arXiv subjects

A. Geminardi

Publications and source records attributed to A. Geminardi.

7 recordsLinked to original sources

arXiv2026

Fast radio burst - persistent radio source systems II. A faint PRS associated with the nearby FRB 20181030A?

Persistent radio sources (PRSs) are the continuum counterparts of fast radio bursts (FRBs), the latter being extragalactic transients of millisecond duration and Jy-level flux density. An FRB-PRS system is thought to be a flaring magnetar surrounded by an highly magnetized, baryon-loaded nebula. We aim to constrain the size of 20181030A-S1, a new PRS candidate, potentially associated with the repeating FRB 20181030A. The latter is localized with $\sim 1'$ uncertainties in the outskirts of NGC 3252, which is a spiral galaxy at a luminosity distance of $20$ Mpc. We report very long baseline interferometric (VLBI) observations using the European VLBI Network at $1.7$ GHz of this PRS candidate at an angular resolution of $20$ milliarcseconds. Our observations reveal the presence of an unresolved radio source (20181030A-S1) at the position of the PRS candidate, confirming its compactness at milli-arcsecond angular scales. A fit to the position of the point-source yields a peak flux density of $280 \pm 30$ $μ$Jy and a transverse physical size constrained to be $R < 0.5$ pc at $68\%$ confidence level (CL). This flux density converts to a spectral luminosity of $(9 \pm 1) \times 10^{25}$ erg s$^{-1}$ Hz$^{-1}$, $\sim 3$ orders of magnitude lower than confirmed PRSs, making 20181030A-S1 the closest and faintest PRS candidate known. Its low luminosity and modest rotation measure are consistent with the $L_ν-$rotation measure (RM) relation followed by confirmed FRB--PRS systems, supporting a common physical origin in magnetar-powered nebulae. We show how a magnetized wind nebula powered by an initially weak ($B_\star \simeq 10^{15}$ G) and young ($t_{\rm age} \simeq 15 - 150$ yrs) magnetar can account for both the observed spectral luminosity and RM of the system. Other possible origin scenarios for 20181030A-S1, in the case in which it is unrelated to the FRB source, are also discussed.

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arXiv2026

Fast radio burst - persistent radio source systems III. The relation bewteen PRS luminosity and FRB rotation measure

Fast radio bursts (FRBs) are millisecond-duration radio transients of extragalactic origin whose physical origin remains uncertain. A small fraction of FRBs are known to repeat, and some of them are associated with persistent radio sources (PRSs), interpreted as synchrotron-emitting nebulae surrounding the FRB source. In the context of magnetar-based models, the rotation measure (RM) of an FRB is expected to correlate with the spectral luminosity of its associated PRS, providing a probe of the physical properties and evolution of the nebula. We investigate the relation between FRB RM and PRS spectral luminosity, constrain the characteristic size of the PRS nebulae, and use the intrinsic scatter of the relation to investigate evolutionary scenarios for FRB--PRS systems. We analyse a sample of 50 FRB sources with known RM, including both PRS detections and spectral luminosity upper limits, using a Bayesian MCMC framework that jointly accounts for detections and non-detections. We model the luminosity--RM relation as $L_ν\propto ζ_e γ_c^2 R^2 {\rm RM}^β$, constraining the characteristic nebular size $R$ and the RM scaling index $β$. For the full sample, we obtain $R = 0.016^{+0.115}_{-0.014}$ pc at $1σ$ confidence for a free $β$, while fixing the RM dependence to the canonical linear scaling, $β= 1$, gives $R = 0.008^{+0.058}_{-0.007}$ pc. The data mildly favour super linear RM scalings, although the inferred values of \b{eta} remain consistent with $β= 1$ within $2σ$. We find a substantial intrinsic scatter in the luminosity--RM relation, significantly larger than previous estimates based on confirmed PRSs alone. Interpreting this scatter within evolutionary models, our results favour scenarios involving efficient particle acceleration and/or rapid nebular expansion.

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arXiv2026

Fast radio burst - persistent radio source systems I. A 1.2 GHz search and catalog definition

We present new $1.26$ GHz observations of 24 fast radio bursts (FRBs; 9 repeaters and 11 one-offs), conducted with the upgraded Giant Meter Wave Radio Telescope (uGMRT) at arc-second angular scale resolution, and combined them with literature data to construct an extended sample of 75 sources with either persistent luminosity measurements or upper limits. Our catalog is complete down to a $10^{29}$ erg s$^{-1}$ Hz$^{-1}$ luminosity threshold, which corresponds to the value of the first-discovered PRS, associated with FRB 20121102. We detect compact radio emission in six FRB fields, two of which are previously known PRSs and four are new candidates. The four new candidates are associated with three new repeating FRBs and one apparently one-off source. We computed the PRS occurrence using the whole catalog of 75 FRBs and found that PRSs more luminous than $10^{29}$ erg s$^{-1}$ Hz$^{-1}$ are rare. Unless otherwise stated, all uncertainties and upper limits are reported at the $95\%$ confidence level (CL). If we only consider confirmed PRSs, we find an occurrence of $f_{\rm all} = 0.06^{+0.09}_{-0.03}$, while including PRS candidates as well yields $f_{\rm all} = 0.14^{+0.22}_{-0.08}$. We also compute the PRS occurrence in the two classes of FRBs: repeating and non-repeating. In the conservative case, i.e., when none of the new PRS candidates are considered to be associated with their FRBs, we find no preferred association between the PRSs and FRB class, with an occurrence of $f_{\rm r} = 0.14^{+0.20}_{-0.09}$ for the sample of repeating FRBs and $f_{\rm nr} \leq 0.12$ for the sample of one-off sources. In the inclusive case, i.e., when all the new candidates are considered confirmed PRSs, we find $f_{\rm r} = 0.27^{+0.21}_{-0.14}$ and $f_{\rm nr} = 0.03^{+0.14}_{-0.02}$, with marginal ($90\%$ CL) evidence that PRSs are preferably associated with repeating FRBs.

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arXiv2026

Einstein Probe discovery of the magnetar EP J223759.5+531421

We report the discovery and early outburst evolution of the new Galactic magnetar EP J223759.5+531421, detected by the Einstein Probe Wide-field X-ray Telescope on 2026 June 28. Follow-up observations with Einstein Probe, XMM--Newton, NuSTAR, SVOM, and IXPE revealed coherent X-ray pulsations at P ~ 6s and an average period derivative of Pdot ~ 2.8x10^{-12} s/s. These values imply a nominal polar dipolar magnetic field of B_dip ~ 2.6x10^{14} Gauss and a characteristic age of tau_c ~ 34 kyr, although the structured timing residuals suggest possible torque variability. The rms pulsed fraction is approximately 20-25% below ~7 keV and decreases at higher energies. The broadband spectra require multiple thermal components and a hard power-law tail. Adopting a distance of 3.3 kpc, the 0.5-30 keV luminosity declined from 1.2x10^{35} to 5.7x10^{34} erg/s during the first month, accompanied by a decrease in the inferred thermal-emitting areas. At this distance, the Galactic latitude b=-4.56 deg corresponds to a height of approximately 0.26 kpc below the Galactic plane, which is difficult to reconcile with the nominal characteristic age under a simple midplane-birth scenario even for an extreme proper motion velocity. Short X-ray bursts independently confirm the magnetar nature of the source. No near-infrared or radio counterpart were detected by GTC, Medicina or FAST, respectively, down to K_s>21 mag and S_{1.25 GHz} <2.3 microJy. These observations demonstrate the potential of the Einstein Probe WXT monitoring to uncover previously quiescent Galactic magnetars and follow their outbursts from their earliest observed stages.

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arXiv2026

The Northern Cross Fast Radio Burst project: VI. The INCART public database

Fast radio bursts (FRBs) are bright (Jansky-level) and short-duration ($\sim 1$ ms) flashes of extragalactic origin. Observations of single events have now been complemented by large-area surveys, delivering FRB catalogues and enabling the first population studies. The Northern Cross (NC) radio interferometer is one of the instruments performing observations of FRBs. In this work, we present the Italian Northern Cross Atlas of Radio Transients ({\tt INCART}), a public platform for the distribution of data products from the NC. {\tt INCART} makes available to the community the FRBs observed by the NC through manageable frequency-time series datasets and catalogues with best-fit physical parameters. The design of {\tt INCART} guarantees the possibility of scientific re-analysis of the FRB properties, in view also of future releases of the processing pipeline. Furthermore, {\tt INCART} focuses on long-term storage optimisation, which is a key aspect of state-of-the-art instrumentation. Public access to the FRB data from the NC maximises the legacy value of the collection, facilitates the synergy with other publicly-available catalogues, and fosters research group collaborations.

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arXiv2025

The Northern Cross Fast Radio Burst project: V. Search for transient radio emission from Galactic magnetars

Context. The radio emission from magnetars is poorly understood and poorly characterized observationally, in particular for what concerns single pulses and sporadic events. The interest in it was boosted by the detection in 2020 of an extremely bright ms radio signal from the Galactic magnetar designated Soft Gamma Repeater (SGR) SGR J1935+2154, 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 (FRBs) and the only one with a sound association with a known progenitor. Aims. We aim to constrain the rate of impulsive radio events from magnetars, by means of an 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 more than 560 hours of observation, setting an upper limit at 95% confidence level of <52 yr$^{-1}$ on the rate of events with energy >10$^{28}$ erg, which is consistent with limits in literature. Furthermore, under some assumptions on the magnetars properties and energetic behavior, we found that our upper limits point towards the fact that the entire population of FRBs observed cannot be explained by radio bursts emitted by magnetars.

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arXiv2024

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$^{\rm th}$ 2022 and December 31$^{\rm st}$ 2023. Their cumulative spectral energy distribution follows a power law with slope $α_E = -1.3 \pm 0.2$ and we measured a repetition rate of $0.19 \pm 0.03$ hr$^{-1}$ for bursts having a fluence of $\mathcal{F} \geq 17$ Jy ms. Furthermore, we report no detections at 1.4 GHz for $\mathcal{F} \geq 20$ 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 .. (abridged)

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