arXiv subjects

Michela Negro

Publications and source records attributed to Michela Negro.

At least 19 recordsLinked to original sources

arXiv2026

A Search for GeV Emission from Magnetar Giant Flare Candidates with Fermi-LAT

The discovery of delayed GeV emission from the extragalactic magnetar giant flare (MGF) GRB 200415A, located in the nearby Sculptor galaxy, revealed for the first time that these rare transients can launch relativistic outflows that power high-energy afterglows. Motivated by the recent identification of additional nearby MGF candidates in the archival data of the Fermi Gamma-ray Burst Monitor, we conduct a search for GeV counterparts with the Fermi Large Area Telescope (LAT). We analysed post-trigger time intervals taken in the range $10^{2}$--$10^{4}$\,s using a maximum-likelihood approach and performed a stacking analysis of all candidates with LAT coverage. In addition, we searched for photon triplets through \chng{a waiting time} analysis to identify events potentially associated with MGFs. We recover the known delayed signal from GRB\,200415A but find no GeV emission from the remaining six candidates. For three events, the earliest emission is unconstrained because the $10^{2}$\,s interval contains zero exposure after standard selections. For the events with LAT coverage, we obtain upper limits at 95\% confidence level of order $F_E \sim 10^{-9}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$ for the individual events, and a stacked population-averaged limit of $\approx2\times10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. We interpret these upper limits within the relativistic fireball framework, where the prompt spectral peaks favor a baryonic-poor regime ($\eta > \eta_*$). For the candidates with hard prompt spectrum and early LAT coverage, the baryonic-poor condition restricts the mass of relativistic ejecta to $M_b \lesssim 1\text{--}4 \times 10^{22}$\,g; the LAT upper limits confirm that the predicted GeV afterglow from such clean outflows falls below current instrumental sensitivity.

astro-ph.HE↗

arXiv2026

A new NASA Pioneer: the Globe Orbiting Soft X-ray Polarimeter (GOSoX)

The Globe Orbiting Soft X-ray Polarimeter (GOSoX) is a spectropolarimeter for the soft X-ray band. GOSoX is based on the Rocket Experiment Demonstration of a Soft X-ray Polarimeter (REDSoX), a NASA-funded sounding rocket payload. Like REDSoX, GOSoX consists of Wolter I X-ray optics from NASA/MSFC, critical-angle transmission (CAT) gratings made at MIT, and multilayer (ML) coated mirrors from LBNL that polarize the X-rays. Colleagues at U. T\"ubingen will adapt commercially available sCMOS sensors for the focal plane. The grating dispersion is matched to the lateral grading of ML mirrors set at 45\deg. GOSoX can measure polarization across the entire 0.2-0.4 keV band with a spectral resolution E/dE $\sim$100. Minimum detectable polarizations (MDPs) of 3-10\% are expected for over a dozen targets in a one-year mission. The mission was selected by NASA for launch in 2030.

astro-ph.IM↗

arXiv2026

4th TDAMM Workshop White Paper

Time-Domain and Multi-Messenger Astrophysics (TDAMM) is entering a new era in which the rate and diversity of transient discoveries will grow rapidly across electromagnetic, gravitational-wave, neutrino, and cosmic-ray facilities. The scientific return from these investments will increasingly depend not on discovery alone, but on the ability to identify, prioritize, and coordinate follow-up observations across a heterogeneous and globally distributed network of observatories. This white paper summarizes the outcomes of the Fourth TDAMM Workshop and assesses the near-term discovery landscape, the infrastructure and tools that support coordinated observations, and the technical, policy, and capability gaps that may limit future progress. The workshop identified three principal challenges: insufficiently scalable and interoperable alert and coordination infrastructure, policies that impede rapid multi-facility observations and rare-event science, and the potential loss of critical high-energy, rapid-response, and spectroscopic capabilities. The white paper identifies the need for sustained support for alert distribution, brokers, standardized observatory metadata, cross-facility coordination platforms, and unified follow-up repositories; expanded joint observing opportunities and funding mechanisms for coordinated analysis; and strategic investment in future TDAMM facilities. The white paper also present a framework for community observing plans that would establish pre-coordinated responses to rare, high-impact events, supported by transparent governance, immediate public data release, and regular community revision. Science overviews and detailed observing strategies are provided for gamma-ray bursts, tidal disruption events, X-ray binaries, novae, supernovae, magnetars, compact binary mergers, and high-energy neutrino sources.

astro-ph.HE↗

arXiv2026

Bridging Ab Initio Symmetries and Global Nuclear Masses with Interpretable Neural Networks

Ab initio theory establishes Wigner's SU(4) and Elliott's SU(3) as dominant symmetries of the nuclear force in light and intermediate-mass nuclei. Previous work shows the relevance of the former symmetry for nuclear binding, whether the latter organizes binding remains elusive. We probe whether both these symmetries organize nuclear masses, aiming at physical insights through interpretable models and predictive capability. From the SU(3) and SU(4) Casimirs we build 3 neural-network models. Two are conventional, a feature-informed NN (FINN) and a Gaussian variant (GINN) with predictive spread, while Wigner-informed network (WINN) is a new design constraining the mass formula to be linear in the operators, learning their (N,Z)-dependent couplings, so that the model is intrinsically explainable. All are trained on AME2016 subtracted by the liquid drop model at 4 data fractions and validated on nuclei new to AME2020, with extrapolation benchmarked against HFB-26 and r-process. The Casimir features carry binding information far beyond the bulk, and SHAP analysis suggests the quadratic SU(4) Casimir as the leading contributor to the residual binding. The WINN yields the best performance, reaching a 0.412 MeV validation error and competitive with state-of-the-art models, and importantly, when trained on the sparsest dataset it outperforms the other NNs trained on the densest. Off the known chart its masses track HFB-26 as closely as WS3 and reproduce the solar abundance peaks of a neutron-star-merger simulation. The WINN's coupling fields reveal an enhanced even-SU(4) contribution toward the neutron dripline, hinting at restoration of Wigner's symmetry. The SU(4) and SU(3) structures reach beyond individual nuclei to organize binding, and embedding symmetry-preserving operators directly in a domain-informed interpretable architecture yields a physically transparent model less hungry for data.

nucl-th↗

arXiv2026

Discriminating blazar emission models with high-energy polarimetry: Multi-band predictions and detectability

Polarimetric properties of blazars provide key constraints on the acceleration mechanisms powering their relativistic jets, the high-energy emission processes involved, and the composition of the jet itself. We present a multi-band polarimetric study spanning from soft X-rays to gamma-ray energies, considering several bands for current and future missions (0.275 keV, 2-8 keV, 0.5-10 keV, 6-35 keV, 0.2-5 MeV, and 1-100 GeV). Our sample is drawn from the RoboPol monitoring programme, a statistically complete gamma-ray sample including low-, intermediate-, and high-synchrotron-peaked blazars. Using spectral energy distribution fitting performed with the Bjet_MCMC tool, we give predictions on the flux expected for each source in the selected energy bands. We model the polarimetric signatures under three competing emission scenarios: leptonic, hadronic, and hybrid. The detectability of each source was assessed by accounting for the instruments' minimum detectable polarisation (MDP) and by computing the probability of the detection in a blind survey. We show that simultaneous multi-wavelength observations can effectively discriminate between competing emission models due to the difference in the expected polarisation degree. Finally, we derive sensitivity requirements for future gamma-ray polarimetric missions aimed at increasing the number of detectable sources.

astro-ph.HE↗

arXiv2026

Detailed Timing, Spectral, and Polarimetric Analysis of Magnetar 1RXS J170849.0-400910

We present a broadband timing, spectral, and polarimetric study of the magnetar 1RXS~J170849.0-400910 using XMM-Newton, NuSTAR, and IXPE. The pulse morphology evolves strongly across 0.5-70 keV. Below 3 keV, the emission is dominated by a broad soft pulse with a leading shoulder that develops into a faint interpulse near 3 keV, while the pulse fraction remains $\approx$25%. The profile becomes increasingly double-peaked between 3 and 20 keV and returns to a single peak at higher energies. The pulse fraction dips to $\sim$20% near 4 keV and rises to $\sim$42% above 25 keV. The phase-averaged spectrum is well described by an absorbed blackbody plus two power-laws, with $kT=0.468\pm0.003$ keV, $\Gamma_{\rm soft}=2.63\pm0.04$, and $\Gamma_{\rm hard}=0.5\pm0.1$. Phase-resolved spectroscopy reveals distinct soft and hard pulse components. The thermal modulation is driven primarily by a factor of $\sim$5 variation in projected emitting area, whereas the soft power-law exhibits two peaks with different phase and energy evolution, suggesting distinct emission regions or mechanisms. The 10-70 keV flux is strongly anticorrelated with the soft power-law photon index, linking spectral hardening to the hard pulse. The polarization degree also varies strongly with phase and energy. In the 2-3 keV band, it is anticorrelated with the intensity profile, consistent with magnetized-atmosphere emission, whereas in the 4-8 keV band it reaches $64\pm10$% during the nonthermal power-law-dominated peak. This high polarization can be reproduced by magnetospheric quantum pair-synchrotron emission. Together, these results reveal an intricate, phase-dependent superposition of emitting regions and radiative processes whose complexity emerges only through broadband, phase-resolved spectropolarimetry.

astro-ph.HE↗

arXiv2026

Magnetar Fireballs and Short Bursts: Curved Spacetime Lensing, QED Effects, Spectra, Polarization, and Impulse Responses

Magnetar short bursts (SBs) are hard X-ray transients of durations $0.01-1$ s peaking at $\sim 10-100$ keV, and are prime targets for new high-energy missions and polarimeters. The recent association of SBs with bright radio bursts in SGR 1935+2154 has broadened interest in SB physics. We present new advanced fireball models combining general relativistic light bending, polarized transport in magnetized photospheres, magnetic photon splitting attenuation, and magnetospheric vacuum birefringence. These models also have relevance to trapped fireballs in magnetar giant flare pulsating tails. We adopt confined flux tube geometries consistent with adiabatic fireballs, and anisotropic/polarized emergent intensities to produce spectra and polarizations, and energy-time Stokes impulse responses. We predict that most fireballs are highly linearly polarized, especially when vacuum birefringence is important. There is rich potential for diagnostics: coexisting direct and lensed delayed images, gaps by occultation of the neutron star surface, and Shapiro+R{\o}mer delay with temporal caustics. These effects can imprint spin phase dependence of the spectral and polarization character of bursts. Predicted signatures depend strongly on viewing geometry, fireball configuration, and photon splitting assumptions, yielding large variance in model high-energy spectral shapes and cutoffs, and energy-dependent polarization. The models can reproduce established double-blackbody SB spectral phenomenology, and we find that the unusual April 2020 radio-associated SB from SGR 1935+2154 is broadly consistent with a footpoint close to the magnetic pole, and possibly near pole-on viewing geometry. Our models motivate reverberation-style analyses for SBs and suggest that high-quality data might constrain source geometry, burst crustal footpoints, and, potentially, neutron star masses and radii.

astro-ph.HE↗

arXiv2026

Detectability of Polarized Gamma-ray Emission from Blazar Flares with COSI

We investigate the detectability of polarized gamma-ray emission from blazar flares with the Compton Spectrometer and Imager (COSI). Using 17 years of Fermi Large Area Telescope observations, we analyze light curves for 1413 blazars and identify a maximum of 787 sources with flaring episodes through Bayesian block analysis. For each flare, we estimate the minimum detectable polarization MDP99 in the COSI energy band (0.2-5 MeV) using instrument response functions under a range of spectral assumptions and background conditions. Under baseline background levels (1 counts/s), and assuming that blazar flare statistics in the MeV band are comparable to those observed at GeV energies, we find that COSI can realistically detect polarization in up to ~6 flares with MDP99<50% over its two-year prime mission depending on different spectral and flare identification assumptions, with only a few most powerful ones reaching MDP99<20%. These expectations are shown to improve when shorter intervals around bright peaks within long flares are considered. We provide a ranked list of the most promising targets, finding that flat-spectrum radio quasars dominate the population of polarization-detectable events. Through its continuous all-sky monitoring in the largely unexplored MeV band, COSI will open a new observational window on blazar variability and deliver the first direct measurements of MeV polarization, offering unique insights into jet geometry and high-energy emission processes.

astro-ph.HE↗

arXiv2026

Label Propagation for Identifying Gamma-Ray Burst Progenitors from Prompt Emission

Gamma-ray bursts (GRBs) are the most energetic bursts of light in our universe, and rapid progenitor association of these events can lead to targeted and optimized follow-up observations, ultimately providing better insights about the physics involved. In this note, we investigate a semi-supervised machine learning algorithm, that utilizes label propagation, as a classification method. Using a dataset of 2512 GRBs we evaluate the method's ability to assign probabilistic class memberships based on a subset of events with known progenitors. Further analysis is ongoing to improve the method and future progress will be made to refine the classification algorithm and the dataset.

astro-ph.HE↗

arXiv2026

IXPE view of the Crab pulsar following the 17 July and 6 August 2025 glitches

The Crab pulsar experienced two relatively small glitches separated by only 20 days in September and October 2025. IXPE observed the source twice, with delay times since the glitch epoch ranging between 35 and 75 days, depending on the observation. We carried out a multi-method analysis to investigate whether there is evidence for significant changes in the polarization properties of the pulsar, underlying possible variations in the pulsar magnetosphere itself following the glitches. Specifically, we performed: (1) phase-averaged polarimetry of the Crab pulsar before and after the glitches, following an approach similar to that adopted in 2019 by PolarLight, a non-imaging CubeSat-class photoelectric polarimeter which observed a change in the X-ray polarization within 100 days after a stronger glitch in July 2019; (2) a comparison, before and after the glitch, of phase-resolved X-ray polarimetry with IXPE, not possible with PolarLight. Furthermore, we investigated, by means of phase-resolved optical (OPTIMA) polarimetry, whether a significant change in the X-to-optical lag was present in the data before and after the glitch. We find no evidence of a change in the polarization for the pulsar emission before and after the glitch, We use the upper limits obtained to estimate the maximum change in magnetic obliquity allowed by the data, using the standard rotating vector model and assuming that the glitch is due to a neutron-star quake. We constrain this maximum change to be no greater than $\pm4^{o}$ at the 95\% confidence level.

astro-ph.HE↗

arXiv2026

Searching for dark matter X-ray lines from the Large Magellanic Cloud with eROSITA

We perform a search for an X-ray monochromatic line arising from dark matter (DM) decay in the halo of the Large Magellanic Cloud. An emission line can be expected from two well-motivated DM candidates: sterile neturinos and axion-like particles (ALPs). We analyze the eROSITA-DE DR1 datasets in the energy range between 1 and 9 keV. No evidence for a DM line is found, and we set lower limits on the DM lifetime. We then recast these bounds into upper limits on the active-sterile neutrino mixing angle $\sin^2(2\theta)$ and on the ALP to photon coupling $g_{a\gamma}$, for DM masses between 2 and 18 keV. These results set new strong constraints for masses below 5 keV.

hep-ph↗

arXiv2026

High-Energy Shock Breakout from Supernovae and Gamma-ray Bursts

Cosmic explosions play a critical role in a broad range of astrophysical fields. Although considerable progress has been made to understand the explosive engines and their progenitors, many of the details are not well understood. One of the most powerful electromagnetic probes of the explosive mechanism and the stellar progenitor is the first burst of photons emitted from this blastwave as it exits the stellar photosphere, known as shock breakout (SBO). Our understanding of SBO has evolved considerably in the past decade. Shock heating as the blastwave propagates through the star and circumstellar material can drastically alter this emission producing a much broader range of potential SBO signals than that predicted by standard analytical approaches. Here we present a semi-analytic approach to model this diverse SBO emission, focused on thermal Bremsstrahlung radiation, which more accurately captures the complexities in Nature over previous treatments. We calculate a range of signals for a range of supernova and gamma-ray burst types. Our models demonstrate how we can use these signals to place constraints on the nature of the explosive engines and better understand the role SBO can play in prompt gamma-ray bursts. We study the implications of these results to historic observations, Einstein Probe transients, and in the context of proposed missions. We find that stripped envelope events can be detected serendipitously with survey telescopes, but type Ia and II SBO detections require fast-pointing X-ray observations in response to early warning alerts from gravitational wave or neutrino detectors.

astro-ph.HE↗

arXiv2026

Magnetic-field Order in the Southwestern Rim of RCW 86 Constrained Using X-Ray Polarimetry

RCW 86 is a supernova remnant whose origin has recently been linked to an off-center explosion within a cavity created by its progenitor star. In the southwestern region, the forward shock is thought to have reached the cavity wall, encountering diverse environmental conditions. We report on the spatially resolved X-ray polarimetric observation of RCW 86 with the Imaging X-ray Polarimetry Explorer (IXPE). In the 2--4.5 keV energy band we find no significant detection of polarization. Employing a dedicated background subtraction procedure and Bayesian spectropolarimetric fitting, we derive 99% upper limits on the polarization degree of the synchrotron component: 15% in higher-statistics regions and 30%--40% in lower-statistics regions. These upper limits on the polarization degree in several regions exclude the possibility of a strongly coherent magnetic field down to the subparsec scale, and that of a moderately coherent one on the scale of the synchrotron features as resolved by IXPE. The results indicate that the shocks in the southwestern rim of RCW 86 propagate more slowly than the unshocked ejecta at their locations, yet exceed the measured proper motion speeds. This behavior is consistent with reflected shocks occurring in tenuous regions of the shocked ejecta, distinct from regions that are radio-bright.

astro-ph.HE↗

arXiv2025

Super-Eddington Accretion Geometry: a Remarkable Stability of the Hidden Ultraluminous X-Ray Source Cygnus X-3

We report on the average and orbital phase-resolved polarization of Cyg X-3 in the hard state during the 2023 Imaging X-ray Polarimetry Explorer (IXPE) observational campaign. We find the polarization degree of $ 21.2 \pm 0.4 \% $ and polarization angle of $ 92.2 \pm 0.5^\circ $, well compatible with the first hard-state IXPE observation in 2022. As the observed polarization depends on both the accretion geometry and the X-ray emission mechanism, which we attribute to reflection from the optically thick envelope surrounding the central source, our result indicates that both are very stable on year-long timescale. We discuss time- and energy-dependent polarization properties and their implications for the geometry and stability of the accretion funnel.

astro-ph.HE↗

arXiv2025

From Rare Events to a Population: Discovering Overlooked Extragalactic Magnetar Giant Flare Candidates in Archival Fermi Gamma-ray Burst Monitor Data

Magnetar giant flares (MGFs) are rare, extremely bright bursts of gamma-rays from highly magnetized neutron stars. These events are challenging to identify because, at extragalactic distances, they can appear similar to other astrophysical phenomena. Only a handful have been confidently identified to date, limiting our understanding of their origin and physical properties. This study focuses on expanding the sample of known events and enabling a more detailed characterization of their observational features and intrinsic properties, while introducing significant improvements in the methods used to identify and analyze them. When applied to archival data from the Gamma-ray Burst Monitor (GBM) on the \Fermi Gamma-ray Space Telescope, this approach added four previously unidentified events the known sample, expanding the total to 13 MGFs. This demonstrates both the effectiveness of the method and the likelihood that additional MGFs remain hidden in existing gamma-ray burst catalogs. We utilize this expanded sample to gain a deeper understanding of the broader population of MGFs. We develop a statistical modeling framework that combines previously considered data with modern observations from Fermi/GBM. The model accounts for instrumental sensitivity and the expected diversity in event characteristics. We infer a volumetric rate of events above $1.2\times10^{44}\,\rm{erg}$ of $R_{MGF}=5.5^{+4.5}_{-2.7}\times10^5\rm{Gpc^{-3}yr^{-1}}$. The results show that individual magnetars must produce multiple flares throughout their lifetimes, reinforcing the idea that these are recurring phenomena rather than singular explosive events. Expanding the sample of known MGFs improves our understanding of magnetars and their role in other astrophysical phenomena, including possible links to fast radio bursts, gravitational waves, and the creation of heavy elements in extreme astrophysical environments.

astro-ph.HE↗

arXiv2025

Embedding Reliability for Unsupervised Classification of Gamma Ray Burst progenitors from Prompt Gamma-ray Emission

We present a statistical method based on scDEED to assess the reliability of a 2D embedding showing a low-dimensional representation of the distribution of Gamma-Ray Bursts (GRBs) detected by the Fermi Gamma-ray Burst Monitor (GBM). The original dataset consists of 12 waterfall plots for each event, which contain key information about the prompt emission of each GRB. The dataset's dimensionality is first reduced to a 30-dimensional latent space using an autoencoder, and subsequently to 2D using UMAP. While the methodology and results are discussed in a previous work (arXiv:2406.03643), here we introduce a statistical approach to evaluate the reliability of the final 2D distribution based on the scDEED algorithm. Our analysis shows that the 2D embedding demonstrates overall good reliability, with more than 90\% of the events classified as trustworthy.

astro-ph.HE↗

arXiv2025

Direct observation of the X-ray counterpart of the H{\alpha} filaments and of the sloshing spiral in the Perseus galaxy cluster

Deep Chandra observations of the Perseus galaxy cluster have allowed for the discovery of X-ray counterparts to the H{\alpha} filamentary structures and of a sloshing spiral. However, both components are extremely faint, and their study is largely hindered by the volume-filling hot intracluster medium (ICM). Using the Poisson General Morphological Component Analysis (pGMCA) algorithm, a blind source separation method adapted for Poissonian statistics, we were able to extract detailed, clean morphological maps of these components. We then introduced a template fitting method to investigate their spectral characteristics. We report the first direct observation of about 1.35 keV low-energy emission from the sloshing spiral, and produce the most detailed and unpolluted map of the X-rays filaments thus far obtained.

astro-ph.HE↗

arXiv2025

GRB 250702B: Discovery of a Gamma-Ray Burst from a Black Hole Falling into a Star

Gamma-ray bursts are the most luminous electromagnetic events in the universe. Their prompt gamma-ray emission has typical durations between a fraction of a second and several minutes. A rare subset of these events have durations in excess of a thousand seconds, referred to as ultra-long gamma-ray bursts. Here, we report the discovery of the longest gamma-ray burst ever seen with a ~25,000 s gamma-ray duration, GRB 250702B, and characterize this event using data from four instruments in the InterPlanetary Network and the Monitor of All-sky X-ray Image. We find a hard spectrum, subsecond variability, and high total energy, which are only known to arise from ultrarelativistic jets powered by a rapidly-spinning stellar-mass central engine. These properties and the extreme duration are together incompatible with all confirmed gamma-ray burst progenitors and nearly all models in the literature. This burst is naturally explained with the helium merger model, where a field binary ends when a black hole falls into a stripped star and proceeds to consume and explode it from within. Under this paradigm, GRB 250702B adds to the growing evidence that helium stars expand and that some ultra-long GRBs have similar evolutionary pathways as collapsars, stellar-mass gravitational wave sources, and potentially rare types of supernovae.

astro-ph.HE↗