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Sandro Mereghetti

Publications and source records attributed to Sandro Mereghetti.

At least 19 recordsLinked to original sources
arXiv2026

GRINTA: a new Probe of the Dynamic Universe

Recent observations of the transient sky at all wavelengths are increasingly revealing the importance of the multi-messenger and multi-wavelength approach. The GRINTA (Gamma-Ray INternational Transient Array Observatory) mission, proposed for launch around the middle of the next decade, is conceived as a small mission with good sensitivity, excellent angular resolution and fast follow-up capability for studying transient sources at timescales from ms to hours, at the same time ensuring optimal integration with multi-messenger networks. The GRINTA mission will carry two complementary payloads to cover in total the 5 keV-10 MeV band, that will detect and localise gamma-ray bursts covering $\sim$ half of the sky and will be able to perform imaging surveys with sub-arcmin resolution. GRINTA will operate in synergy with the most powerful electromagnetic, gravitational wave and neutrino observatories foreseen to be operational after 2035.

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arXiv2026

Detection of an Extremely Luminous Radio Counterpart to the Be/X-ray Binary A0538-66

We present the discovery of radio emission from the Be/X-ray binary A0538-66 with the Australian Square Kilometre Array Pathfinder (ASKAP), and results from a subsequent weekly monitoring campaign with the MeerKAT radio telescope. A0538-66, located in the Large Magellanic Cloud, hosts a neutron star with a short spin period ($P \approx 69$ ms) in a highly eccentric $\approx16.6$-day orbit. Its rare episodes of super-Eddington accretion, rapid optical and X-ray flares, and other peculiar properties make it an interesting system among high-mass X-ray binaries. Our MeerKAT data reveal that it is also one of the most radio-luminous neutron star X-ray binaries observed to date, reaching $\approx 3 \times 10^{22}~\text{erg}~\text{s}^{-1} \text{Hz}^{-1}$, with radio emission that appears to be orbitally modulated. We consider several possible mechanisms for the radio emission, and place A0538-66 in context by comparing it to similar systems.

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arXiv2026

The long quest for vacuum birefringence in magnetars: 1E 1547.0-5408 and the elusive smoking gun

Magnetars are now known to be among the most strongly polarized celestial sources in X-rays. Here we report on the $500\,\mathrm{ks}$ observation of the magnetar 1E 1547.0-5408 performed by the Imaging X-ray Polarimetry Explorer (IXPE) in March 2025. The IXPE spectrum is well reproduced by a single thermal component with blackbody temperature $kT_\mathrm{BB}\sim 0.67\,\mathrm{keV}$ and emission radius $R_\mathrm{BB}\sim 1.2\,\mathrm{km}$. The source exhibits a high linear polarization degree in the $2$--$6\,\mathrm{keV}$ band ($\mathrm{PD}=47.7\pm2.9\%$) with polarization angle $\mathrm{PA}=75^\circ.8 \pm 1.^\circ8$, measured West of celestial North. While $\mathrm{PA}$ does not appear to vary with energy, there is some evidence (at the $1\sigma$ confidence level) of a minimum in $\mathrm{PD}$ between $3$ and $4\,\mathrm{keV}$, compatible with what is expected by partial mode conversion at the vacuum resonance in a magnetized atmosphere. Phase-resolved spectral and polarimetric analyses reveal that X-ray thermal radiation likely originates from a single, fairly small hot spot with a non-uniform temperature distribution. Fitting the phase-dependent $\mathrm{PA}$ measured by IXPE with a rotating vector model (RVM) constrains the source geometry and indicates that both the dipole axis and line-of-sight are misaligned with respect to the spin axis. Under these conditions, the high polarization of the source cannot be regarded as compelling evidence for the presence of vacuum birefringence in the star magnetosphere. Nevertheless, the fact that the RVM successfully reproduces the modulation of the X-ray polarization angle and the behavior of $\mathrm{PD}$ with the energy hint once more to the presence of QED effects in magnetars.

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arXiv2025

INTEGRAL IBIS catalog of magnetar bursts

One of the distinctive properties of magnetars, young neutron stars powered mainly by magnetic energy, is the emission of short ($\lesssim$1 s) bursts of hard X-rays. Such bursts have been observed in nearly all the known magnetars, although at different and time-variable rates of occurrence. In the last two decades, the INTEGRAL satellite has extensively covered with good imaging capabilities the Galactic plane, where most magnetars reside. We present the results of a comprehensive search for magnetar bursts in more than twenty years of archival data of the INTEGRAL IBIS instrument (15 keV - 1 MeV). This led to the detection of 1349 bursts with 30-150 keV fluence in the $\sim2\times10^{-9} - 3\times10^{-6}$ erg cm$^{-2}$ range from 21 of the 34 examined magnetars and candidate magnetars with well known positions. The durations of the bursts, in terms of $T_{90}$, follow a lognormal distribution centered at $\sim0.1$ s. Most of the detected bursts originated from three particularly active sources: 1E 1547-5408, SGR 1806-20, and SGR 1935+2154. The integral distributions of their burst fluences follow power laws with slopes $\beta$= 0.76$\pm$0.04, 0.95$\pm$0.06, and 0.92$\pm$0.10, respectively. The burst spectra are generally well fit with an exponentially cut-off power law with peak energy $E_{peak}$ in the range $\sim20-60$ keV for SGR 1806-20 and SGR 1935+2154, while the bursts of 1E 1547-5408 are slightly harder ($E_{peak}\sim35-100$ keV). A significant anti-correlation between $E_{peak}$ and fluence is found for SGR 1806-20, which provided the largest number of bursts among the sources of our sample.

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arXiv2025

The very faint X-ray transient Swift J174610-290018 at the Galactic center

Very Faint X-ray Transients (VFXTs) are a class of X-ray binary systems that exhibit occasional outbursts with peak X-ray luminosities (L_X< 1e36 erg s^-1) much lower than typical X-ray transients. On 22nd February 2024, during its daily Galactic center monitoring, Swift-XRT detected a VFXT, 7 arcmin from Sgr A* dubbing it Swift J174610--290018. We aim to characterize the outburst that occurred in 2024, and a second, distinct outburst in 2025, to understand the nature and accretion flow properties of this new VFXT. Swift-XRT light curves are used to constrain the duration of the two events. We carried out X-ray spectral analysis exploiting XMM and NuSTAR data. We used Chandra and XMM observations of the last 25 years to constrain the quiescent luminosity of the source. During the 2024 outburst, which lasted about 50 days, the source reached a luminosity in the 2-10 keV band of L_X = 1.2e35 erg s^-1 (assuming it is located at the Galactic center). The 2025 outburst is shorter (about 5 days), and reached L_X = 9e34 erg s^-1. The spectral features of the source include an excess at 6.5-7 keV, which can be associated either with a single reflection line or with the ionized Fe XXV and XXVI lines. The same source was identified in both the XMM and Chandra catalogs of point sources (known as 4XMM J174610.7--290020). During previous detections, the source displayed luminosity levels ranging from L_X= 2e32 to L_X = 3e34 erg s^-1 between 2000 and 2010. Moreover, it exhibited a potential type I X-ray burst in 2004. The analysis of the outbursts and the potential type I burst strongly suggests the neutron star low mass X-ray binary (NS-LMXB) nature of the VFXT. The source can be described by an accretion disk corona (as has been recently proposed by the XRISM/Xtend analysis). This scenario explains the overall low luminosity of this transient and the peculiar iron lines in the spectrum.

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arXiv2025

Classification of Unidentified Extended LHAASO Sources based on their Gamma-Ray Morphology: Prospects for Future IACTs

While Supernova Remnants (SNRs) are widely considered the primary accelerators of cosmic rays (CRs) up to hundreds of TeV, they struggle to account for the CR flux at PeV energies, suggesting the existence of additional PeVatrons. Observations from LHAASO (Large High Altitude Air Shower Observatory) have identified several PeVatron candidates, including some SNRs, pulsar wind nebulae, TeV halos and young massive star clusters (YMSCs). These objects accelerate particles that interact with the surrounding interstellar medium and radiation fields, producing very-high-energy gamma rays (>100 TeV), a key signature of both leptonic and hadronic PeVatrons. We simulate and model the emission of TeV halos and YMSCs, adopting radial emission profiles derived from observational data. Given the current angular resolution of gamma-ray instruments, these profiles often appear similar, making it challenging to distinguish between source classes. We explore how next-generation Imaging Atmospheric Cherenkov Telescopes (IACTs), namely the CTAO (Cherenkov Telescope Array Observatory) and the ASTRI Mini-Array (Astrofisica con Specchi a Tecnologia Replicante Italiana), can classify these sources based on their morphology. We test our classification methods, derived from the profile features of known sources, on simulated CTAO and ASTRI Mini-Array observations of unidentified extended sources from the first LHAASO catalog. We present the results of our analysis to highlight the potential of future IACT observations in identifying the nature of extended gamma-ray sources, refining PeVatron candidate classifications, and improving our understanding of cosmic-ray accelerators.

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arXiv2025

Morphology of Young Massive Stellar Clusters with Next-Generation IACTs

The term PeVatron designates astrophysical objects capable of accelerating particles to PeV energies. Their nature and particle acceleration mechanisms are uncertain, but ultra-high-energy gamma rays are produced when particles accelerated by either leptonic or hadronic PeVatrons interact with the surrounding medium or radiation fields. The atmospheric air shower observatory LHAASO detected photons with energies above 100 TeV from 43 sources in the Galactic Plane, proving the existence of PeVatrons within the Milky Way. In particular, one of the detections was a 1.4 PeV photon in spatial correspondence with Cygnus OB2, providing a strong hint that young massive stellar clusters (YMSCs) can act as PeVatrons. The next-generation Cherenkov telescopes will have unprecedented energy and angular resolution. Therefore, they will be able to resolve spatially YMSCs better than LHAASO. We focused on a sample of 5 YMSCs and their environments visible from either hemisphere with the CTAO or ASTRI Mini-Array. We modeled the gamma-ray emission above 1 TeV. We devised methods for classifying YMSCs that could be detected as unidentified extended TeV sources and estimated the observational time needed to distinguish the morphology of different classes of sources. We study the morphology of the sources in our sample in order to identify the main features. We simulated observations of all sources with the instrument response function (IRF) of CTAO or ASTRI Mini-Array. We compare their emission distribution to the one of the TeV halos observed by HAWC. We parametrize their radial profiles in order to develop methodologies to classify them and to distinguish YMSCs from TeV halos based on their morphology. We expect some feature, such as the emission peak, to be key in differentiating between the two classes of objects. We then test them on a sample of sources of the first LHAASO catalog.

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arXiv2025

Unveiling TeV halos among unidentified extended TeV sources

In recent years, the number of known sources emitting very- and ultra-high-energy gamma-rays has increased significantly thanks to facilities such as LHAASO and HAWC. Many of the observed sources are still unidentified or poorly constrained due to the limited angular resolution of these instruments; however, it is now ascertained that approximately half of them have a pulsar in coincidence. Some of these unidentified extended sources may be the result of the diffusion of leptons accelerated by the pulsar itself or in its nebula to energies exceeding 50 TeV. This new class of sources, called TeV halos, is characterized by a peculiar radial profile that, if properly resolved, is key to distinguishing them from other TeV sources that are associated with a pulsar, such as supernova remnants and pulsar wind nebulae. In this contribution, we consider all the pulsars which are spatially coincident with an unidentified extended TeV source, in order to quantify whether its spin-down power, age and distance allow the pulsar to produce a TeV halo with the observed flux and extension. We also investigate how the next generation of Imaging Atmospheric Cherenkov Telescopes (IACTs), namely the Cherenkov Telescope Array Observatory (CTAO) and the ASTRI Mini-Array, will observe and characterize these TeV halos. We present a set of simulated sources with the expected morphology and spectrum, and we show for which of them we can distinguish between TeV halos and other classes of extended sources.

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arXiv2025

Deep Chandra X-ray observation of the isolated black hole OGLE-2011-BLG-0462

OGLE-2011-BLG-0462 is an isolated black hole of ~7 solar masses at a distance of 1.5 kpc identified thanks to the astrometric microlensing technique. It is the first specimen discovered of the large population of ~10^8 stellar-mass black holes that are believd to wander in the Galaxy. Electromagnetic radiation powered by accretion from the interstellar medium is expected from OGLE-2011-BLG-0462, but has not been detected at any wavelength. We present the results of a deep pointed observation with the Chandra satellite that provides an upper limit of 3x10^29 erg/s on the luminosity of OGLE-2011-BLG-0462 in the 0.5-7 keV energy range. This is about one order of magnitude below the previous limit obtained from shallower observations that serendipitously covered the sky position of this black hole. Our results are briefly compared with models of the source and with the X-ray upper limits for candidate isolated black holes and black holes in wide binary systems.

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arXiv2025

Sometimes they come back: pulsations in A 0538-66 at 69 ms rediscovered with NICER

We report on NICER X-ray observations of the Be X-ray binary A 0538-66 located in the Large Magellanic Cloud. Fast pulsations (69 ms) in this source were discovered in 1980 during a bright outburst in which it reached a luminosity of ~8E38 erg/s, but were never reobserved since then. We clearly detected the pulsations at P=69.3055 +/- 0.0005 ms with a pulsed fraction of ~20% during a short time interval (~11 minutes) on 2023 January 9, when A 0538-66 had a luminosity of ~8E36 erg/s (0.3-10 keV). The pulsations were not detected in other NICER observations (total exposure ~162.7 ks), during which A 0538-66 had a similar or lower luminosity. On 2023 February 8-9 the source exhibited a strong variability, with short flares reaching ~1E38 erg/s, but no periodic pulsations were detected. Assuming the magnetospheric radius lies within the corotation radius during pulsations, we estimate the neutron star magnetic field is below ~2.7E10 G. This would make A 0538-66 the high-mass X-ray binary with the weakest known magnetic field. We discuss implications for magnetic field evolution in accreting pulsars and propose that, alternatively, A 0538-66 has a stronger magnetic field and during the NICER detection, a centrifugal barrier may have been active while part of the plasma accumulated at the magnetosphere sporadically leaked through it via an instability mechanism, allowing accretion onto the polar caps.

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arXiv2025

Multi-wavelength study of the high Galactic latitude supernova remnant candidate G118.4+37.0 associated with the Calvera pulsar

The candidate supernova remnant (SNR) G118.4+37.0 (Calvera's SNR), discovered as a faint radio ring at high Galactic latitude and coincident with extended Fermi/LAT gamma-ray emission, is likely associated to the X-ray pulsar 1RXS J141256.0+792204 (Calvera). Previous XMM-Newton data hinted at soft diffuse X-ray emission inside the ring but lacked sufficient exposure for detailed characterisation. We obtained new XMM-Newton observations, and produced count-rate images, equivalent width and median photon energy maps to identify optimal regions for spectral analysis. We complemented these observations with a reanalysis of Fermi/LAT gamma-ray data and new Telescopio Nazionale Galileo observations aimed to search for Halpha emission. The X-ray diffuse emission is well described by a model of shock-heated plasma with temperature kT ~ 0.15 keV, mildly under-solar N and O abundances and densities ne=0.1-0.7 cm-3. According to our estimates, Calvera's SNR is 10-20 kyr old and lies at a distance of 4-5 kpc. A distinct "Clump" region shows hard emission equally well described by a thermal (kT ~ 1.7 keV) or a non thermal model (Gamma ~ 2.7). The brightest X-ray area is close to the gamma-ray peak and to an isolated Halpha filament. G118.4+37.0 is a middle-aged remnant which expands in a tenuous medium and encountered a denser phase, likely the relic of the wind activity of the massive progenitor star. The estimated SNR distance is consistent within the uncertainties with that estimated for Calvera, confirming that this peculiar pulsar was born in the explosion of a massive star high above the Galactic disk. Our measured ambient density, together with the patchy morphology of the gamma-ray emission and the detection of Halpha filaments indicates that a hadronic origin is compatible with the gamma-ray flux, though a mixed leptonic-hadronic cannot be excluded.

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arXiv2025

Prospects for Time-Domain and Multi-Messenger Science with eXTP

In this new era of time-domain and multi-messenger astronomy, various new transients and new phenomena are constantly being discovered thanks to the rapid advances in observations, which provide the excellent opportunity to study the physics in the extreme environments. The enhanced X-ray Timing and Polarimetry mission (eXTP), planned to be launched in 2030, has several key advantages, including advanced polarimetry, high sensitivity & large effective area, and wide energy range coverage, which make it a groundbreaking project in high-energy astrophysics. In this article, we briefly introduce the potential time-domain and multi-messenger targets for eXTP, including gravitational-wave (GW) counterparts, gamma-ray bursts (GRBs), magnetars and fast radio bursts (FRBs), tidal disruption events (TDEs), supernovae, high energy neutrinos and TeV active galactic nucleus (AGNs), and so on. We discuss the advantages of future eXTP observations for detecting these sources, their detection capabilities, the abilities to distinguish theoretical models, and their applications in gravity and cosmology.

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arXiv2024

The massive fast spinning white dwarf in the HD 49798/RX J0648.0-4418 binary

I review the properties and discuss some of the puzzling aspects of the unique binary system composed of the luminous hot subdwarf HD 49798 and a white dwarf with mass of 1.2 solar masses and spin period of 13.2 s. This is one of the few massive white dwarfs with a dynamically measured mass and the one with the shortest spin period. It emits pulsed X-rays with a very soft spectrum, powered by accretion from the tenuous stellar wind of its companion of sdO spectral type. The current level of mass accretion cannot provide enough angular momentum to explain the small, but precisely measured, spin-up rate of 72 nanoseconds per year, which is instead best interpreted as the result of the radial contraction of this young white dwarf. The higher mass transfer rate expected during the future evolutionary stages of HD 49798 will drive the white dwarf above the Chandrasekhar limit, but the final fate, a type Ia SN explosion or the collapse to a millisecond pulsar, is uncertain.

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arXiv2024

IXPE detection of highly polarized X-rays from the magnetar 1E 1841-045

The Imaging X-ray Polarimetry Explorer (IXPE) observed for the first time highly polarized X-ray emission from the magnetar 1E 1841-045, targeted after a burst-active phase in August 2024. To date, IXPE has observed four other magnetars during quiescent periods, highlighting substantially different polarization properties. 1E 1841-045 exhibits a high, energy-dependent polarization degree, which increases monotonically from ~15% at 2-3 keV up to ~55% at 5.5-8 keV, while the polarization angle, aligned with the celestial North, remains fairly constant. The broadband spectrum (2-79 keV) obtained by combining simultaneous IXPE and NuSTAR data is well modeled by a blackbody and two power-law components. The unabsorbed 2-8 keV flux (~2E-11 erg/cm2/s) is about 10% higher than that obtained from archival XMM-Newton and NuSTAR observations. The polarization of the soft, thermal component does not exceed ~25%, and may be produced by a condensed surface or a bombarded atmosphere. The intermediate power law is polarized at around 30%, consistent with predictions for resonant Compton scattering in the star magnetosphere; while, the hard power law exhibits a polarization degree exceeding 65%, pointing to a synchrotron/curvature origin.

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arXiv2024

GRB241107A: a Giant Flare from a close-by extragalactic Magnetar?

We report the results on the short gamma-ray burst GRB 241107A, obtained with the IBIS instrument on board the INTEGRAL satellite. The burst had a duration of about 0.2 s, a fluence of $8 \times 10^{-7}$ erg cm-2 in the 20 keV-10 MeV range and a hard spectrum, characterized by a peak energy of 680 keV. The position of GRB 241107A has been precisely determined because it fell inside the imaging field of view of the IBIS coded mask instrument. The presence of the nearby galaxy PGC 86046 in the 3 arcmin radius error region, suggests that GRB 241107A might be a giant flare from a magnetar rather than a canonical short GRB. For the 4.1 Mpc distance of PGC 86046, the isotropic energy of $1.6 \times 10^{45}$ erg is in agreement with this hypothesis, that is also supported by the time resolved spectral properties similar to those of the few other extragalactic magnetars giant flares detected so far.

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arXiv2024

INTEGRAL search for magnetar giant flares from the Virgo Cluster and in nearby galaxies with high star formation rate

Giant flares from magnetars can reach, for a fraction of a second, luminosities greater than 10$^{47}$ erg s$^{-1}$ in the hard X-ray/soft $\gamma$-ray range. This makes them visible at distances of several megaparsecs. However, at extragalactic distances (farther than the Magellanic Clouds) they are difficult to distinguish from the short $\gamma$-ray bursts, which occur much more frequently. Since magnetars are young neutron stars, nearby galaxies with a high rate of star formation are optimal targets to search for magnetar giant flares (MGFs). Here we report the results of a search for MGFs in observations of the Virgo cluster and in a small sample of nearby galaxies obtained with the IBIS instrument on the INTEGRAL satellite. From the currently known MGF sample we find that their energy distribution is well described by a power law with slope $\gamma$=2 (with 90% c.l. interval [1.7-2.2]). From the lack of detections in this extensive data set (besides 231115A in M82) we derive a 90% c.l. upper limit on the rate of MGF with $E>3\times10^{45}$ erg of $\sim2\times10^{-3}$ yr$^{-1}$ per magnetar and a lower limit of $R(E)>\sim4\times10^{-4}$ yr$^{-1}$ magnetar$^{-1}$ for $E<10^{45}$ erg.

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arXiv2024

INTEGRAL observations of magnetars

The INTEGRAL satellite has collected a large amount of data on magnetars in our Galaxy, spanning more than 20 years starting from 2003. The large data set obtained with the IBIS/ISGRI instrument at energies above 20 keV allows us to study both the properties and long-term evolution of their persistent hard X-ray emission and the population characteristics of the short bursts emitted during active periods. We are carrying out a comprehensive analysis of the observed magnetars, exploiting the most recent calibrations and analysis software. Here we report on the long term evolution of the hard X-ray flux of the magnetars detected with ISGRI and the results of a sensitive search for short bursts in SGR J1935+2154.

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