Comparison of three measurement methods of saturated hydraulic condutivity

International audienceAfter pointing out the importance of the saturated hydraulic conductivity (ks) measurements and the difficulties and uncertainties that are present, and after recalling salient aspects of three well-known measurement methods of this parameter (i.e. constant-head tension infiltrometer (TI) method, constant-head pressure infiltrometer (PI) method and soil core (SC) estimates method), the results of an investigation on data which were obtained during a measurement campaign on an area of 800 m2, on a sandy loam hillslope, located in Southern Italy, were carried out again here. Three sets of values of ks, obtained with these measurement methods, were analyzed statistically, verifying that the log-normal distribution describes these better than the normal one; moreover, the more significant statistical parameters of each set were compared (average value , amplitude A, coefficient of variation CV and standard deviation SD), individualizing the more significant differences. The greatest value of hydraulic conductivity was found with method (PI), while the smallest with (SC) and the intermediate with (TI); these differences were translated into macroporosity and into the influence of the single measurement method. Moreover, referring to the possible factors affecting the results, the importance can be noted of the structure, the texture and the soil events, in terms of utilization, which can affect the measure of ks leading often to very different values even for similar soils, but with a different history, independently of the coincidence of the measurement points and they can be determining to explain the differences affecting the results obtained in analogous investigations by other researchers. Having confirmed that generalization is not possible, the need was emphasized to adopt the necessary devices relating to the specific measurement method, case by case, and to carefully explain the obtained results, in the light of the peculiarities and the limits of each situation. Finally, the results of similar statistical analysis carried out on a greater number of ks values, measured through the (TI) and (PI) methods are shown in this paper, with some statistical considerations on the increasing of the measurements number

Characterization of the field saturated hydraulic conductivity on a hillslope: measurement techniques, data sensitivity analysis and spatial correlation modelling

International audienceIn the context of studies aiming at the estimation of effective parameters for unsaturated zone modelling, this work tackles the problem of experimental data quality, considering the large collection of data gathered at an experimental site equipped for unsaturated zone hydraulic monitoring in the alluvial basin of a Calabrian river, in the South of Italy. Focusing attention on field saturated hydraulic conductivity, the in-site measurement techniques by tension disc and pressure ring infiltrometers are considered, pointing out the main indications for the correct use of each measuring approach; laboratory techniques are also considered. Statistical data analysis showed that the measurements performed by tension disc infiltrometer supplied values of hydraulic conductivity which are on average lower and more homogeneous than the values provided by the other measurement techniques considered. Sensitivity analysis was then carried out by Monte Carlo simulation on the parameter sampling achieved by field measurement techniques in order to evaluate the influence of any possible small measurement errors on the data. Sensitivity analysis showed that both ring and disc infiltrometer are tools reliable enough for the in situ measurements of field saturated hydraulic conductivity. Finally, after a data merging procedure giving origin to different sets of data, the spatial correlation structure of field saturated hydraulic conductivity is investigated, using well-known geostatistical techniques

Chandra high-resolution spectra of 4U 1630-47: the disappearance of the wind

We present the analysis of six Chandra X-ray high-resolution observations of the black hole low-mass X-ray binary 4U 1630-47 taken during its 2012–2013 outburst. Fe XXVI K α, K β, Fe XXV K α, K β, and Ca XX K α blueshifted absorption lines were identified in the first four observations, which correspond to soft accretion states. The remaining observations, associated to intermediate and possibly hard accretion states, do not show significant absorption features down to equivalent width of 1 eV for both Fe XXVI and Fe XXV. We inferred wind launching radii between 1.2 − 2.0 (1012 cm/n) × 1011 cm and column densities N(H) > 1023 cm−2. In the first four observations, we found that thermal pressure is likely to be the dominant launching mechanism for the wind, although such conclusions depend on the assumed density. We used the spectral energy distributions obtained from our continuum modelling to compute thermal stability curves for all observations using the XSTAR photoionization code. We found that the absence of lines in the transitional state cannot be attributed to an evolution of the plasma caused by thermal instabilities derived from the change in the continuum spectrum. In contrast, the disappearance of the wind could indicate an acceleration of the flow or that the plasma has been exhausted during the soft state

Radio Emission from the Intermediate-mass Black Hole in the Globular Cluster G1

We have used the Very Large Array (VLA) to search for radio emission from the globular cluster G1 (Mayall-II) in M31. G1 has been reported by Gebhardt et al. to contain an intermediate-mass black hole (IMBH) with a mass of ~2 x 10^4 solar masses. Radio emission was detected within an arcsecond of the cluster center with an 8.4 GHz power of 2 x 10^{15} W/Hz. The radio/X-ray ratio of G1 is a few hundred times higher than that expected for a high-mass X-ray binary in the cluster center, but is consistent with the expected value for accretion onto an IMBH with the reported mass. A pulsar wind nebula is also a possible candidate for the radio and X-ray emission from G1; future high-sensitivity VLBI observations might distinguish between this possibility and an IMBH. If the radio source is an IMBH, and similar accretion and outflow processes occur for hypothesized ~ 1000-solar-mass black holes in Milky Way globular clusters, they are within reach of the current VLA and should be detectable easily by the Expanded VLA when it comes on line in 2010.Comment: ApJ Letters, accepted, 11 pages, 1 figur

Millihertz Quasi-periodic Optical Oscillations in 4U 0614+091

We report the discovery of a 1mHz optical quasi-periodic oscillation (QPO) in the candidate ultracompact low-mass X-ray binary 4U 0614+091. The ultra-low frequency QPO has no X-ray counterpart in contemporaneous RXTE/PCA data and is likely a signature of structure in the accretion disk. The QPO can be reasonably fitted with a single sine wave but with a phase jump part way through the observation, indicating that it is not coherent.We also identify a 48 min modulation, approximately consistent with the suggested orbital period of O'Brien (2005) and Shahbaz et al. (2008). If this is indeed orbital, it supports an identification of 4U 0614+091 as an ultra-compact source.Comment: 7 pages, 5 figures, accepted for publication in Monthly Notices of the Royal Astronomical Societ

Rapid X-ray variability of the gamma-ray binary LS I+61 303

The gamma-ray binary LS I+61 303 has been widely monitored at different wavelengths since its discovery more than sixty years ago. However, the nature of the compact object and the peculiar behavior of the system are still largely debated. Aimed at investigating the rapid X-ray variability of LS I+61 303, we have analysed all the archival RXTE/PCA data of the source, taken between 1996 and 2011. The timing analysis yields a periodicity of $P\sim 26.6\pm 0.3$ days, which is statistically compatible with several periodicities reported in the literature for LS I+61 303. Using this period, we performed a data phase-resolved analysis to produce a set of phase-bin-averaged energy spectra and power density spectra. These power density spectra are dominated by weak red noise below 0.1 Hz, and show no signal above this frequency. The amplitude of the red noise varies mildly with the phase, and shows a maximum that coincides with a dip of the X-ray flux and a softer photon index. Aside from low-frequency noise, this analysis does not provide any statistically significant periodic or quasi-periodic timing feature in the RXTE/PCA data of LS I+61 303.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS

Linking Jet Emission, X-ray States and Hard X-ray Tails in the Neutron Star X-ray Binary GX 17+2

We present the results from simultaneous radio (Very Large Array) and X-ray (Rossi-X-ray Timing Explorer) observations of the Z-type neutron star X-ray binary GX~17+2. The aim is to assess the coupling between X-ray and radio properties throughout its three rapidly variable X-ray states and during the time-resolved transitions. These observations allow us, for the first time, to investigate quantitatively the possible relations between the radio emission and the presence of the hard X-ray tails and the X-ray state of the source. The observations show: 1) a coupling between the radio jet emission and the X-ray state of the source, i.e. the position in the X-ray hardness-intensity diagram (HID); 2) a coupling between the presence of a hard X-ray tail and the position in the HID, qualitatively similar to that found for the radio emission; 3) an indication for a quantitative positive correlation between the radio flux density and the X-ray flux in the hard-tail power law component; 4) evidence for the formation of a radio jet associated with the Flaring Branch-to-Normal Branch X-ray state transition; 5) that the radio flux density of the newly-formed jet stabilizes when also the normal-branch oscillation (NBO) in the X-ray power spectrum stabilizes its characteristic frequency, suggesting a possible relation between X-ray variability associated to the NBO and the jet formation. We discuss our results in the context of jet models.Comment: Accepted by Ap

Simultaneous detection of an intrinsic absorber and a compact jet emission in the X-ray binary IGR J17091-3624 during a hard accretion state

We present a detailed analysis of three XMM-Newton observations of the black hole low-mass X-ray binary IGR~J17091-3624 taken during its 2016 outburst. Radio observations obtained with the Australia Telescope Compact Array (ATCA) indicate the presence of a compact jet during all observations. From the best X-ray data fit results we concluded that the observations were taken during a transition from a hard accretion state to a hard-intermediate accretion state. For Observations 1 and 2 a local absorber can be identified in the EPIC-pn spectra but not in the RGS spectra, preventing us from distinguishing between absorption local to the source and that from the hot ISM component. For Observation 3, on the other hand, we have identified an intrinsic ionized static absorber in both EPIC-pn and RGS spectra. The absorber, observed simultaneously with a compact jet emission, is characterized by an ionization parameter of 1.96< log({\xi}) <2.05 and traced mainly by Ne X, Mg XII, Si XIII and Fe XVIII.Comment: 13 pages, 10 figure

Galactic X-ray binary jets

With their relatively fast variability time-scales, Galactic X-ray binaries provide an excellent laboratory to explore the physics of accretion and related phenomena, most notably outflows, over different regimes. After comparing the phenomenology of jets in black hole X-ray binary systems to that of neutron stars, here I discuss the role of the jet at very low Eddington ratios, and present preliminary results obtained by fitting the broadband spectral energy distribution of a quiescent black hole binary with a `maximally jet-dominated' model.Comment: Refereed version, accepted for publication in Astrophysics & Space Scienc