Proper motion of an ionized filament in the interstellar medium observed with the LOFAR radio telescope

Međuzvjezdana materija je tvar koja prožima prostor između zvijezda unutar galaksija, pa tako i prostor unutar naše galaksije po imenu Mliječni put. Sastoji se od plina u ioniziranom, atomskom i molekularnom stanju, kozmičke prašine i visokoenergetskih kozmičkih zraka, te je prožeta magnetskim poljima. U radiopodručju elektromagnetskog spektra, međuzvjezdana materija može se proučavati analizom sinkrotronskog zračenja i njegove polarizacije. Nedavna opažanja pomoću radioteleskopa LOFAR (100 – 200 MHz) otkrila su nekoliko stupnjeva dugačak i vrlo ravni filament koji zakreće ravninu polarizacije pozadinskog sinkrotronskog zračenja. Najvjerojatnije se radi o filamentu ionizirane međuzvjezdane materije koja se nalazi unutar 100 pc od nas. Cilj ovoga rada je izmjeriti vlastito gibanje filamenta analizirajući LOFAR-ova promatranja vremenski razmaknuta jednu i tri, odnosno četiri godine. Diferencijalnom usporedbom položaja filamenta u vremenu nije zabilježen statistički značajan pomak u smjeru okomitom na os izduženosti filamenta, niti u mjerenjima koja su razmaknuta godinu dana, niti u mjerenjima koja su razmaknuta četiri godine. S obzirom na to da kroz razdoblje od četiri godine nema pomaka većeg od pogreške mjerenja (> 18 lučnih sekundi), vlastito gibanje filamenta je manje od 4:5 lučnih sekundi na godinu. Također, moguće je da je dominantan smjer gibanja filamenta paralelno s galaktičkom ravninom te povezano s rotacijom same galaksije.The Galactic interstellar medium (ISM) is the matter that permeates the space between the stars in our Galaxy. It includes gas in ionic, atomic, and molecular form, dust and cosmic rays. It is also pervaded by magnetic fields. The ISM can be studied at radio wavelengths by observing synchrotron emission and its polarization. Recent observation with the LOFAR radio telescope (100 – 200 MHz) revealed a few degrees straight filament, which changes the plane of polarization of the background synchrotron emission. It is likely a filament of an ionized gas located somewhere within 100 pc from us. This work aims to measure the proper motion of the filament by comparing its differential position in the LOFAR observations separated by one, three and four years. The filament does not show any statistically significant shift in the direction perpendicular to its orientation. Since there is no shift greater than the measurement error (> 18 arcseconds) over the course of four years, its proper motion is smaller than 4:5 arcseconds per year. It is also possible that the dominant velocity of the filament is almost parallel to the Galactic plane and connected to the global rotation of the Galaxy

Proper motion of an ionized filament in the interstellar medium observed with the LOFAR radio telescope

Međuzvjezdana materija je tvar koja prožima prostor između zvijezda unutar galaksija, pa tako i prostor unutar naše galaksije po imenu Mliječni put. Sastoji se od plina u ioniziranom, atomskom i molekularnom stanju, kozmičke prašine i visokoenergetskih kozmičkih zraka, te je prožeta magnetskim poljima. U radiopodručju elektromagnetskog spektra, međuzvjezdana materija može se proučavati analizom sinkrotronskog zračenja i njegove polarizacije. Nedavna opažanja pomoću radioteleskopa LOFAR (100 – 200 MHz) otkrila su nekoliko stupnjeva dugačak i vrlo ravni filament koji zakreće ravninu polarizacije pozadinskog sinkrotronskog zračenja. Najvjerojatnije se radi o filamentu ionizirane međuzvjezdane materije koja se nalazi unutar 100 pc od nas. Cilj ovoga rada je izmjeriti vlastito gibanje filamenta analizirajući LOFAR-ova promatranja vremenski razmaknuta jednu i tri, odnosno četiri godine. Diferencijalnom usporedbom položaja filamenta u vremenu nije zabilježen statistički značajan pomak u smjeru okomitom na os izduženosti filamenta, niti u mjerenjima koja su razmaknuta godinu dana, niti u mjerenjima koja su razmaknuta četiri godine. S obzirom na to da kroz razdoblje od četiri godine nema pomaka većeg od pogreške mjerenja (> 18 lučnih sekundi), vlastito gibanje filamenta je manje od 4:5 lučnih sekundi na godinu. Također, moguće je da je dominantan smjer gibanja filamenta paralelno s galaktičkom ravninom te povezano s rotacijom same galaksije.The Galactic interstellar medium (ISM) is the matter that permeates the space between the stars in our Galaxy. It includes gas in ionic, atomic, and molecular form, dust and cosmic rays. It is also pervaded by magnetic fields. The ISM can be studied at radio wavelengths by observing synchrotron emission and its polarization. Recent observation with the LOFAR radio telescope (100 – 200 MHz) revealed a few degrees straight filament, which changes the plane of polarization of the background synchrotron emission. It is likely a filament of an ionized gas located somewhere within 100 pc from us. This work aims to measure the proper motion of the filament by comparing its differential position in the LOFAR observations separated by one, three and four years. The filament does not show any statistically significant shift in the direction perpendicular to its orientation. Since there is no shift greater than the measurement error (> 18 arcseconds) over the course of four years, its proper motion is smaller than 4:5 arcseconds per year. It is also possible that the dominant velocity of the filament is almost parallel to the Galactic plane and connected to the global rotation of the Galaxy

Faraday tomography of LoTSS-DR2 data: I. Faraday moments in the high-latitude outer Galaxy and revealing Loop III in polarisation

Observations of synchrotron emission at low radio frequencies reveal a labyrinth of polarised Galactic structures. However, the explanation for the wealth of structures remains uncertain due to the complex interactions between the interstellar medium and the magnetic field. A multi-tracer approach to the analysis of large sky areas is needed. This paper aims to use polarimetric images from the LOFAR Two metre Sky Survey (LoTSS) to produce the biggest mosaic of polarised emission in the northern sky at low radio frequencies (150 MHz) to date. The large area this mosaic covers allows for detailed morphological and statistical studies of polarised structures in the high-latitude outer Galaxy, including the well-known Loop III region. We produced a 3100 square degree Faraday tomographic cube using a rotation measure synthesis tool. We calculated the statistical moments of Faraday spectra and compared them with data sets at higher frequencies (1.4 GHz) and with a map of a rotation measure derived from extragalactic sources. The mosaic is dominated by polarised emission connected to Loop III. Additionally, the mosaic reveals an abundance of other morphological structures, mainly {narrow and extended} depolarisation canals, which are found to be ubiquitous. We find a correlation between the map of an extragalactic rotation measure and the LoTSS first Faraday moment image. The ratio of the two deviates from a simple model of a Burn slab (Burn 1966) along the line of sight, which highlights the high level of complexity in the magnetoionic medium that can be studied at these frequencies.Comment: 20 pages, 25 figures, accepted for publication in A&

LOFAR Deep Fields: Probing faint Galactic polarised emission in ELAIS-N1

We present the first deep polarimetric study of Galactic synchrotron emission at low radio frequencies. Our study is based on 21 observations of the European Large Area Infrared Space Observatory Survey-North 1 (ELAIS-N1) field using the Low-Frequency Array (LOFAR) at frequencies from 114.9 to 177.4 MHz. These data are a part of the LOFAR Two-metre Sky Survey Deep Fields Data Release 1. We used very low-resolution ($4.3'$) Stokes QU data cubes of this release. We applied rotation measure (RM) synthesis to decompose the distribution of polarised structures in Faraday depth, and cross-correlation RM synthesis to align different observations in Faraday depth. We stacked images of about 150 hours of the ELAIS-N1 observations to produce the deepest Faraday cube at low radio frequencies to date, tailored to studies of Galactic synchrotron emission and the intervening magneto-ionic interstellar medium. This Faraday cube covers $\sim36~{\rm deg^{2}}$ of the sky and has a noise of $27~{\rm \mu Jy~PSF^{-1}~RMSF^{-1}}$ in polarised intensity. This is an improvement in noise by a factor of approximately the square root of the number of stacked data cubes ($\sim\sqrt{20}$), as expected, compared to the one in a single data cube based on five-to-eight-hour observations. We detect a faint component of diffuse polarised emission in the stacked cube, which was not detected previously. Additionally, we verify the reliability of the ionospheric Faraday rotation corrections estimated from the satellite-based total electron content measurements to be of $~\sim0.05~{\rm rad~m^{-2}}$. We also demonstrate that diffuse polarised emission itself can be used to account for the relative ionospheric Faraday rotation corrections with respect to a reference observation.Comment: 15 pages, 15 figures, accepted for publication in A&

Faraday tomography of the interstellar medium at low radio frequencies

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.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

Faraday tomography of the interstellar medium at low radio frequencies

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.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

Proper motion of an ionized filament in the interstellar medium observed with the LOFAR radio telescope

Međuzvjezdana materija je tvar koja prožima prostor između zvijezda unutar galaksija, pa tako i prostor unutar naše galaksije po imenu Mliječni put. Sastoji se od plina u ioniziranom, atomskom i molekularnom stanju, kozmičke prašine i visokoenergetskih kozmičkih zraka, te je prožeta magnetskim poljima. U radiopodručju elektromagnetskog spektra, međuzvjezdana materija može se proučavati analizom sinkrotronskog zračenja i njegove polarizacije. Nedavna opažanja pomoću radioteleskopa LOFAR (100 – 200 MHz) otkrila su nekoliko stupnjeva dugačak i vrlo ravni filament koji zakreće ravninu polarizacije pozadinskog sinkrotronskog zračenja. Najvjerojatnije se radi o filamentu ionizirane međuzvjezdane materije koja se nalazi unutar 100 pc od nas. Cilj ovoga rada je izmjeriti vlastito gibanje filamenta analizirajući LOFAR-ova promatranja vremenski razmaknuta jednu i tri, odnosno četiri godine. Diferencijalnom usporedbom položaja filamenta u vremenu nije zabilježen statistički značajan pomak u smjeru okomitom na os izduženosti filamenta, niti u mjerenjima koja su razmaknuta godinu dana, niti u mjerenjima koja su razmaknuta četiri godine. S obzirom na to da kroz razdoblje od četiri godine nema pomaka većeg od pogreške mjerenja (> 18 lučnih sekundi), vlastito gibanje filamenta je manje od 4:5 lučnih sekundi na godinu. Također, moguće je da je dominantan smjer gibanja filamenta paralelno s galaktičkom ravninom te povezano s rotacijom same galaksije.The Galactic interstellar medium (ISM) is the matter that permeates the space between the stars in our Galaxy. It includes gas in ionic, atomic, and molecular form, dust and cosmic rays. It is also pervaded by magnetic fields. The ISM can be studied at radio wavelengths by observing synchrotron emission and its polarization. Recent observation with the LOFAR radio telescope (100 – 200 MHz) revealed a few degrees straight filament, which changes the plane of polarization of the background synchrotron emission. It is likely a filament of an ionized gas located somewhere within 100 pc from us. This work aims to measure the proper motion of the filament by comparing its differential position in the LOFAR observations separated by one, three and four years. The filament does not show any statistically significant shift in the direction perpendicular to its orientation. Since there is no shift greater than the measurement error (> 18 arcseconds) over the course of four years, its proper motion is smaller than 4:5 arcseconds per year. It is also possible that the dominant velocity of the filament is almost parallel to the Galactic plane and connected to the global rotation of the Galaxy

Multi-tracer analysis of straight depolarisation canals in the surroundings of the 3C 196 field

Context. Faraday tomography of a field centred on the extragalactic point source 3C 196 with the LOw Frequency ARray (LOFAR) revealed an intertwined structure of diffuse polarised emission with straight depolarisation canals and tracers of the magnetised and multi-phase interstellar medium (ISM), such as dust and line emission from atomic hydrogen (HI). Aims. This study aims at extending the multi-tracer analysis of LOFAR data to three additional fields in the surroundings of the 3C 196 field. For the first time, we study the three-dimensional structure of the LOFAR emission by determining the distance to the depolarisation canals. Methods. We used the rolling Hough transform to compare the orientation of the depolarisation canals with that of the filamentary structure seen in HI, and based on starlight and dust polarisation data, with that of the plane-of-the-sky magnetic field. Stellar parallaxes from Gaia complemented the starlight polarisation with the corresponding distances. Results. Faraday tomography of the three fields shows a rich network of diffuse polarised emission at Faraday depths between − 10 and + 15 rad m−2. A complex system of straight depolarisation canals resembles that of the 3C 196 field. The depolarisation canals align both with the HI filaments and with the magnetic field probed by dust. The observed alignment suggests that an ordered magnetic field organises the multiphase ISM over a large area (~20°). In one field, two groups of stars at distances below and above 200 pc, respectively, show distinct magnetic field orientations. These are both comparable with the orientations of the depolarisation canals in the same field. We conclude that the depolarisation canals likely trace the same change in the magnetic field as probed by the stars, which corresponds to the edge of the Local Bubble