The Nature of Nonthermal X-ray Filaments Near the Galactic Center

Recent Chandra and XMM-{\it Newton} observations reported evidence of two X-ray filaments G359.88-0.08 (SgrA-E) and G359.54+0.18 (the ripple filament) near the Galactic center. The X-ray emission from these filaments has a nonthermal spectrum and coincides with synchrotron emitting radio sources. Here, we report the detection of a new X-ray feature coincident with a radio filament G359.90-0.06 (SgrA-F) and show more detailed VLA, Chandra and BIMA observations of the radio and X-ray filaments. In particular, we show that radio emission from the nonthermal filaments G359.90-0.06 (SgrA-F) and G359.54+0.18 (the ripple) has a steep spectrum whereas G359.88-0.08 (SgrA-E) has a flat spectrum. The X-ray emission from both these sources could be due to synchrotron radiation. However, given that the 20 \kms molecular cloud, with its intense 1.2mm dust emission, lies in the vicinity of SgrA-F, it is possible that the X-rays could be produced by inverse Compton scattering of far-infrared photons from dust by the relativistic electrons responsible for the radio synchrotron emission. The production of X-ray emission from ICS allows an estimate of the magnetic field strength of ~0.08 mG within the nonthermal filament. This should be an important parameter for any models of the Galactic center nonthermal filaments.Comment: 14 pages, 9 figures, in Cospar 2004 session E1.4; editors: Cara Rakowski and Shami Chatterjee; "Young Neutron Stars and Supernova Remnants", publication: Advances in Space Research (in press

The Underluminous Nature of Sgr A*

In the last several years, a number of observing campaigns of the massive black hole Sgr A* has been carried out in order to address two important issues: one concerns the underluminous nature of Sgr A* with its bolometric luminosity being several orders of magnitude less than those of its more massive counterparts. It turns out that the angular momentum of the ionized stellar winds from orbiting stars in one or two disks orbiting Sgr A* could be a critical factor in estimating accurately the accretion rate unto Sgr A*. A net angular momentum of ionized gas feeding Sgr A* could lower the Bondi rate. Furthermore, the recent time delay picture of the peak flare emission can be understood in the context of adiabatic expansion of hot plasma. The expansion speed of the plasma is estimated to be sub-relativistic. However, relativistic bulk motion of the plasma could lead to outflow from Sgr A*. Significant outflow from Sgr A* could then act as a feedback which could then reduce Bondi accretion rate. These uncertain factors can in part explain the underluminous nature of Sgr A*. The other issue is related to the emission mechanism and the cause of flare activity in different wavelength bands. Modeling of X-ray and near-IR flares suggests that inverse Compton scattering (ICS) of IR flare photons by the energetic electrons responsible for the submm emission can account for the X-ray flares. A time delay of minutes to tens of minutes is predicted between the peak flaring in the near-IR and X-rays, NOT due to adiabatic expansion of optically thick hot plasma, but to the time taken for IR flare photons to cross the accretion flow before being upscattered.Comment: 4 pages, To appear in Proceedings of "X-ray Astronomy 2009: Present Status, Multi-Wavelength Approach and Future Perspectives", Bologna, Italy, September 7-11, 2009, AIP, eds. A. Comastri, M. Cappi, and L. Angelin

The Origin of Parsec-Scale Gaseous and Stellar Disks in the Galactic Center and AGNs

The Galactic center stellar disk and the circumnuclear ring provide a unique opportunity to study in detail the dynamics and physical conditions of distant molecular disks in the nuclei of galaxies. One of the key questions is how these disks form so close to their host black holes and under what condition they form stars in a tidally stressed environment. We argue that disk formation around a massive black hole is due to partial accretion of extended molecular clouds that temporarily pass through the central region of the Galaxy. The cancellation of angular momentum of the gravitationally focused gas naturally creates a compact gaseous disk. The disk can potentially become gravitationally unstable and form stars. We apply these ideas to explain the origin of sub-parsec megamaser disks found in the nuclei of Seyfert 2 galaxies. We show that an empirical scaling relation between the mass of the black hole and the size of the disk can be understood in the context of the cloud capture scenario. We conclude that the stellar and gas disks found in our Galactic center act as a bridge to further our understanding of more distant mega-maser disks in the nuclei of Seyfert 2 galaxies.Comment: 6 pages, 2 figures, to appear in "The Central Kiloparsec in Galactic Nuclei: Astronomy at High Angular Resolution 2011", open access Journal of Physics: Conference Series (JPCS), published by IOP Publishin

The Galactic Center: An Interacting System of Unusual Sources

The region bounded by the inner tens of light years at the center of the Milky Way contains five principal components that coexist within the central deep gravitational potential well. These constituents are a black hole candidate (Sgr~A*) with a mass equivalent to $2.6\pm0.2\times 10^6$ suns, a surrounding cluster of evolved stars, a complex of young stars, molecular and ionized gas clouds, and a powerful supernova-like remnant. The interaction of these components is responsible for many of the phenomena occurring in this complex and unique portion of the Galaxy. Developing a consistent picture of the primary interactions between the components at the Galactic Center will improve our understanding of the nature of galactic nuclei in general, and will provide with a better defined set of characteristics of black holes. For example, the accretion of stellar winds by Sgr A* appears to produce far less radiation than indicated by estimates based on models of galactic nuclei.Comment: 26 pages (text only); see the published article with ten figures under Science Online Article http://www.physics.nwu.edu/research/zadeh.htm

Proper Motion of the Irradiated Jet HH 399 in the Trifid Nebula

HH 399 is one of the first Herbig Haro flows recognized to be irradiated by the UV radiation of the massive O7.5 star in the Trifid nebula. We present the proper motion of the first irradiated jet based on two epochs of HST observations of HH 399 separated nearly by five years using H$\alpha$ and [SII] line filters. High proper motion with continuous velocities between 200$\pm$55 and 528$\pm24$ \kms are detected in both lines along the 18$''$ extent of the jet axis. The irradiated fully-ionized jet consists of numerous knots along the jet but also shows the evidence for a number of isolated blob-like structures running immediately outside the jet with lower transverse velocities. The transverse velocities combined with radial velocity measurements indicate that the jet axis lies away from the plane of the sky by only few degrees. We argue that the jet is fully ionized based on [SII]/H$\alpha$ line ratio as well as radio continuum emission detected from the full extent of the jet at 3.6cm wavelength. The stellar mass-loss rate producing HH 399 is estimated to be \approx 2\times10^{-6} \msol yr$^{-1}$.Comment: 14 pages, 6 figures, ApJ (in press

Cosmic-Ray Heating of Molecular Gas in the Nuclear Disk: Low Star Formation Efficiency

Understanding the processes occurring in the nuclear disk of our Galaxy is interesting in its own right, as part of the Milky Way Galaxy, but also because it is the closest galactic nucleus. It has been more than two decades since it was recognized that the general phenomenon of higher gas temperature in the inner few hundred parsecs by comparison with local clouds in the disk of the Galaxy. This is one of the least understood characteristics of giant molecular clouds having a much higher gas temperature than dust temperature in the inner few degrees of the Galactic center. We propose that an enhanced flux of cosmic-ray electrons, as evidenced recently by a number of studies, are responsible for directly heating the gas clouds in the nuclear disk, elevating the temperature of molecular gas ($\sim$ 75K) above the dust temperature ($\sim$ 20K). In addition we report the detection of nonthermal radio emission from Sgr B2-F based on low-frequency GMRT and VLA observations. The higher ionization fraction and thermal energy due to the impact of nonthermal electrons in star forming sites have important implications in slowing down star formation in the nuclear disk of our galaxy and nuclei of galaxies.Comment: 12 pages, one figure, ApJL (in press

The Variability of Polarized Radiation from Sgr A*

Sgr A* is variable at radio and submillimeter wavelengths on hourly time scales showing time delays between the peaks of flare emission as well as linearly polarized emission at millimeter and sub-mm wavelengths. To determine the polarization characteristics of this variable source at radio frequencies, we present VLA observations of Sgr A* and report the detection of polarized emission at a level of 0.77\pm0.01% and 0.2\pm0.01% at 43 and 22 GHz, respectively. The change in the time averaged polarization angle between 22 and 43 GHz corresponds to a RM of -2.5\pm0.6 x10^3 rad m{-2} with no phase wrapping (or \sim 5x10^4 rad m^2 with 2\pi phase wrap). We also note a rise and fall time scale of 1.5 -- 2 hours in the total polarized intensity. The light curves of the degree of linearly polarized emission suggests a a correlation with the variability of the total intensity at 43 GHz. The available polarization data at radio and sub-mm wavelengths suggest that the rotation measure decreases with decreasing frequency. This frequency dependence, and observed changes in polarization angle during flare events, may be caused by the reduction in rotation measure associated with the expansion of synchrotron-emitting blobs.Comment: 11 pages, 3 figures, ApJL (in press

Shocked molecular gas towards the SNR G359.1-0.5 and the Snake

We have found a bar of shocked molecular hydrogen (H2) towards the OH(1720 MHz) maser located at the projected intersection of supernova remnant (SNR) G359.1-0.5 and the nonthermal radio filament, known as the Snake. The H2 bar is well aligned with the SNR shell and almost perpendicular to the Snake. The OH(1720 MHz) maser is located inside the sharp western edge of the H2 emission, which is consistent with the scenario in which the SNR drives a shock into a molecular cloud at that location. The spectral-line profiles of 12CO, HCO+ and CS towards the maser show broad-line absorption, which is absent in the 13CO spectra and most probably originates from the pre-shock gas. A density gradient is present across the region and is consistent with the passage of the SNR shock while the H2 filament is located at the boundary between the pre--shocked and post-shock regions.Comment: 8 pages, 12 figures, accepted by the MNRAS, typos fixe

Massive Star Formation in the Molecular Ring Orbiting the Black Hole at the Galactic Center

A ring of dense molecular gas extending 2-7 pc orbits the supermassive black hole Sgr A* at the center of our Galaxy. Using the Green Bank Telescope, we detected water maser lines and both narrow (0.35 km/s) and broad (30 - 50 km/s) methanol emission from the molecular ring. Two of the strongest methanol lines at 44 GHz are confirmed as masers by interferometric observations. These class I methanol masers are collisionally excited and are signatures of early phases of massive star formation in the disk of the Galaxy, suggesting that star formation in the molecular ring is in its early phase. Close inspection of the kinematics of the associated molecular clumps in the HCN (J=1-0) line reveals broad red-shifted wings indicative of disturbance by protostellar outflows from young (few times 10^4 yr), massive stars embedded in the clumps. The thermal methanol profile has a similar shape, with a narrow maser line superimposed on a broad, red-shifted wing. Additional evidence for the presence of young massive protostars is provided by shocked molecular hydrogen and a number of striking ionized and molecular linear filaments in the vicinity of methanol sources suggestive of 0.5-pc scale protostellar jets. Given that the circumnuclear molecular ring is kinematically unsettled and thus is likely be the result of a recent capture, the presence of both methanol emission and broad, red-shifted HCN emission suggests that star formation in the circumnuclear ring is in its infancy.Comment: 13 pages, 4 figures, ApJ Letters (in press