Sandro Mereghetti

A0538–66 is a peculiar Be/X-ray binary in the Large Magellanic Cloud, hosting the fastest spinning neutron star of its class (∼69 ms) and showing the brightest optical bursts of any system of this type. We report four XMM-Newton observations obtained near periastron in 2025, together with MAXI and REM monitoring, and a reanalysis of archival RXTE data. On 30 September 2025 the source reached L _X ≈ 1.5 × 10 ³⁹ erg s ⁻¹ (0.2–10 keV), the highest level since 1980, and in February, May, and August 2026, MAXI detected similarly bright outbursts. During the XMM-Newton observation taken on 30 September 2025, we detected the pulsations, measuring P _spin = 69.25878(2) ms and Ṗ _spin = 6.58(9) × 10 ⁻¹⁰ s s ⁻¹ ; the pulsed fraction was low (≈3% in 1–10 keV), increased with energy, and the soft and hard pulse profiles were shifted by half a cycle. A new detection in RXTE data of 3 December 1998 gives a faster spin period of P = 68.88813(28) ms. The XMM-Newton /RGS spectra show emission lines from photoionised plasma at rest and blueshifted absorption lines that imply an outflow at ∼0.12 c . This indicates a highly super-Eddington accretion rate and an intrinsic L _X ≈ 1.5 × 10 ³⁹ erg s ⁻¹ , and it makes A0538–66 a transient ultra-luminous X-ray pulsar, a class of which only a few objects are known so far. Since 2023 the optical bursts observed by REM have become longer and brighter, resembling those of the 1980s, but most of them lack a luminous X-ray counterpart ( L _X > 10 ³⁷ erg s ⁻¹ ). We interpret these results in terms of a radiation pressure dominated accretion disc whose geometrically thick inner regions intermittently screen the innermost emitting regions.

We report the results of a deep (≈65 ks) Chandra exposure of the globular cluster NGC 6540, obtained by combining three observations carried out in 2023–2024 to investigate the nature of the peculiar X-ray source 3XMM J180608.9–274553. This source was previously observed with XMM–Newton to exhibit a short (≈300 s) and intense X-ray flare whose luminosity and duration are inconsistent with both typical type I X-ray bursts from low-mass X-ray binaries and stellar flares. Our new data indicate three faint X-ray sources near the position of the flare seen by XMM–Newton , only one of which was detected in a previous, much shorter Chandra observation. Based on the properties of these sources – localized at sub-arc-second precision – and of their optical counterparts, we discuss their possible nature and association with 3XMM J180608.9–274553: only one of these newly detected sources is compatible with the position of the flare. We also discuss a few scenarios – such as microlensing-induced amplification and black hole flaring activity analogous to that observed in Sgr A* – to explain the X-ray flare. Although these scenarios are intriguing, current observational evidence makes them both unlikely to be the origin of the XMM–Newton flare.

Magnetars are now known to be among the most strongly polarized celestial sources in X-rays. Here, we report on the 500 ks observation of the magnetar 1E 1547.0−5408 performed by the Imaging X-ray Polarimetry Explorer (IXPE) in 2025 March. The IXPE spectrum is well reproduced by a single thermal component with blackbody temperature kT _BB ∼ 0.67 keV and emission radius R _BB ∼ 1.2 km. The source exhibits a high linear polarization degree in the 2–6 keV band (PD = 47.7% ± 2.9%) with polarization angle PA = 75 . ° 8 ± 1 . ° 8 , measured west of celestial north. While PA does not appear to vary with energy, there is some evidence (at the 1 σ confidence level) of a minimum in PD between 3 and 4 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 nonuniform temperature distribution. Fitting the phase-dependent 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’s magnetosphere. Nevertheless, the fact that the RVM successfully reproduces the modulation of the X-ray polarization angle and the behavior of PD with the energy hint once more at the presence of QED effects in magnetars.

One of the distinctive properties of magnetars, young neutron stars powered mainly by magnetic energy, is the emission of short (≲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 past two decades, the INTEGRAL satellite has extensively covered the Galactic plane with good imaging capabilities, where most magnetars reside. We present the results of a comprehensive search for magnetar bursts in more than 20 yr 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 ∼2 × 10 ⁻⁹ to 3 × 10 ⁻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 ∼0.1 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 β = 0.76±0.04, 0.95±0.06, and 0.92±0.10, respectively. The burst spectra are generally well fit with an exponentially cutoff power law with peak energy E _peak in the range ∼20−60 keV for SGR 1806–20 and SGR 1935+2154, while the bursts of 1E 1547–5408 are slightly harder ( E _peak ∼ 35−100 keV). A significant anticorrelation between E _peak and fluence is found for SGR 1806–20, which provided the largest number of bursts among the sources of our sample.

The magnetar Swift J1555.2–5402 was discovered in outburst on 2021 June 3 by the Burst Alert Telescope on board the Swift satellite. Early X-ray follow-up revealed a spin period P ≃ 3.86 s, a period derivative Ṗ ≃ 3 × 10 ⁻¹¹ s s ⁻¹ , dozens of short bursts, and an unusual flux decline. We report here on the X-ray monitoring of Swift J1555.2–5402 over the first ≃29 months of its outburst with Swift , NICER , NuSTAR , INTEGRAL , and Insight -HXMT, as well as radio observations with Parkes soon after the outburst onset. The observed 0.3–10 keV flux remained at levels ≳10 ⁻¹¹ erg cm ⁻² s ⁻¹ for nearly 500 days before dropping by a factor of ≃10 from its 2021 June peak towards the end of the monitoring campaign. During this time span, the spectrum was dominated by a single blackbody, whose temperature attained an approximately constant value (∼1.2 keV), while the inferred radius shrank from ≈1.7 km to ≈0.3 km (assuming a source distance of 10 kpc). The long-term spin-down rate ( Ṗ ≃ 3.6 × 10 ⁻¹¹ s s ⁻¹ ) is only ∼15% higher than that measured in the first 30 days. No periodic or burst-like radio emission was detected, in line with what had been previously reported using different radio facilities. The persistently high temperature, shrinking hotspot, and prolonged bright flux plateau followed by a fast dimming observed during the outburst evolution pose a challenge for the outburst mechanisms proposed so far.

Context. The Galactic magnetar SGR 1935+2154 has exhibited prolific high-energy (HE) bursting activity in recent years. Aims. Investigating its potential tera-electronvolt counterpart could provide insights into the underlying mechanisms of magnetar emission and very high-energy (VHE) processes in extreme astrophysical environments. We aim to search for a possible tera-electronvolt counterpart to both its persistent and sub-second-scale burst emission. Methods. We analysed over 25 hour of observations from the Large-Sized Telescope prototype (LST−1) of the Cherenkov Telescope Array Observatory (CTAO) during periods of HE activity from SGR 1935+2154 in 2021 and 2022 to search for persistent emission. For bursting emission, we selected and analysed nine 0.1 s time windows centred around known short X-ray bursts, targeting potential sub-second-scale tera-electronvolt counterparts in a low-photon-statistics regime. Results. While no persistent or bursting emission was detected in our search, we establish upper limits for the tera-electronvolt emission of a short magnetar burst simultaneous to its soft gamma-ray flux. Specifically, for the brightest burst in our sample, the ratio between tera-electronvolt and X-ray flux is ≲10 ⁻³ . Conclusions. The non-detection of either persistent or bursting tera-electronvolt emission from SGR 1935+2154 suggests that if such components exist, they may occur under specific conditions not covered by our observations. This aligns with theoretical predictions of VHE components in magnetar-powered fast radio bursts and the detection of MeV–GeV emission in giant magnetar flares. These findings underscore the potential of magnetars, fast radio bursts, and other fast transients as promising candidates for future observations in the low-photon-statistics regime with Imaging Atmospheric Cherenkov Telescopes, particularly with the CTAO.

Context . 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 ≲ 10 ³⁶ erg s ⁻¹ ) much lower than those of typical X-ray transients. On 22 February 2024, during its daily Galactic center monitoring, Swift -XRT detected a VFXT 7 arcmin from Sgr A*, dubbing it Swift J174610–290018. Aims . 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. Methods . Swift -XRT light curves were used to constrain the duration of the two events. We carried out X-ray spectral analysis, exploiting XMM-Newton and NuSTAR data. We used Chandra and XMM-Newton observations from the last 25 years to constrain the quiescent luminosity of the source and to compare the two most recent outbursts with previous detections of the source. Results . During the 2024 outburst, which lasted about 50 days, the source reached a luminosity in the 2–10 keV band ( L _2–10 ) of ≃1.2 × 10 ³⁵ erg s ⁻¹ (assuming it is located at the Galactic center, i.e., at a distance of 8.2 kpc). The 2025 outburst is shorter (about 5 days) and reached L _2–10 ≃ 9 × 10 ³⁴ 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-Newton and Chandra catalogs of point sources (known as 4XMM J174610.7–290020 and 2CXO J174610.7–290019). During previous detections the source displayed luminosity levels ranging from L _2–10 ≃ 2 × 10 ³² to L _2–10 ≃ 3 × 10 ³³ erg s ⁻¹ between 2000 and 2010. Moreover, it exhibited a potential type I X-ray burst in 2004. Conclusions . The analysis of the outbursts and the potential type I burst strongly suggests that the VFXT Swift J174610–290018 is a neutron star low-mass X-ray binary. The source can be described as being an accretion disk corona source (as has been recently proposed by an XRISM/Xtend analysis). This scenario explains the overall low luminosity of this transient and the peculiar iron lines in the spectrum.

Thanks to a recent observation with XMM-Newton, we discovered periodic pulsations at P = 9.6652 ± 0.0002 s in a new ultraluminous X-ray source (ULX) in the galaxy NGC 4631. This source, dubbed as X−8, shows one of the largest spin-up rates ever observed, P ̇ = ( − 9.6 ± 0.5 ) × 1 0 − 8 s s ⁻¹ . These findings indicate that the compact object is a neutron star, and X−8 is a new member of the pulsating ULX class. The 0.3–10 keV luminosity of X−8 is ∼3.4 × 10 ³⁹ erg s ⁻¹ , and its X-ray spectrum can be described by an absorbed disk blackbody or a cut-off power law, similar to what is observed in other pulsating ULXs. We discuss two possible causes for the large spin-up rate: Doppler shift from orbital motion of the neutron star and intrinsic spin-up due to accretion torque. This new ULX pulsar adds a key source to the small known population, and will enable future studies to better constrain the physical mechanisms responsible for their super-Eddington luminosities.