Northern Cross Radiotelescope Research Articles

The Northern Cross Fast Radio Burst project. IV. Multi-wavelength study of the actively repeating FRB 20220912A

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. 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. 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, which included high energy observations in the $0.3$--$10$ keV, $0.4$--$100$ MeV and $0.03$--$30$ GeV energy range. 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 $ 0.2$ and we measured a repetition rate of $0.19 $ 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 $ < -2.3$, with beta being the 408 MHz -- 1.4 GHz spectral index of FRB 20220912A. This is inconsistent with positive beta 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_ $ erg Hz$^ $, 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 $240 36$ mu Jy flux density at 1.2 GHz, centered on the FRB 20220912A coordinates. Finally, we place an upper limit on the gamma to radio burst efficiency eta to be $ 10^9$ at 99.7<!PCT!> confidence level, in the $0.4$ -- $30$ MeV energy range. 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 beta 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 mu Jy beam$^ $ (cit). Finally, the upper limit on the ratio between the gamma and radio burst fluence disfavours a giant flare origin for the radio bursts unlike observed for the Galactic magnetar SGR 1806-20.

The Northern Cross Fast Radio Burst project

Context. Fast radio bursts (FRBs) are millisecond radio transients observed at cosmological distances. The nature of their progenitors is still a matter of debate, although magnetars are invoked by most models. The proposed FRB–magnetar connection was strengthened by the discovery of an FRB-like event from the Galactic magnetar SGR J1935+2154. Aims. In this work we aim to investigate how prevalent magnetars such as SGR J1935+2154 are within FRB progenitors. Methods. To this end, we carried out an FRB search in a sample of seven nearby (< 12 Mpc) galaxies with the Northern Cross Radio Telescope for a total of 692 h. Results. We detected one 1.8 ms burst in the direction of M 101 with a fluence of 58 ± 5 Jy ms. Its dispersion measure of 303 pc cm−3 places it most likely beyond M 101. Considering that no significant detection comes indisputably from the selected galaxies, we place a 38 yr−1 upper limit on the total burst rate (i.e. including the whole sample) at the 95% confidence level. This upper limit constrains the event rate per magnetar to λmag < 0.42 magnetar−1 yr−1 or, if combined with literature observations of a similar sample of nearby galaxies, it yields a joint constraint of λmag < 0.25 magnetar−1 yr−1. We also provide the first constraints on the expected rate of FRBs hypothetically originating from ultra-luminous X-ray (ULX) sources, since some of the galaxies observed during our observational campaign host confirmed ULXs. We obtain < 13 yr−1 per ULX for the total sample of galaxies observed. Conclusions. Our results indicate that bursts with energies E > 1034 erg from magnetars such as SGR J1935+2154 appear more rarely compared to previous observations and further disfavour them as unique progenitors for the cosmological FRB population. This provides support to the idea that there is a greater contribution from a population of more exotic magnetars not born via core-collapsed supernovae.

The northern cross fast radio burst project – II. Monitoring of repeating FRB 20180916B, 20181030A, 20200120E, and 20201124A

ABSTRACT In this work, we report the results of a 19-month fast radio burst observational campaign carried out with the north–south arm of the Medicina Northern Cross radio telescope at 408 MHz in which we monitored four repeating sources: FRB20180916B, FRB20181030A, FRB20200120E, and FRB20201124A. We present the current state of the instrument and the detection and characterization of three bursts from FRB20180916B. Given our observing time, our detections are consistent with the event number we expect from the known burst rate (2.7 ± 1.9 above our 10σ, 38 Jy ms detection threshold) in the 5.2 d active window of the source, further confirming the source periodicity. We detect no bursts from the other sources. We turn this result into a 95 per cent confidence level lower limit on the slope of the differential fluence distribution α to be α > 2.1 and α > 2.2 for FRB20181030A and FRB20200120E, respectively. Given the known rate for FRB20201124A, we expect 1.0 ± 1.1 bursts from our campaign, consistent with our non-detection.

A Digital Beamformer for the PHAROS2 Phased Array Feed

PHased Arrays for Reflector Observing Systems (PHAROS) is a C-band (4–8[Formula: see text]GHz) Phased Array Feed (PAF) receiver designed to operate from the primary focus of a large single-dish radio astronomy antenna. It consists of an array of 220-element Vivaldi antennas ([Formula: see text] polarization), cryogenically cooled at roughly 20[Formula: see text]K along with low noise amplifiers (LNAs), and of analogue beamformers cryogenically cooled at roughly 80[Formula: see text]K. PHAROS2, the upgrade of PHAROS, is a PAF demonstrator developed in the framework of the Square Kilometer Array Advanced Instrumentation Program (SKA AIP) with the goal of investigating the potential of the PAF technologies at high frequencies in view of their possible application on the SKA dish telescopes. The PHAROS2 design includes new cryogenically cooled LNAs with state-of-the-art performance, a digital beamformer capable of synthesizing four beams from a sub-array of 24 single-polarization antenna elements, and a C-band multi-channel Warm Section receiver capable of analogue filtering and down-converting the signals from the antennas to a suitable frequency range at the input of the digital backend, providing an instantaneous bandwidth of 275[Formula: see text]MHz for each signal. In this paper, we describe the design and performance of the PHAROS2 digital backend/beamformer, based on the Italian Tile Processing Module (ITPM) hardware, which was initially developed for the SKA Low Frequency Aperture Array (LFAA). The backend was adapted to perform the beamforming for our PAF application. We describe the implementation of the beamformer on the Field Programmable Gate Arrays (FPGAs) of the ITPM and how the backend was successfully used to synthesize four independent beams, both in the laboratory (across the entire 275[Formula: see text]MHz instantaneous bandwidth) and during on-field observations at the BEST-2 array (across 16[Formula: see text]MHz instantaneous bandwidth), which is a subset of the Northern Cross Radio Telescope (located in the district of Bologna, Italy). The beamformer design allows re-scaling to a greater number of beams and wider bandwidths.

The Northern Cross fast radio burst project – I. Overview and pilot observations at 408 MHz

ABSTRACT Fast radio bursts (FRBs) remain one of the most enigmatic astrophysical sources. Observations have significantly progressed over the last few years, due to the capabilities of new radio telescopes and the refurbishment of existing ones. Here, we describe the upgrade of the Northern Cross radio telescope, operating in the 400–416 MHz frequency band, with the ultimate goal of turning the array into a dedicated instrument to survey the sky for FRBs. We present test observations of the pulsar B0329+54 to characterize the system performance and forecast detectability. Observations with the system currently in place are still limited by modest sky coverage (∼9.4 deg2) and biased by smearing of high dispersion measure events within each frequency channels. In its final, upgraded configuration, however, the telescope will be able to carry out unbiased FRB surveys over a ∼350 deg2 instantaneous field of view up to z ∼ 5, with a (nearly constant) $\sim 760 \, (\tau /{\rm ms})^{-0.5}$ mJy rms sensitivity.

Gridded Vivaldi Antenna Feed System for the Northern Cross Radio Telescope

The design of a broadband antenna feed system for the cylindrical offset parabolic reflector of the Italian Northern Cross radio telescope is described. Its operative frequency band spans from the Low Frequency Array (LOFAR) upper band, i.e. 120-240 MHz, to the present telescope operative band centered at 408 MHz. The proposed configuration consists of a linear array of gridded Vivaldi (tapered slot) radiators inside a wired subreflector. Numerical simulations have been carried out using both commercial software and a specialized Method-of-Moments approach. They show that the designed feed system provides both a good impedance matching and an efficient illumination of the main reflector in the overall frequency band.

Northern Cross Radiotelescope: Test and Measurements of Reliability Performance on Radioreceiver Chains

In this paper, the results of reliability measurements and tests are presented. The systems taken into account concern two different architectures of a receiver chain for implementation in the Northern Cross Radiotelescope. A reliability analysis of the circuit and reliability tests implemented on critical parts of a fiber receiver chain were performed. The target was to chose the receiver chain with the highest value of mean time between failures and minimize faults in the structure. Moreover, since the reliability prediction indicated the front end, which was installed on the focal line of the antenna, as the more stressed block of the entire radioreceiver chain by environmental and climatic factors, we proposed specific development tests. The whole test plan has been characterized, and some results after a combination of temperature and humidity cycles, as well as random vibrations tests, are reported in this paper. The measurement and tests implemented in this paper for the solution of the receiver chain in the Northern Cross Radiotelescope can represent a useful aid for the new design of the international SKA project.

Basic Element for Square Kilometer Array Training (BEST): Evaluation of the Antenna Noise Temperature

In this paper, a rigorous procedure for calculating the antenna noise temperature is described, and applied to the antenna of BEST-1 (basic element for SKA training - version 1), which represents one of the SKA (square kilometer array) demonstrators. The SKA will be a new-generation radio telescope, with a collecting area 50 times larger than the area of today's largest radio telescope. BEST is the Italian reduced-scale SKA demonstrator, based on the re-instrumentation of about 8000 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of the Northern Cross radio telescope, built with cylindrical parabolic antennas. In order to perform the antenna-temperature analysis, an electromagnetic tool to accurately evaluate the antenna pattern in the whole space surrounding the antenna itself is required. We used the commercial software GRASP8, developed by TICRA, to characterize reflector antennas. The antenna temperature was evaluated using the guideline adopted by the Antenna Task Force of the SKA world consortium. For BEST-1 at 408 MHz, we found an antenna temperature equal to 30 K in the zenith direction and 60 K at the horizon. The numerical results have been verified through several celestial calibration radio sources.

Italian SKA test bed based on cylindrical antennas

AbstractA re‐engineering of part of the Northern Cross radiotelescope for concepts verifications and technology developments for a next generation radiotelescope (SKA, Square Kilometre Array) is here described. Such a re‐instrumentation could produce a SKA demonstrator at a low cost and it might be ready to operate late 2007. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Search for radio recombination lines at 408 MHz with the northern cross

Abstract Spectral line search was carried out for the first time with the Northern Cross radiotelescope at Medicina (Italy). The total bandpass of the antenna was sampled and processed in real time with the high resolution digital FFT spectrometer. The radio recombination line of carbon C252α was reliably detected toward Cassiopeia A. The observed line profile is composed of 3 emission components at positions -47 km s−1, -38 km s−1 and 0 km s−1 and relative intensities as 3:2:1. The detection of the component at zero velocity is of special importance since there has been only one measurement of such an emission made at 325 MHz.

A fast PC-interface for synchronized sampling of digital data

We describe a high-speed interface unit for a PC-like microcomputer used for pulsar observations at the Northern Cross radiotelescope. The unit allows sampling synchronized to a master UTC clock and asynchronous output to a storage device with a peak data rate of about 2.5 MB/sec.

The contribution of the Italian Northern Cross Radiotelescope to the Gamma-Ray Observatory Pulsar Timing Network

The observation of pulsars at gamma-ray energy requires updated pulsar parameters in order to fold the gamma-ray data over time intervals of weeks. The Compton Gamma-Ray Observatory Institute has set up an international agreement including several radiotelescopes and pulsar observers in order to provide a suitable timing support at radio wavelengths. In this paper we describe the system used at the Italian Northern Cross for the pulse timing observations related to the GRO mission.

Radioemission from planetary nebulae observed with the pencil beam of the « Northern-Cross » radiotelescope

13 planetary nebulae were observed with the Bologna Northern Cross radiotelescope at a frequency of 408 MHz with the resolving power of 3 × 10 min of arc. The observations were conducted by using, for the first time, the pencil beam resulting from the correlation of both the E-W and the N-S arms of the radiotelescope. The most important result of this research is that we can now confirm that all the nebulae observed at radiofrequency show a thermal spectrum.