The accretion environment of supergiant fast X-ray transients probed withXMM-Newton

Abstract

Context. Supergiant fast X-ray transients are a peculiar class of supergiant X-ray binaries characterized by a remarkable variability in the X-ray domain, widely ascribed to the accretion from a clumpy stellar wind. Aims. In this paper we performed a systematic and homogeneous analysis of the sufficiently bright X-ray flares observed with XMMNewton from the supergiant fast X-ray transients to probe spectral variations on timescales as short as a few hundred of seconds. Our ultimate goal is to investigate if SFXT flares and outbursts are triggered by the presence of clumps and eventually reveal whether strongly or mildly dense clumps are required. Methods. For all sources, we employ a technique developed by our group already exploited in a number of our previous papers, making use of an adaptive rebinned hardness ratio to optimally select the time intervals for the spectral extraction. A total of twelve observations performed in the direction of five SFXTs are reported, providing the largest sample of events available so far. Results. Using the original results reported here and those obtained with our technique from the analysis of two previously published XMM-Newton observations of IGR J17544-2619 and IGR J18410-0535, we show that both strongly and mildly dense clumps can trigger these events. In the former case, the local absorption column density may increase by a factor of ≫3, while in the latter case, the increase is only by a factor ∼2-3 (or lower). An increase in the absorption column density is generally recorded during the rise of the flares/outbursts, while a drop follows when the source achieves the peak flux. In a few cases, a re-increase of the absorption column density after the flare is also detected, and we discovered one absorption event related to the passage of an unaccreted clump in front of the compact object. Overall, there seems to be no obvious correlation between the dynamic ranges in the X-ray fluxes and absorption column densities in supergiant fast X-ray transients, with an indication that lower densities are recorded at the highest fluxes. Conclusions. The spectral variations measured in all sources are in agreement with the idea that the flares/outbursts are triggered by the presence of dense structures in the wind interacting with the X-rays from the compact object (leading to photoionization). The lack of correlation between the dynamic ranges in the X-ray fluxes and absorption column densities can be explained by the presence of accretion inhibition mechanism(s). Based on the knowledge acquired so far on the SFXTs, we propose a classification of the flares/outbursts from these sources to drive future observational investigations. We suggest that the difference between the classes of flares/outbursts is related to the fact that the mechanism(s) inhibiting accretion can be overcome more easily in some sources compared to others. We also investigate the possibility that different stellar wind structures, rather than clumps, could provide the means to temporarily overcome the...

Supergiant Fast X-ray Transients uncovered by the EXTraS project: flares reveal the development of magnetospheric instability in accreting neutron stars

“…We note that the same XMM-Newton observations reported here have been analyzed by Giménez-García et al (2015), Bozzo et al (2017), Pradhan et al (2018) (and references therein). Although their temporal-selected spectra of flares were extracted from intervals much longer than the B.b.s adopted here (so that they cannot be directly compared), some spectra of SFXT flares showed a column density larger than 1.5×10 22 cm −2 (the value we assume for all flares), implying, in principle, a larger conversion factor from count rate to flux.…”

XMM–Newton andINTEGRALanalysis of the Supergiant Fast X-ray Transient IGR J17354−3255

“…A time-resolved spectral analysis performed at 1000s resolution on observation 3 using a simple powerlaw model did not show any significant correlation between the continuum flux and photon index. This result agrees with the findings of Bozzo et al (2017) whose analysis of the hardness ratio vs. the flux shows no significant variation. There is there-fore no evidence that IGR J17354−3255 follows the 'harderwhen-brighter' correlation seen in some other sources.…”

“…The results of our work generally agree with the findings of Bozzo et al (2017). They fit an absorbed powerlaw model and achieved reasonable results.…”

Probing Clumpy Wind Accretion in IGR J18027-2016 with XMM-Newton

“…We investigated the presence of spectral changes associated with the X-ray variability in each XMM-Newton observation by extracting also the energy resolved lightcurves of the source with a time bin corresponding to the spin period measured above in each observation and computing the hardness ratio (HR). Based on our previous analysis on similar sources with the same technique (Bozzo et al 2013b(Bozzo et al , 2017, we selected the energy bands 0.5-3 keV and 3-10 keV, and computed the corresponding hardness ratio (HR) by using a further adaptive rebinning to achieve in each bin of the soft energy band lightcurve a minimum signal-tonoise ratio S/N 10. The MOS and pn source spectra were then extracted by combining HR bins with similar values in each observation, following as closely as possible the observed HR variations.…”

“…In order to provide support to the fact that flares are triggered by the presence of clumps, in the case of the SFXTs it was highlighted that often the rise of flares is accompanied by an increase in the local absorption column density before the onset of a flare (Bozzo et al 2017). This can be interpreted as being due to the clump approaching the NS and contributing to an enhancement of the local absorbing material.…”