Luigi Piro

We present the homogeneous and complete sample of 96 bona-fide gamma-ray bursts (GRBs) detected by the Wide Field cameras (2–28 keV) of BeppoSAX . We complement these data with the Gamma-ray Burst Monitor (40–700 keV) simultaneous observations. We derive the spectral and temporal properties of the prompt emission, and assess the properties of the GRB soft population, namely X-ray flashes (XRFs) in comparison with normal GRBs. We find, on the basis of the spectral shape, that 36 events are XRFs, 40 X-ray rich (XRR), and 20 normal GRBs. We analyze the distribution of the spectral parameters of the Band function, α, β and E _p , finding that the spectral indexes of the three classes are broadly similar. In contrast, the peak energy is the parameter driving the spectra shape, from 8.5 keV for XRF keV to 83 keV for GRBs. The duration ( T ₉₀ ) in the X-ray range is similar in the three classes, clustering around 70 s. Likewise, a similar duration of 25 s is observed in the gamma-ray range. For the 67 events that are detected in both instruments we find that nine events exhibit a soft X-ray precursor taking place from 14 to 105 s before the onset of the gamma-ray burst. About 90% of the events that were identified in real time exhibit an X-ray afterglow, with a similar fraction for the three classes. In the optical and radio the the corresponding fractions are 35% and 33%. All the similarities in the spectrum, duration, and afterglow properties suggest common progenitors for the three classes, where the differences are likely a combination of the effect of different baryon loading, energy, structure, and orientation of the jet with respect to the observer. A comparison with Einstein Probe shows that the latter, thanks to its sensitivity, reaches out to a population of fainter and more numerous events, whose presence was first hinted at by the unique very low-luminosity BeppoSAX GRB 980425.

The candidate gravitational wave (GW) event S250818k was one of only three non-retracted LIGO-Virgo-KAGRA public alerts issued during the fourth observing run of the network (O4) with a binary neutron star (BNS) merger classification probability exceeding one percent. This triggered a prompt search for a potential electromagnetic (EM) counterpart in the large localization error region (949 deg ² projected in the sky at the 90% credible level). The transient supernova (SN) 2025ulz discovered by the Zwicky Transient Facility (ZTF) during the search, attracted a great deal of attention due to a potential spatial and temporal coincidence, and due to its initial fast decay and featureless spectrum. Here, we report on the follow-up of this transient by the Electromagnetic counterparts of gravitational wave sources at the Very Large Telescope (ENGRAVE) Collaboration. We conducted an extensive multi-wavelength observational campaign, which led to the spectral classification of the transient as a type IIb SN, indicating that it is unrelated to the candidate GW event. In this article, we describe our observing strategies, data reduction, and interpretation. All of our results confirm and strengthen our classification of the source, and also show that shock-cooling tails associated with type IIb SNe are one of the most prominent contaminants in kilonova searches.

Context. Active galactic nuclei (AGNs) are one of the most powerful sources in the Universe. The energy liberated by the accretion onto the central supermassive black hole can strongly impact the surrounding environment, up to the host galaxy and beyond. X-ray spectroscopy is a powerful probe of the structure of AGNs and of the nuclear scale outflows. Notwithstanding their ubiquitous presence, such energetic outflows are still poorly characterised. This is mainly due to the degeneracy between their number density n _e and radial location r , intrinsic in the photoionisation equilibrium models which are usually employed to fit the observations. This degeneracy prevents a self-consistent determination of the gas mass and energy rates, and therefore, it does not allow to assess the potential role of these outflows in transporting the accretion-liberated energy outwards. Aims. We analyse a joint XMM-Newton EPIC-pn and NuSTAR observation of NGC 4051, one of the brightest and most variable local AGNs. X-ray observations in the past 25 years have revealed three main warm ionised absorbers in NGC 4051 together with emission lines coming from different accretion disc radii. The high flux of NGC 4051 allows time-resolved spectroscopy to be performed and thus to study the evolution of the absorbers following the variability of the incident ionising continuum. Since the timescale of the gas ionisation variability depends on its number density, constraining it permits breaking of the density–distance degeneracy, finally enabling a precise determination of the key gas properties. Methods. We employ the Time-Evolving PhotoIonisation Device (TEPID) to model the temporal evolution of the outflows. We split the observations in 37 time-resolved spectra, each few kiloseconds long (total duration 160 ks), and we fit them jointly with the time-resolved ionised spectra predicted by TEPID. The column densities and the line-of-sight velocities were derived as well through the analysis of the simultaneous Reflection Grating Spectroscopy (RGS) grating spectra in our previous paper. Results. Thanks to our time-resolved analysis, we fully constrain the number density, n _e = 10 ⁷ cm ⁻³ , and the distance, r = 10 ⁴ gravitational radii ( r _G ), of the absorber with the highest opacity and with intermediate ionisation. This distance is the same of the broad-line region observed in the optical and UV bands and as the soft X-ray emission lines. We obtain upper and lower limits for the other two absorbers. The fastest and most ionised one is at r < 600 r _G , where the broad component of the Fe K α line at 6.4 keV also originates. The slowest and least ionised absorber is at r ≥ 3.8 · 10 ⁵ r _G , the same distance of the cold torus and the narrow Fe K α . Finally, the total energy outflow rate is below 10 ⁻⁴ the bolometric luminosity of the AGN L bol , thus ruling out a meaningful mechanical impact on the host galaxy. These results demonstrate the potential held by time-evolving photoionisation in delivering a clear view of the nuclear outflows and their connection with the AGN structure.

NewAthena is an X-ray astrophysics observatory selected as ESA's next large-class mission to be launched in 2039. Its X-ray Integral Field Unit (X-IFU) instrument is a cryogenic imaging spectrometer based on an array of about 1500 Transition Edge Sensor (TES) microcalorimeters operating at 50 mK and providing high-resolution, spatially resolved spectroscopy. To reduce the instrumental background induced by cosmic particles, the X-IFU includes the Cryogenic AntiCoincidence detector (CryoAC), a TES-based detector composed of four pixels readout by SQUID amplifiers operating in Flux Locked Loop (FLL) configuration. The CryoAC warm electronics consist of the analog Warm Front-End Electronics (WFEE), implementing the FLL, and the Warm Back-End Electronics (WBEE), which provides control, digitization and communication with the X-IFU Instrument Control Unit (ICU). This paper presents experimental activities carried out with the CryoAC warm electronics breadboards. The experimental activities were carried out in two separate cryogenic setups: one using complete CryoAC detector assemblies in a dilution refrigerator, and the other using standalone SQUID assemblies in a pulse-tube cryostat operating at about 3 K. The measurements comprised SQUID V-Φ characterization, acquisition of detector pulses and a comparison of different grounding schemes in the balanced differential readout configuration. The measurements provided valuable operational experience and highlighted aspects of the present implementation that will be considered in future developments of the CryoAC warm electronics.

The Cryogenic Anticoincidence (CryoAC) detector is a critical subsystem designed to reduce the particle background for the X-ray Integral Field Unit (X-IFU) instrument onboard the NewAthena space observatory, the next ESA X-ray Large mission. Advancing this technology to Technology Readiness Level 5 (TRL5) requires a unified validation spanning both cleanroom microfabrication repeatability and mK low temperature operational performance. This work presents the complete development cycle of the Demonstration Model 1.2 (DM 1.2), which is aimed at completing the TRL5 demonstration path featured by all the critical technologies operating simultaneously. First, single-process verification protocols were established for Iridium pulsed laser deposition, Reactive Ion Etching (RIE), and deep silicon trenching via the Bosch process. Second, three identical single-pixel prototypes were fabricated and subjected to mK characterization. Four-wire resistance measurement results confirmed a 2/3 production yield against strict design targets (T_C ~100 mK). Finally, functional testing at a bath temperature of 50 mK using a VTT FAB4 SQUID readout demonstrated excellent performance, including a low-energy threshold of ~0.6 keV, a pixel power dissipation of 5.15 nW, and an energy resolution ΔE_FWHM = 735 eV at 6 keV. These combined achievements successfully validate the entire manufacturing and operational baseline against all primary space mission requirements. This paper has to be considered as a review of the CryoAC technology path toward the TRL5 achievement; main findings will be reported and discussed. Details are relegated to other papers.

We performed a search for fast X-ray transients (FXTs), defined for the purposes of this work as transients with durations of longer than one second and shorter than one day. We used data from the Wide Field Camera (WFC) instrument on board the Beppo SAX X-ray observatory collected between June 1996 and April 2002. The WFC sensitivity ranged from 10 ⁻⁹ erg s ⁻¹ cm ⁻² (2-10 keV), for a timescale of 10 s, to a few times 10 ⁻¹¹ erg s ⁻¹ cm ⁻² , for a timescale of 10 ⁵ s. The WFC location accuracy was 0.7-4.9′ at a 68% confidence. We focused our search on gamma-ray bursts (GRBs), X-ray flashes (XRFs), and X-ray flares from high-mass X-ray binaries and stellar flares. Overall, 149 FXTs were detected. Of these, 63 are new to the literature. 38 flares were identified with 22 nearby stars. Three stars in the sample have never been seen flaring before in X-rays or optical (NLTT 51688, GR Dra and UCAC4 255-003783). We find that the MeV transient GRO J1753+57 is most likely the same object as GR Dra, rather than an active galactic nucleus (AGN) as previously thought. Eleven flares were detected from known high-mass X-ray binaries with irregular wind accretion. 100 GRBs were identified. Of these, 24 had not been published before. We classified 38% of the X-ray detected GRBs as XRFs with a relatively high X-ray-to-gamma-ray flux ratio. The gamma rays were measured with the Beppo SAX Gamma Ray Burst Monitor. The duration and spectral hardness distribution of all FXTs are bimodal, separating the group roughly in transients that are either shorter or longer than 1 ksec and with relatively hard and soft spectra, respectively. We identified the “short” FXTs as GRBs and XRFs and the “long” FXTs as flares from nearby late-type stars and X-ray binaries. The Beppo SAX-WFC FXT sample is found to be consistent with the one observed by Einstein Probe when the sensitivity of the two instruments is taken into account. These results suggest that the bulk of the population of faints events disclosed by Einstein Probe could represent the extension of a similar population to lower luminosities.

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.

The X-ray Integral Field Unit is the X-ray imaging spectrometer on board one of ESA’s next large missions, Athena. Athena is set to investigate the theme of the Hot and Energetic Universe, with a launch planned in the late-2030s. Based on a high-sensitivity transition edge sensor (TES) detector array operated at very low temperature (50 mK), X-IFU will provide spatially resolved high-resolution spectroscopy of the X-ray sky in the 0.2 to 12 keV energy band, with an energy resolution goal of 4 eV up to 7 keV [3 eV design goal]. This paper presents the current calibration plan of the X-IFU. It provides the requirements applicable to the X-IFU calibration, describes the overall calibration strategy, and details the procedure and sources needed for the ground calibration of each parameter or characteristics of the X-IFU.

The 1–100-pc region embedding long-duration gamma-ray bursts (GRBs) has been hitherto unexplored, as extremely high ionization by the GRB prevents the application of optical absorption spectroscopy at such distances. Here we show that the ionizing flux of the GRB imprints a unique time- and spatially dependent ionization structure on the gas, which can be probed by X-ray absorption. Application of this model to a selected sample of seven bright GRB X-ray afterglow spectra observed by XMM-Newton EPIC-pn enables an independent, quantitative estimation of the density (log( n ) ≈ 2–4) and distances (5–100 pc) of the ionized absorber directly from the GRB X-ray spectrum, thereby allowing us to locate the absorbing medium of this representative sample of long-duration GRBs in the region of the density–size diagram populated by star-forming regions versus other gravitationally bound objects in the Universe. Our results provide one of the most direct links between long-duration GRBs and star formation and show the potential of high-resolution X-ray spectroscopy as a powerful probe of star-forming regions that embed GRBs up to the highest redshifts.

We present a high-resolution X-ray spectroscopic study of the narrow-line Seyfert 1 galaxy NGC 4051 using two XMM-Newton high-resolution Reflection Grating Spectrometer observations. The spectra reveal three distinct layers of photoionized gas flowing outward from the central black hole: a low-ionization phase (LIP), a higher-ionization phase (HIP), and a high-velocity and high-ionization phase (HVIP). Each absorber leaves characteristic imprints on the soft X-ray spectrum. While the LIP and HVIP are fully consistent with being in ionization equilibrium with the central radiation field over the course of the ∼250 ks spanned by the two observations, the HIP shows a significant change in ionization (3.8 σ ), suggesting nonequilibrium. By modeling the two spectra with our time-dependent photoionization code ( TEPID ), we constrain the density of the HIP gas to log n H = 7 . 7 − 0.9 + 0.2 and estimate its distance to be about R = 0.4 5 − 0.09 + 0.80 lt-day from the black hole, corresponding to R = 400 0 − 800 + 7000 gravitational radii. In contrast, the narrow soft X-ray emission lines remain constant, consistent with an origin in the more extended narrow-line region. Our results show the value of combining high-resolution and time-resolved spectroscopy to probe the structure, physical conditions, and variability of active galactic nucleus outflows.