Andrea Mattana

Context. The radio emission from magnetars is poorly understood and poorly characterized observationally, particularly for what concerns single pulses and sporadic events. Interest in this type of radio emission has been boosted by the detection of an extremely bright millisecond radio signal from the Galactic magnetar designated as SGR J1935+2154 in 2020, which occurred almost simultaneously with a typical magnetar short burst of X-rays. As of now, this event remains the Galactic radio pulse that is the most reminiscent of fast radio bursts, and it is the only one that has a sound association with a known progenitor. Aims. We aim to constrain the rate of impulsive radio events from magnetars by means of intensive monitoring using a high-sensitivity radio telescope. Methods. We performed a long-term campaign on seven Galactic magnetars (plus one candidate) using the Northern Cross transit radio telescope (in Medicina, Italy), searching for short timescales and dispersed radio pulses. Results. We obtained no detections in ∼560 hours of observation, setting an upper limit at a 95% confidence level of < 52 yr⁻¹ on the rate of events with energy ≳10²⁸ erg, which is consistent with limits in the literature. Furthermore, under some assumptions regarding the properties and energetic behavior of magnetars, we find that our upper limits point toward the fact that the entire population of observed fast radio bursts cannot be explained by radio bursts emitted by magnetars.

Precise polarimetric measurements of radio signals from the sky are crucial for achieving many scientific goals of modern and future radio telescopes. A key figure of merit (FoM) used to quantify the polarization purity of a radio polarimeter is the intrinsic cross-polarization ratio (IXR, introduced by Carozzi and Woan¹). This FoM is independent of the coordinate systems and is commonly used to assess the polarimetric performance of low-frequency phased aperture arrays, such as low frequency aperture array (LOFAR),² murchison widefield array (MWA),³ long wavelenght array (LWA),⁴ and the upcoming low-frequency component of the Square Kilometre Array (SKA-Low).⁵ Polarization calibration of such arrays is challenging due to their wide fields of view and the scarcity of polarized calibrators at these frequencies. In the literature, the various works investigating polarization performance using IXR are mostly relying on electromagnetic antenna simulations [e.g., aperture tile in focus (APERTIF),⁶ MWA,⁷ and SKA-Low⁸]. More recently, all-sky observations from the LOFAR-LBA Swedish station have been used to estimate the instrumental Stokes parameters across the sky and enable the computation of its IXR.⁹

Context. Fast radio bursts (FRBs) are energetic, millisecond-duration radio pulses observed at extragalactic distances and whose origins are still a subject of heated debate. A fraction of the FRB population have shown repeating bursts, however it’s still unclear whether these represent a distinct class of sources. Aims. We investigated the bursting behaviour of FRB 20220912A, one of the most active repeating FRBs known thus far. In particular, we focused on its burst energy distribution, linked to the source energetics, and its emission spectrum, with the latter directly related to the underlying emission mechanism. Methods. We monitored FRB 20220912A at 408 MHz with the Northern Cross radio telescope and at 1.4 GHz using the 32-m Medicina Grueff radio telescope. Additionally, we conducted 1.2 GHz observations taken with the upgraded Giant Meter Wave Radio Telescope (uGMRT) searching for a persistent radio source coincident with FRB 20220912A, and included high energy observations in the 0.3–10 keV, 0.4–100 MeV and 0.03–30 GeV energy range. Results. We report 16 new bursts from FRB 20220912A at 408 MHz during the period between October 16^th 2022 and December 31^st 2023. Their cumulative spectral energy distribution follows a power law with slope α_E = −1.3 ± 0.2 and we measured a repetition rate of 0.19 ± 0.03 hr⁻¹ for bursts having a fluence of ℱ ≥ 17 Jy ms. Furthermore, we report no detections at 1.4 GHz for ℱ ≥ 20 Jy ms. These non-detections imply an upper limit of β < −2.3, with β being the 408 MHz – 1.4 GHz spectral index of FRB 20220912A. This is inconsistent with positive β values found for the only two known cases in which an FRB has been detected in separate spectral bands. We find that FRB 20220912A shows a decline of four orders of magnitude in its bursting activity at 1.4 GHz over a timescale of one year, while remaining active at 408 MHz. The cumulative spectral energy distribution (SED) shows a flattening for spectral energy E_ν ≥ 10³¹ erg Hz⁻¹, a feature seen thus far in only two hyperactive repeaters. In particular, we highlight a strong similarity between FRB 20220912A and FRB 20201124A, with respect to both the energy and repetition rate ranges. We also find a radio continuum source with 240 ± 36 μJy flux density at 1.2 GHz, centered on the FRB 20220912A coordinates. Finally, we place an upper limit on the γ to radio burst efficiency η to be η < 1.5 × 10⁹ at 99.7% confidence level, in the 0.4–30 MeV energy range. Conclusions. The strong similarity between the cumulative energy distributions of FRB 20220912A and FRB 20201124A indicate that bursts from these sources are generated via similar emission mechanisms. Our upper limit on β suggests that the spectrum of FRB 20220912A is intrinsically narrow-band. The radio continuum source detected at 1.2 GHz is likely due to a star formation environment surrounding the FRB, given the absence of a source compact on millisecond scales brighter than 48 μJy beam⁻¹. Finally, the upper limit on the ratio between the γ and radio burst fluence disfavours a giant flare origin for the radio bursts unlike observed for the Galactic magnetar SGR 1806-20.

A fiber delay measurement and tracking system,which consists of low-cost, off-the-shelf instruments and components, and finds application in several areas, such as Telecommunications, Radar and Radio Astronomy, is presented and tested in an on-the-field realisticscenario. Through a comparison performed with a correspondent high-cost version of the same system, and thanks to an accurate, physical-model-based postprocessing procedure, a precision of 0.1 ps is estimated for the fiber delay over a monitored distance of around 100 m.

The signal reception chain is an essential element for achieving the square kilometer array-low (SKA-low) system requirements in terms of high sensitivity and dynamic range. The balance between gain, linearity, and low power consumption, as well as the cost, are fundamental parameters that influence the selection of the most suitable technology for SKA-low. Further factors, such as low self-generated radio frequency (RF) interference, high reliability, robustness under extreme environment, and last but not least, the distance between the antennas and the acquisition systems, have impacts on the selection for both architecture and receiver system design. The selected technology for the SKA-low RF signal transportation is RF-over-fiber systems, where the preamplified RF signal picked up by the antennas is carried via analogue modulation over optical fiber. The rationales behind the selection are reported, along with descriptions on the development of the receiver prototypes. The prototypes were deployed and installed on the demonstrator arrays at the selected SKA-low site in Western Australian. Particular attention has been put on the thermal characterization of the receiver system under the actual operating temperature on site, especially when both transmitting part and the optical medium are subjected to external ambient temperature variations. Performance issues encountered in the demonstrator arrays are also discussed along with some proposals for future activities.

The Square Kilometre Array (SKA) will be the world’s largest and most sensitive radio telescope ever built, covering the huge fractional bandwidth from 50 MHz to 15.3 GHz. It will bring a revolution in astronomy and astrophysics in the coming decades.¹ When completed, it will consist of two telescopes: the SKA-Mid for the highest frequency end of the bandwidth (above 350 MHz, in the Karoo region of South Africa) and the SKA-Low for the lowest (below 350 MHz, in the Murchison Desert of Western Australia).

We present the Engineering Development Array 2, which is one of two instruments built as a second generation prototype station for the future Square Kilometre Low-Frequency Array. The array is comprised of 256 dual-polarization dipole antennas that can work as a phased array or as a standalone interferometer. We describe the design of the array and the details of design changes from previous generation instruments, as well as the motivation for the changes. Using the array as an imaging interferometer, we measure the sensitivity of the array at five frequencies ranging from 70 to 320 MHz.

The design and development process for the Square Kilometre Array (SKA) radio telescope’s Low Frequency Aperture Array component was progressed during the SKA pre-construction phase by an international consortium, with the goal of meeting requirements for a critical design review. As part of the development process a full-sized prototype SKA Low ‘station’ was deployed – the Aperture Array Verification System 1 (AAVS1). We provide a system overview and describe the commissioning results of AAVS1, which is a low frequency radio telescope with 256 dual-polarisation log-periodic dipole antennas working as a phased array. A detailed system description is provided, including an in-depth overview of relevant sub-systems, ranging from hardware, firmware, software, calibration, and control sub-systems. Early commissioning results cover initial bootstrapping, array calibration, stability testing, beam-forming, and on-sky sensitivity validation. Lessons learned are presented, along with future developments.

The Sardinia Aperture Array Demonstrator (SAD) is an Italian facility, which is composed of 128 prototypical Vivaldi antennas, specifically designed to operate across the 50–500 MHz frequency range. As well known, one of the major burden at low frequency are the radio frequency interferences, thus after several accurate measurement campaigns we realized that a specific signal conditioning is needed in order to feed the digital beamformer with the proper signal level. In this paper, we report the results of the preliminary tests that we carried out in order to design an ad hoc receiving chain for the SAD array.

We present the first Southern-Hemisphere all-sky imager and radio-transient monitoring system implemented on two prototype stations of the low-frequency component of the Square Kilometre Array (SKA-Low). Since its deployment, the system has been used for real-time monitoring of the recorded commissioning data. Additionally, a transient searching algorithm has been executed on the resulting all-sky images. It uses a difference imaging technique to enable identification of a wide variety of transient classes, ranging from human-made radio-frequency interference to genuine astrophysical events. Observations at the frequency 159.375 MHz and higher in a single coarse channel ( 0.926 MHz) were made with 2 s time resolution, and multiple nights were analysed generating thousands of images. Despite having modest sensitivity ( few Jy beam–1), using a single coarse channel and 2-s imaging, the system was able to detect multiple bright transients from PSR B0950+08, proving that it can be used to detect bright transients of an astrophysical origin. The unusual, extreme activity of the pulsar PSR B0950+08 (maximum flux density 155 Jy beam–1) was initially detected in a ‘blind’ search in the 2020 April 10/11 data and later assigned to this specific pulsar. The limitations of our data, however, prevent us from making firm conclusions of the effect being due to a combination of refractive and diffractive scintillation or intrinsic emission mechanisms. The system can routinely collect data over many days without interruptions; the large amount of recorded data at 159.375 and 229.6875 MHz allowed us to determine a preliminary transient surface density upper limit of for a timescale and limiting flux density of 2 s and 42 Jy, respectively. In the future, we plan to extend the observing bandwidth to tens of MHz and improve time resolution to tens of milliseconds in order to increase the sensitivity and enable detections of fast radio bursts below 300 MHz.