The Ultraviolet/Optical Continuum of Hercules X-1/HZ Herculis

Active galactic nuclei (AGNs) are known to exhibit optical/ultraviolet variability and most of them can be well modelled by the damped random walks. Physical processes that are not related to the accretion disc, such as tidal disruption events (TDEs) or moving foreground dusty clouds, can cause flare-like and eclipse-like features in the optical light curve. Both long-term and high-cadence monitoring are needed to identify such features. By combining the Sloan Digital Sky Survey (SDSS), Panoramic Survey Telescope and Rapid Response System, and the Zwicky Transient Facility (ZTF) survey, we are able to identify a rare sample (11) out of the SDSS quasar catalogue ($\sim\!\! 83\, 000$). These quasars exhibit more or less constant brightness but show rapid optical variation in the epochs of ZTF Public Data Release 2. To investigate the possible origins of these flare/eclipse-like variabilities, we propose the second-epoch spectroscopic observations with the Gran Telescopio CANARIAS. We find that the change in accretion rate plays a significant role in these quasar variabilities. Among them, we identify two changing-look AGN candidates: SDSS J1427+2930 and SDSS J1420+3757. The luminosity change of the former may be caused by the enhanced supermassive black hole’s accretion or the TDE, while the latter is more related to the change in the accretion rate.

We present the X-ray spectral and temporal analysis of the black hole X-ray transient Swift J1658.2–4242 observed by AstroSat. Three epochs of data have been analysed using the JeTCAF model to estimate the mass accretion rates and to understand the geometry of the flow. The best-fitting disc mass accretion rate ($\dot{m}_{\rm d}$) varies between $0.90^{+0.02}_{-0.01}$ and $1.09^{+0.04}_{-0.03}$$\dot{M}_{\rm Edd}$ in these observations, while the halo mass accretion rate changes from $0.15^{+0.01}_{-0.01}$ to $0.25^{+0.02}_{-0.01}$$\dot{M}_{\rm Edd}$. We estimate the size of the dynamic corona that varies substantially from $64.9^{+3.9}_{-3.1}$ to $34.5^{+2.0}_{-1.5}$ rg and a moderately high jet/outflow collimation factor stipulates isotropic outflow. The inferred high disc mass accretion rate and bigger corona size indicate that the source might be in the intermediate to soft spectral state of black hole X-ray binaries. The mass of the black hole estimated from different model combinations is ∼14 M⊙. In addition, we compute the quasi-periodic oscillation (QPO) frequencies from the model-fitted parameters, which match the observed QPOs. We further calculate the binary parameters of the system from the decay profile of the light curve and the spectral parameters. The estimated orbital period of the system is 4.0 ± 0.4 h by assuming the companion as a mid or late K-type star. Our analysis using the JeTCAF model sheds light on the physical origin of the spectrotemporal behaviour of the source, and the observed properties are mainly due to the change in both the mass accretion rates and absorbing column density.

This thesis contains the results of two investigations: one into the nature of stars with degenerate neutron cores and the other into the interpretation of the phenomenology of luminous low-mass X-ray binaries (LMXBs) displaying slow quasi-periodic oscillations (QPOs) in their X-ray flux. A star with a degenerate neutron core would be a red giant or supergiant. In this thesis we investigate the structure of such a supergiant, particularly examining the energy production and seeking an identifying observational signature. This star is convective from near the photosphere down to the base of the envelope just outside the degenerate core (radius 10 km). The star's luminosity comes from the rp-process in a convective burning region within 100 km of the base of the envelope. The convection brings fuel for the rp-process into the burning region from throughout the envelope and deposits the products of rp-burning back into the envelope, including the photosphere. After about 10^5 years, the abundances of Br, Rb, Y, and Nb at the surface of the star will be about 200 times greater than their solar abundances, and that of Mo, over 1000 times solar. A suitable observational signature would be the strong enhancement of absorption lines for these elements in the star's spectrum. As many as 10 of the nearest 100 red supergiants (those within 5 kpc) could have neutron cores. The other investigation concerns a model of rapid accretion onto an unmagnetized neutron star with radius somewhat less than 6GM/c^2. This model is applied to the phenomenology of a class of LMXBs displaying slow (∼ 6 Hz) QPOs in X-ray flux. These sources are highly luminous (approximately Eddington) and display what appears to be three modes ("branches") of accretion. In this model, at low accretion rates, the neutron star lies within the inner edge of the accretion disk, and matter is dripped onto the neutron star from the inner edge. As the accretion rate increases, the transition from the "horizontal branch" to the "normal branch" occurs when the disk thickens and its inner edge touches the star and forms a boundary layer. The formation of a boundary layer changes the structure of the inner disk and the spectral character of the escaping X-rays. The transition from the normal branch to the "flaring branch" occurs when the boundary layer covers the whole surface of the neutron star and radiation escapes primarily through convective instabilities. This thesis presents an exploration of this model, with an emphasis on establishing the plausibility that a neutron star could indeed lie inside an accretion disk accreting at the observed rate and that a change of mass accretion rate could push the inner radius onto the surface of the star.

SS Cyg has long been recognized as the prototype of a group of dwarf novae that show only outbursts. However, this object has entered a quite anomalous event in 2021, which at first appeared to be standstill, i.e., an almost constant luminosity state observed in Z Cam-type dwarf novae. This unexpected event gives us a great opportunity to reconsider the nature of standstill in cataclysmic variables. We have observed this anomalous event and its forerunner, a gradual and simultaneous increase in the optical and X-ray flux during quiescence, through many optical telescopes and the X-ray telescopes NICER and NuSTAR. We have not found any amplification of the orbital hump during quiescence before the anomalous event, which suggests that the mass transfer rate did not significantly fluctuate on average. The estimated X-ray flux was not enough to explain the increment of the optical flux during quiescence via X-ray irradiation of the disk and the secondary star. It would be natural to consider that viscosity in the quiescent disk was enhanced before the anomalous event, which increased mass accretion rates in the disk and raised not only the optical flux but also the X-ray flux. We suggest that enhanced viscosity also triggered the standstill-like phenomenon in SS Cyg, which is considered to be a series of small outbursts. The inner part of the disk would always stay in the outburst state and only its outer part would be unstable against the thermal–viscous instability during this phenomenon, which is consistent with the observed optical color variations. This scenario is in line with our X-ray spectral analyses which imply that the X-ray-emitting inner accretion flow became hotter than usual and vertically expanded, and that it became denser and was cooled down after the onset of the standstill-like state.

Radiative feedback from populations II stars played a vital role in early structure formation. Particularly, photons below the Lyman limit can escape the star forming regions and produce a background ultraviolet (UV) flux which consequently may influence the pristine halos far away from the radiation sources. These photons can quench the formation of molecular hydrogen by photo-detachment of $\rm H^{-}$. In this study, we explore the impact of such UV radiation on fragmentation in massive primordial halos of a few times $\rm 10^{7}$~M${_\odot}$. To accomplish this goal, we perform high resolution cosmological simulations for two distinct halos and vary the strength of the impinging background UV field in units of $\rm J_{21}$. We further make use of sink particles to follow the evolution for 10,000 years after reaching the maximum refinement level. No vigorous fragmentation is observed in UV illuminated halos while the accretion rate changes according to the thermal properties. Our findings show that a few 100-10, 000 solar mass protostars are formed when halos are irradiated by $\rm J_{21}=10-500$ at $\rm z>10$ and suggest a strong relation between the strength of UV flux and mass of a protostar. This mode of star formation is quite different from minihalos, as higher accretion rates of about $\rm 0.01-0.1$ M$_{\odot}$/yr are observed by the end of our simulations. The resulting massive stars are the potential cradles for the formation of intermediate mass black holes at earlier cosmic times and contribute to the formation of a global X-ray background.

Compact radio jets are ubiquitous in stellar-mass black-hole binaries in\ntheir hard spectral state. Empirical relations between the radio and\nnarrow-band X-ray fluxes have been used to understand the connection between\ntheir accretion discs and jets. However, a narrow-band (e.g., 1--10 or 3--9\nkeV) X-ray flux can be a poor proxy for either the bolometric luminosity or the\nmass accretion rate. Here, we study correlations between the radio and\nunabsorbed broad-band X-ray fluxes, the latter providing good estimates of the\nbolometric flux. We consider GX 339--4, the benchmark object for the main\nbranch of the correlation, and H1743--322, the first source found to be an\noutlier of the correlation. The obtained power-law dependencies of the radio\nflux on the bolometric flux have significantly different indices from those\nfound for the narrow X-ray bands. Also, the radio/bolometric flux correlations\nfor the rise of the outbursts are found to be significantly different from\nthose for the outburst decline. This points to a possible existence of a jet\nhysteresis in the radio/X-ray source evolution, in addition to that seen in the\nhardness/flux diagram of low-mass X-ray binaries. The correlation during the\nrise of the outbursts is similar for both GX 339--4 and H1743--322. The\ncorrelation for the decline of the outbursts for H1743--322 lies below that of\nGX 339--4 at intermediate X-ray fluxes, whereas it approaches the standard\ncorrelation at lower X-ray luminosities. We also compare these correlations to\nthose for the high-mass X-ray binaries Cyg X-1 and Cyg X-3.\n

Cataclysmic variables are very close binary stars in which the primary component is a white dwarf. The secondary star, usually a low-mass main-sequence star, is filling its Roche lobe and transferring material to the white dwarf. This material has too much angular momentum to fall directly on to the white dwarf and instead forms an accretion disc, which makes up the third major component of the cataclysmic variable. A magnetic field can be associated with each of the two stars and the disc. In the case of the white dwarf the field is frozen in. It can be strong enough to disrupt all or part of the accretion disc and force the accreting matter to fall in along field lines. The fields associated with the red dwarf and accretion disc appear to be dynamo generated. They cannot be measured directly but must be inferred from other properties of the systems. In cataclysmic variables, conservative mass transfer would lead to the secondary star shrinking within its Roche lobe until it expanded on a nuclear timescale. For almost all systems this exceeds the age of the Galaxy by several orders of magnitude. Nor can simple mass loss drive the observed mass transfer and an alternative angular momentum loss mechanism is required. At the shortest periods gravitational radiation suffices but at longer periods magnetic braking seems to be more likely. Any determination of the mass transfer rate can then constrain the field strength and structure and ultimately the nature of any magnetic dynamo. In the absence of viscosity, transferred material would accumulate in a ring around the white dwarf. Viscosity allows this ring to spread out into a disc so that matter accretes while angular momentum is returned to the orbit. Some form of radial magnetic field in the disc provides the most viable source of viscosity. It appears to be necessary to generate this field by dynamo action within the disc itself. Dwarf novae undergo periodic outbursts that are due to changes in accretion rate through the disc. This time dependent behaviour probes the strength of the radial and azimuthal field and hence any disc dynamo.

We investigate the optical variability of 7658 quasars from SDSS Stripe 82. Taking advantage of a larger sample and relatively more data points for each quasar, we estimate variability amplitudes and divide the sample into small bins of redshift, rest-frame wavelength, black hole mass, Eddington ratio, and bolometric luminosity, respectively, to investigate the relationships between variability and these parameters. An anti-correlation between variability and rest-frame wavelength is found. The variability amplitude of radio-quiet quasars shows almost no cosmological evolution, but that of radio-loud ones may weakly anti-correlate with redshift. In addition, variability increases as either luminosity or Eddington ratio decreases. However, the relationship between variability and black hole mass is uncertain; it is negative when the influence of Eddington ratio is excluded, but positive when the influence of luminosity is excluded. The intrinsic distribution of variability amplitudes for radio-loud and radio-quiet quasars are different. Both radio-loud and radio-quiet quasars exhibit a bluer-when-brighter chromatism. Assuming that quasar variability is caused by variations of accretion rate, the Shakura–Sunyaev disk model can reproduce the tendencies of observed correlations between variability and rest-frame wavelength, luminosity as well as Eddington ratio, supporting that changes of accretion rate play an important role in producing the observed optical variability. However, the predicted positive correlation between variability and black hole mass seems to be inconsistent with the observed negative correlation between them in small bins of Eddington ratio, which suggests that other physical mechanisms may still need to be considered in modifying the simple accretion disk model.

Numerous eruptive variable young stellar objects (YSOs), mostly Class I systems, were recently detected by the near-infrared Vista Variables in the Via Lactea (VVV) survey. We present an exploratory near-infrared spectroscopic variability study of 14 eruptive YSOs. The variations were sampled over one-day and one-to-two-year intervals and analysed in combination with VVV light curves. CO overtone absorption features are observed on three objects with FUor-like spectra: all show deeper absorption when they are brighter. This implies stronger emission from the circumstellar disc with a steeper vertical temperature gradient when the accretion rate is higher. This confirms the nature of fast VVV FUor-like events, in line with the accepted picture for classical FUors. The absence of Brγ emission in a FUor-like object declining to pre-outburst brightness suggests that reconstruction of the stellar magnetic field is a slow process. Within the one-day time-scale, 60 per cent of H2-emitting YSOs show significant but modest variation, and 2/6 sources have large variations in Brγ. Over year-long time-scales, H2 flux variations remain modest despite up to 1.8 mag variation in Ks. This indicates that emission from the molecular outflow usually arises further from the protostar and is unaffected by relatively large changes in accretion rate on year-long time-scales. Two objects show signs of on/off magnetospheric accretion traced by Brγ emission. In addition, a 60 per cent inter-night brightening of the H2 outflow is detected in one YSO.

Some different correlations between optical-ultraviolet (UV) variability and other quasar properties, such as luminosity, black hole mass and rest-frame wavelength, were discovered. The positive correlation between optical-UV variability amplitude and black hole mass was first found by Wold et al., and this was confirmed by Wilhite et al. We suggest that the accretion disc model can explain these correlations, provided the optical-UV variability is triggered by the change of accretion rate. The disc temperature of accretion discs decreases with increasing black hole mass, which leads to systematical spectral shape difference with black hole mass even if the black hole is accreting at the same rate . The observed positive correlation between optical-UV variability and black hole mass can be well reproduced by our model calculations, if the mean accretion rate with variation of . We also found that the observed correlations of optical-UV variability amplitude with luminosity or rest-frame wavelength can be qualitatively explained by this accretion disc model.

This letter reports statistically significant changes in the equivalent widths of ${\rm Mg\, {\small II}}$ and ${\rm Ca\, {\small II}}$ lines in the dusty and polluted white dwarf WD 0106–328, based on six epochs of spectroscopy using the Very Large Telescope and Keck spanning 25 yr. Furthermore, the ratio of these two equivalent widths may also vary, with a 7 per cent probability of being constant. Between 2000 and 2025, both Mg and Ca have experienced decreases in accretion rates of approximately 20 per cent and 60 per cent, respectively, but with individual variation during the interim. These metal abundance decreases are the first empirical corroboration of diffusion theory in white dwarfs, which predict sinking time-scales on the order of days for this star. However, the persistent atmospheric metals require a more gradual, circumstellar process, where one possibility is viscous spreading in an ionized disc of metals, consistent with $\alpha \approx 0.1$ within that formalism. The combination of optical and ultraviolet spectroscopy with the Hubble Space Telescope detects all the major rock-forming elements (O, Mg, Si, Fe), and demonstrates that Fe dominates the accreted material by mass, and that it is delivered mostly as pure metal from within a differentiated parent body. This inference is consistent with the possibility that chemically segregated accretion may result from a combination of planetary assembly, fragmentation, and disc evolution, and can be observed on relatively short time-scales.

We present an i-band photometric study of over 800 young stellar objects in the OB association Cep OB3b, which samples time-scales from one minute to 10 yr. Using structure functions we show that on all time-scales (τ) there is a monotonic decrease in variability from Class I to Class II through the transition disc (TD) systems to Class III, i.e. the more evolved systems are less variable. The Class Is show an approximately power-law increase (τ0.8) in variability from time-scales of a few minutes to 10 yr. The Class II, TDs, and Class III systems show a qualitatively different behaviour with most showing a power-law increase in variability up to a time-scale corresponding to the rotational period of the star, with little additional variability beyond that time-scale. However, about a third of the Class IIs shows lower overall variability, but their variability is still increasing at 10 yr. This behaviour can be explained if all Class IIs have two primary components to their variability. The first is an underlying roughly power-law variability spectrum, which evidence from the infrared suggests is driven by accretion rate changes. The second component is approximately sinusoidal and results from the rotation of the star. We suggest that the systems with dominant longer time-scale variability have a smaller rotational modulation either because they are seen at low inclinations or have more complex magnetic field geometries. We derive a new way of calculating structure functions for large simulated data sets (the ‘fast structure function’), based on fast Fourier transforms.

We present results of a comprehensive analysis of the X-ray pulsar X Persei over the period 1996 to 2011, encompassing the quite low state and subsequent strong outburst activity. Using data from the RXTE and Swift observatories we detected several consecutive outbursts, during which the source luminosity increased by a factor of ~5 up to L_x~1.2x10^{35} erg/s. Previously the source was observed in a high state only once. The source spectrum in a standard energy band (4-25 keV) is independent of the flux change and can be described by a model that includes both thermal and non-thermal components. With the help of the INTEGRAL observatory data we registered the highly significant cyclotron absorption line in the source spectrum and, for the first time, significantly detected a hard X-ray emission from the pulsar up to ~160 keV. We also report drastic changes of the pulse period during the outburst activity: a long episode of spin-down was changed to spin-up with a rate of Pdot/P = -(3-5)x10^{-4} yr^{-1}, that is several times higher than previous rates of spin-up and spin-down. To search for a correlation between the X-ray and optical lightcurves we took data from the AAVSO International Database. No significant correlation between optical and X-ray fluxes at any time lag from dozens of days to years was found.

The Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) has been providing high-cadence, high-resolution, full-disk UV-visible/extreme ultraviolet (EUV) images since 2010, with the best time coverage among all the solar missions. A number of codes have been developed to extract plasma differential emission measures (DEMs) from AIA images. Although widely used, they cannot effectively constrain the DEM at flaring temperatures with AIA data alone. This often results in much higher X-ray fluxes than observed. One way to solve the problem is by adding more constraint from other data sets (such as soft X-ray images and fluxes). However, the spatial information of plasma DEMs are lost in many cases. In this Letter, we present a different approach to constrain the DEMs. We tested the sparse inversion code and show that the default settings reproduce X-ray fluxes that could be too high. Based on the tests with both simulated and observed AIA data, we provided recommended settings of basis functions and tolerances. The new DEM solutions derived from AIA images alone are much more consistent with (thermal) X-ray observations, and provide valuable information by mapping the thermal plasma from ∼0.3 to ∼30 MK. Such improvement is a key step in understanding the nature of individual X-ray sources, and particularly important for studies of flare initiation.

We describe the first X-ray monitoring of a symbiotic star during phases of enhanced activity. AG Dra is a Pop II object with a composite spectrum, characterized by a cool K-type component, prominent high ionization emission lines and a strong UV continuum which is attributed to a hot dwarf companion. Periodic variability of the UV radiation during minimum could be attributed to the orbital motion of the system. In April 1980 HEAO-2 detected an intense, soft X-ray flux from AG Dra, stronger than in other symbiotic stars. After one major outburst of November 1980, which continued until 1983, two more outbursts occurred in February 1985 and January 1986, and coordinated X-ray (EXOSAT) and ultraviolet (IUE) observations were organized to study the behaviour of AG Dra during different activity phases. EXOSAT observations made during decline after the 1985 outburst, revealed a weak X-ray flux in the Thin Lexan filter of the Low Energy dedtector. Observations made during minimum, in June and November 1985, at phases 0.22 and 0.50 of the UV light curve, disclosed the presence of an intense X-ray flux, which was not occulted in November. AG Dra was again observed with EXOSAT in February 1986 when the stellar luminosity was still at maximum. No X-ray flux was detected, in spite of the prominent, high ionization UV spectrum observed with IUE.A detailed discussion of the X-ray and ultraviolet results on AG Dra in the light of possible models is in progress.