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We report on the Swift monitoring of the candidate supergiant fast X-ray transient (SFXT) IGR J16418-4532, for which both orbital and spin periods are known (~3.7d and ~1250s, respectively). Our observations, for a total of ~43ks, span over three orbital periods and represent the most intense and complete sampling of the light curve of this source with a sensitive X-ray instrument. With this unique set of observations we can address the nature of this transient. By applying the clumpy wind model for blue supergiants to the observed X-ray light curve, and assuming a circular orbit, 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 ~5E16 g to 1E21g. Our data suggest, based on the X-ray behaviour, that this is an intermediate SFXT.
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We report on the Swift observations of the candidate supergiant fast X-ray transient (SFXT) IGR J16418-4532, which has an orbital period of ~3.7 d. Our monitoring, for a total of ~43 ks, spans over three orbits and represents the most intense and complete sampling along the orbital period of the light curve of this source. If one assumes a circular orbit, the X-ray emission from this source can be explained by accretion from a spherically symmetric clumpy wind from a blue supergiant, composed of clumps with different masses, ranging from ~5x10^16 g to 10^21g.
We report on the Swift/X-ray Telescope (XRT) monitoring of the field of view around the candidate supergiant fast X-ray transient (SFXT) IGR J17354-3255, which is positionally associated with the AGILE/GRID gamma-ray transient AGL J1734-3310. Our observations, which cover 11 days for a total on-source exposure of about 24 ks, span 1.2 orbital periods (P_orb=8.4474 d) and are the first sensitive monitoring of this source in the soft X-rays. These new data allow us to exploit the timing variability properties of the sources in the field to unambiguously identify the soft X-ray counterpart of IGR J17354-3255. The soft X-ray light curve shows a moderate orbital modulation and a dip. We investigated the nature of the dip by comparing the X-ray light curve with the prediction of the Bondi-Hoyle-Lyttleton accretion theory, assuming both spherical and nonspherical symmetry of the outflow from the donor star. We found that the dip cannot be explained with the X-ray orbital modulation. We propose that an eclipse or the onset of a gated mechanism is the most likely explanation for the observed light curve.
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 (18.52d) and spin period (21s) are known. We report on the first complete monitoring of the X-ray activity along an entire orbital period of a Supergiant Fast X-ray Transient. These Swift observations, lasting 28d, 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 5x 10^{21}g.
IGR J17503-2636 is a hard X-ray transient discovered by INTEGRAL on 2018 August 11. This was the first ever reported X-ray emission from this source. Following the discovery, follow-up observations were carried out with Swift, Chandra, NICER, and NuSTAR. We report in this paper the analysis and results obtained from all these X-ray data. Based on the fast variability in the X-ray domain, the spectral energy distribution in the 0.5-80 keV energy range, and the reported association with a highly reddened OB supergiant at ~10 kpc, we conclude that IGR J17503-2636 is most likely a relatively faint new member of the supergiant fast X-ray transients. Spectral analysis of the NuSTAR data revealed a broad feature in addition to the typical power-law with exponential roll-over at high energy. This can be modeled either in emission or as a cyclotron scattering feature in absorption. If confirmed by future observations, this feature would indicate that IGR J17503-2636 hosts a strongly magnetized neutron star with B~2e12 G.
IGR J11215-5952 is a hard X-ray transient source discovered in April 2005 with INTEGRAL and a confirmed member of the new class of High Mass X-ray Binaries, the Supergiant Fast X-ray Transients (SFXTs). Archival INTEGRAL data and RXTE observations showed that the outbursts occur with a periodicity of ~330 days. Thus, IGR J11215-5952 is the first SFXT displaying periodic outbursts, possibly related to the orbital period. We performed a Target of Opportunity observation with Swift with the main aim of monitoring the source behaviour around the time of the fifth outburst, expected on 2007 Feb 9. The source field was observed with Swift twice a day (2ks/day) starting from 4th February, 2007, until the fifth outburst, and then for ~5 ks a day afterwards, during a monitoring campaign that lasted 23 days for a total on-source exposure of ~73 ks. This is the most complete monitoring campaign of an outburst from a SFXT. The spectrum during the brightest flares is well described by an absorbed power law with a photon index of 1 and N_H~1 10^22 cm^-2. A 1-10 keV peak luminosity of ~10^36 erg s^-1 was derived (assuming 6.2 kpc, the distance of the optical counterpart). These Swift observations are a unique data-set for an outburst of a SFXT, thanks to the combination of sensitivity and time coverage, and they allowed a study of IGR J11215-5952 from outburst onset to almost quiescence. We find that the accretion phase lasts longer than previously thought on the basis of lower sensitivity instruments observing only the brightest flares. The observed phenomenology is consistent with a smoothly increasing flux triggered at the periastron passage in a wide eccentric orbit with many flares superimposed, possibly due to episodic or inhomogeneous accretion.
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