A Constraint on the Organization of the Galactic Center Magnetic Field Using Faraday Rotation

We present new 6 and 20 cm Very Large Array (VLA) observations of polarized continuum emission of roughly 0.5 square degrees of the Galactic center (GC) region. The 6 cm observations detect diffuse linearly-polarized emission throughout the region with a brightness of roughly 1 mJy per 15"x10" beam. The Faraday rotation measure (RM) toward this polarized emission has structure on degree size scales and ranges from roughly +330 rad/m2 east of the dynamical center (Sgr A) to -880 rad/m2 west of the dynamical center. This RM structure is also seen toward several nonthermal radio filaments, which implies that they have a similar magnetic field orientation and constrains models for their origin. Modeling shows that the RM and its change with Galactic longitude are best explained by the high electron density and strong magnetic field of the GC region. Considering the emissivity of the GC plasma shows that while the absolute RM values are indirect measures of the GC magnetic field, the RM longitude structure directly traces the magnetic field in the central kiloparsec of the Galaxy. Combining this result with previous work reveals a larger RM structure covering the central ~2 degrees of the Galaxy. This RM structure is similar to that proposed by Novak and coworkers, but is shifted roughly 50 pc west of the dynamical center of the Galaxy. If this RM structure originates in the GC region, it shows that the GC magnetic field is organized on ~300 pc size scales. The pattern is consistent with a predominantly poloidal field geometry, pointing from south to north, that is perturbed by the motion of gas in the Galactic disk.Comment: Accepted to ApJ. emulateapj style, 14 pages, 15 figure

The magnetic environment in the central region of nearby galaxies

The central regions of galaxies harbor some of the most extreme physical phenomena, including dense stellar clusters, non-circular motions of molecular clouds and strong and pervasive magnetic field structures. In particular, radio observations have shown that the central few hundred parsecs of our Galaxy has a striking magnetic field configuration. It is not yet clear whether these magnetic structures are unique to our Milky Way or a common feature of all similar galaxies. Therefore, we report on (a) a new radio polarimetric survey of the central 200 pc of the Galaxy to better characterize the magnetic field structure and (b) a search for large-scale and organized magnetized structure in the nuclear regions of nearby galaxies using data from the Very Large Array (VLA) archive. The high angular resolution of the VLA allows us to study the central 1 kpc of the nearest galaxies to search for magnetized nuclear features similar to what is detected in our own Galactic center. Such magnetic features play a important role in the nuclear regions of galaxies in terms of gas transport and the physical conditions of the interstellar medium in this unusual region of galaxies.Comment: 8 pages; Proceedings for "The Universe under the Microscope" (AHAR 2008), held in Bad Honnef (Germany) in April 2008, to be published in Journal of Physics: Conference Series by Institute of Physics Publishing, R. Schoedel, A. Eckart, S. Pfalzner, and E. Ros (eds.

Gauge thresholds in the presence of oblique magnetic fluxes

We compute the one-loop partition function and analyze the conditions for tadpole cancellation in type I theories compactified on tori in the presence of internal oblique magnetic fields. We check open - closed string channel duality and discuss the effect of T-duality. We address the issue of the quantum consistency of the toroidal model with stabilized moduli recently proposed by Antoniadis and Maillard (AM). We then pass to describe the computation of one-loop threshold corrections to the gauge couplings in models of this kind. Finally we briefly comment on coupling unification and dilaton stabilization in phenomenologically more viable modelsComment: 34 pages, 2 figures; references added, major changes to the discussion of the model proposed by Antoniadis and Maillar

Fading hard X-ray emission from the Galactic Centre molecular cloud Sgr B2

The centre of our Galaxy harbours a 4 million solar mass black hole that is unusually quiet: its present X-ray luminosity is more than 10 orders of magnitude less than its Eddington luminosity. The observation of iron fluorescence and hard X-ray emission from some of the massive molecular clouds surrounding the Galactic Centre has been interpreted as an echo of a past flare. Alternatively, low-energy cosmic rays propagating inside the clouds might account for the observed emission, through inverse bremsstrahlung of low energy ions or bremsstrahlung emission of low energy electrons. Here we report the observation of a clear decay of the hard X-ray emission from the molecular cloud Sgr B2 during the past 7 years thanks to more than 20 Ms of INTEGRAL exposure. The measured decay time is compatible with the light crossing time of the molecular cloud core . Such a short timescale rules out inverse bremsstrahlung by cosmic-ray ions as the origin of the X ray emission. We also obtained 2-100 keV broadband X-ray spectra by combining INTEGRAL and XMM-Newton data and compared them with detailed models of X-ray emission due to irradiation of molecular gas by (i) low-energy cosmic-ray electrons and (ii) hard X-rays. Both models can reproduce the data equally well, but the time variability constraints and the huge cosmic ray electron luminosity required to explain the observed hard X-ray emission strongly favor the scenario in which the diffuse emission of Sgr B2 is scattered and reprocessed radiation emitted in the past by Sgr A*. Using recent parallax measurements that place Sgr B2 in front of Sgr A*, we find that the period of intense activity of Sgr A* ended between 75 and 155 years ago.Comment: Accepted for publication in ApJ. 10 pages, 5 figure

On the role of stochastic Fermi acceleration in setting the dissipation scale of turbulence in the interstellar medium

We consider the dissipation by Fermi acceleration of magnetosonic turbulence in the Reynolds Layer of the interstellar medium. The scale in the cascade at which electron acceleration via stochastic Fermi acceleration (STFA) becomes comparable to further cascade of the turbulence defines the inner scale. For any magnetic turbulent spectra equal to or shallower than Goldreich-Sridhar this turns out to be $\ge 10^{12}$cm, which is much larger than the shortest length scales observed in radio scintillation measurements. While STFA for such spectra then contradict models of scintillation which appeal directly to an extended, continuous turbulent cascade, such a separation of scales is consistent with the recent work of \citet{Boldyrev2} and \citet{Boldyrev3} suggesting that interstellar scintillation may result from the passage of radio waves through the galactic distribution of thin ionized boundary surfaces of HII regions, rather than density variations from cascading turbulence. The presence of STFA dissipation also provides a mechanism for the non-ionizing heat source observed in the Reynolds Layer of the interstellar medium \citep{Reynolds}. STFA accommodates the proper heating power, and the input energy is rapidly thermalized within the low density Reynolds layer plasma.Comment: 12 Pages, no figures. Accepted for publication in MNRA

High-Resolution, Wide-Field Imaging of the Galactic Center Region at 330 MHz

We present a wide field, sub-arcminute resolution VLA image of the Galactic Center region at 330 MHz. With a resolution of ~ 7" X 12" and an RMS noise of 1.6 mJy/beam, this image represents a significant increase in resolution and sensitivity over the previously published VLA image at this frequency. The improved sensitivity has more than tripled the census of small diameter sources in the region, has resulted in the detection of two new Non Thermal Filaments (NTFs), 18 NTF candidates, 30 pulsar candidates, reveals previously known extended sources in greater detail, and has resulted in the first detection of Sagittarius A* in this frequency range. A version of this paper containing full resolution images may be found at http://lwa.nrl.navy.mil/nord/AAAB.pdf.Comment: Astronomical Journal, Accepted 62 Pages, 21 Figure

Evidence for a Weak Galactic Center Magnetic Field from Diffuse Low Frequency Nonthermal Radio Emission

New low-frequency 74 and 330 MHz observations of the Galactic center (GC) region reveal the presence of a large-scale (6\arcdeg\times 2\arcdeg) diffuse source of nonthermal synchrotron emission. A minimum energy analysis of this emission yields a total energy of $\sim (\phi^{4/7}f^{3/7})\times 10^{52}$ ergs and a magnetic field strength of $\sim 6(\phi/f)^{2/7}$ \muG (where $\phi$ is the proton to electron energy ratio and $f$ is the filling factor of the synchrotron emitting gas). The equipartition particle energy density is $1.2(\phi/f)^{2/7}$ \evcm, a value consistent with cosmic-ray data. However, the derived magnetic field is several orders of magnitude below the 1 mG field commonly invoked for the GC. With this field the source can be maintained with the SN rate inferred from the GC star formation. Furthermore, a strong magnetic field implies an abnormally low GC cosmic-ray energy density. We conclude that the mean magnetic field in the GC region must be weak, of order 10 \muG (at least on size scales \ga 125\arcsec).Comment: 12 pages, 1 JPEG figure, uses aastex.sty; Accepted for publication, ApJL (2005, published

Single hole dynamics in the t-J model on a square lattice

We present quantum Monte Carlo (QMC) simulations for a single hole in a t-J model from J=0.4t to J=4t on square lattices with up to 24 x 24 sites. The lower edge of the spectrum is directly extracted from the imaginary time Green's function. In agreement with earlier calculations, we find flat bands around $(0,\pm\pi)$, $(\pm\pi,0)$ and the minimum of the dispersion at $(\pm\pi/2,\pm\pi/2)$. For small J both self-consistent Born approximation and series expansions give a bandwidth for the lower edge of the spectrum in agreement with the simulations, whereas for J/t > 1, only series expansions agree quantitatively with our QMC results. This band corresponds to a coherent quasiparticle. This is shown by a finite size scaling of the quasiparticle weight $Z(\vec k)$ that leads to a finite result in the thermodynamic limit for the considered values of $J/t$. The spectral function $A(\vec k, \omega)$ is obtained from the imaginary time Green's function via the maximum entropy method. Resonances above the lowest edge of the spectrum are identified, whose J-dependence is quantitatively described by string excitations up to J/t=2