Odot Yr Research Articles

XMM‐NewtonSpectra of Intermediate‐Mass Black Hole Candidates: Application of a Monte Carlo Simulated Model

We present a systematic spectral analysis of six ultraluminous X-ray sources (NGC1313 X-1/X-2, IC342 X-1, HoIX X-1, NGC5408 X-1 and NGC3628 X-1) observed with XMM-Newton Observatory. These extra-nuclear X-ray sources in nearby late-type galaxies have been considered as intermediate-mass black hole candidates. We have performed Monte-Carlo simulations of Comptonized multi-color black-body accretion disks. This unified and self-consistent spectral model assumes a spherically symmetric, thermal corona around each disk and accounts for the radiation transfer in the Comptonization. We find that the model provides satisfactory fits to the XMM-Newton spectra of thesources. The characteristic temperatures of the accretion disks (T_in), for example, are in the range of ~ 0.05-0.3 keV, consistent with the intermediate-mass black hole interpretation. We find that the black hole mass is typically about a few times 10^3 M_\odot and has an accretion rate ~ 10^{-6} - 10^{-5} M_\odot yr^{-1}. For the spectra considered here, we find that the commonly used multi-color black-body accretion disk model with an additive power law component, though not physical, provides a good mathematical approximation to the Monte-Carlo simulated model. However, the latter model provides additional constraints on the properties of the accretion systems, such as the disk inclination angles and corona optical depths.

Kinematics and parameters of the gas in the vicinity of TW Hya

The following conclusions about the kinematics and parameters of the gas in the vicinity of TW Hya have been drawn from an analysis of optical and ultraviolet line profiles and intensities. The accreting matter rises in the magnetosphere to a distance z>R * above the disk plane and falls to the star near its equator almost perpendicular to its plane. The matter outflows from a disk region with an outer radius of ≤0.5 AU. The [OI], [SII], and H2 lines originate in the disk atmosphere outside the outflow region, where the turbulent gas velocity is close to the local speed of sound. In the formation region of the forbidden lines, T⋍8500 K and N e⋍5×106 cm−3, and the hydrogen is almost neutral: x e<0.03. The absorption features observed in the blue wings of some of the ultraviolet lines originate in the part of the wind that moves almost perpendicular to the disk plane, i.e., in the jet of TW Hya. The V z gas velocity component in the jet decreases with increasing distance from the jet axis from 200 to 30 km s−1. The matter outflowing from the inner disk boundary, moves perpendicular to the disk plane in the formation region of blue absorption line components, at a distance of ∼0.5 AU from the axis of symmetry of the disk. This region of the wind is collimated into the jet at a distance of <3 AU from the disk plane. The gas temperature in the formation region of absorption components is ⋍2×104 K, and the gas density is <3×106 cm−3. This region of the jet is on the order of several AU away from the disk plane, while free recombination in the jet begins even farther from the disk. The mass-loss rate for TW Hya is $$\dot M_w < 7 \times 10^{ - 10} M_ \odot yr^{ - 1}$$ , which is a factor of 3lower than the mean accretion rate. The relative abundance of silicon and aluminum in the jet gas is at least an order of magnitude lower than its standard value.

Wolf-Rayet stars, black holes, and gamma-ray bursters in close binaries

We consider the evolutionary status of observed close binary systems containing black holes and Wolf-Rayet (WR) stars. When the component masses and the orbital period of a system are known, the reason for the formation of a WR star in an initial massive system of two main-sequence stars can be established. Such WR stars can form due to the action of the stellar wind from a massive OB star (MOB≥50M⊙), conservative mass transfer between components with close initial masses, or the loss of the common envelope in a system with a large (up to ∼25) initial component mass ratio. The strong impact of observational selection effects on the creation of samples of close binaries with black holes and WR stars is demonstrated. We estimate theoretical mass-loss rates for WR stars, which are essential for our understanding the observed ratio of the numbers of carbon and nitrogen WR stars in the Galaxy \(\dot M_{WR} (M_ \odot yr^{ - 1} ) = 5 \times 10^{ - 7} (M_{WR} /M_ \odot )^{1.3} \). We also estimate the minimum initial masses of the components in close binaries producing black holes and WR stars to be ∼25M⊙. The spatial velocities of systems with black holes indicate that, during the formation of a black hole from a WR star, the mass loss reaches at least several solar masses. The rate of formation of rapidly rotating Kerr black holes in close binaries in the Galaxy is ∼3×10−6 yr−1. Their formation may be accompanied by a burst of gamma radiation, possibly providing clues to the nature of gamma-ray bursts. The initial distribution of the component mass ratios for close binaries is dN∼dq=dM2/M1 in the interval 0.04≲q0≤1, suggesting a single mechanism for their formation.

The structure of molecular clumps around high-mass young stellar objects

We have used the IRAM 30-m and FCRAO 14-m telescopes to observe the molecular clumps associated with 12 ultracompact (UC) HII regions in the J=6-5, 8-7 and 13-12 rotational transitions of methyl-acetylene (CH3C2H). Under the assumption of LTE and optically thin emission, we have derived temperature estimates ranging from 30 to 56 K. We estimate that the clumps have diameters of 0.2-1.6 pc, H_2 densities of 10^5-10^6 {cm^{-3}}, and masses of 10^2-2 10^4 M_\odot. We compare these values with those obtained by other authors from different molecular tracers and find that the H_2 density and the temperature inside the clumps vary respectively like n_{H_2} ~ R^{-2.6} and T ~ R^{-0.5}, with R distance from the centre. We also find that the virial masses of the clumps are ~3 times less than those derived from the CH3C2H column densities: we show that a plausible explanation is that magnetic fields play an important role to stabilise the clumps, which are on the verge of gravitational collapse. Finally, we show that the CH3C2H line width increases for decreasing distance from the clump centre: this effect is consistent with infall in the inner regions of the clumps. We conclude that the clumps around UC HII regions are likely to be transient (~10^(5) yr) entities, remnants of isothermal spheres currently undergoing gravitational collapse: the high mass accretion rates (~10^{-2} M_\odot yr^{-1}) lead to massive star formation at the centre of such clumps.

Are Seyfert Narrow‐Line Regions Powered by Radio Jets?

We argue that the narrow line regions of Seyfert galaxies are powered by the transport of energy and momentum by the radio-emitting jets and consequently that the ratio of the radio power to jet energy flux is much smaller than is usually assumed for radio galaxies. This can be partially attributed to the smaller ages ($\sim 10^6 yrs$) of Seyferts compared to radio galaxies but one also requires that either the magnetic energy density is more than an order of magnitude below the equipartition value, or more likely, that the internal energy densities of Seyfert jets are dominated by thermal plasma. If Seyfert jets are initially dominated by relativistic plasma, then an analysis of the data on jets in five Seyfert galaxies shows that all but one of these would have mildly relativistic jet velocities near 100 pc in order to power the respective narrow-line regions. However, observations of jet-cloud interactions in the NLR provide additional information on jet velocities and composition via the momentum budget. Our analysis of a jet-cloud interaction in NGC 1068, implies a shocked jet pressure much larger than the minimum pressure of the radio knot, a velocity $\sim 0.06 c$ and a jet temperature $\sim 10^9 K$ implying mildly relativistic electrons but thermal protons. The jet mass flux at this point $\sim 0.5 M_\odot yr^{-1}$, is an order of magnitude higher than the mass accretion rate into the black hole, strongly indicating entrainment. The initial jet mass flux $\sim 0.02 M_\odot yr^{-1}$, comparable to the mass accretion rate and is consistent with the densities inferred for accretion disc coronae from high energy observations, together with an initially mildly relativistic velocity and an initial jet radius of order 10 gravitational radii.