63 system near apastron

The PSR B1259--63 system contains a 47 ms radio pulsar in a highly eccentric binary with a Be-star companion. Strongly time variable X-ray emission was reported from this system as the pulsar was near apastron in 1992-early 1993. The variability was primarily deduced from an apparent non-detection of the \psr system during a first pre-apastron \ros observation in February~1992. We have re-analyzed the \ros observations of the \psr system. Contrary to the results of a previous analysis, we find that the \psr system was detected by \ros during the first off-axis February~1992 observation. The intensity of the soft X-ray emission of the \psr system before and after the 1992 apastron appears to vary at most by a factor $\sim 2$. Our results sensibly constrain theoretical models of X-ray emission from the \psr system.

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