Vito Sguera

Context. We have investigated the properties of the supergiant donors in two types of high-mass X-ray binaries (HMXBs, i.e. the so-called supergiant fast X-ray transients (SFXTs) and the classical X-ray sources), by collecting all available information from the literature. Aims. A prevalence of O-type versus B-type supergiant donors in SFXTs was previously suggested in the literature, possibly indicative of faster winds in SFXTs than in classical, persistent sources. Our aim is to investigate whether there is evidence for a dichotomy in the stellar luminosity class, since B-type supergiants are also known to be hosted in SFXTs. Methods. Our collection of the relevant literature allowed us to build a robust sample of both types of HMXBs with a secure stellar classification. Results. We have found that the SFXTs showing the most extreme, transient X-ray emission host only O-type or B-type stars with a Ib and Iab luminosity class, whereas classical sources have mostly Ia donors. Conclusions. This is robust evidence that might explain the two different behaviours of HMXBs at X-rays since the stellar luminosity class appears to be linked to the wind properties (the supergiant wind terminal velocity, the mass-loss rate, and the wind strength Q parameter), indicative of faster and less dense winds in SFXTs than in classical sources.

Context. On April 14, 2009, the AGILE satellite detected a γ -ray flare from an unknown transient source. Subsequent X-ray follow-up observations with Swift and INTEGRAL identified the supergiant fast X-ray transient (SFXT) IGR J17354−3255 as the best candidate counterpart, based on positional coincidence and a similar temporal behaviour. Aside from this hint, no SFXT has been firmly detected at high energies to date. Overall, SFXTs comprise a subclass of high-mass X-ray binaries (HMXBs) that host a massive OB supergiant star as a companion donor. They tend to display the most extreme X-ray variability among HMXBs. These systems might be able to emit photons at MeV-TeV energies in the form of fast flares lasting from hours to a few days, with a low-duty cycle. Aims. In this work, we analyse archival AGILE data to search for γ -ray flares consistent with IGR J17354−3255 and evaluate their possible physical origin. Methods. We identified a transient source, AGL J1736−3250, which emitted 19 γ -ray flares and was seen to be positionally consistent with IGR J17354−3255. Most flares, detected on a 1 d timescale, concentrate most of their emission on two, four, and six hour timescales, resembling those observed in the X-ray band from IGR J17354−3255. Results. An orbital phase analysis revealed that approximately half of the γ -ray activity occurs around the apastron passage of the compact object hosted in the binary system. We also incorporated archival Swift and INTEGRAL observations to provide phase-folded light curves at lower energies. Our collected results strongly support a physical association between IGR J17354−3255 and AGL J1736−3250, offering evidence that SFXTs could constitute a new class of Galactic high-energy transient emitters.

We present a new X-ray study and multiwavelength spectral energy distribution (SED) modeling of the young pulsar wind nebula (PWN) powered by the energetic pulsar PSR J1747–2809, inside the composite supernova remnant (SNR) G0.9+0.1, located in the Galactic center region. The source is detected by NuSTAR up to 30 keV with evidence for the synchrotron burnoff effect in the changing spatial morphology with increasing energy. The broadband 2–30 keV spectrum of PWN G0.9+0.1 is modeled by a single power law with photon index Γ = 2.11 ± 0.07. We combined the new X-ray data with multiwavelength observations in radio, GeV, and TeV gamma rays and modeled the SED, applying one-zone and multizone leptonic models. The comparison of the models is successful, as we obtained physically compatible results in the two cases. Through the one-zone model, we constrain the age of the system to ∼2.2 kyr, as well as reproduce the observed PWN and SNR radio sizes. In both the one-zone and multizone leptonic models, the electron injection spectrum is well described by a single power law with spectral index p ∼ 2.6 and a maximum electron energy of ∼2 PeV, suggesting the source could be a leptonic PeVatron candidate. We estimate the average magnetic field to be B _PWN ∼ 20 μ G. We also report the serendipitous NuSTAR detection of renewed X-ray activity from the very faint X-ray transient XMMU J174716.1–281048 and characterize its spectrum.

We present new broad-band X-ray results aimed at the identification and characterization of four poorly studied hard X-ray transients discovered by INTEGRAL: IGR J16426+6536, IGR J09446−2636, IGR J21268+6203, and IGR J02447+7046. The key properties and X-ray behavior of these sources have remained largely unknown until now. We investigated the temporal, spectral, and energetic characteristics of their hard X-ray outbursts detected above 20 keV by INTEGRAL. In addition, we performed a systematic analysis of unpublished archival soft X-ray observations below 10 keV, enabling a full exploration of their large INTEGRAL error circles in order to search for the most likely soft X-ray counterparts. Within their arcsecond-sized X-ray error circles, we identified single optical/near-infrared counterparts for each source. We analyzed their photometric properties to constrain the nature of the systems. Our results show that the X-ray properties of these four transients are inconsistent with the previously proposed extragalactic AGN origin and instead support a Galactic nature for all of them. Specifically, we propose a very faint X-ray transient classification for IGR J16426+6536, a nearby flaring star for IGR J09446−2636 and IGR J21268+6203, and finally a γ -ray binary nature for IGR J02447+7046.

Context. 1LHAASO J1740+0948u is a very-high-energy (VHE) source initially reported in the first catalogue by the LHAASO Collaboration, with no previous identifications and no counterpart at other wavelengths. It is detected by the KM2A instrument only, i.e. at energies above 25 TeV, with a 17.1σ significance, and also above 100 TeV at a 9.4σ level. It is located (σ _RA,Dec ~ 0.02° at 95% confidence) at 0.22° from PSR J1740+1000, a faint radio and gamma-ray pulsar placed well above the Galactic plane ( b = 20.4°) that displays a long X-ray tail. Despite the offset, the two sources are likely associated with each other, since no other object has been found nearby at such a high Galactic latitude. Aims. We aim to study the diffuse X-ray emission around PSR J1740+1000 and its tail-like pulsar wind nebula (PWN) with XMM-Newton to investigate the origin of 1LHAASO J1740+0948u through a multi-wavelength spectral energy distribution (SED) fitting, testing different scenarios. Methods. We analysed ~500 ks of XMM-Newton observations of PSR J1740+1000. We studied, for the first time, the diffuse emission in two different regions: one centred on the pulsar and the other located inside the 1LHAASO J1740+0948u source region. We also studied the X-ray tail and how its emission evolves as a function of the distance from the pulsar. We then performed a fit of the SED, including the spectrum of 1LHAASO J1740+0948u and the X-ray data obtained from either the analysis of the PWN or the diffuse emission, to understand whether one of the two X-ray sources could be related to the VHE emission and attempt a source classification. Results. The X-ray analysis of the diffuse emission resulted in upper limits in the range of 0.5-10 keV. The tail-like PWN is best fitted with an absorbed power law with Γ = 1.76 ± 0.06 in the 0.5-8 keV range, with no significant detection of spectral variations with distance. The SED modelling, assuming the VHE emission to be only due to the X-ray tail, constrains its magnetic field to B = 6.8 ± 1.9 μG, which is in line with previous results. However, we do not find a good fit that could explain both the X-rays of the tail and the LHAASO spectrum with reasonable parameters, hinting that the VHE emission likely comes from an older X-ray-faint electron population. We then performed a SED fitting of the VHE spectrum combined with the upper limits on the diffuse X-ray emission, constraining the magnetic field to be as low as B ≤ 1.2 μG. We suggest that 1LHAASO J1740+0948u could represent either the relic PWN of PSR J1740+1000 or its pulsar halo. Based on our best-fit results, we estimated the energy density and obtained values ranging from 0.03 to 0.67 eV/cm ³ , depending on the spectral index of the electron distribution. These very low values suggest a halo-like nature for 1LHAASO J1740+0948u, but deeper multi-wavelength observations are required to confirm this hypothesis.

We present new broad-band X-ray results aimed at the identification and characterization of four poorly studied hard X-ray transients discovered by INTEGRAL: IGR J16426+6536, IGR J09446-2636, IGR J21268+6203, and IGR J02447+7046. The key properties and X-ray behavior of these sources have remained largely unknown until now. We investigated the temporal, spectral, and energetic characteristics of their hard X-ray outbursts detected above 20 keV by INTEGRAL. In addition, we performed a systematic analysis of unpublished archival soft X-ray observations below 10 keV, enabling a full exploration of their large INTEGRAL error circles in search of the most likely soft X-ray counterparts. Within their arcsecond-sized X-ray error circles, we identified single optical/near-infrared counterparts for each source. We analyzed their photometric properties to constrain the nature of the systems. Our results show that the X-ray properties of these four transients are inconsistent with the previously proposed extragalactic AGN origin, and instead support a Galactic nature for all of them. Specifically, we propose a very faint X-ray transient classification for IGR J16426+6536, a nearby flaring star for IGR J09446-2636 and IGR J21268+6203, finally a gamma-ray binary nature for IGR J02447+7046.

We report on the results of X-ray observations (XMM–Newton, INTEGRAL and Swift) of two hard X-ray sources, IGR J17503-2636 and IGR J17507-2647, whose nature is not fully elucidated in the literature. Three XMM–Newton observations covered the field of IGR J17503-2636, in 2020 and twice in 2023. The analysis of the two XMM–Newton observations performed in September 2023, six days apart, did not detect IGR J17503-2636, allowing us to pose the most stringent 3σ upper limit on the source flux to date (∼9.5 × 10⁻¹⁴ erg cm⁻² s⁻¹, 2–10 keV, flux corrected for absorption). This value implies that the amplitude of the X-ray flux variability exceeds a factor of ∼2100, compared with the discovery outburst in 2018. A candidate X-ray periodicity at 0.335397(3) seconds has been barely detected (significance of ∼3.8σ) from IGR J17503-2636 with XMM–Newton (pulsed fraction of (10 ± 1)%). The new data, put into the context of previous literature, allow us to propose a new classification of IGR J17503-2636 as a symbiotic X-ray binary, rather than a candidate supergiant fast X-ray transient. IGR J17507-2647 was formerly reported below 10 keV only during Chandra observations performed in 2009. We report here on two XMM–Newton observations that serendipitously covered the source field in 2020 and in 2023, finding a stable X-ray emission, both in X-ray flux and spectral shape. The long-term, persistent X-ray emission has also been probed by several Swift/XRT short observations and by INTEGRAL data spanning several years. We have detected an iron line in the emission (with centroid energy in the range of 6.3–6.6 keV), never reported before in the IGR J17507-2647 spectrum. The source properties favor the identification with a cataclysmic variable.

We report new X-ray results from the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), Swift, Chandra, and XMM–Newton observations of the hitherto poorly studied unidentified X-ray transient IGR J17419−2802. We studied in detail the temporal, spectral, and energetic properties of three hard X-ray outbursts detected above 20 keV by INTEGRAL. They are all characterized by an average X-ray luminosity of 3 × 10³⁵ erg s⁻¹ and a constrained duration of a few days. This marks a peculiarly short and faint X-ray transient nature for IGR J17419−2802. From archival unpublished soft X-ray observations, we found that the source spends most of the time undetected at very low X-ray fluxes (down to < 4.7 × 10⁻¹⁴ erg cm⁻² s⁻¹) for a dynamic range > 2000 when in outburst. We provided an accurate arcsecond-sized source error circle. Inside it, we pinpointed the best candidate near-infrared counterpart whose photometric properties are compatible with a late-type spectral nature. Based on our new findings, we suggest that IGR J17419−2802 is a new member of the very faint X-ray transients (VFXTs) class. Detailed investigations of VFXT outbursts above 20 keV are particularly rare. In this respect, our reported INTEGRAL outbursts are among the best studied to date; in particular, their constrained duration of a few days is among the shortest ever measured for a VFXT.