Strange attractors in periodically-kicked degenerate Hopf bifurcations

We prove that spiral sinks (stable foci of vector fields) can be transformed into strange attractors exhibiting sustained, observable chaos if subjected to periodic pulsatile forcing. We show that this phenomenon occurs in the context of periodically-kicked degenerate supercritical Hopf bifurcations. The results and their proofs make use of a new multi-parameter version of the theory of rank one maps developed by Wang and Young.Comment: 16 page

Chandra Observations of the Galactic Center and Nearby Edge-on Galaxies

I review our recent Chandra surveys of the center region of the Milky Way and other nearby edge-on galaxies. Our Galactic center survey provides an unprecedented high-resolution, broad-band X-ray panorama of a 0.8x2 square degree swath along the Galactic plane. Our preliminary analysis has led a detection of about 1000 discrete sources. We find that the diffuse X-ray emission dominates over the contribution from faint discrete sources and is globally associated with distinct interstellar structures observed at radio and mid-infrared wavelengths. We study how high-energy activities in the center region affect the immediate vicinity and may influence other aspects of the Galaxy. We have further observed nearby edge-on late-type disk galaxies in fields of low foreground Galactic extinction to gain external perspectives of the global disk/halo interaction. We have detected a giant diffuse X-ray-emitting corona around the galactic disk of NGC 4631. Extraplanar diffuse X-ray emission is also detected around NGC 3556. These X-ray-emitting coronae morphologically resemble the radio halos of these galaxies, indicating a close connection between outflows of hot gas, cosmic rays, and magnetic field from the galactic disks. There is only marginal evidence for extraplanar diffuse X-ray emission in NGC 4244 -- a galaxy with an extremely low star formation rate. In general, the extraplanar diffuse X-ray emission is evidently related to recent massive star forming activities in the galactic disks, especially in their central regions.Comment: 7 pages plus figures, based on an invited talk presented at Proc. Symposium "New Vision of the X-ray Universe in the XMM-Newton and Chandra Era", a complete and high-resolution version can be found at http://xray.astro.umass.edu/wqd/papers/gcs/qwang_E3.p

Ultraluminous X-ray Source 1E 0953.8+6918 (M81 X-9): An Intermediate Mass Black Hole Candidate and its Environs

We present a ROSAT and ASCA study of the Einstein source X-9 and its relation to a shock-heated shell-like optical nebula in a tidal arm of the M81 group of interacting galaxies. Our ASCA observation of the source shows a flat and featureless X-ray spectrum well described by a multi-color disk blackbody model. The source most likely represents an optically thick accretion disk around an intermediate mass black hole in its high/soft state, similar to other variable ultraluminous X-ray sources observed in nearby disk galaxies. Using constraints derived from both the innermost stable orbit around a black hole and the Eddington luminosity, we find that the black hole is fast-rotating and that its mass is between $\sim 20/({\rm cos} i) {\rm M}_{\odot} - 110/({\rm cos} i)^{1/2} {\rm M}_{\odot}$, where $i$ is the inclination angle of the disk. The inferred bolometric luminosity of the accretion disk is $\sim (8 \times 10^{39} {\rm ergs s^{-1}})/({\rm cos} i)^{1/2}$. Furthermore, we find that the optical nebula is very energetic and may contain large amounts of hot gas, accounting for a soft X-ray component as indicated by archival ROSAT PSPC data. The nebula is apparently associated with X-9; the latter may be powering the former and/or they could be formed in the same event (e.g., a hypernova). Such a connection, if confirmed, could have strong implications for understanding both the birth of intermediate mass black holes and the formation of energetic interstellar structures.Comment: Accepted for publication in MNRA

Confronting feedback simulations with observations of hot gas in elliptical galaxies

Elliptical galaxies comprise primarily old stars, which collectively generate a long-lasting feedback via stellar mass-loss and Type Ia SNe. This feedback can be traced by X-ray-emitting hot gas in and around such galaxies, in which little cool gas is typically present. However, the X-ray-inferred mass, energy, and metal abundance of the hot gas are often found to be far less than what are expected from the feedback, particularly in so-called low L_X/L_B ellipticals. This "missing" stellar feedback is presumably lost in galaxy-wide outflows, which can play an essential role in galaxy evolution (e.g., explaining the observed color bi-modality of galaxies). We are developing a model that can be used to properly interpret the X-ray data and to extract key information about the dynamics of the feedback and its interplay with galactic environment.Comment: To be published in Highlights of Astronomy, Vol 15, XXVIIth IAU General Assembly, JD

Extra-planar Diffuse Hot Gas Around Normal Disk Galaxies

I review results from {\sl Chandra} observations of nearby normal edge-on galaxies (Sd to Sa types). These galaxies have a broad range of star formation rate, but none of them is dominated by a nuclear starburst. The galaxies are all in directions of low Galactic foreground absorption. Extra-Planar diffuse soft X-ray emission is detected unambiguously from all the galaxies, except for N4244 (Sd), which is low in both the stellar mass and the star formation rate. The thermal nature of the X-ray-emitting gas is well established, although its chemical and ionization states remain largely uncertain. The X-ray luminosity of the gas is proportional to the star formation rate and to the stellar mass of the galaxies. But the luminosity accounts for at most a few percent of the expected supernova mechanical energy input. Therefore, there is a ``missing'' energy problem for spiral galaxies. Much of the energy in late-type spirals may be converted and radiated in lower energy bands. But early-type ones most likely have outflows, which are powered primarily by Type Ia supernovae in galactic bulges. These galactic outflows may strongly affect both the dynamics and cooling of the intergalactic gas accretion, hence the evolution of the galaxies.Comment: 8 pages. To appear in Extraplanar Gas, ed. R. Braun. a version with full resolution plots can be found http://www.astro.umass.edu/~wqd/papers/extraplanar.p