V. Sguera

THE FOURTH IBIS/ISGRI SOFT GAMMA-RAY SURVEY CATALOG

In this paper, we report on the fourth soft gamma-ray source catalog obtained with the IBIS gamma-ray imager on board the INTEGRAL satellite. The scientific data set is based on more than 70 Ms of high-quality observations performed during the first five and a half years of the Core Program and public observations. Compared to previous IBIS surveys, this catalog includes a substantially increased coverage of extragalactic fields, and comprises more than 700 high-energy sources detected in the energy range 17-100 keV, including both transients and faint persistent objects that can only be revealed with longer exposure times. A comparison is provided with the latest Swift/BAT survey results.

Swift/XRT monitoring of the supergiant fast X-ray transient IGR J18483−0311 for an entire orbital period

IGR J18483 0311 is an X‐ray pulsar with transient X‐ray activity, belonging to the new class of High Mass X‐ray Binaries called Supergiant Fast X‐ray Transients. This system is one of the two members of this class, together with IGR J11215‐5952, where both the orbital (18.52 d) and spin period (21 s) are known. We report on the firs t complete monitoring of the X‐ray activity along an entire orbital period of a Supergiant Fast X‐ray Transient. These Swift observations, lasting 28 days, cover more than one entire orbital phase consecutively. They are a unique data-set, which allows us to constrain the different mechanisms proposed to explain the nature of this new class of X‐ray transients. We applied the new clumpy wind model for blue supergiants developed by Ducci et al. (2009), to the observed X‐ray light curve. Assuming an eccentricity of e = 0.4, the X‐ray emission from this source can be explained in terms of the accretion from a spherically symmetric clumpy wind, composed of clumps with different masses, ranging from 10 18 g to 5×10 21 g.

Dissecting the region of 3EG J1837-0423 and HESS J1841-055 with INTEGRAL

3EG J1837-0423 and HESS J1841-055 are two unidentified and peculiar high-energy sources located in the same region of the sky, separated by ~1.4°. Specifically, 3EG J1837-0423 is a transient MeV object detected by EGRET only once during flaring activity that lasted a few days while HESS J1841-055 is a highly extended TeV source. We attempted to match the high-energy emission from the unidentified sources 3EG J1837-0423 and HESS J1841-055 with X-rays (4-20 keV) and soft γ-rays (20-100 keV) candidate counterparts detected through deep International Gamma-Ray Astrophysics Laboratory observations of the sky region. As a result we propose the Supergiant Fast X-ray Transient (SFXT) AX J1841.0-0536 as a possible candidate counterpart of 3EG J1837-0423, based on spatial proximity and transient behavior. Alternatively, AX J1841.0-0536 could be responsible for at least a fraction of the entire TeV emission from the extended source HESS J1841-055, based on a striking spatial correlation. In either case, the proposed association is also supported from an energetic standpoint by a theoretical scenario where AX J1841.0-0536 is a low magnetized pulsar which, due to accretion of massive clumps from the supergiant companion donor star, undergoes sporadic changes to transient Atoll-states where a magnetic tower can produce transient jets and as a consequence high-energy emission. In either case (by association with 3EG J1837-0423 or alternatively with HESS J1841-055), AX J1841.0-0536 might be the prototype of a new class of Galactic transient MeV/TeV emitters.

Discovery of a 30-d period in the supergiant fast X-ray transient SAX J1818.6?1703

SAX J1818.6−1703 has been characterized as a supergiant fast X-ray transient system on the basis of several INTEGRAL/IBIS detections since the original BeppoSAX Wide Field Camera detection. Using IBIS/ISGRI, Swift/BAT and archival observations, we show that, in fact, SAX J1818.6−1703 exhibits emission on a period of 30 ± 0.1 d, with a high degree of recurrence. SAX J1818.6−1703 is therefore the second supergiant fast X-ray transient shown to exhibit periodic outbursts, but with a considerably shorter period than the other known system, IGR J11215−5952.

The IBIS soft gamma-ray sky after 1000 INTEGRAL orbits

We report here an all-sky soft gamma-ray source catalog based on IBIS observations performed during the first 1000 orbits of INTEGRAL. The database for the construction of the source list consists of all good quality data available from launch in 2002 up to the end of 2010. This corresponds to

New insights on accretion in supergiant fast X-ray transients from XMM–Newton and INTEGRAL observations of IGR J17544−2619

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Discovery of the 51.47-d orbital period in the supergiant fast X-ray transient XTE J1739−302 with INTEGRAL

110 Ms of scientific public observations with a concentrated coverage on the Galactic Plane and extragalactic deep exposures. This new catalog includes 939 sources above a 4.5 sigma significance threshold detected in the 17-100 keV energy band, of which 120 represent previously undiscovered soft gamma-ray emitters. The source positions are determined, mean fluxes are provided in two main energy bands, and are reported together with the overall source exposure. Indicative levels of variability are provided, and outburst times and durations are given for transient sources. Comparison is made with previous IBIS catalogs, and those from other similar missions.

X-ray pulsations from the region of the supergiant fast X-ray transient IGR J17544−2619

XMM–Newton observations of the supergiant fast X-ray transient IGR J17544?2619 are reported and placed in the context of an analysis of archival INTEGRAL/IBIS data that provide a refined estimate of the orbital period at 4.9272?±?0.0004?d. A complete outburst history across the INTEGRAL mission is reported. Although the new XMM–Newton observations (each lasting ?15 ks) targeted the peak flux in the phase-folded hard X-ray light curve of IGR J17544?2619, no bright outbursts were observed, the source spending the majority of the exposure at intermediate luminosities of the order of several 1033?erg?s?1 (0.5–10 keV) and displaying only low level flickering activity. For the final portion of the exposure, the luminosity of IGR J17544?2619 dropped to ?4?×?1032?erg?s?1 (0.5–10 keV), comparable with the lowest luminosities ever detected from this source, despite the observations being taken near to periastron. We consider the possible orbital geometry of IGR J17544?2619 and the implications for the nature of the mass transfer and accretion mechanisms for both IGR J17544?2619 and the supergiant fast X-ray transients (SFXTs) population. We conclude that accretion under the ‘quasi-spherical accretion’ model provides a good description of the behaviour of IGR J17544?2619 and suggests an additional mechanism for generating outbursts based upon the mass accumulation rate in the hot shell (atmosphere) that forms around the neutron star under the quasi-spherical formulation. Hence, we hope to aid in explaining the varied outburst behaviours observed across the SFXT population with a consistent underlying physical model.

Using the ROSAT catalogue to find counterparts for unidentified objects in the first Fermi/LAT catalogue

Timing analysis of ~12.4 Ms of INTEGRAL/IBIS data has revealed a period of 51.47 ± 0.02 d in the supergiant fast X-ray transient source XTE J1739-302/IGR J17391-3021 that can be interpreted as an orbital period. An outburst history showing 35 epochs of activity has been produced, showing X-ray outbursts throughout the orbit of XTE J1739-302. Possible indications of an enhanced equatorial density region within the supergiant stellar wind are present in the phase-folded light curve. It is found that many orbital configurations are possible within this system with eccentricities of up to e ~ 0.8.

INTEGRAL and XMM–Newton observations of IGR J16418−4532: evidence of accretion regime transitions in a supergiant fast X-ray transient

Phase-targeted RXTE observations have allowed us to detect a transient 71.49 0.02 s signal that is most likely to be originating from the supergiant fast X-ray transient IGR J17544 2619. The phase-folded light curve shows a possible double-peaked structure with a pulsed flux of 4.8 10 12 erg cm 2 s 1 (3 10 keV). Assuming the signal to indicate the spin period of the neutron star in the system, the provisional location of IGR J17544 2619 on the Corbet diagram places the system within the classical wind-fed supergiant XRB region. Such a result illustrates the growing trend of supergiant fast X-ray transients to span across both of the original classes of HMXB in Porb Pspin space.