arXiv subjects

Vito Sguera

Publications and source records attributed to Vito Sguera.

7 recordsLinked to original sources

arXiv2026

Evidence for a luminosity-class dichotomy in the supergiant donors of persistent high-mass X-ray binaries and supergiant fast X-ray transients

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. 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. Our collection of the relevant literature allowed us to build a robust sample of both types of HMXBs with a secure stellar classification. 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. 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.

astro-ph.HE↗

arXiv2026

AGILE detection of transient {\gamma}-ray emission from the region of the supergiant fast X-ray transient source IGR J17354-3255

Context. On April 14, 2009, the AGILE satellite detected a {\gamma}-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 {\gamma}-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 {\gamma}-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 {\gamma}-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.

astro-ph.HE↗

arXiv2026

New Hard X-Ray and Multiwavelength Study of the PeVatron Candidate PWN G0.9+0.1 in the Galactic Center Region

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 $\Gamma=2.11\pm0.07$. We combined the new X-ray data with the multiwavelength observations in radio, GeV, and TeV gamma rays and modeled the SED, applying a one-zone and a multi-zone leptonic model. 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 $\sim2.2$ kyr, as well as reproduce the observed PWN and SNR radio sizes. In both the one-zone and multi-zone leptonic models, the electron injection spectrum is well-described by a single power law with spectral index $p \sim 2.6$ and a maximum electron energy of $\sim2$ PeV, suggesting the source could be a leptonic PeVatron candidate. We estimate the average magnetic field to be $B_{\rm PWN} \sim 20\ \mu$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.

astro-ph.HE↗

arXiv2023

Capturing the lowest luminosity state of the Supergiant Fast X-ray Transient XTE J1739-302

We report here on the results of the analysis of Chandra, XMM-Newton and NuSTAR recent observations of the Supergiant Fast X-ray Transient XTEJ1739-302. The source was caught in a low X-ray luminosity state, from a few $10^{31}$ to $10^{34}$ erg/s (0.5-10 keV). In particular, a very low X-ray luminosity was captured during an XMM-Newton observation performed in October 2022, at a few $10^{31}$ erg/s (0.5-10 keV), never observed before in XTEJ1739-302. The XMM-Newton spectrum could be well fitted either by an absorbed, steep power law model (photon index of 3.5) or by a collisionally-ionized diffuse gas with a temperature of 0.7 keV, very likely produced by shocks in the supergiant donor wind. These observations covered different orbital phases, but all appear compatible with the low luminosity level expected from the orbital INTEGRAL light curve. The absorbing column density is variable in the range $10^{22}-10^{23}$ cm$^{-2}$. A broad-band X-ray spectrum could be investigated at $10^{34}$ erg/s (0.5-30 keV) for the first time in XTEJ1739-302 with not simultaneous (but at similar orbital phases) Chandra and NuSTAR data, showing a power law spectral shape with a photon index of about 2.2 and an absorbing column density of $\sim$$10^{23}$ cm$^{-2}$. Remarkably, owing to the XMM-Newton observation, the amplitude of the X-ray variability has increased to five orders of magnitude, making XTEJ1739-302 one of the most extreme SFXTs.

astro-ph.HE↗

arXiv2021

INTEGRAL View of TeV Sources: A Legacy for the CTA Project

Investigations that were carried out over the last two decades with novel and more sensitive instrumentation have dramatically improved our knowledge of the more violent physical processes taking place in galactic and extra-galactic Black-Holes, Neutron Stars, Supernova Remnants/Pulsar Wind Nebulae, and other regions of the Universe where relativistic acceleration processes are in place. Simultaneous and/or combined observations with gamma-ray satellites and ground based high-energy telescopes, have clarified the scenario of the mechanisms responsible for high energy photon emission by leptonic and hadronic accelerated particles in the presence of magnetic fields. Specifically, the European Space Agency INTEGRAL soft gamma-ray observatory has detected more than 1000 sources in the soft gamma-ray band, providing accurate positions, light curves and time resolved spectral data for them. Space observations with Fermi-LAT and observations that were carried out from the ground with H.E.S.S., MAGIC, VERITAS, and other telescopes sensitive in the GeV-TeV domain have, at the same time, provided evidence that a substantial fraction of the cosmic sources detected are emitting in the keV to TeV band via Synchrotron-Inverse Compton processes, in particular from stellar galactic BH systems as well as from distant black holes. In this work, employing a spatial cross correlation technique, we compare the INTEGRAL/IBIS and TeV all-sky data in search of secure or likely associations. Although this analysis is based on a subset of the INTEGRAL all-sky observations, we find that there is a significant correlation: 39 objects show emission in both soft gamma-ray and TeV wavebands. The full INTEGRAL database, now comprising almost 19 years of public data available, will represent an important legacy that will be useful for the Cherenkov Telescope Array (CTA) and other ground based large projects.

astro-ph.HE↗

arXiv2020

Advances in Understanding High-Mass X-ray Binaries with INTEGRAL and Future Directions

High mass X-ray binaries are among the brightest X-ray sources in the Milky Way, as well as in nearby Galaxies. Thanks to their highly variable emissions and complex phenomenology, they have attracted the interest of the high energy astrophysical community since the dawn of X-ray Astronomy. In more recent years, they have challenged our comprehension of physical processes in many more energy bands, ranging from the infrared to very high energies. In this review, we provide a broad but concise summary of the physical processes dominating the emission from high mass X-ray binaries across virtually the whole electromagnetic spectrum. These comprise the interaction of stellar winds with the high gravitational and magnetic fields of compact objects, the behaviour of matter under extreme magnetic and gravity conditions, and the perturbation of the massive star evolutionary processes by presence in a binary system. We highlight the role of the INTEGRAL mission in the discovery of many of the most interesting objects in the high mass X-ray binary class and its contribution in reviving the interest for these sources over the past two decades. We show how the INTEGRAL discoveries have not only contributed to significantly increase the number of high mass X-ray binaries known, thus advancing our understanding of the population as a whole, but also have opened new windows of investigation that stimulated the multi-wavelength approach nowadays common in most astrophysical research fields. We conclude the review by providing an overview of future facilities being planned from the X-ray to the very high energy domain that will hopefully help us in finding an answer to the many questions left open after more than 18 years of INTEGRAL scientific observations.

astro-ph.HE↗

arXiv2012

BeppoSAX observations of the X-ray pulsar MAXI J1409-619 in low state: discovery of cyclotron resonance features

The transient 500 s X-ray pulsar MAXI J1409-619 was discovered by the slit cameras aboard MAXI on October 17, 2010, and soon after accurately localized by Swift. We found that the source position was serendipitously observed in 2000 during BeppoSAX observations of the Galactic plane. Two sources are clearly detected in the MECS: one is consistent with the position of IGR J14043-6148 and the other one with that of MAXI J1409-619. We report on the analysis of this archival BeppoSAX/MECS observation integrated with newly analyzed observation from ASCA and a set of high-energy observations obtained from the offset fields of the BeppoSAX/PDS instrument. For the ON-source observation, the 1.8-100 keV spectrum is fit by an absorbed power law with a photon index Gamma = 0.87_{-0.19}^{+0.29}, corresponding to 2-10 and 15-100 keV unabsorbed fluxes of 2.7E-12 and 4E-11 erg/cm2/s, respectively, and a 2-10 keV luminosity of 7E+34 erg/s for a 15 kpc distance. For a PDS offset field observation, performed about one year later and showing a 15-100 keV flux of 7E-11 erg/cm2/s, we clearly pinpoint three spectral absorption features at 44, 73, and 128 keV, resolved both in the spectral fit and in the Crab ratio. We interpret these not harmonically spaced features as due to cyclotron resonances. The fundamental energy of 44 +/- 3 keV corresponds to a magnetic field strength at the neutron star surface of 3.8E12 (1+z) G, where z is the gravitational redshift. We discuss the nature of the source in the light of its possible counterpart.

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