1,359 research outputs found
Results from some calibration experiments conducted at Strasbourg Superconducting gravity station
No full textThe research deals with a calibration experiment performed in Strasbourg (France) superconducting gravity station by means of two FG5 AGs (#206 and #211) and some new generation spring meters (Scintrex Ltd. Autograv-CG3M and -CG5M and Microg-LaCoste gPhone). We discuss the results in terms of precision and accuracy of the SG calibration by means of different metrological approaches. Our main results turn out that spring gravimeters are not suitable to replace AG meters for SG calibration. Owing to the time variability of their sensitivity, the spring meters cannot be used as a stable reference for the SGs. On the contrary, the spring gravimeters could take advantage from measuring in well calibrated superconducting stations. In fact a monthly record session in a superconducting gravity observatory could provide a complete definition of the sensor's transfer function in the tidal band, both in phase and amplitude
COMPARISON OF NOISE LEVELS OF THE NEWEST GENERATION OF RELATIVE GRAVIMETERS
No full textSince the beginning of the Global Geodynamics Project in 1997, the number of superconducting gravimeters (SGs) has increased to reach 25 operating sites today. Data from the network allows a comparison of the noise levels of the different contributing stations. Knowledge of the noise levels of each station is important in a number of studies that combine the data to determine global Earth parameters. We cite for example the stacking of the data to determine the period of the free core nutation and the Chandler wobble, and the use of the data in the search for elusive signals, like the gravity variations associated with the translational mode of the inner core. We use a standardized processing procedure to evaluate the combined instrument plus site noise in the long-period seismic band (0.3 mHz - 1 mHz) and in the sub-seismic frequency band (0.03 mHz – 0.3 mHz), based on computing residual power spectral densities (PSDs) over a quiet time period. The experience at Strasbourg (France) has shown improvements from the T005 full-size instrument to the C026 compact model in terms of noise and drift, while the most recent type Observatory SG OSG044 at Bad-Homburg (Germany) does not show any improvement with respect to the compact CD30. Besides the development of new models of SGs by GWR, a new generation of portable Earth tide meters was developed by Micro-g LaCoste. A 6-month inter-comparison performed at Strasbourg shows that the newest generation of portable Earth tide meter (gPhone-54) is 20 dB noisier than the SG C026 and that its instrumental drift cannot be modeled by a simple polynomial function, preventing the use of the gPhone for long-term gravity studies
300-days of parallel gravity record with the gPhone-054 spring gravimeter and the GWR-C026 superconducting gravimeter in Strasbourg (France): a comparative study
No full textA wide set of geodynamical and hydrological phenomena, involving underground mass redistribution and/or change of the Earth’s figure, affects the gravity field, sometimes inducing “slow” and “small” temporal gravity changes, the detection of which relies on instruments with high sensitivity, long-term stability and a very low drift.
Here we report on the results of a comparative analysis carried out on more than ten months of co-located record collected with a new generation spring gravimeter, the gPhone-054, owned by the IGN of Madrid (Spain), and the GWR-C026 superconducting gravimeter (SG-C026) at the J9 gravity station in Strasbourg (France). The Microg-LaCoste gPhone is a portable Earth tide gravimeter equipped with a 0.1 μGal resolution feedback. The core sensor is the patented LaCoste & Romberg (LR) zero-length spring suspension system. The gPhone is essentially a LR, model G meter, but with significant upgrades: it has an improved thermal system (a double-oven) for increased temperature stability. Moreover the instrument should have a “true” vacuum seal making it almost insensitive to the buoyancy changes due to atmospheric pressure fluctuations.
We test the performances of the gPhone-054 in terms of resolution, accuracy, noise level and long-term stability (drift) with respect to the SG-C026. Our comparative analysis is performed in a wide spectral domain, ranging from the body tides to the seismic band. This study demonstrates that the SGs have better performances in the whole analyzed spectral band. Focusing on the gPhone-054 instrumental drift observed during this study, it still remains a critical point preventing the study of the long-term gravity changes. In fact the drift was large and even not linear, sometimes requiring a high degree (> 4) polynomial fitting to be reduced; the latter makes hard to distinguish real time gravity changes from the instrumental drift. We observed a drift rate evolution characterized by a decrease from 50 μGal/day to 15 μGal/day, after about 1 month of operation. Moreover we tried to improve the drift modeling by using frequent (about every 5 days) absolute gravity (AG) measurements collected within about forty days; the measurements have been carried out with FG5#211, but unfortunately during that time no significant gravity changes have been detected which would have helped us to discriminate short-term drift excursions from real gravity changes by superimposing the gPhone data onto the AG points
Cataclysmic variables from a ROSAT/2MASS selection. I, Four new intermediate polars
No full textWe report the first results from a new search for cataclysmic variables (CVs) using a combined X-ray (ROSAT)/infrared (2MASS) target selection that discriminates against background active galactic nuclei. Identification spectra were obtained at the Isaac Newton Telescope for a total of 174 targets, leading to the discovery of 12 new CVs. Initially devised to find short-period low-mass-transfer CVs, this selection scheme has been very successful in identifying new intermediate polars. Photometric and spectroscopic follow-up observations identify four of the new CVs as intermediate polars: 1RXS J063631.9+353537 P(orb)similar or equal to 201 min, P-spin= 1008.3408 s or 930.5829 s), 1RXS J070407.9+262501 (P(orb)similar or equal to 250 min, P-spin= 480.708 s) 1RXS J173021.5-055933 (P-orb= 925.27 min, P-spin= 128.0 s), and 1RXS J180340.0+401214 (P-orb= 160.21 min, P-spin= 1520.51 s). RX J1730, also a moderately bright hard X-ray source in the INTEGRAL/IBIS Galactic plane survey, resembles the enigmatic AE Aqr. It is likely that its white dwarf is not rotating at the spin equilibrium period, and the system may represent a short-lived phase in CV evolution
New periodic variable stars coincident with ROSAT sources discovered using SuperWASP
No full textWe present optical lightcurves of 428 periodic variable stars coincident with ROSAT X-ray sources, detected using the first run of the SuperWASP photometric survey. Only 68 of these were previously recognised as periodic variables. A further 30 of these objects are previously known pre-main sequence stars, for which we detect a modulation period for the first time. Amongst the newly identified periodic variables, many appear to be close eclipsing binaries, their X-ray emission is presumably the result of RS CVn type behaviour. Others are probably BY Dra stars, pre-main sequence stars and other rapid rotators displaying enhanced coronal activity. A number of previously catalogued pulsating variables (RR Lyr stars and Cepheids) coincident with X-ray sources are also seen, but we show hat these are likely to be misclassifications. We identify four objects which are probable low mass eclipsing binary stars, based on
their very red colour and light curve morphology
Integral simplex using decomposition with primal cutting planes
No full textThis paper concentrates on the addition of cutting planes to the integral simplex using decomposition (ISUD) of Zaghrouti et al. (Oper Res 62(2):435–449, 2014). This method solves the set partitioning problem by iteratively improving an existing feasible solution. We present the algorithm in a primal language and relate it to existing augmenting methods. The resulting theoretical properties, stronger than the ones already known, simplify termination proofs and deepen the geometrical insights on ISUD in particular. We show that primal cuts, that is, cutting planes that are tight at the current feasible integer solution, can be used to improve the performance of the algorithm, and further that such cutting planes are enough to solve each augmentation problem. We propose efficient separation procedures for well-known polyhedral inequalities, namely primal clique and odd-cycle cuts. Numerical results demonstrate the effectiveness of primal cutting planes; tests are performed on small and large-scale set partitioning problems from aircrew and bus-driver scheduling instances up to 1600 constraints and 570,000 variables
ROSAT X-ray observations of four planetary nebulae
No full textWe report ROSAT Position Sensitive Proportional Counter (PSPC) observations of four planetary nebulae. The observations show that all four nebulae have two-component spectra. A two-component model consisting of a blackbody and a Raymond-Smith thermal plasma is fitted to the observed ROSAT PSPC spectra of NGC 1535, Abell 36, and NGC 7293. For all three cases, the temperatures of the blackbody component are apparently higher than those of the stellar photospheres, indicating nonblackbody atmospheres. In all cases, the temperatures of the high-energy component can be attributed to a hot plasma associated with the possible stellar coronae or the "hot bubbles" predicted by the interacting winds model. The spectral energy distribution of NGC 1535 is constructed and modeled by incorporating our X-ray spectrum into existing data in UV, optical, near-infrared, and far-infrared wavelengths. © 1996. The American Astronomical Society. All rights reserved.published_or_final_versio
The Brera Multi-scale Wavelet ROSAT HRI source catalogue
No full textWe present the Brera Multi-scale Wavelet ROSAT HRI source catalogue
(BMW-HRI) derived from all ROSAT HRI pointed observations with
exposure times longer than 100 s available in the
ROSAT public archives.
The data were analyzed automatically using a wavelet
detection algorithm suited to the detection and characterization of both
point-like and extended sources.
This algorithm is able to detect and disentangle sources in very crowded fields
and/or in the presence of extended or bright sources.
Images have been also visually inspected after the analysis
to ensure verification.
The final catalogue, derived from 4303 observations, consists of 29 089 sources detected
with a detection probability of ≥
.
For each source, the primary catalogue entries provide name, position,
count rate, flux and extension along with the relative errors.
In addition, results of cross-correlations with existing
catalogues at different wavelengths (FIRST, IRAS, 2MASS and GSC2)
are also reported.
Some information is available on the web via the DIANA Interface.
As an external check, we compared our catalogue with the
previously available ROSHRICAT catalogue (both in its short and long
versions) and we were able to recover, for the short version, ~ of the entries.
We computed the sky coverage of the entire HRI data set by means of simulations.
The complete BMW-HRI catalogue provides a sky coverage of 732 deg2 down to a
limiting flux of ~
and of 10 deg2 down to ~.
We were able to compute the cosmological log(N)-log(S) distribution
down to a flux of
Evaluation Of Source Counts And Upper Limits
No full textWe present a method for determining source counts, S/N, and upper limits at specified positions in a crowded ROSAT PSPC field using a combination of publicly available software packages. The algorithm is based on the so-called `Local DETECT' method of source detection, and improves upon currently available software, to permit a meaningful comparison of non-detections with detected sources. We also present a recipe to obtain source counts and S/N in the case of point sources which overlap significantly
In-flight calibration of the ROSAT HRI ultraviolet sensitivity
No full textComparing measured and estimated count rates of a few selected sample stars, we confirm the validity and provide the in-flight calibration of the ROSAT HRI UV/visible effective area model in Zombeck et al. The count rate estimates for Betelgeuse derived with this model are in agreement with the measured HRI upper limit. This result is also confirmed in an erratum by Berghöfer et al. aimed at revising their previous calculation, which was overestimated by more than 2 orders of magnitude. Adopting this ROSAT HRI UV/visible effective area model and measured UV/visible spectra of a set of sample stars covering the range of Teff 3000-40,000 K, we have built the calibration curves to estimate UV/visible contamination count rates for any star of known Teff, mv, and NH
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