Probing X-ray burst -- accretion disk interaction in low mass X-ray binaries through kilohertz quasiperiodic oscillations

The intense radiation flux of Type I X-ray bursts is expected to interact with the accretion flow around neutron stars. High frequency quasiperiodic oscillations (kHz QPOs), observed at frequencies matching orbital frequencies at tens of gravitational radii, offer a unique probe of the innermost disk regions. In this paper, we follow the lower kHz QPOs, in response to Type I X-ray bursts, in two prototypical QPO sources, namely 4U 1636-536 and 4U 1608-522, as observed by the Proportional Counter Array of the Rossi X-ray Timing Explorer. We have selected a sample of 15 bursts for which the kHz QPO frequency can be tracked on timescales commensurable with the burst durations (tens of seconds). We find evidence that the QPOs are affected for over ~200 s during one exceptionally long burst and ~100 s during two others (although at a less significant level), while the burst emission has already decayed to a level that would enable the pre-burst QPO to be detected. On the other hand, for most of our burst-kHz QPO sample, we show that the QPO is detected as soon as the statistics allow and in the best cases, we are able to set an upper limit of ~20 s on the recovery time of the QPO. This diversity of behavior cannot be related to differences in burst peak luminosity. We discuss these results in the framework of recent findings that accretion onto the neutron star may be enhanced during Type I X-ray bursts. The subsequent disk depletion could explain the disappearance of the QPO for ~100 s, as possibly observed in two events. However, alternative scenarios would have to be invoked for explaining the short recovery timescales inferred from most bursts. Clearly the combination of fast timing and spectral information of Type I X-ray bursts holds great potential in the study of the dynamics of the inner accretion flow around neutron stars.Comment: 8 pages, 9 figures, appears in Astronomy & Astrophysics, Volume 567, id.A80, published 07/201

ECLAIRs: A microsatellite for the prompt optical and X-ray emission of Gamma-Ray Bursts

The prompt gamma-ray emission of Gamma-Ray Bursts (GRBs) is currently interpreted in terms of radiation from electrons accelerated in internal shocks in a relativistic fireball. On the other hand, the origin of the prompt (and early afterglow) optical and X-ray emission is still debated, mostly because very few data exist for comparison with theoretical predictions. It is however commonly agreed that this emission hides important clues on the GRB physics and can be used to constrain the fireball parameters, the acceleration and emission processes and to probe the surroundings of the GRBs. ECLAIRs is a microsatellite devoted to the observation of the prompt optical and X-ray emission of GRBs. For about 150 GRBs/yr, independent of their duration, ECLAIRs will provide high time resolution high sensitivity spectral coverage from a few eV up to ~50 keV and localization to ~ 5'' in near real time. This capability is achieved by combining wide field optical and X-ray cameras sharing a common field of view (>~ 2.2 steradians) with the coded-mask imaging telescopes providing the triggers and the coarse localizations of the bursts. Given the delays to start ground-based observations in response to a GRB trigger, ECLAIRs is unique in its ability to observe the early phases (the first ~20 sec) of all GRBs at optical wavelengths. Furthermore, with its mode of operation, ECLAIRs will enable to search for optical and X-ray precursors expected from theoretical grounds. Finally ECLAIRsis proposed to operate simultaneously with GLAST on a similar orbit. This combination will both provide unprecedented spectral coverage from a few eV up to ~200 GeV for ~100 GRBs/yr, as well as accurate localization of the GLAST GRBs to enable follow-up studies

Simultaneous BeppoSAX and Rossi X-ray Timing Explorer observations of 4U1812-12

4U1812-12 is a faint persistent and weakly variable neutron star X-ray binary. It was observed by BeppoSAX between April 20th and 21st, 2000 in a hard spectral state with a bolometric luminosity of ~2x10^36 ergs/s. Its broad band energy spectrum is characterized by the presence of a hard X-ray tail extending above ~100 keV. It can be represented as the sum of a dominant hard Comptonized component (electron temperature of ~36 keV and optical depth ~3) and a weak soft component. The latter component which can be fitted with a blackbody of about 0.6 keV and equivalent radius of ~2 km is likely to originate from the neutron star surface. We also report on the first measurement of the power density spectrum of the source rapid X-ray variability, as recorded during a simultaneous snapshot observation performed by the Rossi X-ray Timing Explorer. As expected for a neutron star system in such hard spectral state, its power density spectrum is characterized by the presence of a ~0.7 Hz low frequency quasi-periodic oscillation together with three broad noise components, one of which extends above ~200 Hz.Comment: 6 pages, 3 figures, accepted for publication in A&

X-ray sources and their optical counterparts in the globular cluster M 22

Using XMM-Newton EPIC imaging data, we have detected 50 low-luminosity X-ray sources in the field of view of M 22, where 5 +/- 3 of these sources are likely to be related to the cluster. Using differential optical photometry, we have identified probable counterparts to those sources belonging to the cluster. Using X-ray spectroscopic and timing studies, supported by the optical colours, we propose that the most central X-ray sources in the cluster are cataclysmic variables, millisecond pulsars, active binaries and a blue straggler. We also identify a cluster of galaxies behind this globular cluster.Comment: 11 pages, 7 figures, accepted for publication in A&

Simultaneous BeppoSAX and RXTE observations of the X-ray burst sources GX 3+1 and Ser X-1

We have obtained spectral and timing data on GX 3+1 and Ser X-1. Both sources were observed simultaneously with BeppoSAX and RXTE. The RXTE data is used to provide power spectra and colour-colour diagrams in order to constrain the state (and thus track $\dot M$) the sources are in. The BeppoSAX data provide the broad-band spectra. The spectra of both sources are reasonably well-fit using a model consisting of a disk-blackbody, a comptonized component and a Fe line, absorbed by interstellar absorption. The electron temperature (kT$_{\rm e}$) of the Comptonizing plasma is in both cases $\sim$2.5 keV. This implies that no strong high-energy tail from the Comptonized component is present in either of the sources. We discuss the similarities between these burst sources and the luminous X-ray sources located in globular clusters. We find that the spectral parameters of the comptonized component provide information about the mass-accretion rate, which agrees well with estimates from the timing and spectral variations.Comment: 8 pages, accepted by A&

On the high coherence of kilo-Hz Quasi-Periodic Oscillations

We have carried out a systematic study of the properties of the kilo-Hertz quasi-periodic oscillations (QPO) observed in the X-ray emission of the neutron star low-mass X-ray binary 4U1608-52, using archival data obtained with the Rossi X-ray Timing Explorer. We have investigated the quality factor, Q, of the oscillations (defined as the ratio of the frequency of the QPO peak to its full width at half maximum). In order to minimise the effect of long-term frequency drifts, power spectra were computed over the shortest times permitted by the data statistics. We show that the high Q of ~200 reported by Berger et al. (1996) for the lower frequency kilo-Hz QPO in one of their observations is by no means exceptional, as we observe a mean Q value in excess of 150 in 14 out of the 21 observations analysed and Q can remain above 200 for thousands of seconds. The frequency of the QPO varies over the wide range 560--890 Hz and we find a systematic trend for the coherence time of the QPO, estimated as tau=Q /(pi nu), to increase with the frequency, up to a maximum level at ~ 800 Hz, beyond which it appears to decrease, at frequencies where the QPO weakens. There is a more complex relationship between tau and the QPO root mean squared amplitude (RMS), in which positive and negative correlations can be found. A higher-frequency QPO, revealed by correcting for the frequency drift of the 560-890 Hz one, has a much lower Q (~10) which does not follow the same pattern. We discuss these results in the framework of competing QPO models and show that those involving clumps orbiting within or above the accretion disk are ruled out.Comment: Accepted for publication in MNRAS, 8 pages, 6 figures, 3 Table