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The soft X-ray excess in the spectra of active galactic nuclei is characterized by similar electron temperatures of 0.1–0.3 keV and similar photon indices around 2.2–3, if fitted with inverse Comptonization. It remains a puzzle why both values are not sensitive to the black hole mass nor the accretion rate. Supposing that the scattering-dominated surface layer of an accretion disk can act as a warm corona, we construct a vertical one-zone model to understand what determines its temperature. By solving the equations of (1) the condition for the effective optical depth, (2) the energy balance, and (3) the dominance of the Compton cooling over the bound–free cooling, we could reproduce the basic observational features of the soft excess, provided that anomalous heating (excess heating other than what is expected by local energy dissipation) takes place in the warm corona in agreement with similar studies done so far. The similar temperatures can be understood, since both the anomalous heating and Compton cooling rates are proportional to the dissipation rate of the accretion energy, while similar photon indices are a natural consequence of the fact that observed photons are finally emitted from the layer of Compton y ∼ 1. The soft excess is not observed in black hole binaries, since disk temperatures are too high for the Compton scattering to work as cooling. The derived temperatures are somewhat of an underestimation, however. This may indicate a necessity of multi-zone corona structure. The stability of the warm corona and its consequences are briefly discussed. [ABSTRACT FROM AUTHOR]

Radiation hydrodynamical equations with Compton scattering are presented in a convenient way; equations are available in the subrelativistic regime of |$k_{\rm B}T/(m_{\rm e}c^2) \lesssim 0.1$|⁠ , |$h\nu /(m_{\rm e}c^2) \lesssim 0.1$| (or |$k_{\rm B}T_{\rm rad}/(m_{\rm e}c^2) \lesssim 0.1$|⁠), and |$v/c \lesssim 0.1$|⁠ , where T is the electron temperature, ν the photon frequency, T rad the photon temperature, and v the fluid bulk velocity. Several basic concepts and radiation hydrodynamical phenomena under Compton scattering are briefly discussed; e.g. electron-scattering opacity and the Eddington luminosity, optical depth and a static atmosphere, the photon trapping radius, the magic terminal speed, the pseudo-photosphere of radiative winds, the explicit form of basic equations for steady flows, radiation acoustic wave, and so on. [ABSTRACT FROM AUTHOR]

We present analyses of an Intermediate Polar, IGR J15094-6649, based on the archival optical data obtained from the Transiting Exoplanet Survey Satellite (TESS) and X-ray data obtained from the Suzaku, NuSTAR , and Neil Gehrels Swift Observatory (Swift). Present analysis confirms and refines the previously reported spin period of IGR J15094-6649 as 809.49584 ± 0.00075 s. Clear evidence of a beat period of 841.67376 ± 0.00082 s is found during the long-term TESS optical observations, which was not evident in the earlier studies. The dominance of X-ray and optical spin pulse unveils the disc-fed dominance accretion, however, the presence of an additional beat frequency indicates that part of the accreting material also flows along the magnetic field lines. The energy-dependent spin pulsations in the low (<10 keV) energy band are due to the photoelectric absorption in the accretion flow. However, the complex absorbers may be responsible to produce low amplitude spin modulations via Compton scattering in the hard (>10 keV) energy band and indicate that the height of the X-ray emitting region may be negligible. The observed double-humped X-ray profiles with a pronounced dip are indicative of the photoelectric absorption in the intervening accretion stream. Analysis of the X-ray spectra reveals the complexity of the X-ray emission, being composed of multitemperature plasma components with a soft excess, reflection, and suffers from strong absorption. [ABSTRACT FROM AUTHOR]

Relativistic Fe K emission lines from accretion discs and from disc winds encode key information about black holes, and their accretion and feedback mechanisms. We show that these two processes can in principle produce indistinguishable line profiles, such that they cannot be disentangled spectrally. We argue that it is likely that in many cases both processes contribute to the net line profile, and their relative contributions cannot be constrained purely by Fe K spectroscopy. In almost all studies of Fe K emission to date, a single process (either disc reflection or wind Compton scattering) is assumed to dominate the total line profile. We demonstrate that fitting a single process emission model (pure reflection or pure wind) to a hybrid line profile results in large systematic biases in the estimates of key parameters, such as mass outflow rate and spin. We discuss various strategies to mitigate this effect, such as including high-energy data covering the Compton hump, and the implications for future X-ray missions. [ABSTRACT FROM AUTHOR]

We present the results of our study of the transient X-ray pulsar GRO J2058+42 in a wide energy range in a state with a luminosity erg s . We have found that the pulse profile of the source, along with the pulse fraction, changed significantly in comparison with the previous NuSTAR observations performed when the pulsar was brighter approximately by a factor of 10. The position of the cyclotron line at keV in a narrow phase interval is consistent with the observations in the high state. Spectral analysis has shown that at high luminosities erg s the spectrum has a shape typical of accreting pulsars, whereas a two-component model should be used to describe the spectrum when the luminosity drops approximately by an order of magnitude, to erg s . This behavior fits into the model in which the low-energy part of the spectrum is formed in a hot spot, while the high-energy one is formed as a result of resonant Compton scattering by the infalling matter in the accretion channel above the neutron star surface. [ABSTRACT FROM AUTHOR]

Irradiation of the accretion disc causes reflection signatures in the observed X-ray spectrum, encoding important information about the disc structure and density. A Type I X-ray burst will strongly irradiate the accretion disc and alter its properties. Previous numerical simulations predicted the evolution of the accretion disc due to an X-ray burst. Here, we process time-averaged simulation data of six time intervals to track changes in the reflection spectrum from the burst onset to just past its peak. We divide the reflecting region of the disc within r ≲ 50 km into six to seven radial zones for every time interval and compute the reflection spectra for each zone. We integrate these reflection spectra to obtain a total reflection spectrum per time interval. The burst ionizes and heats the disc, which gradually weakens all emission lines. Compton scattering and bremsstrahlung rates increase in the disc during the burst rise, and the soft excess at <3 keV rises from ≈4 to ≈38 per cent of the total emission at the burst peak. A soft excess is expected to be ubiquitous in the reflection spectra of X-ray bursts. Structural disc changes such as inflation because of heating or drainage of the inner disc due to Poynting–Robertson drag affect the strength of the soft excess. Further studies on the dependence of the reflection spectrum characteristics to changes in the accretion disc during an X-ray burst may lead to probes of the disc geometry. [ABSTRACT FROM AUTHOR]

We analytically study the dynamical and thermal properties of the optically thin gases at the parsec-scale when they are spherically accreted on to low luminous active galactic nuclei. The falling gases are irradiated by the central X-ray radiation with the Compton temperature of 5–15 × 107 K. The radiative heating/cooling and the bulge stellar potential in galaxies are taken into account. We analyse the effect of accretion rate, luminosity, gas temperature, and Compton temperature on steady solutions of dynamical and thermal properties. The steady solutions are obviously different from Bondi solution. Compared to our models, the Bondi model underestimates the accretion rate. We give the boundary between thermal stability and instability. The boundary is significantly affected by Compton temperature. When Compton temperature is higher, the falling gases tend to become thermally unstable. When thermal instability takes place in the irradiated gases, the gases become two phases (i.e. hot gases and cool gases) and the hot gases may become outflows. This effect may reduce the accretion rates. [ABSTRACT FROM AUTHOR]

We develop a model describing the dynamical and observed properties of disc-dominated tidal disruption events (TDEs) around black holes with the lowest masses (M ≲ few × 106M⊙). TDEs around black holes with the lowest masses are most likely to reach super-Eddington luminosities at early times in their evolution. By assuming that the amount of stellar debris that can form into a compact accretion disc is set dynamically by the Eddington luminosity, we make a number of interesting and testable predictions about the observed properties of bright soft-state X-ray TDEs and optically bright, X-ray dim TDEs. We argue that TDEs around black holes of the lowest masses will expel the vast majority of their gravitationally bound debris into a radiatively driven outflow. A large-mass outflow will obscure the innermost X-ray producing regions, leading to a population of low black hole mass TDEs that are only observed at optical and UV energies. TDE discs evolving with bolometric luminosities comparable to their Eddington luminosity will have near constant (i.e. black hole mass independent) X-ray luminosities, of order L X, max ≡ LM ∼ 1043 − 1044 erg s−1. The range of luminosity values stems primarily from the range of allowed black hole spins. A similar X-ray luminosity limit exists for X-ray TDEs in the hard (Compton scattering dominated) state, and we therefore predict that the X-ray luminosity of the brightest X-ray TDEs will be at the scale LM (a) ∼ 1043 − 1044 erg s−1, independent of black hole mass and accretion state. These predictions are in strong agreement with the properties of the existing population (∼40 sources) of observed TDEs. [ABSTRACT FROM AUTHOR]

We extend the relativistic time-dependent thin-disc TDE model to describe non-thermal (2−10 keV) X-ray emission produced by the Compton up-scattering of thermal disc photons by a compact electron corona, developing analytical and numerical models of the evolving non-thermal X-ray light curves. In the simplest cases, these X-ray light curves follow power-law profiles in time. We suggest that TDE discs act in many respects as scaled-up versions of XRB discs, and that such discs should undergo state transitions into harder accretion states. XRB state transitions typically occur when the disc luminosity becomes roughly one per cent of its Eddington value. We show that if the same is true for TDE discs then this, in turn, implies that TDEs with non-thermal X-ray spectra should come preferentially from large-mass black holes. The characteristic hard-state transition mass is M HS ≃ 2 × 107M⊙. Hence, subpopulations of thermal and non-thermal X-ray TDEs should come from systematically different black hole masses. We demonstrate that the known populations of thermal and non-thermal X-ray TDEs do indeed come from different distributions of black hole masses. The null-hypothesis of identical black hole mass distributions is rejected by a two-sample Anderson-Darling test with a p -value <0.01. Finally, we present a model for the X-ray rebrightening of TDEs at late times as they transition into the hard state. These models of evolving TDE light curves are the first to join both thermal and non-thermal X-ray components in a unified scenario. [ABSTRACT FROM AUTHOR]

We examined the behaviour of blazar OJ 287 based on 12 year observation data in optic, UV, X-ray, and gamma ray. The optical, UV, and X-ray data of OJ 287 were obtained from Swift archive, which provide more or less simultaneous optical–X-ray observations, while gamma ray data were obtained from Fermi archive. The occurrence of outburst episode in 2007 can be confirmed in Swift/UVOT data. In addition, episodes of significant brightening (possibly the outburst(s) of this decade) can also be seen in 2015–2016 and 2016–2017. Long-term flux variability in gamma ray and X-ray data is not always concurrent with those of optical and UV. Interestingly, the high peaks appeared almost simultaneous in optic, UV and X-ray in the episode of brightening in 2016–2017. X-ray spectral fittings show that non-thermal process predominates most of the X-ray spectra. Simultaneous optical–X-ray spectral fittings show the spectral evolution and suggest the combination of thermal and non-thermal component before brightening episode in 2015–2016. We obtained gamma ray spectra with broader peak at rather high frequency compared to that expected from simple synchrotron self-Compton mechanism. During brightening 2016–2017, the optical–X-ray fluxes are higher compared to those other periods. The shape of the X-ray and gamma ray spectra also have inverted trend than those of other periods. [ABSTRACT FROM AUTHOR]

Spectral-timing analysis of the fast variability observed in X-rays is a powerful tool to study the physical and geometrical properties of the accretion/ejection flows in black hole (BH) binaries. The origin of type-B quasi-periodic oscillations (QPO), predominantly observed in BH candidates in the soft-intermediate state, has been linked to emission arising from the relativistic jet. In this state, the X-ray spectrum is characterized by a soft-thermal blackbody-like emission due to the accretion disc, an iron emission line (in the 6–7 keV range), and a power-law-like hard component due to inverse-Compton scattering of the soft-photon source by hot electrons in a corona or the relativistic jet itself. The spectral-timing properties of MAXI J1348−630 have been recently studied using observations obtained with the NICER observatory. The data show a strong type-B QPO at ∼4.5 Hz with increasing fractional rms amplitude with energy and positive lags with respect to a reference band at 2–2.5 keV. We use a variable-Comptonization model that assumes a sinusoidal coherent oscillation of the Comptonized X-ray flux and the physical parameters of the corona at the QPO frequency, to fit simultaneously the energy-dependent fractional rms amplitude and phase lags of this QPO. We show that two physically connected Comptonization regions can successfully explain the radiative properties of the QPO in the full 0.8–10 keV energy range. [ABSTRACT FROM AUTHOR]

A fundamental difference between a neutron star (NS) and a black hole (BH) is the absence of a physical surface in the latter. For this reason, any remaining kinetic energy of the matter accreting on to a BH is advected inside its event horizon. In the case of an NS, on the contrary, accreting material is decelerated on the NS surface, and its kinetic energy is eventually radiated away. Copious soft photons produced by the NS surface will affect the properties of the Comptonized component dominating spectra of X-ray binaries in the hard state. Thus, parameters of the Comptonized spectra – the electron temperature kT e and the Compton y -parameter, could serve as an important tool for distinguishing BHs from NSs. In this paper, we systematically analyse heretofore the largest sample of spectra from the BH and NS X-ray binaries in the hard state for this purpose, using archival RXTE /PCA and RXTE /HEXTE observations. We find that the BHs and NSs occupy distinctly different regions in the y − kT e plane with NSs being characterized by systematically lower values of y -parameter and electron temperature. Due to the shape of the boundary between BHs and NSs on the y − kT e plane, their 1D y and kT e distributions have some overlap. A cleaner one parameter diagnostic of the nature of the compact object in X-ray binaries is provided by the Compton amplification factor A , with the boundary between BHs and NSs lying at A ≈ 3.5–4. This is by far the most significant detection of the imprint of the event horizon on the X-ray spectra for stable stellar-mass BHs. [ABSTRACT FROM AUTHOR]

We study reflected X-ray emission that returns to the accretion disc in the strong gravitational fields around black holes using General Relativistic ray-tracing and radiative transfer calculations. Reflected X-rays that are produced when the inner regions of the disc are illuminated by the corona are subject to strong gravitational light bending, causing up to 47 per cent of the reflected emission to be returned to the disc around a rapidly spinning black hole, depending upon the scale height of the corona. The iron Kα line is enhanced relative to the continuum by 25 per cent, and the Compton hump by up to a factor of 3. Additional light traveltime between primary and secondary reflections increases the reverberation time lag measured in the iron K band by 49 per cent, while the soft X-ray lag is increased by 25 per cent and the Compton hump response time is increased by 60 per cent. Measured samples of X-ray reverberation lags are shown to be consistent with X-rays returning to the accretion disc in strong gravity. Understanding the effects of returning radiation is important in interpreting reverberation observations to probe black holes. Reflected X-rays returning to the disc can be uniquely identified by blueshifted returning iron K line photons that are Compton scattered from the inner disc, producing excess, delayed emission in the 3.5–4.5 keV energy range that will be detectable with forthcoming X-ray observatories, representing a unique test of General Relativity in the strong field limit. [ABSTRACT FROM AUTHOR]

Aims. We investigate a two-temperature advective transonic accretion disc around a black hole and analyse its spectrum in the presence of radiative processes such as bremsstrahlung, synchrotron, and inverse-Comptonisation. The aim is to link the emergent spectra with constants of motion of the accretion disc fluid, however, the number of unknowns in two-temperature theory exceeds the number of equations for a given set of constants of motion. We intend to remove the degeneracy using a general methodology and obtain a unique solution, along with its spectrum. Methods. We used hydrodynamic equations (continuity, momentum, and energy conservation equation) to obtain sonic points and solutions. To solve these equations of motion we used the 4th order Runge-Kutta method. For the spectral analysis, general and special relativistic effects were taken into consideration. The system is, nonetheless, degenerate and we remove the degeneracy by choosing the solution with maximum entropy, as dictated by the second law of thermodynamics. Results. We obtained a unique transonic solution for a given set of constants of motion. The entropy expression is a tool used to make a selection between the degenerate solutions. We found that Coulomb coupling is a weak energy exchange term, which allows protons and electrons to settle down into two different temperatures, justifying, hence, our study of two-temperature flows. The information of the electron flow allows us to model the spectra. We show that the spectra of accretion solutions depend on the associated constants of motion. At low accretion rates, bremsstrahlung is important. A fraction of the bremsstrahlung photons may be of higher energy than the neighbouring electrons, energising them through the process of Compton scattering. Synchrotron emission, on the other hand, provides soft photons, which can be inverse-Comptonised to produce a hard power law part in the spectrum. Luminosity increases with the increase in the accretion rate of the system, as well as with the increase in BH mass. However, the radiative efficiency of the flow has almost no dependence on the BH mass, but it sharply rises with the increase in the accretion rate. The spectral index, however, hardens with the increase in the accretion rate, while it does not change much with the variation in BH mass. In addition to the constants of motion, the value of the plasma beta parameter and magnitude of magnetic dissipation in the system also helps in shaping the spectrum. A shocked solution exists in two-temperature accretion flows in a limited region of the parameter space. We find that a shocked solution is always brighter than a solution without a shock. Conclusions. An accreting system in two-temperature regime admits multiple solutions for the same set of constants of motion, producing widely different spectra. Comparing the observed spectrum with that derived from a randomly chosen accretion solution would give us a wrong estimation of the accretion parameters of the system. The form of entropy measurement we obtained helped us to remove the degeneracy of the solutions and allowed us to understand the physics of the system, shorn of arbitrary assumptions. In this work, we show how the spectra and luminosities of an accreting system depend on the constants of motion, producing solutions ranging from radiatively inefficient flows to luminous flows. An increase in BH mass quantitatively changes the system, making the system more luminous, while the spectral bandwidth also increases. A higher BH mass system spans the range from radio to gamma-rays. However, increasing the accretion rate around a BH of certain mass has little influence on the frequency range of the spectra. [ABSTRACT FROM AUTHOR]

Recently, a retrograde neutron star is proposed for the classical wind-fed X-ray pulsar, GX 301-2, to explain the orbital spin-up to spin-down reversal near periastron, based on the stream model invoked to explain the pre-periastron flare of GX 301-2 previously. We study in detail three rare spin-up events detected by Fermi /GBM and find that the spin derivatives are correlated with the Swift /BAT fluxes, following a relation of |$\dot{\nu }\propto F^{0.75\pm 0.05}$|⁠. All the spin-up events of GX 301-2 started about 10 d after the periastron, which is the time needed for tidally stripped gas to reach the neutron star. The slow rotation of the optical companion implies that the accreted matter is likely to have angular momentum in the direction of the orbital motion, as in a Roche lobe-like overflow. As a result, the spin-up events of GX 301-2 would favour accretion of a prograde disc to a prograde neutron star. We also find that the flare of intrinsic X-ray emission of GX 301-2 happened 0.4 d before periastron, while the flare of low-energy emission (2–10 keV) happened about 1.4 d before periastron. The preceding low-energy flare can be explained by stronger absorption of the intrinsic X-ray emission closer to the periastron. This finding weakened the need of the stream model. The pulse fraction of GX 301-2 near periastron is reduced heavily, which is likely caused by Compton-scattering process. Compton reflection from the optical companion might be responsible for the observed orbital spin reversal of GX 301-2. [ABSTRACT FROM AUTHOR]

We present NuSTAR observation of the atoll type neutron star (NS) low-mass X-ray binary (LMXB) Serpens X-1 (Ser X-1) performed on 2018 February 17. We observed Ser X-1 in a soft X-ray spectral state with 3–79 keV luminosity of L X ∼ 0.4 × 1038 erg s−1 (⁠|${\sim}23{{\ \rm per\ cent}}$| of the Eddington luminosity), assuming a distance of 7.7 kpc. A positive correlation between intensity and hardness ratio suggests that the source was in the banana branch during this observation. The broad-band 3–30 keV NuSTAR energy spectrum can be well described either by a three-component continuum model consisting of a disc blackbody, a single temperature blackbody, and a power law or by a two-component continuum model consisting of a disc blackbody and a Comptonization component. A broad iron line ∼5–8 keV and the Compton back-scattering hump peaking at ∼10–20 keV band are clearly detected in the X-ray spectrum. These features are best interpreted by a self-consistent relativistic reflection model. Fits with relativistically blurred disc reflection model suggests that the inner disc radius R in is truncated prior to the ISCO at (1.9–2.5)  R ISCO (⁠|${\simeq}11.4\!-\!15\, R_{g}\: \text{or}\: 26\!-\!34$| km) and the accretion disc is viewed at a low inclination of i ≃ 16°−20°. The disc is likely to be truncated either by a boundary layer or by the magnetosphere. Based on the measured flux and the mass accretion rate, the maximum radial extension for the boundary layer is estimated to be ∼6.4  Rg from the NS surface. The truncated inner disc in association with pressure from a magnetic field sets an upper limit of B ≤ 1.9 × 109 G. [ABSTRACT FROM AUTHOR]

We have obtained new solutions and methods for the process of thermal Comptonization. We modify the solution to the kinetic equation of Sunyaev and Titarchuk to allow its application up to mildly relativistic electron temperatures and optical depths |$\gtrsim {1}$|⁠. The solution can be used for spectral fitting of X-ray spectra from astrophysical sources. We also have developed an accurate Monte Carlo method for calculating spectra and timing properties of thermal Comptonization sources. The accuracy of our kinetic equation solution is verified by comparison with the Monte Carlo results. We also compare our results with those of other publicly available methods. Furthermore, based on our Monte Carlo code, we present distributions of the photon emission times and the evolution of the average photon energy for both up and down scattering. [ABSTRACT FROM AUTHOR]

SAX J1748.9−2021 is a transient accretion powered millisecond X-ray pulsar located in the globular cluster NGC 6440. We report on the spectral and timing analysis of SAX J1748.9−2021 performed on AstroSat data taken during its faint and short outburst of 2017. We derived the best-fitting orbital solution for the 2017 outburst and obtained an average local spin frequency of 442.361098(3) Hz. The pulse profile obtained from 3 to 7 and 7 to 20 keV energy bands suggest constant fractional amplitude ∼0.5 per cent for fundamental component, contrary to previously observed energy pulse profile dependence. Our AstroSat observations revealed the source to be in a hard spectral state. The 1–50 keV spectrum from SXT (Soft X-ray Telescope) and LAXPC (Large Area X-ray Proportional Counter) on-board AstroSat can be well described with a single temperature blackbody and thermal Comptonization. Moreover, we found that the combined spectra from XMM–Newton (EPIC-PN) and AstroSat (SXT + LAXPC) indicated the presence of reflection features in the form of iron (Fe Kα) line that we modelled with the reflection model xillvercp. One of the two X-ray burst observed during the AstroSat /LAXPC observation showed hard X-ray emission (>30 keV) due to Compton up-scattering of thermal photons by the hot corona. Time-resolved analysis performed on the bursts revealed complex evolution in emission radius of blackbody for second burst suggestive of mild photospheric radius expansion. [ABSTRACT FROM AUTHOR]

We present a detailed analysis of the spectral properties of the black hole transient GRS 1716−249, based on the archival Swift and NuSTAR observations taken during the outburst of this source in 2016–2017. The first six NuSTAR observations show that the source is in a canonical hard state, where the spectrum is dominated by a power-law continuum. The seventh NuSTAR observation is taken during the intermediate state where both a disc thermal component and a power-law continuum are shown. All of our observations show a broad emission-line feature in the iron band and a Compton hump above 10 keV. We model the broad-band spectra using a high-density disc reflection model, where the soft X-ray emission in the hard state is interpreted as part of the disc reflection component. This model enables us to constrain the disc density parameter of GRS 1716−249 in the range of 1019–1020 cm−3. We only obtain an upper limit of the inner disc radius using high-density disc reflection spectroscopy and the results indicate either a non-truncated disc or a slightly truncated disc with R in ≲ 20  r g. [ABSTRACT FROM AUTHOR]

Quasi-periodic oscillation (QPO), particularly of low frequency (LF), is a very obvious feature of outbursting black hole candidates. The association of QPOs in a specific spectral state and their transition with states make them a key ingredient in understanding the underlying physical processes that produce them. Observations have revealed that generally, in the hard spectral state of the outburst, the size of the Compton cloud is relatively bigger, which produces low-frequency QPOs (LFQPOs). In progressive days increased cooling shrinks the area of the cloud, the inner edge of the disc comes close to the black holes, and produces higher frequency QPOs. However, rotating black holes with higher spin values are likely to produce LFQPOs even if their inner edge of the disc is closer to the hole. Here, for the first time, we address the issue, solving hydrodynamic flow equations in the presence of qualitative turbulent pressure and cooling in pseudo-Kerr geometry. Increasing turbulence slackens the infalling flow, thus the infall time becomes longer, producing LFQPOs. Our study discovers that the effect of turbulence modifies LFQPOs value significantly, by a factor of a few lower throughout the angular momentum distribution of the flow. We find a strong correlation between the turbulence and the spin parameter of the hole. Finally, we discuss the observed results in light of the present solution. [ABSTRACT FROM AUTHOR]

Blueshifted absorption lines are seen in high inclination black hole binary systems in their disc-dominated states, showing these power an equatorial disc wind. While some contribution from magnetic winds remain a possibility, thermal and thermal-radiative winds are expected to be present. We show results from radiation hydrodynamic simulations that show that the additional radiation force from atomic features (bound–free and lines) is important along with electron scattering. Together, these increase the wind velocity at high inclinations, so that they quantitatively match the observations in H1743−322, unlike purely thermal winds that are too slow. We highlight the role played by shadowing of the outer disc from the (subgrid) inner disc Compton heated layer, and show that the increase in shadow from the higher Compton temperature after the spectral transition to the hard state leads to strong suppression of the wind. Thermal-radiative winds explain all of the spectral features (and their disappearance) in this simplest wind system and magnetic winds play only a minor role. We speculate that thermal-radiative winds can explain all the spectral features seen in the more complex (larger disc size) binaries, GRO J1655−40 and GRS 1915+105, without requiring magnetic winds. [ABSTRACT FROM AUTHOR]

We present here the timing and spectral analysis of the accreting millisecond pulsar (AMXP) Swift J1756.9−2508 during its recent outburst in 2018 using Swift and NUSTAR observations. The simultaneous fitting of the Swift and NUSTAR spectra indicates that the source was in the hard state with a cut-off energy of about 74.58 keV. We also study in detail the pulse profile of the AMXP and its dependence on energy. The colour–colour diagram of the source is different from those previously reported. We performed phase- and time-resolved spectral analysis using NUSTAR data. Pulse phase-resolved spectra were fitted with a power-law model and significant changes in the spectral parameters with pulse phase were observed. The orbital phase and time-resolved spectra were fitted with a cut-off power-law model. The column density and photon index obtained from orbital phase spectral analysis were found to show some anticorrelation with the flux. Through time-resolved spectral analysis, we observed that the spectral parameters show positive correlation with each other and with the flux. We do not observe a softening of the spectrum with time. No emission lines or Compton bump were observed in the spectrum of the AMXP. [ABSTRACT FROM AUTHOR]

MAXI J1820+070, a black hole candidate first detected in early 2018 March, was observed by XMM–Newton during the outburst rise. In this letter we report on the spectral and timing analysis of the XMM–Newton X-ray and UV data, as well as contemporaneous X-ray data from the Swift satellite. The X-ray spectrum is well described by a hard thermal Comptonization continuum. The XMM–Newton X-ray light curve shows a pronounced dipping interval, and spectral analysis indicates that it is caused by a moderately ionized partial covering absorber. The XMM–Newton /OM U -filter data do not reveal any signs of the 17 h orbital modulation that was seen later on during the outburst decay. The UV/X-ray cross-correlation function shows a complex shape, with a peak at positive lags of about 4 s and a precognition dip at negative lags, which is absent during the X-ray dipping episode. Such shape could arise if the UV emission comes partially from synchrotron self-Compton emission near the black hole, as well as from reprocessing of the X-rays in the colder accretion disc further out. [ABSTRACT FROM AUTHOR]

We report on a NuSTAR and Swift / XRT observation of the newly discovered X-ray transient MAXI J1820+070. Swift / XRT and NuSTAR have concurrently observed the newly detected source on 2018 March 14. We have simultaneously fitted the broad-band spectra obtained from Swift / XRT and NuSTAR. The observed joint spectra in the energy range 0.6–78.0 keV are well modeled with a weak disc black-body emission, dominant thermal Comptonization, and relativistic reflection fraction. We have detected a fluorescent Iron-Kα line relativistically broadened and a Compton hump at ∼30 keV. We constrain the inner disc radius as well as the disc inclination angle, and their values are found to be 4.1 |$^{+0.8}_{-0.6}$| R ISCO (where R ISCO ≡ radius of the innermost stable circular orbit) or 5.1 |$^{+1.0}_{-0.7}$| r g (where rg ≡ gravitational radius) and 29.8 |$^{+3.0}_{-2.7}$| °, respectively. The best-fitting broad-band spectra suggest that the source was in the hard state and evolving. The source emission is best described by weak thermal emission along with strong thermal Comptonization from a relatively cold, optically thick, geometrically thin and ionized accretion disc. X-ray spectral modeling helps us to understand the accretion and ejection properties in the vicinity of the compact object. [ABSTRACT FROM AUTHOR]

In this work, we present a spectral and temporal analysis of Swift /XRT and NuSTAR observations of GRS 1716–249 during its recent 2016–2017 outburst. This low-mass X-ray binary underwent an extraordinary outburst after a long quiescence of 23 yr, since its last major outburst in 1993. The source was observed over two different epochs during 2017 April 7 and 10. The best-fitting joint spectral fitting in the energy range 0.5–79.0 keV indicates that the spectrum is best described by relatively cold, weak disc blackbody emission, dominant thermal Comptonization emission, and a relativistically broadened fluorescent iron K  α emission line. We observed a clear indication of a Compton hump around 30 keV. We also detected an excess feature of 1.3 keV. Assuming a lamp-post geometry of the corona, we constrained the inner disc radius for both observations to 11.92 |$^{+8.62}_{-11.92}$| R ISCO (i.e. an upper limit) and 10.39 |$^{+9.51}_{-3.02}$| R ISCO (where R ISCO ≡ radius of the innermost stable circular orbit) for the first epoch (E1) and second epoch (E2), respectively. A significant (5σ) type C quasi-periodic oscillation (QPO) at 1.20 ± 0.04 Hz is detected for the first time for GRS 1716–249, which drifts to 1.55 ± 0.04 Hz (6σ) at the end of the second observation. The derived spectral and temporal properties show a positive correlation between the QPO frequency and the photon index. [ABSTRACT FROM AUTHOR]

GX 339–4 is a black hole X-ray binary that is a key focus of accretion studies, since it goes into outburst roughly every 2–3 yr. Tracking of its radio, infrared (IR), and X-ray flux during multiple outbursts reveals tight broad-band correlations. The radio emission originates in a compact, self-absorbed jet; however, the origin of the X-ray emission is still debated: jet base or corona? We fit 20 quasi-simultaneous radio, IR, optical, and X-ray observations of GX 339–4 covering three separate outbursts in 2005, 2007, 2010–2011, with a composite corona+jet model, where inverse Compton emission from both regions contributes to the X-ray emission. Using a recently proposed identifier of the X-ray variability properties known as power-spectral hue, we attempt to explain both the spectral and evolving timing characteristics, with the model. We find the X-ray spectra are best fit by inverse Compton scattering in a dominant hot corona (kT e ∼ hundreds of keV). However, radio and IR-optical constraints imply a non-negligible contribution from inverse Compton scattering off hotter electrons (kT e ≥ 511 keV) in the base of the jets, ranging from a few up to ∼50 per cent of the integrated 3–100 keV flux. We also find that the physical properties of the jet show interesting correlations with the shape of the broad-band X-ray variability of the source, posing intriguing suggestions for the connection between the jet and corona. [ABSTRACT FROM AUTHOR]

We study the properties of two-temperature accretion flow around a non-rotating black hole in presence of various dissipative processes where pseudo-Newtonian potential is adopted to mimic the effect of general relativity. The flow encounters energy loss by means of radiative processes acted on the electrons and at the same time, flow heats up as a consequence of viscous heating effective on ions. We assumed that the flow is exposed with the stochastic magnetic fields that leads to Synchrotron emission of electrons and these emissions are further strengthen by Compton scattering. We obtain the two-temperature global accretion solutions in terms of dissipation parameters, namely, viscosity (α) and accretion rate (m), and find for the first time in the literature that such solutions may contain standing shock waves. Solutions of this kind aremultitransonic in nature, as they simultaneously pass through both inner critical point (xin) and outer critical point (xout) before crossing the black hole horizon. We calculate the properties of shock-induced global accretion solutions in terms of the flow parameters.We further show that two-temperature shocked accretion flow is not a discrete solution, instead such solution exists for wide range of flow parameters.We identify the effective domain of the parameter space for standing shock and observe that parameter space shrinks as the dissipation is increased. Since the post-shock region is hotter due to the effect of shock compression, it naturally emits hard X-rays, and therefore, the two-temperature shocked accretion solution has the potential to explain the spectral properties of the black hole sources. [ABSTRACT FROM AUTHOR]

We model X-ray reflection from white dwarfs (WDs) in magnetic cataclysmic variables (mCVs) using a Monte Carlo simulation. A point source with a power-law spectrum or a realistic post-shock accretion column (PSAC) source irradiates a cool and spherical WD. The PSAC source emits thermal spectra of various temperatures stratified along the column according to the PSAC model. In the point-source simulation, we confirm the following: a source harder and nearer to the WD enhances the reflection; higher iron abundance enhances the equivalent widths (EWs) of fluorescent iron Kα1, 2 lines and their Compton shoulder, and increases the cut-off energy of a Compton hump; significant reflection appears from an area that is more than 90° apart from the position right under the point X-ray source because of theWD curvature. The PSAC simulation reveals the following: a more massive WD basically enhances the intensities of the fluorescent iron Kα1, 2 lines and the Compton hump, except for some specific accretion rate, because the more massive WD makes a hotter PSAC from which higher-energy X-rays are preferentially emitted; a larger specific accretion rate monotonically enhances the reflection because it makes a hotter and shorter PSAC; the intrinsic thermal component hardens by occultation of the cool base of the PSAC by the WD.We quantitatively estimate the influences of the parameters on the EWs and the Compton hump with both types of source. We also calculate X-ray modulation profiles brought about by the WD spin. These depend on the angles of the spin axis from the line of sight and from the PSAC, and on whether the two PSACs can be seen. The reflection spectral model and the modulation model involve the fluorescent lines and the Compton hump and can directly be compared to the data, which allows us to estimate these geometrical parameters with unprecedented accuracy. [ABSTRACT FROM AUTHOR]

We use frequency-resolved spectroscopy to examine the energy spectra of the prominent low-frequency quasi-periodic oscillation (QPO) and its harmonic in GX 339-4. We track the evolution of these spectra as the source makes a transition from a bright low/hard to hard intermediate state. In the hard/intermediate states, the QPO and time-averaged spectra are similar and the harmonic is either undetected or similar to the QPO. By contrast, in the softer states, the harmonic is dramatically softer than the QPO spectrum and the time-averaged spectrum, and the QPO spectrum is dramatically harder than the time-averaged spectrum. Clearly, the existence of these very different spectral shaped components mean that the timeaveraged spectra are complex, as also seen by the fact that the softer spectra cannot be well described by a disc, Comptonization and its reflection. We use the frequency-resolved spectra to better constrain the model components, and find that the data are consistent with a timeaveraged spectrum which has an additional low-temperature, optically thick Comptonization component. The harmonic can be described by this additional component alone, while the QPO spectrum is similar to that of the hard Comptonization and its reflection. Neither QPO nor harmonic shows signs of the disc component even when it is strong in the time-averaged spectrum. This adds to the growing evidence for inhomogeneous Comptonization in black hole binaries. While the similarity between the harmonic and QPO spectra in the intermediate state can be produced from the angular dependence of Compton scattering in a single region, this cannot explain the dramatic differences seen in the soft state. Instead, we propose that the soft Compton region is located predominantly above the disc while the hard Compton is from the hotter inner flow. Our results therefore point to multiple possible mechanisms for producing harmonic features in the power spectrum. The dominant mechanism in a given observation is likely a function of both inclination angle and inner disc radius. [ABSTRACT FROM AUTHOR]

Abstract: The effective compton cross section in a non-degenerate plasma ) is investigated in a wide range of temperatures. The results show a decreasing behavior with temperatures especially for . The results may be important in phenomena like accretion discs or ultra-relativistic blast waves in GRB models, where the emitted radiation has to pass through a medium containing high-energy electrons. [Copyright &y& Elsevier]

X-ray light-curves of three X-ray pulsars, SMC X-1, LMC X-4, and Her X-1, folded with their respective super-orbital periods, are shown to be well reproduced by a model in which X-rays from a compact object towards us are periodically obscured by a precessing ring at the outermost part of an accretion disk around the central object. A situation is considered in which matter from a companion star flows into a gravitational field of a compact star carrying a certain amount of specific angular momentum and first forms a geometrically thick ring-tube along the Keplerian circular orbit. For the model to well fit to the observations, it is necessary that the optical depth of the ring-tube for Compton scattering, τ ≃ 1 ∼ 2, the ring matter temperature, T ≃ 105 ∼ 106 K, and the ionization parameter, ξ ≃ 102 erg cm s−1 due to X-ray heating from the central X-ray source.From simple energetics- and perturbation-arguments, we find that a precession of such a ring is rather stable and can be excited in the T and ξ ranges. The time during which matter accumulates in the ring is estimated to be ∼106 s, and is shown to be comparable to the time for an accretion disk to extend from the ring. It is discussed that in the above T and ξ ranges, the ring-tube matter could become thermally unstable. Then, relatively high density regions in the ring-tube further cool down and tend to shrink to the tube center. The flow across the ring circulating flow should excite turbulent motions, and angular momenta of the matter would be effectively transferred across the tube. Finally, a steady flow should be established from the companion star through the accretion ring to the accretion disk towards the central compact star. [ABSTRACT FROM AUTHOR]

To what extent can the one-dimensional slim disk model reproduce the multi-dimensional results of global radiation-hydrodynamic simulations of super-Eddington accretion? With this question in mind, we perform a systematic simulation study of accretion flow onto a non-spinning black hole for a variety of black hole masses of (10–107)  M ⊙ and mass accretion rates of (1.4 × 102–5.6 × 103)  L Edd/ c 2 (with L Edd and c being the Eddington luminosity and the speed of light). In order to adequately resolve large-scale outflow structure, we extensively expand a simulation box to cover the space of 3000  r S (with r S being the Schwarzschild radius), larger than those in most previous studies, so that we can put relatively large angular momentum on the gas injected from the outer simulation boundary. The adopted Keplerian radius, at which the centrifugal force balances the gravitational force, is r K = 300  r S. The injected mass first falls and is accumulated at around this radius and then slowly accretes toward the central black hole via viscosity. We simulate such accretion processes, taking inverse and bulk Compton scattering into account. The simulated accretion flow is in a quasi-steady state inside r qss ∼ 200  r S. Within this radius the flow properties are, on the whole, in good agreement with those described by the slim disk model except that the radial density profile of the underlying disk is much flatter, ρ ∝  r −0.73 (cf. ρ ∝  r −3/2 in the slim disk model), due probably to efficient convection. We find very weak outflow from inside r ∼ 200  r S, unlike the previous studies. [ABSTRACT FROM AUTHOR]