X-ray Irradiated Disk Research Articles

Clockwise evolution in the hardness–intensity diagram of the black hole X-ray binary Swift J1910.2−0546

ABSTRACT We present a detailed study of optical data from the 2012 outburst of the candidate black hole X-ray binary Swift J1910.2−0546 using the Faulkes Telescope and Las Cumbres Observatory (LCO). We analyse the peculiar spectral state changes of Swift J1910.2−0546 in different energy bands, and characterize how the optical and UV emission correlates with the unusual spectral state evolution. Using various diagnostic tools like the optical/X-ray correlation and spectral energy distributions, we disentangle the different emission processes contributing towards the optical flux of the system. When Swift J1910.2−0546 transitions to the pure hard state, we find significant optical brightening of the source along with a dramatic change in the optical colour due to the onset of a jet during the spectral state transition. For the rest of the spectral states, the optical/UV emission is mostly dominated by an X-ray irradiated disc. From our high cadence optical study, we have discovered a putative modulation. Assuming that this modulation arises from a superhump, we suggest Swift J1910.2−0546 to have an orbital period of 2.25–2.47 h, which would make it the shortest orbital period black hole X-ray binary known to date. Finally, from the state transition luminosity of the source, we find that the distance to the source is likely to be ∼4.5–20.8 kpc, which is also supported by the comparative position of the source in the global optical/X-ray correlation of a large sample of black hole and neutron star X-ray binaries.

Swift and SALT observations of the multiple outbursts of MAXI J1957+032

The new recurrent X-ray transient MAXI J1957+032 has had four X-ray outbursts within 16 months, all very briefly detected (lasting <5 days). During the most recent event (Sep/Oct 2016), we obtained with SALT the first optical spectrum of the transient counterpart, showing the classic blue continuum of an X-ray irradiated disc in an LMXB and no other features. At high Galactic latitude below the plane (-13 deg) reddening is low but there is no quiescent counterpart visible on any of the existing sky surveys, nor any other known X-ray source in the region. Swift monitoring of 3 of the 4 events is presented, showing rapidly fading X-ray outbursts together with significant UVOT detections in the UV (W1,M2,W2), U and B bands. The optical properties are most like those of the short-period LMXBs, which, combined with the softening witnessed during the decay to quiescence would place the system at d <13 kpc. The short duration and short recurrence time of the outbursts are reminiscent of the AMXPs, which exhibit peak luminosities of 1 % L_Edd. Assuming this peak luminosity would place MAXI J1957+032 at a distance of d= 5-6 kpc.

Far-ultraviolet and X-ray irradiated protoplanetary disks: a grid of models

Context. Planets are thought to eventually form from the mostly gaseous (~99% of the mass) disks around young stars. The density structure and chemical composition of protoplanetary disks are affected by the incident radiation field at optical, FUV, and X-ray wavelengths, as well as by the dust properties. Aims. The effect of FUV and X-rays on the disk structure and the gas chemical composition are investigated. This work forms the basis of a second paper, which discusses the impact on diagnostic lines of, e.g., C+, O, H2O, and Ne+ observed with facilities such as Spitzer and Herschel. Methods. A grid of 240 models is computed in which the X-ray and FUV luminosity, minimum grain size, dust size distribution, and surface density distribution are varied in a systematic way. The hydrostatic structure and the thermo-chemical structure are calculated using ProDiMo. Results. The abundance structure of neutral oxygen is stable to changes in the X-ray and FUV luminosity, and the emission lines will thus be useful tracers of the disk mass and temperature. The C+ abundance distribution is sensitive to both X-rays and FUV. The radial column density profile shows two peaks, one at the inner rim and a second one at a radius r=5-10 AU. Ne+ and other heavy elements have a very strong response to X-rays, and the column density in the inner disk increases by two orders of magnitude from the lowest (LX = 1e29 erg/s) to the highest considered X-ray flux (LX = 1e32 erg/s). FUV confines the Ne+ ionized region to areas closer to the star at low X-ray luminosities (LX = 1e29 erg/s). H2O abundances are enhanced by X-rays due to higher temperatures in the inner disk and higher ionization fractions in the outer disk. The line fluxes and profiles are affected by the effects on these species, thus providing diagnostic value in the study of FUV and X-ray irradiated disks around T Tauri stars. (abridged)

Far-ultraviolet and X-ray irradiated protoplanetary disks: a grid of models

Most of the mass in protoplanetary disks is in the form of gas. The study of the gas and its diagnostics is of fundamental importance in order to achieve a detailed description of the thermal and chemical structure of the disk. The radiation from the central star (from optical to X-ray wavelengths) and viscous accretion are the main source of energy and dominates the disk physics and chemistry in its early stages. This is the environment in which the first phases of planet formation will proceed. We investigate how stellar and disk parameters impact the fine-structure cooling lines [NeII], [ArII], [OI], [CII] and H2O rotational lines in the disk. These lines are potentially powerful diagnostics of the disk structure and their modelling permits a thorough interpretation of the observations carried out with instrumental facilities such as Spitzer and Herschel. Following Aresu et al. (2011), we computed a grid of 240 disk models, in which the X-ray luminosity, UV-excess luminosity, minimum dust grain size, dust size distribution power law and surface density distribution power law, are systematically varied. We solve self-consistently for the disk vertical hydrostatic structure in every model and apply detailed line radiative transfer to calculate line fluxes and profiles for a series of well known mid- and far-infrared cooling lines. The [OI] 63 micron line flux increases with increasing FUV luminosity when Lx < 1e30 erg/s, and with increasing X-ray luminosity when LX > 1e30 erg/s. [CII] 157 micron is mainly driven by FUV luminosity via C+ production, X-rays affect the line flux to a lesser extent. [NeII] 12.8 micron correlates with X-rays; the line profile emitted from the disk atmosphere shows a double-peaked component, caused by emission in the static disk atmosphere, next to a high velocity double-peaked component, caused by emission in the very inner rim. (abridged)

THE NATURE OF THE UV/OPTICAL EMISSION OF THE ULTRALUMINOUS X-RAY SOURCE IN HOLMBERG II

We report on UV and X-ray spectroscopy and broad-band optical observations of the ultraluminous X-ray source in Holmberg II. Fitting various stellar spectral models to the combined, non-simultaneous data set, we find that normal metallicity stellar spectra are ruled out by the data, while low metallicity, Z = 0.1 Z_{\odot}, late O-star spectra provide marginally acceptable fits, if we allow for the fact that X-ray ionization from the compact object may reduce or eliminate UV absorption/emission lines from the stellar wind. By contrast, an irradiated disk model fits both UV and optical data with chi^2/dof=175.9/178, and matches the nebular extinction with a reddening of E(B-V)=0.05^{+0.05}_{-0.04}. These results suggest that the UV/optical flux of Holmberg II X-1 may be dominated by X-ray irradiated disk emission.

Ne II FINE-STRUCTURE LINE EMISSION FROM THE OUTFLOWS OF YOUNG STELLAR OBJECTS

The flux and line shape of the fine-structure transitions of \NeII\ and \NeIII\ at 12.8 and 15.55\,$\mu$m and of the forbidden transitions of \OI\ $\lambda6300$ are calculated for young stellar objects with a range of mass-loss rates and X-ray luminosities using the X-wind model of jets and the associated wide-angle winds. For moderate and high accretion rates, the calculated \NeII\ line luminosity is comparable to or much larger than produced in X-ray irradiated disk models. All of the line luminosities correlate well with the main parameter in the X-wind model, the mass-loss rate, and also with the assumed X-ray luminosity --- and with one another. The line shapes of an approaching jet are broad and have strong blue-shifted peaks near the effective terminal velocity of the jet. They serve as a characteristic and testable aspect of jet production of the neon fine-structure lines and the \OI\ forbidden transitions.

Molecular hydrogen emission from protoplanetary disks: UV and X-ray irradiated disk model with dust evolution

We have modelled a detailed physical structure of protoplanetary disks, taking into account X-ray and UV irradiation from a central star, as well as dust size growth and settling towards the disk midplane. In addition, we have calculated the level populations and line emission of molecular hydrogen in the disks. As a result, we reproduce the observed strong H 2 line flux if the disks are influenced by strong UV and X-ray irradiation. Also, the dust evolution changes the physical properties of the disk, and thus the H 2 line ratios.

Role of emission angular directionality in spin determination of accreting black holes with a broad iron line

Spin of an accreting black hole can be determined by spectroscopy of the emission and absorption features produced in the inner regions of an accretion disc. We discuss the method employing the relativistic line profiles of iron in the X-ray domain, where the emergent spectrum is blurred by general relativistic effects. Precision of spectra fitting procedure could be compromised by inappropriate account of the angular distribution of the disc emission. Often a unique profile is assumed, invariable over the entire range of radii in the disc and energy in the spectral band. We study how sensitive the spin determination is to the assumptions about the intrinsic angular distribution of the emitted photons. We find that the uncertainty of the directional emission distribution translates to 20% uncertainty in determination of the marginally stable orbit. By assuming a rotating black hole in the centre of an accretion disc, we perform radiation transfer computations of an X-ray irradiated disc atmosphere to determine the directionality of outgoing X-rays in the 2-10 keV energy band. We implemented the simulation results as a new extension to the KY software package for X-ray spectra fitting of relativistic accretion disc models. Although the parameter space is rather complex, leading to a rich variety of possible outcomes, we find that on average the isotropic directionality reproduces our model data to the best precision. Our results also suggest that an improper usage of limb darkening can partly mimic a steeper profile of radial emissivity. We demonstrate these results on the case of XMM-Newton observation of the Seyfert galaxy MCG-6-30-15, for which we construct confidence levels of chi squared statistics, and on the simulated data for the future X-ray IXO mission.

Results from DROXO

The infrared [Ne II] and [Ne III] fine structure lines at 12.81um and 15.55um are predicted to trace the circumstellar disk gas subject to X-ray heating and ionization. We investigate the origin of these lines by comparing observations with models of X-ray irradiated disks and by searching for empirical correlations between the line luminosities and stellar and circumstellar parameters. We measure neon line fluxes and X-ray luminosities for 28 young stellar objects in the Rho Ophiuchi star formation region for which good quality infrared spectra and X-ray data have been obtained, the former with the Spitzer IRS and the latter with the Deep Rho Ophiuchi XMM-Newton Observation. We detect the [Ne II] and the [Ne III] lines in 10 and 1 cases, respectively. Line luminosities show no correlation with X-ray emission. The luminosity of the [Ne II] line for one star, and that of both the [Ne II] and [Ne III] lines for a second star, match the predictions of published models of X-ray irradiated disks; for the remaining 8 objects the [Ne II] emission is 1-3 dex higher than predicted on the basis of their L_X. Class I objects show significantly stronger [Ne II] lines than Class II and Class III ones. A correlation is moreover found between the [Ne II] line emission and the disk mass accretion rates. This might point toward a role of accretion-generated UV emission in the generation of the line or to other mechanisms related to mass inflows from circumstellar disks and envelopes and/or to the associated mass outflows (winds and jets). We conclude that the X-ray luminosity is not the only parameter that determines the [Ne II] emission. Explaining the strong [Ne II] emission of Class I objects likely requires the inclusion in the models of additional physical components such as the envelope, inflows, and outflows.

C2dSpitzerIRS Spectra of Disks around T Tauri Stars. III. [NeII], [FeI], and H2Gas‐Phase Lines

We present a survey of mid-infrared gas-phase lines toward a sample of 76 circumstellar disks around low mass pre-main sequence stars from the Spitzer Cores to Disks legacy program. We report the first detections of [Ne II] and [Fe I] toward classical T Tauri stars in ~20% respectively ~9% of our sources. The observed [Ne II] line fluxes and upper limits are consistent with [Ne II] excitation in an X-ray irradiated disk around stars with X-ray luminosities L_X=10^{29}-10^{31} erg s^{-1}. [Fe I] is detected at ~10^-5-10^-4 L_Sun, but no [S I] or [Fe II] is detected down to ~10^{-6} L_Sun. The [Fe I] detections indicate the presence of gas-rich disks with masses of >~0.1 M_J. No H_2 0-0 S(0) and S(1) disk emission is detected, except for S(1) toward one source. These data give upper limits on the warm (T~100-200K) gas mass of a few Jovian masses, consistent with recent T Tauri disk models which include gas heating by stellar radiation. Compact disk emission of hot (T>~500K) gas is observed through the H_2 0-0 S(2) and/or S(3) lines toward ~8% of our sources. The line fluxes are, however, higher by more than an order of magnitude than those predicted by recent disk models, even when X-ray and excess UV radiation are included. Similarly the [Ne II]/H_2 0-0 S(2) ratios for these sources are lower than predicted, consistent with the presence of an additional hot molecular gas component not included in current disk models. Oblique shocks of stellar winds interacting with the disk can explain many aspects of the hot gas emission, but are inconsistent with the non-detection of [S I] and [Fe II] lines.

A Search for Fallback Disks in Four Young Supernova Remnants

We report on our search for the optical/infrared counterparts to the central compact objects in four young supernova remnants: Pup A, PKS 1209−52, RCW 103, and Cas A. The X-ray point sources in these supernova remnants are excellent targets for probing the existence of supernova fallback disks, since irradiation of a disk by a central X-ray source should lead to an infrared excess. We used ground-based optical and near-infrared imaging and Spitzer Space Telescope mid-infrared imaging to search for optical/infrared counterparts at the X-ray point source positions measured by the Chandra X-Ray Observatory. We did not detect any counterparts, and hence find no evidence for fallback disks around any of these sources. In PKS 1209−52, we are able to exclude a nearby optical/infrared candidate counterpart. In RCW 103, a blend of 3 faint stars at the X-ray source position prevents us from deriving useful limits. For the other targets, the upper limits on the infrared/X-ray flux ratio are as deep as (1.0–1.7)×10 −4 . Comparing these limits to the ratio of ≈ 6 × 10 −5 measured for 4U 0142+61 (a young pulsar recently found with an X-ray irradiated dust disk), we conclude that the non-detection of any disks around young neutron stars studied here are consistent with their relatively low X-ray luminosities, although we note that a similar dust disk around the neutron star in Pup A should be detectable by deeper infrared observations. Subject headings: ISM: individual (Puppis A, PKS 1209−52, RCW 103, Cassiopeia A) — supernova remnants — X-rays: stars — stars: neutron

The vertical structure and ultraviolet spectrum of X-ray irradiated accretion disks in active galactic nuclei

Motivated by recent work indicating that the UV continuum in active galactic nuclei may be produced by reradiation of energy absorbed from X-rays irradiating an accretion disk, we present a calculation of the vertical structures and ultraviolet spectra of X-ray-irradiated accretion disks around massive nonrotating black holes. After finding the radial dependence of vertically integrated quantities for these disks, we solve the equations of hydrostatic equilibrium, energy balance, and frequency-dependent radiation transfer as functions of altitude. To solve the last set of equations, we use a variable Eddington factor method. We include electron scattering, free-free and H I, He I, and He II bound-free opacities, and the corresponding continuum cooling processes. The incident X-ray flux heats a thin layer of material 3-4 scale heights above the midplane of the disk. This X-ray-heated skin has two layers: (1) a radiation pressure-supported region in which the UV flux is created, and, immediately above this layer, (2) a warmer zone, optically thin to UV radiation, formed where the X-ray ionization parameter is large. In the lower layer, the gas pressure is nearly independent of altitude, but the temperature increases upward. The fraction of the incident hard X-ray flux that emerges in the UV falls with increasing ${u{m}{705F}}$ --> (the accretion rate in Eddington units). At frequencies below the Lyman edge, the slope of the continuum (d ln L?/d ln ?) varies from -1.6 to 0.8 as ${u{m}{705F}}$ -->/m -->8 increases from 0.001 to 1. Here m8 is the mass of the central black hole in units of 108 M?. In all cases examined (0.003?${u{m}{705F}}$ -->?0.3 and 0.27 ? m8 ? 27), the Lyman edge appears in emission. The amplitude of the Lyman edge feature increases with m8 but is relatively independent of ${u{m}{705F}}$ -->. The amplitude of the Lyman edge emission feature increases with disk inclination. Compton scattering in disk coronae can smooth the Lyman edge feature only if ?c 0.5, where ?c is the Thomson depth of the coronae. While the overall spectral shape predicted by X-ray irradiation may be compatible with observations, the Lyman edge emission feature it predicts is not. This finding raises questions for many otherwise plausible models in which X-ray irradiation plays a major role.

The Vertical Structure and Ultraviolet Spectrum of X‐Ray–irradiated Accretion Disks in Active Galactic Nuclei

Motivated by recent work indicating that the UV continuum in AGN may be produced by reradiation of energy absorbed from X-rays irradiating an accretion disk, we present a calculation of the vertical structures and ultraviolet spectra of X-ray irradiated accretion disks around massive non-rotating black holes. After finding the radial dependence of vertically-integrated quantities for these disks, we solve the equations of hydrostatic equilibrium, energy balance, and frequency-dependent radiation transfer as functions of altitude. To solve the last set of equations, we use a variable Eddington factor method. We include electron scattering, free-free, and HI, HeI, and HeII bound-free opacities and the corresponding continuum cooling processes. While the overall spectral shape predicted by X-ray irradiation may be compatible with observations, the Lyman edge emission feature it predicts is not. This finding raises questions for many otherwise plausible models in which X-ray irradiation plays a major role.

X-ray irradiated accretion disks and bimodal states

X-ray irradiation plays an important role in the structure of an outer accretion disk. The authors examine the outer portion of an accretion disk, carefully taking into account X-ray irradiation within the context of α-viscosity. Bimodal states observed in the X-ray sources Cyg X-1 and GX 339-4 are explained naturally in terms of an accretion disk irradiated by X-rays originating at the inner portion of the disk.