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Register a new userThick-disc model to explain the spectral state transition in NGC 247
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Jing Guo
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We propose the thick-disc model of Gu et al. 2016 to interpret the transition between soft ultraluminous state (SUL) and supersoft ultraluminous (SSUL) state in NGC 247. As accretion rate increases, the inner disc will puff up and act as shield to block the innermost X-ray emission regions and absorb both soft and hard X-ray photons. The absorbed X-ray emission will be re-radiated as a much softer blackbody X-ray spectrum. Hence NGC 247 shows flux dips in the hard X-ray band and transits from the SUL state to the SSUL state. The $sim 200$s transition timescale can be explained by the viscous timescale. According to our model, the inner disc in the super-soft state is thicker and has smaller viscous timescale than in the soft state. X-ray flux variability, which is assumed to be driven by accretion rate fluctuations, might be viscous time-scale invariant. Therefore, in the SSUL state, NGC 247 is more variable. The bolometric luminosity is saturated in the thick disc; the observed radius-temperature relation can therefore be naturally explained.
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Soft Ultra-Luminous X-ray (ULXs) sources are a subclass of the ULXs that can switch from a supersoft spectral state, where most of the luminosity is emitted below 1 keV, to a soft spectral state with significant emission above 1 keV. In a few systems, dips have been observed. The mechanism behind this state transition and the dips nature are still debated. To investigate these issues, we obtained a long XMM-Newton monitoring campaign of a member of this class, NGC 247 ULX-1. We computed the hardness-intensity diagram for the whole dataset and identified two different branches: the normal branch and the dipping branch, which we study with four and three hardness-intensity resolved spectra, respectively. All seven spectra are well described by two thermal components: a colder ($kT_{rm bb}$ $sim$ 0.1-0.2 keV) black-body, interpreted as emission from the photo-sphere of a radiatively-driven wind, and a hotter ($kT_{rm disk}$ $sim$ 0.6 keV) multicolour disk black-body, likely due to reprocessing of radiation emitted from the innermost regions. In addition, a complex pattern of emission and absorption lines has been taken into account based on previous high-resolution spectroscopic results. We studied the evolution of spectral parameters and the flux of the two thermal components along the two branches and discuss two scenarios possibly connecting the state transition and the dipping phenomenon. One is based on geometrical occultation of the emitting regions, the other invokes the onset of a propeller effect.
(Abridged) We have used the atmospheric parameters, [alpha/Fe] abundances and radial velocities, determined from the Gaia-ESO Survey GIRAFFE spectra of FGK-type stars (iDR1), to provide a chemo-kinematical characterisation of the disc stellar populations. We focuss on a subsample of 1016 stars with high quality parameters, covering the volume |Z|<4.5kpc and R in the range 2-13kpc. We have identified a thin to thick disc separation in the [alpha/Fe] vs [M/H] plane, thanks to the presence of a low-density region in the number density distribution. The thick disc stars seem to lie in progressively thinner layers above the Galactic plane, as metallicity increases and [alpha/Fe] decreases. The thin disc population presents a constant value of the mean distance to the plane at all metallicities. Our data confirm the already known correlations between V_phi and [M/H] for the two discs. For the thick disc sequence, a study of the possible contamination by thin disc stars suggests a gradient up to 64km/s/dex. The distributions of V_phi, V_Z, and orbital parameters are analysed for the chemically separated samples. Concerning the gradients with galactocentric radius, we find for the thin disc a flat behaviour of V_phi, a [M/H] gradient of -0.058dex/kpc and a small positive [alpha/Fe] gradient. For the thick disc, flat gradients in [M/H] and [alpha/Fe] are derived. Our chemo-kinematical analysis suggests a picture in which the thick disc seems to have experienced a settling process, during which its rotation increased progressively, and, possibly, the V_phi dispersion decreased. At [M/H]-0.25dex and [alpha/Fe]0.1dex, the mean characteristics of the thick disc in distance to the Galactic plane, V_phi, V_phi dispersion and eccentricity agree with those of the thin disc stars, suggesting a possible connection between these populations at a certain epoch of the disc evolution.
Most ultraluminous X-ray sources (ULXs) are believed to be stellar mass black holes or neutron stars accreting beyond the Eddington limit. Determining the nature of the compact object and the accretion mode from broadband spectroscopy is currently a challenge, but the observed timing properties provide insight into the compact object and details of the geometry and accretion processes. Here we report a timing analysis for an 800 ks XMM-Newton campaign on the supersoft ultraluminous X-ray source, NGC 247 ULX-1. Deep and frequent dips occur in the X-ray light curve, with the amplitude increasing with increasing energy band. Power spectra and coherence analysis reveals the dipping preferentially occurs on $sim 5$ ks and $sim 10$ ks timescales. The dips can be caused by either the occultation of the central X-ray source by an optically thick structure, such as warping of the accretion disc, or from obscuration by a wind launched from the accretion disc, or both. This behaviour supports the idea that supersoft ULXs are viewed close to edge-on to the accretion disc.
Many Seyfert galaxies are known to exhibit Fe-K broad emission line features in their X-ray energy spectra. The observed lines have three distinct features; (1) the line profiles are skewed and show significant low-energy tails, (2) the Fe-K band have low variability, which produces a broad and deep dip in the root-mean-square (rms) spectra, and (3) photons in this band have time lags behind those in the adjacent energy bands with amplitudes of several $R_g/c$, where $R_g$ is the gravitational radius. The relativistic light bending model is proposed to explain these observed features, where a compact X-ray source (lamp post) above an extreme Kerr black hole illuminates the innermost area of the accretion disc. In this paper, we critically examine the relativistic light bending model by computing the rms spectra and the lag features using a ray-tracing technique, when a lamp post moves vertically on the black hole spin axis. As a result, we found that the observed deep rms dip requires that the iron is extremely overabundant ($gtrsim10$ solar), whereas the observed lag amplitude is consistent with the normal iron abundance. Furthermore, disappearance of the lag in the high-flux state requires a source height as high as $sim40,R_g$, which contradicts the relativistically broad emission line feature. Our simulations agree with the data that the reverberation feature moves to lower frequencies with larger source height, however, if this scenario is correct, the simulations predict detection of a clear Fe-K lag at low frequencies, which is not constrained in the data. Therefore, we conclude that the relativistic light bending model may not explain the characteristic Fe-K spectral variations in Seyfert galaxies.
We search for LBV stars in galaxies outside the Local Group. Here we present a study of two bright $Halpha$ sources in the NGC 247 galaxy. Object j004703.27-204708.4 ($M_V=-9.08 pm 0.15^m$) shows the spectral lines typical for well-studied LBV stars: broad and bright emission lines of hydrogen and helium He I with P Cyg profiles, emission lines of iron Fe II, silicon Si II, nitrogen N II and carbon C II, forbidden iron [Fe II] and nitrogen [N II] lines. The variability of the object is $Delta B = 0.74pm0.09^m$ and $Delta V = 0.88pm0.09^m$, which makes it reliable LBV candidate. The star j004702.18-204739.93 ($M_V=-9.66 pm 0.23^m$) shows many emission lines of iron Fe II, forbidden iron lines [Fe II], bright hydrogen lines with broad wings, and also forbidden lines of oxygen [O I] and calcium [Ca II] formed in the circumstellar matter. The study of the light curve of this star also did not reveal significant variations in brightness ($Delta V = 0.29pm0.09^m$). We obtained estimates of interstellar absorption, the photosphere temperature, as well as bolometric magnitudes $M_text{bol}=-10.5^{+0.5}_{-0.4}$ and $M_text{bol}=-10.8^{+0.5}_{-0.6}$, which corresponds to bolometric luminosities $log(L_text{bol}/L_{odot})=6.11^{+0.20}_{-0.16}$ and $6.24^{+0.20}_{-0.25}$ for j004703.27-204708.4 and j004702.18-204739.93 respectively. Thus, the object j004703.27-204708.4 remains a reliable LBV candidate, while the object j004702.18-204739.93 can be classified as B[e]-supergiant.
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