443 research outputs found
Jets in neutron star X-ray binaries: a comparison with black holes
(Abridged) We present a comprehensive study of the relation between radio and
X-ray emission in neutron star X-ray binaries, use this to infer the general
properties of the disc-jet coupling in such systems, and compare the results
quantitatively with those already established for black hole systems. There are
clear qualitative similarities between the two classes of object: hard states
below about 1% of the Eddington luminosity produce steady jets, while transient
jets are associated with outbursting and variable sources at the highest
luminosities. However, there are important quantitative differences: the
neutron stars are less radio-loud for a given X-ray luminosity (regardless of
mass corrections), and they do not appear to show the strong suppression of
radio emission in steady soft states which we observe in black hole systems.
Furthermore, in the hard states the correlation between radio and X-ray
luminosities of the neutron star systems is steeper than the relation observed
in black holes by about a factor of two. This result strongly suggests that the
X-ray emission in the black hole systems is radiatively inefficient, with an
approximate relation of the form L_X \propto \dot{m}^2, consistent with both
advection-dominated models and jet-dominated scenario. On the contrary the jet
power in both classes of object scales linearly with accretion rate. This
constitutes some of the first observational evidence for the radiatively
inefficient scaling of X-ray luminosity with accretion rate in accreting black
hole systems.Comment: Accepted for publication in MNRA
The Black Hole Candidate LSI+61303
In recent years, fundamental relationships for the black hole X-ray binaries
have been established between their X-ray luminosity and the photon index
of their X-ray spectrum. For the moderate-luminosity regime, an
anti-correlation between and has been observed. In this article,
aimed to verify if the moderate luminous X-ray binary system LSI +61303 is a
black hole, we analyse observations of LSI +61303. We compare the
derived vs distribution, first with the statistical trend for
black hole X-ray binaries, then with the trend of the pulsar PSR B1259-63, and
finally with the individual trends of the black hole X-ray binaries Swift
J1357.2-0933 and V404 Cygni. We find that the system PSR B1259-63 shows a
positive correlation between and , whereas in contrast LSI +61303
shows the same anti-correlation as for black hole X-ray binaries. Moreover, the
trend of LSI +61303 in the / plane overlaps with
that of the two black holes Swift J1357.2-0933 and V404 Cygni. All three
systems, Swift J1357.2-0933, V404 Cygni and LSI +61303 well trace the last part
of the evolution of accreting black holes at moderate-luminosity until their
drop to quiescence.Comment: 5 pages, 4 figures, MNRAS accepte
X-ray Observation of SS 433 with RXTE
Apart from regular monitoring by ASM, the compact object SS 433 was observed
with RXTE several times last two/three years. We present the first analysis of
these observations. We also include the results of the recent exciting TOO
campaign made during donour inferior (orbital phase ) and superior
() conjunctions which took place on Oct. 2nd, 2003, and on March
13th, 2004 respectively, when the jet itself was directly pointing towards us
(i.e., precessional phase ). Generally, we found that two distinct
lines fit the spectra taken on all these days. We present some of the
light-curves and the X-ray spectra, and show that the Doppler shifts of the
emitted lines roughly match those predicted by the kinematic model for the
jets. We find that the line with a higher energy can be best identified with a
FeXXVI Ly- transition while the line with lower energy can be
identified with a FeXXV (1s2p - 1s) transition. We observe that the X-ray
flux on March 13th, 2004 (when the base of the jet is exposed) is more than
twice compared to that on Oct. 2nd, 2003 (when the base is covered by the
companion). We find the flux to continue to remain high at least till another
orbital period. We believe that this is because SS 433 was undergoing a weak
flaring activity during the recent observation.Comment: 15 pages, 6 figures, submitted for publication in MNRAS (April, 2004
The influence of spin on jet power in neutron star X-ray binaries
We investigate the role of the compact object in the production of jets from
neutron star X-ray binaries. The goal is to quantify the effect of the neutron
star spin, if any, in powering the jet. We compile all the available measures
or estimates of the neutron star spin frequency in jet-detected neutron star
X-ray binaries. We use as an estimate of the ranking jet power for each source,
the normalisation of the power law which fits the X-ray/radio and
X-ray/infrared luminosity correlations L_(radio/IR) proportional to
L_(X)^(Gamma) (using infrared data for which there is evidence for jet
emission). We find a possible relation between spin frequency and jet power
(Spearman rank 97%), when fitting the X-ray/radio luminosity correlation using
a power law with slope 1.4; Gamma=1.4 is observed in 4U 1728-34 and is
predicted for a radiatively efficient disc and a total jet power proportional
to the mass accretion rate. If we use a slope of 0.6, as observed in Aql X-1,
no significant relation is found. An indication for a similar positive
correlation is also found for accreting millisecond X-ray pulsars (Spearman
rank 92%), if we fit the X-ray/infrared luminosity correlation using a power
law with slope 1.4. While our use of the normalisation of the luminosity
correlations as a measure of the ranking jet power is subject to large
uncertainties, no better proxy for the jet power is available. However, we urge
caution in over-interpreting the spin-jet power correlations, particularly
given the strong dependence of our result on the (highly uncertain) assumed
power law index of the luminosity correlations. We discuss the results in the
framework of current models for jet formation in black holes and young stellar
objects and speculate on possible different jet production mechanisms for
neutron stars depending on the accretion mode.Comment: Accepted for publication in MNRA
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
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
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
- …