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Register a new userFast flaring observed from XMMU J053108.3-690923 by eROSITA: a supergiant fast X-ray transient in the Large Magellanic Cloud
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Supergiant fast X-ray transients (SFXTs) are a peculiar class of supergiant high-mass X-ray binary (HMXB) systems characterised by extreme variability in the X-ray domain. In current models, this is mainly attributed to the clumpy nature of the stellar wind coupled with gating mechanisms involving the spin and magnetic field of the neutron star. We studied the X-ray properties of the supergiant HMXB XMMU J053108.3-690923 in the Large Magellanic Cloud to understand its nature. We performed a detailed temporal and spectral analysis of the eROSITA and XMM-Newton data of XMMU J053108.3-690923. We confirm the putative pulsations previously reported for the source with high confidence, certifying its nature as a neutron star in orbit with a supergiant companion. We identify the extremely variable nature of the source in the form of flares seen in the eROSITA light curves. The source flux exhibits a total dynamic range of more than three orders of magnitude, which confirms its nature as an SFXT, and is the first such direct evidence from a HMXB outside our Galaxy exhibiting a very high dynamic range in luminosity as well as a fast flaring behaviour. We detect changes in the hardness ratio during the flaring intervals where the hardness ratio reaches its minimum during the peak of the flare and increases steeply shortly afterwards. This is also supported by the results of the spectral analysis carried out at the peak and off-flare intervals. This scenario is consistent with the presence of dense structures in the supergiant wind of XMMU J053108.3-690923 where the clumpy medium becomes photoionised at the peak of the flare leading to a drop in the photo-electric absorption. Further, we provide an estimate of the clumpiness of the medium and the magnetic field of the neutron star assuming a spin equilibrium condition.
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IGR J08408-4503 is a supergiant fast X-ray transient discovered in 2006 with a confirmed association with a O8.5Ib(f) supergiant star, HD 74194. We report on the analysis of two outbursts caught by Swift/BAT on 2006 October 4 and 2008 July 5, and followed up at softer energies with Swift/XRT. The 2008 XRT light curve shows a multiple-peaked structure with an initial bright flare that reached a flux of ~1E-9 erg/cm2/s (2-10 keV), followed by two equally bright flares within 75 ks. The spectral characteristics of the flares differ dramatically, with most of the difference, as derived via time-resolved spectroscopy, being due to absorbing column variations. We observe a gradual decrease of the NH, derived with a fit using absorbed power law model, as time passes. We interpret these NH variations as due to an ionization effect produced by the first flare, resulting in a significant decrease in the measured column density towards the source. The durations of the flares, as well as the times of the outbursts suggest that the orbital period is ~35 days, if the flaring activity is interpreted within the framework of the Sidoli et al 2007 model with the outbursts triggered by the neutron star passage inside an equatorial wind inclined with respect to the orbital plane.
We present the most recent results from our investigation on Supergiant Fast X-ray Transients, a class of High-Mass X-ray Binaries, with a possible counterpart in the gamma-ray energy band. Since 2007 Swift has contributed to this new field by detecting outbursts from these fast transients with the BAT and by following them for days with the XRT. Thus, we demonstrated that while the brightest phase of the outburst only lasts a few hours, further activity is observed at lower fluxes for a remarkably longer time, up to weeks. Furthermore, we have performed several campaigns of intense monitoring with the XRT, assessing the fraction of the time these sources spend in each phase, and their duty cycle of inactivity.
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 J18483-0311 is a supergiant fast X-ray transient whose compact object is located in a wide (18.5 d) and eccentric (e~0.4) orbit, which shows sporadic outbursts that reach X-ray luminosities of ~1e36 erg/s. We investigated the timing properties of IGR J18483-0311 and studied the spectra during bright outbursts by fitting physical models based on thermal and bulk Comptonization processes for accreting compact objects. We analysed archival INTEGRAL data collected in the period 2003-2010, focusing on the observations with IGR J18483-0311 in outburst. We searched for pulsations in the INTEGRAL light curves of each outburst. We took advantage of the broadband observing capability of INTEGRAL for the spectral analysis. We observed 15 outbursts, seven of which we report here for the first time. This data analysis almost doubles the statistics of flares of this binary system detected by INTEGRAL. A refined timing analysis did not reveal a significant periodicity in the INTEGRAL observation where a ~21s pulsation was previously detected. Neither did we find evidence for pulsations in the X-ray light curve of an archival XMM-Newton observation of IGR J18483-0311. In the light of these results the nature of the compact object in IGR J18483-0311 is unclear. The broadband X-ray spectrum of IGR J18483-0311 in outburst is well fitted by a thermal and bulk Comptonization model of blackbody seed photons by the infalling material in the accretion column of a neutron star. We also obtained a new measurement of the orbital period using the Swift/BAT light curve.
Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries associated with OB supergiant companions and characterised by an X-ray flaring behaviour whose dynamical range reaches 5 orders of magnitude on timescales of a few hundred to thousands of seconds. Current investigations concentrate on finding possible mechanisms to inhibit accretion in SFXTs and explain their unusually low average X-ray luminosity. We present the Swift observations of an exceptionally bright outburst displayed by the SFXT IGR J17544-2619 on 2014 October 10 when the source achieved a peak luminosity of $3times10^{38}$ erg s$^{-1}$. This extends the total source dynamic range to $gtrsim$10$^6$, the largest (by a factor of 10) recorded so far from an SFXT. Tentative evidence for pulsations at a period of 11.6 s is also reported. We show that these observations challenge, for the first time, the maximum theoretical luminosity achievable by an SFXT and propose that this giant outburst was due to the formation of a transient accretion disc around the compact object.
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