The JetCurry Code. I. Reconstructing Three-Dimensional Jet Geometry from Two-Dimensional images

We present a reconstruction of jet geometry models using numerical methods based on a Markov ChainMonte Carlo (MCMC) and limited memory Broyden-Fletcher-Goldfarb-Shanno (BFGS) optimized algorithm. Our aim is to model the three-dimensional geometry of an AGN jet using observations, which are inherently two-dimensional. Many AGN jets display complex hotspots and bends over the kiloparsec scales. The structure of these bends in the jets frame may be quite different than what we see in the sky frame, transformed by our particular viewing geometry. The knowledge of the intrinsic structure will be helpful in understanding the appearance of the magnetic field and hence emission and particle acceleration processes over the length of the jet. We present the method used, as well as a case study based on a region of the M87 jet.Comment: Submitted to ApJ on Feb 01, 201

High Energy Variability Of Synchrotron-Self Compton Emitting Sources: Why One Zone Models Do Not Work And How We Can Fix It

With the anticipated launch of GLAST, the existing X-ray telescopes, and the enhanced capabilities of the new generation of TeV telescopes, developing tools for modeling the variability of high energy sources such as blazars is becoming a high priority. We point out the serious, innate problems one zone synchrotron-self Compton models have in simulating high energy variability. We then present the first steps toward a multi zone model where non-local, time delayed Synchrotron-self Compton electron energy losses are taken into account. By introducing only one additional parameter, the length of the system, our code can simulate variability properly at Compton dominated stages, a situation typical of flaring systems. As a first application, we were able to reproduce variability similar to that observed in the case of the puzzling `orphan' TeV flares that are not accompanied by a corresponding X-ray flare.Comment: to appear in the 1st GLAST symposium proceeding

The Event Horizon of M87

The 6 billion solar mass supermassive black hole at the center of the giant elliptical galaxy M87 powers a relativistic jet. Observations at millimeter wavelengths with the Event Horizon Telescope have localized the emission from the base of this jet to angular scales comparable to the putative black hole horizon. The jet might be powered directly by an accretion disk or by electromagnetic extraction of the rotational energy of the black hole. However, even the latter mechanism requires a confining thick accretion disk to maintain the required magnetic flux near the black hole. Therefore, regardless of the jet mechanism, the observed jet power in M87 implies a certain minimum mass accretion rate. If the central compact object in M87 were not a black hole but had a surface, this accretion would result in considerable thermal near-infrared and optical emission from the surface. Current flux limits on the nucleus of M87 strongly constrain any such surface emission. This rules out the presence of a surface and thereby provides indirect evidence for an event horizon.Comment: 9 pages, 2 figures, submitted to Ap

The HI Environment of Nearby Lyman-alpha Absorbers

We present the results of a VLA and WSRT search for HI emission from the vicinity of seven nearby clouds, which were observed in Lya absorption with HST toward Mrk335, Mrk501 and PKS2155-304. We searched a volume of 40' x 40' x 1000 km/s. The HI mass sensitivity (5 sigma) varies from 5x10^6 to 5x10^8 Msun. We detected HI emission in the vicinity of four out of seven absorbers. The closest galaxy is a small dwarf galaxy at a projected distance of 68/h kpc from the sight line toward Mrk335. It has the same velocity (V=1970 km/s) as one of the absorbers, and has an HI mass of only 4x10^7 Msun. We found a more luminous galaxy at the velocity (V=5100 km/s) of one of the absorbers toward PKS2155-304, 230/h kpc from the sight line. Two other, stronger absorbers toward PKS2155-304 at V=17,000 km/s are associated with a loose group of three bright spiral galaxies, at projected distances of 300 to 600/h kpc. These results support the conclusion that most nearby Lya forest clouds trace the large-scale structures outlined by optically luminous galaxies. We do not find any evidence for a physical association between an absorber and its closest galaxy.Comment: 4 Tables, 11 Figures, to be published in Astron J. (Oct 1996) Vol 11

New Constraints on Quantum Gravity from X-ray and Gamma-Ray Observations

One aspect of the quantum nature of spacetime is its "foaminess" at very small scales. Many models for spacetime foam are defined by the accumulation power $\alpha$, which parameterizes the rate at which Planck-scale spatial uncertainties (and thephase shifts they produce) may accumulate over large path-lengths. Here $\alpha$ is defined by theexpression for the path-length fluctuations, $\delta \ell$, of a source at distance $\ell$, wherein $\delta \ell \simeq \ell^{1 - \alpha} \ell_P^{\alpha}$, with $\ell_P$ being the Planck length. We reassess previous proposals to use astronomical observations ofdistant quasars and AGN to test models of spacetime foam. We show explicitly how wavefront distortions on small scales cause the image intensity to decay to the point where distant objects become undetectable when the path-length fluctuations become comparable to the wavelength of the radiation. We use X-ray observations from {\em Chandra} to set the constraint $\alpha \gtrsim 0.58$, which rules out the random walk model (with $\alpha = 1/2$). Much firmer constraints canbe set utilizing detections of quasars at GeV energies with {\em Fermi}, and at TeV energies with ground-based Cherenkovtelescopes: $\alpha \gtrsim 0.67$ and $\alpha \gtrsim 0.72$, respectively. These limits on $\alpha$ seem to rule out $\alpha = 2/3$, the model of some physical interest.Comment: 11 pages, 9 figures, ApJ, in pres