4,035 research outputs found
Large X-ray Flares from LMC X-4: Discovery of Milli-hertz Quasi-periodic Oscillations and QPO-modulated Pulsations
We report the discovery of milli-hertz (mHz) quasi-periodic oscillations
(QPOs) and QPO-modulated pulsations during large X-ray flares from the
high-mass X-ray binary pulsar LMC X-4 using data from the Rossi X-Ray Timing
Explorer (RXTE). The lightcurves of flares show that, in addition to ~74 mHz
coherent pulsations, there exist two more time-varying temporal structures at
frequencies of ~0.65-1.35 and ~2-20 mHz. These relatively long-term structures
appear in the power density spectra as mHz QPOs and as well-developed sidebands
around the coherent pulse frequency as well, indicating that the amplitudes of
the coherent pulsation is modulated by those of the mHz QPOs. One interesting
feature is that, while the first flare shows symmetric sidebands around the
coherent pulse frequency, the second flare shows significant excess emission in
the lower-frequency sidebands due to the ~2-20 mHz QPOs. We discuss the origin
of the QPOs using a combination of the beat-frequency model and a modified
version of the Keplerian-frequency model. According to our discussion, it seems
to be possible to attribute the origin of the ~0.65-1.35 and ~2-20 mHz QPOs to
the beating between the rotational frequency of the neutron star and the
Keplerian frequency of large accreting clumps near the corotation radius and to
the orbital motion of clumps at Keplerian radii of 2-10 times 10^9 cm,
respectively.Comment: 12 pages, including 4 figures; accepted by ApJ Letter
Long-Term X-ray Monitoring of 1E 1740.7-2942 and GRS 1758-258
We report on long-term observations of the Galactic-bulge black hole
candidates 1E 1740.7-2942 and GRS 1758-258 with the Rossi X-Ray Timing
Explorer. 1E 1740.7-2942 has been observed 77 times and GRS 1758-258 has been
observed 82 times over the past 1000 days. The flux of each object has varied
by no more than a factor of 2.5 during this period, and the indices of the
energy spectra have varied by no more than 0.4. The power spectra are similar
to other black-hole candidates: flat-topped noise, breaking to a power law.
Each object has exhibited a brightening that lasted for several months, and we
have a found a time lag between the photon power-law index and the count rate.
In both sources, the spectrum is softest during the decline from the
brightening. This behavior can be understood in the context of thin-disk and
advection-dominated accretion flows coexisting over a wide range of radii, with
the implication that both sources have low-mass companions and accrete via
Roche-lobe overflow.Comment: Accepted for publication in The Astrophysical Journa
ProduktivitÀt, Narbendichte und Vegetation einer Kurzrasenweide
Productivity, sward structure and floristic diversity of a continuous grazed pasture on organic permanent grassland were determined in 2015. Daily pasture growth reached
less than 50 kg dry matter per hectare in May and stayed low due to unfavorable weather conditions in 2015 (cold spring, dry and hot summer). Forage quality was quite high (> 6,5 MJ NEL/kg dm) most of the grazing season. Tiller density was moderate during summer but increased after sufficient rain in late summer. Perennial ryegrass, Kentucky bluegrass and white clover were the dominant plant species and yielded more the 85 % of dry matter
Excited-state dynamics of [Ru(SâSbpy)(bpy)2]2+to form long-lived localized triplet states
The novel photosensitizer [Ru( SâS bpy)(bpy) 2 ] 2+ harbors two distinct sets of excited states in the UV/Vis region of the absorption spectrum located on either bpy or SâS bpy ligands. Here, we address the question of whether following excitation into these two types of states could lead to the formation of different long-lived excited states from where energy transfer to a reactive species could occur. Femtosecond transient absorption spectros- copy identifies the formation of the final state within 80 fs for both excitation wavelengths. The recorded spectra hint at very similar dynamics following excitation toward either the parent or sulfur- decorated bpy ligands, indicating ultrafast interconversion into a unique excited-state species regardless of the initial state. Non-adiabatic surface hopping dynamics simulations show that ultrafast spinâorbit-mediated mixing of the states within less than 50 fs strongly increases the localization of the excited electron at the SâS bpy ligand. Extensive structural relaxation within this sulfurated ligand is possible, via SâS bond cleavage that results in triplet state energies that are lower than those in the analogue [Ru(bpy)3 ]2+ . This structural relaxation upon localization of the charge on SâS bpy is found to be the reason for the formation of a single long-lived species independent of the excitation wavelength
The Infrared Counterpart of the Microquasar GRS 1758-258
We present revised infrared (2.2 micron) astrometry of the field containing
the Galactic microquasar GRS 1758-258, using observations at the Keck I 10-m
telescope. We find three candidates for the microquasar within a 3 sigma error
circle, but none within 2 sigma. We show that if the 18.4 day X-ray period of
GRS 1758-258 is due to a binary orbit, then only one of the three candidates,
an early K-type giant, is large enough to power the microquasar via Roche lobe
overflow. We therefore identify this star as the infrared counterpart of GRS
1758-258, which we classify as a low mass X-ray binary. Long term infrared
monitoring of this source should provide further information about the
microquasar system, including a confirmation of the X-ray period and an
estimate of the compact object's mass.Comment: 4 pages, 2 figures; accepted for publication in ApJ Letter
Discovery of a Third Harmonic Cyclotron Resonance Scattering Feature in the X-ray Spectrum of 4U 0115+63
We have discovered a third harmonic cyclotron resonance scattering feature
(CRSF) in observations of the recent outburst of 4U 0115+63 with the Rossi
X-ray Timing Explorer (RXTE). The spectrum in a narrow pulse phase range shows
CRSFs at 12.40+0.65/-0.35, 21.45+0.25/-0.38, and 33.56+0.70/-0.90 keV. With
centroid energy ratios to the fundamental of 1.73+/-0.08 and 2.71+/-0.13, the
CRSFs are not harmonically spaced. Strong variability of the continuum and
CRSFs with pulse phase indicate a complex emission geometry near the neutron
star polar cap. In addition, one RXTE observation, which spanned periastron
passage, revealed a strong 2 mHz quasi-periodic oscillation (QPO). This is
slower by two orders of magnitude than the beat-frequency QPO expected in this
system and slower by a factor of more than 5 compared with other QPOs seen in
accreting X-ray pulsars.Comment: To appear in The Astrophysical Journal Letters. 4 pages, 5 figures.
Uses "emulateapj.sty". Revised version includes new figures and additions to
the analysi
Gamma-Ray Imaging Observations of Point Source Emission From the Galactic Center Region
The first coded-aperture images of the galactic center region at energies above 30 keV
reveal two strong 7-ray sources. One source is tentatively identified as the X-ray source
1El740.7-2942. If this source is at the distance of the galactic center, it is one of the most
luminous objects in the galaxy at energies from 35 to 200 keV. The second source is consistent
in location with the X-ray source GX354-0. No significant flux was detected from
the direction of the galactic nucleus (Sgr A*)
- âŠ