Session R11: TeV Gamma-Ray Astronomy and AGNs

Discovry of TeV Gamma-Ray Emission from the Cygnus Region of the Galaxy with Milagro

The diffuse gamma radiation arising from the interaction of cosmic ray particles with matter and radiation in the Galaxy is one of the few probes available to study the origin of the cosmic rays. Milagro is a water Cherenkov detector that continuously views the entire overhead sky. The large field-of-view combined with the long observation time makes Milagro the most sensitive instrument available for the study of large, low surface brightness sources such as the diffuse gamma radiation arising from interactions of cosmic radiation with interstellar matter. Here we present a gamma-ray image of the Cygnus Region at energies near 12 TeV using a new background rejection technique. We have discovered both an extended source and a large area of diffuse gamma-ray emission. The new extended source, MGRO 2019+37, has an extent of 0.33$\pm$0.14 degrees and a flux given by $E^{2} dN/dE =(3.49 \pm 0.47_{stat} \pm 1.05_{sys})\times 10^{-12} TeV cm^{-2} s^{-1}$ at the median detected energy of 12 TeV assuming a differential source spectrum of $E^{-2.6}$. The flux form the diffuse emission from the Cygnus Region at 12 TeV is $E^2dN/dE = (4.18 \pm 0.52_{stat} \pm 1.26_{sys}) \times 10^{-12} TeV cm^{-2} s^{-1} sr^{-1}$ assuming a differential source spectrum of $E^{-2.6}$.   

Survey of the Galactic Plane at 12 TeV with Milagro

The Milagro Gamma-Ray Observatory is a water Cherenkov detector that operates continuously, detecting extensive air showers from the overhead sky. The large field of view and long observation time of Milagro is ideal for surveying large regions of the Northern Hemisphere sky and for detecting gamma rays at the highest energies. The emission from the entire inner Galaxy as visible from the Northern Hemisphere -- Galactic latitude $|b| < 5^\circ$ and Galactic longitude $l\in [30^\circ, 100^\circ]$ -- is detected at a median energy of 12 TeV with a significance of $>$ 7 standard deviations above the isotropic background. The flux of this region exceeds that predicted from cosmic ray interactions with matter and radiation. However, discrete sources will contribute to this flux and 6 locations within this inner Galaxy region are more than 4.5 standard deviations above the isotropic background. Given the size of the inner Galaxy selected and the search method, fewer than one location is expected above 4.5 standard deviations by chance, if the background is isotropic. The flux of the inner Galaxy and of these 6 excess locations will be reported.   

A Survey of EGRET Sources by Milagro Observatory

The Milagro gamma-ray observatory employs a water-Cherenkov technique to continuously monitor the northern sky for TeV gamma-ray emission from astrophysical sources. Milagro has a high duty-cycle $(\sim90\%)$ and wide aperture ($\sim2$ sr). About six and half years of Milagro data are used to search for gamma-ray emission from the EGRET sources in the northern sky. Constraints on the fluxes at 3 TeV and 15 TeV, assuming various power law spectra, will be presented. Different background rejection variables are used for different energy ranges. We compare Milagro fluxes with the fluxes measured by EGRET and their extrapolation to Milagro energies.   

Gamma-Rays Produced in Cosmic-Ray Interactions and the TeV-band Spectrum of RX J1713-3946

We study the individual contribution to diffuse $\gamma$-ray emission from the secondary products in hadronic interactions generated by cosmic rays (CRs), in addition to the contribution of $\pi^0$ decay. For that purpose we employ the Monte Carlo particle collision code DPMJET3.04 to determine the multiplicity spectra of various secondary particles with $\gamma$'s as the final decay state, that result from inelastic collisions between cosmic-ray protons and Helium nuclei and the interstellar medium with standard composition. We thus derive an easy-to-use $\gamma$-ray production matrix for cosmic ray with energies up to about 10 PeV. The production matrices are used to interpret the $\gamma$-ray spectra of diffuse galactic emission and supernova remnants (SNR) and also to the GeV excess in diffuse galactic $\gamma$-rays. We also test the hypothesis that the TeV-band $\gamma$-ray emission of the shell-type SNR RX J1713-3946 observed with HESS is caused by shock-accelerated hadronic cosmic rays. Using the $\chi^2$ statistics we find that a continuously softening spectrum is strongly preferred, in contrast to expectations. A hardening spectrum has about 1\% probability to explain the HESS data, but then only if a hard cut-off at 50-100 TeV is imposed on the particle spectrum.   

Optical, Gamma-ray and X-ray monitoring of Markarian 421 in 2005-2006 season

Multifrequency observations of Mrk421 were taken in 2005-2006 season. The data set shows strong variability in gamma-ray, X-ray, and optical frequencies. This campaign is unique in the high level of gamma-ray coverage obtained given the dedication of the Whipple 10m telescope to this blazar monitoring program. Our observations also include a large number of optical observations made with the FLWO 1.2m telescope. Lightcurves and correlation functions will be presented. We discuss the implications of these observations for discriminating between various leptonic emission models and for constraining physical parameters of the inner jet of Mrk421.   

X-ray and TeV Gamma-Ray Emission from Parallel Electron-Positron or Electron-Proton Beams in BL Lac Objects

In this contribution we discuss models of the X-rays and TeV gamma-ray emission from BL Lac objects based on parallel electron-positron or electron-proton beams that form close to the central black hole owing to the strong electric fields generated by the accretion disk and possibly also by the black hole itself. Fitting the energy spectrum of the BL Lac object Mrk 501, we obtain tight constrains on the beam properties. Launching a sufficiently energetic beam requires rather strong magnetic fields close to the black hole 100-1000 G. However, the model fits imply that the magnetic field in the emission region is only 0.02 G. Thus, the particles are accelerated close to the black hole and propagate a considerable distance before instabilities trigger the dissipation of energy through synchrotron and self-Compton emission. We discuss various approaches to generate enough power to drive the jet and, at the same time, to accelerate particles to 20 TeV energies. Although the parallel beam model has its own problems, it explains some of the long-standing problems that plague models based on Fermi type particle acceleration, like the presence of a very high minimum Lorentz factor of accelerated particles. We conclude with a brief discussion of the implications of the model for the difference between the processes of jet formation in BL Lac type objects and in quasars.   

A Model of MBH Accretion Disk Dynamo, Jet and Radio Lobe

Stirling Colgate, Hui Li, Philipp Kronberg, LANL. The minimum total energy of radio lobes occurs at equipartition of magnetic energy and relativistic electron energy. Typical total energies, $10^{61}$ ergs challenge Massive Black Hole formation, $10^{62}$ ergs. This large coherent magnetic flux is substantiated by rotation measures and polarization, and relativistic electrons by Compton x-rays from the cosmic radiation. This coherent flux is produced from a helical flux pinch driven by Keplerian winding, of a quadrupole poloidal magnetic field. The poloidal field is created by a coherent $\alpha \omega$ dynamo in the disk produced by plumes driven by star-disk collisions. Toroidal flux inside the disk is advected through the event horizon, but the poloidal flux accumulates inside and outside the disk. Outside the disk it saturates the helicity production at a value well below the dynamical Keplerian stress. The winding of the external poloidal flux produces the helical jet with a small flux core, stabilized by relativistic run-away current carriers, starved by preferential loss. After current carrier loss the flux core becomes pinch unstable producing the solenoidal current configuration approximating radio lobes.   

Influence of Local Angular Momentum Transport and Ring Configurations in the Strong Gravity of a Central Object*

Differentially rotating plasma structures in the prevalent gravity of a central object (e.g. black hole) can acquire a configuration characterized by a radial sequence of rings\footnote{B. Coppi and F. Rousseau, \textit{ Ap. J. } \textbf{641 (1)}, 458 (2006)} in the presence of a ``seed'' vertical magnetic field, rather than one typical of a gaseous disk. A sequence of pairs of counter-streaming toroidal current channels is associated with these rings and no poloidal currents are produced if a torque is absent. When a local transport process of angular momentum is present, loops of poloidal currents associated with the resulting torque are formed.\footnote{B. Coppi, MIT-LNS Report 06/05 (Cambridge, MA, 2006)} In particular, the vertical current densities are up- down antisymmetric. A jet that could emerge from the innermost ring would, in fact, have antisymmetric vertical current densities relative to the equatorial plane. When a small radial velocity, resulting for instance from accretion, is present we argue that matter will flow along the X-lines and the O-lines of a weakly spiraling ring configuration that is envisioned instead of the strictly axisymmetric$^{1}$ configuration found when only a toroidal velocity is present. *Sponsored in part by the U.S. Department of Energy.