Eliana Palazzi

The Euclid spacecraft, located at the second Lagrangian point of the Sun-Earth system, hosts the Near-Infrared Spectrometer and Photometer (NISP) instrument. NISP is equipped with a mosaic of 16 HgCdTe-based detectors (Teledyne’s H2RG) to acquire near-infrared photometric and spectroscopic data. To meet the spacecraft’s constraints on computational resources and telemetry bandwidth, the near-infrared signal is processed on board via a dedicated hardware-software architecture designed to fulfil Euclid ’s stringent data quality requirements. A custom application software, running on the two NISP data-processing units, implements the NISP signal estimator: an ad hoc algorithm that delivers accurate flux measurements and simultaneously estimates the quality of signal estimation through the quality factor (QF) parameter. This paper further investigates the properties of the NISP signal estimator by evaluating its performance during the early flight operations of the NISP instrument. First, we revisit the assumptions on which the inference of the near-infrared signal is based and investigate the origin of the main systematics of the signal estimator through Monte Carlo simulations. Then, we explain how we tested the flight performance of the NISP signal estimator. Results indicate a systematic bias lower than 0.01 e ⁻ s ⁻¹ for 99% of the NISP pixel array, which is well within the noise budget of the estimated signal. We also derived an analytical expression for the variance of the NISP signal estimator, demonstrating its validity, particularly when the covariance matrix is not pre-computed. Finally, we provide a robust statistical framework to interpret the QF parameter, analyse its dependence on the signal estimator bias, and show its sensitivity to cosmic-ray hits on NISP detectors. Our findings corroborate previous results on the NISP signal estimator and suggest a leading-order correction based on the agreement between flight data and simulations.

Aims. We confirm the compact nature and constrain the radio spectra of candidate persistent radio sources (PRSs) associated with repeating fast radio bursts (FRBs), and we increase the number of PRSs with spectral indices measured using very long baseline interferometry (VLBI). Methods. We performed observations with the European VLBI Network (EVN) at 5 and 8 GHz targeting two candidate PRSs that were identified in a recent VLA survey of repeating FRBs. We measured the flux densities and upper limits at milliarcsecond resolution and combined them with published data at lower frequencies to derive spectral constraints. Results. We detect a compact source associated with FRB 20190417A at 5 GHz with a flux density of 150 ± 45 μJy, but no detection is obtained at 8 GHz. The source is unresolved on milliarcsecond scales, implying a projected physical size ≲15 pc and a brightness temperature T _b > 3.8 × 10 ⁵ K, confirming its non-thermal nature. The combination of our measurement with the 1.4 GHz VLBI data yields a spectral index α = −0.19 ± 0.29, consistent with a nearly flat spectrum. This makes FRB 20190417A only the second PRS with a spectral index constrained using VLBI data. The inferred luminosity places the source on the proposed L _ν –|RM| relation. When this source is included, the scatter is σ _Δ = 0.67, corresponding to α̂ | ϵ | ∼ 1.5. This value is roughly consistent with scenarios involving forward shocks in the free-expansion phase or young pulsar wind nebulae. For the candidate PRS associated with FRB 20181030A, we report 5 σ upper limits of 80 μJy at 5 GHz and 150 μJy at 8 GHz, corresponding to L _5 GHz ≲ 3.8 × 10 ²⁵ erg s ⁻¹ Hz ⁻¹ , and implying a steep spectral index ( α ≲ −1.2) if the VLA emission arises from a compact component. Conclusions. Our results highlight the importance of VLBI in isolating compact emission from FRB engines, and they provide one of the few spectral constraints for PRSs at milliarcsecond resolution. The consistency of FRB 20190417A with the L _ν –|RM| relation supports a nebular origin for the persistent emission.

Context. The long gamma-ray burst GRB 180728A at a redshift of z = 0.1171 stands out due to its high isotropic energy of E _γ, iso ≈ 2.5 × 10 ⁵¹ erg, in contrast with most events at redshift z < 0.2, but it is comparable to the bulk of luminous bursts more common at higher redshift. Aims. We aim to study the properties of GRB 180728A’s prompt emission, afterglow, and associated supernova (SN 2018fip), comparing them with other GRB-SN events. Methods. This study employs a dense photometric and spectroscopic follow-up of the afterglow and the SN up to 80 days after the burst. We used image subtraction to remove the presence of a nearby bright star, and modelled both the afterglow and the supernova. Results. This event lies on the E _p, i – E _γ, iso plane occupied by classical collapsar events, and the prompt emission is one of the most energetic at z < 0.2 after GRB 030329 and GRB 221009A. The afterglow of GRB 180728A is less luminous than that of most long GRBs, showing a shallow early phase that steepens after about 5 hours (0.2 days). The GRB exploded in an irregular low-mass blue star-forming galaxy, which is typical of low- z collapsar events. Because of the relatively faint afterglow, the light curve bump of SN 2018fip dominates the optical emission already after approximately 3 days and is one of the best sampled to date. The strong suppression below ∼4000 Å and a largely featureless continuum in the early 6–9 day spectra favour aspherical two-component ejecta with a high-velocity collimated component (> 20 000 km s ⁻¹ ), that is dominant early on and a more massive low-velocity component that dominates at much later epochs. Conclusions. Our findings indicate that asymmetries need to be considered in order to better understand GRB-SNe. In any case, SN 2018fip shares many characteristics with typical GRB-SNe. Its kinetic energy is below the common range of 10 ⁵² –10 ⁵³ erg and does not correlate with the high energy of the GRB, highlighting the complexity and diversity of the GRB-SN energy budget partition.

We present detailed, multi-wavelength analysis of GRB 210704A: a Fermi/GBM discovered and Fermi/LAT detected gamma-ray burst (GRB). The burst is dominated by a short (≈2 s) pulse followed by weaker, softer emission. We line stack our afterglow spectrum and determine the most likely redshift to be z = 2.34. This is corroborated by the photometric redshift of the extended source underlying the GRB. The spectral energy distribution fit parameters, late-time imaging, as well as the GRB’s energetics, spectral lag, and location point to a collapsar nature. Follow-up observations reveal excess optical/infrared emission with respect to a standard afterglow, peaking around T₀ + 7 d (2 d in the rest frame). The excess is extremely luminous (M_r = −22.0 mag) and rapidly evolving. Strikingly, it resembles the emission seen in recently discovered Einstein Probe fast X-ray transients EP241021a and EP240414a, as well as the population of luminous fast blue optical transients (LFBOTs). This provides a link between these sources and GRBs. Fermi/LAT observations imply a high Lorentz factor, making this a case where LFBOT-like emission is also associated with a powerful successfully launched jet. We model the excess as likely coming from an energetic refreshed shock.

The AT2017gfo kilonova transient remains a unique multi-messenger event thanks to its proximity (z=0.00987) and the possibility to investigate time-resolved spectra, providing evidence of r-process nucleosynthesis. The kilonova signal was extensively studied in the spectral and time domains, giving key insights into the chemical composition and physical properties of the ejecta. Here, we report the discovery of a novel correlation between two fundamental observables: the peak energy of the EF_E spectrum, Ep, and the isotropic-equivalent luminosity, Liso. In particular, we show that up to about 2.5 days after the merger, the AT2017gfo spectrum evolves according to: log10[Ep/eV] = -0.13 (+0.02/-0.02) + 0.62 (+0.02/-0.02) * log10[Liso/(1e41 erg/s)] (68% C.L.) while in subsequent epochs, Ep remains almost constant with Liso, flattening around 1 eV. Exploiting simulations from a state-of-the-art radiative transfer code, we demonstrate that our kilonova model inherently predicts this peculiar correlation, suggesting a new diagnostic tool for comparing observables against simulations. Future kilonova observations will provide additional insight into the physics behind the Ep-Liso correlation.

The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015–2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14 000 deg² of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.

Aims. We present the discovery of a fourth persistent radio source (PRS) associated with a fast radio burst (FRB). Methods. Following previous indications of a candidate PRS associated with FRB 20240114A, we performed deep Very Long Baseline Array observations at 5 GHz to test the presence of a compact radio source within the uncertainty position of this FRB (±200 mas). Results. We detected a component ∼50 mas northwards of the nominal position provided by the PRECISE collaboration. The corresponding radio luminosity together with the Faraday rotation measure provided by previous observations of the FRB locate this PRS in the expected region of the radio luminosity versus Faraday rotation measure relation for the nebular model, further supporting its validity. Comparison of the measured flux density with the values collected at a lower frequency by previous studies indicates a possible steepening of the radio spectrum in the 1–5 GHz range. Optical observations performed with the Large Binocular Telescope could reveal that the FRB and its PRS lie at ∼1 kpc from the centre of the host galaxy, which is a dwarf sub-solar metallicity starburst galaxy with a star-formation rate of ∼1 M_⊙ yr⁻¹ and a stellar mass of M ∼ 10⁸ M_⊙.

GRB 191019A was a long gamma-ray burst (GRB) lasting ~65 s and, as such, originally thought to be linked to a core-collapse supernova. However, even though follow-up observations identified the optical counterpart close to the bright nucleus of a nearby ancient galaxy (z = 0.248), no associated supernova was found. This led to the suggestion that the burst was caused by the merger of two compact stellar objects, likely in a dense circumnuclear environment. By using a recently developed diagnostic tool based on prompt emission temporal properties, we noticed that GRB 191019A falls among those long GRBs which are associated with compact mergers and with evidence of kilonova light. We thus reanalyzed unpublished GROND multicolor ( g ′ r ′ i ′ z ′ J H K s ) data obtained between 0.4 and 15 days posttrigger. Image subtraction confirmed the optical counterpart in all four optical bands, with GROND tracking its fading until 1.5 days postburst. Incorporating publicly available Swift-XRT data, a joint fit of an afterglow plus a kilonova model revealed a better match than an afterglow-only scenario. The resulting kilonova properties resemble those of AT2017gfo associated with the binary neutron star merger GW170817, with a total ejected mass of ~0.06 M _⊙. Contrary to previous findings inferring a high-density circumburst environment (n ₀ ~ 10⁷⁻⁸ cm⁻³), our analysis finds standard conditions (n ₀ ~ 1 cm⁻³), suggesting the long duration of GRB 191019A was intrinsic rather than due to jet interaction with a dense external medium.

The statistical distribution and evolution of key properties of active galactic nuclei (AGN), such as their accretion rate, mass, and spin, remains a subject of open debate in astrophysics. The ESAEuclidspace mission, launched on July 1 2023, promises a breakthrough in this field. We create detailed mock catalogues of AGN spectra from the rest-frame near-infrared down to the ultraviolet – including emission lines – to simulate whatEuclidwill observe for both obscured (type 2) and unobscured (type 1) AGN. We concentrate on the red grisms of the NISP instrument, which will be used for the wide-field survey, opening a new window for spectroscopic AGN studies in the near-infrared. We quantify the efficiency in the redshift determination as well as in retrieving the emission line flux of the Hα+[N II] complex, asEuclidis mainly focused on this emission line, given that it is expected to be the brightest one in the probed redshift range. Spectroscopic redshifts are measured for 83% of the simulated AGN in the interval where the Hαis visible (i.e. 0.89 < z < 1.83 at a line flux of > 2 × 10⁻¹⁶erg s⁻¹cm⁻², encompassing the peak of AGN activity atz ≃ 1 − 1.5) within the spectral coverage of the red grism. Outside this redshift range, the measurement efficiency decreases significantly. Overall, a spectroscopic redshift iscorrectly determined for about 90% of type 2 AGN down to an emission line flux of roughly 3 × 10⁻¹⁶erg s⁻¹cm⁻², and for type 1 AGN down to 8.5 × 10⁻¹⁶erg s⁻¹cm⁻². Recovered values for black hole mass show a small offset with respect to the input values by about 10%, but the agreement is good overall. With such a high spectroscopic coverage atz < 2, we will be able to measure AGN demography, scaling relations, and clustering from the epoch of the peak of AGN activity down to the present-day Universe for hundreds of thousands of AGN with homogeneous spectroscopic information.