Erratum: Optical and soft X-ray light-curve analysis during the 2022 eruption of U Scorpii: Structural changes in the accretion disk

X-ray and optical orbital modulation of EXO 0748-676: A co-variability study using XMM-Newton

The recently discovered non-magnetic cataclysmic variable TUVO-21acq was serendipitously observed in a quiescent state during an XMM-Newton observing campaign for the starburst galaxy NGC 4945. Data from this campaign was combined with archival serendipitous XMM observations to examine its X-ray and UV/optical characteristics. TUVO-21acq was found to have X-ray flux F X(0.4–3.0 keV) ∼ 1.3 × 10−14 erg s−1 cm−2 and features similar to other quiescent dwarf novae, with X-ray luminosity L X(0.4–3.0 keV) ∼ 8 × 1030 erg s−1 and a white dwarf mass MWD=0.78−0.27+0.37M⊙ . Its UV/optical spectrum was variable between observations, possibly due to changes in the accretion disk or the visibility of the bright spot.

X-ray observations of unabsorbed active galactic nuclei provide an opportunity to explore the innermost regions of supermassive black hole accretion discs. Our goal in this paper is to investigate the central environment of a Seyfert 1.5 galaxy IRAS 05078+1626. We studied the time-averaged spectrum obtained with the EPIC and RGS instruments. A power-law continuum (photon index ~ 1.75) dominates the 2-10 keV energy range. A narrow iron K alpha spectral line is clearly seen, presumably originating in a distant torus, but no broad relativistic component was detected. However, the power law and the iron K alpha line alone do not provide a satisfactory fit in the soft X-ray band whose spectrum can be explained by the combination of three components: a) a cold photoelectric absorber with column density ~ 10^(21) cm^(-2). This gas could be located either in outer parts of the accretion disc, at the rim of the torus or farther out in the host galaxy; b) a warm absorber with high ionization parameter (log(xi) ~ 2.2) and column density ~ 10^(24) cm^(-2); c) an ionized reflection where the reflecting gas could be either in the inner wall of a warm absorber cone or in an ionized accretion disc. The first X-ray spectroscopic measurement of IRAS05078+1626 unveils some of the standard ingredients in Seyfert galaxies, such as a power-law primary continuum, modified by reflection from the accretion disc and by the effect of complex, multi-phase obscuration. However, data constrains the accretion disc, if present, not to extend closer than to 60 gravitational radii from the black hole.

<p indent="0mm">The optical variability analysis of quasars based on fractal Brownian motion in the Sloan Digital Sky Survey (SDSS). The violent activities of the high energy variability can be observed in multiple wavelength bands of quasars, and studying quasar variability can contribute to a deeper understanding of the physical mechanism of the emission from the central region. Many studies have shown that the great majority of quasar variability is considered to originate in the accretion disk’s thermal emission, but some people do not agree with this view. The Hurst exponent of SDSS quasar variability is calculated using the rescaled range analysis method to investigate the fractal properties of variability in quasars from the SDSS stripe 82. The Hurst exponent results of variability show that the quasar variability is self-similar and long-range correlated. The variability obeys the biased random walk model, and its variation is similar to fractal Brownian motion. The power spectral exponent obtained from the Hurst exponent conversion shows that most of the variabilities can be approximately fitted by the damped random walk model, but a small number of samples have large deviations.

Context.A detailed study of the close interacting binary V4142 Sgr, based on photometric and spectroscopic analyses, indicates that this system belongs to the enigmatic class of Algol-like variables exhibiting a long photometric cycle of an unknown nature.Aims.By performing photometric data-mining and spectroscopic observations covering the orbital cycle, we obtained the orbital parameters and the stellar properties of the binary system, along with the physical properties of the accretion disk located around the hot star. In addition, we gained insights into the evolutive path of the system.Methods.We modeled the light curve through an inverse method, using a theoretical light curve of the binary system that considers the light curve contribution of both stars and the accretion disk of the hot star to obtain the fundamental parameters. To constrain the main stellar parameters, the mass ratio was fixed in addition to the donor temperature, using the obtained values from our spectroscopic analysis, which includes deblending methods to isolate the spectral lines of the stellar components. The system parameters were compared with a grid of binary star evolutive models in order to gain insights into the evolutionary history of the system.Results.The orbital period and the long cycle were re-calculated, with the following result: 30d​​.633 ± 0d​​.002 and 1201 ± 14 days. The spectral analysis reveals Hαdouble emission with a persistentV ≤ Rasymmetry, which is considered evidence of a possible wind emerging from the hotspot region. In addition, a cold and evolved donor star ofMd = 1.11 ± 0.2 M⊙,Td = 4500 ± 125 K and aRd = 19.4 ± 0.2 R⊙as well as a rejuvenated B-dwarf companion ofMg = 3.86 ± 0.3 M⊙,Tg = 14 380 ± 700 K, andRg = 6.35 ± 0.2 R⊙were found. The gainer is surrounded by a concave and geometrically thick disk, creating its own atmosphere around the main component of a radial extension, ℛd= 22.8 ± 0.3R⊙, contributing ∼1.4 percent of the total luminosity of the system at theV-band at orbital phase 0.25. The disk is characterized by a hot-spot roughly placed where the stream hits the disk and an additional bright-spot separated 102.​​° 5 ± 0.​​° 04 degrees along the disk edge rim in the direction of the orbital motion. The system is seen under an inclination of 81.​​° 5 ± 0.​​° 3 and at a distance ofdGaia = 1140 ± 35 pc. Doppler maps of the emission lines reveal sites of enhanced line emission in the second and third velocity quadrants. The former would correspond to a hotspot and the latter to a bright spot detected by the light curve analysis. We find that the system comes from an initially shorter orbital period binary that inverted its mass ratio due to mass exchange. A plausible model scenario indicates that, at present, the K-type giant should have been depleted of hydrogen in its core, while the companion would have gained about 2M⊙through a process lasting about 2 Myr.

Accreting compact object systems such as X-ray binaries (XRBs; neutron stars and black holes up to tens of solar masses) and active galactic nuclei (AGN; supermassive black holes up to 1e9 solar masses) exhibit variability in their luminosity on many timescales ranging from milliseconds to hundreds of days. Current studies on accretion disks seek to determine how the changes in black hole mass, the rate at which mass accretes onto the central black hole, and the internal and external disk environment affect the observed variability in various bandwidths of light. The fundamental structure of the accretion flow process itself, how it changes and results in the observed variability is not well known. This thesis employs novel methods from nonlinear dynamics to connect the ubiquitously complex variability of XRBs and AGN to the accretion properties and emission and probe commonalities across decades of mass. The methodologies used surpass the capabilities of traditional time series analysis techniques commonly used in astrophysics, such as the power density spectrum and other second order measures of variability, to probe the underlying dynamics. We study the longest light curves to date of XRBs by combining data from multiple all-sky monitors in the 2-20 keV bandpass; the most well-sampled and high precision optical light curves of AGN from the Kepler satellite; and the largest samples to date of long-term monitoring in the hard X-ray of AGN with the Swift/BAT telescope. We find evidence for chaos in a neutron star XRB connected to its superorbital period of 120 days and possibly related to a precession in the accretion disk. We similarly find higher levels of determinism correlate with specific spectral states of XRBs. Among AGN, we find that Type 1 AGN are more likely to exhibit nonlinear behavior than Type 2, obscured AGN are more likely to exhibit stochastic behavior than unobscured AGN, and radio loud sources are more deterministic than radio quiet, with a possible anti-correlation between increased luminosity and nonlinearity. Overall, we hypothesize that specific configurations of the accretion flow onto a compact object result in specific modulations of the emission that can be characterized as deterministic (e.g., possibly related to the presence of a jet-like process), nonlinear (e.g., due to a modulating warp or precession in the accretion disk), or stochastic (e.g., possibly related to the presence of strong outflows or hard X-ray coronal component).

We report on Rossi X-Ray Timing Explorer observations of four type I X-ray bursters, namely, 1E 1724-3045, GS 1826-238, SLX 1735-269, and KS 1731-260. The first three were in a low state, with 1-200 keV X-ray luminosities in the range ~0.05-0.1LEdd (LEdd: Eddington luminosity for a neutron star, LEdd = 2.5 × 1038 ergs s-1), whereas KS 1731-260 was in a high state, with luminosity ~0.35LEdd. The low-state sources have very similar power spectra, displaying high-frequency noise up to ~200 Hz. For KS 1731-260, its power spectrum is dominated by noise at frequencies 20 Hz; in addition a quasi-periodic oscillation at 1200 Hz is detected in a segment of the observation. The 1-200 keV spectra of the low-state sources are all consistent with resulting from thermal Comptonization with an electron temperature (kTe) around 25-30 keV. For KS 1731-260, the spectrum is also dominated by thermal Comptonization, but with a much lower kTe ~ 3 keV and no significant hard X-ray emission. With the exception of GS 1826-238, they each have an underlying soft component, carrying at most ~25% of the total 1-200 keV luminosity. For all sources, we have detected an iron Kα line at 6.4 keV (although it is weak and marginal in 1E 1724-3045). A reflection component is present in the spectra of GS 1826-238 and SLX 1735-269, and for both we find that the reflecting medium subtends only a small solid angle (Ω/2π ~ 0.15, 0.28). The origin of the line and the reflection component is most likely to be irradiation of the accretion disk by the X-ray source. We suggest a model in which the region of main energy release, where hard X-rays are produced, would be an optically thin boundary layer merged with an advection-dominated accretion flow (ADAF) and would be responsible for the rapid variability observed. The soft component observed probably represents the unscattered emission from an optically thick accretion disk of variable inner radius. When the accretion rate increases, the inner disk radius shrinks and the strength of the reflected component and associated iron line increase. At the same time, the Comptonization region cools off in response to an increased cooling flux from the accretion disk and from the reprocessed/reflected component, thus leading progressively to a quenching of the hard X-ray emission. If low-state neutron stars (NSs) accrete via ADAFs, the observation of X-ray bursts, indicating that all the accreting matter actually accumulates onto the NS surface, argues against the existence of strong winds from such accretion flows. Finally, we discuss two criteria recently proposed to distinguish between nonquiescent black holes (BHs) and NSs that are not contradicted by existing observations. The first one states that, when thermal Comptonization is responsible for the hard X-ray emission, only BHs have kTe larger than ~50 keV. However, this criterion is weakened by the fact that there are NSs displaying nonattenuated power laws extending up to at least 200 keV, possibly implying nonthermal Comptonization or thermal Comptonization with kTe larger than 50 keV. The second criterion stipulates that only BHs are capable of emitting hard X-ray tails with 20-200 keV luminosities 1.5 × 1037 ergs s-1.

We investigate the spectral features of accretion flows composed of an outer cool, optically thick disk and inner hot, optically thin, advection dominated accretion flows (ADAF) within the framework of disk and corona with mass evaporation (Liu et al. 2002a, ApJ, 575, 117). In this work, both the magnetic field and Compton scattering of soft photons from the disk by electrons in the corona are included to calculate the evaporation rates at different distances. The disk is truncated at the distance where the evaporation rate equals to the accretion rate [$\dot m_{\rm evap}$($r_{\rm tr}$) $= \dot m$]. For a series of accretion rates, the corresponding truncation radii are calculated out, with which we are able to calculate the emergent spectra from the inner ADAF $+$ outer disk $+$ corona. At very low accretion rates, the spectra are similar to that of a pure ADAF because the disk is truncated at large distances. The disk component becomes important at high accretion rates since the truncation occurs at small distances. An anti-correlation between the Eddington ratio, $\xi \equiv L_{0.5-25 \ {\rm keV}}/L_{\rm Edd}$, and the hard X-ray photon index, $\Gamma_{3-25 \ {\rm keV}}$, in low/hard states is predicted by the model. Comparing the theoretical results with observations, we find that our model can reproduce the anti-correlation between the Eddington ratio, $\xi$, and the hard X-ray photon index observed for the X-ray binary XTE J1118$+$480.

We find a significant anti-correlation between the hard X-ray photon index and the Eddington ratio L_Bol/L_Edd for a sample of Low-Ionization Nuclear Emission-line Regions (LINERs) and local Seyfert galaxies, compiled from literatures with Chandra or XMM-Newton observations. This result is in contrast with the positive correlation found in luminous active galactic nuclei (AGNs), while it is similar to that of X-ray binaries (XRBs) in low/hard state. Our result is qualitatively consistent with the spectra produced from advection dominated accretion flows (ADAFs). It implies that the X-ray emission of low-luminosity active galactic nuclei (LLAGNs) may originate from the Comptonization process in ADAF, and the accretion process in LLAGNs may be similar to that of XRBs in the low/hard state, which is different from that in luminous AGNs.

Despite the long history of studies of active galactic nuclei (AGN), details of the structure of the accretion flow onto supermassive black holes are far from clear. Work presented in this thesis is directed at unveiling properties of AGN structure through broadband X-ray spectroscopy, with particular emphasis on the hard X-ray band (photon energies above 10 keV). With its unprecedented sensitivity in this energy band, the NuSTAR telescope provides the key observational diagnostics of the properties of the AGN X-ray source, the corona, and the surrounding gas in the accretion disk, the broad-line region, and the torus. The first study presented in this thesis focuses on measurements of the optical depth and the temperature of the plasma in the corona of an obscured AGN. Fitting theoretical spectral models for coronal emission to the NuSTAR data constrained these two basic physical parameters under the assumption of either spherical or disk-like geometry for the corona. The remainder of the thesis is dedicated to studies of the anisotropic obscuring structure broadly referred to as the torus. One of them is a case study of three heavily obscured AGN with spectra dominated by the X-ray light scattered and reprocessed in the torus, where it is possible to constrain one of the basic torus properties -- its globally averaged column density. The following study presents the calculation of a new spectral model for reprocessing of the intrinsic X-ray continuum within the torus. Its added flexibility compared to previously available models allows for both the average column density of the torus and its covering factor to be constrained from broadband X-ray spectra of a wide variety of AGN. The final part of the thesis in based on a large survey of the local obscured AGN population performed with NuSTAR. Spectral modeling of more than a hundred individual AGN, including both old and new spectral models, is presented. From analyses of the X-ray data for a large and representative AGN sample, for the first time, it is found that their tori preferentially have high covering factors and average column densities close to unity optical depth for Compton scattering.

Hercules X-1 was observed for extended periods during its low state by the Extreme Ultraviolet Explorer (EUVE). These observations yield low-state light curves in the extreme ultraviolet (EUV) which are compared with a composite model here. The model includes reflection of soft X-rays off the companion HZ Her, including the shadowing of HZ Her by the accretion disc, and emission from the accretion disc surface. Four different geometries for the accretion disc were adopted, all derived from the RXTE All-Sky Monitor (ASM) 35-day light-curve modelling. Three were thin disc models for different system inclinations, i, and the fourth was a disc with a thick inner ring for i= 85°. With the HZ Her reflection model, with no free parameters except normalization, and a simple model for the disc emission, the models fit the data well. The disc emission accounts for about half of the EUV flux, depending on which accretion disc geometry is used. The disc geometry that best fits the EUV light curves is the disc with a thick inner ring, which is the same model that gives the best fit to the RXTE/ASM light curve.

We examine theoretically the behaviour of the inner accretion disk in GRS 1915+105 when soft X-ray dips are present in the X-ray light curve. We assume the presence of a radial shock in the accretion disk, as in some of the Two Component Advective Flow (TCAF) solutions. We discuss the behaviour of the flux tubes inside a TCAF (which we name Magnetized TCAF or MTCAF model for brevity) and compare various competing forces on the flux tubes. In this MTCAF model, we find that the magnetic tension is the strongest force in a hot plasma of temperature $\gsim 10^{10}$K and as a result, magnetic flux tubes entering in this region collapse catastrophically, thereby occasionally evacuating the inner disk. We postulate that this magnetic `rubber-band' effect induced evacuated disk matter produces the blobby components of outflows and IR/radio jets. We derive the size of the post-shock region by equating the time scale of the Quasi-Periodic Oscillations to the infall time of accreting matter in the post-shock region and found the shock location to be $\sim 45-66 r_g$. We calculate the transition radius $r_{tr}$, where the Keplerian disk deviates into a sub-Keplerian flow, to be $\sim 320r_g$. Based on the derived X-ray spectral parameters, we calculate the mass of this region to be $\sim10^{18}$g. We conclude that during the X-ray dips the matter in the post-shock region, which manifests itself as the thermal-Compton component in the X-ray spectrum, is ejected, along with some sub-Keplerian matter in the pre-shock region.

Low-mass X-ray binaries (LMXBs) are binary systems that consist of a compact object (i.e., a black hole or a neutron star) and a low-mass star (less than 1 solar mass), known as companion star, orbiting around a common center of mass. If the two components are close enough, the compact object accretes matter from the companion star forming an accretion disc that produces strong emission in X-rays. A hot electron plasma, called the corona, surrounds the compact object and the accretion disc. Energetic electrons in the corona interact with the photons coming from the accretion disc via inverse Compton scattering to produce the hard X-rays observed in these systems. The emission from the accretion disc and the corona varies with time, producing different spectral states. In this thesis, I study the evolution of the spectral and timing properties of LMXBs with a black hole as a compact object, the so-called black-hole low-mass X-ray binaries (BH LMXBs). The study of these properties allowed me to infer the nature of the compact object of these systems, as well as shed light on some phenomena that are not very well studied, such as Failed-Transition outbursts and reflares.

The primary X-ray emission from active galactic nuclei (AGNs), described by a power-law, irradiates the accretion disc producing reflection features in the spectrum. The reflection features arising from the inner regions of the disc can be significantly modified by the relativistic effects near the black hole. We investigate the relationship between the relativistic reflection fraction Rf, defined as the ratio of the coronal intensity that illuminates the accretion disc to the coronal intensity observed directly, and the hard X-ray photon index Γ of a Nuclear Spectroscopic Telescope Array (NuSTAR) sample of Seyfert 1 galaxies. The X-ray spectra are modelled using relxill code that helps to directly obtain the reflection fraction of a relativistically smeared reflection component. The parameter Rf depends on the amount of Comptonized X-ray emission intercepted by the inner accretion disc. We found a positive correlation between Γ and Rf in our sample. Seed photons from a larger area of an accretion disc entering the corona will result in increased cooling of the coronal plasma, giving rise to steeper X-ray spectrum. The corona irradiating the larger area of the disc will result in higher reflection fraction. Thus, the observed Rf –Γ relation is most likely related to the variations in the disc–corona geometry of AGNs.

view Abstract Citations (27) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Ultraviolet/Optical Continuum of Hercules X-1/HZ Herculis Vrtilek, S. D. ; Cheng, F. H. Abstract We present observations of the binary X-ray pulsar system Hercules X-1/HZ Herculis taken with the International Ultraviolet Explorer (lUE) and several ground-based observatories as part of a multi- wavelength campaign conducted in 1993 August. The 1993 campaign found the source in an unusual state, with substantially reduced X-ray flux, and also revealed a significant reduction in ultraviolet flux observed around eclipse ingress and egress, which implies an absence of the normally observed excess attributed to X-ray heating of the disk. Comprehensive modeling of the ultraviolet and optical continuum flux suggests that the ultraviolet and optical light curves can be reproduced by assuming variations in Mdot alone; however, changes in accretion rate are insufficient to account for the observed drop in X-ray flux. Shifts in disk inclination angle and changes in mass accretion rate can consistently explain the observed changes in X-ray, ultraviolet, and optical fluxes. Changes in mass accretion rate and disk inclination angle may be related to shifts in the 35 day precession period, with the average mass accretion rate corresponding to the average 35 day period. The rapid fluctuations in soft X-ray flux, observed several times during the campaign, may be caused by rapid variation of the mass accretion rate. Publication: The Astrophysical Journal Pub Date: July 1996 DOI: 10.1086/177475 Bibcode: 1996ApJ...465..915V Keywords: STARS: INDIVIDUAL CONSTELLATION NAME: HERCULES X-1; STARS: INDIVIDUAL CONSTELLATION NAME: HZ HERCULIS; ULTRAVIOLET: STARS full text sources ADS | data products SIMBAD (3) MAST (1) INES (1)