7,704 research outputs found
High-Frequency Quasi-Periodic Oscillations from GRS 1915+105
We report the results of a systematic timing analysis of all archival Rossi
X-Ray Timing Explorer (RXTE) observations of the bright black-hole binary GRS
1915+105 in order to detect high-frequency quasi-periodic oscillations (HFQPO).
We produced power-density spectra in two energy bands and limited the analysis
to the frequency range 30-1000 Hz. We found 51 peaks with a single trial
significance larger than 3 sigma. As all but three have centroid frequencies
that are distributed between 63 and 71 Hz, we consider most of them significant
regardless of the number of trials involved. The average centroid frequency and
FWHM are 67.3 +/- 2.0 Hz and 4.4 +/- 2.4 Hz respectively. Their fractional rms
varies between 0.4% and 2% (total band detections) and between 0.5% and 3%
(hard ban detections). As GRS 1915+105 shows large variability on time scales
longer than 1s, we analysed the data in 16s intervals and found that the
detections are limited to a specific region in the colour-colour diagram,
corresponding to state B of the source, when the energy spectrum is dominated
by a bright accretion disk component. However, the rms spectrum of the HFQPO is
very hard and does not show a flattening up to 40 keV, where the fractional rms
reaches 11%. We discuss our findings in terms of current proposed models and
compare them with the results on other black-hole binaries and neutron-star
binaries.Comment: 9 pages, 6 figures, Accepted for publication in MNRA
X rays from old open clusters: M 67 and NGC 188
We have observed the old open clusters M 67 and NGC 188 with the ROSAT PSPC.
In M 67 we detect a variety of X-ray sources. The X-ray emission by a
cataclysmic variable, a single hot white dwarf, two contact binaries, and some
RS CVn systems is as expected. The X-ray emission by two binaries located below
the subgiant branch in the Hertzsprung Russell diagram of the cluster, by a
circular binary with a cool white dwarf, and by two eccentric binaries with
orbital period > 700 d is puzzling. Two members of NGC 188 are detected,
including the FK Com type star D719. Another possible FK Com type star,
probably not a member of NGC 188, is also detected.Comment: 10 pages, 5 figures. Accepted for publication on Astronomy &
Astrophysic
Quasi-Periodic Oscillations and energy spectra from the two brightest Ultra-Luminous X-ray sources in M82
Ultra-Luminous X-ray sources are thought to be accreting black holes that
might host Intermediate Mass Black Holes (IMBH), proposed to exist by
theoretical studies, even though a firm detection (as a class) is still
missing. The brightest ULX in M82 (M82 X-1) is probably one of the best
candidates to host an IMBH. In this work we analyzed the data of the recent
release of observations obtained from M82 X-1 taken by XMM-Newton. We performed
a study of the timing and spectral properties of the source. We report on the
detection of (46+-2) mHz Quasi-Periodic Oscillations (QPOs) in the power
density spectra of two observations. A comparison of the frequency of these
high-frequency QPOs with previous detections supports the 1:2:3 frequency
distribution as suggested in other studies. We discuss the implications if the
(46+-2) mHz QPO detected in M82 X-1 is the fundamental harmonic, in analogy
with the High-Frequency QPOs observed in black hole binaries. For one of the
observations we have detected for the first time a QPO at 8 mHz (albeit at a
low significance), that coincides with a hardening of the spectrum. We suggest
that the QPO is a milli-hertz QPO originating from the close-by transient ULX
M82 X-2, with analogies to the Low-Frequency QPOs observed in black hole
binaries.Comment: 9 pages (with 4 figures and 4 tables). Accepted for publication in
MNRAS (26/09/13
Sunflower under conventional and organic farming systems: results from a long term experiment in Central Italy
Sunflower productivity under organic and conventional agricultural systems was studied between 2002 and 2004 in the frame of the MASCOT experiment (Mediterranean Arable Systems COmparison Trial), established in 2001. The aim was to compare organic and conventional management systems for a typical arable crop rotation of Central Italy in the long-term. Sunflower was cultivated as a part of a five-year stockless arable crop rotation (sugar beet-common wheatsunflower-pigeon bean-durum wheat). In the organic system, red clover (Trifolium pratense) is interseeded in common and durum wheat and used as a green manure for sunflower or sugar beet. Grain yield of organically-grown sunflower was lower by 41%, 17% and 44% in 2002, 2003 and 2004 respectively, but no significant differences in percent seed oil content were found between the conventional and organic sunflower in two years out of three
On the modulation of low frequency Quasi-Periodic Oscillations in black-hole transients
We studied the properties of the low-frequency quasi-periodic oscillations
detected in a sample of six black hole candidates (XTE J1550-564, H 1743-322,
XTE J1859+226, 4U 1630-47,GX 339-4, XTE J1650-500) observed by the Rossi XTE
satellite. We analyzed the relation between the full width half maximum and the
frequency of all the narrow peaks detected in power density spectra where a
type-C QPO is observed. Our goal was to understand the nature of the modulation
of the signal by comparing the properties of different harmonic peaks in the
power density spectrum. We find that for the sources in our sample the width of
the fundamental and of the first harmonic are compatible with a frequency
modulation, while that of the sub-harmonic is independent of frequency,
possibly indicating the presence of an additional modulation in amplitude. We
compare our results with those obtained earlier from GRS 1915+105 and XTE
J1550-564.Comment: 8 pages, 3 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society Main Journa
Tracing the power-law component in the energy spectrum of black hole candidates as a function of the QPO frequency
We investigated the relation between the centroid frequency of the
quasi-periodic oscillation observed in the power density spectra of a sample of
galactic black-hole candidates with the power-law photon index obtained from
spectral fits. Our aim is to avoid inner accretion disk radius determination
directly from spectral fits, given the uncertainties of the absolute values
obtained in that way, but to base our analysis on the likely association of QPO
frequency to a characteristic radius. We used archival RXTE data of GRS
1915+105 and published parameters for GRO 1655-40, XTE J1550-564, XTE J1748-288
and 4U 1630-47. While for low values of the QPO frequency, the two parameters
are clearly correlated for each source, there is evidence for a turnoff in the
correlation above a characteristic frequency, different for different sources.
We discuss the possible nature of this turnoff.Comment: 11 pages, 10 figures. Accepted for publication on Astronomy &
Astrophysic
ASCA observations of the galactic bulge hard x-ray source GRS 1758--258
GRS 1758-258 is one of the few persistent hard X-ray emitters (E>100 keV) in
the Galaxy. Using the ASCA satellite, we have obtained the first detailed data
on GRS 1758-258 in the 1-10 keV range, where previous observations were
affected by confusion problems caused by the nearby strong source GX5-1. The
spectrum is well described by a power law with photon index 1.7 without strong
Fe emission lines. A prominent soft excess, as observed with ROSAT when the
hard X-ray flux was in a lower intensity state, was not detected. However, the
presence of a soft spectral component, accounting for at most 5% of the 0.1-300
keV flux, cannot be excluded. The accurate measurement of interstellar
absorption (N_H=(1.5+-0.1) x 10^22 cm -2) corresponds to an optical extinction
which definitely excludes the presence of a massive companion.Comment: 7 pages, AAS latex [11pt,aaspptwo,flushrt,tighten], + 1.ps figure
Accepted for pubblication in ApJ, 09 02 96 Also available at
http://fy.chalmers.se/~haardt/personal/curr.html Figures 1 and 2 available
upon request at [email protected]
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