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Register a new userThe Peculiar X-ray Transient IGR 16358-4726
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The new transient IGR 16358-4726 was discovered on 2003 March 19 with INTEGRAL. We detected the source serendipitously during our 2003 March 24 observation of SGR 1627-41 with the Chandra X-ray Observatory at the 1.7 x 10^{-10} ergs s^{-1} cm^{-2} flux level (2-10 keV) with a very high absorption column (N_H=3.3(1) x 10^{23} cm^{-2}) and a hard power law spectrum of index 0.5(1). We discovered a very strong flux modulation with a period of 5880(50) s and peak-to-peak pulse fraction of 70(6)% (2-10 keV), clearly visible in the x-ray data. The nature of IGR 16358-4726 remains unresolved. The only neutron star systems known with similar spin periods are low luminosity persistent wind-fed pulsars; if this is a spin period, this transient is a new kind of object. If this is an orbital period, then the system could be a compact Low Mass X-ray Binary (LMXB).
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The hard X-ray source IGR J11215-5952 is a peculiar transient, displaying very short X-ray outbursts every 165 days. We obtained high-resolution spectra of the optical counterpart, HD 306414, at different epochs, spanning a total of three months, before and around the 2007 February outburst with the combined aims of deriving its astrophysical parameters and searching for orbital modulation. We fit model atmospheres generated with the fastwind code to the spectrum. We also cross-correlated each individual spectrum to the best-fit model to derive radial velocities. From its spectral features, we classify HD 306414 as B0.5 Ia. From the model fit, we find Teff = 24 700 K and log g = 2.7, in good agreement with the morphological classification. Using the interstellar lines in its spectrum, we estimate a distance to HD 306414 d > 7 kpc. Assuming this distance, we derive R* = 40 Rsol and Mspect = 30 Msol (consistent, within errors, with Mevol = 38 Msol). Radial velocity changes are not dominated by the orbital motion, and we find an upper limit on the semi-amplitude for the optical component Kopt < 11 +- 6 km/s. Large variations in the depth and shape of photospheric lines suggest the presence of strong pulsations, which may be the main cause of the radial velocity changes. Very significant variations, uncorrelated with those of the photospheric lines are seen in the shape and position of the Halpha emission feature around the time of the X-ray outburst, but large excursions are also observed at other times. HD 306414 is a normal B0.5 Ia supergiant. Its radial velocity curve is dominated by an effect that is different from binary motion, and is most likely stellar pulsations. The data available suggest that the X-ray outbursts are caused by the close passage of the neutron star in a very eccentric orbit, perhaps leading to localised mass outflow. (abridged).
Symbiotic X-ray Binaries (SyXBs) are a recently discovered subclass of Low Mass X-ray Binaries. Their growing number makes them an important evolutionary channel of X-ray Binaries. Our goal is to perform spectral analysis and classification of the proposed counterparts to IGR J16358-4726 and IGR J16393-4643 and to establish their nature as X-ray systems. We used the ESO/UT1 ISAAC spectrograph to observe the proposed counterparts to the two sources, obtaining K-band medium resolution spectra (R = 500) with a S/N > 140. Data reduction was performed with the standard procedure. We classified them by means of comparison with published atlases. We performed SED fitting in order to refine the spectral classification. The two counterparts clearly exhibit the typical features of late-type stars, notably strong CO absorption bands in the red part of the spectrum. With information from previous X-ray studies, we classify the two systems as two new members of the SyXB class. For IGR J16393-4643, we considered the most probable counterpart to the system, although three other objects cannot be completely discarded. For this system, we compared our findings with available orbital solutions, constraining the orbital parameters and the mass of the companion star. By including two more systems, we increased to eight the number of known SyXBs, which emerges as a non-negligible category of galactic X-ray binaries.
We present detailed spectral and timing analysis of the hard x-ray transient IGR J16358-4726 using multi-satellite archival observations. A study of the source flux time history over 6 years, suggests that lower luminosity transient outbursts can be occuring in intervals of at most 1 year. Joint spectral fits of the higher luminosity outburst using simultaneous Chandra/ACIS and INTEGRAL/ISGRI data reveal a spectrum well described by an absorbed power law model with a high energy cut-off plus an Fe line. We detected the 1.6 hour pulsations initially reported using Chandra/ACIS also in the INTEGRAL/ISGRI light curve and in subsequent XMM-Newton observations. Using the INTEGRAL data we identified a spin up of 94 s (dP/dt = 1.6E-4), which strongly points to a neutron star nature for IGR J16358-4726. Assuming that the spin up is due to disc accretion, we estimate that the source magnetic field ranges between 10^13 - 10^15 G, depending on its distance, possibly supporting a magnetar nature for IGR J16358-4726.
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.
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 days which we interpret as the orbital period of the binary system. In addition 11 outbursts (9 of which are previously unpublished) have been found between MJD 52652 to MJD 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 dinamical ranges are >100, still our reported values are significantly greater than that of classical persistent variable supergiant HMXBs (<20), supporting the idea that IGRJ16465-4507 is an intermediate SFXT system, much like few other similar cases reported in the literature.
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