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Register a new userThe Gaia spectrophotometric standard stars survey -- IV. Results of the absolute photometry campaign
مسح علمي قيا للنجوم المعيارية الطيفية البصرية -- IV. نتائج الحملة الفوتومترية المطلقة
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نقدم فوتومتريا Johnson-Kron-Cousins BVRI لـ 228 نجمة مرشحة لإستخدامها كمعيار طبقات الإشعاع للبيانات الخاصة بجايا. تم جمع البيانات في إطار حملة مراقبة تستمر على مدى عشر سنوات مع هدف بناء شبكة خارجية لمعايير الإشعاع لجايا، وحصلنا على فوتومتريا مطلقة وفوتومتريا نسبية لمراقبة الثبات، وطبقات الإشعاع. تم استخدام الإصدارات المبدئية من الجداول الإشعاعية لتعيين الإصدارين الأول والثاني من جايا. يركز هذا البحث على الإطارات التي تم التقاطها في ظروف سماء جيدة (حوالي 9100). سيتم استخدام الفوتومتريا للتحقق من جداول الإشعاع الأرضية لنجوم معيار طبقات الإشعاع لجايا ولتصحيح الطيف الذي تم الحصول عليه في ظروف غير فوتومترية بشكل صحيح لأخذ الصفات الصغيرة. يربط الفوتومتريا المطلقة المقدمة هنا بنظام نجوم معيار لاندولت بدقة تبلغ حوالي 1٪ أو أفضل، بحسب الحزمة الفوتومترية. وبعد مقارنة واسعة مع مصادر مختلفة من الأدب، تبين توافقاً عاماً تبلغ حوالي 1٪، وهذا يبدو أنه الحد الحالي في دقة تعيين الإشعاع عبر مختلف العينات والتقنيات في الأدب. ويبلغ دقة الفوتومتريا الخاصة بجايا حالياً ما بين 0.1٪ أو أفضل، لذلك تناقش هنا الأفكار المختلفة لتحسين دقة تعيين الفوتومتريا.
We present Johnson-Kron-Cousins BVRI photometry of 228 candidate spectrophotometric standard stars for the external (absolute) flux calibration of Gaia data. The data were gathered as part of a ten-year observing campaign with the goal of building the external grid of flux standards for Gaia and we obtained absolute photometry, relative photometry for constancy monitoring, and spectrophotometry. Preliminary releases of the flux tables were used to calibrate the first two Gaia releases. This paper focuses on the imaging frames observed in good sky conditions (about 9100). The photometry will be used to validate the ground-based flux tables of the Gaia spectrophotometric standard stars and to correct the spectra obtained in non-perfectly photometric observing conditions for small zeropoint variations. The absolute photometry presented here is tied to the Landolt standard stars system to $simeq$1 per cent or better, depending on the photometric band. Extensive comparisons with various literature sources show an overall $simeq$1 per cent agreement, which appears to be the current limit in the accuracy of flux calibrations across various samples and techniques in the literature. The Gaia photometric precision is presently of the order of 0.1 per cent or better, thus various ideas for the improvement of photometric calibration accuracy are discussed.
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We describe two ground based observing campaigns aimed at building a grid of approximately 200 spectrophotometric standard stars (SPSS), with an internal ~1% precision and tied to Vega within ~3%, for the absolute flux calibration of data gathered by Gaia, the ESA astrometric mission. The criteria for the selection and a list of candidates are presented, together with a description of the survey strategy and the adopted data analysis methods. We also discuss a short list of notable rejected SPSS candidates and difficult cases, based on identification problems, literature discordant data, visual companions, and variability. In fact, all candidates are also monitored for constancy (within pm5 mmag, approximately). In particular, we report on a CALSPEC standard, 1740346, that we found to be a delta Scuti variable during our short-term monitoring (1-2 h) campaign.
We present the results of the short-term constancy monitoring of candidate Gaia Spectrophotometric Standard Stars (SPSS). We obtained time series of typically 1.24 hour - with sampling periods from 1-3 min to a few hours, depending on the case - to monitor the constancy of our candidate SPSS down to 10 mmag, as required for the calibration of Gaia photometric data. We monitored 162 out of a total of 212 SPSS candidates. The observing campaign started in 2006 and finished in 2015, using 143 observing nights on nine different instruments covering both hemispheres. Using differential photometry techniques, we built light curves with a typical precision of 4 mmag, depending on the data quality. As a result of our constancy assessment, 150 SPSS candidates were validated against short term variability, and only 12 were rejected because of variability including some widely used flux standards such as BD+174708, SA 105-448, 1740346, and HD 37725.
We present the flux tables of the spectro-photometric standard stars (SPSS) used to calibrate in flux the Gaia DR2 and (E)DR3 data releases. The latest SPSS grid version contains 112 stars, whose flux tables agree to better than 1% with the CALSPEC spectra of 11 flux standards for the calibration of the Hubble Space Telescope. The synthetic magnitudes computed on the SPSS spectra also agree to better than 1% with the Landolt magnitudes of 37 stars in common. The typical spreads in both comparisons are of the order of 1%. These uncertainties already meet the initial requirements for the Gaia SPSS project, but further improvements are expected in the next SP
We present precise photometry and spectroscopy for 23 candidate spectrophotometric standard white dwarfs. The selected stars are distributed in the Northern hemisphere and around the celestial equators and are all fainter than r ~ 16.5 mag. This network of stars, when established as standards, together with the three Hubble Space Telescope primary CALSPEC white dwarfs, will provide a set of spectrophotometric standards to directly calibrate data products to better than 1%. These new faint standard white dwarfs will have enough signal-to-noise ratio in future deep photometric surveys and facilities to be measured accurately while still avoiding saturation in such surveys. They will also fall within the dynamic range of large telescopes and their instruments for the foreseeable future. This paper discusses the provenance of the observational data for our candidate standard stars. The comparison with models, reconciliation with reddening, and the consequent derivation of the full spectral energy density distributions for each of them is reserved for a subsequent paper.
We describe the preliminary results of a ground-based observing campaign aimed at building a grid of approximately 200 spectro-photometric standard stars (SPSS), with an internal $simeq 1$% accuracy (and sub-percent precision), tied to CALSPEC Vega and Sirius systems within $simeq 1$%, for the absolute flux calibration of data gathered by {it Gaia}, the European Space Agency (ESA) astrometric mission. The criteria for the selection and a list of candidates are presented, together with a description of the surveys strategy and the adopted data analysis methods. All candidates were also monitored for constancy (within $pm 5$ mmag, approximately). The present version of the grid contains about half of the final sample, it has already reached the target accuracy but the precision will substantially improve with future releases. It will be used to calibrate the {it Gaia} (E)DR3 release of spectra and photometry.
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