424 research outputs found
NELIOTA: The wide-field, high-cadence lunar monitoring system at the prime focus of the Kryoneri telescope
We present the technical specifications and first results of the ESA-funded,
lunar monitoring project "NELIOTA" (NEO Lunar Impacts and Optical TrAnsients)
at the National Observatory of Athens, which aims to determine the
size-frequency distribution of small Near-Earth Objects (NEOs) via detection of
impact flashes on the surface of the Moon. For the purposes of this project a
twin camera instrument was specially designed and installed at the 1.2 m
Kryoneri telescope utilizing the fast-frame capabilities of scientific
Complementary Metal-Oxide Semiconductor detectors (sCMOS). The system provides
a wide field-of-view (17.0' 14.4') and simultaneous observations in
two photometric bands (R and I), reaching limiting magnitudes of 18.7 mag in 10
sec in both bands at a 2.5 signal-to-noise level. This makes it a unique
instrument that can be used for the detection of NEO impacts on the Moon, as
well as for any astronomy projects that demand high-cadence multicolor
observations. The wide field-of-view ensures that a large portion of the Moon
is observed, while the simultaneous, high-cadence, monitoring in two
photometric bands makes possible, for the first time, the determination of the
temperatures of the impacts on the Moon's surface and the validation of the
impact flashes from a single site. Considering the varying background level on
the Moon's surface we demonstrate that the NELIOTA system can detect NEO impact
flashes at a 2.5 signal-to-noise level of ~12.4 mag in the I-band and R-band
for observations made at low lunar phases ~0.1. We report 31 NEO impact flashes
detected during the first year of the NELIOTA campaign. The faintest flash was
at 11.24 mag in the R-band (about two magnitudes fainter than ever observed
before) at lunar phase 0.32. Our observations suggest a detection rate of events .Comment: Accepted for publication in A&
Brief communication: A first hydrological investigation of extreme August 2023 floods in Slovenia, Europe
Extreme floods occurred from 4 to 6 August 2023 in Slovenia causing three casualties and causing total direct and indirect damage, including post-disaster needs according to the Post-Disaster Needs Assessment (PDNA), close to EUR 10 billion. The atypical summer weather conditions combined with the high air and sea temperatures in the Mediterranean and the high soil moisture led to the most extreme flood event in Slovenia in recent decades. The return periods of both daily and sub-daily precipitation extremes and peak discharges reached 250–500 years, and the runoff coefficient of a typical torrential and mostly forested mesoscale catchment was around 0.5. In addition, flooding, soil erosion, mass movements and river sediment transport processes caused major damage to buildings (more than 12 000 houses) and diverse infrastructure.</p
CoBiToM Project -- II: Evolution of contact binary systems close to the orbital period cut-off
Ultra-short orbital period contact binaries (Porb < 0.26 d) host some of the
smallest and least massive stars. These systems are faint and rare, and it is
believed that they have reached a contact configuration after several Gyrs of
evolution via angular momentum loss, mass transfer and mass loss through
stellar wind processes. This study is conducted in the frame of Contact
Binaries Towards Merging (CoBiToM) Project and presents the results from light
curve and orbital analysis of 30 ultra-short orbital period contact binaries,
with the aim to investigate the possibility of them being red nova progenitors,
eventually producing merger events. Approximately half of the systems exhibit
orbital period modulations, as a result of mass transfer or mass loss
processes. Although they are in contact, their fill-out factor is low (less
than 30 per cent), while their mass ratio is larger than the one in longer
period contact binaries. The present study investigates the orbital stability
of these systems and examines their physical and orbital parameters in
comparison to those of the entire sample of known and well-studied contact
binaries, based on combined spectroscopic and photometric analysis. It is found
that ultra-short orbital period contact binaries have very stable orbits, while
very often additional components are gravitationally bound in wide orbits
around the central binary system. We confirmed that the evolution of such
systems is very slow, which explains why the components of ultra-short orbital
period systems are still Main Sequence stars after several Gyrs of evolution
Period changes in six semi-detached Algol-type binaries
Six semi-detached Algol-type binaries lacking a period analysis were chosen
to test for a presence of a third body. The O-C diagrams of these binaries were
analyzed with the least-squares method by using all available times of minima.
Also fourteen new minima, obtained from our observations, were included in the
present research. The light-time effect was adopted as a main factor for the
detailed description of the long-term period changes. Third bodies were found
with orbital periods from 46 up to 84 years, and eccentricities from 0.0 to
0.78 for the selected binaries. The mass functions and the minimal masses of
such bodies were also calculated.Comment: 14 pages, 8 figure
Precursor flares in OJ 287
We have studied three most recent precursor flares in the light curve of the
blazar OJ 287 while invoking the presence of a precessing binary black hole in
the system to explain the nature of these flares. Precursor flare timings from
the historical light curves are compared with theoretical predictions from our
model that incorporate effects of an accretion disk and post-Newtonian
description for the binary black hole orbit. We find that the precursor flares
coincide with the secondary black hole descending towards the accretion disk of
the primary black hole from the observed side, with a mean z-component of
approximately z_c = 4000 AU. We use this model of precursor flares to predict
that precursor flare of similar nature should happen around 2020.96 before the
next major outburst in 2022.Comment: to appear in the Astrophysical Journa
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