Increase In Accretion Rate Research Articles

On The Low Frequency Quasi Periodic Oscillations Of X-Ray Sources

Based on the interpretation of the twin kilohertz Quasi Periodic Oscillations (kHz QPOs) of X-ray spectra of Low Mass X-Ray Binaries (LMXBs) ascribed to the Keplerian and the periastron precession frequencies at the inner disk respectively, we ascribe the low frequency (0.1–10 Hz) Quasi Periodic Oscillations (LFQPO) and HBO (15–60 Hz QPO for Z sources or Atoll sources) to the periastron precession at some outer disk radius. It is assumed that both radii are correlated by a scaling factor of 0.4. The conclusions obtained include: All QPO frequencies increase with increasing accretion rate. The theoretical relations between HBO (LFQPO) frequency and the kHz QPO frequencies are similar to the measured empirical formula.

Massive planets in FU Orionis discs: implications for thermal instability models

FU Orionis are young stellar objects undergoing episodes of enhanced luminosity, which are generally ascribed to a sudden increase of mass accretion rate in the surrounding protostellar disc. Models invoking a thermal instability in the disc are able to reproduce many features of the outburst, but cannot explain the rapid rise time-scale observed in many cases. Here we explore the possibility (originally suggested by Clarke & Syer 1996) that the thermal instability is triggered away from the disc inner edge (at a distance of $\approx 10R_{\odot}$ from the central protostar) due to the presence of a massive planet embedded in the disc. We have constructed simple, one-dimensional, time-dependent models of the disc evolution, taking into account both the interaction between the disc and the planet, and the thermal evolution of the disc. We are indeed able to reproduce rapid rise outbursts (with rise time-scale $\approx 1$ yr), with a planet mass $M_{\mathrm{s}}=10-15M_{\mathrm{Jupiter}}$. We show that the luminosity and the duration of the outbursts are increasing functions of planet mass. We also show that the inward migration of the planet is significantly slowed once it reaches the radius where it is able to trigger FU Orionis outbursts, thus suggesting that a single planet may be involved in triggering several outbursts.

Heavy Elements Synthesized in an Accretion Disk around a Black Hole Associated with Gamma-Ray Bursts

We examine nucleosynthesis inside an accretion disk with accretion rates 0.01 - 1M s - 1 associated with gamma-ray bursts. The black hole mass is fixed to be 3M. The chemical composition of accreting gas far from the black hole is taken to be that of an oxygen-rich layer of a massive star before the collapse. The disk consists of five layers characterized by dominant elements: 1 6 O, 2 8 Si, 5 4 Fe (and 5 6 Ni), 4 He, and nucleons. As the accretion rate increases, the individual layers shift outwards with keeping the overall profiles of composition. A similar situation may be realized in an accretion disk around an intermediate mass black hole produced in a collapsing Population III supermassive star.

The use of wetlands in the Mississippi Delta for wastewater assimilation: a review

The use of wetlands for treatment of wastewaters has a number of important ecological and economic benefits. Adding nutrient rich treated wastewater effluent to selected coastal wetlands results in the following benefits: (1) improved effluent water quality; (2) increased accretion rates to help offset subsidence; (3) increased productivity of vegetation; and (4) financial and energy savings of capital not invested in conventional tertiary treatment systems. We present as case studies results from several wetlands that are receiving secondarily treated wastewater in coastal Louisiana. At one site where sedimentation accumulation was measured, rates of accretion increased significantly after wastewater application began in the treatment site (from 7.8 to 11.4 mm yr −1) and approached the estimated rate of regional relative sea level rise (RSLR) (12.0 mm yr −1). No corresponding increase was observed in an adjacent control site. This suggests that the application of nutrient-rich wastewater can help coastal wetlands survive sea level rise. In the same site, surface water nutrient reduction, from the effluent inflow to outflow (1600 m), ranged from 100% for nitrate-nitrogen (NO 3-N) to 66% for total phosphorus (P). At a second site, a forested wetland that has been receiving wastewater effluent for 50 years, N and P were both reduced by more than 90%. Nutrient reduction is due to three main pathways: burial, denitrification and plant uptake. Dendrochronological analysis at the second site revealed that stem growth increased significantly in the treatment site after wastewater applications began, and was significantly greater than an adjacent control site. Similar increases in productivity have been measured in a number of wetland treatment sites. Economic analyses comparing conventional and wetland systems indicate savings range from $500,000 to $2.6 million. In addition there are substantial energy savings.

Relaxation Oscillations of a Luminous Accretion Disk in a Microquasar: Spectral Evolution and Observational Implications

To understand the busting behaviour of microquasars, we performed time evolution of luminous accretion disks and the spectrum evolution, taking into account the inverse Cornpton scattering, gravitational redshift, etc. The spectra during the burst became a factor of 2-3 harder as the mass accretion rate increases, due to the inverse Compton process inside the disk. This trend is roughly consistent with the observational properties such as burst duration, luminosity, and maximum temperature of microquasar GRS1915+105.

Variability of the Hα emission of Cygnus X-1 and its connection with the soft X-ray radiation

High-resolution Halpha monitoring of Cyg X-1, HD226868 was carried out during 1996-2002 and the resultant spectra analysed in conjunction with 1.5-12 keV X-ray monitoring. We demonstrate that the Halpha line-profiles have complex variability on different timescales, controlled in particular by the orbital period and the focused wind model of mass loss. We find that long-term variability of the mass loss by the supergiant and short-term variability due to clumpy structure of the stellar wind dominate during the low/hard X-ray state and that X-ray photoionization has a relatively small influence on the line-profile shape and EW variability. During the high/soft X-ray state and flaring the effect of photoionization the line-profile and EW of Halpha increases but is still unable to describe the loose anti-correlation between EW and the low energy X-ray emission. We propose that variability of the mass loss by the supergiant can change wind velocities in the Stromgren zone around the accretion disc of the secondary, leading to an increase in accretion rate through the disc.

SMC X-1 as an Intermediate-Stage Flaring X-Ray Pulsar

We present Rossi X-ray Timing Explorer observations of the X-ray pulsar SMC X-1. The source is highly variable on short time scales (< 1 h), exhibiting apparent X-ray flares occupying a significant fraction (~3 %) of the total observing time, with a recurrence time of ~100 s. The flares seem to occur over all binary orbital phases, and correlate with the overall variability in the light curve. We find a total of 323 discrete flares which have a mean full width half maximum of ~18 s. The detailed properties of SMC X-1 do not vary significantly between the flares and the normal state, suggesting that the flare may be an extension of the normal state persistent emission with increased accretion rates. The flares resemble Type II X-ray bursts from GRO J1744--28. We discuss the origin of the SMC X-1 flares in terms of a viscous instability near the inner edge of the accretion disk around a weakly magnetized X-ray pulsar, and find this is consistent with the interpretation that SMC X-1 is in fact an intermediate-stage source like GRO J1744--28.

Extreme Ultraviolet ExplorerPhase‐resolved Spectroscopy of V834 Centauri

The Extreme Ultraviolet Explorer (EUVE) satellite was employed for 5.46 days, beginning on 1999 February 9.03 UT, to acquire phase-resolved EUV photometric and spectroscopic observations of the AM Her type cataclysmic variable V834 Centauri. The resulting data are superior to those obtained by EUVE beginning on 1993 May 28.14 UT, because the source was approximately 3 times brighter, the observation was 4 times longer and dithered, and ASCA observed the source simultaneously. Although we do not understand the EUV light curves in detail, they are explained qualitatively by a simple model of accretion from a ballistic stream along the field lines of a tilted ([β,ψ] ≈ [10°,40°]) magnetic dipole centered on the white dwarf. In 1993, when the EUV flux was lower, accretion was primarily along the ≈ ψ ≈ 40° field line, whereas in 1999, when the EUV flux was higher, accretion took place over a broad range of azimuths extending from ≈ ψ ≈ 40° to ≈ 76°. These changes in the accretion geometry could be caused by an increase in the mass accretion rate and/or the clumpiness of the flow. The 75-140 A EUVE spectra are well described by either a blackbody or a pure H stellar atmosphere absorbed by a neutral hydrogen column density, but constraints on the size of the EUV emission region and its UV brightness favor the blackbody interpretation. The mean 1999 EUV spectrum is best fitted by an absorbed blackbody with temperature kT ≈ 17.6 eV, hydrogen column density NH ≈ 7.4 × 1019 cm-2, fractional emitting area f ≈ 10-3, 70-140 A flux ≈ 3.0 × 10-11 ergs cm-2 s-1, and luminosity Lsoft ≈ 7.2 × 1032(d/100 pc)2 ergs s-1. The ratio of the EUV to X-ray luminosities is Lsoft/Lhard ≈ 40, signaling that some mechanism other than irradiation (e.g., blob heating) dominates energy input into the accretion spot. The 1999 short-wavelength (SW) hardness ratio variation can be explained by minor variations in kT and/or NH, but instead of tracking the SW count rate variation, the hardness ratio variation was sinusoidal, with a minimum (maximum) when the accretion spot was on the near (far) side of the white dwarf, consistent with the trend expected for an atmosphere with an inverted temperature distribution.

Wind Accretion and State Transitions in Cygnus X‐1

We present the results of a spectroscopic monitoring program (from 1998 to 2002) of the H-alpha emission strength in HDE 226868, the optical counterpart of the black hole binary, Cyg X-1. The H-alpha profiles consist of (1) a P Cygni component associated with the wind of the supergiant, (2) emission components that attain high velocity at the conjunctions and that probably form in enhanced outflows both towards and away from the black hole, and (3) an emission component that moves in anti-phase with the supergiant's motion. We argue that the third component forms in accreted gas near the black hole, and the radial velocity curve of the emission is consistent with a mass ratio of M_X / M_opt = 0.36 +/- 0.05. We find that there is a general anti-correlation between the H-alpha emission strength and X-ray flux in the sense that when the H-alpha emission is strong (W_\lambda < -0.5 Angstroms) the X-ray flux is weaker and the spectrum harder. On the other hand, there is no correlation between H-alpha emission strength and X-ray flux when H-alpha is weak. During the low/hard X-ray state, the strong wind is fast and the accretion rate is relatively low, while in the high/soft state the weaker, highly ionized wind attains only a moderate velocity and the accretion rate increases. We argue that the X-ray transitions from the normal low/hard to the rare high/soft state are triggered by episodes of decreased mass loss rate in the supergiant donor star.

Does the Slim‐Disk Model Correctly Consider Photon‐trapping Effects?

We investigate the photon-trapping effects in the super-critical black hole accretion flows by solving radiation transfer as well as the energy equations of radiation and gas. It is found that the slim-disk model generally overestimates the luminosity of the disk at around the Eddington luminosity (L_E) and is not accurate in describing the effective temperature profile, since it neglects time delay between energy generation at deeper inside the disk and energy release at the surface. Especially, the photon-trapping effects are appreciable even below L ~ L_E, while they appear above ~ 3L_E according to the slim disk. Through the photon-trapping effects, the luminosity is reduced and the effective temperature profile becomes flatter than r^{-3/4} as in the standard disk. In the case that the viscous heating is effective only around the equatorial plane, the luminosity is kept around the Eddington luminosity even at very large mass accretion rate, Mdot>>L_E/c^2. The effective temperature profile is almost flat, and the maximum temperature decreases in accordance with rise in the mass accretion rate. Thus, the most luminous radius shifts to the outer region when Mdot/(L_E/c^2) >> 10^2. In the case that the energy is dissipated equally at any heights, the resultant luminosity is somewhat larger than in the former case, but the energy-conversion efficiency still decreases with increase of the mass accretion rate, as well. The most luminous radius stays around the inner edge of the disk in the latter case. Hence, the effective temperature profile is sensitive to the vertical distribution of energy production rates, so is the spectral shape. Future observations of high L/L_E objects will be able to test our model.

On the spectral evolution of Cygnus X–2 along its color-color diagram

We report on the results of a broad band (0.1-200 keV) spectral study of Cyg X-2 using two BeppoSAX observations taken in 1996 and 1997, respectively, for a total effective on-source time of ~100 ks. The color-color (CD) and hardness-intensity (HID) diagrams show that the source was in the horizontal branch (HB) and normal branch (NB) during the 1996 and 1997 observation, respectively. Five spectra were selected around different positions of the source in the CD/HID, two in the HB and three in the NB. These spectra are fit to a model consisting of a disk blackbody, a Comptonization component, and two Gaussian emission lines at ~1 keV and ~6.6 keV, respectively. The addition of a hard power-law tail with photon index ~2, contributing ~1.5% of the source luminosity, improves the fit of the spectra in the HB. We interpret the soft component as the emission from the inner accretion disk, with inner temperature, k T_in, varying between 0.8 and 1.7 keV and inner radius, R_in, varying between 26 and 11 km (assuming an inclination angle of the system of 60 degrees). The Comptonization component is probably emitted by hot plasma (electron temperature k T_e varying between 3 and ~20 keV, optical depth \tau ~ 11-0.4, seed-photon temperature k T_W ~ 1-2.4 keV) surrounding the NS. The changes in the parameters of the blackbody component indicate that the inner rim of the disk approaches the NS surface when the source moves from the HB to the NB, i.e. as the (inferred) mass accretion rate increases. The parameters of the Comptonized component also change significantly when the source moves from the HB to the NB. We discuss possible scenarios which can explain these changes.

A comment on the colour-colour diagrams of low-mass X-ray binaries

Disc accreting neutron stars come in two distinct varieties, atolls and Z sources, named after their differently shaped tracks on a colour-colour diagram as the source luminosity changes. Here we present analysis of three transient atoll sources showing that there is an additional branch in the colour-colour diagram of atoll sources which appears at very low luminosities. This new branch connects to the top of previously known C-shaped (atoll) path, forming a horizontal track where the average source flux decrease from right to left. This turns the C-shape into a Z. Thus both atolls and Z sources share the same topology on the colour-colour diagram, and evolve in similar way as a function of increasing averaged mass accretion rate. This strongly favours models in which the underlying geometry of these sources changes in similar ways. A possible scenario is one where the truncated disc approaches the neutron star when the accretion rate increases, but that in the atolls the disc is truncated by evaporation (similarly to black holes) whereas in the Z sources it is truncated by the magnetic field.

Massive pre-main sequence stars in the Magellanic Clouds

We give an overview of the search and discovery of Sequence (PMS) stars in the Magellanic Clouds. For the first time the irregular optical brightness variability of Pre-Main Sequence stars, generally attributed to variable dust obscuration of the central star, has been used as an initial selection of young stellar objects. We discuss 21 PMS candidates in the Large Magellanic Cloud (LMC) and 7 PMS candidates in the Small Magellanic Cloud (SMC) detected in this way. These stars have bolometric luminosities which correspond to the most massive Galactic Herbig AeBe (HAeBe) stars, or even more massive. The location of the LMC and SMC objects is correlated with higher densities of cold interstellar dust as measured by IRAS in the far infrared. We derive the fundamental parameters from spectroscopy and find that there location in the HR-diagram would indicate that the stars are more luminous when compared to Galactic HAeBe stars of the same spectral type. There may be a trend with metallicity, in that the SMC PMS stars can be even more luminous than the LMC PMS stars. It indicates a possible increase of the proto-stellar accretion rate with decreasing metallicity. In the optical these HAeBe candidates have a higher probability to be found in small (10 pc) clusters then other field stars of similar colour and magnitude. We present a case study of a cluster around one of the LMC HAeBe stars, using high resolution SUSI imaging. The H α -emitters detected in this cluster are also located in an HR-diagram above the Galactic Palla-Stahler birthline corresponding to an average protostellar mass accretion rate of 10 -5 M ⊙ yr -1 .

Dust–Gas Accretion onto a Luminous Object

Spherical accretion of dust and gas onto a gravitating object is examined under the influence of central radiation. We found that, as long as the central luminosity is less than the Eddington one for dust, the gas flow is almost Bondi type, while the dust flow is greatly affected by central radiation. For non-luminous sources, in the outer region where the gravity on the dust is balanced by the drag force, the infall velocity of dust is generally larger than the gas velocity. Both gas and dust freefall in the inner region. As the dust-to-gas ratio increases, the gas accretion rate slightly decreases, while the dust accretion rate increases. As a result, the total accretion rate increases. When the central luminosity is considered, the infall velocity of dust becomes smaller and smaller, as the central luminosity increases. Although the gas accretion rate slightly increases with luminosity, the dust accretion rate linearly decreases due to the radiation pressure. As a result, the total accretion rate decreases with luminosity. Approximate expressions for dust and gas accretion rates were obtained.

Emission‐Line Diagnostics of T Tauri Magnetospheric Accretion. II. Improved Model Tests and Insights into Accretion Physics

We present new radiative transfer models of magnetospheric accretion in T Tauri stars. Hydrogen and Na I line profiles were calculated, including line damping and continuum opacity for a grid of models spanning a large range of infall rates, magnetospheric geometries, and gas temperatures. We also calculated models for rotating magnetospheres and show that for typical T Tauri rotation rates, the line profiles are not significantly affected. We show that line-damping wings can produce significant high-velocity emission at Hα, and to a lesser extent in higher Balmer lines, in much better agreement with observations than previous models. We present comparisons to specific objects spanning a wide range of accretion activity and find that in most cases the models successfully reproduce the observed emission profile features. Blueshifted absorption components cannot be explained without including a wind outside of the magnetosphere, and true P Cygni Balmer line profiles in the few objects with extreme accretion activity indicate both absorption and emission from a wind. We constrain the range of gas temperatures required to explain observational diagnostics like profile shapes, line ratios, and continuum emission. The exact heating mechanism remains unclear but is probably linked to the accretion process itself. In order to explain observed correlations between line emission and accretion luminosity, we find that the size of the emitting region must be correlated with the accretion rate. We suggest that such a correlation may manifest itself in reality via nonaxisymmetric accretion, where the number and/or width of discrete funnel flows increase with increasing accretion rate, a scenario also indicated by accretion shock models.

Updrift river mouth migration on cuspate deltas: two examples from the coast of Tuscany (Italy)

The configuration of the Arno and Ombrone deltas (Italy) is evaluated. The terminal courses of the rivers forming these deltas face the dominant wave orthogonal. This configuration came about during rapid delta progradation as a consequence of increased river sediment input due to widespread deforestation within the catchment area, a prevailing phenomenon from the Early Middle Ages to the XVIII century. The rivers attained and maintained their directions as the result of the increased accretion rate on the less exposed downdrift side of the delta. On the more exposed updrift side, delta growth caused the shoreline to gradually rotate so that it directly faced the waves approaching the coast. Here, due to the lower refraction, wave energy per shore unit increased whilst accretion rate decreased. This rotation of the shoreline-induced longshore inversion on the updrift side, whereas today cusp erosion is restoring the original drift direction.

Stability of respiratory quotient and growth outcomes of very low birth weight infants.

Nutritional management of very low birth weight (VLBW) infants involves promoting growth at rates that mimic intrauterine rates. Nutritional intake at the recommended energy level to promote growth results in fat accretion at levels that exceed intrauterine rates for fat accretion. The respiratory quotient (RQ), the ratio of carbon dioxide produced to oxygen consumed during oxidation, provides a measure of the percentage of substrates used for energy. An RQ of greater than 0.9 indicates carbohydrate is used to meet energy needs, allowing the majority of fat intake to be stored as new tissue. The purpose of this study was to examine the stability of the RQ across time in relation to nutritional intake and growth in VLBW infants. Subjects were 9 enterally fed VLBW infants. Measurements to determine the RQ were obtained weekly for 3 weeks by indirect calorimetry. Nutritional intake and growth velocity were examined. There was no significant difference in the RQ across 3 weeks. The mean (+/- SD) RQs for each of the 3 weeks were 1.08 (+/- 0.04), 1.06 (+/- 0.05), and 1.06 (+/- 0.07), respectively. No significant differences were found across the 3 weeks for any of the macronutrient variables. Growth velocity for the period was 15.7 g/kg/day. By discharge from the NICU, 66% of the infants had weights less than the 10th percentile on an intrauterine growth reference. An RQ greater than 1.0 indicates VLBW infants are depositing excess fat. However, increased accretion rates of fat did not improve the growth outcomes of these VLBW infants.

HEXTE studies of Sco X-1 spectra: Detections of hard X-ray tails beyond 200 keV

Using the HEXTE experiment on-board the RXTE satellite, we performed a search for hard X-ray tails in Sco X-1 spectra. We found strong evidence for the presence of such a non-thermal component on several occasions. Using the PCA/ RXTE we were able to track the position of the source along the Z diagram, and we observed that the presence of the hard X-ray tail is not confined to a particular region. However, we found a correlation between the power law index of the non-thermal component and the position of the source in the Z diagram, suggesting that the hard X-ray spectrum (i.e., E > 50 keV) becomes flatter as the mass accretion rate increases. We were also able to study the temporal variation of the appearance/absence of the hard X-ray component. With our derived luminosities, we were also able to test the idea that X-ray luminosities can be used to distinguish between X-ray binary systems containing neutron stars and black holes.

RXTEStudies of X‐Ray Spectral Variations with Accretion Rate in 4U 1915−05

We present the results of detailed spectral studies of the ultra-compact low mass X-ray binary (LMXB) 4U 1915-05 carried out with the Rossi X-ray Timing Explorer (RXTE) during 1996. 4U 1915-05 is an X-ray burster (XRB) known to exhibit a ~199-day modulation in its 2--12 keV flux. Observations were performed with the PCA and HEXTE instruments on RXTE at roughly one-month intervals to sample this long-term period and study accretion rate-related spectral changes. We obtain good fits with a model consisting of a blackbody and an exponentially cut-off power law. The spectral parameters are strongly correlated with both the broad-band (2--50 keV) luminosity and the position in the color-color diagram, with the source moving from a low hard state to a high soft state as the accretion rate increases. The blackbody component appears to drive the spectral evolution. Our results are consistent with a geometry in which the soft component arises from an optically thick boundary layer and the hard component from an extended Comptonizing corona. Comparing our results with those of a similar study of the brighter source 4U 1820-30 (Bloser et al. 2000), we find that the two ultra-compact LMXBs occupy similar spectral states even though the transitions occur at very different total luminosities.

X-ray spectra of a large sample of quasars with ASCA

The results from an X-ray spectral analysis of a large sample of quasars, observed with ASCA, are presented. Data reduction leaves 62 quasars, 35 radio-loud and 27 radio-quiet, suitable for spectral analysis. Differences are found between the radio-quiet quasars (RQQs) and the radio-loud quasars (RLQs), in agreement with previous studies. A correlation between Gamma and optical H-beta was also found for the radio-quiet quasars in this sample, whereby the steepest X-ray spectra tend to be found in those objects with narrow H-beta widths. The correlation is significant at >99% confidence, confirming the well-known trend between Gamma and H-beta FWHM in Seyfert 1s (Brandt et al. 1997), but at higher luminosities. Other spectral complexities are observed from this sample. A soft X-ray excess, with blackbody temperatures in the range 100 - 300 eV, is seen in many low z radio-quiet quasars. In most cases the temperatures are probably too hot to originate directly from the disk. Iron K line emission features are also found in the RQQs; but often from partially ionised material. Indeed in the highest luminosity RQQs there is neither evidence for iron line emission nor the reflection component expected from disk models. These observations can be explained by an increase in the quasar accretion rate with luminosity, leading to an increase in the ionisation state of the surface layers of the disk. The occurrence of ionised or `warm' absorbers is rare in this sample, with only 5 detections. However excess neutral X-ray absorption is found towards several of the high z, predominantly radio-loud, quasars. Although found to increase with quasar redshift, this `intrinsic' absorption may be associated with radio-loud AGN.