- evn2024@mpifr.de
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Contribution
Speakers
- Mr. Vladislavs BEZRUKOVS
Primary authors
- Mr. Vladislavs BEZRUKOVS (Ventspils International Radioastronomy centre)
Co-authors
- Mr. Jānis ŠTEINBERGS (Ventspils International Radio Astronomy Center)
- Mrs. Karina SKIRMANTE (Ventspils International Radio Astronomy Center)
- Artis ABERFELDS (Latvia Republic)
- Dr. Ross BURNS (RIKEN)
- Dr. Dmitrijs BEZRUKOVS (Engineering Research Institute Ventspils International Radio Astronomy Centre (ERI VIRAC) of Ventspils University of Applied Sciences (VUAS))
- Mr. Arturs ORBIDANS (Ventspils International Radio Astronomy Center)
- Ms. Aija KALNINA (Engineering Research Institute Ventspils International Radio Astronomy Centre (ERI VIRAC) of Ventspils University of Applied Sciences (VUAS))
- Dr. Kolotkov DMITRII (University of Warwick)
- Dr. Valery NAKARIAKOV (University of Warwick)
Content
Flares on the Sun and other stars result from rapid explosive releases of free magnetic energy stored in the solar/stellar coronae. Solar flares and connected with them coronal mass ejections (CME) have a crucial impact on physical conditions in the near-Earth space, known as “space weather”, and on the functioning of various infrastructure facilities such as communication and navigation systems, energy supply lines, high-latitude flights, etc., with the financial risks up to billions of euros worldwide. Other stars (including solar-type ones) have been observed to host flares far more energetic than the largest known solar flares. The occurrence of such ferocious “superflares” on the Sun would lead to devastating and long-lasting consequences for our civilisation. Therefore, the study of stellar superflares, processes operating in them, and assessment of the Sun’s capability to produce similar events and their expected occurrence rate are among the prioritised research directions of modern heliophysics and astrophysics. Observations in the radio band are one of the most promising tools for studying solar/stellar flares. The advantages of the radio band are justified by the facts that radio emission is generated directly in the solar or stellar corona, i.e. where the energy release processes take place; radio emission is highly sensitive to parameters of the magnetic field and accelerated particles and, therefore, allows for determining these parameters. Usually, an accurate determination of radio emission source parameters requires simultaneous observations at several frequencies. Therefore, the VIRAC with its rich complex of radio-band instruments, such as solar radiopolarimeters at the radiotelescope RT-32 and the LOFAR LV614 antenna array, is one of the most suitable facilities for flare studies. In addition, the high temporal resolution traditionally available in the radio band makes it possible to track rapid changes in the characteristics of the magnetic field and accelerated particles in flares. Moreover, single-baseline radio interferometers, e.g., the VIRAC RT-16 and RT-32 complex, offer interesting opportunities in this emerging research field. Compared to single-dish radio telescopes they have a much deeper sensitivity to radio continuum emission thanks to the signal correlation process and spatial filtering of the background sky emission. The use of large, multi-telescope arrays, like EVN is shared among the international research community, and hence usually prohibits monitoring observations and regular observations needed to observe stellar flares at the timescales of their variability. We exploring opportunity to use two radio telescopes, RT-16 and RT-32, situated in Irbene, Latvia for employing a single-baseline interferometer capable of leading global research in the field of solar and stellar flares in radio band. The two radio telescopes, of 32 m and 16 m diameters, are capable of interferometric observations under the operation of the VIRAC. The telescopes are separated by 800 m, which provides rapid increase in sensitivity compared to the single-dish observations, reduces RFI influence and enhances credibility of observations. Such a configuration of accessible radio telescopes is now uncommon due to the recent move of focus to large arrays, therefore presenting a unique opportunity to make progress and innovation in the research field. An in-house interferometer also has the benefit of flexible time allocation to provide the monitoring observations necessary to detect rapid activity on stellar flares. The early testing of interferometric observations using two-element Irbene complex has demonstrated that the interferometer can reach close to its theoretical sensitivity limit of 6 mJy in a 2-minute integration. After calibration using a preliminary data processing pipeline developed in the IVARS project (see report by J.Šteinbergs), the stability of the interferometer was confirmed to be suitably stable for the detection of potential stellar flares events. As on solar-type stars the flare occurrence rate is rather low, on average 0.1–0.2 per day, we focus on stellar flares on rapidly rotating red dwarfs, in which the observed flare rate is much higher than on the Sun. The selected objects are M-type main-sequence stars situated at a distance of up to 25 pc. On red dwarfs, flares are observed at a rate of 1–2 per day in the X-ray, UV, and optical bands. The radio brightness temperature of flares on these objects reaches 10^13 K. Typical values of the spectral density of the radiation flux varies as 50-200 mJy (cf. 6 mJy the expected sensitivity of Irbene interferometer). Thus, the choice of perspective targets is dictated by high flare activity, high radio brightness, and a fairly high position of the star in the Northern Hemisphere (declination): e.g. CR Dra (Dec=+55, M5.6V), DO Cep (Dec=+57, M4.0V), EV Lac (Dec=+44, M4.0V), AD Leo (Dec=+19, M3.5eV). The observed occurrence rate, 1-2 flares per day, allows us to expect that a series of observations in the tracking mode with the highest possible time resolution of one of these targets within one week will give us the opportunity to register several flares. The high time resolution will allow us to detect short-period Quasi-Periodic Pulsations (QPP) with periods about a minute, which are the most common QPP periods in solar flares and non-stationary processes in QPPs (period drift, amplitude modulation, multiple periodicities). Observations are carried out in both polarisations in the 6.7 GHz band corresponding to the typical gyrosynchrotron emission, with Irbene radiotelescopes RT-32 and RT-16 in the interferometer mode. This observational programme does not require high spatial resolution, since the selected flare stars are sufficiently isolated in the radio band. This work has been supported by project “Multi-Wavelength Study of Quasi-Periodic Pulsations in Solar and Stellar Flares. (lzp-2022/1-0017).