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Register a new userThe nature of the absorber in the INTEGRAL highly obscured sgHMXB IGR J17252-3616
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INTEGRAL played a key role in discovering obscured sgHMXB in the Galaxy. We used XMM-Newton to perform X-ray wind tomography of a specific of these systems, IGR J17252-3616, featuring eclipses of the accreting pulsar. The X-ray band (0.2-10 keV) reveals vital information on the geometry of the surrounding gas probing simultaneously the absorption and the fluorescence emission. The XMM observations were scheduled to cover as many orbital phases as possible. Timing analysis allows the derivation of an accurate orbital solution and of the system parameters. Spectral analysis revealed remarkable variations of the absorbing column density along the orbit and of the Fe K$alpha$ fluorescence line around the eclipse. The combination of these observables revealed a highly asymmetric and unprecedentedly extended structure in the stellar wind extending up to 2-3 stellar radii. The observations can be modeled in terms of three independent components: i) the unperturbed stellar wind ii) the contribution of a highly asymmetric hydrodynamic wind tail-like structure and iii) a cusp of material close to the neutron star. These dynamical structures are imaged for the first time in a sgHMXB and explain the source of the high obscuration.
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The discovery of the X-ray source IGR J17252-3616 by INTEGRAL was reported on 9 February 2004. Regular monitoring by INTEGRAL shows that IGR J17252-3616 is a persistent hard X-ray source with an average count rate of 0.96 counts/s (~6.4 mCrab) in the 20-60 keV energy band. A follow-up observation with XMM-Newton, which was performed on 21 March 21 2004, showed that the source is located at R.A.(2000.0)=17h25m11.4 and Dec.=-36degr1658.6 with an uncertainty of 4. The only infra-red counterpart to be found within the XMM-Newton error circle was 2MASS J17251139-3616575, which has a Ks-band magnitude of 10.7 and is located 1 away from the XMM-Newton position. The analysis of the combined INTEGRAL and XMM-Newton observations shows that the source is a binary X-ray pulsar with a spin period of 413.7 s and an orbital period of 9.72 days. The spectrum can be fitted with a flat power law plus an energy cut off (Gamma~0.02,Ecut~8.2 keV) or a Comptonized model (kTe~5.5 keV, tau~7.8). The spectrum also indicates a large hydrogen column density of Nh~15x1e22 atoms/cm-2 suggesting an intrinsic absorption. The Fe Kalpha line at 6.4 keV is clearly detected. Phase-resolved spectroscopy does not show any variation in the continuum except the total emitted flux. The absorption is constant along the pulse phase. This source can be associated with EXO 1722-363 as both systems show common timing and spectral features. The observations suggest that the source is a wind-fed accreting pulsar accompanied by a supergiant star.
We report a Hitomi observation of IGR J16318-4848, a high-mass X-ray binary system with an extremely strong absorption of N_H~10^{24} cm^{-2}. Previous X-ray studies revealed that its spectrum is dominated by strong fluorescence lines of Fe as well as continuum emission. For physical and geometrical insight into the nature of the reprocessing material, we utilize the high spectroscopic resolving power of the X-ray microcalorimeter (the soft X-ray spectrometer; SXS) and the wide-band sensitivity by the soft and hard X-ray imager (SXI and HXI) aboard Hitomi. Even though photon counts are limited due to unintended off-axis pointing, the SXS spectrum resolves Fe K{alpha_1} and K{alpha_2} lines and puts strong constraints on the line centroid and width. The line width corresponds to the velocity of 160^{+300}_{-70} km s^{-1}. This represents the most accurate, and smallest, width measurement of this line made so far from any X-ray binary, much less than the Doppler broadening and shift expected from speeds which are characteristic of similar systems. Combined with the K-shell edge energy measured by the SXI and HXI spectra, the ionization state of Fe is estimated to be in the range of Fe I--IV. Considering the estimated ionization parameter and the distance between the X-ray source and the absorber, the density and thickness of the materials are estimated. The extraordinarily strong absorption and the absence of a Compton shoulder component is confirmed. These characteristics suggest reprocessing materials which are distributed in a narrow solid angle or scattering primarily with warm free electrons or neutral hydrogen.
Context. One of the most striking discoveries of the INTEGRAL observatory is the existence of a previously unknown population of X-ray sources in the inner arms of the Galaxy. The investigations of the optical/NIR counterparts of some of them have provided evidence that they are highly absorbed high mass X-ray binaries hosting supergiants. Aims. We aim to identify the optical/NIR counterpart of one of the newly discovered INTEGRAL sources, IGR J16283-4838, and determine the nature of this system. Methods. We present optical and NIR observations of the field of IGR J16283-4838, and use the astrometry and photometry of the sources within it to identify its counterpart. We obtain its NIR spectrum, and its optical/NIR spectral energy distribution by means of broadband photometry. We search for the intrinsic polarization of its light, and its short and long-term photometric variability. Results. We demonstrate that this source is a highly absorbed HMXB located beyond the Galactic center, and that it may be surrounded by a variable circumstellar medium.
INTEGRAL regularly scans the Galactic plane to search for new objects and in particular for absorbed sources with the bulk of their emission above 10-20 keV. The first new INTEGRAL source was discovered on 2003 January 29, 0.5 degree from the Galactic plane and was further observed in the X-rays with XMM-Newton. This source, IGR J16318-4848, is intrinsically strongly absorbed by cold matter and displays exceptionally strong fluorescence emission lines. The likely infrared/optical counterpart indicates that IGR J16318-4848 is probably a High Mass X-Ray Binary neutron star or black hole enshrouded in a Compton thick environment. Strongly absorbed sources, not detected in previous surveys, could contribute significantly to the Galactic hard X-ray background between 10 and 200 keV.
The third INTEGRAL/IBIS survey has revealed several new hard X-ray sources, which are still unclassified. To identify these sources, we need to find their counterparts at other wavelengths and then study their nature. The capability of XRT on board Swift to localize the sources with a positional accuracy of few arcseconds allows the search for optical/UV, infrared and radio counterparts to be more efficient and reliable. We analysed all XRT observations available for three unidentified INTEGRAL sources, IGR J18249-3243, IGR J19443+2117 and IGR J22292+6647, localized their soft X-ray counterparts and searched for associations with objects in the radio band. We also combined X-/gamma-ray data, as well as all the available radio, infrared and optical/UV information, in order to provide a broad-band spectral characterization of each source and investigate its nature. All three sources are found to be bright and repeatedly observed radio objects, although poorly studied. The X-/gamma-ray spectrum of each source is well described by power laws with photon indices typical of AGN; only IGR J19443+2117 may have absorption in excess of the Galactic value, while IGR J22292+6647 is certainly variable at X-ray energies. IGR J18249-3243 has a complex radio morphology and a steep radio spectrum; the other two sources show flatter radio spectra and a more compact morphology. Overall, their radio, optical/UV and infrared characteristics, as well as their X-/gamma-ray properties, point to an AGN classification for all three objects.
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