Gaia science alerts and the observing facilities of the Serbian-Bulgarian mini-network telescopes

The astrometric European Space Agency (ESA) Gaia mission was launched in December 19, 2013. One of the tasks of the Gaia mission is production of an astrometric catalog of over one billion stars and more than 500000 extragalactic sources. The quasars (QSOs), as extragalactic sources and radio emitters, are active galactic nuclei objects (AGNs) whose coordinates are well determined via Very Long Baseline Interferometry (VLBI) technique and may reach sub-milliarcsecond accuracy. The QSOs are the defining sources of the quasi-inertial International Celestial Reference Frame (ICRF) because of their core radio morphology, negligible proper motions (until sub-milliarcsecond per year), and apparent point-like nature. Compact AGNs, visible in optical domain, are useful for a direct link of the future Gaia optical reference frame with the most accurate radio one. Apart from the above mentioned activities, Gaia has other goals such as follow-up of transient objects. One of the most important Gaia's requirements for photometric alerts is a fast observation and reduction response, that is, submition of observations within 24 hours. For this reason we have developed a pipeline. In line with possibilities of our new telescope (D(cm)/F(cm)=60/600) at the Astronomical Station Vidojevica (ASV, of the Astronomical Observatory in Belgrade), we joined the Gaia-Follow-Up Network for Transients Objects (Gaia-FUN-TO) for the photometric alerts. Moreover, in view of the cooperation with Bulgarian colleagues (in the frst place, SV), one of us (GD) initiated a local mini-network of Serbian { Bulgarian telescopes useful for the Gaia-FUN-TO and other astronomical purposes. During the next year we expect a new 1.4 m telescope at ASV site. The speed of data processing (from observation to calibration server) could be one day. Here, we present an overview of our activities in the Gaia-FUN-TO which includes establishing Serbian { Bulgarian mini-network (of five telescopes at three sites, ASV in Serbia, Belogradchik and Rozhen in Bulgaria), the Gaia-FUN-TO test observations, and some results. [Projekat Ministarstva nauke Republike Srbije, br. 176011: Dynamics and kinematics of celestial bodies and systems, br. 176004: Stellar physics, i br. 176021: Visible and invisible matter in nearby galaxies: theory and observations

Envelopes of Cometary Orbits

We discuss cometary orbits from the standpoint of Nonstandard (Leibnitz) analysis, a relatively new branch of mathematics. In particular, we consider parabolic cometary paths. It appears that, in a sense, every parabola is an ellipse

Corrections to the Hipparcos proper motions in declination for 807 stars

We used the data on latitude variations obtained from observations with 10 classical photographic zenith tubes (PZT) in order to improve the Hipparcos proper motions in declinations µδ for 807 stars. Part of observing programmes, carried out during the last century for the purpose of studying the Earth's rotation, were realized by using PZT instruments. These observations were performed within in the intervals (tens of years) much longer than that of the Hipparcos mission (less than 4 years). In addition, the annual number of observations for every PZT programme star is several hundreds on the average. Though the accuracy of the star coordinates in the Hipparcos Catalogue is by two orders of magnitude better than that of the star coordinates from the PZT observations, the large number of observations performed a much longer time interval makes it possible to correct the Hipparcos proper motions and to improve their accuracy with respect to the accuracy given in the Hipparcos Catalogue. Long term examinations of latitude and time variations were used to form the Earth Orientation Catalogue (EOC-2), aimed at a more accurate determination of positions and proper motions for the stars included. Our method of calculating the corrections of the proper motions in declination from the latitude variations is different from the method used in obtaining the EOC-2 Catalogue. Comparing the results we have established a good agreement between our µδ and the EOC-2 ones for the star sample used in the present paper

Corrected μβ for stars of Hipparcos catalogue from independent latitude observations over many decades

During the last century, there were many so-called independent latitude (IL) stations with the observations which were included into data of a few international organizations (like Bureau International de l'Heure - BIH, International Polar Motion Service - IPMS) and the Earth rotation programmes for determining the Earth Orientation Parameters - EOP. Because of this, nowadays, there are numerous astrometric ground-based observations (made over many decades) of some stars included in the Hipparcos Catalogue (ESA 1997). We used these latitude data for the inverse investigations - to improve the proper motions in declination μβ of the mentioned Hipparcos stars. We determined the corrections Δμβ and investigated agreement of our μβ and those from the catalogues Hipparcos and new Hipparcos (van Leeuwen 2007). To do this we used the latitude variations of 7 stations (Belgrade, Blagoveschtschensk, Irkutsk, Poltava, Pulkovo, Warsaw and Mizusawa), covering different intervals in the period 1904.7 - 1992.0, obtained with 6 visual and 1 floating zenith telescopes (Mizusawa). On the other hand, with regard that about two decades have elapsed since the Hipparcos ESA mission observations (the epoch of Hipparcos catalogue is 1991.25), the error of apparent places of Hipparcos stars has increased by nearly 20 mas because of proper motion errors. Also, the mission lasted less than four years which was not enough for a sufficient accuracy of proper motions of some stars (such as double or multiple ones). Our method of calculation, and the calculated μβ for the common IL/Hipparcos stars are presented here. We constructed an IL catalogue of 1200 stars: there are 707 stars in the first part (with at least 20 years of IL observations) and 493 stars in the second one (less than 20 years). In the case of μβ of IL stars observed at some stations (Blagoveschtschensk, Irkutsk, Mizusawa, Poltava and Pulkovo) we find the formal errors less than the corresponding Hipparcos ones and for some of them (stations Blagoveschtschensk and Irkutsk) even less than the new Hipparcos ones

Comparison of proper motions in declination for 387 Gaia DR2 and HIPPARCOS stars from ILS observations over many decades

International audienceContext. The second solution of the Gaia catalog, which has been available since April 2018, plays an important role in the realization of the future Gaia reference frame. Since 1997, the reference frame has been materialized by the optical Hipparcos positions of about 120 000 stars. The Hipparcos has been compared with and linked to the International Celestial Reference Frame (ICRF). The ICRF is materialized by means of the radio positions of extragalactic sources using very large baseline interferometry observations. Both, the Hipparcos and Gaia missions belong to the European Space Agency, and it is important to note that the Gaia catalog is going to replace the Hipparcos catalog. Aims. It has been shown that the International Latitude Service zenith telescope data pertaining to ground-based surveys that span a time baseline of about 80 yr, and which are also key when measuring proper motions, could be useful for the accurate determination of µ δ for 387 ILS stars. Therefore, in this study we aim first to reduce these stars to the Hipparcos reference system; second, to made our original catalog of µ δ , which we refer to as the ILS catalog, for these 387 bright stars; third, to present comparison results of the four catalogs by pairs (the ILS, Hipparcos or HIP, new Hipparcos or NHIP, and Gaia DR2); and fourth, to analyze the differences in µ δ between pairs of catalogs to characterize the µ δ errors for these catalogs with a special focus on the Gaia DR2 and ILS catalogs. Methods. At seven ILS sites around the world at latitude 39. • 1, a set of seven telescopes was used to monitor the latitude variation via observations of the same stars for about 80 yr. Here, the inverse task was applied to improve µ δ values of the 387 Hipparcos stars using the previously mentioned observations. Due to the specific Horrebow-Talcott method of the measured star pair, it is difficult to determine µ δ for each single star. However, we achieved this by developing the original method and in combination with the Hipparcos data. We used the previously developed least squares method and formula to determine the coefficients, which describe the systematic part of differences in µ δ between the pairs of catalogs. Results. We calculated the coefficients with the aforementioned formula (in line with the coordinates, stellar magnitude, and color index of every star) to compare ILS, HIP, NHIP, and Gaia DR2 data of µ δ against each other by using the set of 387 stars. The presented differences of µ δ show that the systematic errors in the four catalogs are nearly at the same level of 0.1 mas yr −1. This means that the DR2 and ILS µ δ values are in good agreement with each other, and with values from the Hipparcos and new Hipparcos catalogs. Also, the random errors of differences are small ones; they are near 1 mas yr −1 for ILS-HIP and ILS-NHIP, and about 2 mas yr −1 for ILS-DR2, HIP-DR2, and NHIP-DR2. It is important to note that there is a similar level of proper motion formal errors in Hipparcos and new Hipparcos catalogs

Corrections of proper motions in declination by using ILS data

There are nowadays numerous astrometric ground{based observations of some stars referred to Hipparcos Catalogue, made at many observatories during the last century. We used the data on latitude variations, covering the period 1899.7 - 1979.0, of visual Zenith Telescopes (ZT) of International Latitude Service (ILS), to improve the Hipparcos proper motions in declination for stars observed at seven ILS stations: Carloforte, Cincinnati, Gaithersburg Kitab, Mizusawa, Tschardjui and Ukiah. About 15 years elapsed since the HIPPARCOS ESA mission (ESA 1997) observations (1991.25 is the epoch of this catalogue), and with the errors of the Hipparcos proper motions close to 1 mas/yr, the error of apparent places of stars is now more than 15 mas; so that it exceeds the error of the Hipparcos positions by one order of magnitude (which is about 1 mas). Also, for some Hipparcos stars, the errors of proper motions are much larger than the averaged value itself, even not realistic at all (Vondrák et al. 1998); the Hipparcos astronomical satellite mission lasted less than four years, not enough to get a sufficient accuracy of the proper motions. To improve the accuracy of the proper motions for some Hipparcos stars, the ground-based data were used and some new catalogues were published (such as ARIHIP, EOC-2, etc) during the last decade. Our investigations are in accordance with the Earth Orientation Catalogue - EOC (Vondrák and Ron 2003) one, based on the Earth rotation programmes ground{based data, but we used different method here. Our results yield better proper motions in declination for stars common to ILS and HIPPARCOS and a good agreement with those from EOC-2

Envelopes of cometary orbits

We discuss cometary orbits from the standpoint of Nonstandard (Leibnitz) analysis, a relatively new branch of mathematics. In particular, we consider parabolic cometary paths. It appears that, in a sense, every parabola is an ellipse

Short-term and long-term flux variability of extragalactic objects useful for the future Gaia CRF

International audienceSome Active Galactic Nuclei (AGN) objects (quasars - QSOs, blazars, for example) are well known for their rapid flux variability across the whole electromagnetic spectrum. They are variable on diverse time-scales, from minutes through months to even decades. There are three classes of variability: intraday - IDV (from a few minutes to several hours), short-term - STV (from several days to months), and long-term variability - LTV (from months to decades). In case of IDV, the flux changes is by a few tenths of magnitude, but in cases of STV and LTV it could be more than magnitude (even few magnitudes). Photometry is a powerful tool to investigate AGNs by measuring their variability time-scales, amplitude and duty cycle. Quasars have got a fundamental role in the evolution of their host galaxies, and they are used to build the International Celestial Reference Frame (ICRF). It is of importance that their proper motions are negligible because of their extreme distances. The visual-wavelength Gaia astrometry of the micro-arcsecond domain has got significant positional offsets with the radio VLBI positions of QSOs. We did optical observations of QSOs (visible in both, optical and radio domain, and important for ICRF - Gaia CRF link) to study their flux and color variability on short-term and long-term time-scales. Some results of five objects (1535+231, 1556+335, 1607+604, 1722+119, and 1741+597) are presented, here

Focal length determination for the 60cm telescope at Astronomical station Vidojevica

The focal length of a telescope is an important parameter in determining the angular pixel size. This parameter is used for the purpose of determining the relative coordinates (angular separation and positional angle) of double and multiple stars, and the precise coordinates of extragalactic radio sources (ERS) that are visible at optical wavelengths. At the Astronomical Station Vidojevica we have collected observations of these objects using two CCD cameras, Apogee Alta U42 and SBIG ST-10ME, attached to the 60 cm telescope. Its nominal focal length is 600 cm as given by the manufacturer. To determine the telescope focal length more precisely for both attached detectors, we used angular-separation measurements from CCD images taken at Astronomical Station Vidojevica. The obtained focal lengths are: F42 = (5989 ± 7) mm using the CCD camera Apogee Alta U42 attached to the telescope, and F10 = (5972 ± 4) mm with the CCD camera SBIG ST-10ME attached to the telescope