| Sažetak | Promatranja radiointerferometrom LOFAR (skraćeno od engl. LOw Frequency ARray) otkrila su bogatu morfologiju polariziranog sinkrotronskog zračenja naše galaksije. Otkrivene strukture raspetljane su RM sintezom (skraćeno od engl. Rotation Measure synthesis), tehnikom u radiopolarimetriji koja razdvaja promatrano polarizirano zračenje prema količini Faradayeve rotacije. To nam onda omogućava proučavanje relativne raspodjele magnetsko-ionske međuzvjezdane tvari (engl. InterStellar Medium, skraćeno ISM) kao funkciju Faradayeve dubine, tj. Faradayevu tomografiju. U ovom radu napravljena je multifrekvencijska analiza struktura detektiranih Faradayevom tomografijom u širem području polja 3C196. Najupečatljiviji oblici su depolarizirani kanali koje možemo vidjeti na slikama maksimalnog polariziranog intenziteta. Koristeći RHT (skraćeno od engl. Rolling Hough Transform) algoritam za detekciju ravnih linija na slikama, dobiveno je da je orijentacija depolariziranih kanala vrlo slična orijentaciji filamenata neutralnog vodika i orijentaciji komponenti magnetskog polja u ravnini neba. Poravnanje između ta tri različita pokazatelja ISM-a govori nam da uređeno magnetsko polje ima važnu ulogu u oblikovanju različitih faza ISM-a na velikom području (∼ 20◦). Uz to, kut polarizacije svjetlosti zvijezda korelira s orijentacijom depolariziranih kanala u jednom od
promatranih polja, omogućavajući nam da po prvi puta odredimo udaljenosti do detektiranih Faradayevih struktura. Uz prethodnu metodu, pomoću pulsara koji su vidljivi u Faradayevom spektru LoTSS-a (skraćeno od engl. LOFAR Two-metre Sky Survey) te RM mape naše galaksije, dodatno se procijenila udaljenost do područja ISM-a koja stvaraju Faradayeve strukture te istražila
najveća udaljenost s koje potječe opaženo sinkrotronsko zračenje, tzv. horizont polarizacije. |
| Sažetak (engleski) | The direct way to address cosmological questions regarding Cosmic Dawn (CD) and the Epoch of Reionization (EoR) is by observing the neutral hydrogen line at a wavelength of 21 cm (1420 MHz), which allows us to investigate the evolution of neutral hydrogen (HI) throughout the history of the universe (Mesinger, 2019). Today, this cosmological signal is observed
at frequencies ranging from 30 to 200 MHz using various radio interferometers such as LOFAR (LOw Frequency ARray, van Haarlem et al., 2013), and soon, SKA (Square Kilometer Array, Koopmans et al., 2015). Detecting the cosmological signal is not straightforward due to foreground radiation at low radio frequencies, which, in terms of fluctuations, is orders of magnitude stronger than the cosmological signal itself. The dominant foreground radiation comes from our Galaxy as synchrotron radiation, produced by the spiralling motion of ultrarelativistic charged particles, mainly electrons, along the magnetic field lines. It dominates at frequencies below 10 GHz (Pacholczyk, 1970; Rybicki & Lightman, 1986). It is intrinsically linearly polarized, with a polarization degree of about 70% (Le Roux, 1961). The intensity of synchrotron
radiation depends on a density of ultrarelativistic cosmic electrons, the strength of the magnetic field component perpendicular to the line of sight, and the exponent in the energy distribution of ultrarelativistic cosmic electrons (Mesinger, 2019). The spectral index of the radiation is different at lower and higher frequencies due to the ageing of the energy spectrum of cosmic electrons. As cosmic electrons traverse the interstellar medium (ISM), they lose energy through
interactions with interstellar matter, magnetic fields, and radiation. The energy loss through synchrotron radiation is more significant for higher-energy particles because the radiation power is proportional to the square of the electron’s kinetic energy. In addition to variations in the spectral index across the entire sky, variations in the brightness temperature (intensity defined by the Rayleigh-Jeans law) of Galactic synchrotron radiation reflect spatial fluctuations in the density of cosmic electrons and the strength of the magnetic field in the ISM. |
