arXiv subjects

T. Belloni

Publications and source records attributed to T. Belloni.

At least 127 records · Page 7Linked to original sources

arXiv2000

Relativistic Iron Emission and Disk Reflection in Galactic Microquasar XTE J1748-288

We report evidence for an Fe K-alpha fluorescence line feature and disk reflection in the Very High, High, and Low State X-ray spectra of the galactic microquasar XTE J1748-288 during its June~1998 outburst. Spectral analyses are made on data gathered throughout the outburst by the Rossi X-ray Timing Explorer Proportional Counter Array. Gaussian line, relativistic disk emission line, and ionized disk reflection models are fit to the data. In the Very High State the line profile appears strongly redshifted, consistent with disk emission from the innermost stable orbits around a maximally rotating Kerr black hole. In the High State the line profile is less redshifted and increasingly prominent. The Low State line profile is very strong (~0.5 keV equivalent width) and centered at 6.7 +/- 0.10 keV; disk line emission model fits indicate that the inner edge of the disk fluctuates between ~20 and ~100 gravitational radii in this state. The disk reflection fraction is traced through the outburst; reflection from an ionized disk is preferred in the VHS and HS, and reflection from a relatively neutral disk is preferred in the LS. We discuss the implications of our findings for the binary system dynamics and accretion flow geometry in XTE J1748-288.

astro-ph↗

arXiv2000

A Comptonization Model for Phase Lag Variability in GRS 1915+105

In this Letter we propose a simple thermal comptonization model to account for the observed properties of the phase lags associated to the ``plateau'' intervals of GRS 1915+105. By invoking a temperature stratification in a corona and assuming that the optical depth of the comptonizing region increases as the disk inner radius moves inward, we are able to reproduce both the observed colors and time lags in the continuum.

astro-ph↗

arXiv2000

Unusual quiescent X-ray activity from XTE J0421+560 (CI Cam)

We report on BeppoSAX observations of the X-ray transient XTE J0421+560 in quiescence 156, 541, and ~690 days after the maximum of the 1998 April outburst. In the first observation the source was soft with a power-law photon index of 4.0 (+1.9 -0.9) and absorption, NH, of (1.1 +4.9 -1.1) X 10^21 atom/cm2. In the second observation, the source brightened by a factor ~15 in the 1-10 keV energy range, became significantly harder with a photon index of 1.86 (+0.27 -0.32) and was strongly absorbed with NH = (4.0 +/- 0.8) X 10^23 atom/cm2. There is evidence for a narrow emission line in both spectra at \~7 keV. In the third observation, the source had faded by a factor ~$8 from the previous observation to below the BeppoSAX detection level. It is possible that these variations result from orbital motion of a compact object around the B[e] star companion with the intense, absorbed, spectrum arising during passage through dense circumstellar material. If this is the case, the system may be continuing to exhibit periodic activity.

astro-ph↗

arXiv2000

Disk mass accretion rate and infrared flares in GRS 1915+105

We have analyzed in detail a set of Rossi X-ray Timing Explorer (RXTE) observations of the galactic microquasar GRS 1915+105 corresponding to times when quasi-periodic oscillations in the infrared have been reported. From time-resolved spectral analysis, we have estimated the mass accretion rate through the (variable) inner edge of the accretion disk. We compare this accretion rate to an estimate of the mass/energy outflow rate in the jet. We discuss the possible implications of these results in terms of disk-instability and jet ejection, and in particular note an apparent anti-correlation between the accretion and ejection rates, implying that the gas expelled in the jet must leave the accretion disk before reaching its innermost radius.

astro-ph↗

arXiv2000

Phase lag variability associated with the 0.5-10 Hz QPO in GRS 1915+105

We have used Rossi X-ray Timing Explorer data to measure the lags between soft (2-5 keV) and hard (5-13 keV) photons and to study the aperiodic variability of the superluminal black hole candidate GRS 1915+105 during low-flux states. The power density spectra exhibit quasi-periodic oscillations (QPO) whose frequency increases with increasing count rate and varies in the frequency range 0.6-8 Hz. A correlation between the QPO frequency and the phase lag spectra is reported for the first time. This correlation is found for both the phase lag continuum and the phase lag at the QPO frequency. We find that as the QPO frequency moves to higher values the phase lags reverse sign from positive to negative. The absolute value of the lag always increases with photon energy. The negative (soft) lags are associated with a softer energy spectrum, whereas the positive (hard) lags are seen when the source is harder. We describe a possible scenario that may account for the change in the sign of the lags.

astro-ph↗

arXiv2000

X-ray/optical observations of XTE J0421+560/CI Cam in quiescence

We report on a BeppoSAX observation of the transient X-ray source XTE J0421+560 during quiescence performed ~150 days after the 1998 April outburst. The source had an unabsorbed 0.5-10 keV flux of 6.7x10^{-12} erg/cm2/s and was still remarkably soft with most of the emission below 2 keV. The X-ray spectrum can be fit with the same two-temperature model as the outburst observations. There is evidence for the presence of an Iron emission feature at ~7 keV. We report also on a series of optical observations performed using the 72 cm Teramo-Normale Telescope (TNT) of the Teramo Observatory, and the 1.5 m Loiano Telescope of the Bologna Observatory. The optical spectrum includes very strong Balmer emission lines, He I, [Fe II], and [O I] features. From the observed L_x/Lopt~10^{-3} the quiescent optical emission cannot be due to re-processing of the X-rays, but has to be generated in the optical companion or the circumstellar material. Moreover, the quiescent X-ray luminosity cannot be due to the optical star if it is of spectral type OB. Although the nature of the compact object present in the XTE J0421+560/CI Cam system cannot be firmly established, we speculate that it is most probably a white dwarf.

astro-ph↗

arXiv2000

A model-independent analysis of the variability of GRS 1915+105

We analyzed 163 observations of the microquasar GRS 1915+105 made with the Rossi X-ray Timing Explorer (RXTE) in the period 1996-1997. For each observation, we produced light curves and color-color diagrams. We classified the observations in 12 separate classes, based on their count rate and color characteristics. From the analysis of these classes, we reduced the variability of the source to transitions between three basic states: a hard state corresponding to the non-observability of the innermost parts of the accretion disk, and two softer states with a fully observable disk. These two soft states represent different temperatures of the accretion disk, related to different local values of the accretion rate. The transitions between these states can be extremely fast. The source moves between these three states following certain patterns and avoiding others, giving rise to a relatively large but limited number of variability classes. These results are the first step towards a linking of the properties of this exceptional source with standard black-hole systems and with accretion disk models.

astro-ph↗

arXiv1999

The Timing Evolution of 4U 1630-47 During its 1998 Outburst

We report on the evolution of the timing of 4U1630-47 during its 1998 outburst using data obtained with the Rossi X-ray Timing Explorer (RXTE). The count rate and position in hardness-intensity, color-color diagrams and simple spectral fits are used to track the concurrent spectral changes. The source showed seven distinct types of timing behavior, most of which show differences with the canonical black hole spectral/timing states. In marked contrast to previous outbursts, we find quasi-periodic oscillation (QPO) signals during nearly all stages of the outburst with frequencies between 0.06Hz and 14Hz and a remarkable variety of other characteristics. In particular we find large (up to 23% rms) amplitude QPO on the early rise. Later, slow 0.1Hz semi- regular short (~ 5 sec), 9 to 16% deep dips dominate the light curve. At this time there are two QPOs, one stable near 13.5Hz and the other whose frequency drops from 6--8Hz to ~4.5 Hz during the dips. BeppoSAX observations during the very late declining phase show 4U1630-47 in a low state.

astro-ph↗

arXiv1999

Linear and nonlinear time series analysis of the black hole candidate Cygnus X-1

We analyze the variability in the X-ray lightcurves of the black hole candidate Cygnus X-1 by linear and nonlinear time series analysis methods. While a linear model describes the over-all second order properties of the observed data well, surrogate data analysis reveals a significant deviation from linearity. We discuss the relation between shot noise models usually applied to analyze these data and linear stochastic autoregressive models. We debate statistical and interpretational issues of surrogate data testing for the present context. Finally, we suggest a combination of tools from linear andnonlinear time series analysis methods as a procedure to test the predictions of astrophysical models on observed data.

chao-dyn↗

arXiv1999

Inner accretion disk disappearance during a radio flare in GRS 1915+105

Simultaneous X-ray (BeppoSAX, 0.1-300 keV) and radio (Ryle Telescope, 15 GHz) observations of the superluminal galactic source GRS 1915+105 in high flux state were carried out starting October 1996. Here we present the simultaneous radio/X-ray light curves of the flare event and the spectral and timing analysis of the relevant BeppoSAX data. We obtain evidence for a temporary disappearance and subsequent restoring of the inner portion of the accretion disk during the flare from the spectral analysis of the X-ray data. We find a variable power law photon index positively correlated to the variability of the exponential cut-off energy. In addition, frequency and intensity variations of a $\sim$4-5~Hz quasi-periodic oscillation (QPO) are detected during the same time interval. The centroid frequency of the QPO is found to be correlated to the spectral power law photon index, and to the X-ray flux from the disk and the high energy flux from the power law. These observational results are discussed in the framework of the disk-instability model proposed for this source.

astro-ph↗

arXiv1999

On the nature of XTE J0421+560/CI Cam

We present the results of the analysis of RXTE, BATSE and optical/infrared data of the 1998 outburst of the X-ray transient system XTE J0421+560 (CI Cam). The X-ray outburst shows a very fast decay (initial e-folding time ~0.5 days, slowing down to ~2.3 days). The X-ray spectrum in the 2-25 keV band is complex, softening considerably during decay and with strongly variable intrinsic absorption. A strong iron emission line is observed. No fast time variability is detected (<0.5 % rms in the 1-4096 Hz band at the outburst peak). The analysis of the optical/IR data suggests that the secondary is a B[e] star surrounded by cool dust and places the system at a distance of >~ 2 kpc. At this distance the peak 2-25 keV luminosity is ~4 x 10^37 erg/s. We compare the properties of this peculiar system with those of the Be/NS LMC transient A 0538-66 and suggest that CI Cam is of similar nature. The presence of strong radio emission during outburst indicates that the compact object is likely to be a black hole or a weakly magnetized neutron star.

astro-ph↗

arXiv1999

A state transition of GX 339-4 observed with RXTE

We report the results of observations with the Rossi X-ray Timing Explorer of the black-hole candidate GX 339-4 during the state transition of 1998. We find that both the X-ray spectrum and the characteristic of the time variability after the transition are typical of a high/soft state. We cannot exclude that the source went through an Intermediate State before entering the high state. We discuss the results and compare them with other known black hole candidates.

astro-ph↗

arXiv1999

The Nuclear Spectral Energy Distribution of NGC 4395, The Least Luminous Type 1 Seyfert Galaxy

We present X-ray (ROSAT), infrared, and radio observations of NGC 4395, which harbors the optically least luminous type 1 Seyfert nucleus discovered thus far. In combination with published optical and ultraviolet spectra, we have used these data to assemble the broadband spectral energy distribution (SED) of the galaxy's nucleus. Interestingly, the SED of NGC 4395 differs markedly from the SEDs of both quasars and typical low-luminosity active galactic nuclei, which may be a manifestation of the different physical conditions (i.e., black hole masses, accretion rates, and/or accretion modes) that exist in these objects. The nuclear X-ray source in NGC 4395 is variable and has an observed luminosity of just ~ 10^38 ergs/s. Although this emission could plausibly be associated with either a weak active nucleus or a bright stellar-mass binary system, the optical and ultraviolet emission-line properties of the nucleus strongly suggest that the X-rays arise from a classical AGN.

astro-ph↗

arXiv1999

BeppoSAX observations of the black hole candidates LMC X-1 and LMC X-3

We describe BeppoSAX observations of the black hole candidates LMC X--1 and LMC X--3 performed in Oct. 1997. Both sources can be modelled by a multicolor accretion disk spectrum, with temperature $\sim 1$ keV. However, there is some evidence that a thin emitting component coexists with the thick disk at these temperatures. In the direction of LMC X--1, we detected a significant emission above 10 keV, which we suspect originates from the nearby source PSR 0540-69. For LMC X--1, we estimate an absorbing column density of $\simeq 6\times 10^{21}$ cm$^{-2}$, which is almost ten times larger than that found for LMC X--3. In both sources, we find no indication of emission or absorption features whatsoever.

astro-ph↗

arXiv1998

High-Energy Observations of the Binary Millisecond Pulsar PSR J0218+4232

We report the detection of pulsed X-ray emission ($4.9σ$) from the binary millisecond pulsar PSR J0218+4232 in a 100 ks ROSAT HRI observation. The pulse profile shows a sharp main pulse and an indication for a second weaker pulse at $\sim$ 0.47 phase separation. The pulsed fraction is 37 \pm 13%. PSR J0218+4232 was several times in the field of view of the high-energy $γ$-ray telescope EGRET and a source positionally consistent with the pulsar was detected above 100 MeV. Spatial and timing analyses of EGRET data indicate that the source is probably multiple: Between 0.1 GeV and 1 GeV PSR J0218+4232 is the most likely counterpart, while the BL Lac 3C66A is the best candidate above 1 GeV. If part of the EGRET signal truly belongs to the pulsar, then this would be the first millisecond $γ$-ray pulsar.

astro-ph↗

arXiv1998

The 1998 outburst of the X-ray transient 4U 1630-47 observed with BeppoSAX

We report results of five pointed BeppoSAX observations of the black hole candidate 4U 1630-47 during part of its 1998 outburst when the 2-10 keV source luminosity decreased from 2.1x10^38 erg/s (for an assumed distance of 10 kpc) by a factor of ~7. The 2-200 keV spectrum can be modeled by a soft multi-temperature disk blackbody and a hard power-law. During four of the observations, there is evidence for deviations from a pure power-law shape >~20 keV which may be due to reflection. As the outburst decayed a number of spectral trends are evident: (1) the amount of photoelectric absorption decreased, (2) the spectrum became harder (i.e. the power-law index became smaller), (3) the temperature of the disk blackbody decreased and (4) the inferred inner disk radius increased by a factor of ~4. The change in the accretion disk inner radius during the outburst is in contrast to results from other soft X-ray transients. We find that the disk blackbody temperature depends on the inner disk radius roughly as T=r^(-3/4). For a standard Shakura-Sunyaev disk model this implies that the inferred mass accretion rate at the inner disk radius did not change appreciably during the part of the outburst observed by BeppoSAX.

astro-ph↗

arXiv1998

X rays from old open clusters: M 67 and NGC 188

We have observed the old open clusters M 67 and NGC 188 with the ROSAT PSPC. In M 67 we detect a variety of X-ray sources. The X-ray emission by a cataclysmic variable, a single hot white dwarf, two contact binaries, and some RS CVn systems is as expected. The X-ray emission by two binaries located below the subgiant branch in the Hertzsprung Russell diagram of the cluster, by a circular binary with a cool white dwarf, and by two eccentric binaries with orbital period > 700 d is puzzling. Two members of NGC 188 are detected, including the FK Com type star D719. Another possible FK Com type star, probably not a member of NGC 188, is also detected.

astro-ph↗

arXiv1998

Optical Identification of ROSAT sources in M 67: activity in an old cluster

We present optical identification and high-resolution spectroscopy of ROSAT sources in the field of the old open cluster M 67. For the first time it is possible to analyze coronal and chromospheric activity of active stars in a solar-age cluster, and to compare it with field stars. ROSAT observed the high X-ray luminosity tail of the cluster sources. In agreement with what expected from studies of field stars, most of the detected X-ray sources are binaries, preferably with short periods and eccentric orbits. In addition, several of the M 67 ROSAT sources have peculiar locations in the cluster colour-magnitude diagram. This is most likely due to rather complex evolutionary histories, involving the presence of mass transfer or large mass losses. The X-ray luminosity of the sources does not scale with the stellar parameters in an obvious way. In particular, no relationship is found between coronal emission and stellar magnitude or binary period. The Ca II K chromospheric flux from most of the counterparts is in excess to that of single stars in the cluster by one order of magnitude. The X-ray luminosity of the sources in the old M 67 is one order of magnitude lower than the most active active binaries in the field, but comparable to that of the much younger binaries in the Hyades.

astro-ph↗