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On Synergies Between Ground-Based and Space-Based Observations: the WSO-UV Project

B. Shustov, M. Sachkov, and A. Shugarov

Institute of Astronomy, Russian Academy of Sciences, Moscow, Russia, [email protected]

Abstract. Ground-based support is an essential condition for the success of almost any space astronomical project. Optical telescopes are widely used to support space-based projects. In the first part of the paper, the main areas of the support are briefly considered: cooperative observations – both synchronous and asynchronous, ground testing of service systems (for example, fine guidance systems), astrometric (ballistic) support, etc. Some examples are briefly considered. Special attention is paid to ground support for the WSO-UV (Spektr-UF) space observatory. Specific examples of both science and technical support (new spectrographs designed for cooperative observations, ground testing of the guide sensor system) as well as organizational issues are discussed. The call for applications that require preliminary ground-based observation has been announced. Seven applications were selected.

Keywords: telescopes; space vehicles DOI:10.26119/978-5-6045062-0-2 2020 12

1 Ground-Based Observational Support for Space Astronomy We enter a new epoch in science on Universe, epoch of multi-channel astronomy. A comprehensive studies electromagnetic radiation, gravitational waves, and el- ementary particles, such as neutrinos and high-energy cosmic rays emitted by the same extraterrestrial sources, in order to obtain information about processes occurring in space, is an essence of multi-channel approach. The multi-channel astronomy paradigm is becoming firmly embedded in science. For example we can refer to (presumably) first professional book on the issue (Multichannel Astronomy, Ed: A. M. Cherepashchuk, Publisher: Vek-2, Moscow, ISBN: 978-5- 85099-198-2, appeared in December 2018). The multi-channel approach to observations of electromagnetic radiation also implies multi-wave (all-wave) approach, i.e. observations at all wavelengths of Ground-Based Observations to Support the WSO-UV Project the electromagnetic spectrum. Multi-wavelength concept inevitably means the involvement of space-based observation facilities, since the possibility of ground- based observations is strongly limited by the transparency of the Earth’s atmo- sphere. Ground-based and space-based observations of astronomical objects tend to be much more correlated than previously. Astronomers make joint observations all the time, but they did and still do so in a generally un-coordinated way. However, the understanding of the necessity and inevitability of coordinated observations is growing. We share the opinion expressed in O’Meara (2019): “Science..in many cases not only benefits from ground/space coordination, but requires it. Our current mode of siloed operations slows, hampers, and in some cases, halts scientific progress”. This position is supported by many authors of Decadal Survey on Astronomy and Astrophysics 2020 (Astro2020)1. The authors consider broad scope of issues: from Solar system (Kofman et al. 2020; Roth et al. 2020) to stars (Clementini et al. 2020) and cosmology (Cuby et al. 2019). In some projects, ground-based and space-based telescopes are inseparable components of the project. For example, in the “Spektr-R” project, which op- erated in the space-Earth interferometer mode. The new project of space interferometer “Millimetron” (Kardashev 2017) and the recently proposed radio space-ground interferometer for BH shadow observations (Mikheeva et al. 2020) will work in the same way. Optical ground-based telescopes are actively used to support space-based astronomical projects for various purposes. There are several types of the support: – cooperative observations; – ground testing of optical scientific and service instruments; – astrometric (ballistic) support. In this paper we discuss some examples the of support. Of course, cooperative observations are the most important part of the support. They could be synchronous or asynchronous. For synchronous observations (the concept makes sense for observations of transients) the following requirement must be met: the time interval for overlapping observations (with different tools) is shorter than the time scale of changes of the object. A good example of (almost) synchronous observations – optical observations of the gamma-ray burst sources on the optical MASTER network (see e.g. Troja et al. (2017)). Asynchronous observations with optical telescopes are most numerous. For example, the 6-m telescope BTA at SAO observatory, as it is reported by D.Kud- ryavtsev, is used for support for Spectrum-RG (SRG) space observatory, namely

1 See for details https://www.nationalacademies.org/our-work/ decadal-survey-on-astronomy-and-astrophysics-2020-astro2020

13 Shustov et al. for optical identification of distant galaxy clusters and X–ray quasars; observations of active galactic nuclei using RATAN-600 and Radioastron; monitoring of bright sources with RATAN-600 during the Planck mission. Few more examples follow. In Zaznobin et al. (2019) results of optical iden- tifications and spectroscopic redshift measurements for galaxy clusters from the second Planck catalogue of Sunyaev-Zeldovich sources are presented. The observations were carried out with the 1.5-m Russian-Turkish telescope (RTT-150), the 1.6-m Sayan Observatory AZT-33IK telescope, the 3.5-m Calar Alto telescope, and the 6-m SAO RAS telescope (BTA). The 1.6-m AZT-33VM and AZT-33IK telescopes of the Sayan Observatory of the ISTP SB RAS are included in the ground-based optical support complex of the SRG Observatory. The first two spectra of distant quasars (z 4) that were detected by eROSITA were observed (Khorunzhev et al. 2020). ∼ As to examples of ground testing of optical scientific and service instruments we refer to Section 3 of this paper, where we describe some results of testing the WSO-UV Fine Guidance System with ground-based telescopes Both RTT-150 and AZT-33VM telescopes perform high-precision (accuracy of 0.1 and 0.3 arcsec correspondingly, difference is due to different focal length of the telescopes) astrometric monitoring of the SRG spacecraft. Next sections of this paper are devoted to space observatory WSO-UV. For many astrophysical tasks included in the Core program of the mission ground- based spectroscopic observations in spectral domain of 300 – 1000 nm with high resolution (R = 50000) are required. For these purposes existing facilities of the SAO RAS, including NES, ESPRI and CAES spectrographs are being upgrading or constructed (Panchuk et al. 2018). We consider this way - to prepare dedicated ground-based instruments well beforehand – as a very promising one.

2 WSO-UV Project: General Information and State of Art

World Space Observatory - Ultraviolet (WSO-UV) aka “Spektr-UF” is a space astrophysical observatory for observations of astronomical objects in the far and near UV range (115–310 nm). Unlike the HST the space complex consists of uniﬁed platform (bus) “Navigator” and payload (P/L). Platform “Navigator” provides important service systems – attitude control (pointing and stabilisa- tion), radio link and power supply. Concept of the HST implies unique design of service subsystems integrated with the telescope. “Navigator” is a serial product. It was successfully used in space for ﬁve times, including astrophysical missions Spektr-R (Radioastron) and Spektr-RG. The latter at the moment is operating

14 Ground-Based Observations to Support the WSO-UV Project in space. P/L is essentially a complex of scientiﬁc equipments. We brieﬂy remind major features of the project (see for more details Shustov et al. (2018))

2.1 Complex of Scientiﬁc Equipment (CSE) The observatory has a complex of scientiﬁc equipment (CSE), which includes:

– T-170M telescope (aperture 170 cm, focal ratio 10, field of view diameter 30’, diffraction image quality in the center of the field); – a block of spectrographs (WUVS), including three channels: vacuum ultraviolet echelle spectrograph VUVES (115–176 nm, R 50000), UV echelle spectrograph UVES (174–310 nm, R 50000) and a∼ spectrograph with a long slit LSS (115–305 nm, R 1000);∼ – field cameras unit (FCU), including∼ a far-UV channel (FUV) for obtaining direct images in the range of 115–175 nm and a near-UV channel (NUV) for obtaining direct images in the range of 174–305 nm; – scientific data management unit (SDMU); – scientific equipment “Konus-UV” that is a small gamma-ray detector devel- oped at the Ioffe Institute of Physics and Technology of the Russian Academy of Sciences and which has been successfully used in space in other projects.

Fig. 1 shows the location of the fields of view (for spectrographs – that of slits) of scientific instruments and three guide sensors on the focal surface of the T-170M telescope. Work on the creation of the CSE continues as planned. Manutacturing of WUVES goes on. Current characteristics of FCU (see Tabl. 1) differ from those presented in Shustov et al. (2018). There is either a new pro- posal to include in the CSE an additional spectrograph for studying exospheres of Earth-like exoplanets. Space Research Institute and Institute of astronomy (both of the RAS) in cooperation with the Japanese aerospace Agency JAXA and a number of leading universities in Japan are working on creating a UV- spectrograph UVSPEX (Tavrov et al. 2018). Question about whether or not the UVSPEX spectrograph will be included in the CSE of the WSO-UV Observa- tory is discussed at the level of responsible persons in Roscosmos and JAXA. The main principle is – the inclusion of a new device in the CSE should not violate the project implementation schedule.

2.2 Cooperation Since Russia leads the major work on the creation of the WSO-UV Observatory, the project can be considered as national. According to preliminary estimates, up to 2 thousands of Russian scientists (astronomers, physicists, etc.) are eager to use the observatory in their research work.

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Fig. 1. Location of the ﬁelds of view of scientiﬁc instruments and three sensors of the Fine Guidance System (FGS S1, FGS S2 and FGS S3) on the focal surface of the T-170M telescope.

Table 1. Comparison of the characteristics of FCU (FUV and NUV channels) and Hub- ble telescope cameras – SBC (HST/ACS/SBC) and UVIS (HST/WFC3/UVIS)

Parameter FUV NUV SBC UVIS Detector type MCP CCD MCP, MAMA CCD Range, nm 115–176 174–305 (1000) 115–170 200–1000 2 Effective area, m 0.068 0.27∗ 0.18 0.45 FoV, ” ∅121 ∅451 34.59 30.8 162 162 × × Scale, ”/pixel 0.047 0.146 0.033 0.030 0.0395 × Detector size, mm ∅40 ∅37 25 61 61 × Effective image area, pixels ∅2.6k ∅3.1k 1k 1k 4k 4k × × Number of filters up to 10 up to 15 6 62 Number of prisms up to 2 no 2 no ∗ filters transmittance not included

The primary contractor of the project is Lavochkin Co. (it is responsible for the space complex as a whole, the platform “Navigator”, T-170M telescope, ground control system). The main scientific organization is the Institute of As- tronomy of the Russian Academy of Sciences (CSE, ground-based scientific complex). Basic partners are: VNIIEF (WUVES), IKI RAS (SDMU, electronics units of spectrographs and field cameras, FGS), LPI (UV detectors of WUVES, FCU),

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“Luch” Co. (coating of mirrors), LZOS (main and secondary mirrors of the T- 170M telescope). The cooperation is working rhythmically. The main international project partner, Spain, supplies a detector for the FUV channel of the FCU, participates in the creation of the science ground segment. Russia and Spain are members of the WSO-UV international consor- tium. As noted above, in in recent years, Japan has demonstrating an interest in participating in the project.

2.3 Ground Segment, Orbit, Launch, Launch Date The ground-based scientiﬁc complex (GSC) has adopted a symmetric scheme interaction between the project’s international partners – Russia and Spain. For scientiﬁc information processing a dedicated software has been prepared. Two- way communication lines were tested. Recently there have been made some changes in the choice of working orbit and launch vehicle. A new launcher “Angara-5” is selected for the launch of WSO-UV. In the current version of the Federal Space Program launch is scheduled for 2025. New orbit is geosynchronous with an inclination of 35◦. The previously considered orbit with an inclination of 51.6◦ turned out to be less favourable in terms of the radiation situation and the duration of radio visibil- ity. Since the launcher has a certain energy reserve, it was decided to change the inclination to ensure continuous communication with the spacecraft from the Russian and Spanish ground stations.

3 Testing of WSO-UV Fine Guidance System with Ground-Based Telescopes

Precise pointing (accuracy 0.100) and stabilization (accuracy 3σ 0.100) of T- 170M telescope is provided∼ by Fine Guidance System (FGS). A special∼ Master Catalog (MC) will be used as a catalog of guiding stars for the FGS (see for MC description Chupina et al. (2008)). The MC meets the following requirements: – a dense (not less than 3 stars in FoV of each FGS sensor) grid of guiding stars over the whole sky; – high accuracy of star coordinates; – the proximity of the observation epoch to the launch time of the observatory; – completeness up to 17m in FGS-compatible photometric band. The MC was obtained from the 2MASS catalog by reducing its infrared stellar magnitudes J to optical RJ . The system of optical values RU from the UCAC2 catalog was chosen as the reference photometric system for this reduction The following sky areas were selected to test the functioning of the SDG: galactic

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Fig. 2. Section of the ASCC103 sky area. a) observed with Zeiss-1000 (SAO RAS); b) same region in the MC; c) image of a star 17m FGS. Exposure -1 s, S/N = 5.3; d) photometric proﬁle of the image c). pole, MW plane, open star cluster, outer regions of globular clusters, vicinity of a very bright star. To verify the engineering model of the FGS and photometric system of the Master Catalog ground based observations with Zeiss-1000 (SAO RAS) and Zeiss-2000 (Terskol observatory) as well as laboratory simulations were carried out (see Fig 2). The main results were presented at COSPAR-2014 conference (Shugarov et al. 2014).

4 WSO-UV: First Call for Applications In the ﬁrst two years of operation of the WSO-UV, the observational time will be divided between three Programs:

– Core Program, approximately 50% of the total time; – National Programs of the participating countries (approximately 40% of the total observation time, in proportion to ﬁnancial contributions of partner countries in the project); – Open Program (includes applications of high scientiﬁc value, regardless of whether the applicant represents the partner country of the project).

After completing the Core Program the share of the Open Program and National Programs will be substantially increased.

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All the proposals accepted for the Core Program are expected to be very ambitious tasks which require substantial observation time. The main research areas of the Core Program are described in some detail in Boyarchuk et al. (2016); Sachkov et al. (2018). Here we only brieﬂy recall the main elements of the Core Program.

Table 2. Selected application for the ﬁrst Call.

Partner (country) Theme of application Russia The WSO-UV survey of exoplanet atmospheres. Spain Magnetospheric accretion or boundary layer. Understanding the formation of Herbig Ae/Be stars. Russia Accretion processes in magnetic cataclysmic variables. Spain Lyman-alpha observation of cool, main sequence stars with high radial velocities. Spain Ultraviolet spectroscopy survey of T Tauri stars to understand planetary systems formation. Russia Studies of stellar activity of Sun-like stars and the stellar age-activity relation. Russia UV-spectoscopy of star grazing comets.

Currently, the Core Program of the WSO-UV includes:

– census of baryonic content (including metals) and chemical evolution of dif- fuse matter in the Universe and in galaxies; – physics of energetic processes (instabilities, outbursts, accretion onto com- pact objects etc.); – formation and early evolution of stars and circumstellar disks and astro- chemistry in presence of strong UV radiation ﬁelds; – atmospheres (exospheres) of exoplanets.

In 2018-2019, the initial call for applications, requiring preliminary ground support (relatively long-term observations with ground-based telescopes) was announced in Russia and Spain (Sachkov 2018). 7 application (see Tabl. 2) have been selected from 19 received.

5 Conclusions We conclude in the following:

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1. Astronomy is evolving towards multi-messenger time domain science. 2. The main factor of success in this way is a deep cooperation between ground- based and space-based observational programs. 3. International as well as national cooperation is absolutely necessary. 4. It is time to getting ready to work with the WSO-UV observatory.

Acknowledgements. The work was supported by RFBR grant No. 19-29-11027.

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