arXiv subjects

P. Romano

Publications and source records attributed to P. Romano.

At least 19 recordsLinked to original sources

arXiv2026

NOCTURNE. VI. Hour-timescale radio variability in early-stage active galactic nucleus SDSS J122844.81+501751.2

Recently, a sample of radio-weak active galactic nuclei (AGN) was discovered to exhibit extreme Jy-level radio flares at 37~GHz. The origin of these extraordinary flares remains unknown. All sources host low-mass supermassive black holes, and most of them can be classified as narrow-line Seyfert 1 galaxies. We employed multiwavelength observations to investigate one of these sources, SDSS J122844.81+501751.2, during a period of high 37~GHz activity. Our data include radio observations from Metsähovi Radio Observatory (MRO) at 37~GHz, OVRO at 15~GHz, and JVLA at 15, 22, and 33~GHz, as well as optical, UV, and X-ray observations from \textit{Swift}/XRT and UVOT. We studied the variability of the multiwavelength light curves (LC) using statistical tests, as well as derived the $e$-folding timescales of the flares. Utilising the timescales and the observed flare properties, we estimated the variability brightness temperatures and Doppler factors associated with the flares. Most individual LCs do not show statistically significant variability, which is likely due to the low number of measurements. The shortest timescale we estimated is only 1.1~hours, measured between MRO 37~GHz and JVLA 33~GHz detections. This is the fastest intrinsic radio variability reported in an AGN so far. This yielded an extreme variability brightness temperature of $T_\mathrm{B}$ = 4.20 $~\times$ 10$^{18}$~K and a variability Doppler factor of 438. We discuss magnetic eruptions near the black hole as a possible explanation for the extreme radio flares, and show that both incoherent and coherent processes can produce flares at these timescales. These flares challenge our current understanding of AGN radio variability. Whereas magnetic eruptions offer a viable explanation for the flares, more observations, preferably broadband and including polarisation, will be required to confirm their origin. (abridged)

astro-ph.GA↗

arXiv2026

A bright X-ray flaring event from the intermediate supergiant fast X-ray transient IGR J00370+6122 observed with NuSTAR

We present an analysis of a long NuSTAR observation of the intermediate supergiant fast X-ray transient (SFXT) IGR J00370+6122, obtained around the peak of its orbital modulation. The observation has a net exposure ~93 ks. During the observation, the source displayed a bright flaring event that was resolved into numerous short flares with durations of 10-1000 s. The 3-70 keV luminosity spanned a dynamic range of ~200, from ~1E34 to ~1.8E36 erg/s; the maximum was reached for only ~40 s in total. The spectrum is well described by an absorbed double-Comptonisation model, with parameters consistent with those found in other X-ray pulsars accreting at similarly low luminosities. This is the first detection of a double-hump continuum in IGR J00370+6122, which extends this spectral state to the class of intermediate SFXTs. We searched for the previously reported ~346 s and ~674 s periodicities and find no significant signal at either period, which makes their interpretation as the neutron star spin period less likely. Using ~21 yr of Swift/BAT monitoring, we refine the orbital period of the system to P_orb=15.6654 +/- 0.0008 d. Finally, we discuss the observed flaring activity in the framework of quasi-spherical settling accretion, which is expected at the observed luminosities.

astro-ph.HE↗

arXiv2026

Multi-mission Investigation of X-ray Superorbital Modulation in the Supergiant High Mass X-ray Binary 4U 1538-52

Superorbital modulations has been detected in the supergiant High-Mass X-ray binary 4U 1538-52 using long-term monitoring with the Neil Gehrels Swift Observatory Burst Alert Telescope (BAT). The source also exhibits a long-term pulse period evolution as seen with Rossi X-ray Timing Explorer (RXTE), INTEGRAL, and Fermi Gamma-Ray Burst Monitor (GBM) that appears uncorrelated with changes in its X-ray flux. To investigate the mechanisms causing these superorbital modulations and its possible dependence on pulse period changes, we analyzed long-term monitoring with Swift-BAT and Monitor of All Sky X-ray Image Gas Slit Camera (MAXI-GSC) to construct dynamic power spectra and superorbital intensity profiles. In addition, we used pointed X-ray observations from Nuclear Spectroscopic Telescope Array (NuSTAR) and Neutron Star Interior Composition Explorer mission (NICER) to investigate the pulsation and spectral properties across different superorbital and orbital phase intervals. We find the presence of superorbital modulations in the MAXI-GSC 2-20 keV lightcurves, consistent with the periodicity observed with the Swift-BAT lightcurves. However, no significant changes are detected in the pulse profiles or spectral parameters across different superorbital, orbital, or pulse-change intervals. This lack of spectral or timing variations with orbital and superorbital phases suggests that the mechanisms driving the observed superorbital modulation and pulse period changes are likely associated with large-scale stellar wind structures, such as Co-Rotating Interaction regions, within the stellar wind of the supergiant companion.

astro-ph.HE↗

arXiv2026

Investigating Active Galactic Nuclei variability with the Cherenkov Telescope Array Observatory

Blazars, a type of active galactic nuclei (AGN) with relativistic jets pointed at the observer, exhibit flux variability across the electromagnetic spectrum due to particle acceleration in their jets. Power spectral density (PSD) studies show breaks at specific frequencies, particularly in X-rays, linked to the accretion regime and black hole mass. However, very-high-energy gamma-ray PSD breaks remain unexplored due to current instrument limitations. The Cherenkov Telescope Array Observatory (CTAO), with up to ten times greater sensitivity compared to current generation instruments, will allow precise PSD reconstruction and unprecedented study of blazar flares. These flares reveal key insights into particle acceleration, photon production, and jet properties. The AGN monitoring and flare programs in CTAO's Key Science Project aim to deepen our understanding of blazar emissions.

astro-ph.HE↗

arXiv2025

Solar flare forecasting with foundational transformer models across image, video, and time-series modalities

We present a comparative study of transformer-based architectures for solar flare forecasting using heterogeneous data modalities, including images, video sequences, and time-series observations. Our analysis evaluates three recent foundational models - SigLIP2 for image encoding, VideoMAE for spatio-temporal video representation, and Moirai2 for multivariate time-series forecasting - applied to publicly available datasets of solar magnetograms from the SDO/HMI mission and soft X-ray fluxes acquired by GOES satellites. All models are trained and validated under consistent data splits and evaluation criteria, with the goal of assessing the strengths and limitations of transformer backbones across spatial and temporal representations of solar activity. We investigate multiple loss formulations (weighted BCE, focal, and score-oriented) and training balance strategies to mitigate class imbalance typical of flare datasets. Results show that while both SigLIP2 and VideoMAE achieve typical performance on image and video data (True Skill Statistic TSS~0.60-0.65), the time-series model Moirai2 reaches superior forecasting skill (TSS~0.74) using irradiance-based temporal evolution alone. These findings highlight the potential of pretrained transformer architectures and cross-modal learning for advancing operational space weather forecasting, paving the way toward unified multimodal models that integrate visual and temporal information.

astro-ph.IM↗

arXiv2025

Ultra-luminous X-ray pulsars as sources of TeV neutrinos

We explored the expected properties of the neutrino emission from accreting neutron stars in X-ray binaries using numerical simulations. The simulations are based on a model in which neutrinos are produced by the decay of charged pions and kaons, generated in inelastic collisions between protons accelerated up to TeV energies in the magnetosphere of a magnetized (B~1E12 G) neutron star and protons of the accretion disc. Our results show that this process can produce strong neutrino emission up to a few tens of TeV when the X-ray luminosity is above ~1E39 erg/s, as in ultra-luminous X-ray (ULX) pulsars. We show that neutrinos from a transient Galactic ULX pulsar with L_x ~ 5E39 erg/s can be detected with kilometre-scale detectors such as IceCube if the source is within about 3-4 kpc. We also derived an upper limit on the neutrino flux from the Galactic ULX pulsar Swift J0243.6+6124 using IceCube data, a result that has not been previously reported. Our findings establish a new benchmark for future astrophysical neutrino observations, critical for interpreting data from current and upcoming instruments with significantly improved sensitivity.

astro-ph.HE↗

arXiv2025

Swift/XRT monitoring of the orbital and superorbital modulations in 4U 1909+07

We report on an observational campaign performed with Swift/XRT on the wind-fed supergiant X-ray binary 4U 1909+07 to investigate the nature of the orbital and superorbital modulation of its X-ray emission. A total of 137 XRT observations have been carried out, summing up to a total effective exposure time of 114 ks and covering a total of 66 orbital and 19 superorbital cycles of the source. The XRT data folded on the orbital period of the source confirmed and improved the previously reported variability in intensity and absorption column density, which can be ascribed to the neutron star accreting from the wind of its B supergiant companion across a fairly circular orbit. The XRT data folded on the superorbital period did not provide evidence of significant variations in either the absorption column density and/or the power-law photon index. This may be due to a significant weakening of the superorbital modulation during the times when the XRT observations were carried out, as confirmed by the BAT dynamic power spectrum. We discuss the implications of these findings within the corotating interaction region model proposed to interpret the superorbital variability in wind-fed supergiant X-ray binaries.

astro-ph.HE↗

arXiv2025

Metis Observations of Alfvénic Outflows Driven by Interchange Reconnection in a Pseudostreamer

This study presents observations of a large pseudostreamer solar eruption and, in particular, the post-eruption relaxation phase, as captured by Metis onboard the Solar Orbiter on October 12, 2022, during its perihelion passage. Utilizing total brightness data, we observe the outward propagation of helical features up to 3 solar radii along a radial column that appears to correspond to the stalk of the pseudostreamer. The helical structures persisted for more than 3 hours following a jet-like coronal mass ejection associated with a polar crown prominence eruption. A notable trend is revealed: the inclination of these features decreases as their polar angle and height increase. Additionally, we measured their helix pitch. Despite a 2-minute time cadence limiting direct correspondence among filamentary structures in consecutive frames, we find that the Metis helical structure may be interpreted as a consequence of twist (nonlinear torsional Alfvén waves) and plasma liberated by interchange reconnection. A comparison was performed of the helix parameters as outlined by fine-scale outflow features with those obtained from synthetic white-light images derived from the high-resolution magnetohydrodynamics simulation of interchange reconnection in a pseudostreamer topology by Wyper et al. (2022). A remarkable similarity between the simulation-derived images and the observations was found. We conjecture that these Metis observations may represent the upper end in spatial and energy scale of the interchange reconnection process that has been proposed recently as the origin of the Alfvénic solar wind.

astro-ph.SR↗

arXiv2024

Probing the emission mechanism and nature of the pulsating compact object in the X-ray binary SAX J1324.4-6200

Recently, there has been renewed interest in the Be X-ray binary (Be/XRB) SAX J1324.4-6200 because of its spatial coincidence with a gamma-ray source detected by Fermi/LAT. To explore more thoroughly its properties, new observations were carried out in 2023 by NuSTAR, XMM-Newton, and Swift, jointly covering the energy range 0.2-79 keV. The X-ray spectrum of SAX J1324.4-6200 fits well with an absorbed power law with a high energy cut-off. We measured a NuSTAR spin period of 175.8127 +/- 0.0036 s and an XMM-Newton spin period of 175.862 +/- 0.025 s. All the available spin period measurements of SAX J1324.4-6200, spanning 29 years, are correlated with time, resulting in a remarkably stable spin-down of dP/dt=(6.09 +/- 0.06)*1E-9 s/s. If SAX J1324.4-6200 hosts an accretion powered pulsar, accretion torque models indicate a surface magnetic field of ~1E12-1E13 G. The X-ray properties emerging from our analysis strenghten the hypothesis that SAX J1324.4-6200 belongs to the small group of persistent Be/XRBs. We also performed radio observations with the Parkes Murriyang telescope, to search for radio pulsations. However, no radio pulsations compatible with the rotational ephemeris of SAX J1324.4-6200 were detected. We rule out the hypothesis that SAX J1324.4-6200 is a gamma-ray binary where the emission is produced by interactions between the pulsar and the companion winds. Other models commonly used to account for the production of gamma-rays in accreting pulsars cannot reproduce the bright emission from SAX J1324.4-6200. We examined other mechanisms for the gamma-ray emission and noted that there is a ~0.5% chance probability that an unknown extragalactic AGN observed through the Galactic plane may coincidentally fall within the Fermi/LAT error circle of the source and be the responsible of the gamma-ray emission. [Abridged]

astro-ph.HE↗

arXiv2024

Study of solar brightness profiles in the 18-26 GHz frequency range with INAF radio telescopes II. Evidence for coronal emission

One of the most important objectives of solar physics is the physical understanding of the solar atmosphere, the structure of which is also described in terms of the density (N) and temperature (T) distributions of the atmospheric matter. Several multi-frequency analyses show that the characteristics of these distributions are still debated, especially for the outer coronal emission. We aim to constrain the T and N distributions of the solar atmosphere through observations in the centimetric radio domain. We employ single-dish observations from two of the INAF radio telescopes at the K-band frequencies (18 - 26 GHz). We investigate the origin of the significant brightness temperature ($T_B$) level that we detected up to the upper corona ($\sim 800$ Mm of altitude with respect to the photospheric solar surface). To probe the physical origin of the atmospheric emission and to constrain instrumental biases, we reproduced the solar signal by convolving specific 2D antenna beam models. The analysis of the solar atmosphere is performed by adopting a physical model that assumes the thermal bremsstrahlung as the emission mechanism, with specific T and N distributions. The modelled $T_B$ profiles are compared with those observed by averaging solar maps obtained during the minimum of solar activity (2018 - 2020). The T and N distributions are compatible (within $25\%$ of uncertainty) with the model up to $\sim 60$ Mm and $\sim 100$ Mm of altitude, respectively. The analysis of the role of the antenna beam pattern on our solar maps proves the physical nature of the atmospheric emission in our images up to the coronal tails seen in our $T_B$ profiles. The challenging analysis of the coronal radio emission at higher altitudes, together with the data from satellite instruments will require further multi-frequency measurements.

astro-ph.SR↗

arXiv2024

Study of solar brightness profiles in the 18-26 GHz frequency range with INAF radio telescopes I: solar radius

The Sun is an extraordinary workbench, from which several fundamental astronomical parameters can be measured with high precision. Among these parameters, the solar radius $R_{\odot}$ plays an important role in several aspects, such as in evolutionary models. Despite the efforts in obtaining accurate measurements of $R_{\odot}$, the subject is still debated and measurements are puzzling and/or lacking in many frequency ranges. We aimed to determine the mean, equatorial, and polar radii of the Sun ($R_c$, $R_{eq}$, and $R_{pol}$) in the frequency range 18.1 - 26.1 GHz. We employed single-dish observations from the newly-appointed Medicina "Gavril Grueff" Radio Telescope and the Sardinia Radio Telescope (SRT) throughout 5 years, from 2018 to mid-2023, in the framework of the SunDish project for solar monitoring. Two methods to calculate the radius at radio frequencies are considered and compared. To assess the quality of our radius determinations, we also analysed the possible degrading effects of the antenna beam pattern on our solar maps, using two 2D-models. We carried out a correlation analysis with the evolution of the solar cycle through the calculation of Pearson's correlation coefficient $ρ$. We obtained several values for the solar radius - ranging between 959 and 994 arcsec - and $ρ$, with typical errors of a few arcsec. Our $R_{\odot}$ measurements, consistent with values reported in literature, suggest a weak prolatness of the solar limb ($R_{eq}$ > $R_{pol}$), although $R_{eq}$ and $R_{pol}$ are statistically compatible within 3$σ$ errors. The correlation analysis using the solar images from Grueff shows (1) a positive correlation between the solar activity and the temporal variation of $R_c$ (and $R_{eq}$) at all observing frequencies, and (2) a weak anti-correlation between the temporal variation of $R_{pol}$ and the solar activity at 25.8 GHz.

astro-ph.SR↗

arXiv2024

NuSTAR and Swift observations of two supergiant fast X-ray transients: AX J1841.0-0536 and SAX J1818.6-1703

Supergiant fast X-ray transients are wind-fed binaries hosting neutron star accretors, which display a peculiar variability in the X-ray domain. Different models have been proposed to explain this variability and the strength of the compact object magnetic field is generally considered a key parameter to discriminate among possible scenarios. We present here the analysis of two simultaneous observational campaigns carried out with Swift and NuSTAR targeting the supergiant fast X-ray transient sources AX J1841.0-0536 and SAX J1818.6-1703. A detailed spectral analysis is presented for both sources, with the main goal of hunting for cyclotron resonant scattering features that can provide a direct measurement of the neutron star magnetic field intensity. AX J1841.0-0536 was caught during the observational campaign at a relatively low flux. The source broad-band spectrum was featureless and could be well described by using a combination of a hot blackbody and a power-law component with no measurable cut-off energy. In the case of SAX J1818.6-1703, the broad-band spectrum presented a relatively complex curvature which could be described by an absorbed cut-off power-law (including both a cut-off and a folding energy) and featured a prominent edge at $\sim$7 keV, compatible with being associated to the presence of a "screen" of neutral material partly obscuring the X-ray source. The fit to the broad-band spectrum also required the addition of a moderately broad ($\sim$1.6 keV) feature centered at $\sim$14 keV. If interpreted as a cyclotron resonant scattering feature, our results would indicate for SAX J1818.6-1703 a relatively low magnetized neutron star ($\sim$1.2$\times$10$^{12}$ G).

astro-ph.HE↗

arXiv2023

Swift/XRT observations of superorbital modulations in wind-fed supergiant X-ray binaries

We present the first Swift/XRT long-term monitoring of 2S 0114+650, a wind-fed supergiant X-ray binary for which both orbital and superorbital periods are known (P_orb~11.6d and P_sup~0.8d). Our campaign, summing up to ~ 79ks, is the most intense and complete sampling of the X-ray light curve of this source with a sensitive pointed X-ray instrument, and covers 17 orbital, and 6 superorbital cycles. The combination of flexibility, sensitivity, and soft X-ray coverage of XRT allowed us to confirm previously reported spectral changes along the orbital cycle of the source and unveil the variability in its spectral parameters as a function of the superorbital phase. For completeness, we also report on a similar analysis carried out by exploiting XRT archival data on three additional wind-fed supergiant X-ray binaries IGR J16418-4532, IGR J16479-4514, and IGR J16493-4348. For these sources, the archival data provided coverage along several superorbital cycles but our analysis could not reveal any significant spectral variability.

astro-ph.HE↗

arXiv2023

Eruptive events with exceptionally bright emission in HI Ly-alpha observed by the Metis coronagraph

Metis, the coronagraph on board Solar Orbiter, provides for the first time coronagraphic imaging in the ultraviolet HI Ly-alpha line and, simultaneously, in polarized visible light, thus providing a host of information on the properties of CMEs and solar eruptions like their overall dynamics, time evolution, mass content, and outflow propagation velocity in the expanding corona. We analyzed in this work six CMEs observed by Metis between April and October 2021, which are characterized by a very strong HI Ly-alpha emission. We studied in particular the morphology, kinematics, and the temporal and radial evolution of the emission of such events, focusing on the brightest UV features. The kinematics of the eruptive events under consideration were studied by determining the height-time profiles of the brightest parts on the Metis plane of the sky. Furthermore, the 3D position in the heliosphere of the CMEs were determined by employing co-temporal images from two other coronagraphs: LASCO/C2 onboard SOHO, and COR2 onboard STEREO-A. Finally, the radiometrically calibrated Metis images of the bright UV features were analyzed to provide estimates of their volume and density. From the kinematics and radiometric analyses, we obtained indications of the temperatures of the bright UV cores of these events. The analysis of these strong UV-emitting features associated with coronal mass ejections demonstrates the capabilities of the current constellation of space coronagraphs, Metis, LASCO/C2, and COR2, in providing a complete characterization of the structure and dynamics of eruptive events in their propagation phase from their inception up to several solar radii. Furthermore, we show how the unique capabilities of the Metis instrument to observe these events in both HI Ly-alpha line and polarized VL radiation allow plasma diagnostics on the thermal state of these events.

astro-ph.SR↗

arXiv2023

Swift, NuSTAR, and INTEGRAL observations of the symbiotic X-ray binary IGR J16194-2810

We report on a simultaneous observational campaign with both Swift/XRT and NuSTAR targeting the symbiotic X-ray binary IGR J16194-2810. The main goal of the campaign was to investigate the possible presence of cyclotron scattering absorption features in the broad-band spectrum of the source, and help advance our understanding of the process of neutron star formation via the accretion-induced collapse of a white dwarf. The 1-30 keV spectrum of the source, as measured during our campaign, did not reveal the presence of any statistically significant absorption feature. The spectrum could be well described using a model comprising a thermal black-body hot component, most likely emerging from the surface of the accreting neutron star, and a power-law with no measurable cut-off energy (and affected by a modest absorption column density). Compared to previous analyses in the literature, we could rule out the presence of a colder thermal component emerging from an accretion disk, compatible with the idea that IGR J16194-2810 is a wind-fed binary (as most of the symbiotic X-ray binaries). Our results were strengthened by exploiting the archival XRT and INTEGRAL data, extending the validity of the spectral model used up to 0.3-40 keV and demonstrating that IGR J16194-2810 is unlikely to undergo significant spectral variability over time in the X-ray domain.

astro-ph.HE↗

arXiv2023

Bright blazar flares with CTA

The TeV extragalactic sky is dominated by blazars, radio-loud active galactic nuclei with a relativistic jet pointing towards the Earth. Blazars show variability that can be quite exceptional both in terms of flux (orders of magnitude of brightening) and time (down to the minute timescale). This bright flaring activity contains key information on the physics of particle acceleration and photon production in the emitting region, as well as the structure and physical properties of the jet itself. The TeV band is accessed from the ground by Cherenkov telescopes that image the pair cascade triggered by the interaction of the gamma ray with the Earth's atmosphere. The Cherenkov Telescope Array (CTA) represents the upcoming generation of imaging atmospheric Cherenkov telescopes, with a significantly higher sensitivity and larger energy coverage with respect to current instruments. It will thus provide us with unprecedented statistics on blazar light-curves and spectra. In this contribution we present the results from realistic simulations of CTA observations of bright blazar flares, taking as input state-of-the-art numerical simulations of blazar emission models and including all relevant observational constraints.

astro-ph.HE↗

arXiv2023

Modelling the expected very high energy gamma-ray emission from accreting neutron stars in X-ray binaries

The detection of gamma-ray emission from accreting pulsars in X-ray binaries (XRBs) has long been sought after. For some high-mass X-ray binaries (HMXBs), marginal detections have recently been reported. Regardless of whether these will be confirmed or not, future telescopes operating in the gamma-ray band could offer the sensitivity needed to achieve solid detections and possibly spectra. In view of future observational advances, we explored the expected emission above 10 GeV from XRBs, based on the Cheng & Ruderman model, where gamma-ray photons are produced by the decay of pion-0 originated by protons accelerated in the magnetosphere of an accreting pulsar fed by an accretion disc. We improved this model by considering, through Monte Carlo simulations, the development of cascades inside of and outside the accretion disc, taking into account pair and photon production processes that involve interaction with nuclei, X-ray photons from the accretion disc, and the magnetic field. We produced grids of solutions for different input parameter values of the X-ray luminosity (L_x), magnetic field strength (B), and for different properties of the region where acceleration occurs. We found that the gamma-ray luminosity spans more than five orders of magnitude, with a maximum of ~1E35 erg/s. The gamma-ray spectra show a large variety of shapes: some have most of the emission below ~100 GeV, others are harder (emission up to 10-100 TeV). We compared our results with Fermi/LAT and VERITAS detections and upper-limits of two HMXBs: A0535+26 and GROJ1008-57. More consequential comparisons will be possible when more sensitive instruments will be operational in the coming years.

astro-ph.HE↗

arXiv2023

Probing AGN variability with the Cherenkov Telescope Array

Relativistic jets launched by Active Galactic Nuclei are among the most powerful particle accelerators in the Universe. The emission over the entire electromagnetic spectrum of these relativistic jets can be extremely variable with scales of variability from less than few minutes up to several years. These variability patterns, which can be very complex, contain information about the acceleration processes of the particles and the area(s) of emission. Thanks to its sensitivity, five-to twenty-times better than the current generation of Imaging Atmospheric Cherenkov Telescopes depending on energy, the Cherenkov Telescope Array will be able to follow the emission from these objects with a very accurate time sampling and over a wide spectral coverage from 20 GeV to > 20 TeV and thus reveal the nature of the acceleration processes at work in these objects. We will show the first results of our lightcurve simulations and long-term behavior of AGN as will be observed by CTA, based on state-of-art particle acceleration models.

astro-ph.HE↗