Fast X-ray transients towards the Galactic bulge with the Rossi X-ray Timing Explorer

Smith, David M.; Markwardt, C. B.; Swank, J. H.; Negueruela, I.

doi:10.1111/j.1365-2966.2012.20836.x

Cited by 34 publications

(29 citation statements)

References 68 publications

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“…Drave et al (2010) reported the discovery of a P orb = 51.47 ± 0.02 d orbital period based on ∼12.4 Ms of INTEGRAL data. We note however that this period was not independently confirmed by an RXTE investigation (Smith et al 2012) and a hint of a periodicity at P orb = 12.8658 ± 0.0073 d (1/4 of the value above) was reported by Romano et al (2009e).…”

Section: Xte J1739−302: the Sfxt Class Prototypementioning

confidence: 58%

The 100-monthSwiftcatalogue of supergiant fast X-ray transients

Romano¹,

Krimm²,

Palmer³

et al. 2014

A&A

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Context. Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries (HMXBs) that are defined by their hard X-ray flaring behaviour. During these flares they reach peak luminosities of 10 36 -10 37 erg s −1 for a few hours (in the hard X-ray), which are much shorter timescales than those characterizing Be/X-ray binaries. Aims. We investigate the characteristics of bright flares (detections in excess of 5σ) for a sample of SFXTs and their relation to the orbital phase. Methods. We have retrieved all Swift/BAT Transient Monitor light curves and collected all detections in excess of 5σ from both dailyand orbital-averaged light curves in the time range of 2005 February 12 to 2013 May 31 . We also considered all on-board detections as recorded in the same time span and selected those in excess of 5σ and within 4 arcmin of each source in our sample.Results. We present a catalogue of over a thousand BAT flares from 11 SFXTs, down to 15-150 keV fluxes of ∼6 × 10 −10 erg cm −2 s −1(daily timescale) and ∼1.5 × 10 −9 erg cm −2 s −1 (orbital timescale, averaging ∼800 s); the great majority of these flares are unpublished. The catalogue spans 100 months. This population is characterized by short (a few hundred seconds) and relatively bright (in excess of 100 mCrab, 15-50 keV) events. In the hard X-ray, these flares last generally much less than a day. Clustering of hard X-ray flares can be used to indirectly measure the length of an outburst, even when the low-level emission is not detected. We construct the distributions of flares, of their significance (in terms of σ), and of their flux as a function of orbital phase to infer the properties of these binary systems. In particular, we observe a trend of clustering of flares at some phases as P orb increases, which is consistent with a progression from tight circular or mildly eccentric orbits at short periods to wider and more eccentric orbits at longer orbital periods. Finally, we estimate the expected number of flares for a given source for our limiting flux and provide the recipe for calculating them for the limiting flux of future hard X-ray observatories.

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Section: Xte J1739−302: the Sfxt Class Prototypementioning

confidence: 58%

The 100-monthSwiftcatalogue of supergiant fast X-ray transients

Romano¹,

Krimm²,

Palmer³

et al. 2014

A&A

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“…However, it is noteworthy that the ∼12 years monitoring campaigns carried out with the RXTE/PCA on several SFXTs also feature plateau. Although the plateau in the PCA data are less prominent than those observed by XRT, in both cases these features are due to the brightest SFXT outbursts which are detected as rare events and span a relatively limited range in luminosity (Smith et al, 2012). If consolidated by future XRT monitoring observations, this could be interpreted in terms of those peculiar source states discussed above during which the highest mass accretion rate is achieved.…”

Section: Discussionmentioning

confidence: 99%

Supergiant fast X-ray transients as an under-luminous class of supergiant X-ray binaries

Bozzo

Romano

Ducci

et al. 2015

Advances in Space Research

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The usage of cumulative luminosity distributions, constructed thanks to the long-term observations available through wide field hard X-ray imagers, has been recently exploited to study the averaged high energy emission (>17 keV) from Supergiant Fast X-ray Transients (SFXTs) and classical Supergiant High Mass X-ray Binaries (SgXBs). Here, we take advantage of the long term monitorings now available with Swift/XRT to construct for the first time the cumulative luminosity distributions of a number of SFXTs and the classical SgXB IGR J18027-2016 in the soft X-ray domain with a high sensitivity focusing X-ray telescope (0.3-10 keV).By complementing previous results obtained in the hard X-rays, we found that classical SgXBs are characterized by cumulative distributions with a single knee around ∼10 36 -10 37 erg s −1 , while SFXTs are found to be systematically sub-luminous and their distributions are shifted at significantly lower luminosities (a factor of ∼10-100). As the luminosity states in which these sources spend most of their time are typically below the sensitivity limit of large field of view hard X-ray imagers, we conclude that soft X-ray monitorings carried out with high sensitivity telescopes are particularly crucial to reconstruct the complete profile of the SFXT cumulative luminosity distributions.The difference between the cumulative luminosity distributions of classical SgXBs and SFXTs is interpreted in terms of accretion from a structured wind in the former sources and the presence of magnetic/centrifugal gates or a quasi-spherical settling accretion regime in the latter.

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“…Other authors (Smith et al 2012;Paizis & Sidoli 2014) have used the longer baseline of relatively less sensitive RXTE and INTEGRAL data available-hence geared to best detect the bright flares-to construct cumulative luminosity distributions. In this paper we exploited the higher sensitivity XRT data to construct differential count rate (flux and luminosity) distributions, instead, searching for faint features that originate in different populations of flares in the soft X-ray emission.…”

Section: Differential Luminosity Distributionsmentioning

confidence: 99%

Soft X-ray characterisation of the long-term properties of supergiant fast X-ray transients

Romano

Ducci

Mangano

et al. 2014

A&A

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Context. Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries (HMXBs) that are characterised by a hard X-ray (≥ 15 keV) flaring behaviour. These flares reach peak luminosities of 10 36 -10 37 erg s −1 and last a few hours in the hard X-rays. Aims. We investigate the long-term properties of SFXTs by examining the soft (0.3−10 keV) X-ray emission of the three least active SFXTs in the hard X-ray and by comparing them with the remainder of the SFXT sample. Methods. We performed the first high-sensitivity soft X-ray long-term monitoring with Swift/XRT of three relatively unexplored SFXTs, IGR J08408−4503, IGR J16328−4726, and IGR J16465−4507, whose hard X-ray duty cycles are the lowest measured among the SFXT sample. We assessed how long each source spends in each flux state and compared their properties with those of the prototypical SFXTs.Results. The behaviour of IGR J08408−4503 and IGR J16328−4726 resembles that of other SFXTs, and it is characterised by a relatively high inactivity duty cycle (IDC) and pronounced dynamic range (DR) in the X-ray luminosity. We found DR ∼ 7400, IDC ∼ 67% for IGR J08408−4503, and DR ∼ 750, IDC ∼ 61% for IGR J16328−4726 (in all cases the IDC is given with respect to the limiting flux sensitivity of XRT, that is 1−3 × 10 −12 erg cm −2 s −1 ). In common with all the most extreme SFXT prototypes (IGR J17544−2619, XTE J1739−302, and IGR J16479−4514), IGR J08408−4503 shows two distinct flare populations. The first one is associated with the brightest outbursts (X-ray luminosity L X 10 35−36 erg s −1 ), while the second comprises dimmer events with typical luminosities of L X 10 35 erg s −1 . This double-peaked distribution of the flares as a function of the X-ray luminosity seems to be a ubiquitous feature of the extreme SFXTs. The lower DR of IGR J16328−4726 suggests that this is an intermediate SFXT. IGR J16465−4507 is characterised by a low IDC ∼ 5% and a relatively narrow DR ∼ 40, reminiscent of classical supergiant HMXBs. The duty cycles measured with XRT are found to be comparable with those reported previously by BAT and INTEGRAL, when the higher limiting sensitivities of these instruments are taken into account and sufficiently long observational campaigns are available. By making use of these new results and those we reported previously, we prove that no clear correlation exists between the duty cycles of the SFXTs and their orbital periods. Conclusions. The unique sensitivity and scheduling flexibility of Swift/XRT allowed us to carry out an efficient long-term monitoring of the SFXTs, following their activity across more than 4 orders of magnitude in X-ray luminosity. While it is not possible to exclude that particular distributions of the clump and wind parameters may produce double-peaked differential distributions in the X-ray luminosities of the SFXTs, the lack of a clear correlation between the duty cycles and orbital periods of these sources make it difficult to interpret their peculiar variability by only using arguments related to the properti...

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Fast X-ray transients towards the Galactic bulge with the Rossi X-ray Timing Explorer

Cited by 34 publications

References 68 publications

The 100-monthSwiftcatalogue of supergiant fast X-ray transients

The 100-monthSwiftcatalogue of supergiant fast X-ray transients

Supergiant fast X-ray transients as an under-luminous class of supergiant X-ray binaries

Soft X-ray characterisation of the long-term properties of supergiant fast X-ray transients

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