We present detailed multiwavelength analysis of GRB 210704A: a Fermi Gamma-ray Burst Monitor discovered and Fermi Large Area Telescope (LAT) detected gamma-ray burst (GRB). The burst is dominated by a short (approximate to 2 s) pulse followed by weaker softer emission. We line stack our afterglow spectrum and determine the most likely redshift to be z = 2 . 34 . This is corroborated by the photometric redshift of the extended source underlying the GRB. The spectral energy distribution fit parameters, late-time imaging, as well as the GRB's energetics, spectral lag, and location point to a collapsar nature. Follow-up observations reveal excess optical/infrared emission with respect to a standard afterglow, peaking around T-0 + 7 d (2 d in the rest frame). The excess is extremely luminous (M-r = -22.0 mag) and rapidly evolving. Strikingly, it resembles the emission seen in recently discovered Einstein Probe fast X-ray transients EP241021a and EP240414a, as well as the population of luminous fast blue optical transients (LFBOTs). This provides a link between these sources and GRBs. Fermi /LAT observations imply a high Lorentz factor, making this a case where LFBOT-like emission is also associated with a powerful successfully launched jet. We model the excess as likely coming from an energetic refreshed shock.

We present a spectroscopic survey of young stellar objects (YSOs) in Sh2-284, a distant ( 4.5 kpc), low-metallicity (Z 1/3 Zsun) star-forming region (SFR) toward the Galactic anticenter. Candidate YSOs were selected using mid-infrared Spitzer/IRAC data with optical and near-infrared photometry. Follow-up spectroscopy was conducted with MODS at the Large Binocular Telescope. We characterize the stellar and accretion properties of the disk-bearing population in a metal-poor environment, probing stellar masses from 2.95 Msun to 0.35 Msun. This work constitutes the first wide-field ( 45'x45') spectroscopic investigation of YSOs in Sh2-284, providing a comprehensive view of star formation in this Galactic environment. We provide tentative first estimates of iron abundance for three low-mass targets and lithium abundance for a limited number of objects, offering an initial characterization of the chemical properties of these stars. We adopt a multi-diagnostic approach based on nine tracers, exploiting the spectral coverage of the instrument, using H_alpha, H_beta, H_gamma, CaII infrared triplet, Pa_eta, Pa_zeta, and Pa_epsilon emission lines. We derive key accretion properties and compare them with those measured in nearby, solar-metallicity SFRs to explore potential metallicity-driven differences in accretion behavior and disk evolution. We tentatively find indications of a flattening in the flux-flux relations of the metal-poor YSOs compared to solar-metallicity samples, a behavior that is recovered across diagnostics. Our observations indicate that the selected disk-bearing YSOs in this metal-poor environment exhibit resilient accretion activity, spanning from 2.3x10^(-10) Msun/yr and 1.0x10^(-6) Msun/yr, and a median rate of 2.2x10^(-8) Msun/yr. Overall, this demonstrates that gas accretion can be efficiently sustained in sub-solar metallicity environments.

Context. RR Lyrae (RRL) stars are widely considered tracers of ancient (> 10 Gyr) metal-poor stellar populations. However, recent kinematic and photometric studies suggest the existence of a metal-rich RRL subpopulation associated with the thin disk and intermediate ages (similar to 2-5 Gyr), therefore challenging canonical evolutionary models. Aims. We aim to provide the first spectroscopic confirmation of a member of this elusive population. Specifically, we target a metal-rich RRL candidate recently identified photometrically as a member of the intermediate-age open cluster Trumpler 5 (similar to 2.5 Gyr). Methods. We obtained high-resolution spectroscopy using PEPSI at the LBT and GHOST at Gemini South telescope. We measured radial velocities (RVs) from multiple epochs to constrain cluster membership and derived detailed chemical abundances (Mg, Ca, Sc, Ti, Mn, Fe, Cu, Zn, Y, and Ba) to compare the RRL's composition with that of red clump stars in the cluster. Results. The RRL's systemic velocity (V-gamma = 50.57(-0.36)(+0.78) km s(-1)) is in excellent agreement with the cluster mean (V = 50.76 +/- 0.49 km s(-1)). Combining RVs, proper motions, and parallax, the probability of the star being a background interloper is negligible (similar to 0.002%, better than 4 sigma). We derived a metallicity of [Fe/H] = -0.40 +/- 0.05, which matches the cluster value. While most abundance ratios (Mg, Ti, Mn, Cu, and Zn) align with cluster members, the RRL exhibits significant depletion in Ca, Sc, Y, and Ba. Notably, [Sc/Fe] is underabundant by similar to 0.6 dex relative to the cluster stars, following trends seen in field metal-rich RRLs. Conclusions. We provide strong constraints on the membership status between an RRL variable and an intermediate-age open cluster. Cluster membership enables accurate measurement of RRL age and chemical anomalies relative to its host, particularly in Sc and neutron-capture elements. These anomalies further reinforce a nonstandard formation channel for this RRL, possibly indicating binary interactions and mass transfer.

The Large Binocular Telescope (LBT) is a world-leading astronomical observatory, where the Italian partnership has played an important role in increasing the telescope’s productivity, both through an optimized observing strategy and through peer-reviewed publications that are well recognized by the international astronomical community. This manuscript provides an updated overview of the active and past instruments at LBT, together with key usage statistics. In particular, we analyze the operational performance recorded in the LBT Italia night logs during INAF’s observing time and assess the scientific impact of each instrument. Between 2014 and 2025, LBT Italia produced an average of 14 refereed publications per year, based on an annual average of 311 h of on-sky time. This corresponds to approximately 2.2 nights of telescope time per publication. The results of this analysis are placed in an international context to evaluate the competitiveness of LBT, and we outline future perspectives for scientific exploitation.

Context. Anomalous Cepheids (ACs) are pulsating variable stars, and are less studied compared to the well-known Classical Cepheids (CCs) and RR Lyrae stars. The ACs are metal poor ([Fe/H] < 1.5) and follow distinct period-luminosity (PL) and period-Wesenheit (PW) relations that can be used for distance measurements, and they can pulsate in the fundamental (F) and first overtone (1O) modes. Aims. Our goal is to evaluate the precision and accuracy of distances obtained via PL and PW relations of ACs and thus to assess if they could be used to establish a cosmic distance scale independent from CCs. To this aim, we derived new, precise PL and PW relations for the F mode, the 1O mode, and, for the first time, the combined F+1O mode ACs in the Magellanic Clouds. We investigated the wavelength dependence of these relations and applied them to calculate the distances of various stellar systems in the Local Group hosting ACs, as well as to confirm the classification of these variable stars. Methods. We analyzed near-infrared (NIR) time series photometry in the Y, J, and K-s bands for about 200 ACs in the Magellanic Clouds acquired during 2009-2018 in the context of the VISTA survey of the Magellanic Clouds system (VMC), a European Southern Observatory public survey. The VMC NIR photometry was complemented with optical data from Gaia DR3 and the Optical Gravitational Lensing Experiment IV survey, which also provided the identification, periods, and pulsation mode for the investigated ACs. Custom templates generated from our best light curves were used to derive precise intensity-averaged mean magnitudes for 118 and 75 ACs in the Large (LMC) and Small Magellanic Clouds (SMC), respectively. Results. Optical and NIR mean magnitudes were used to derive multiband PL and PW relations, which were calibrated with the geometric distance modulus to the LMC based on eclipsing binaries. We investigated the dependence of PL relations on wavelength, finding that slopes increase and dispersion decreases when going from optical to NIR bands. We calculated the LMC distance modulus through calibrated AC PW relations in the Milky Way using Gaia parallaxes, the LMC-SMC relative distance modulus, and we confirmed the AC nature of a few new pulsators in Galactic globular clusters. We derived a distance modulus for the Draco dwarf spheroidal galaxy of 19.425 +/- 0.048 mag, which is in agreement with recent literature determinations, but a discrepancy of 0.1 mag with RR Lyrae-based distance hints at possible metallicity effects on the AC PL and PW relations. Future spectroscopic surveys and Gaia DR4 will refine the AC distance scale and assess metallicity effects on PLRs and PWRs.

We report the discovery of the ultracool dwarf binary system J1250+0455AB, a low-mass (M-tot < 0.2 M-circle dot) system in which the components straddle the M/L dwarf boundary. The binary was resolved through near-infrared adaptive optics imaging with LUCI1-SOUL on the Large Binocular Telescope, revealing a projected angular separation of 0.17 +/- 0.015 arcsec, which, combined with a system distance of 71 +/- 5.8 pc, corresponds to a physical separation of 12.2 +/- 1.5 AU at a position angle of 84.8 +/- 0.2 degrees. We estimated the orbital period of J1250+0455AB to be 156 +/- 8 yr, the bolometric luminosities of the primary and secondary luminosities as log(L-bol/L-circle dot) = -3.45 +/- 0.04 and -3.58 +/- 0.04, respectively, with the spectral types of M9 and L0 determined through binary template fitting and spectrophotometric relations. This binary system is part of a hierarchical triple with a separation of 10.44 arcsec from its primary. We estimated the age of the system from the rotational period of the primary star as 0.56( -0.06)(+0.07) Gyr. Using evolutionary models, for each component we estimate the mass [0.079 +/- 0.002 M-circle dot / 0.072 +/- 0.003 M-circle dot], effective temperature [2350 +/- 38 K / 2200 +/- 43 K], and radius [0.113 +/- 0.003 R-circle dot / 0.108 +/- 0.002 R-circle dot]. Based on the system's binding energy, total mass, and separation, J1250+0455AB is predicted to be a highly stable system, remaining bound for >10 Gyr. J1250+0455AB extends the growing population of UCD benchmark systems, providing a new system for refining evolutionary theories at the lowest stellar masses into the substellar regime

Context. Identifying black holes is essential for our understanding of the development of stars and can reveal novel principles of physics. Gravitational microlensing provides an exceptional opportunity to examine an undetectable population of black holes in the Milky Way. In particular, long-lasting events are likely to be associated with massive lenses, including black holes. Aims. We present an analysis of the Gaia18ajz microlensing event reported by the Gaia Science Alerts system. Gaia18ajz is a long-timescale event exhibiting features indicative of the annual microlensing parallax effect. Our objective is to estimate its lens parameters based on the best-fitting model. Methods. We used photometric data obtained from the Gaia satellite and terrestrial observatories to investigate a variety of microlensing models and calculate the most probable mass and distance to the lens, taking into consideration a Galactic model as a prior. Subsequently, we applied a mass-brightness relation to evaluate the likelihood that the lens is a main sequence star. We also describe the DarkLensCode (DLC), an open- source routine that computes the distribution of probable lens mass, distance, and luminosity employing the Galaxy priors on stellar density and velocity for microlensing events with detected microlensing parallax. Results. We modelled the Gaia18ajz event and found its two possible models, the most probable Einstein timescales for which are 316(+36) (-30) days and 299(+25) (-22) days. Applying Galaxy priors for stellar density and motion, we calculated a most probable lens mass of 4.9(-2.3)(+5.4) M (R) located at 1.14(+0.75) (-0.57) kpc, and a less probably mass of 11.1(+10.3) (-4.7) M (R) located at 1.31(-0.60)(+0.80) kpc. Our analysis of the blended light suggests that the lens is likely a dark remnant of stellar evolution rather than a main sequence star.

X-ray observations of the optically selected z=6.025 QSO CFHQS J164121+375520 (hereafter J1641) revealed that its flux dropped by a factor ≳7 from 2018, when it was a bright and soft X-ray source, to 2021. Such a strong variability amplitude has not been observed before among z>6 QSOs, and the underlying physical mechanism was unclear. We carried out a new X-ray and rest-frame UV monitoring campaign of J1641 over 2022-2024. We detected J1641 with Chandra in the 2-7 keV band, while no significant emission is detected at softer X-ray energies, making J1641 an X-ray changing look QSO at z>6. Comparing with the 2018 epoch, the 0.5-2 keV flux dropped dramatically by a factor >20. We ascribe this behaviour to intervening, and still ongoing, obscuration by Compton-thick gas intercepting our line of sight between 2018 and 2021. The screening material could be an inner disk or a failed nuclear wind that increased their thickness. Another possibility is that we have witnessed an occultation event due to dust-free clouds located at sub-pc/pc scales, similar to those recently invoked to explain the remarkable X-ray weakness of AGN discovered by JWST. These interpretations are also consistent with the lack of strong variations of the QSO rest-frame UV lightcurve over the same period. Future monitoring of J1641 and the possible discovery of other X-ray changing look QSOs at z>6 will provide us with precious information about the physics of rapid supermassive black-hole growth at high redshift.

We performed a detailed spectroscopic analysis of three extremely metal-poor RR Lyrae stars, exploring uncharted territories at these low metallicities for this class of stars. Using high-resolution spectra acquired with HARPS-N at TNG, UVES at VLT, and PEPSI at LBT, and employing Non-Local Thermodynamic Equilibrium (NLTE) spectral synthesis calculations, we provide abundance measurements for Fe, Al, Mg, Ca, Ti, Mn, and Sr. Our findings indicate that the stars have metallicities of [Fe/H] = -3.40 +/- 0.05, -3.28 +/- 0.02, and -2.77 +/- 0.05 for HD 331986, DO Hya, and BPS CS 30317-056, respectively. Additionally, we derived their kinematic and dynamical properties to gain insights into their origins. Interestingly, the kinematics of one star (HD 331986) is consistent with the Galactic disc, while the others exhibit Galactic halo kinematics, albeit with distinct chemical signatures. We compared the [Al/Fe] and [Mg/Mn] ratios of the current targets with recent literature estimates to determine whether these stars were either accreted or formed in situ, finding that the adopted chemical diagnostics are ineffective at low metallicities ([Fe/H] less than or similar to -1.5). Finally, the established horizontal branch evolutionary models, indicating that these stars arrive at hotter temperatures on the Zero-Age Horizontal Branch (ZAHB) and then transition into RR Lyrae stars as they evolve, fully support the existence of such low-metallicity RR Lyrae stars. As a consequence, we can anticipate detecting more of them when larger samples of spectra become available from upcoming extensive observational campaigns.

We present new deep, wide-field imaging data from the Large Binocular Telescope (LBT) in g and r bands from the Smallest Scale of Hierarchy Survey (SSH) that reveal previously undetected tidal features and stellar streams in the outskirts of six dwarf irregular galaxies (NGC 5238, UGC 6456, UGC 6541, UGC 7605, UGC 8638, and UGC 8760) with stellar masses in the range 1.2 x 10(7) M-circle dot to 1.4 x 10(8) M-circle dot. The six dwarfs are located 1-2 Mpc away from large galaxies, which implies that the observed distortions are unlikely to be due to tidal effects from a nearby, massive companion. At the distances of similar to 3-4 Mpc at which the dwarfs lie, the identified tidal features are all resolved into individual stars in the LBT images and appear to consist of a population older than 1-2 Gyr. This excludes the possibility that they result from irregular and asymmetric star formation episodes that are common in gas-rich dwarf galaxies. The most plausible explanation is that we witness the hierarchical merging assembly of these dwarfs with their satellite populations. This scenario is also supported by the peculiar morphology and disturbed velocity field of their HI component. From the SSH sample, we estimate that a fraction of similar to 13% of the late-type dwarfs show signs of merging with satellites. This is in agreement with other recent independent studies and theoretical predictions within the Lambda CDM cosmological framework.

Relics of ancient accretion events experienced by the Milky Way are predominantly located within the stellar halo of our Galaxy. However, debris from different objects display overlapping distributions in dynamical spaces, making it extremely challenging to properly disentangle their contribution to the build-up of the Galaxy. To shed light on this chaotic context, we initiated a program aimed at the homogeneous chemical tagging of the local halo of the Milky Way, focusing on the component in retrograde motion, since this is expected to host a large fraction of stars accreted from past mergers. The A Walk on the Retrograde Side (WRS) project targets retrograde halo stars in the solar neighborhood with accurate 6D phase space information available, measuring the precise chemical abundance of several chemical elements from high-resolution spectroscopy. In this first paper, we present the project and the analysis of high-resolution spectra obtained with UVES at VLT and at LBT for 186 stars. We obtained accurate radial velocity and chemical abundances for several elements for all the target stars. In particular, we focus on the chemical composition of a specific subset of substructures that have been dynamically identified in the literature. Our study reveals that two among the more recently discovered structures in the retrograde halo, namely, Antaeus/L-RL64 and ED-3, have identical chemical patterns and similar integrals of motion, suggesting a common origin. In turn, the abundance patterns of this unified system differ from that of Gaia-Enceladus, confirming that it is an independent structure. Finally, Sequoia exhibits a different chemistry with respect to that of Gaia-Enceladus at [Fe/H] < −1.5 dex, showcasing an excess of stars with lower Mg and Ca in the common metallicity range.

Thanks to the rapidly increasing time-domain facilities, we are entering a golden era of research on gamma-ray bursts (GRBs). In this Letter, we report our observations of GRB 240529A with the Burst Optical Observer and Transient Exploring System, the 1.5 m telescope at Observatorio de Sierra Nevada, the 2.5 m Wide Field Survey Telescope of China, the Large Binocular Telescope, and the Telescopio Nazionale Galileo. The prompt emission of GRB 240529A shows two comparable energetic episodes separated by a quiescence time of roughly 400 s. Combining all available data on the GRB Coordinates Network, we reveal the simultaneous apparent X-ray plateau and optical rebrightening around 103-104 s after the burst. Rather than the energy injection from the magnetar as widely invoked for similar GRBs, the multiwavelength emissions could be better explained as two shocks launched from the central engine separately. The optical peak time and our numerical modeling suggest that the initial bulk Lorentz factor of the later shock is roughly 50, which indicates that the later jet should be accretion driven and have a higher mass loading than a typical one. The quiescence time between the two prompt emission episodes may be caused by the transition between different accretion states of a central magnetar or black hole, or the fallback accretion process. A sample of similar bursts with multiple emission episodes in the prompt phase and sufficient follow-up could help to probe the underlying physics of GRB central engines.

Relics of ancient accretion events experienced by the Milky Way are predominantly located within the stellar halo of our Galaxy. However, debris from different objects display overlapping distributions in dynamical spaces, making it extremely challenging to properly disentangle their contribution to the build-up of the Galaxy. To shed light on this chaotic context, we initiated a program aimed at the homogeneous chemical tagging of the local halo of the Milky Way, focusing on the component in retrograde motion, since this is expected to host a large fraction of stars accreted from past mergers. The A Walk on the Retrograde Side (WRS) project targets retrograde halo stars in the solar neighborhood with accurate 6D phase space information available, measuring the precise chemical abundance of several chemical elements from high-resolution spectroscopy. In this first paper, we present the project and the analysis of high-resolution spectra obtained with UVES at VLT and at LBT for 186 stars. We obtained accurate radial velocity and chemical abundances for several elements for all the target stars. In particular, we focus on the chemical composition of a specific subset of substructures that have been dynamically identified in the literature. Our study reveals that two among the more recently discovered structures in the retrograde halo, namely, Antaeus/L-RL64 and ED-3, have identical chemical patterns and similar integrals of motion, suggesting a common origin. In turn, the abundance patterns of this unified system differ from that of Gaia-Enceladus, confirming that it is an independent structure. Finally, Sequoia exhibits a different chemistry with respect to that of Gaia-Enceladus at [Fe/H] < -1.5 dex, showcasing an excess of stars with lower Mg and Ca in the common metallicity range.

ABSTRACTClassical Cepheids (DCEPs) are the most important standard candles in the extra-galactic distance scale thanks to the period–luminosity ($\rm PL$), period–luminosity–colour ($\rm PLC$), and period–Wesenheit ($\rm PW$) relations that hold for these objects. The advent of the Gaia mission, and in particular the Early Data Release 3 (EDR3), provided accurate parallaxes to calibrate these relations. In order to fully exploit Gaia measurements, the zero point (ZP) of Gaia parallaxes should be determined with an accuracy of a few $\rm \mu as$. The individual ZP corrections provided by the Gaia team depend on the magnitude and the position on the sky of the target. In this paper, we use an implicit method that relies on the Cepheid $\rm PL$ and $\rm PW$ relations to evaluate the ensemble Gaia parallax zero point. The best inferred estimation of the offset value needed to additionally correct (after the Gaia team correction) the Gaia parallaxes of the present DCEP sample amounts to $\rm -22\pm 4\, \mu as$. This value is in agreement with the most recent literature values and confirms that the correction proposed by the Gaia team overcorrected the parallaxes. As a further application of our results, we derive an estimate of the Large Magellanic Cloud distance ($\rm \mu _0=18.49\pm 0.06\, mag$) in very good agreement with the currently accepted value obtained through geometric methods.

Through adaptive optics (AO) imaging with the SOUL+LUCI instrument at the Large Binocular Telescope we were able to resolve, for the first time, individual stars in the gas-rich galaxy DDO68 C. This system was already suggested to be interacting with the extremely metal-poor dwarf DDO68, but its nature has remained elusive so far because of the presence of a bright foreground star close to its line of sight that hampers a detailed study of its stellar population and distance. In our study, we turned this interloper star into an opportunity to have a deeper insight on DDO68 C, using it as a guide star for the AO correction. Although the new data do not allow for a direct distance measurement through the red giant branch tip method, the combined analysis of the resolved-star color–magnitude diagram, of archival GALEX far-UV and near-UV photometry, and of H α data provides a self-consistent picture in which DDO68 C is at the same ∼13 Mpc distance as its candidate companion DDO68. These results indicate that DDO68 is a unique case of a low-mass dwarf, less massive than the Magellanic Clouds, interacting with three satellites (DDO68 C and two previously confirmed accreting systems), providing useful constraints on cosmological models and a potential explanation for its anomalous extremely low metallicity.

We present the results of our analysis of Gaia19dke, an extraordinary microlensing event in the Cygnus constellation that was first spotted by the {\gaia} satellite. This event featured a strong microlensing parallax effect, which resulted in multiple peaks in the light curve. We conducted extensive photometric, spectroscopic, and high-resolution imaging follow-up observations to determine the mass and the nature of the invisible lensing object. Using the Milky Way priors on density and velocity of lenses, we found that the dark lens is likely to be located at a distance of $D_L =(3.05^{+4.10}_{-2.42})$kpc, and has a mass of $M_L =(0.51^{+3.07}_{-0.40}) M_\odot$. Based on its low luminosity and mass, we propose that the lens in Gaia19dke event is an isolated white dwarf.

Gamma-ray bursts (GRBs) are classified into long and short events. Long GRBs (LGRBs) are associated with the end states of very massive stars, while short GRBs (SGRBs) are linked to the merger of compact objects. GRB 200826A was a peculiar event, because by definition it was an SGRB, with a rest-frame duration of ∼0.5 s. However, this event was energetic and soft, which is consistent with LGRBs. The relatively low redshift (z = 0.7486) motivated a comprehensive, multiwavelength follow-up campaign to characterize its host, search for a possible associated supernova (SN), and thus understand the origin of this burst. To this aim we obtained a combination of deep near-infrared (NIR) and optical imaging together with spectroscopy. Our analysis reveals an optical and NIR bump in the light curve whose luminosity and evolution are in agreement with several SNe associated to LGRBs. Analysis of the prompt GRB shows that this event follows the E p,i–E iso relation found for LGRBs. The host galaxy is a low-mass star-forming galaxy, typical of LGRBs, but with one of the highest star formation rates, especially with respect to its mass ( logM*/M⊙=8.6 , SFR ∼ 4.0 M ⊙ yr−1). We conclude that GRB 200826A is a typical collapsar event in the low tail of the duration distribution of LGRBs. These findings support theoretical predictions that events produced by collapsars can be as short as 0.5 s in the host frame and further confirm that duration alone is not an efficient discriminator for the progenitor class of a GRB.

Explaining the existence of super massive black holes (SMBHs) with MBH ≳ 108 M⊙ at z ≳ 6 is a persistent challenge to modern astrophysics. Multiwavelength observations of z ≳ 6 quasi-stellar objects (QSOs) reveal that, on average, their accretion physics is similar to that of their counterparts at lower redshift. However, QSOs showing properties that deviate from the general behavior can provide useful insights into the physical processes responsible for the rapid growth of SMBHs in the early universe. We present X-ray (XMM-Newton, 100 ks) follow-up observations of a z ≈ 6 QSO, J1641+3755, which was found to be remarkably X-ray bright in a 2018 Chandra dataset. J1641+3755 is not detected in the 2021 XMM-Newton observation, implying that its X-ray flux decreased by a factor ≳7 on a notably short timescale (i.e., ≈115 rest-frame days), making it the z > 4 QSO with the largest variability amplitude. We also obtained rest-frame ultraviolet (UV) spectroscopic and photometric data with the Large Binocular Telescope (LBT). Surprisingly, comparing our LBT photometry with archival data, we found that J1641+3755 became consistently brighter in the rest-frame UV band from 2003 to 2016, while no strong variation occurred from 2016 to 2021. Its rest-frame UV spectrum is consistent with the average spectrum of high-redshift QSOs. Multiple narrow absorption features are present, and several of them can be associated with an intervening system at z = 5.67. Several physical causes can explain the variability properties of J1641+3755, including intrinsic variations of the accretion rate, a small-scale obscuration event, gravitational lensing due to an intervening object, and an unrelated X-ray transient in a foreground galaxy in 2018. Accounting for all of the z > 6 QSOs with multiple X-ray observations separated by more that ten rest-frame days, we found an enhancement of strongly (i.e., by a factor > 3) X-ray variable objects compared to QSOs at later cosmic times. This finding may be related to the physics of fast accretion in high-redshift QSOs.