83 research outputs found
Near-infrared jet emission in the microquasar XTE J1550-564
Context: Microquasars are accreting Galactic sources that are also observed
to launch relativistic jets. A key signature of the ejection is non-thermal
radio emission. The level of this jet component at high frequencies is still
poorly constrained. Aims: The X-ray binary and microquasar black hole candidate
XTE J1550-564 exhibited a faint X-ray outburst in April 2003 during which it
stayed in the X-ray low/hard state. We took optical and near-infrared (NIR)
observations with the ESO/NTT telescope during this outburst to disentangle the
various contributions to the spectral energy distribution (SED) and investigate
the presence of a jet component. Methods: Photometric and spectroscopic
observations allowed us to construct an SED and also to produce a high
time-resolution lightcurve. Results: The SED shows an abrupt change of slope
from the NIR domain to the optical. The NIR emission is attributed to
non-thermal synchrotron emission from the compact, self-absorbed jet that is
known to be present in the low/hard state. This is corroborated by the fast
variability, colours, lack of prominent spectral features and evidence for
intrinsic polarisation. The SED suggests the jet break from the optically thick
to the thin regime occurs in the NIR. Conclusions: The simultaneous optical-NIR
data allow an independent confirmation of jet emission in the NIR. The
transition to optically thin synchrotron occurs at NIR frequencies or below,
which leads to an estimated characteristic size greater than 2x10^8cm and
magnetic field less than 5T for the jet base, assuming a homogeneous one-zone
synchrotron model.Comment: Accepted for publication in Astronomy and Astrophysics (Section 7.
Stellar structure and evolution; 8 pages, 6 figures
Pentaquark baryons in SU(3) quark model
We study the SU(3) group structure of pentaquark baryons which are made of
four quarks and one antiquark. The pentaquark baryons form {1}, {8}, {10},
{10}-bar, {27}, and {35} multiplets in SU(3) quark model. First, the flavor
wave functions of all the pentaquark baryons are constructed in SU(3) quark
model and then the flavor SU(3) symmetry relations for the interactions of the
pentaquarks with three-quark baryons and pentaquark baryons are obtained.Comment: REVTeX, 36 pages, 8 figures, references added, section for mass sum
rules is added, to appear in Phys. Rev.
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A new generation of 99.999% enriched 28Si single crystals for the determination of Avogadro's constant
A metrological challenge is currently underway to replace the present definition of the kilogram. One prerequisite for this is that the Avogadro constant, NA, which defines the number of atoms in a mole, needs to be determined with a relative uncertainty of better than 2ââĂââ10â8. The method applied in this case is based on the x-ray crystal density experiment using silicon crystals. The first attempt, in which silicon of natural isotopic composition was used, failed. The solution chosen subsequently was the usage of silicon highly enriched in 28Si from Russia. First, this paper reviews previous efforts from the very first beginnings to an international collaboration with the goal of producing a 28Si single crystal with a mass of 5âkg, an enrichment greater than 0.9999 and of sufficient chemical purity. Then the paper describes the activities of a follow-up project, conducted by PTB, to produce a new generation of highly enriched silicon in order to demonstrate the quasi-industrial and reliable production of more than 12âkg of the 28Si material with enrichments of five nines. The intention of this project is also to show the availability of 28Si single crystals as a guarantee for the future realisation of the redefined kilogram
Evidence for a compact jet dominating the broadband spectrum of the black hole accretor XTE J1550-564
[abridged] The black hole X-ray binary XTE J1550-564 was monitored
extensively at X-ray, optical and infrared wavelengths throughout its outburst
in 2000. We show that it is possible to separate the optical/near-infrared
(OIR) jet emission from the OIR disc emission. Focussing on the jet component,
we find that as the source fades in the X-ray hard state, the OIR jet emission
has a spectral index consistent with optically thin synchrotron emission (alpha
~ -0.6 to -0.7, where F_nu \propto nu^alpha). This jet emission is tightly and
linearly correlated with the X-ray flux; L_OIR,jet \propto L_X^(0.98 +- 0.08)
suggesting a common origin. This is supported by the OIR, X-ray and OIR to
X-ray spectral indices being consistent with a single power law (alpha =
-0.73). Ostensibly the compact, synchrotron jet could therefore account for ~
100 % of the X-ray flux at low luminosities in the hard state. At the same
time, (i) an excess is seen over the power law decay of the X-ray flux at the
point in which the jet would start to dominate, (ii) the X-ray spectrum
slightly softens, which seems to be due to a high energy cut-off or break
shifting to a lower energy, and (iii) the X-ray rms variability increases. This
may be the strongest evidence to date of synchrotron emission from the compact,
steady jet dominating the X-ray flux of an X-ray binary. For XTE J1550-564,
this is likely to occur within the luminosity range ~ (2 e-4 - 2 e-3) L_Edd on
the hard state decline of this outburst. However, on the hard state rise of the
outburst and initially on the hard state decline, the synchrotron jet can only
provide a small fraction (~ a few per cent) of the X-ray flux. Both thermal
Comptonization and the synchrotron jet can therefore produce the hard X-ray
power law in accreting black holes.Comment: MNRAS accepted, 12 pages, 9 figure
Modelling spectral and timing properties of accreting black holes: the hybrid hot flow paradigm
The general picture that emerged by the end of 1990s from a large set of
optical and X-ray, spectral and timing data was that the X-rays are produced in
the innermost hot part of the accretion flow, while the optical/infrared (OIR)
emission is mainly produced by the irradiated outer thin accretion disc. Recent
multiwavelength observations of Galactic black hole transients show that the
situation is not so simple. Fast variability in the OIR band, OIR excesses
above the thermal emission and a complicated interplay between the X-ray and
the OIR light curves imply that the OIR emitting region is much more compact.
One of the popular hypotheses is that the jet contributes to the OIR emission
and even is responsible for the bulk of the X-rays. However, this scenario is
largely ad hoc and is in contradiction with many previously established facts.
Alternatively, the hot accretion flow, known to be consistent with the X-ray
spectral and timing data, is also a viable candidate to produce the OIR
radiation. The hot-flow scenario naturally explains the power-law like OIR
spectra, fast OIR variability and its complex relation to the X-rays if the hot
flow contains non-thermal electrons (even in energetically negligible
quantities), which are required by the presence of the MeV tail in Cyg X-1. The
presence of non-thermal electrons also lowers the equilibrium electron
temperature in the hot flow model to <100 keV, making it more consistent with
observations. Here we argue that any viable model should simultaneously explain
a large set of spectral and timing data and show that the hybrid
(thermal/non-thermal) hot flow model satisfies most of the constraints.Comment: 26 pages, 13 figures. To be published in the Space Science Reviews
and as hard cover in the Space Sciences Series of ISSI - The Physics of
Accretion on to Black Holes (Springer Publisher
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