147 research outputs found
Long-term periodicity in LSI+61303 as beat frequency between orbital and precessional rate
Context: In the binary system LSI+61303 the peak flux density of the radio
outburst, which is related to the orbital period of 26.4960 +/- 0.0028d,
exibits a modulation of 1667 +/- 8 d. The radio emission at high spatial
resolution appears structured in a precessing jet with a precessional period of
27-28 d. Aims: How close is the precessional period of the radio jet to the
orbital period? Any periodicity in the radio emission should be revealed by
timing analysis. The aim of this work is to establish the accurate value of the
precessional period. Methods: We analyzed 6.7 years of the Green Bank
Interferometer database at 2.2 GHz and 8.3 GHz with the Lomb-Scargle and phase
dispersion minimization (PDM) methods and performed simulations. Results: The
periodograms show two periodicities, P1 = 26.49 +/- 0.07 d (\nu1=0.03775
d^{-1}) and P2 = 26.92 +/- 0.07 d (\nu2 = 0.03715 d^{-1}). Whereas radio
outbursts have been known to have nearly orbital occurrence P1 with timing
residuals exhibiting a puzzling sawtooth pattern, we probe in this paper that
they are actually periodical outbursts and that their period is Paverage=
(2/(\nu1 + \nu2)= 26.70 +/- 0.05 d. The period Paverage as well as the
long-term modulation Pbeat=1/(\nu1 - \nu2)=1667 +/- 393 d result from the beat
of the two close periods, the orbital P1 and the precessional P2 periods.
Conclusions: The precessional period, indicated by the astrometry to be of
27--28 d, is P2=26.92 d. The system \lsi seems to be one more case in astronomy
of beat, i.e., a phenomenon occurring when two physical processes create stable
variations of nearly equal frequencies. The very small difference in frequency
creates a long-term variation of period 1/(\nu1-\nu2). The long-term modulation
of 1667 d results from the beat of the two close orbital and precessional
rates.Comment: 8 pages, 4 figures, accepted for publication in A&
Long-term OVRO monitoring of LSI+61303: confirmation of the two close periodicities
Context: The gamma-ray binary LSI+61303 shows multiple periodicities. The
timing analysis of 6.7 yr of GBI radio data and of 6 yr of Fermi-LAT GeV
gamma-ray data both have found two close periodicities P1(GBI) = 26.49 \pm 0.07
d, P2(GBI)=26.92 \pm 0.07 d and P1(gamma)=26.48 \pm 0.08 d, P2(gamma) = 26.99
\pm 0.08 d. Aims: The system LSI+61303 is the object of several continuous
monitoring programs at low and high energies. The frequency difference between
f1 and f2 of only 0.0006 d(-1) requires long-term monitoring because the
frequency resolution in timing analysis is related to the inverse of the
overall time interval. The Owens Valley Radio Observatory (OVRO) 40 m telescope
has been monitoring the source at 15 GHz for five years and overlaps with
Fermi-LAT monitoring. The aim of this work is to establish whether the two
frequencies are also resolved in the OVRO monitoring. Methods: We analysed OVRO
data with the Lomb-Scargle method. We also updated the timing analysis of
Fermi-LAT observations. Results: The periodograms of OVRO data confirm the two
periodicities P1(OVRO) = 26.5 \pm 0.1 d and P2(OVRO) = 26.9 \pm 0.1 d.
Conclusions: The three indipendent measurements of P1 and P2 with GBI, OVRO,
and Fermi-LAT observations confirm that the periodicities are permanent
features of the system LSI+61303. The similar behaviours of the emission at
high (GeV) and low (radio) energy when the compact object in LSI+61303 is
toward apastron suggest that the emission is caused by the same periodically
(P1) ejected population of electrons in a precessing (P2) jet.Comment: 4 pages, 7 figures, A&A Letters in pres
Discovery of a periodical apoastron GeV peak in LS I +61{\deg}303
Aims. The aim of this paper is to analyse the previously discovered
discontinuity of the periodicity of the GeV -ray emission of the
radio-loud X-ray binary LS I +61{\deg}303 and to determine its physical origin.
Methods. We used wavelet analysis to explore the temporal development of
periodic signals. The wavelet analysis was first applied to the whole data set
of available Fermi-LAT data and then to the two subsets of orbital phase
intervals and . We also performed a
Lomb-Scargle timing Analysis. We investigated the similarities between GeV
-ray emission and radio emission by comparing the folded curves of the
Fermi-LAT data and the Green Bank Interferometer radio data. Results. During
the epochs when the timing analysis fails to determine the orbital periodicity,
the periodicity is present in the two orbital phase intervals and . That is, there are two periodical signals, one
towards periastron (i.e., ) and another one towards apoastron
(). The apoastron peak seems to be affected by the same
orbital shift as the radio outbursts and, in addition, reveals the same two
periods and that are present in the radio data. Conclusions. The
-ray emission of the apoastron peak normally just broadens the emission
of the peak around periastron. Only when it appears at , because of the orbital shift, it is enough detached from the first peak
to become recognisable as a second orbital peak, which is the reason why the
timing analysis fails. Two -ray peaks along the orbit are predicted by
the two-peak accretion model for an eccentric orbit, that was proposed by
several authors for LS I +61{\deg}303.Comment: 7 pages, 4 figures, A&A (2014
Long-term OVRO monitoring of LS I +61º 303: confirmation of the two close periodicities
Context. The gamma-ray binary LS I +61° 303 shows multiple periodicities. The timing analysis of 6.7 yr of GBI radio data and of 6 yr of Fermi-LAT GeV gamma-ray data both have found two close periodicities P_(1,GBI) = 26.49 ± 0.07 d, P_(2,GBI) = 26.92 ± 0.07 d and P_(1,γ) = 26.48 ± 0.08 d, P_(2,γ) = 26.99 ± 0.08 d.
Aims. The system LS I +61°303 is the object of several continuous monitoring programs at low and high energies. The frequency difference between ν_1 and ν_2 of only 0.0006 d^(-1) requires long-term monitoring because the frequency resolution in timing analysis is related to the inverse of the overall time interval. The Owens Valley Radio Observatory (OVRO) 40 m telescope has been monitoring the source at 15 GHz for five years and overlaps with Fermi-LAT monitoring. The aim of this work is to establish whether the two frequencies are also resolved in the OVRO monitoring.
Methods. We analysed OVRO data with the Lomb-Scargle method. We also updated the timing analysis of Fermi-LAT observations.
Results. The periodograms of OVRO data confirm the two periodicities and .
Conclusions. The three independent measurements of P_1 and P_2 with GBI, OVRO, and Fermi-LAT observations confirm that the periodicities are permanent features of the system LS I +61°303. The similar behaviours of the emission at high (GeV) and low (radio) energy when the compact object in LS I +61°303 is toward apastron suggest that the emission is caused by the same periodically (P_1) ejected population of electrons in a precessing (P_2) jet
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 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
Considerations of VLBI transmitters on Galileo satellites
For directly linking the dynamical reference frame of satellite orbits to the quasi-inertial reference frame of extra-galactic radio sources, observations of satellites with the Very Long Baseline Interferometry (VLBI) technique are the only conceivable method. Hence, the satellite observations should be embedded in VLBI network sessions during which also natural radio sources are observed. For this reason, it would be most practical if the artificial signal generated at the satellite for VLBI observations covers the same frequency bands as regularly observed by VLBI radio telescopes and should have a similar flux density across the observed bandwidth as these natural sources. The use of satellites of Global Navigation Satellite Systems (GNSS) such as the Galileo system is advisable because they are well monitored in terms of precise orbit determination and the altitude allows common visibilities of many VLBI telescopes. So far, signal generation on a GNSS satellite dedicated to VLBI observations has not been realized yet, partly because suitable signal generation equipment has not been considered in depth. In addition, many aspects, such as legal implications and technical complications, have not yet been addressed. In this publication, we compiled various aspects of generating an artificial VLBI signal on a GNSS satellite. We describe the legal and technical aspects of generating and emitting an artificial signal on a Galileo satellite suitable for VLBI observations including a design study for the necessary equipment on the satellite. Since geodetic VLBI is currently in a transition period from traditional observations at S and X band to the broadband VLBI Global Observing System (VGOS), the proposed equipment generates a signal suitable for both frequency setups. We have also considered the restrictions for installation on a satellite, such as power consumption, weight, and size. The equipment mainly consists of three devices: noise source, amplifier, and antenna. A diode is used as the noise source. This noise is amplified by a set of low noise amplifiers and then radiated by a spiral antenna. The diode and the amplifiers were chosen from the market, but the antenna was newly designed and simulated. The output signal of this chain was tested using a VLBI baseband data simulator, then correlated and fringe-fitted for validation. The instrumentation proposed here is easy to be constructed, but will still have to be tested in the laboratory together with the instruments on the actual satellite
Transpulmonary thermodilution for hemodynamic measurements in severely burned children
Abstract
Introduction
Monitoring of hemodynamic and volumetric parameters after severe burns is of critical importance. Pulmonary artery catheters, however, have been associated with many risks. Our aim was to show the feasibility of continuous monitoring with minimally invasive transpulmonary thermodilution (TPTD) in severely burned pediatric patients.
Methods
This prospective cohort study was conducted in patients with severe burns over 40% of the total body surface area (TBSA) who were admitted to the hospital within 96 hours after sustaining the injury. TPTD measurements were performed using the PiCCO system (Pulsion Medical Systems, Munich, Germany). Cardiac Index (CI), Intrathoracic Blood Volume Index (ITBVI) (Stewart-Hamilton equation), Extravascular Lung Water Index (EVLWI) and Systemic Vascular Resistance Index (SVRI) measurements were recorded twice daily. Statistical analysis was performed using one-way repeated measures analysis of variance with the post hoc Bonferroni test for intra- and intergroup comparisons.
Results
Seventy-nine patients with a mean age (±SD) of 9 ± 5 years and a mean TBSA burn (±SD) of 64% ± 20% were studied. CI significantly increased compared to level at admission and was highest 3 weeks postburn. ITBVI increased significantly starting at 8 days postburn. SVRI continuously decreased early in the perioperative burn period. EVLWI increased significantly starting at 9 days postburn. Young children (0 to 5 years old) had a significantly increased EVLWI and decreased ITBVI compared to older children (12 to 18 years old). EVLWI was significantly higher in patients who did not survive burn injury.
Conclusions
Continuous PiCCO measurements were performed for the first time in a large cohort of severely burned pediatric patients. The results suggest that hyperdynamic circulation begins within the first week after burn injury and continues throughout the entire intensive care unit stay
Evidence for periodic accretion–ejection in LS I +61°303
The stellar binary system LS I +61°303, composed of a compact object in an eccentric orbit around a B0 Ve star, emits from radio up to γ-ray energies. The orbital modulation of radio spectral index, X-ray, and GeV γ-ray data suggests the presence of two peaks. This two-peaked profile is in line with the accretion theory predicting two accretion–ejection events for LS I +61°303 along the 26.5 d orbit. However, the existing multiwavelength data are not simultaneous. In this paper, we report the results of a campaign covering radio, X-ray, and γ-ray observations of the system along one single orbit. Our results confirm the two predicted events along the orbit and in addition show that the positions of radio and γ-ray peaks are coincident with X-ray dips as expected for radio and γ-ray emitting ejections depleting the X-ray emitting accretion flow. We discuss future observing strategies for a systematic study of the accretion–ejection physical processes in LS I +61°303
Evidence for periodic accretion–ejection in LS I +61°303
The stellar binary system LS I +61°303, composed of a compact object in an eccentric orbit around a B0 Ve star, emits from radio up to γ-ray energies. The orbital modulation of radio spectral index, X-ray, and GeV γ-ray data suggests the presence of two peaks. This two-peaked profile is in line with the accretion theory predicting two accretion–ejection events for LS I +61°303 along the 26.5 d orbit. However, the existing multiwavelength data are not simultaneous. In this paper, we report the results of a campaign covering radio, X-ray, and γ-ray observations of the system along one single orbit. Our results confirm the two predicted events along the orbit and in addition show that the positions of radio and γ-ray peaks are coincident with X-ray dips as expected for radio and γ-ray emitting ejections depleting the X-ray emitting accretion flow. We discuss future observing strategies for a systematic study of the accretion–ejection physical processes in LS I +61°303
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