98 research outputs found
Fast-neutron induced background in LaBr3:Ce detectors
The response of a scintillation detector with a cylindrical 1.5-inch LaBr3:Ce
crystal to incident neutrons has been measured in the energy range En = 2-12
MeV. Neutrons were produced by proton irradiation of a Li target at Ep = 5-14.6
MeV with pulsed proton beams. Using the time-of-flight information between
target and detector, energy spectra of the LaBr3:Ce detector resulting from
fast neutron interactions have been obtained at 4 different neutron energies.
Neutron-induced gamma rays emitted by the LaBr3:Ce crystal were also measured
in a nearby Ge detector at the lowest proton beam energy. In addition, we
obtained data for neutron irradiation of a large-volume high-purity Ge detector
and of a NE-213 liquid scintillator detector, both serving as monitor detectors
in the experiment. Monte-Carlo type simulations for neutron interactions in the
liquid scintillator, the Ge and LaBr3:Ce crystals have been performed and
compared with measured data. Good agreement being obtained with the data, we
present the results of simulations to predict the response of LaBr3:Ce
detectors for a range of crystal sizes to neutron irradiation in the energy
range En = 0.5-10 MeVComment: 28 pages, 10 figures, 4 Table
High precision microlensing maps of the Galactic bulge
We present detailed maps of the microlensing optical depth and event density
over an area of 195 sq. deg towards the Galactic bulge. The maps are computed
from synthetic stellar catalogues generated from the Besancon Galaxy Model,
which comprises four stellar populations and a three-dimensional extinction map
calibrated against the Two-Micron All-Sky Survey. The optical depth maps have a
resolution of 15 arcminutes, corresponding to the angular resolution of the
extinction map. We compute optical depth and event density maps for all
resolved sources above I=19, for unresolved (difference image) sources
magnified above this limit, and for bright standard candle sources in the
bulge. We show that the resulting optical depth contours are dominated by
extinction effects, exhibiting fine structure in stark contrast to previous
theoretical optical depth maps. Optical depth comparisons between Galactic
models and optical microlensing survey measurements cannot safely ignore
extinction or assume it to be smooth. We show how the event distribution for
hypothetical J and K-band microlensing surveys, using existing ground-based
facilities such as VISTA, UKIRT or CFHT, would be much less affected by
extinction, especially in the K band. The near infrared provides a substantial
sensitivity increase over current I-band surveys and a more faithful tracer of
the underlying stellar distribution, something which upcoming variability
surveys such as VVV will be able to exploit. Synthetic population models offer
a promising way forward to fully exploit large microlensing datasets for
Galactic structure studies.Comment: 8 pages, submitted to MNRA
The EROS2 search for microlensing events towards the spiral arms: the complete seven season results
The EROS-2 project has been designed to search for microlensing events
towards any dense stellar field. The densest parts of the Galactic spiral arms
have been monitored to maximize the microlensing signal expected from the stars
of the Galactic disk and bulge. 12.9 million stars have been monitored during 7
seasons towards 4 directions in the Galactic plane, away from the Galactic
center. A total of 27 microlensing event candidates have been found. Estimates
of the optical depths from the 22 best events are provided. A first order
interpretation shows that simple Galactic models with a standard disk and an
elongated bulge are in agreement with our observations. We find that the
average microlensing optical depth towards the complete EROS-cataloged stars of
the spiral arms is , a number that is
stable when the selection criteria are moderately varied. As the EROS catalog
is almost complete up to , the optical depth estimated for the
sub-sample of bright target stars with () is easier to interpret. The set of microlensing events
that we have observed is consistent with a simple Galactic model. A more
precise interpretation would require either a better knowledge of the distance
distribution of the target stars, or a simulation based on a Galactic model.
For this purpose, we define and discuss the concept of optical depth for a
given catalog or for a limiting magnitude.Comment: 22 pages submitted to Astronomy & Astrophysic
Discovery of a peculiar Cepheid-like star towards the northern edge of the Small Magellanic Cloud
For seven years, the EROS-2 project obtained a mass of photometric data on
variable stars. We present a peculiar Cepheid-like star, in the direction of
the Small Magellanic Cloud, which demonstrates unusual photometric behaviour
over a short time interval. We report on data of the photometry acquired by the
MARLY telescope and spectroscopy from the EFOSC instrument for this star,
called EROS2 J005135-714459(sm0060n13842), which resembles the unusual Cepheid
HR 7308. The light curve of our target is analysed using the Analysis of
Variance method to determine a pulsational period of 5.5675 days. A fit of
time-dependent Fourier coefficients is performed and a search for proper motion
is conducted. The light curve exhibits a previously unobserved and spectacular
change in both mean magnitude and amplitude, which has no clear theoretical
explanation. Our analysis of the spectrum implies a radial velocity of 104 km
s and a metallicity of -0.40.2 dex. In the direction of right
ascension, we measure a proper motion of 17.46.0 mas yr using EROS
astrometry, which is compatible with data from the NOMAD catalogue. The nature
of EROS2 J005135-714459(sm0060n13842) remains unclear. For this star, we may
have detected a non-zero proper motion for this star, which would imply that it
is a foreground object. Its radial velocity, pulsational characteristics, and
photometric data, however, suggest that it is instead a Cepheid-like object
located in the SMC. In such a case, it would present a challenge to
conventional Cepheid models.Comment: Correction of typos in the abstrac
R Coronae Borealis stars in the Galactic Bulge discovered by EROS-2
Rare types of variable star may give unique insight into short-lived stages
of stellar evolution. The systematic monitoring of millions of stars and
advanced light curve analysis techniques of microlensing surveys make them
ideal for discovering also such rare variable stars. One example is the R
Coronae Borealis (RCB) stars, a rare type of evolved carbon-rich supergiant. We
have conducted a systematic search of the EROS-2 database for the Galactic
catalogue Bulge and spiral arms to find Galactic RCB stars. The light curves of
100 million stars, monitored for 6.7 years (from July 1996 to February
2003), have been analysed to search for the main signature of RCB stars, large
and rapid drops in luminosity. Follow-up spectroscopy has been used to confirm
the photometric candidates. We have discovered 14 new RCB stars, all in the
direction of the Galactic Bulge, bringing the total number of confirmed
Galactic RCB stars to about 51. After reddening correction, the colours and
absolute magnitudes of at least 9 of the stars are similar to those of
Magellanic RCB stars. This suggests that these stars are in fact located in the
Galactic Bulge, making them the first RCB stars discovered in the Bulge. The
localisation of the 5 remaining RCBs is more uncertain: 4 are either located
behind the Bulge at an estimated maximum distance of 14 kpc or have an unusual
thick circumstellar shell; the other is a DY Per RCB which may be located in
the Bulge, even if it is fainter than the known Magellanic DY Per. From the
small scale height found using the 9 new Bulge RCBs,
pc (95% C.L.), we conclude that the RCB stars follow a disk-like distribution
inside the Bulge.Comment: 20 pages, 26 figures, Accepted in A&
Removing the Microlensing Blending-Parallax Degeneracy Using Source Variability
Microlensing event MACHO 97-SMC-1 is one of the rare microlensing events for
which the source is a variable star, simply because most variable stars are
systematically eliminated from microlensing studies. Using observational data
for this event, we show that the intrinsic variability of a microlensed star is
a powerful tool to constrain the nature of the lens by breaking the degeneracy
between the microlens parallax and the blended light. We also present a
statistical test for discriminating the location of the lens based on the
\chi^2 contours of the vector \Lambda, the inverse of the projected velocity.
We find that while SMC self lensing is somewhat favored over halo lensing,
neither location can be ruled out with good confidence.Comment: 15 text pages + 2 tables + 7 figures. Published in the Astrophysical
Journa
Search for new resonant states in 10C and 11C and their impact on the cosmological lithium problem
The observed primordial 7Li abundance in metal-poor halo stars is found to be
lower than its Big-Bang nucleosynthesis (BBN) calculated value by a factor of
approximately three. Some recent works suggested the possibility that this
discrepancy originates from missing resonant reactions which would destroy the
7Be, parent of 7Li. The most promising candidate resonances which were found
include a possibly missed 1- or 2- narrow state around 15 MeV in the compound
nucleus 10C formed by 7Be+3He and a state close to 7.8 MeV in the compound
nucleus 11C formed by 7Be+4He. In this work, we studied the high excitation
energy region of 10C and the low excitation energy region in 11C via the
reactions 10B(3He,t)10C and 11B(3He,t)11C, respectively, at the incident energy
of 35 MeV. Our results for 10C do not support 7Be+3He as a possible solution
for the 7Li problem. Concerning 11C results, the data show no new resonances in
the excitation energy region of interest and this excludes 7Be+4He reaction
channel as an explanation for the 7Li deficit.Comment: Accepted for publication in Phys. Rev. C (Rapid Communication
Blending in Gravitational Microlensing Experiments: Source Confusion And Related Systematics
Gravitational microlensing surveys target very dense stellar fields in the
local group. As a consequence the microlensed source stars are often blended
with nearby unresolved stars. The presence of `blending' is a cause of major
uncertainty when determining the lensing properties of events towards the
Galactic centre. After demonstrating empirical cases of blending we utilize
Monte Carlo simulations to probe the effects of blending. We generate
artificial microlensing events using an HST luminosity function convolved to
typical ground-based seeing, adopting a range of values for the stellar density
and seeing. We find that a significant fraction of bright events are blended,
contrary to the oft-quoted assumption that bright events should be free from
blending. We probe the effect that this erroneous assumption has on both the
observed event timescale distribution and the optical depth, using realistic
detection criteria relevent to the different surveys. Importantly, under this
assumption the latter quantity appears to be reasonably unaffected across our
adopted values for seeing and density. The timescale distribution is however
biased towards smaller values, even for the least dense fields. The dominant
source of blending is from lensing of faint source stars, rather than lensing
of bright source stars blended with nearby fainter stars. We also explore other
issues, such as the centroid motion of blended events and the phenomena of
`negative' blending. Furthermore, we breifly note that blending can affect the
determination of the centre of the red clump giant region from an observed
luminosity function. This has implications for a variety of studies, e.g.
mapping extinction towards the bulge and attempts to constrain the parameters
of the Galactic bar through red clump giant number counts. (Abridged)Comment: 18 pages, 10 figures. MNRAS (in press
Gamma ray production cross sections in proton induced reactions on natural Mg, Si and Fe targets over the proton energy range 30 up to 66 MeV
Gamma-ray excitation functions have been measured for 30, 42, 54 and 66 MeV
proton beams accelerated onto C + O (Mylar), Mg, Si, and Fe targets of
astrophysical interest at the separate-sector cyclotron of iThemba LABS in
Somerset West (Cape Town, South Africa). A large solid angle, high energy
resolution detection system of the Eurogam type was used to record Gamma-ray
energy spectra. Derived preliminary results of Gamma-ray line production cross
sections for the Mg, Si and Fe target nuclei are reported and discussed. The
current cross section data for known, intense Gamma-ray lines from these nuclei
consistently extend to higher proton energies previous experimental data
measured up to Ep ~ 25 MeV at the Orsay and Washington tandem accelerators.
Data for new Gamma-ray lines observed for the first time in this work are also
reported.Comment: 11 pages, 6 figures. IOP Institute of Physics Conference Nuclear
Physics in Astrophysics VII, 28th EPF Nuclear Physics Divisional Conference,
May 18-22 2015, York, U
Using microlensed quasars to probe the structure of the Milky Way
This paper presents an investigation into the gravitational microlensing of
quasars by stars and stellar remnants in the Milky Way. We present predictions
for the all-sky microlensing optical depth, time-scale distributions and event
rates for future large-area sky surveys. As expected, the total event rate
increases rapidly with increasing magnitude limit, reflecting the fact that the
number density of quasars is a steep function of magnitude. Surveys such as
Pan-STARRS and LSST should be able to detect more than ten events per year,
with typical event durations of around one month. Since microlensing of quasar
sources suffers from fewer degeneracies than lensing of Milky Way sources, they
could be used as a powerful tool for recovering the mass of the lensing object
in a robust, often model-independent, manner. As a consequence, for a subset of
these events it will be possible to directly `weigh' the star (or stellar
remnant) that is causing the lensing signal, either through higher order
microlensing effects and/or high-precision astrometric observations of the lens
star (using, for example, Gaia or SIM-lite). This means that such events could
play a crucial role in stellar astronomy. Given the current operational
timelines for Pan-STARRS and LSST, by the end of the decade they could
potentially detect up to 100 events. Although this is still too few events to
place detailed constraints on Galactic models, consistency checks can be
carried out and such samples could lead to exciting and unexpected discoveries.Comment: 11 pages, 8 figures. MNRAS (in press). Minor revisions according to
referee's report; mainly presentational issues and clarification of a few
items in the discussion; results and conclusions remain unchange
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