45 research outputs found
An abrupt drop in the coherence of the lower kilohertz QPO in 4U 1636-536
Using archival data from the Rossi X-ray Timing Explorer (RXTE), we study in
a systematic way the variation of the quality factor and amplitude of the lower
and upper kHz quasi-periodic oscillations (QPO) in the low-mass X-ray binary
4U1636-536, over a frequency range from ~550 Hz to ~1200 Hz. When represented
in a quality factor versus frequency diagram, the upper and lower QPOs follow
two different tracks, suggesting that they are distinct phenomena, although not
completely independent because the frequency difference of the two QPOs, when
detected simultaneously, remains within ~60 Hz of half the neutron star spin
frequency (at 581 Hz). The quality factor of the lower kHz QPO increases with
frequency up to a maximum of ~200 at ~850 Hz, then drops precipitously to Q~50
at the highest detected frequencies ~920 Hz. A ceiling of the lower QPO
frequencies at 920 Hz is also clearly seen in a frequency versus count rate
diagram.
The quality factor provides a measure of the coherence of the underlying
oscillator. For exponentially damped sinusoidal shots, the highest Q observed
corresponds to an oscillator coherence time of ~ 0.1 seconds. All existing QPO
models face challenges in explaining such a long coherence time and the
significantly different behaviours of the quality factors of the upper and
lower QPOs reported here. It is therefore difficult to be certain of the
implications of the abrupt change in the lower QPO at ~850 Hz. We discuss
various possible causes, including that the drop in coherence is ultimately
caused by effects related to the innermost stable circular orbit (abridged).Comment: Accepted for publication in MNRAS, 7 pages, 5 figures (1 in color
The coherence of kHz quasi-periodic oscillations in the X-rays from accreting neutron stars
We study in a systematic way the quality factor of the lower and upper kHz
QPOs in a sample of low luminosity neutron star X-ray binaries, showing both
QPOs varying over a wide frequency range. The sample includes 4U 1636-536, 4U
1608-522, 4U 1735-44, 4U 1728-34, 4U 1820-303 and 4U 0614+09. We find that all
sources except 4U 0614+091 show evidence of a drop in the quality factor of
their lower kHz QPOs at high frequency. For 4U 0614+091 only the rising part of
the quality factor versus frequency curve has been sampled so far. At the same
time, in all sources but 4U 1728-34, the quality factor of the upper kilo-Hz
QPO increases all the way to the highest detectable frequencies. We show that
the high-frequency behaviours of both the lower and upper kHz QPO quality
factors are consistent with what is expected if the drop is produced by the
approach of an active oscillating region to the innermost stable circular
orbit: the existence of which is a key feature of General Relativity in the
strong field regime. Within this interpretation, our results imply
gravitational masses around 2 solar masses for the neutron stars in those
systems.Comment: Accepted for publication in MNRAS, 8 pages, 8 figures, figure 1 in
colo
An Optically Dark GRB Observed by HETE-2: GRB 051022
GRB 051022 was detected at 13:07:58 on 22 October 2005 by HETE-2. The
location of GRB 051022 was determined immediately by the flight localization
system. This burst contains multiple pulses and has a rather long duration of
about 190 seconds. The detections of candidate X-ray and radio afterglows were
reported, whereas no optical afterglow was found. The optical spectroscopic
observations of the host galaxy revealed the redshift z = 0.8. Using the data
derived by HETE-2 observation of the prompt emission, we found the absorption
N_H = 8.8 -2.9/+3.1 x 10^22 cm^-2 and the visual extinction A_V = 49 -16/+17
mag in the host galaxy. If this is the case, no detection of any optical
transient would be quite reasonable. The absorption derived by the Swift XRT
observations of the afterglow is fully consistent with those obtained from the
early HETE-2 observation of the prompt emission. Our analysis implies an
interpretation that the absorbing medium could be outside external shock at R ~
10^16 cm, which may be a dusty molecular cloud.Comment: 6 pages, 2 figures, accepted for publication in PASJ lette
Atypical pathogens in hospitalized patients with community-acquired pneumonia: A worldwide perspective
Background: Empirical antibiotic coverage for atypical pathogens in community-acquired pneumonia (CAP) has long been debated, mainly because of a lack of epidemiological data. We aimed to assess both testing for atypical pathogens and their prevalence in hospitalized patients with CAP worldwide, especially in relation with disease severity. Methods: A secondary analysis of the GLIMP database, an international, multicentre, point-prevalence study of adult patients admitted for CAP in 222 hospitals across 6 continents in 2015, was performed. The study evaluated frequency of testing for atypical pathogens, including L. pneumophila, M. pneumoniae, C. pneumoniae, and their prevalence. Risk factors for testing and prevalence for atypical pathogens were assessed through univariate analysis. Results: Among 3702 CAP patients 1250 (33.8%) underwent at least one test for atypical pathogens. Testing varies greatly among countries and its frequency was higher in Europe than elsewhere (46.0% vs. 12.7%, respectively, p < 0.0001). Detection of L. pneumophila urinary antigen was the most common test performed worldwide (32.0%). Patients with severe CAP were less likely to be tested for both atypical pathogens considered together (30.5% vs. 35.0%, p = 0.009) and specifically for legionellosis (28.3% vs. 33.5%, p = 0.003) than the rest of the population. Similarly, L. pneumophila testing was lower in ICU patients. At least one atypical pathogen was isolated in 62 patients (4.7%), including M. pneumoniae (26/251 patients, 10.3%), L. pneumophila (30/1186 patients, 2.5%), and C. pneumoniae (8/228 patients, 3.5%). Patients with CAP due to atypical pathogens were significantly younger, showed less cardiovascular, renal, and metabolic comorbidities in comparison to adult patients hospitalized due to non-atypical pathogen CAP. Conclusions: Testing for atypical pathogens in patients admitted for CAP in poorly standardized in real life and does not mirror atypical prevalence in different settings. Further evidence on the impact of atypical pathogens, expecially in the low-income countries, is needed to guidelines implementation