Search

We report on two epochs of simultaneous near-infrared (IR) and X-ray observations with a sub-second time resolution of the low mass X-ray binary black hole candidate Swift J1753.5--0127 during its long 2005--2016 outburst. Data were collected strictly simultaneously with VLT/ISAAC (K$_{S}$ band, 2.2 $\mu m$) and RXTE (2-15 keV) or \textit{XMM-Newton} (0.7-10 keV). A clear correlation between the X-ray and the IR variable emission is found during both epochs but with very different properties. In the first epoch, the near-IR variability leads the X-ray by $ \sim 130 \, ms$. This is the opposite of what is usually observed in similar systems. The correlation is more complex in the second epoch, with both anti-correlation and correlations at negative and positive lags. Frequency-resolved Fourier analysis allows us to identify two main components in the complex structure of the phase lags: the first component, characterised by a few seconds near-IR lag at low frequencies, is consistent with a combination of disc reprocessing and a magnetised hot flow; the second component is identified at high frequencies by a near-IR lag of $\approx$0.7 s. Given the similarities of this second component with the well-known constant optical/near-IR jet lag observed in other black hole transients, we tentatively interpret this feature as a signature of a longer-than-usual jet lag. We discuss the possible implications of measuring such a long jet lag in a radio-quiet black hole transient., Comment: 10 pages, 7 figures, accepted for publication in A&A

Black hole X-ray binaries in their hard and hard-intermediate states display hard and soft time lags between broadband noise variations (high-energy emission lagging low-energy and vice versa), which could be used to constrain the geometry of the disk and Comptonising corona in these systems. Comptonisation and reverberation lag models, which are based on light-travel delays, can imply coronae which are very large (hundreds to thousands of gravitational radii, $R_{g}$) and in conflict with constraints from X-ray spectral modelling and polarimetry. Here we show that the observed large and complex X-ray time lags can be explained by a model where fluctuations are generated in and propagate through the blackbody-emitting disk to a relatively compact ($\sim$10 $R_{g}$) inner corona. The model naturally explains why the disk variations lead coronal variations with a Fourier-frequency dependent lag at frequencies $<1$ Hz, since longer variability time-scales originate from larger disk radii. The propagating fluctuations also modulate successively the coronal seed photons from the disk, heating of the corona via viscous dissipation and the resulting reverberation signal. The interplay of these different effects leads to the observed complex pattern of lag behaviour between disk and power-law emission and different power-law energy bands, the energy-dependence of power-spectral shape and a strong dependence of spectral-timing properties on coronal geometry. The observed spectral-timing complexity is thus a natural consequence of the response of the disk-corona system to mass-accretion fluctuations propagating through the disk., Comment: 24 pages, 12 figures. Submitted to MNRAS

Most of the Active Galactic Nuclei (AGN) are radio-quiet (RQ) and, differently from radio-loud (RL) AGN, do not show signature of large-scale and powerful jets. The physical origin of their radio emission remains then broadly unclear. The observation of flat/inverted radio spectra at GHz frequencies seems to support however the presence of an unresolved synchrotron self-absorbed region in the close environment of the supermassive black hole. Its size could be as small as that of the X-ray corona. Since synchrotron self absorption decreases strongly with frequency, these sources need to be observed in the millimetric (mm) domain. We report here a 12h simultaneous mm-X-ray observation of the RQ AGN MCG+08-11-11 by NOEMA and NuSTAR. The mm flux shows a weak but clear increase along the pointing with a fractional variability of $2.0\pm0.1$\%. The 3-10 keV flux of NuSTAR also increases and shows a fractional variability of $7.0\pm1.5$\%. A structure function analysis shows a local maximum in the mm light curve corresponding to 2-3\% of variability on timescale of $\sim2\times10^4$ seconds (100-300 $R_g$ light crossing time). Assuming an optically thick mm emitting medium, this translates into an upper limit of its size of $\sim$1300 $R_g$. The observation of fast variability in radio-mm and X-ray wavelengths, as well as a similar variability trend, well support the mm emission to be emitted by a region close, and potentially related to, the X-ray corona like an outflow/weak jet., Comment: Accepted in A&A Letters. 5 pages main text, 3 pages of appendices, 12 figures

We report on the first simultaneous high-time resolution X-ray and infrared (IR) observations of a neutron star low mass X-ray binary in its hard state. We performed $\approx 2\,$h of simultaneous observations of 4U 1728-34 using HAWK-I@VLT, XMM-Newton and NuSTAR. The source displayed significant X-ray and IR variability down to sub-second timescales. By measuring the cross-correlation function between the infrared and X-ray lightcurves, we discovered a significant correlation with an infrared lead of $\approx 30-40\,$ms with respect to the X-rays. We analysed the X-ray energy dependence of the lag, finding a marginal increase towards higher energies. Given the sign of the lag, we interpret this as possible evidence of Comptonization from external seed photons. We discuss the origin of the IR seed photons in terms of cyclo-synchrotron radiation from an extended hot flow. Finally, we also observed the IR counterpart of a type-I X-ray burst, with a delay of $\approx7.2\,$s. Although some additional effects may be at play, by assuming that this lag is due to light travel time between the central object and the companion star, we find that 4U 1728-34 must have an orbital period longer than $3\,$h and an inclination higher than 8$^\circ$., Comment: Accepted for publication in MNRAS, 11 pages 7 figures

The Compton Spectrometer and Imager (COSI) is a NASA Small Explorer (SMEX) satellite mission in development with a planned launch in 2027. COSI is a wide-field gamma-ray telescope designed to survey the entire sky at 0.2-5 MeV. It provides imaging, spectroscopy, and polarimetry of astrophysical sources, and its germanium detectors provide excellent energy resolution for emission line measurements. Science goals for COSI include studies of 0.511 MeV emission from antimatter annihilation in the Galaxy, mapping radioactive elements from nucleosynthesis, determining emission mechanisms and source geometries with polarization measurements, and detecting and localizing multimessenger sources. The instantaneous field of view for the germanium detectors is >25% of the sky, and they are surrounded on the sides and bottom by active shields, providing background rejection as well as allowing for detection of gamma-ray bursts and other gamma-ray flares over most of the sky. In the following, we provide an overview of the COSI mission, including the science, the technical design, and the project status., Comment: 8 pages

The Compton Spectrometer and Imager (COSI) is a selected Small Explorer (SMEX) mission launching in 2027. It consists of a large field-of-view Compton telescope that will probe with increased sensitivity the under-explored MeV gamma-ray sky (0.2-5 MeV). We will present the current status of cosipy, a Python library that will perform spectral and polarization fits, image deconvolution, and all high-level analysis tasks required by COSI's broad science goals: uncovering the origin of the Galactic positrons, mapping the sites of Galactic nucleosynthesis, improving our models of the jet and emission mechanism of gamma-ray bursts (GRBs) and active galactic nuclei (AGNs), and detecting and localizing gravitational wave and neutrino sources. The cosipy library builds on the experience gained during the COSI balloon campaigns and will bring the analysis of data in the Compton regime to a modern open-source likelihood-based code, capable of performing coherent joint fits with other instruments using the Multi-Mission Maximum Likelihood framework (3ML). In this contribution, we will also discuss our plans to receive feedback from the community by having yearly software releases accompanied by publicly-available data challenges.

The transient X-ray source MAXI J1810-222 was discovered in 2018 and has been active ever since. A long combined radio and X-ray monitoring campaign was performed with ATCA and Swift respectively. It has been proposed that MAXI J1810-222 is a relatively distant black hole X-ray binary, albeit showing a very peculiar outburst behaviour. Here, we report on the spectral study of this source making use of a large sample of NICER observations performed between 2019 February and 2020 September. We detected a strong spectral absorption feature at $\sim$1 keV, which we have characterised with a physical photoionisation model. Via a deep scan of the parameters space, we obtained evidence for a spectral-state dependent outflow, with mildly relativistic speeds. In particular, the soft and intermediate states point to a hot plasma outflowing at 0.05-0.15 $c$. This speeds rule-out thermal winds and, hence, they suggest that such outflows could be radiation pressure or (most likely) magnetically-driven winds. Our results are crucial to test current theoretical models of wind formation in X-ray binaries., Comment: 6 pages, 10 figures, accepted for publication in MNRAS Letter

The Commission Femmes et Astronomie conducted a statistical study that aims at mapping the presence of women in French professional Astronomy today, and set a starting point for studying its evolution with time. For the year 2021, we proceeded with a sub-set of 8 astronomy and astrophysics institutes, hosting a total of 1060 employees, among which PhD students, post-doctoral researchers, and academic, technical, and administrative staff, representing around 25% of the community. We have investigated how the percentage of women vary with career stage, level of responsibility, job security, and level of income. The results of this preliminary study seem to illustrate the leaky pipeline, with one major bottleneck being the access to permanent positions. It appears that the proportion of women steadily decreases with the security of jobs, with the career stage, with the qualification level and with the income level., Comment: 6 pages, 5 figures

In our self-similar, analytical, magneto-hydrodynamic (MHD) accretion-ejection solution, the density at the base of the outflow is explicitly dependent on the disk accretion rate - a unique property of this class of solutions. We had earlier found that the ejection index $p >\sim 0.1 (\dot{M}_{acc} \propto r^p ) $ is a key MHD parameter that decides if the flow can cause absorption lines in the high resolution X-ray spectra of black hole binaries. Here we choose 3 dense warm solutions with $p = 0.1, 0.3, 0.45$ and carefully develop a methodology to generate spectra which are convolved with the Athena and XRISM response functions to predict what they will observe seeing through such MHD outflows. In this paper two other external parameters were varied - extent of the disk, $\rm{r_o|_{max}} = 10^5, \, 10^6 \,\, \rm{r_G}$, and the angle of the line of sight, $i \sim 10 - 25^{\circ}$. Resultant absorption lines (H and He-like Fe, Ca, Ar) change in strength and their profiles manifest varying degrees of asymmetry. We checked if a) the lines and ii) the line asymmetries are detected, in our suit of synthetic Athena and XRISM spectra. Our analysis shows that Athena should detect the lines and their asymmetries for a standard 100 ksec observation of a 100 mCrab source - lines with equivalent width as low as a few eV should be detected if the 6-8 keV counts are larger than $10^4 - 10^5$ even for the least favourable simulated cases., Comment: 18 pages, 13 figures in the main body and 3 figures in the appendix. Accepted for publication in MNRAS

Low Frequency Quasi-Periodic Oscillations or LF QPOs are ubiquitous in BH X-ray binaries and provide strong constraints on the accretion-ejection processes. Although several models have been proposed so far, none has been proven to reproduce all observational constraints and no consensus has emerged yet. We make the conjecture that disks are threaded by a large scale vertical magnetic field that splits it into two radial zones. In the inner Jet Emitting Disk (JED), a near equipartition field allows to drive powerful self-collimated jets, while beyond a transition radius, the disk magnetization is too low and a Standard Accretion Disk (SAD) is settled. In a series of papers, this hybrid JED-SAD disk configuration has been shown to successfully reproduce most multi-wavelength (radio and X-rays) observations, as well as the concurrence with the LFQPOs for the archetypal source GX 339-4. We first analyze the main QPO scenarios provided in the literature: 1) a specific process occurring at the transition radius, 2) the accretion-ejection instability and 3) the solid-body Lense-Thirring disk precession. We recall their main assumptions and shed light on some severe theoretical issues that question the capability to reproduce LF QPOs. We then argue that none of these models could be operating under the JED-SAD physical conditions. We finally propose an alternative scenario where LF QPOs would be the disk response to an instability triggered in the jets, near a magnetic recollimation zone. Such a situation could account for most Type-C QPO phenomenology and is consistent with the global behavior of black hole binaries. The calculation of this non-destructive jet instability remains however to be done. If the existence of this instability is numerically confirmed, then it could also naturally account for the jet wobbling phenomenology seen in various accreting sources., Comment: 11 pages, 1 figure, accepted for publication in Astronomy & Astrophysics

The spectral evolution of transient X-ray binaries can be reproduced by an interplay between two flows separated at a radius $R_J$: a standard accretion disk (SAD) in the outer parts and a jet-emitting disk (JED) in the inner parts. In the previous papers of this series, we recover the spectral evolution in both X-rays and radio for four outbursts of GX339-4 by playing independently with the two parameters: $R_J$ and the accretion rate $\dot{M}_{in}$. In this paper, we compare the time evolution of $R_J$ and $\dot{M}_{in}$ for the four outbursts. We show that despite the undeniable differences between the time evolution of each outburst, a unique pattern in the $\dot{M}_{in}-R_J$ plane seems to be followed by all cycles within the JED-SAD model. We call this pattern a fingerprint, and show that even the 'failed' outburst considered follows it. We also compute the radiative efficiency in X-rays during the cycles and consider its impact on the radio--X-ray correlation. Within the JED-SAD paradigm, we find that the accretion flow is always radiatively efficient in the hard states, with between $15\%$ and $40\%$ of the accretion power being radiated away at any given time. Moreover, we show that the radiative efficiency evolves with the accretion rate because of key changes in the JED thermal structure. These changes give birth to two different regimes with different radiative efficiencies: the thick disk and the slim disk. While the existence of these two regimes is intrinsically linked to the JED-SAD model, we show direct observational evidence of the presence of two different regimes using the evolution of the X-ray power-law spectral index, a model-independent estimate. We then argue that these two regimes could be the origin of the gap in X-ray luminosity in the hard state, the wiggles and different slopes seen in the radio--X-ray correlation, and even the existence of outliers., Comment: replaced after language editing

The Compton Spectrometer and Imager (COSI) is a 0.2-5 MeV Compton telescope capable of imaging, spectroscopy, and polarimetry of astrophysical sources. Such capabilities are made possible by COSI's germanium cross-strip detectors, which provide high efficiency, high resolution spectroscopy and precise 3D positioning of photon interactions. Science goals for COSI include studies of 0.511 MeV emission from antimatter annihilation in the Galaxy, mapping radioactive elements from nucleosynthesis, determining emission mechanisms and source geometries with polarization, and detecting and localizing multimessenger sources. The instantaneous field of view (FOV) for the germanium detectors is >25% of the sky, and they are surrounded on the sides and bottom by active shields, providing background rejection as well as allowing for detection of gamma-ray bursts or other gamma-ray flares over >50% of the sky. We have completed a Phase A concept study to consider COSI as a Small Explorer (SMEX) satellite mission, and here we discuss the advances COSI-SMEX provides for astrophysics in the MeV bandpass., Comment: 9 pages, 4 figures, 1 table, author affiliations provided on final page. arXiv admin note: substantial text overlap with arXiv:1908.04334

Understanding the mechanisms of accretion-ejection during X-ray binaries outbursts has been a problem for several decades. It is still not clear yet what controls the spectral evolution of these objects from the hard to the soft states and then back to the hard states at the end of the outburst, tracing the well-known hysteresis cycle in the hardness-intensity diagram. Moreover, the link between the spectral states and the presence/absence of radio emission is still highly debated. In a series of papers, we developed a model composed of a truncated outer standard accretion disk (SAD, from the solution of Shakura and Sunyaev) and an inner jet emitting disk (JED). In this paradigm, the JED plays the role of the hot corona while simultaneously explaining the presence of a radio jet. Our goal is to apply for the first time direct fitting procedures of the JED-SAD model to the hard states of four outbursts of GX 339-4 observed during the 2000-2010 decade by RXTE, combined with simultaneous or quasi simultaneous ATCA observations. We built JED-SAD model tables usable in Xspec as well as a reflection model table based on the Xillver model. We apply our model to the 452 hard state observations obtained with RXTE/PCA. We were able to correctly fit the X-ray spectra and simultaneously reproduce the radio flux. We show that the functional dependency of the radio emission on the model parameters (mainly the accretion rate and the transition radius between the JED and the SAD) is similar between all the rising phases of the different outbursts of GX 339-4. But it is significantly different from the functional dependency obtained in the decaying phases. This result strongly suggests a change in the radiative and/or dynamical properties of the ejection between the beginning and the end of the outburst. We discuss possible scenarios that could explain these differences., Comment: Accepted A&A

X-ray binaries in outburst typically show two canonical X-ray spectral states, i.e. hard and soft states, in which the physical properties of the accretion flow and of the jet are known to change. Recently, the JED-SAD paradigm has been proposed for black hole X-ray binaries, aimed to address the accretion-ejection interplay in these systems. According to this model, the accretion flow is composed by an outer standard Shakura-Sunyaev disk (SAD) and an inner hot Jet Emitting Disk (JED). The JED produces both the hard X-ray emission, effectively playing the role of the hot corona, and the radio jets. In this paper, we use the JED-SAD model to describe the evolution of the accretion flow in the black hole transient MAXI J1820+070 during its hard and hard-intermediate states. Contrarily to the previous applications of this model, the Compton reflection component has been taken into account. We use eight broadband X-rays spectra, including NuSTAR, NICER and Swift data, providing a total spectral coverage of 0.8-190 keV. The data were directly fitted with the JED-SAD model. Our results suggest that the optically thick disk (i.e. the SAD) does not extend down to the ISCO in any of the considered epochs. In particular, as the system evolves towards the hard/intermediate state, we find that the inner radius decreases from $\sim$60 R$_{\rm G}$ in the first observation down to $\sim$30 R$_{\rm G}$ in the last one. This trend is accompanied by an increase of the mass-accretion rate. In all hard-intermediate state observations, two reflection components, characterized by different values of ionization, are required to adequately explain the data. These components likely originate from different regions of the SAD. We show that a flared outer disk could, in principle, explain the double reflection component., Comment: 15 pages, 11 figures. Accepted for publication in A&A

ESA's INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) was launched on 17 Oct 2002 at 06:41 CEST. Since then, it has been providing long, uninterrupted observations (up to about 47 hr, or 170 ksec, per satellite orbit of 2.7 days) with a large field-of-view (fully coded: 100 deg^2), msec time resolution, keV energy resolution, polarization measurements, as well as additional coverage in the optical. This is realized by two main instruments in the 15 keV to 10 MeV range, the spectrometer SPI (spectral resolution 3 keV at 1.8 MeV) and the imager IBIS (angular resolution 12 arcmin FWHM), complemented by X-ray (JEM-X; 3-35 keV) and optical (OMC; Johnson V-band) monitors. All instruments are co-aligned to simultaneously observe the target region. A particle radiation monitor (IREM) measures charged particle fluxes near the spacecraft. The Anti-coincidence subsystems of the main instruments are also efficient all-sky gamma-ray detectors, which provide omni-directional monitoring above ~75 keV. INTEGRAL can also rapidly (within a couple of hours) re-point and conduct Target of Opportunity observations. INTEGRAL has build an impressive legacy: e.g. discovery of >600 new high-energy sources; first-ever direct detection of 56Ni and 56Co radio-active decay lines from a Type Ia supernova; new insights on positron annihilation in the Galactic bulge and disk; pioneering gamma-ray polarization studies. INTEGRAL is also a successful in multi-messenger astronomy: INTEGRAL found the first prompt electromagnetic radiation in coincidence with a binary neutron star merger. More than 1750 papers based on INTEGRAL data have been published in refereed journals. Here we give a comprehensive update of the satellite status after more than 18 years of operations in a harsh space environment, and an account of the successful Ground Segment., Comment: 29 pages, 20 figures, accepted for publication in special issue of New Astronomy Reviews

We report on a multi-epoch campaign of rapid optical/X-ray timing observations of the superbright 2018 outburst of MAXI J1820+070, a black hole low-mass X-ray binary system. The observations spanned 80 days in the initial hard-state, and were taken with NTT/ULTRACAM and GTC/HiPERCAM in the optical (ugriz filters at time resolutions of 8--300 Hz) and with ISS/NICER in X-rays. We find (i) a growing anti-correlation between the optical and X-ray lightcurves, (ii) a steady, positive correlation at an optical lag of 0.2 s (with a longer lag at longer wavelengths) present in all epochs, and (iii) a curious positive correlation at \textit{negative} optical lags in the last, X-ray softest epoch, with longer wavelengths showing a greater correlation and a more negative lag. To explain these we postulate the possible existence of two synchrotron-emitting components; a compact jet and a hot flow. In our model, the significance of the jet decreases over the outburst, while the hot flow remains static (thus, relatively, increasing in significance). We also discuss a previously discovered quasi-periodic oscillation and note how it creates coherent optical time lags, stronger at longer wavelengths, during at least two epochs., Comment: 19 pages, 13 figures. Accepted for publication to MNRAS

INTEGRAL is an ESA mission in fundamental astrophysics that was launched in October 2002. It has been in orbit for over 18 years, during which it has been observing the high-energy sky with a set of instruments specifically designed to probe the emission from hard X-ray and soft gamma-ray sources. This paper is devoted to the subject of black hole binaries, which are among the most important sources that populate the high-energy sky. We present a review of the scientific literature based on INTEGRAL data, which has significantly advanced our knowledge in the field of relativistic astrophysics. We briefly summarise the state-of-the-art of the study of black hole binaries, with a particular focus on the topics closer to the INTEGRAL science. We then give an overview of the results obtained by INTEGRAL and by other observatories on a number of sources of importance in the field. Finally, we review the main results obtained over the past 18 years on all the black hole binaries that INTEGRAL has observed. We conclude with a summary of the main contributions of INTEGRAL to the field, and on the future perspectives., Comment: 44 pages, 11 figures. Accepted for publication on New Astronomy Reviews

Blueshifted X-ray absorption lines (preferentially from Fe XXV and Fe XXVI present in the 6-8 keV range) indicating the presence of massive hot disk winds in Black Hole (BH) X-ray binaries (XrB) are most generally observed during the soft states. It has been recently suggested that the non-detection of such hot wind signatures in the hard states could be due to the thermal instability of the wind in the ionisation domain consistent with Fe XXV and Fe XXVI. Studying the wind thermal stability requires however a very good knowledge of the spectral shape of the ionizing Spectral Energy Distribution (SED). We discuss in this paper the expected evolution of the disk wind properties during an entire outburst by using the RXTE observations of GX 339-4 during its 2010-2011 outburst. While GX 339-4 never showed signatures of a hot wind in the X-rays, the dataset used is optimal to illustrate our purposes. We compute the corresponding stability curves of the wind using the SED obtained with the Jet-Emitting Disk model. We show that the disk wind can transit from stable to unstable states for Fe XXV and Fe XXVI ions on a day time scale. While the absence of wind absorption features in hard states could be explained by this instability, their presence in soft states seems to require changes of the wind properties (e.g. density) during the spectral transitions between hard and soft states. We propose that these changes could be partly due to the variation of heating power release at the accretion disk surface through irradiation by the central X-ray source. The evolution of the disk wind properties discussed in this paper could be confirmed through the daily monitoring of the spectral transition of a high-inclination BH XrB., Comment: 13 pages, 8 figures. Accepted in Astronomy & Astrophysics. Compared to first version, addition of a few references and some changes in the title and the text

We present the results regarding the analysis of the fast X-ray/infrared (IR) variability of the black-hole transient MAXI J1535$-$571. The data studied in this work consist of two strictly simultaneous observations performed with XMM-Newton (X-rays: 0.7$-$10 keV), VLT/HAWK-I ($K_{\rm s}$ band, 2.2 $\mu$m) and VLT/VISIR ($M$ and $PAH2$_$2$ bands, 4.85 and 11.88 $\mu$m respectively). The cross-correlation function between the X-ray and near-IR light curves shows a strong asymmetric anti-correlation dip at positive lags. We detect a near-IR QPO (2.5 $\sigma$) at $2.07\pm0.09$ Hz simultaneously with an X-ray QPO at approximately the same frequency ($f_0=2.25\pm0.05$). From the cross-spectral analysis a lag consistent with zero was measured between the two oscillations. We also measure a significant correlation between the average near-IR and mid-IR fluxes during the second night, but find no correlation on short timescales. We discuss these results in terms of the two main scenarios for fast IR variability (hot inflow and jet powered by internal shocks). In both cases, our preliminary modelling suggests the presence of a misalignment between disk and jet., Comment: 12 pages, 7 figures. Accepted for publication in MNRAS

Understanding the persistent emission is crucial for studying type I X-ray bursts, which provide insight into neutron star properties. Although accretion disc coronae appear to be common in many accreting systems, their fundamental properties remain insufficiently understood. Recent work suggests that Type I X-ray bursts from accreting neutron stars provide an opportunity to probe the characteristics of coronae. Several studies have observed hard X-ray shortages from the accretion disk during an X-ray burst implying strong coronal cooling by burst photons. Here, we use the plasma emission code EQPAIR to study the impact of X-ray bursts on coronae, and how the coronal and burst properties affect the coronal electron temperatures and emitted spectra. Assuming a constant accretion rate during the burst, our simulations show that soft photons can cool coronal electrons by a factor of $\gtrsim 10$ and cause a reduction of emission in the $30$-$50$ keV band to $\lesssim 1\%$ of the pre-burst emission. This hard X-ray drop is intensified when the coronal optical depth and aspect ratio is increased. In contrast, depending on the properties of the burst and corona, the emission in the $8$-$24$ keV band can either increase, by a factor of $\gtrsim20$, or decrease, down to $\lesssim 1\%$ of the pre-burst emission. An increasing accretion rate during the X-ray burst reduces the coronal cooling effects and the electron temperature drop can be mitigated by $\gtrsim60\%$. These results indicate that changes of the hard X-ray flux during an X-ray burst probe the geometrical properties of the corona., Comment: 12 pages, 7 figures, MNRAS in press

Multi-wavelength spectral energy distributions of Low Mass X-ray Binaries in the hard state are determined by the emission from a jet, for frequencies up to mid-infrared, and emission from the accretion flow in the optical to X-ray range. In the last years, the flat radio-to-mid-IR spectra of Black Hole (BH) X-ray binaries was described using the internal shocks model, which assumes that the fluctuations in the velocity of the ejecta along the jet are driven by the fluctuations in the accretion flow, described by the X-ray Power Density Spectrum (PDS). In this work we attempt to apply this model for the first time to a Neutron Star (NS) LMXB, i.e. 4U 0614+091. We used the multi-wavelength data set obtained in 2006, comprising data from radio to X-ray, and applied a model which includes an irradiated disc model for the accretion flow and an updated version of the internal shocks code for the ejection. The new version of the code allows to change the geometry of the jet for the case of non-conical jets. Only two alternative scenarios provide a satisfactory description of the data: using the X-ray PDS but in a non-conical geometry for the jet, or either using a conical geometry but with a "flicker-noise" PDS. Both scenarios would imply some differences with the results obtained with similar models on BH X-ray binaries, shedding light on the possibility that jets in NS and BH binaries might somehow have a different geometry or a different coupling with the accretion flow., Comment: 18 pages, 12 figures. Accepted for publication by MNRAS

We proposed that the spectral evolution of transient X-ray binaries (XrB) is due to an interplay between two flows: a standard accretion disk (SAD) in the outer parts and a jet-emitting disk (JED) in the inner parts. We showed in previous papers that the spectral evolution in X-ray and radio during the 2010-2011 outburst of GX339-4 can be recovered. We now investigate the presence of low frequency quasi-periodic oscillations (LFQPOs) during an X-ray outburst, and address the possible correlation between the frequencies of these LFQPOs and the transition radius between the two flows, rJ. We select X-ray and radio data form 3 outbursts of GX339-4. We use the method detailed in paper IV to obtain $r_J(t)$ and $\dot{m}_{in}(t)$ for each outburst to reproduce the correlated evolution of the X-ray spectra and the radio emission for 3 different activity cycles of GX339-4. We also independently search and report the detection of 7 new LFQPOs in addition to the literature. We show that the frequency of Type C QPOs can be linked to the dynamical JED-SAD transition radius rJ, rather than the radius of optically thin-thick transition. The scaling factor q such that $\nu_{QPO} \simeq \nu_K (r_J) / q$ is $q \simeq 70-140$, consistent during the 4 cycles and similar to previous studies. The JED-SAD hybrid disk configuration not only provides a successful paradigm allowing us to describe XrB cycles, but also matches the QPO frequencies evolution. QPOs provide an indirect way to probe the JED-SAD transition radius, where an undetermined process produces secular variability. The demonstrated relation between the transition radius links Type C QPOs to the transition between the two flows, tying it to the inner magnetized structure of the jets. This direct connection between the jets' structure and the process responsible for Type C QPOs could naturally explain their puzzling multi-wavelength behavior., Comment: Accepted in publication in A&A, replaced since language editing

The black hole transient GRS 1716-249 was monitored from the radio to the gamma-ray band during its 2016-2017 outburst. This paper focuses on the Spectral Energy Distribution (SED) obtained in 2017 February-March, when GRS 1716-249 was in a bright hard spectral state. The soft gamma-ray data collected with the INTEGRAL/SPI telescope show the presence of a spectral component which is in excess of the thermal Comptonisation emission. This component is usually interpreted as inverse Compton emission from a tiny fraction of non-thermal electrons in the X-ray corona. We find that hybrid thermal/non-thermal Comptonisation models provide a good fit to the X/gamma-ray spectrum of GRS 1716-249. The best-fit parameters are typical of the bright hard state spectra observed in other black hole X-ray binaries. Moreover, the magnetised hybrid Comptonisation model BELM provides an upper limit on the intensity of the coronal magnetic field of about 1E+06 G. Alternatively, this soft gamma-ray emission could originate from synchrotron emission in the radio jet. In order to test this hypothesis, we fit the SED with the irradiated disc plus Comptonisation model combined with the jet internal shock emission model ISHEM. We found that a jet with an electron distribution of p~2.1 can reproduce the soft gamma-ray emission of GRS 1716-249. However, if we introduce the expected cooling break around 10 keV, the jet model can no longer explain the observed soft gamma-ray emission, unless the index of the electron energy distribution is significantly harder (p<2)., Comment: 14 pages, 8 figures. Accepted for publication in MNRAS

A soft X-ray excess above the 2-10 keV power law extrapolation is generally observed in AGN X-ray spectra. Presently there are two competitive models to explain it: blurred ionized reflection and warm Comptonisation. In the latter case, observations suggest a corona temperature $\sim$ 1 keV and a corona optical depth $\sim$ 10. Moreover, radiative constraints from spectral fits with Comptonisation models suggest that most of the accretion power should be released in the warm corona. The disk below is basically non-dissipative, radiating only the reprocessed emission from the corona. The true radiative properties of such a warm and optically thick plasma are not well-known, however. For instance, the importance of the Comptonisation process, the potential presence of strong absorption/emission features or the spectral shape of the output spectrum have been studied only very recently. We present in this paper simulations of warm and optically thick coronae using the TITAN radiative transfer code coupled with the NOAR Monte-Carlo code, the latter fully accounting for Compton scattering of continuum and lines. Illumination from above by a hard X-ray emission and from below by an optically thick accretion disk is taken into account as well as (uniform) internal heating. Our simulations show that for a large part of the parameter space, the warm corona with sufficient internal mechanical heating is dominated by Compton cooling and neither strong absorption nor emission lines are present in the outgoing spectra. In a smaller part of the parameter space, the calculated emission agrees with the spectral shape of the observed soft X-ray excess. Remarkably, this also corresponds to the conditions of radiative equilibrium of an extended warm corona covering almost entirely a non-dissipative accretion disk. These results confirm the warm Comptonisation as a valuable model for the soft X-ray excess., Comment: Accepted for publication in A&A. 13 pages, 8 Figures

We test the two-corona accretion scenario for active galactic nuclei in the case of the `bare' Seyfert 1 galaxy HE 1143-1810. We perform a detailed study of the broad-band UV--X-ray spectral properties and of the short-term variability. We present results of a joint XMM-Newton and NuSTAR monitoring of the source, consisting of 5x20 ks observations, each separated by 2 days, performed in December 2017. The source is variable in flux among the different observations, and a correlation is observed between the UV and X-ray emission. Moderate spectral variability is observed in the soft band. The time-averaged X-ray spectrum exhibits a cut-off at $\sim 100$ keV consistent with thermal Comptonization. We detect an iron K$\alpha$ line consistent with being constant during the campaign and originating from a mildly ionized medium. The line is accompanied by a moderate, ionized reflection component. A soft excess is clearly present below 2 keV and is well described by thermal Comptonization in a `warm' corona with a temperature of $\sim 0.5$ keV and a Thomson optical depth of $\sim 17-18$. For the hot hard X-ray emitting corona, we obtain a temperature of $\sim 20$ keV and an optical depth of $\sim 4$ assuming a spherical geometry. A fit assuming a jet-emitting disc (JED) for the hot corona also provides a nice description of the broad-band spectrum. In this case, the data are consistent with an accretion rate varying between $\sim 0.7$ and $\sim 0.9$ in Eddington units and a transition between the outer standard disc and the inner JED at $\sim 20$ gravitational radii. The broad-band high-energy data agree with an accretion flow model consisting of two phases: an outer standard accretion disc with a warm upper layer, responsible for the optical--UV emission and the soft X-ray excess, and an inner slim JED playing the role of a hard X-ray emitting hot corona., Comment: accepted for publication in A&A. Due to the description of standard procedures of data reduction and analysis, the text partly overlaps with arXiv:1805.06229, by the same authors. The latter paper describes a similar observational campaign, with the same instrumental setup, on a different object and reporting different results

We present results from the first multi-epoch X-ray/IR fast-photometry campaign on the black-hole transient GX 339--4, during its 2015 outburst decay. We studied the evolution of the power spectral densities finding strong differences between the two bands. The X-ray power spectral density follows standard patterns of evolution, plausibly reflecting changes in the accretion flow. The IR power spectral density instead evolves very slowly, with a high-frequency break consistent with remaining constant at $0.63 \pm 0.03$ Hz throughout the campaign. We discuss this result in the context of the currently available models for the IR emission in black-hole transients. While all models will need to be tested quantitatively against this unexpected constraint, we show that an IR emitting relativistic jet which filters out the short-timescales fluctuations injected from the accretion inflow appears as the most plausible scenario., Comment: Accepted for publication in ApJ Letter. 9 pages, 3 figures

Since the birth of X-ray Astronomy, spectacular advances have been seen in the imaging, spectroscopic and timing studies of the hot and violent X-ray Universe, and further leaps forward are expected in the future. On the other hand, polarimetry is very much lagging behind: after the measurements of the Crab Nebula and Scorpius X-1, obtained by OSO-8 in the 70s, no more observations have been performed in the classical X-ray band, even if some interesting results have been obtained in hard X-rays and in soft gamma-rays. The NASA/ASI mission IXPE, scheduled for the launch in 2021, is going to provide for the first time imaging X-ray polarimetry in the 2-8 keV band thanks to its photoelectric polarimeter, coupled with ~25'' angular resolution X-ray mirrors. Its orders of magnitude improvement in sensitivity with respect to the OSO-8 Bragg polarimeter implies scientifically meaningful polarimetric measurements for at least the brightest specimens of most classes of X-ray sources. In 2027, the Chinese-led mission eXTP should also be launched. In addition to timing and spectroscopic instruments, eXTP will have on board photoelectric polarimeters very similar to those of IXPE, but with a total effective area 2-3 times larger. Building on IXPE results, eXTP will increase the number of sources for which significant polarimetric measurements could be obtained. However, further progresses, such as exploring a broader energy range, considering a larger effective area, improving the angular resolution, and performing wide-field polarization measurements, are needed to reach a mature phase for X-ray polarimetry. In the first part of this White Paper we will discuss a few scientific cases in which a next generation X-ray Polarimetry mission can provide significant advances. In the second part, a possible concept for a medium-class Next Generation X-ray Polarimetry (NGXP) mission will be sketched., Comment: White Paper submitted in response to the ESA Voyage 2050 call, 20 pages + title page + references + list of team members, 10 figures

We report on simultaneous sub-second optical and X-ray timing observations of the low mass X-ray binary black hole candidate MAXI J1820+070. The bright 2018 outburst rise allowed simultaneous photometry in five optical bands ($ugriz_s$) with HiPERCAM/GTC (Optical) at frame rates over 100 Hz, together with NICER/ISS observations (X-rays). Intense (factor of two) red flaring activity in the optical is seen over a broad range of timescales down to $\sim$10 ms. Cross-correlating the bands reveals a prominent anti-correlation on timescales of $\sim$seconds, and a narrow sub-second correlation at a lag of $\approx$+165 ms (optical lagging X-rays). This lag increases with optical wavelength, and is approximately constant over Fourier frequencies of $\sim$0.3-10 Hz. These features are consistent with an origin in the inner accretion flow and jet base within $\sim$5000 Gravitational radii. An additional $\sim$+5 s lag feature may be ascribable to disc reprocessing. MAXI J1820+070 is the third black hole transient to display a clear $\sim$0.1s optical lag, which may be common feature in such objects. The sub-second lag $variation$ with wavelength is novel, and may allow constraints on internal shock jet stratification models., Comment: MNRAS Letters, In Press

1RXS J180408.9--342058 is a low mass X-ray binary hosting a neutron star, which shows X-ray activity at very different mass-accretion regimes, from very faint to almost the Eddington luminosity. In this work, we present a comprehensive X-ray study of this source using data from the Neil Gehrels Swift Observatory, NuSTAR and INTEGRAL/JEM-X. In order to follow the spectral evolution, we analysed the 2015 outburst using Swift data and three Nustar observations. Besides the canonical hard and soft spectral states, we identified the rarely observed intermediate state. This was witnessed by the appeareance of the accretion disk emission in the spectrum (at $kT_{\rm disk}$ $\sim$0.7 keV) and the simultaneous cooling of the hot corona. In addition, we also unveiled a hard tail above 30 keV in this state. In the hard state, a thermal Comptonization model with two seed photons populations ($kT_{\rm s,1}\sim 1.5$ keV and $kT_{\rm s,2}\sim 0.4$ keV, respectively) and a hot Comptonising plasma, represents the physically best motivated scenario to describe the data. We also estimated a reflection fraction below 20% in all states, while no constraints on the inclination and only lower limits on the inner disk radius could be inferred. Finally, we studied a number of type-I X-ray bursts displayed from the source, one of them at the Eddington limit (observed with JEM-X). Their characteristics, combined with the clocked behaviour observed during the intermediate state, point out H/He composition for the accreted material, which makes unlikely the helium dwarf nature for the companion., Comment: Accepted for publication in MNRAS

Transients XrB exhibit different spectral shapes during their evolution. In luminosity-color diagrams, their X-ray behavior forms unexplained q-shaped cycles. We proposed a framework where the innermost regions of the accretion disk evolve as a response to variations imposed in the outer regions. These variations lead not only to modifications of the inner disk accretion rate $\dot m_{in}$ but also to the evolution of the transition radius $r_J$ between two regions. The outermost region is a standard accretion disk (SAD), whereas the innermost region is a jet-emitting disk (JED) where all the disk angular momentum is carried away vertically by two self-confined jets. In the previous papers of this series, it has been shown that such a configuration reproduces the typical spectral properties of the five canonical XrB states. The aim of this paper is now to replicate all X-ray spectra and radio emission observed during GX 339-4 2010-2011 outburst. We use the 2T plasma code presented in papers II and III, and design an automatic fitting procedure that gives the parameters $(\dot m_{in},r_J)$ that best fit each X-ray spectrum. We use RXTE/PCA X-ray data spread over 438 days, together with radio observations at 9 GHz (ATCA). We obtain the time distributions of $\dot m_{in}$ and $r_J$ that uniquely reproduce the X-ray luminosity and the spectral shape of the whole cycle. Using the classical self-absorbed jet synchrotron emission model, the JED-SAD configuration reproduces also very satisfactorily the radio properties, in particular the switch-off and -on events and the radio-X-ray correlation. Within the JED-SAD framework, radio emission can be used to constrain the underlying disk configuration. If this result is confirmed using other outbursts from GX 339-4 or other X-ray binaries, then radio could be indeed used as another means to indirectly probe disk physics., Comment: Accepted for publication in A&A, 11 pages, 7 figures

In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s., Comment: Accepted for publication on Sci. China Phys. Mech. Astron. (2019)

In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced 'spectral-timing-polarimetry' techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process., Comment: Accepted for publication on Sci. China Phys. Mech. Astron. (2019)

It is widely believed that the primary X-ray emission of AGN is due to the Comptonisation of optical-UV photons from a hot electron corona, while the origin of the 'soft-excess' is still uncertain and matter of debate. A second Comptonisation component, called warm corona, was therefore proposed to account for the soft-excess, and found in agreement with the optical-UV to X-ray emission of a sample of Seyfert galaxies. In this context, we exploit the broadband XMM-Newton and NuSTAR simultaneous observations of the Seyfert galaxy NGC 4593 to further test the so called "two corona model". The NGC 4593 spectra are well reproduced by the model, from the optical/UV to the hard X-rays. Moreover, the data reveal a significant correlation between the hot and the warm corona parameters during our monitoring campaign., Comment: 11 pages, 11 figures. Accepted for publication in MNRAS

The black hole candidate MAXI J1836-194 was discovered in 2011 when it went into an outburst, and was the subject of numerous, quasi-simultaneous, multi-wavelength observations in the radio, infrared, optical and X-rays. In this paper, we model its multi-wavelength radio to optical spectral energy distributions (SEDs) with an internal shock jet model. The jet emission is modelled on five dates of the outburst, during which the source is in the hard and hard intermediate X-ray spectral states. The model assumes that fluctuations of the jet velocity are driven by the variability in the accretion flow which is traced by the observed X-ray timing properties of the source. While the global shape of the SED is well reproduced by this model for all the studied observations, the variations in bolometric flux and typical energies require at least two parameters to evolve during the outburst. Here we investigate variations of the jet power and mean Lorentz factor, which are both found to increase with the source luminosity. Our results are compatible with the evolution of the jet Lorentz factor reported in earlier studies of this source. However, due to the large degeneracy of the parameters of the ishem model, our proposed scenario is not unique., Comment: 13 pages, 8 figures

We present the spectral and timing analysis of X-ray observations performed on the Galactic black hole transient GRS 1716-249 during the 2016-2017 outburst. The source was almost continuously observed with the Neil Gehrels Swift Observatory from December 2016 until October 2017. The X-ray hardness ratio and timing evolution indicate that the source approached the soft state three times during the outburst, even though it never reached the canonical soft spectral state. Thus, GRS 1716-249 increases the number of black hole transients showing outbursts with "failed" state transition. During the softening events, XRT and BAT broadband spectral modeling, performed with thermal Comptonization plus a multicolor disc black-body, showed a photon index (Gamma < 2) and an inner disc temperature (kTin = 0.2-0.5 keV) characteristic of the hard intermediate state. This is in agreement with the root mean square amplitude of the flux variability (rms > 10%). We find that, coherently with a scenario in which the disc moves closer to the compact object, the accretion disc inner radius decreases with the increase of the inner disc temperature, until a certain point when the temperature starts to increase at constant radius. This, in addition with the spectral analysis results, suggests that either the accretion disc reached the innermost stable circular orbit during the hard intermediate state or the hot accretion flow might re-condensate in an inner mini-disc. We report on the radio observations performed during the outburst finding that GRS 1716-249 is located on the radio-quiet "outlier" branch of the radio/X-ray luminosity plane., Comment: 18 pages, 7 figures, accepted in MNRAS

Using the simultaneous Infra-Red (IR) and X-ray light curves obtained by Kalamkar et al. (2016), we perform a Fourier analysis of the IR/X-ray timing correlations of the black hole X-ray binary (BHB) GX 339-4. The resulting IR vs X-ray Fourier coherence and lag spectra are similar to those obtained in previous studies of GX 339-4 using optical light curves. In particular, above 1 Hz, the lag spectrum features an approximately constant IR lag of about 100 ms. We model simultaneously the radio to IR Spectral Energy Distribution (SED), the IR Power Spectral Density (PSD), and the coherence and lag spectra using the jet internal shock model ISHEM assuming that the fluctuations of the jet Lorentz factor are driven by the accretion flow. It turns out that most of the spectral and timing features, including the 100 ms lag, are remarkably well reproduced by this model. The 100 ms time-scale is then associated with the travel time from the accretion flow to the IR emitting zone. Our exploration of the parameter space favours a jet which is at most mildly relativistic ($\bar{\Gamma}< 3$), and a linear and positive relation between the jet Lorentz factor and X-ray light curve i.e. $\Gamma(t)-1\propto L_{X}(t)$. The presence of a strong Low Frequency Quasi Periodic Oscillation (LFQPO) in the IR light curve could be caused by jet precession driven by Lense-Thirring precession of the jet-emitting accretion flow. Our simulations confirm that this mechanism can produce an IR LFQPO similar to that observed in GX 339-4., Comment: 20 pages, 15 figures, accepted for publication in MNRAS

We report on the results of optical, near-infrared (NIR) and mid-infrared observations of the black hole X-ray binary candidate (BHB) MAXI J1535-571 during its 2017/2018 outburst. During the first part of the outburst (MJD 58004-58012), the source shows an optical-NIR spectrum that is consistent with an optically thin synchrotron power-law from a jet. After MJD 58015, however, the source faded considerably, the drop in flux being much more evident at lower frequencies. Before the fading, we measure a de-reddened flux density of $\gtrsim$100 mJy in the mid-infrared, making MAXI J1535-571 one of the brightest mid-infrared BHBs known so far. A significant softening of the X-ray spectrum is evident contemporaneous with the infrared fade. We interpret it as due to the suppression of the jet emission, similar to the accretion-ejection coupling seen in other BHBs. However, MAXI J1535-571 did not transition smoothly to the soft state, instead showing X-ray hardness deviations, associated with infrared flaring. We also present the first mid-IR variability study of a BHB on minute timescales, with a fractional rms variability of the light curves of $\sim 15-22 \%$, which is similar to that expected from the internal shock jet model, and much higher than the optical fractional rms ($\lesssim 7 \%$). These results represent an excellent case of multi-wavelength jet spectral-timing and demonstrate how rich, multi-wavelength time-resolved data of X-ray binaries over accretion state transitions can help refining models of the disk-jet connection and jet launching in these systems., Comment: 20 pages, 8 figures. Accepted for publication in the ApJ

It has been suggested that the cycles of activity of X-ray Binaries (XrB) are triggered by a switch in the dominant disk torque responsible for accretion (paper I). As the disk accretion rate increases, the disk innermost regions would change from a jet-emitting disk (JED) to a standard accretion disk (SAD). While JEDs have been proven to successfully reproduce hard states (paper II), the existence of an outer cold SAD introduces an extra non local cooling term. We investigate the thermal structure and associated spectra of such a hybrid disk configuration. We use the 2T plasma code elaborated in paper II, allowing to compute outside-in the disk local thermal equilibrium with self-consistent advection and optically thin-to-thick transitions, in both radiation and gas supported regimes. The non-local inverse Compton cooling introduced by the external soft photons is computed by the BELM code. This additional term has a profound influence on JED solutions, allowing a smooth temperature transition from the outer SAD to the inner JED. We explore the full parameter space in disk accretion rate and transition radius, and show that the whole domain in X-ray fluxes and hardness ratios covered by standard XrB cycles is well reproduced by such hybrid configurations. Precisely, a reasonable combination of these parameters allows to reproduce the 3-200 keV spectra of five canonical XrB states. Along with X-ray signatures, JED-SAD configurations also naturally account for the radio emission whenever it is observed. By varying only the transition radius and the accretion rate, hybrid disk configurations combining an inner JED and an outer SAD are able to reproduce simultaneously the X-ray spectral states and radio emission of X-ray binaries during their outburst. Adjusting these two parameters, it is then possible to reproduce a full cycle. This will be shown in a forthcoming paper (paper IV)., Comment: Accepted for publication in A&A

We present the analysis of five joint XMM-Newton/NuSTAR observations, 20 ks each and separated by 12 days, of the broad-line radio galaxy 3C 382. The data were obtained as part of a campaign performed in September-October 2016 simultaneously with VLBA. The radio data and their relation with the X-ray ones will be discussed in a following paper. The source exhibits a moderate flux variability in the UV/X-ray bands, and a limited spectral variability especially in the soft X-ray band. In agreement with past observations, we find the presence of a warm absorber, an iron K$\alpha$ line with no associated Compton reflection hump, and a variable soft excess well described by a thermal Comptonization component. The data are consistent with a "two-corona" scenario, in which the UV emission and soft excess are produced by a warm ($kT \simeq 0.6$ keV), optically thick ($\tau \simeq 20$) corona consistent with being a slab fully covering a nearly passive accretion disc, while the hard X-ray emission is due to a hot corona intercepting roughly 10 per cent of the soft emission. These results are remarkably similar to those generally found in radio-quiet Seyferts, thus suggesting a common accretion mechanism., Comment: 13 pages, 11 figures. Accepted for publication in MNRAS

This is a basic introduction to the physics of compact objects in the context of High Time Resolution Astrophysics (HTRA). The main mechanisms of energy release and the properties of relevant radiation processes are briefly reviewed. As a specific example, the top models for the multi-wavelength variability of accreting black holes are unveiled., Comment: To appear in HIGH TIME RESOLUTION ASTROPHYSICS - XXVII Canary Islands Winter School of Astrophysics (ed. Tariq Shahbaz. Jorge Casares Vel\'azquez, Teodoro Mu\~noz Darias

We elaborate on the paradigm proposed in Ferreira et al. (2006), where the increase and decrease in the disk accretion rate is accompanied by a modification of the disk magnetization $\mu \propto B_z^2/\dot{m}_{in}$, which in turn determines the dominant torque allowing accretion. For $\mu>0.1$, the accretion flow produces jets that vertically, carry away the disk angular momentum (jet-emitting disk or JED). The goal of this paper is to investigate the spectral signatures of the JED configurations. We have developed a two-temperature plasma code that computes the disk local thermal equilibria, taking into account the advection of energy in an iterative way. Our code addresses optically thin-to-thick transitions, both radiation and gas supported regimes and computes in a consistent way the emitted spectrum from a steady-state disk. The optically thin emission is obtained using the BELM code, which provides accurate spectra for bremsstrahlung and synchrotron emission processes as well as for their local Comptonization. For a range in radius and accretion rates, JEDs exhibit three thermal equilibria, one thermally unstable and two stables. Due to the existence of two thermally stable solutions, a hysteresis cycle is naturally obtained. However, standard outbursting X-ray binary cycles cannot be reproduced. Another striking feature of JEDs is their ability to reproduce luminous hard states. Showing that when the loss of angular momentum and power in jets is consistently taken into account, accretion disks have spectral signatures that are consistent with hard states, even up to high luminosities. The reproduction of soft states being well performed by standard accretion disks (SAD), this study argues for the existence of hybrid disk configuration: JED and SAD. A study of such hybrid configuration will be presented in a forthcoming paper III., Comment: Accepted for publication in A&A

Plasma accreted onto the surface of a neutron star can ignite due to unstable thermonuclear burning and produce a bright flash of X-ray emission called a Type-I X-ray burst. Such events are very common; thousands have been observed to date from over a hundred accreting neutron stars. The intense, often Eddington-limited, radiation generated in these thermonuclear explosions can have a discernible effect on the surrounding accretion flow that consists of an accretion disk and a hot electron corona. Type-I X-ray bursts can therefore serve as direct, repeating probes of the internal dynamics of the accretion process. In this work we review and interpret the observational evidence for the impact that Type-I X-ray bursts have on accretion disks and coronae. We also provide an outlook of how to make further progress in this research field with prospective experiments and analysis techniques, and by exploiting the technical capabilities of the new and concept X-ray missions ASTROSAT, NICER, HXMT, eXTP, and STROBE-X., Comment: 70 pages, 14 figures, 7 tables. Accepted to Space Science Reviews

Relativistic plasma jets are observed in many accreting black holes. According to theory, coiled magnetic fields close to the black hole accelerate and collimate the plasma, leading to a jet being launched. Isolating emission from this acceleration and collimation zone is key to measuring its size and understanding jet formation physics. But this is challenging because emission from the jet base cannot be easily disentangled from other accreting components. Here, we show that rapid optical flux variations from a Galactic black-hole binary are delayed with respect to X-rays radiated from close to the black hole by ~0.1 seconds, and that this delayed signal appears together with a brightening radio jet. The origin of these sub-second optical variations has hitherto been controversial. Not only does our work strongly support a jet origin for the optical variations, it also sets a characteristic elevation of <~10$^3$ Schwarzschild radii for the main inner optical emission zone above the black hole, constraining both internal shock and magnetohydrodynamic models. Similarities with blazars suggest that jet structure and launching physics could potentially be unified under mass-invariant models. Two of the best-studied jetted black hole binaries show very similar optical lags, so this size scale may be a defining feature of such systems., Comment: Authors' version of Letter published in Nature Astronomy on Oct 30, including supplementary information. Definitive version to be available at the journal website (nature.com/natastron)

The X-ray spectra of many active galactic nuclei (AGN) show a soft X-ray excess below 1-2 keV on top of the extrapolated high- energy power law. The origin of this component is uncertain. It could be a signature of relativistically blurred, ionized reflection, or the high-energy tail of thermal Comptonization in a warm (kT $\sim$ 1 keV), optically thick ($\tau\simeq$ 10-20) corona producing the optical/UV to soft X-ray emission. The purpose of the present paper is to test the warm corona model on a statistically significant sample of unabsorbed, radio-quiet AGN with XMM-newton archival data, providing simultaneous optical/UV and X-ray coverage. The sample has 22 objects and 100 observations. We use two thermal comptonization components to fit the broad-band spectra, one for the warm corona emission and one for the high-energy continuum. In the optical-UV, we also include the reddening, the small blue bump and the Galactic extinction. In the X-rays, we include a WA and a neutral reflection. The model gives a good fit (reduced $\chi^2 <1.5$) to more than 90% of the sample. We find the temperature of the warm corona to be uniformly distributed in the 0.1-1 keV range, while the optical depth is in the range $\sim$10-40. These values are consistent with a warm corona covering a large fraction of a quasi-passive accretion disc, i.e. that mostly reprocesses the warm corona emission. The disk intrinsic emission represents no more than 20% of the disk total emission. According to this interpretation, most of the accretion power would be released in the upper layers of the accretion flow., Comment: 19 pages, 12 figures. Accepted in A&A

Hard X-rays observed in Active Galactic Nuclei (AGNs) are thought to originate from the Comptonization of the optical/UV accretion disk photons in a hot corona. Polarization studies of these photons can help to constrain the corona geometry and the plasma properties. We have developed a ray-tracing code that simulates the Comptonization of accretion disk photons in coronae of arbitrary shape, and use it here to study the polarization of the X-ray emission from wedge and spherical coronae. We study the predicted polarization signatures for the fully relativistic and various approximate treatments of the elemental Compton scattering processes. We furthermore use the code to evaluate the impact of non-thermal electrons and cyclo-synchrotron photons on the polarization properties. Finally, we model the NuSTAR observations of the Seyfert I galaxy Mrk 335 and predict the associated polarization signal. Our studies show that X-ray polarimetry missions such as NASA's Imaging X-ray Polarimetry Explorer (IXPE) and the X-ray Imaging Polarimetry Explorer (XIPE) proposed to ESA will provide valuable new information about the physical properties of the plasma close to the event horizon of AGN black holes., Comment: Accepted for publication in Astrophysical Journal, 38 pages, 16 figures, and 1 tables

Whilst astronomy as a science is historically founded on observations at optical wavelengths, studying the Universe in other bands has yielded remarkable discoveries, from pulsars in the radio, signatures of the Big Bang at submm wavelengths, through to high energy emission from accreting, gravitationally-compact objects and the discovery of gamma-ray bursts. Unsurprisingly, the result of combining multiple wavebands leads to an enormous increase in diagnostic power, but powerful insights can be lost when the sources studied vary on timescales shorter than the temporal separation between observations in different bands. In July 2015, the workshop "Paving the way to simultaneous multi-wavelength astronomy" was held as a concerted effort to address this at the Lorentz Center, Leiden. It was attended by 50 astronomers from diverse fields as well as the directors and staff of observatories and spaced-based missions. This community white paper has been written with the goal of disseminating the findings of that workshop by providing a concise review of the field of multi-wavelength astronomy covering a wide range of important source classes, the problems associated with their study and the solutions we believe need to be implemented for the future of observational astronomy. We hope that this paper will both stimulate further discussion and raise overall awareness within the community of the issues faced in a developing, important field., Comment: 52 pages, 15 figures, accepted, invited review (to appear in New Astronomy Reviews), v3: updated figure and text

We studied X-ray reverberation lags in the BHXRB GX 339-4 at the end of the 2014-2015 outburst. We analysed data from a XMM-Newton campaign covering the end of the transition from the soft to the hard state, and the decrease of luminosity in the hard state. During all the observations we detected, at high frequencies, significant disc variability, responding to variations of the power law emission with an average time delay of $\sim0.009\pm 0.002$ s. These new detections of disc thermal reverberation add to those previously obtained and suggest the lag to be always present in hard and hard-intermediate states. Our study reveals a net decrease of lag amplitude as a function of luminosity. We ascribe this trend to variations of the inner flow geometry. A possible scenario implies a \emph{decrease} of the inner disc truncation radius as the luminosity increases at the beginning of the outburst, followed by an \emph{increase} of the inner disc truncation radius as the luminosity decreases at the end of the outburst. Finally, we found hints of FeK reverberation ($\sim3\sigma$ significance) during the best quality observation of the XMM monitoring. The lag at the FeK energy has similar amplitude as that of the thermally reprocessed component, as expected if the same irradiated region of the disc is responsible for producing both the thermalized and reflected component. This finding suggests FeK reverberation in BHXRBs to be at the reach of current detectors provided observations of sufficiently long exposure are available., Comment: Accepted for publication in MNRAS

The corona, a hot cloud of electrons close to the centre of the accretion disc, produces the hard X-ray power-law continuum commonly seen in luminous Active Galactic Nuclei (AGN). The continuum has a high-energy turnover, typically in the range of one to several 100 keV and is suggestive of Comptonization by thermal electrons. We are studying hard X-ray spectra of AGN obtained with NuSTAR after correction for X-ray reflection and under the assumption that coronae are compact, being only a few gravitational radii in size as indicated by reflection and reverberation modelling. Compact coronae raise the possibility that the temperature is limited and indeed controlled by electron-positron pair production, as explored earlier (Paper I). Here we examine hybrid plasmas in which a mixture of thermal and nonthermal particles is present. Pair production from the nonthermal component reduces the temperature leading to a wider temperature range more consistent with observations., Comment: 6 pages, 6 Figures, Accepted for publication in MNRAS