Hard X-ray emission of Sco X-1

We study hard X-ray emission of the brightest accreting neutron star Sco X-1 with INTEGRAL observatory. Up to now INTEGRAL have collected ~4 Msec of deadtime corrected exposure on this source. We show that hard X-ray tail in time average spectrum of Sco X-1 has a power law shape without cutoff up to energies ~200-300 keV. An absence of the high energy cutoff does not agree with the predictions of a model, in which the tail is formed as a result of Comptonization of soft seed photons on bulk motion of matter near the compact object. The amplitude of the tail varies with time with factor more than ten with the faintest tail at the top of the so-called flaring branch of its color-color diagram. We show that the minimal amplitude of the power law tail is recorded when the component, corresponding to the innermost part of optically thick accretion disk, disappears from the emission spectrum. Therefore we show that the presence of the hard X-ray tail may be related with the existence of the inner part of the optically thick disk. We estimate cooling time for these energetic electrons and show that they can not be thermal. We propose that the hard X-ray tail emission originates as a Compton upscattering of soft seed photons on electrons, which might have initial non-thermal distribution.Comment: 9 pages, 7 figures, Accepted for publication in MNRA

An XMM-Newton and NuSTAR study of IGR J18214-1318: a non-pulsating high-mass X-ray binary with a neutron star

IGR J18214-1318, a Galactic source discovered by the International Gamma-Ray Astrophysics Laboratory, is a high-mass X-ray binary (HMXB) with a supergiant O-type stellar donor. We report on the XMM-Newton and NuSTAR observations that were undertaken to determine the nature of the compact object in this system. This source exhibits high levels of aperiodic variability, but no periodic pulsations are detected with a 90% confidence upper limit of 2% fractional rms between 0.00003-88 Hz, a frequency range that includes the typical pulse periods of neutron stars (NSs) in HMXBs (0.1-10$^3$ s). Although the lack of pulsations prevents us from definitively identifying the compact object in IGR J18214-1318, the presence of an exponential cutoff with e-folding energy $\lesssim30$ keV in its 0.3-79 keV spectrum strongly suggests that the compact object is an NS. The X-ray spectrum also shows a Fe K$\alpha$ emission line and a soft excess, which can be accounted for by either a partial-covering absorber with $N_{\mathrm{H}}\approx10^{23}$ cm$^{-2}$ which could be due to the inhomogeneous supergiant wind, or a blackbody component with $kT=1.74^{+0.04}_{-0.05}$ keV and $R_{BB}\approx0.3$ km, which may originate from NS hot spots. Although neither explanation for the soft excess can be excluded, the former is more consistent with the properties observed in other supergiant HMXBs. We compare IGR J18214-1318 to other HMXBs that lack pulsations or have long pulsation periods beyond the range covered by our observations.Comment: 15 pages, 12 figures, 4 table

Chandra Observations of Eight Sources Discovered by INTEGRAL

We report on 0.3-10 keV observations with the Chandra X-ray Observatory of eight hard X-ray sources discovered within 8 degrees of the Galactic plane by the INTEGRAL satellite. The short (5 ks) Chandra observations of the IGR source fields have yielded very likely identifications of X-ray counterparts for three of the IGR sources: IGR J14091-6108, IGR J18088-2741, and IGR J18381-0924. The first two have very hard spectra in the Chandra band that can be described by a power-law with photon indices of Gamma = 0.6+/-0.4 and -0.7(+0.4)(-0.3), respectively (90% confidence errors are given), and both have a unique near-IR counterpart consistent with the Chandra position. IGR J14091-6108 also displays a strong iron line and a relatively low X-ray luminosity, and we argue that the most likely source type is a Cataclysmic Variable (CV), although we do not completely rule out the possibility of a High Mass X-ray Binary. IGR J18088-2741 has an optical counterpart with a previously measured 6.84 hr periodicity, which may be the binary orbital period. We also detect five cycles of a possible 800-950 s period in the Chandra light curve, which may be the compact object spin period. We suggest that IGR J18088-2741 is also most likely a CV. For IGR J18381-0924, the spectrum is intrinsically softer with Gamma = 1.5(+0.5)(-0.4), and it is moderately absorbed, nH = (4+/-1)e22 cm-2. There are two near-IR sources consistent with the Chandra position, and they are both classified as galaxies, making it likely that IGR J18381-0924 is an Active Galactic Nucleus (AGN). For the other five IGR sources, we provide lists of nearby Chandra sources, which may be used along with further observations to identify the correct counterparts, and we discuss the implications of the low inferred Chandra count rates for these five sources.Comment: Accepted by ApJ, 14 page

Identifying IGR J14091-6108 as a magnetic CV with a massive white dwarf using X-ray and optical observations

INTEGRAL Gamma-Ray (IGR) J14091−6108 is a Galactic X-ray source known to have an iron emission line, a hard X-ray spectrum, and an optical counterpart. Here, we report on X-ray observations of the source with XMM–Newton and NuSTAR as well as optical spectroscopy with European Southern Obseratory/Very Large Telescope and National Optical Astronomy Observatory/Southern Astrophysical Research Telescope. In the X-rays, this provides data with much better statistical quality than the previous observations, and this is the first report of the optical spectrum. Timing analysis of the XMM data shows a very significant detection of 576.3 ± 0.6 s period. The signal has a pulsed fraction of 30 ± 3 per cent in the 0.3–12 keV range and shows a strong drop with energy. The optical spectra show strong emission lines with significant variability in the lines and continuum, indicating that they come from an irradiated accretion disc. Based on these measurements, we identify the source as a magnetic cataclysmic variable of intermediate polar (IP) type where the white dwarf spin period is 576.3 s. The X-ray spectrum is consistent with the continuum emission mechanism being due to thermal bremsstrahlung, but partial covering absorption and reflection are also required. In addition, we use the IP mass model, which suggests that the white dwarf in this system has a high mass, possibly approaching the Chandrasekhar limit

NuSTAR observations of the supergiant X-ray pulsar IGR J18027-2016: accretion from the stellar wind and possible cyclotron absorption line

We report on the first focused hard X-ray view of the absorbed supergiant system IGR J18027−2016 performed with the Nuclear Spectroscopic Telescope Array observatory. The pulsations are clearly detected with a period of P_(spin)=139.866(1) s and a pulse fraction of about 50–60 per cent at energies from 3 to 80 keV. The source demonstrates an approximately constant X-ray luminosity on a time-scale of more than dozen years with an average spin-down rate of P ≃ 6x10^(-10) s s^(-1). This behaviour of the pulsar can be explained in terms of the wind accretion model in the settling regime. The detailed spectral analysis at energies above 10 keV was performed for the first time and revealed a possible cyclotron absorption feature at energy ∼23 keV. This energy corresponds to the magnetic field B ≃ 3x10^(12) G at the surface of the neutron star, which is typical for X-ray pulsars