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We present the results of combined INTEGRAL and XMM-Newton observations of the supergiant fast X-ray transient (SFXT) IGR J17354$-$3255. Three XMM-Newton observations of lengths 33.4 ks, 32.5 ks and 21.9 ks were undertaken, the first an initial pointing to identify the correct source in the field of view and the latter two performed around periastron. Simultaneous INTEGRAL observations across $\sim66\%$ of the orbital cycle were analysed but the source was neither detected by IBIS/ISGRI nor by JEM-X. The XMM-Newton light curves display a range of moderately bright X-ray activity but there are no particularly strong flares or outbursts in any of the three observations. We show that the spectral shape measured by XMM-Newton can be fitted by a consistent model throughout the observation, suggesting that the observed flux variations are driven by obscuration from a wind of varying density rather than changes in accretion mode. The simultaneous INTEGRAL data rule out simple extrapolation of the simple powerlaw model beyond the XMM-Newton energy range., Comment: 13 pages, 9 figures, This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society Published by Oxford University Press

We present the results of a 30ks XMM-Newton observation of the supergiant fast X-ray transient (SFXT) SAX J1818.6-1703 - the first in-depth soft X-ray study of this source around periastron. INTEGRAL observations shortly before and after the XMM-Newton observation show the source to be in an atypically active state. Over the course of the XMM-Newton observation, the source shows a dynamic range of ~100 with a luminosity greater than 1$\times$10$^{35}$ erg s$^{-1}$ for the majority of the observation. After an ~6 ks period of low luminosity (~10$^{34}$ erg s$^{-1}$) emission, SAX J1818.6-1703 enters a phase of fast flaring activity, with flares 250 s long, separated by ~2ks. The source then enters a larger flare event of higher luminosity and ~8 ks duration. Spectral analysis revealed evidence for a significant change in spectral shape during the observation with a photon index varying from {\Gamma} ~ 2.5 during the initial low luminosity emission phase, to {\Gamma} ~ 1.9 through the fast flaring activity, and a significant change to {\Gamma} ~ 0.3 during the main flare. The intrinsic absorbing column density throughout the observation (n$_H$ ~ 5$\times$10$^{23}$ cm$^{-2}$) is among the highest measured from an SFXT, and together with the XMM-Newton and INTEGRAL luminosities, consistent with the neutron star encountering an unusually dense wind environment around periastron. Although other mechanisms cannot be ruled out, we note that the onset of the brighter flares occurs at 3$\times$10$^{35}$ erg s$^{-1}$, a luminosity consistent with the threshold for the switch from a radiative-dominated to Compton cooling regime in the quasi-spherical settling accretion model., Comment: 11 pages, 8 figures, Accepted for publication in MNRAS

We characterized the broad-band X-ray spectra of Swift J1745-26 during the decay of the 2013 outburst using INTEGRAL ISGRI, JEM-X and Swift XRT. The X-ray evolution is compared to the evolution in optical and radio. We fit the X- ray spectra with phenomenological and Comptonization models. We discuss possible scenarios for the physical origin of a ~50 day flare observed both in optical and X- rays ~170 days after the peak of the outburst. We conclude that it is a result of enhanced mass accretion in response to an earlier heating event. We characterized the evolution in the hard X-ray band and showed that for the joint ISGRI-XRT fits, the e-folding energy decreased from 350 keV to 130 keV, while the energy where the exponential cut-off starts increased from 75 keV to 112 keV as the decay progressed.We investigated the claim that high energy cut-offs disappear with the compact jet turning on during outburst decays, and showed that spectra taken with HEXTE on RXTE provide insufficient quality to characterize cut-offs during the decay for typical hard X-ray fluxes. Long INTEGRAL monitoring observations are required to understand the relation between the compact jet formation and hard X-ray behavior. We found that for the entire decay (including the flare), the X-ray spectra are consistent with thermal Comptonization, but a jet synchrotron origin cannot be ruled out., Comment: Accepted for publication by MNRAS

XMM-Newton observations of the supergiant fast X-ray transient IGR$~$J17544$-$2619 are reported and placed in the context of an analysis of archival INTEGRAL/IBIS data that provides a refined estimate of the orbital period at 4.9272$\pm$0.0004 days. A complete outburst history across the INTEGRAL mission is reported. Although the new XMM-Newton observations (each lasting $\sim$15 ks) targeted the peak flux in the phase-folded hard X-ray light curve of IGR$~$J17544$-$2619, no bright outbursts were observed, the source spending the majority of the exposure at intermediate luminosities of the order of several 10$^{33}\,$erg$\,$s$^{-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 $\sim$4$\times$10$^{32}\,$erg$\,$s$^{-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$-$2619 and the implications for the nature of the mass transfer and accretion mechanisms for both IGR$~$J17544$-$2619 and the SFXT population. We conclude that accretion under the `quasi-spherical accretion' model provides a good description of the behaviour of IGR$~$J17544$-$2619, and suggest an additional mechanism for generating outbursts based upon the mass accumulation rate in the hot shell (atmosphere) that forms around the NS 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., Comment: 12 pages, 5 figures, accepted for publication in MNRAS

Temporal analysis of INTEGRAL/IBIS data has revealed a 5.7195\pm0.0007 day periodicity in the supergiant fast X-ray transient (SFXT) source AX J1845.0-0433, which we interpret as the orbital period of the system. The new-found knowledge of the orbital period is utilised to investigate the geometry of the system by means of estimating an upper limit for the size of the supergiant (<27 R_{\sun}) as well as the eccentricity of the orbit (\epsilon<0.37)., Comment: 6 pages, 6 figures, accepted for publication in MNRAS

We report on a broad-band X-ray study (0.5-60 keV) of the poorly known candidate Supergiant Fast X-ray Transient (SFXT) IGR J18462-0223, and on optical and near-infrared (NIR) followup observations of field objects. The out-of-outburst X-ray state has been investigated for the first time with archival INTEGRAL/IBIS, ASCA, Chandra and Swift/XRT observations. This allowed us to place stringent 3 sigma upper limits on the soft (0.5-10 keV) and hard (18-60 keV) X-ray emission of 2.9x10^-13 erg cm^-2 s^-1 and 8x10^-12 erg cm^-2 s^-1, respectively; the source was also detected during an intermediate soft X-ray state with flux equal to 1.6x10^-11 erg cm^-2 s^-1 (0.5-10 keV). In addition, we report on the INTEGRAL/IBIS discovery of three fast hard X-ray flares (18-60 keV) having a duration in the range 1-12 hours: the flaring behavior was also investigated in soft X-rays (3-10 keV) with archival INTEGRAL/JEM-X observations. The duty cycle (1.2%) and the dynamic ranges (> 1,380 and > 190 in the energy bands 0.5-10 keV and 18-60 keV, respectively) were measured for the first time. Archival UKIDSS JHK NIR data, together with our deep R-band imaging of the field, unveiled a single, very red object inside the intersection of the Swift/XRT and XMM-Newton error circles: this source has optical/NIR photometric properties compatible with a very heavily absorbed blue supergiant located at about 11 kpc, thus being a strong candidate counterpart for IGR J18462-0223. NIR spectroscopy is advised to confirm the association. Finally, a hint of a possible orbital period was found at about 2.13 days. If confirmed by further studies, this would make IGR J18462-0223 the SFXT with the shortest orbital period among the currently known systems., Comment: accepted for publication in A&A, 9 pages, 7 figures, 2 tables

We report on combined INTEGRAL and XMM-Newton observations of the supergiant fast X-ray transient IGR J16418-4532. The observations targeted the X-ray eclipse region of IGR J16418-4532s orbit with continuous INTEGRAL observations across ~25% of orbital phase and two quasi-simultaneous XMM-Newton observations of length 20ks and 14ks, occurring during and just after the eclipse respectively. An enhanced INTEGRAL emission history is provided with 19 previously unreported outbursts identified in the archival 18-60 keV data set. The XMM-Newton eclipse observation showed prominent Fe-emission and a flux of 2.8*10^-13 erg cm^-2 s^-1 (0.5 - 10 keV). Through the comparison of the detected eclipse and post eclipse flux, the supergiant mass loss rate through the stellar wind was determined as \dot{M}_{w} = 2.3-3.8*10^-7 M_{\odot} yr^-1. The post eclipse XMM-Newton observation showed a dynamic flux evolution with signatures of the X-ray pulsation, a period of flaring activity, structured nH variations and the first ever detection of an X-ray intensity dip, or 'off-state', in a pulsating supergiant fast X-ray transient. Consideration is given to the origin of the X-ray dip and we conclude that the most applicable of the current theories of X-ray dip generation is that of a transition between Compton cooling dominated and radiative cooling dominated subsonic accretion regimes within the 'quasi-spherical' model of wind accretion. Under this interpretation, which requires additional confirmation, the neutron star in IGR J16418-4532 possesses a magnetic field of ~10^14 G, providing tentative observational evidence of a highly magnetised neutron star in a supergiant fast X-ray transient for the first time. The implications of these results on the nature of IGR J16418-4532 itself and the wider SFXT class are discussed., Comment: 17 pages, 10 figures, accepted for publication in MNRAS

In this paper we present the identification of two periodic X-ray signals coming from the direction of the Small Magellanic Cloud (SMC). On detection with the Rossi X-ray Timing Explorer (RXTE), the 175.4s and 85.4s pulsations were considered to originate from new Be/X-ray binary (BeXRB) pulsars with unknown locations. Using rapid follow-up INTEGRAL and XMM-Newton observations, we show the first pulsar (designated SXP175) to be coincident with a candidate high-mass X-ray binary (HMXB) in the northern bar region of the SMC undergoing a small Type II outburst. The orbital period (87d) and spectral class (B0-B0.5IIIe) of this system are determined and presented here for the first time. The second pulsar is shown not to be new at all, but is consistent with being SXP91.1 - a pulsar discovered at the very beginning of the 13 year long RXTE key monitoring programme of the SMC. Whilst it is theoretically possible for accreting neutron stars to change spin period so dramatically over such a short time, the X-ray and optical data available for this source suggest this spin-up is continuous during long phases of X-ray quiescence, where accretion driven spin-up of the neutron star should be minimal., Comment: 13 pages, 16 figures, accepted for publication in MNRAS

We report on temporal and spectral analysis of the INTEGRAL fast transient candidate IGR J16328-4726 observed with Beppo SAX in 1998 and more recently with INTEGRAL. The MECS X-ray data show a frequent micro activity typical of the intermediate state of Supergiant Fast X-ray Transients and a weak flare with duration of ~4.6 ks. The X-ray emission in the 1.5-10 keV energy range is well described through the different time intervals by an absorbed power law model. Comparing spectra from the lower emission level up to the peak of the flare, we note that while the power-law photon index was constant (~2), the absorption column density varied by a factor of up to ~6-7, reaching the value of ~2x10^{23}cm^{-2} at the peak of the flare. Analysis of the long-term INTEGRAL/IBIS light curve confirms and refines the proposed ~10.07 day period, and the derived ephemeris places the Beppo SAX observations away from periastron. Using the near and the mid-IR available observations, we constructed a spectral infrared distribution for the counterpart of IGR J16328-4726, allowing us to identify its counterpart as a High Mass OB type star, and to classify this source as a firm HMXB. Following the standard clumpy wind theory, we estimated the mass and the radius of the clump responsible of the flare. The obtained values of M~4x10^{22}g and R~4.4x10^6 km are in agreement with expected values from theoretical predictions., Comment: arXiv admin note: text overlap with arXiv:1106.5125 by other authors

Phase-targeted RXTE observations have allowed us to detect a transient 71.49 \pm 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., Comment: 7 pages, 6 figures, Accepted for publication in Astronomy and Astrophysics main journal

In the last few years Fermi and AGILE observations have indicated the existence of a possible population of transient MeV-GeV sources located on the Galactic plane and characterized by fast flares lasting only a very few days. Notably, no blazar-like counterparts are known within their error boxes so they could represent a completely new class of Galactic transient high energy emitters. The task of identifying their counterparts at lower energies remains very challenging. Despite this difficulty, INTEGRAL observations have provided intriguing hints that reliable candidate counterparts for these unidentified MeV-GeV transients could be found among the members of the recently discovered class of Supergiant Fast X-ray Transients (SFXTs). In this context, to date the best test case is represented by the association between the two sources IGR J17354-3255 and AGL J1734-3310. We will discuss their possible physical link and implications stemming from this association., Comment: Contributed talk at the INTEGRAL conference "The Extreme and Variable High Energy Sky", September 19-23, 2011, Chia Laguna, Sardegna (Italy). Accepted for publication on PoS (PoS(Extremesky 2011)011). 6 pages, 1 figure

We present spectral and temporal results from INTEGRAL long-term monitoring of the unidentified X-ray source IGR J17354-3255. We show that it is a weak persistent hard X-ray source spending a major fraction of the time in an out-of-outburst state with average 18-60 keV X-ray flux of about 1.1 mCrab, occasionally interspersed with fast X-ray flares (duration from a few hours to a few days) with a dynamic range as high as 200. From archival Swift/XRT observations, we also show that the dynamic range from non-detection to highest level of measured X-ray activity is >300. Our IBIS timing analysis strongly confirms the 8.4 days orbital period previously detected with Swift/BAT, in addition we show that the shape of the orbital profile is rather smooth and appears to be dominated by low level X-ray emission rather than by bright outbursts, the measured degree of outburst recurrence is about 25 per cent. The spectral and temporal characteristics of IGR J17354-3255 are highly indicative of a Supergiant High Mass X-ray Binary nature (SGXB). However, our inferred dynamic ranges both at soft and hard X-rays are significantly greater than those of classical SGXB systems, but instead are typical of intermediate Supergiant Fast X-ray Transient (SFXTs). Finally, we note for the first time that the observed fast flaring X-ray behaviour of IGR J17354-3255 is very similar to that detected with AGILE from the spatially associated MeV source AGL J1734-3310, suggesting a possible physical link between the two objects., Comment: accepted for publication in MNRAS, 8 pages, 9 figures, 1 table

SFXTs are a new class of HMXB unveiled by INTEGRAL. They are extreme systems characterised by very short outbursts (a few hours) and extreme X-ray luminosity dynamic ranges (~10^{4}). Ten confirmed systems are currently known and have shown parallels with both Sg-XRBs and Be-XRBs. Temporal studies across all timescales are key to understanding both the place of SFXTs within the HMXB hierarchy and the accretion mechanisms at work within the objects. Here we present the discovery of a new 51.47 \pm 0.02 day orbital period in the SFXT XTE J1739$-$302 using INTEGRAL observations. We also present a higher time resolution study of the SFXTs SAX J1818.6-1703, IGR J16479-4514 and IGR J16465-4507 using RXTE that shows newly discovered flaring activity., Comment: Proceedings of the 8th INTEGRAL workshop "The Restless Gamma-ray Universe", September 27-30, 2010, Dublin, Ireland, 7 pages, 3 figures

We present X-ray and optical data on the Be/X-ray binary (BeXRB) pulsar IGR J01054-7253 = SXP11.5 in the Small Magellanic Cloud (SMC). Rossi X-ray Timing Explorer (RXTE) observations of this source in a large X-ray outburst reveal an 11.483 +/- 0.002s pulse period and show both the accretion driven spin-up of the neutron star and the motion of the neutron star around the companion through Doppler shifting of the spin period. Model fits to these data suggest an orbital period of 36.3 +/- 0.4d and Pdot of (4.7 +/- 0.3) x 10^{-10} ss^{-1}. We present an orbital solution for this system, making it one of the best described BeXRB systems in the SMC. The observed pulse period, spin-up and X-ray luminosity of SXP11.5 in this outburst are found to agree with the predictions of neutron star accretion theory. Timing analysis of the long-term optical light curve reveals a periodicity of 36.70 +/- 0.03d, in agreement with the orbital period found from the model fit to the X-ray data. Using blue-end spectroscopic observations we determine the spectral type of the counterpart to be O9.5-B0 IV-V. This luminosity class is supported by the observed V-band magnitude. Using optical and near-infrared photometry and spectroscopy, we study the circumstellar environment of the counterpart in the months after the X-ray outburst., Comment: 12 pages, 13 figures and 3 tables. This paper has been accepted for publication in MNRAS

Timing analysis of ~12.4 Ms of INTEGRAL/IBIS data has revealed a period of 51.47 +/- 0.02 days 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 lightcurve. It is found that many orbital configurations are possible within this system with eccentricities of up to e ~0.8 valid., Comment: 8 pages, 6 figures, 2 tables, MNRAS in press

Timing analysis of the INTEGRAL-IBIS and Swift-BAT light curves of the Supergiant Fast X-ray Transient (SFXT) IGR J16465-4507 has identified a period of 30.32+/-0.02 days which we interpret as the orbital period of the binary system. In addition 11 outbursts (9 of which are previously unpublished) have been found between MJD 52652 to MJD 54764, all of which occur close to the region of the orbit we regard as periastron. From the reported flux outbursts, we found a dynamical range in the interval ~30-80. Although in this regard IGR J16465-4507 cannot be considered a classical SFXT for which typical dinamical ranges are >100, still our reported values are significantly greater than that of classical persistent variable supergiant HMXBs (<20), supporting the idea that IGRJ16465-4507 is an intermediate SFXT system, much like few other similar cases reported in the literature., Comment: 5 Pages, 7 Figures, 1 Table