A Model for the X-Ray and UV Emission from Seyfert Galaxies and Galactic Black Holes

We propose that the X--ray emission from radio quiet AGN and galactic black holes is due to Comptonization of soft thermal photons emitted by the underlying accretion disk in localized structures (blobs). The power per unit area produced by the blobs, impinging on the disk, can easily dominate the radiation internally produced by the disk. In this case the electron temperature and the high energy spectrum can be determined in a similar way as in the previously studied homogeneous model (Haardt \& Maraschi 1991). However in the present model: a) the emitted spectrum is largely independent of the {\it fraction} of gravitational power dissipated in the blobs; b) the X--ray spectrum can be harder depending on a form factor of the blobs; c) the UV (or soft X--ray for galactic objects) luminosity that is not intercepted by the blobs can be larger than the X--ray luminosity. In the framework of a simplified accretion disk $\alpha-\Omega$ dynamo model, we make order of magnitude estimates of the number of active blobs, their size, luminosity and hence their compactness, finding values in agreement with what is observed. The expected UV to X--ray spectra and correlations of X--ray and UV light curves are discussed.Comment: 10 pages. Plain Tex. Accepted in ApJ Letter

Formation and evolution of coronal rain observed by SDO/AIA on February 22, 2012

The formation and dynamics of coronal rain are currently not fully understood. Coronal rain is the fall of cool and dense blobs formed by thermal instability in the solar corona towards the solar surface with acceleration smaller than gravitational free fall. We aim to study the observational evidence of the formation of coronal rain and to trace the detailed dynamics of individual blobs. We used time series of the 171 \AA\, and 304 \AA\, spectral lines obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) above active region AR 11420 on February 22, 2012. Observations show that a coronal loop disappeared in the 171 \AA\ channel and appeared in the 304 \AA\ line$\text{}\text{}$ more than one hour later, which indicates a rapid cooling of the coronal loop from 1 MK to 0.05 MK. An energy estimation shows that the radiation is higher than the heat input, which indicates so-called catastrophic cooling. The cooling was accompanied by the formation of coronal rain in the form of falling cold plasma. We studied two different sequences of falling blobs. The first sequence includes three different blobs. The mean velocities of the blobs were estimated to be 50 km s$^{-1}$, 60 km s$^{-1}$ and 40 km s$^{-1}$. A polynomial fit shows the different values of the acceleration for different blobs, which are lower than free-fall in the solar corona. The first and second blob move along the same path, but with and without acceleration, respectively. We performed simple numerical simulations for two consecutive blobs, which show that the second blob moves in a medium that is modified by the passage of the first blob. Therefore, the second blob has a relatively high speed and no acceleration, as is shown by observations. The second sequence includes two different blobs with mean velocities of 100 km s$^{-1}$ and 90 km s$^{-1}$, respectively.Comment: 8 pages, 8 figures, Accepted in A&

Blobs in recurring EUV jets

In this paper, we report our discovery of blobs in the recurrent and homologous jets that occurred at the western edge of NOAA active region 11259 on 2011 July 22. The jets were observed in the seven extreme-ultraviolet (EUV) filters of the Atmospheric Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO). Using the base-difference images of the six filters (94, 131, 171, 211, 193, and 335 {\AA}), we carried out the differential emission measure (DEM) analyses to explore the thermodynamic evolutions of the jets. The jets were accompanied by cool surges observed in the H$\alpha$ line center of the ground-based telescope in the Big Bear Solar Observatory. The jets that had lifetimes of 20$-$30 min recurred at the same place for three times with interval of 40$-$45 min. Interestingly, each of the jets intermittently experienced several upward eruptions at the speed of 120$-$450 km s$^{-1}$. After reaching the maximum heights, they returned back to the solar surface, showing near-parabolic trajectories. The falling phases were more evident in the low-$T$ filters than in the high-$T$ filters, indicating that the jets experienced cooling after the onset of eruptions. We identified bright and compact blobs in the jets during their rising phases. The simultaneous presences of blobs in all the EUV filters were consistent with the broad ranges of the DEM profiles of the blobs ($5.5\le \log T\le7.5$), indicating their multi-thermal nature. The median temperatures of the blobs were $\sim$2.3 MK. The blobs that were $\sim$3 Mm in diameter had lifetimes of 24$-$60 s. To our knowledge, this is the first report of blobs in coronal jets. We propose that these blobs are plasmoids created by the magnetic reconnection as a result of tearing-mode instability and ejected out along the jets.Comment: 22 pages, 10 figure

Cold Feedback in Cooling-Flow Galaxy Clusters

We put forward an alternative view to the Bondi-driven feedback between heating and cooling of the intra-cluster medium (ICM) in cooling flow galaxies and clusters. We adopt the popular view that the heating is due to an active galactic nucleus (AGN), i.e. a central black hole accreting mass and launching jets and/or winds. We propose that the feedback occurs with the entire cool inner region (5-30 kpc). A moderate cooling flow does exist here, and non-linear over-dense blobs of gas cool fast and are removed from the ICM before experiencing the next major AGN heating event. Some of these blobs may not accrete on the central black hole, but may form stars and cold molecular clouds. We discuss the conditions under which the dense blobs may cool to low temperatures and feed the black hole.Comment: 6 pages, no figures, to appear in the Proceedings of "Heating vs. Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching (Germany

Phase Separation by Entanglement of Active Polymerlike Worms

We investigate the aggregation and phase separation of thin, living T. tubifex worms that behave as active polymers. Randomly dispersed active worms spontaneously aggregate to form compact, highly entangled blobs, a process similar to polymer phase separation, and for which we observe power-law growth kinetics. We find that the phase separation of active polymerlike worms does not occur through Ostwald ripening, but through active motion and coalescence of the phase domains. Interestingly, the growth mechanism differs from conventional growth by droplet coalescence: the diffusion constant characterizing the random motion of a worm blob is independent of its size, a phenomenon that can be explained from the fact that the active random motion arises from the worms at the surface of the blob. This leads to a fundamentally different phase-separation mechanism that may be unique to active polymers.Comment: 4 pages, 4 figure

A Lyman-alpha blob in the GOODS South field: evidence for cold accretion onto a dark matter halo

We report on the discovery of a z = 3.16 Lyman-alpha emitting blob in the GOODS South field. The blob has a total Ly-alpha luminosity of ~ 10^(43) erg s^(-1) and a diameter larger than 60 kpc. The available multi-wavelength data in the GOODS field consists of 13 bands from X-rays (Chandra) to infrared (Spitzer). Unlike other discovered Ly-alpha blobs, this blob shows no obvious continuum counter-part in any of the broad-bands. In particular, no optical counter-parts are found in the deep HST/ACS imaging available. For previously published blobs, AGN (Active Galactic Nuclei) or 'superwind' models have been found to provide the best match with the data. We here argue that the most probable origin of the extended Ly-alpha emission from the blob in the GOODS South field is cold accretion onto a dark matter halo.Comment: 4 pages, 2 tables, 2 figures, Accepted to A&A Letters, minor changes to tex

Hydrodynamics of Suspensions of Passive and Active Rigid Particles: A Rigid Multiblob Approach

We develop a rigid multiblob method for numerically solving the mobility problem for suspensions of passive and active rigid particles of complex shape in Stokes flow in unconfined, partially confined, and fully confined geometries. As in a number of existing methods, we discretize rigid bodies using a collection of minimally-resolved spherical blobs constrained to move as a rigid body, to arrive at a potentially large linear system of equations for the unknown Lagrange multipliers and rigid-body motions. Here we develop a block-diagonal preconditioner for this linear system and show that a standard Krylov solver converges in a modest number of iterations that is essentially independent of the number of particles. For unbounded suspensions and suspensions sedimented against a single no-slip boundary, we rely on existing analytical expressions for the Rotne-Prager tensor combined with a fast multipole method or a direct summation on a Graphical Processing Unit to obtain an simple yet efficient and scalable implementation. For fully confined domains, such as periodic suspensions or suspensions confined in slit and square channels, we extend a recently-developed rigid-body immersed boundary method to suspensions of freely-moving passive or active rigid particles at zero Reynolds number. We demonstrate that the iterative solver for the coupled fluid and rigid body equations converges in a bounded number of iterations regardless of the system size. We optimize a number of parameters in the iterative solvers and apply our method to a variety of benchmark problems to carefully assess the accuracy of the rigid multiblob approach as a function of the resolution. We also model the dynamics of colloidal particles studied in recent experiments, such as passive boomerangs in a slit channel, as well as a pair of non-Brownian active nanorods sedimented against a wall.Comment: Under revision in CAMCOS, Nov 201