Revisiting a fundamental test of the disc instability model for X-ray binaries

We revisit a core prediction of the disc instability model (DIM) applied to X-ray binaries. The model predicts the existence of a critical mass transfer rate, which depends on disc size, separating transient and persistent systems. We therefore selected a sample of 52 persistent and transient neutron star and black hole X-ray binaries and verified if observed persistent (transient) systems do lie in the appropriate stable (unstable) region of parameter space predicted by the model. We find that, despite the significant uncertainties inherent to these kinds of studies, the data are in very good agreement with the theoretical expectations. We then discuss some individual cases that do not clearly fit into this main conclusion. Finally, we introduce the transientness parameter as a measure of the activity of a source and show a clear trend of the average outburst recurrence time to decrease with transientness in agreement with the DIM predictions. We therefore conclude that, despite difficulties in reproducing the complex details of the lightcurves, the DIM succeeds to explain the global behaviour of X-ray binaries averaged over a long enough period of time.Comment: 12 pages, 4 figures. Accepted for publication in MNRAS. Version 2: some typos corrected and references adde

Radio and X-ray observations during the outburst decay of the Black Hole Candidate XTE J1908+094

Obtaining simultaneous radio and X-ray data during the outburst decay of soft X-ray transients is a potentially important tool to study the disc - jet connection. Here we report results of the analysis of (nearly) simultaneous radio (VLA or WSRT) and Chandra X-ray observations of XTE J1908+094 during the last part of the decay of the source after an outburst. The limit on the index of a radio - X-ray correlation we find is consistent with the value of 0.7 which was found for other black hole candidates in the low/hard state. Interestingly, the limit we find seems more consistent with a value of 1.4 which was recently shown to be typical for radiatively efficient accretion flow models. We further show that when the correlation-index is the same for two sources one can use the differences in normalisation in the radio - X-ray flux correlation to estimate the distance towards the sources if the distance of one of them is accurately known (assuming black hole spin and mass and jet Lorentz factor differences are unimportant or minimal). Finally, we observed a strong increase in the rate of decay of the X-ray flux. Between March 23, 2003 and April 19, 2003 the X-ray flux decayed with a factor ~5 whereas between April 19, 2003 and May 13, 2003, the X-ray flux decreased by a factor ~750. The source (0.5-10 keV) luminosity at the last Chandra observation was L~3x10^32 (d/8.5 kpc)^2 erg s^-1.Comment: 7 pages, 3 figures, accepted for publication by MNRA

A population of isolated hard X-ray sources near the supernova remnant Kes 69

Recent X-ray observations of the supernova remnant IC443 interacting with molecular clouds have shown the presence of a new population of hard X-ray sources related to the remnant itself, which has been interpreted in terms of fast ejecta fragment propagating inside the dense environment. Prompted by these studies, we have obtained a deep {\sl XMM-Newton} observation of the supernova remnant (SNR) Kes 69, which also shows signs of shock-cloud interaction. We report on the detection of 18 hard X-ray sources in the field of Kes 69, a significant excess of the expected galactic source population in the field, spatially correlated with CO emission from the cloud in the remnant environment. The spectra of 3 of the 18 sources can be described as hard power laws with photon index <2 plus line emission associated to K-shell transitions. We discuss the two most promising scenarios for the interpretation of the sources, namely fast ejecta fragments (as in IC443) and cataclysmic variables. While most of the observational evidences are consistent with the former interpretation, we cannot rule out the latter.Comment: 9 pages, 5 figures, A&A in pres

Quiescent X-ray variability from the neutron star transient Aql X-1

A number of studies have revealed variability from neutron star low-mass X-ray binaries during quiescence. Such variability is not well characterised, or understood, but may be a common property that has been missed due to lack of multiple observations. One such source where variability has been observed is Aql X-1. Here, we analyse 14 Chandra and XMM-Newton observations of Aql X-1 in quiescence, covering a period of approximately 2 years. There is clear variability between the epochs, with the most striking feature being a flare-like increase in the flux by a factor of 5. Spectral fitting is inconclusive as to whether the power-law and/or thermal component is variable. We suggest that the variability and flare-like behaviour during quiescence is due to accretion at low rates which might reach the neutron star surface.Comment: 8 pages, 5 figures, accepted for publication in MNRA

Constraining Jet/Disk Geometry and Radiative Processes in Stellar Black Holes XTE J1118+480 and GX 339-4

We present results from modeling of quasi-simultaneous broad band (radio through X-ray) observations of the galactic stellar black hole (BH) transient X-ray binary (XRB) systems XTE J1118+480 and GX 339-4 using an irradiated disc + compact jet model. In addition to quantifying the physical properties of the jet, we have developed a new irradiated disc model which also constrains the geometry and temperature of the outer accretion disc by assuming a disc heated by viscous energy release and X-ray irradiation from the inner regions. For the source XTE J1118+480, which has better spectral coverage of the two in optical and near-IR (OIR) wavelengths, we show that the entire broad band continuum can be well described by an outflow-dominated model + an irradiated disc. The best-fit radius of the outer edge of the disc is consistent with the Roche lobe geometry of the system, and the temperature of the outer edge of the accretion disc is similar to those found for other XRBs. Irradiation of the disc by the jet is found to be negligible for this source. For GX 339-4, the entire continuum is well described by the jet-dominated model only, with no disc component required. For the two XRBs, which have very different physical and orbital parameters and were in different accretion states during the observations, the sizes of the jet base are similar and both seem to prefer a high fraction of non-thermal electrons in the acceleration/shock region and a magnetically dominated plasma in the jet. These results, along with recent similar results from modeling other galactic XRBs and AGNs, may suggest an inherent unity in diversity in the geometric and radiative properties of compact jets from accreting black holes.Comment: Accepted for publication in MNRAS. Uses mn2e.cls. 17 pages, 4 figures, 3 table

An intense state of hard X-ray emission of Cyg X-1 observed by INTEGRAL coincident with TeV measurements

We present INTEGRAL light curves and spectra of the black-hole binary Cyg X-1 during a bright event that occurred in 2006 September, and which was simultaneous with a detection at 0.15-1 TeV energies by the MAGIC telescope. We analyse the hard X-ray emission from 18 to 700 keV with the INTEGRAL data taken on 2006 September 24-26. We present the light curves and fit the high energy spectrum with various spectral models. Despite variations in the flux by a factor of ~2 in the the 20-700 keV energy band, the shape of the energy spectrum remained remarkably stable. It is very well represented by an e-folded power law with the photon index of 1.4 and a high energy cut-off around 130-140 keV. The spectrum is also well described by thermal Comptonisation including a moderate reflection component, with a reflectionamplitude R around 0.4. The temperature of the hot Comptonising electrons is ~70 keV and their Thomson optical depth is ~2.5. These spectral properties are typical of those observed in the low/hard state. This shows that Cyg X-1 may stay in the low hard state at least up to the flux level of 2 Crab, which corresponds to ~2-3 percent of the Eddington luminosity. It is the first time a persistent high-mass black-hole binary is observed at a few percent of the Eddington luminosity with a stable low/hard state spectrum over a period of a few days. The TeV detection coincides with the peak of a small X-ray flare just after a very fast rise in hard X-ray flux. In contrast, the source remained undetected by MAGIC at the peak of a larger X-ray flare occurring one day later and corresponding to the maximum of the X-ray luminosity of the whole outburst. We do not find any obvious correlation between the X-ray and TeV emission.Comment: 8 pages, 8 figures, accepted for publication in Astronomy and Astrophysics (major changes following the referee's comments

Thermal emission from the stellar-mass black hole binary XTE J1118+480 in the low/hard state

We report on the detection of a thermal-disk component from the stellar-mass black hole binary XTE J1118+480 in the canonical low/hard state. The presence of a thermal component with a temperature of approximately 0.21keV in the Chandra spectra of XTE J1118+480 is found at more than the 14 sigma confidence level. Based on this evidence we argue that the accretion disk in XTE J1118+480 is not truncated far from the central black hole in contrast with previous claims.Comment: Accepted for publication in MNRAS, 5 pages, 5 figure

Investigating the disc-jet coupling in accreting compact objects using the black hole candidate Swift J1753.5-0127

In studies of accreting black holes in binary systems, empirical relations have been proposed to quantify the coupling between accretion processes and ejection mechanisms. These processes are probed respectively by means of X-ray and radio/optical-infrared observations. The relations predict, given certain accretion conditions, the expected energy output in the form of a jet. We investigated this coupling by studying the black hole candidate Swift J1753.5-0127, via multiwavelength coordinated observations over a period of ~4 years. We present the results of our campaign showing that, all along the outburst, the source features a jet that is fainter than expected from the empirical correlation between the radio and the X-ray luminosities in hard spectral state. Because the jet is so weak in this system the near-infrared emission is, unusually for this state and luminosity, dominated by thermal emission from the accretion disc. We briefly discuss the importance and the implications of a precise determination of both the slope and the normalisation of the correlations, listing some possible parameters that broadband jet models should take into account to explain the population of sources characterized by a dim jet. We also investigate whether our data can give any hint about the nature of the compact object in the system, since its mass has not been dynamically measured.Comment: Accepted for publication in MNRA

Multiwavelength Observations of LS I +61 303 with VERITAS, Swift and RXTE

We present results from a long-term monitoring campaign on the TeV binary LSI +61 303 with VERITAS at energies above 500 GeV, and in the 2-10 keV hard X-ray bands with RXTE and Swift, sampling nine 26.5 day orbital cycles between September 2006 and February 2008. The binary was observed by VERITAS to be variable, with all integrated observations resulting in a detection at the 8.8 sigma (2006/2007) and 7.3 sigma (2007/2008) significance level for emission above 500 GeV. The source was detected during active periods with flux values ranging from 5 to 20% of the Crab Nebula, varying over the course of a single orbital cycle. Additionally, the observations conducted in the 2007-2008 observing season show marginal evidence (at the 3.6 sigma significance level) for TeV emission outside of the apastron passage of the compact object around the Be star. Contemporaneous hard X-ray observations with RXTE and Swift show large variability with flux values typically varying between 0.5 and 3.0*10^-11 ergs cm^-2 s^-1 over a single orbital cycle. The contemporaneous X-ray and TeV data are examined and it is shown that the TeV sampling is not dense enough to detect a correlation between the two bands.Comment: 30 pages, 5 figures, 2 table, Accepted for publication in The Astrophysical Journa

STAT5-and hypoxia-dependent upregulation of AXL

Internal tandem duplication in Fms-like tyrosine kinase 3 (FLT3-ITD) is the most frequent mutation observed in acute myeloid leukemia (AML) and correlates with poor prognosis. FLT3 tyrosine kinase inhibitors are promising for targeted therapy. Here, we investigated mechanisms dampening the response to the FLT3 inhibitor quizartinib, which is specific to the hematopoietic niche. Using AML primary samples and cell lines, we demonstrate that convergent signals from the hematopoietic microenvironment drive FLT3-ITD cell resistance to quizartinib through the expression and activation of the tyrosine kinase receptor AXL. Indeed, cytokines sustained phosphorylation of the transcription factor STAT5 in quizartinib-treated cells, which enhanced AXL expression by direct binding of a conserved motif in its genomic sequence. Likewise, hypoxia, another well-known hematopoietic niche hallmark, also enhanced AXL expression. Finally, in a xenograft mouse model, inhibition of AXL significantly increased the response of FLT3-ITD cells to quizartinib exclusively within a bone marrow environment. These data highlight a new bypass mechanism specific to the hematopoietic niche that hampers the response to quizartinib through combined upregulation of AXL activity. Targeting this signaling offers the prospect of a new therapy to eradicate resistant FLT3-ITD leukemic cells hidden within their specific microenvironment, thereby preventing relapses from FLT3-ITD clones