Ultraluminous X-ray sources are extragalactic, off-nucleus, point sources in galaxies, and have X-ray luminosities in excess of 3 × 10(39) ergs per second. They are thought to be powered by accretion onto a compact object. Possible explanations include accretion onto neutron stars with strong magnetic fields, onto stellar-mass black holes (of up to 20 solar masses) at or in excess of the classical Eddington limit, or onto intermediate-mass black holes (10(3)-10(5) solar masses). The lack of sufficient energy resolution in previous analyses has prevented an unambiguous identification of any emission or absorption lines in the X-ray band, thereby precluding a detailed analysis of the accretion flow. Here we report the presence of X-ray emission lines arising from highly ionized iron, oxygen and neon with a cumulative significance in excess of five standard deviations, together with blueshifted (about 0.2 times light velocity) absorption lines of similar significance, in the high-resolution X-ray spectra of the ultraluminous X-ray sources NGC 1313 X-1 and NGC 5408 X-1. The blueshifted absorption lines must occur in a fast-outflowing gas, whereas the emission lines originate in slow-moving gas around the source. We conclude that the compact object in each source is surrounded by powerful winds with an outflow velocity of about 0.2 times that of light, as predicted by models of accreting supermassive black holes and hyper-accreting stellar-mass black holes.
There have recently been several reports of apparently periodic variations in the light curves of quasars, e.g. PG 1302−102 by Graham et al. (2015a). Any quasar showing periodic oscillations in brightness would be a strong candidate to be a close binary supermassive black hole and, in turn, a candidate for gravitational wave studies. However, normal quasars -powered by accretion onto a single, supermassive black hole -usually show stochastic variability over a wide range of timescales. It is therefore important to carefully assess the methods for identifying periodic candidates from among a population dominated by stochastic variability. Using a Bayesian analysis of the light curve of PG 1302−102, we find that a simple stochastic process is preferred over a sinusoidal variations. We then discuss some of the problems one encounters when searching for rare, strictly periodic signals among a large number of irregularly sampled, stochastic time series, and use simulations of quasar light curves to illustrate these points. From a few thousand simulations of steep spectrum ('red noise') stochastic processes, we find many simulations that display few-cycle periodicity like that seen in PG 1302−102. We emphasise the importance of calibrating the false positive rate when the number of targets in a search is very large.
In recent work with high-resolution grating spectrometers (RGS) aboard XMM-Newton Pinto et al. (2016) have discovered that two bright and archetypal ultraluminous X-ray sources (ULXs) have strong relativistic winds in agreement with theoretical predictions of high accretion rates. It has been proposed that such winds can become optically thick enough to block and reprocess the disk X-ray photons almost entirely, making the source appear as a soft thermal emitter or ultraluminous supersoft Xray source (ULS). To test this hypothesis we have studied a ULX where the wind is strong enough to cause significant absorption of the hard X-ray continuum: NGC 55 ULX. The RGS spectrum of NGC 55 ULX shows a wealth of emission and absorption lines blueshifted by significant fractions of the light speed (0.01 − 0.20)c indicating the presence of a powerful wind. The wind has a complex dynamical structure with the ionization state increasing with the outflow velocity, which may indicate launching from different regions of the accretion disk. The comparison with other ULXs such as NGC 1313 X-1 and NGC 5408 X-1 suggests that NGC 55 ULX is being observed at higher inclination. The wind partly absorbs the source flux above 1 keV, generating a spectral drop similar to that observed in ULSs. The softening of the spectrum at lower (∼ Eddington) luminosities and the detection of a soft lag agree with the scenario of wind clumps crossing the line of sight, partly absorbing and reprocessing the hard X-rays from the innermost region.
We present a detailed X-ray timing analysis of the highly variable NLS1 galaxy, IRAS 13224-3809. The source was recently monitored for 1.5 Ms with XMM-Newton which, combined with 500 ks archival data, makes this the best studied NLS1 galaxy in X-rays to date. We apply standard time-and Fourier-domain techniques in order to understand the underlying variability process. The source flux is not distributed lognormally, as expected for all types of accreting sources. The first non-linear rms-flux relation for any accreting source in any waveband is found, with rms ∝ flux 2/3 . The light curves exhibit significant strong non-stationarity, in addition to that caused by the rms-flux relation, and are fractionally more variable at lower source flux. The power spectrum is estimated down to ∼ 10 −7 Hz and consists of multiple peaked components: a lowfrequency break at ∼ 10 −5 Hz, with slope α < 1 down to low frequencies; an additional component breaking at ∼ 10 −3 Hz. Using the high-frequency break we estimate the black hole mass M BH = [0.5 − 2] × 10 6 M , and mass accretion rate in Eddington units, m Edd ∼ > 1. The broadband PSD and accretion rate make IRAS 13224-3809 a likely analogue of Very-high/Intermediate state black hole X-ray binaries. The non-stationarity is manifest in the PSD with the normalisation of the peaked components increasing with decreasing source flux, as well as the low-frequency peak moving to higher frequencies. We also detect a narrow coherent feature in the soft band PSD at 7×10 −4 Hz, modelled with a Lorentzian the feature has Q ∼ 8 and an rms ∼ 3 %. We discuss the implication of these results for accretion of matter onto black holes.
We report the detection of a 78.1 ± 0.5 day period in the X-ray light curve of the extreme ultraluminous X-ray source NGC 5907 ULX1 ( L X,peak ∼ 5 × 10 40 erg s−1), discovered during an extensive monitoring program with Swift. These periodic variations are strong, with the observed flux changing by a factor of ∼3–4 between the peaks and the troughs of the cycle; our simulations suggest that the observed periodicity is detected comfortably in excess of 3σ significance. We discuss possible origins for this X-ray period, but conclude that at the current time we cannot robustly distinguish between orbital and super-orbital variations.
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