The pulsar IGR J00291+5934 is the fastest-known accretion-powered X-ray pulsar, discovered during a transient outburst in 2004. In this paper, we report on INTEGRAL and Swift observations during the ...2015 outburst, which lasts for ~25 d. The source has not been observed in outburst since 2008, suggesting that the long-term accretion rate has decreased by a factor of two since discovery. The averaged broad-band (0.1–250 keV) persistent spectrum in 2015 is well described by a thermal Comptonization model with a column density of NH ≈ 4 × 1021 cm-2, a plasma temperature of kTe ≈ 50 keV, and a Thomson optical depth of τT ≈ 1. Pulsations at the known spin period of the source are detected in the INTEGRAL data up to the ~150 keV energy band. We also report on the discovery of the first thermonuclear burst observed from IGR J00291+5934, which lasts around 7 min and occurs at a persistent emission level corresponding to roughly 1.6% of the Eddington accretion rate. The properties of the burst suggest it is powered primarily by helium ignited at a depth of yign ≈ 1.5 × 109 g cm-2 following the exhaustion by steady burning of the accreted hydrogen. The Swift/BAT data from the first ~20 s of the burst provide indications of a photospheric radius expansion phase. Assuming this is the case, we infer a source distance of d = 4.2 ± 0.5 kpc.
In this paper, we investigated the long-term evolution of the pulse period in the high-mass X-ray binary LMC X-4 by taking advantage of more than 43 yr of measurements in the X-ray domain. Our ...analysis revealed for the first time that the source is displaying near-periodical variations of its spin period on a time-scale of roughly 6.8 yr, making LMC X-4 one of the known binary systems showing remarkable long-term spin torque reversals. We discuss different scenarios to interpret the origin of these torque reversals.
Aims.
LS 5039 is an enigmatic high-mass gamma-ray binary which hosts a powerful O6.5V companion, but the nature of the compact object is still to be established using multi-wavelength observations.
...Methods.
We analyzed phase-resolved multi-instrument spectra of nonthermal emission from LS 5039 in order to produce reliable spectral models, which can be further employed to select between various scenarios and theoretical models of the binary.
Results.
The combined phase-resolved hard X-ray and MeV-range gamma-ray spectra obtained with
XMM-Newton
, Suzaku,
NuSTAR
, INTEGRAL, and COMPTEL indicate a meaningful spectral hardening above 50 keV. The spectral break observed in both major phases of the binary may indicate the presence of an upturn in the spectrum of accelerated leptons which could originate from the interaction of wind from the O6.5V companion star with the relativistic outflow from a yet unidentified compact object.
Swift J1734.5-3027 is a hard X-ray transient discovered by Swift while undergoing an outburst in September 2013. Archival observations showed that this source underwent a previous episode of enhanced ...X-ray activity in 2013 May−June. In this paper we report on the analysis of all X-ray data collected during the outburst in 2013 September, the first that could be intensively followed up by several X-ray facilities. Our dataset includes INTEGRAL, Swift, and XMM-Newton observations. From the timing and spectral analysis of these observations, we show that a long Type-I X-ray burst took place during the source outburst, making Swift J1734.5-3027 a new member of the class of bursting neutron star low-mass X-ray binaries. The burst lasted for about 1.9 ks and reached a peak flux of (6.0 ± 1.8) × 10-8 erg cm-2 s-1 in the 0.5−100 keV energy range. The estimated burst fluence in the same energy range is (1.10 ± 0.10) × 10-5 erg cm-2. By assuming that a photospheric radius expansion took place during the first ~200 s of the burst and that the accreted material was predominantly composed by He, we derived a distance to the source of 7.2 ± 1.5 kpc.
ABSTRACT
After two major outbursts in 2006 and 2011, on 2017 May 16 the magnetar CXOU J164710.2−455216, hosted within the massive star cluster Westerlund I, emitted a short (∼20 ms) burst, which ...marked the onset of a new active phase. We started a long-term monitoring campaign with Swift (45 observations), Chandra (five observations), and NuSTAR (four observations) from the activation until 2018 April. During the campaign, Swift Burst Alert Telescope (BAT) registered the occurrence of multiple bursts, accompanied by two other enhancements of the X-ray persistent flux. The long time span covered by our observations allowed us to study the spectral and the timing evolution of the source. After ∼11 months since the 2017 May outburst onset, the observed flux was ∼15 times higher than its historical minimum level and a factor of ∼3 higher than the level reached after the 2006 outburst. This suggests that the crust has not fully relaxed to the quiescent level, or that the source quiescent level has changed following the multiple outburst activities in the past 10 yr or so. This is another case of multiple outbursts from the same source on a yearly time-scale, a somehow recently discovered behaviour in magnetars.
The Wilkes Subglacial Basin, in the hinterland of the Transantarctic Mountains, represents one of the least understood continental-scale features in Antarctica. Aeromagnetic data suggests that this ...basin may be imposed on a Ross age back arc region adjacent to the East Antarctic Craton. However, the evolution of the deeper crustal structure is disputed. Here, we present new airborne gravity data that reveals the crustal architecture of the northern Wilkes Subglacial Basin. Our gravity models indicate that the crust under the northern Wilkes Subglacial Basin is 30–35km thick, i.e. ca 5–10km thinner than imaged under the Transantarctic Mountains, and ~15km thinner than predicted from some flexural and seismic models in the southern Wilkes Basin. We suggest that crustal thickening under northern Victoria Land reflects Ross-age (ca 500Ma) orogenic events. Airy isostatic anomalies along both flanks of the Wilkes Basin reveal major inherited tectonic structures, which likely controlled the basin location, supporting aeromagnetic interpretations of the Wilkes Subglacial Basin as a structurally controlled basin. The positive anomaly along the western margin of the basin defines the boundary between the East Antarctic Craton and the Ross Orogen, and the anomaly along its eastern flank likely reflects high-grade rocks of the central Wilson Terrane. Our models indicate that the crust is ~5km thinner beneath the northern Wilkes Basin, compared to formerly contiguous segments of the Delamerian Orogen in south-eastern Australia. The thinner crust may be linked to: i) back-arc basin formation or orogenic collapse processes and segmentation within the Ross\Delamerian Orogen, ii) Jurassic to Cretaceous extension prior to break-up between Australia and East Antarctica, iii) Cenozoic glacial erosion or most likely, iv) a combination of these processes.
► The Wilkes Subglacial Basin, East Antarctica, is structurally controlled. ► The Wilkes Subglacial Basin has thinner crust than the Transantarctic Mountains. ► Erosion contributes to crustal thinning of the northern Wilkes Subglacial Basin.
ABSTRACT
The Accreting Millisecond X-ray Pulsar IGR J17591–2342 is a Low Mass X-ray Binary (LMXB) system that went in outburst on 2018 August and it was monitored by the NICER observatory and ...partially by other facilities. We aim to study how the spectral emission of this source evolved during the outburst by exploiting the whole X-ray data repository of simultaneous observations. The continuum emission of the combined broad-band spectra is on average well described by an absorbed Comptonization component scattering blackbody-distributed photons peaking at (0.8 ± 0.5) keV by a moderately optically thick corona (τ = 2.3 ± 0.5) with temperature of (34 ± 9) keV. A blackbody component with temperature and radial size of (0.8 ± 0.2) keV and (3.3 ± 1.5) km, respectively, is required by some of the spectra and suggests that part of the central emission, possibly a fraction of the neutron star surface, is not efficiently scattered by the corona. The continuum at low energies is characterized by significant residuals suggesting the presence of an absorption edge of O viii and of emission lines of Ne ix ions. Moreover, broad Fe i and Fe xxv Kα emission lines are detected at different times of the outburst, suggesting the presence of reflection in the system.
The long-term X-ray light curves of classical supergiant X-ray binaries and supergiant fast X-ray transients show relatively similar super-orbital modulations, which are still lacking a sound ...interpretation. We propose that these modulations are related to the presence of corotating interaction regions (CIRs) known to thread the winds of OB supergiants. To test this hypothesis, we couple the outcomes of three-dimensional (3D) hydrodynamic models for the formation of CIRs in stellar winds with a simplified recipe for the accretion onto a neutron star. The results show that the synthetic X-ray light curves are indeed modulated by the presence of the CIRs. The exact period and amplitude of these modulations depend on a number of parameters governing the hydrodynamic wind models and on the binary orbital configuration. To compare our model predictions with the observations, we apply the 3D wind structure previously shown to well explain the appearance of discrete absorption components in the UV time series of a prototypical B0.5I-type supergiant. Using the orbital parameters of IGRJ 16493-4348, which has the same B0.5I donor spectral type, the period and modulations in the simulated X-ray light curve are similar to the observed ones, thus providing support to our scenario. We propose that the presence of CIRs in donor star winds should be considered in future theoretical and simulation efforts of wind-fed X-ray binaries.
Abstract
In a search for the counterpart to the Fermi-LAT source 3FGL J0838.8−2829, we performed a multiwavelength campaign: in the X-ray band with Swift and XMM–Newton; in the infrared and optical ...with OAGH, ESO-NTT and IAC80; and in the radio with ATCA observations. We also used archival hard X-ray data obtained by INTEGRAL. We report on three X-ray sources consistent with the position of the Fermi-LAT source. We confirm the identification of the brightest object, RX J0838−2827, as a magnetic cataclysmic variable that we recognize as an asynchronous system (not associated with the Fermi-LAT source). RX J0838−2827 is extremely variable in the X-ray and optical bands, and timing analysis reveals the presence of several periodicities modulating its X-ray and optical emission. The most evident modulations are interpreted as being caused by the binary system orbital period of ∼1.64 h and the white dwarf spin period of ∼1.47 h. A strong flux modulation at ∼15 h is observed at all energy bands, consistent with the beat frequency between spin and orbital periods. Optical spectra show prominent Hβ, He i and He ii emission lines that are Doppler-modulated at the orbital period and at the beat period. Therefore, RX J0838−2827 accretes through a disc-less configuration and could be either a strongly asynchronous polar or a rare example of a pre-polar system on its way to reaching synchronism. Regarding the other two X-ray sources, XMM J083850.4−282759 showed a variable X-ray emission, with a powerful flare lasting for ∼600 s, similar to what is observed in transitional millisecond pulsars during the subluminous disc state: this observation possibly means that this source can be associated with the Fermi-LAT source.
We report on the results of the multiwavelength campaign carried out after the discovery of the INTEGRAL transient IGR J17329-2731. The optical data collected with the SOAR telescope allowed us to ...identify the donor star in this system as a late M giant at a distance of 2.7-1.2+3.4 kpc. The data collected quasi-simultaneously with XMM–Newton and NuSTAR showed the presence of a modulation with a period of 6680 ± 3 s in the X-ray light curves of the source. This unveils that the compact object hosted in this system is a slowly rotating neutron star. The broadband X-ray spectrum showed the presence of a strong absorption (≫1023 cm−2) and prominent emission lines at 6.4 keV, and 7.1 keV. These features are usually found in wind-fed systems, in which the emission lines result from the fluorescence of the X-rays from the accreting compact object on the surrounding stellar wind. The presence of a strong absorption line around ~21 keV in the spectrum suggests a cyclotron origin, thus allowing us to estimate the neutron star magnetic field as ~2.4 × 1012 G. All evidencethus suggests IGR J17329-2731 is a symbiotic X-ray binary. As no X-ray emission was ever observed from the location of IGR J17329-2731 by INTEGRAL (or other X-ray facilities) during the past 15 yr in orbit and considering that symbiotic X-ray binaries are known to be variable but persistent X-ray sources, we concluded that INTEGRAL caught the first detectable X-ray emission from IGR J17329-2731 when the source shined as a symbiotic X-ray binary. The Swift XRT monitoring performed up to ~3 months after the discovery of the source, showed that it maintained a relatively stable X-ray flux and spectral properties.
PDF
