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: 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 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 studied time variability and spectral evolution of the Galactic black hole transient Swift J174510.8–262411 during the first phase of its outburst. INTEGRAL and Swift observations collected from 2012 September 16 until October 30 have been used. The total squared fractional rms values did not drop below 5 per cent and quasi-periodic oscillations (QPOs), when present, were type-C, indicating that the source never made the transition to the soft-intermediate state. Even though the source was very bright (up to 1 Crab in hard X-rays), it showed a so called failed outburst as it never reached the soft state. XRT and IBIS broad-band spectra, well represented by a hybrid thermal/non-thermal Comptonization model, showed physical parameters characteristic of the hard and intermediate states. In particular, the derived temperature of the geometrically thin disc blackbody was about 0.6 keV at maximum. We found a clear decline of the optical depth of the corona electrons (close to values of 0.1), as well as of the total compactness ratio h / s . The hard-to-hard/intermediate state spectral transition is mainly driven by the increase in the soft photon flux in the corona, rather than small variations of the electron heating. This, associated with the increasing of the disc temperature, is consistent with a disc moving towards the compact object scenario, i.e. the truncated-disc model. Moreover, this scenario is consistent with the decreasing fractional squared rms and increasing of the noise and QPO frequency. In our final group of observations, we found that the contribution from the non-thermal Comptonization to the total power supplied to the plasma is $0.59^{+0.02}_{-0.05}$ and that the thermal electrons cool to kT e 〈 26 keV.

Description: We present a new way of describing the flares from Sgr A* with a self-consistent calculation of the particle distribution. All relevant radiative processes are taken into account in the evolution of the electron distribution and resulting spectrum. We present spectral modelling for new X-ray flares observed by NuSTAR , together with older observations in different wavelengths, and discuss the changes in plasma parameters to produce a flare. We show that under certain conditions, the real particle distribution can differ significantly from standard distributions assumed in most studies. We conclude that the flares are likely generated by magnetized plasma consistent with our understanding of the accretion flow. Including non-thermal acceleration, injection, escape, and cooling losses produces a spectrum with a break between the infrared and the X-ray, allowing a better simultaneous description of the different wavelengths. We favour the non-thermal synchrotron interpretation, assuming the infrared flare spectrum used is representative. We also consider the effects on Sgr A*'s quiescent spectrum in the case of a density increase due to the G2 encounter with Sgr A*.

Description: We characterized the broad-band X-ray spectra of Swift J1745–26 during the decay of the 2013 outburst using INTEGRAL ISGRI, JEM-X and Swift XRT. The X-ray evolution is compared to the evolution in optical and radio. We fit the X-ray spectra with phenomenological and Comptonization models. We discuss possible scenarios for the physical origin of an ~50 d flare observed both in optical and X-rays ~170 d after the peak of the outburst. We conclude that it is a result of enhanced mass accretion in response to an earlier heating event. We characterized the evolution in the hard-X-ray band and showed that for the joint ISGRI–XRT fits, the e-folding energy decreased from 350 to 130 keV, while the energy where the exponential cut-off starts increased from 75 to 112 keV as the decay progressed. We investigated the claim that high-energy cut-offs disappear with the compact jet turning on during outburst decays, and showed that spectra taken with HEXTE on RXTE provide insufficient quality to characterize cut-offs during the decay for typical hard-X-ray fluxes. Long INTEGRAL monitoring observations are required to understand the relation between the compact jet formation and hard-X-ray behaviour. We found that for the entire decay (including the flare), the X-ray spectra are consistent with thermal Comptonization, but a jet synchrotron origin cannot be ruled out.

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 results from the spectral fitting of the candidate black hole X-ray binary Swift J1753.5-0127 in an accretion state previously unseen in this source. We fit the 0.7–78 keV spectrum with a number of models, however the preferred model is one of a multitemperature disc with an inner disc temperature k T in  = 0.252 ± 0.003 keV scattered into a steep power-law with photon index $\Gamma =6.39^{+0.08}_{-0.02}$ and an additional hard power-law tail ( = 1.79 ± 0.02). We report on the emergence of a strong disc-dominated component in the X-ray spectrum and we conclude that the source has entered the soft state for the first time in its ~10 yr prolonged outburst. Using reasonable estimates for the distance to the source (3 kpc) and black hole mass (5 M ), we find the unabsorbed luminosity (0.1–100 keV) to be 0.60 per cent of the Eddington luminosity, making this one of the lowest luminosity soft states recorded in X-ray binaries. We also find that the accretion disc extended towards the compact object during its transition from hard to soft, with the inner radius estimated to be $R_{\mathrm{in}}=28.0^{+0.7}_{-0.4} R_g$ or ~12 R g , dependent on the boundary condition chosen, assuming the above distance and mass, a spectral hardening factor f  = 1.7 and a binary inclination i  = 55°.

Description: We have investigated the complex multiwavelength evolution of GRO J1655–40 during the rise of its 2005 outburst. We detected two hard X-ray flares, the first one during the transition from the soft state to the ultra-soft state, and the second one in the ultra-soft state. The first X-ray flare coincided with an optically thin radio flare. We also observed a hint of increased radio emission during the second X-ray flare. To explain the hard flares without invoking a secondary emission component, we fit the entire data set with the eqpair model. This single, hybrid Comptonization model sufficiently fits the data even during the hard X-ray flares if we allow reflection fractions greater than unity. In this case, the hard X-ray flares correspond to a Comptonizing corona dominated by non-thermal electrons. The fits also require absorption features in the soft and ultra-soft state which are likely due to a wind. In this work we show that the wind and the optically thin radio flare co-exist. Finally, we have also investigated the radio to optical spectral energy distribution, tracking the radio spectral evolution through the quenching of the compact jet and rise of the optically thin flare, and interpreted all data using state transition models.

Description: We report on FORS2 (FOcal Reducer/low dispersion Spectrograph 2) spectroscopy aiming at the identification of four Galactic plane sources discovered by INTEGRAL , IGR J18088–2741, IGR J18381–0924, IGR J17164–3803, and IGR J19173+0747, complemented by XMM–Newton spectroscopy for IGR J18381–0924. The presence of broad H i and He i emission lines and a flat Balmer decrement Hα/Hβ show that IGR J18088–2741 is a cataclysmic variable located beyond 8 kpc. For IGR J18381–0924, the detection of redshifted Hα and O i emission signatures and the absence of narrow forbidden emission lines point towards a low-luminosity Seyfert 1.9 nature at z = 0.031 ± 0.002. Its XMM–Newton spectrum, best fitted by an absorbed = 1.19 ± 0.07 power law combined with a $z=0.026_{-0.008}^{+0.016}$ redshifted iron emission feature, is in agreement with this classification. The likely IGR J17164–3803 optical counterpart is an M2 III star at 3–4 kpc which, based on the X-ray spectrum of the source, is the companion of a white dwarf in an X-ray faint symbiotic system. Finally, we challenge the accepted identification of IGR J19173+0747 as a high-mass X-ray binary. Indeed, the USNO optical counterpart is actually a blend of two objects located at the most likely 3 kpc distance, both lying within the error circle of the Swift position. The first is a cataclysmic variable, which we argue is the real nature of IGR J19173+0747. However, we cannot rule out the second one which we identify as an F3 V star which, if associated with IGR J19173+0747, likely belongs to a quiescent X-ray binary.