An INTEGRAL overview of High Mass X-ray Binaries: classes or transitions?

We analyzed in a systematic way the public INTEGRAL observations spanning from December 2002 to September 2016, to investigate the hard X-ray properties of about 60 High Mass X-ray Binaries (HMXBs). We considered both persistent and transient sources, hosting either a Be star (Be/XRBs) or a blue supergiant companion (SgHMXBs, including Supergiant Fast X-ray Transients, SFXTs), a neutron star or a black hole. INTEGRAL X-ray light curves (18-50 keV), sampled at a bin time of about 2 ks, were extracted for all HMXBs to derive the cumulative distribution of their hard X-ray luminosity, their duty cycle, the range of variability of their hard X-ray luminosity. This allowed us to obtain an overall and quantitative characterization of the long-term hard X-ray activity of the HMXBs in our sample. Putting the phenomenology observed with INTEGRAL into context with other known source properties (e.g. orbital parameters, pulsar spin periods) together with observational constraints coming from softer X-rays (1-10 keV), enabled the investigation of the way the different HMXB sub-classes behave (and sometimes overlap). For given source properties, the different sub-classes of massive binaries seem to cluster in a suggestive way. However, for what concerns supergiant systems (SgHMXBs versus SFXTs), several sources with intermediate properties exist, suggesting a smooth transition between the two sub-classes.Comment: 27 pages, 17 figures, 3 tables; accepted for publication in Monthly Notices of the Royal Astronomical Society (accepted 2018 August 30. Received 2018 August 22; in original form 2018 May 16

Lontano, lontano nel tempo. un viaggio nell'universo a cavallo della luce

Mostra temporanea che accosta reperti archeologici all'intero del Museo di Storia Naturale di Milano con oggetti astronomici la cui distanza spaziale si traduce in distanza temporale a causa della limitati velocità della luce. La mostra sfrutta le connessioni multidisciplinari per illustrate concetti scientifici apparentemente difficili e distanti dal pubblico. L’accostamento tra reperti archeologici e le sorgenti celesti è uno degli esempi più concreti per tastare la distanza degli oggetti celesti e creare connessioni e sinergie tra diversi ambiti culturali: un metodo per inserire l’astronomia in una percezione più quotidiana e connessa agli aspetti comuni

EXTraS discovery of a peculiar flaring X-ray source in the Galactic globular cluster NGC 6540

We report the discovery of a flaring X-ray source in the globular cluster NGC 6540, obtained during the EXTraS project devoted to a systematic search for variability in archival data of the XMM-Newton satellite. The source had a quiescent X-ray luminosity of the order of ~10^32 erg/s in the 0.5-10 keV range (for a distance of NGC 6540 of 4 kpc) and showed a flare lasting about 300 s. During the flare, the X-ray luminosity increased by more than a factor 40, with a total emitted energy of ~10^36 erg. These properties, as well as Hubble Space Telescope photometry of the possible optical counterparts, suggest the identification with a chromospherically active binary. However, the flare luminosity is significantly higher than what commonly observed in stellar flares of such a short duration, leaving open the possibility of other interpretations.Comment: To appear in Astronomy and Astrophysic

INTEGRAL @ INAF-IASF Milano: from Archives to Science

In this work, we give an overview of the INTEGRAL archival activities performed at INAF-IASF Milano, along with highlights of the scientific results obtained on High Mass X-ray Binaries. A few years after the launch of INTEGRAL, in order to increment and ease the exploitation of the data at INAF-IASF Milano, we prepared and maintained an INTEGRAL archive named ’GOLIA’. This gave us full control over what was being done and how, and enabled unexpected discoveries and systematic studies, stimulating new investigations and collaborations. Indeed, GOLIA allowed us to discover the (currently only) outburst periodicity from a Supergiant Fast X-ray Transient: IGRJ11215-5952. Starting from this discovery, and thanks to the population study approach offered by such an archive, we undertook a journey to understand Supergiant Fast X-ray Transients within the wider scenario of High Mass X-ray Binaries, tackling several long-term aspects: cumulative luminosity distributions, energy and temporal properties of bright flares, large-scale wind structures. Given the very productive outcome of such an approach, we decided to build a second generation archive in IASF Milano, named ANITA (A New InTegral Archive), with improved hardware and software performances

Wind accretion: Theory and observations

A review of wind accretion in high-mass X-ray binaries is presented. We focus on different regimes of quasi-spherical accretion onto the neutron star (NS): the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the NS magnetosphere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. These two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg s-1. In the subsonic case, which sets in at lower luminosities, a hot quasi-spherical shell must form around the magnetosphere, and the actual accretion rate onto NS is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. In turn, two regimes of subsonic accretion are possible, depending on plasma cooling mechanism (Compton or radiative) near the magnetopshere. The transition from the high-luminosity with Compton cooling to the lowluminosity (Lx ≲ 3 × 1035 erg s-1) with radiative cooling can be responsible for the onset of the off states repeatedly observed in several low-luminosity slowly accreting pulsars, such as Vela X-1, GX 301-2, and 4U 1907+09. The triggering of the transitionmay be due to a switch in the X-ray beam pattern in response to a change in the optical depth in the accretion column with changing luminosity. We also show that in the settling accretion theory, bright X-ray flares (~1038-1040 erg) observed in supergiant fast X-ray transients (SFXT) can be produced by sporadic capture of magnetized stellar wind plasma. At sufficiently low accretion rates, magnetic reconnection can enhance the magnetospheric plasma entry rate, resulting in copious production of X-ray photons, strong Compton cooling and ultimately in unstable accretion of the entire shell. A bright flare develops on the free-fall time scale in the shell, and the typical energy released in an SFXT bright flare corresponds to the mass of the shell

“Destinazione Futuro”, un esperimento INAF di riflessione condivisa riguardo il tempo che verrà. Report edizione anno 2021

Il 21 maggio del 2021, in occasione dell’avvicinarsi di due importanti ricorrenze a cavallo tra scienza e fantascienza (in agosto dell’anno ricorre il centenario dalla nascita di Gene Roddenberry, creatore di Star Trek e il ventennale dalla morte di Fred Hoyle, celebre scienziato e scrittore di fantascienza) si è lanciato un progetto per ascoltare il grande pubblico su una serie di questioni riguardanti la visione personale del futuro, mantenendo un registro intermedio tra scienza e fantascienza. Le risposte pervenute (frutto di più di duecento compilazioni) sono state valutate dagli organizzatori e in parte condivise con alcuni esperti in varie discipline, che sono stati intervistati per una serie di articoli pubblicati - uno alla settimana - durante l’estate 2021 sulla testata EduINAF. L’iniziativa si inserisce in un dialogo tra INAF e il pubblico più vasto, che in un periodo assai critico - a motivo delle restrizioni causate dalla pandemia COVID19 - ha potuto esprimere le proprie speranze e anche i disagi, ritrovando un Ente in ascolto e non chiuso nel proprio sapere, che anzi ha fatto degli input del pubblico uno strumento di ulteriore lavoro con selezionati esperti. Considerata la risposta molto positiva e il buon numero di compilazioni del questionario, si concorda di lasciare aperta la possibilità di produrre ulteriori interviste, contattando successivamente altri esperti, in modo che i dati raccolti possano venire usati in modi ancora diversi, accogliendo gli spunti che essi offrono per un lavoro capace di condurre in innumerevoli direzioni, molte delle quali ancora inesplorate

Gruppo INDACO: Report della divulgazione per i progetti ASTRI e CTA Anni 2021-2022

Il gruppo INDACO (INAF per la Divulgazione di ASTRI e CTA Observatory) si è costituito alla fine del 2020 per occuparsi della divulgazione dei due progetti, ASTRI e CTA Observatory, per conto di INAF. Dopo i primi due anni di lavoro, facciamo il punto di quanto affrontato e raggiunto finora e delle prospettive future