ALBERT

Description: 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.

Description: We report the results of the deepest Galactic plane (| b | 〈 17 $_{.}^{\circ}$ 5) survey in the 67.9 and 78.4 keV nuclear de-excitation lines of titanium-44 ( 44 Ti) performed using the data acquired with the IBIS/ISGRI instrument onboard the INTEGRAL satellite during 12 yr of operation. The peak sensitivity of our survey reached an unprecedented level of 4.8 x 10 –6 ph cm –2  s –1 (3) that improves the sensitivity of the survey done by Compton Gamma-Ray Observatory /COMPTEL by a factor of ~5. As a result, constraining upper limits for all sources from the catalogue of Galactic supernova remnants (SNRs; Green 2014 ) are derived. These upper limits can be used to estimate the exposure needed to detect 44 Ti emission from any known SNR using existing and prospective X- and gamma-ray telescopes. Among the youngest Galactic SNRs, only Cas A shows significant 44 Ti emission flux in good agreement with the NuSTAR measurements. We did not detect any other sources of titanium emission in the Galactic plane at significance level higher than 5 confirming previous claims of the rarity of such 44 Ti-producing SNRs.

Description: Studying the population of faint hard X-ray sources along the plane of the Galaxy is challenging because of high extinction and crowding, which make the identification of individual sources more difficult. IGR J18293–1213 is part of the population of persistent sources which have been discovered by the INTEGRAL satellite. We report on NuSTAR and Swift /XRT observations of this source, performed on 2015 September 11. We detected three eclipsing intervals in the NuSTAR light curve, allowing us to constrain the duration of these eclipses, $\Delta t= 30.8^{+6.3}_{-0.0}$  min, and the orbital period of the system, T = 6.92 ± 0.01 h. Even though we only report an upper limit on the amplitude of a putative spin modulation, the orbital period and the hard thermal bremsstrahlung spectrum of IGR J18293–1213 provide strong evidence that this source is a magnetic cataclysmic variable. Our NuSTAR and Swift /XRT joint spectral analysis places strong constraints on the white dwarf mass $M_{\rm wd} = 0.78^{+0.10}_{-0.09}$  M . Assuming that the mass to radius ratio of the companion star M * / R * = 1 (solar units) and using T , t , and M wd , we derived the mass of the companion star M * = 0.82 ± 0.01 M , the orbital separation of the binary system a = 2.14 ± 0.04 R , and its orbital inclination compared to the line of sight $i=(72 {^{\circ}_{.}} 2^{+2.4}_{-0.0})\pm 1 {^{\circ}_{.}} 0$ .

Description: We present a high-quality hard X-ray spectrum of the ultraluminous X-ray source (ULX) NGC 5643 X-1 measured with NuSTAR in 2014 May–June. We have obtained this spectrum by carefully separating the signals from the ULX and from the active nucleus of its host galaxyNGC 5643 located 0.8 arcmin away. Together with long XMM–Newton observations performed in 2009 July and 2014 August, the NuSTAR data confidently reveal a high-energy cutoff in the spectrum of NGC 5643 X-1 above ~10 keV, which is a characteristic signature of ULXs. The NuSTAR and XMM–Newton data are consistent with the source having a constant luminosity ~1.5  x  10 40 erg s –1 (0.2–12 keV) in all but the latest observation (2014 August) when it brightened to ~3  x  10 40 erg s –1 . This increase is associated with the dominant, hard spectral component (presumably collimated emission from the inner regions of a supercritical accretion disc), while an additional, soft component (with a temperature ~0.3 keV if described by multicolour disc emission), possibly associated with a massive wind outflowing from the disc, is also evident in the spectrum but does not exhibit significant variability.

Description: We use a sample of 151 local non-blazar active galactic nuclei (AGN) selected from the INTEGRAL all-sky hard X-ray survey to investigate if the observed declining trend of the fraction of obscured (i.e. showing X-ray absorption) AGN with increasing luminosity is mostly an intrinsic or selection effect. Using a torus-obscuration model, we demonstrate that in addition to negative bias, due to absorption in the torus, in finding obscured AGN in hard X-ray flux-limited surveys, there is also positive bias in finding unobscured AGN, due to Compton reflection in the torus. These biases can be even stronger taking into account plausible intrinsic collimation of hard X-ray emission along the axis of the obscuring torus. Given the AGN luminosity function, which steepens at high luminosities, these observational biases lead to a decreasing observed fraction of obscured AGN with increasing luminosity even if this fraction has no intrinsic luminosity dependence. We find that if the central hard X-ray source in AGN is isotropic, the intrinsic (i.e. corrected for biases) obscured AGN fraction still shows a declining trend with luminosity, although the intrinsic obscured fraction is significantly larger than the observed one: the actual fraction is larger than ~85 per cent at L 10 42.5 erg s –1 (17–60 keV), and decreases to 60 per cent at L 10 44  erg s –1 . In terms of the half-opening angle of an obscuring torus, this implies that 30° in lower luminosity AGN, and 45° in higher luminosity ones. If, however, the emission from the central supermassive black hole is collimated as d L /d cos α, the intrinsic dependence of the obscured AGN fraction is consistent with a luminosity-independent torus half-opening angle ~ 30°.

Description: We report on the results of the extensive multi-wavelength campaign from optical to GeV -rays of the 2014 periastron passage of PSR B1259–63, which is a unique high-mass -ray emitting binary system with a young pulsar companion. Observations demonstrate the stable nature of the post-periastron GeV flare and prove the coincidence of the flare with the start of rapid decay of the Hα equivalent width, usually interpreted as a disruption of the Be stellar disc. Intensive X-ray observations reveal changes in the X-ray spectral behaviour happening at the moment of the GeV flare. We demonstrate that these changes can be naturally explained as a result of synchrotron cooling of monoenergetic relativistic electrons injected into the system during the GeV flare.

Description: We report the results of a spectral and timing analysis of the poorly studied transient X-ray pulsar 2S 1553–542 using data collected with the NuSTAR and Chandra observatories and the Fermi /GBM instrument during an outburst in 2015. The properties of the source at high energies (〉30 keV) are studied for the first time and the sky position has been essentially improved. The source broad-band spectrum has a quite complicated shape and can be reasonably described by a composite model with two continuum components – a blackbody emission with the temperature about 1 keV at low energies and a power law with an exponential cut-off at high energies. Additionally, an absorption feature at ~23.5 keV is discovered both in phase-averaged and phase-resolved spectra and interpreted as the cyclotron resonance scattering feature corresponding to the magnetic field strength of the neutron star B  ~ 3  x  10 12  G. Based on the Fermi /GBM data, the orbital parameters of the system were substantially improved, which allowed us to determine the spin period of the neutron star P  = 9.27880(3) s and a local spin-up $\dot{P} \simeq -7.5 \times 10^{-10}$  s s –1 due to the mass accretion during the NuSTAR observations. Assuming accretion from the disc and using standard torque models, we estimated the distance to the system as d  = 20 ± 4 kpc.

Description: 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 Ms 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 10 with the faintest tail at the top of the so-called flaring branch of its colour–colour 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 disc, 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 disc. We estimate cooling time for these energetic electrons and show that they cannot 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.

Description: Here, we report on observations of two hard X-ray sources that were originally discovered with the INTEGRAL satellite: IGR J04059+5416 and IGR J08297–4250. We use the Chandra X-ray Observatory to localize the sources and then archival near-IR images to identify the counterparts. Both sources have counterparts in the catalogue of extended 2 Micron All-Sky Survey sources, and the counterpart to IGR J04059+5416 has been previously identified as a galaxy. Thus, we place IGR J04059+5416 in the class of active galactic nuclei (AGN), and we suggest that IGR J08297–4250 is also an AGN. If this identification is correct, the near-IR images suggest that the host galaxy of IGR J08297–4250 may be merging with a smaller nearby galaxy. For IGR J04059+5416, the 0.3–86 keV spectrum from Chandra and INTEGRAL is consistent with an absorbed power-law with a column density of $N_{\rm H} = (3.1^{+2.0}_{-1.5})\times 10^{22}$  cm –2 and a photon index of  = 1.4 ± 0.7, and we suggest that it is a Seyfert galaxy. For IGR J08297–4250, the photon index is similar,  = 1.5 ± 0.8, but the source is highly absorbed ( $N_{\rm H} = (6.1^{+10.1}_{-4.3})\times 10^{23}$  cm –2 ).

Description: We present NuSTAR spectral and timing studies of the supergiant fast X-ray transient (SFXT) IGR J17544–2619. The spectrum is well described by an ~1 keV blackbody and a hard continuum component, as expected from an accreting X-ray pulsar. We detect a cyclotron line at 17 keV, confirming that the compact object in IGR J17544–2619 is indeed a neutron star. This is the first measurement of the magnetic field in an SFXT. The inferred magnetic field strength, B = (1.45 ± 0.03) x 10 12 G (1 + z ) is typical of neutron stars in X-ray binaries, and rules out a magnetar nature for the compact object. We do not find any significant pulsations in the source on time-scales of 1–2000 s.