We here report on the multiwavelength study which led us to the identification of X-ray source IGR J16194-2810 as a new Symbiotic X-ray Binary (SyXB), that is, a rare type of Low Mass X-ray Binary ...(LMXB) composed of a M-type giant and a compact object. Using the accurate X-ray position allowed by Swift/XRT data, we pinpointed the optical counterpart, a M2 III star. Besides, the combined use of the spectral information afforded by XRT and INTEGRAL/IBIS shows that the 0.5-200 keV spectrum of this source can be described with an absorbed Comptonization model, usually found in LMXBs and, in particular, in SyXBs. No long-term (days to months) periodicities are detected in the IBIS data. The time coverage afforded by XRT reveals shot-noise variability typical of accreting Galactic X-ray sources, but is not good enough to explore the presence of X-ray short-term (seconds to hours) oscillations in detail. By using the above information, we infer important parameters for this source such as its distance (~3.7 kpc) and X-ray luminosity (~$1.4\times10^{35}$ erg s-1 in the 0.5-200 keV band), and we give a description for this system (typical of SyXBs) in which a compact object (possibly a neutron star) accretes from the wind of its M-type giant companion. We also draw some comparisons between IGR J16194-2810 and other sources belonging to this subclass, finding that this object resembles SyXBs 4U 1700+24 and 4U 1954+31.
Timing analysis of ∼12.4 Ms of INTEGRAL/IBIS data has revealed a period of 51.47 ± 0.02 d in the supergiant fast X-ray transient source XTE J1739−302/IGR J17391−3021 that can be interpreted as an ...orbital period. An outburst history showing 35 epochs of activity has been produced, showing X-ray outbursts throughout the orbit of XTE J1739−302. Possible indications of an enhanced equatorial density region within the supergiant stellar wind are present in the phase-folded light curve. It is found that many orbital configurations are possible within this system with eccentricities of up to e∼ 0.8.
Phase-targeted RXTE observations have allowed us to detect a transient 71.49 ± 0.02 s signal that is most likely to be originating from the supergiant fast X-ray transient IGR J17544−2619. The ...phase-folded light curve shows a possible double-peaked structure with a pulsed flux of ~4.8 × 10-12 erg cm-2 s-1 (3−10 keV). Assuming the signal to indicate the spin period of the neutron star in the system, the provisional location of IGR J17544−2619 on the Corbet diagram places the system within the classical wind-fed supergiant XRB region. Such a result illustrates the growing trend of supergiant fast X-ray transients to span across both of the original classes of HMXB in Porb − Pspin space.
There are a total of 1451 gamma-ray emitting objects in the Fermi Large Area Telescope First Source Catalogue. The point source location accuracy of typically a few arcmin has allowed the ...counterparts for many of these sources to be found at other wavelengths, but even so there are 630 which are described as having no plausible counterpart at 80 per cent confidence. In order to help identify the unknown objects, we have cross-correlated the positions of these sources with the ROSAT All Sky Survey Bright Source Catalogue. In this way, for Fermi sources which have a possible counterpart in soft X-rays, we can use the much smaller ROSAT error box to search for identifications. We find a strong correlation between the two samples and calculate that there are about 60 sources with a ROSAT counterpart. Using the ROSAT error boxes we provide tentative associations for half of them, demonstrate that the majority of these are either blazars or blazar candidates, and give evidence that most belong to the BL Lac class. Given that they are X-ray selected and most are high synchrotron peaked objects, which indicates the presence of high-energy electrons, these sources are also good candidates for TeV emission, and therefore good probes of the extragalactic background light.
Timing analysis of the INTEGRAL-IBIS and Swift-BAT light curves of the supergiant fast X-ray transient (SFXT) IGR J16465−4507 has identified a period of 30.32 ± 0.02 d which we interpret as the ...orbital period of the binary system. In addition 11 outbursts (nine of which are previously unpublished) have been found between MJD 52652 and 54764, all of which occur close to the region of the orbit we regard as periastron. From the reported flux outbursts, we found a dynamical range in the interval ∼30–80. Although in this regard IGR J16465−4507 cannot be considered a classical SFXT for which typical dynamical ranges are >100, still our reported values are significantly greater than that of classical persistent variable supergiant HMXBs (<20), supporting the idea that IGR J16465−4507 is an intermediate SFXT system, much like few other similar cases reported in the literature.
We report the results from archival XMM–Newton and INTEGRAL observations of the Supergiant Fast X-ray Transient (SFXT) IGR J18483−0311 in quiescence. The 18–60 keV hard X-ray behaviour of the source ...is presented here for the first time; it is characterized by a spectral shape (Γ∼ 2.5) similar to that during outburst activity, and the lowest measured luminosity level is ∼1034 erg s−1. The 0.5–10 keV luminosity state, measured by XMM–Newton during the apastron passage, is about one order of magnitude lower and it is reasonably fitted by an absorbed blackbody model yielding parameters consistent with previous measurements. In addition, we find evidence (∼3.5σ significance) of an emission-like feature at ∼3.3 keV in the quiescent 0.5–10 keV source spectrum. The absence of any known or found systematic effects, which could artificially introduce the observed feature, gives us confidence about its non-instrumental nature. We show that its physical explanation in terms of atomic emission line appears unlikely, and conversely we attempt to ascribe it to an electron cyclotron emission line which would imply a neutron star magnetic field of the order of ∼3 × 1011 G. Importantly, such direct estimation is in very good agreement with that independently inferred by us in the framework of accretion from a spherically symmetric stellar wind. If firmly confirmed by future longer X-ray observations, this would be the first detection ever of a cyclotron feature in the X-ray spectrum of an SFXT, with important implications on theoretical models.
Context.IGR J18483-0311 is a poorly known transient hard X-ray source discovered by INTEGRAL during observations of the Galactic Center region performed between 23–28 April 2003. Aims.To detect new ...outbursts from IGR J18483-0311 using INTEGRAL and archival Swift XRT observations and finally to characterize the nature of this source using the optical/near-infrared (NIR) information available through catalogue searches. Methods.We performed an analysis of light curves and spectra of INTEGRAL and archival Swift XRT data as well as of optical/NIR catalogues. Results.We report on 5 newly discovered outbursts from IGR J18483-0311 detected by INTEGRAL. For two of them it was possible to constrain a duration of the order of a few days. The strongest outburst reached a peak flux of ~120 mCrab (20–100 keV); its broad band JEM–X/ISGRI spectrum (3–50 keV) is best fitted by an absorbed cutoff power law with $\Gamma=1.4\pm0.3$, cutoff energy of $22^{\rm +7.5}_{\rm -4.5}$ keV and $N_{\rm H}=9^{+5}_{-4}\times10^{22}$ cm-2. Timing analysis of INTEGRAL data allowed us to identify periodicities of 18.52 days and 21.0526 seconds which are likely the orbital period of the system and the spin period of the X-ray pulsar respectively. Swift XRT observations of IGR J18483-0311 provided a very accurate source position which strongly indicates a highly reddened star in the USNO–B1.0 and 2MASS catalogues as its possible optical/NIR counterpart. Conclusions.The X-ray spectral shape, the periods of 18.52 days and 21.0526 s, the high intrinsic absorption, the location in the direction of the Scutum spiral arm and the highly reddened optical object as possible counterpart, all favour the hypothesis that IGR J18483-0311 is a HMXB with a neutron star as compact companion. The system is most likely a Be X-ray binary, but a Supergiant Fast X-ray Transient nature can not be entirely excluded.
3EG J1837-0423 and HESS J1841-055 are two unidentified and peculiar high-energy sources located in the same region of the sky, separated by ~1.4DG. Specifically, 3EG J1837-0423 is a transient MeV ...object detected by EGRET only once during flaring activity that lasted a few days while HESS J1841-055 is a highly extended TeV source. We attempted to match the high-energy emission from the unidentified sources 3EG J1837-0423 and HESS J1841-055 with X-rays (4-20 keV) and soft gamma -rays (20-100 keV) candidate counterparts detected through deep International Gamma-Ray Astrophysics Laboratory observations of the sky region. As a result we propose the Supergiant Fast X-ray Transient (SFXT) AX J1841.0-0536 as a possible candidate counterpart of 3EG J1837-0423, based on spatial proximity and transient behavior. Alternatively, AX J1841.0-0536 could be responsible for at least a fraction of the entire TeV emission from the extended source HESS J1841-055, based on a striking spatial correlation. In either case, the proposed association is also supported from an energetic standpoint by a theoretical scenario where AX J1841.0-0536 is a low magnetized pulsar which, due to accretion of massive clumps from the supergiant companion donor star, undergoes sporadic changes to transient Atoll-states where a magnetic tower can produce transient jets and as a consequence high-energy emission. In either case (by association with 3EG J1837-0423 or alternatively with HESS J1841-055), AX J1841.0-0536 might be the prototype of a new class of Galactic transient MeV/TeV emitters.
Here, we report the results of our analysis of recent
Chandra
,
XMM-Newton
, and
NuSTAR
observations of the supergiant fast X-ray transient XTE J1739−302. The source was caught in a low X-ray ...luminosity state, from a few 10
31
–10
34
erg s
−1
(0.5–10 keV). A very low X-ray luminosity was captured during an
XMM-Newton
observation performed in October 2022, at a few 10
31
erg s
−1
(0.5–10 keV), which had never been observed before in XTE J1739−302. The
XMM-Newton
spectrum could be well fitted either by an absorbed, steep power-law model (photon index of 3.5) or by a collisionally ionized diffuse gas with a temperature of 0.7 keV that would very likely have been produced by shocks in the supergiant donor wind. These observations covered different orbital phases, but they all appear compatible with the low luminosity level expected from the orbital INTEGRAL light curve. The absorbing column density is variable in the range between 10
22
and 10
23
cm
−2
. The broadband X-ray spectrum was feasibly investigated at 10
34
erg s
−1
(0.5–30 keV) for the first time in XTE J1739−302 with non-simultaneous (albeit at similar orbital phases)
Chandra
and
NuSTAR
data, showing a power-law spectral shape with a photon index of ∼2.2 and an absorbing column density of ∼10
23
cm
−2
. Remarkably, owing to the
XMM-Newton
observation, the amplitude of the X-ray variability now exceeds five orders of magnitude, making XTE J1739−302 one of the most extreme SFXTs.
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