arXiv subjects

H. Lesch

Publications and source records attributed to H. Lesch.

41 records · Page 3Linked to original sources

arXiv1997

The radio spectra of galactic nuclei

We present a model for the inverted radio spectra of active active galactic nuclei as well as the central regions of normal galaxies. The model is based on the unified scenario for active galaxies, stating that the central engines of active galaxies consists of a supermassive black hole surrounded by an accretion disk and a radio jet. The nuclei of normal (i. e. less active) galaxies are supposed to be scaled-down versions of the same phenomenon. We show that the radio emission of a jet component, becoming optically thin to the radio emission of a monoenergetic pair plasma at decreasing frequencies as it moves outward and expands, is well suited to explain the observational results. We present a model calculation for the special case of the nucleus of M~81.

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arXiv1997

On the nonthermal emission in active galactic nuclei

We consider the role of centrifugal force on the energetics of electrons moving along the magnetic field lines of spinning active galactic nuclei. We find the energy gained by charged particles against inverse Compton scattering and/ synchrotron radiation losses, become quite significant in the region close to the light cylinder. The particles accelerated by the centrifugal force become a part of the jet material. The scattering of UV-photons against the energetic electrons lead to the generation of X-ray and gamma-ray photons within the light cylinder, and outside, they radiate the non-thermal optical radiation via the synchrotron emission.

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arXiv1996

Dissipation of Magnetic Fields in the Galactic Halo

This paper shows that magnetic reconnection could be an important heating process in cosmic gases. In any volume where magnetized plasmas collide, the dissipation of magnetic energy via reconnection seems to be unavoidable. Since most cosmic plasmas are highly conductive, the magnetic field lines are transported with the gas and no dissipation occurs for the most part of the volume. This ideal frozen-in property of the magnetic field is broken in small volumes if field gradients with different field polarity appear, in which localized dissipative effects, e.g. anomalous resistivity, become important. On the base of X-ray measurements exhibiting a clear connection of infalling high-velocity clouds (HVC) with ROSAT "hotspots" we perform resistive magnetohydrodynamic simulations to investigate the capabilities of magnetic dissipation as a major heating process in the interaction zone of the cloud with the halo. The main result is that in the physical environment of a galactic halo heating by externally driven magnetic reconnection cannot be suppressed by thermal conduction and/or radiative cooling. Thus, the gas reaches the maximum temperature given by the magnetic field pressure in the interaction zone of the HVC with the galactic halo.

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arXiv1996

The large-scale structure of the diffuse radio halo of the Coma cluster at 1.4 GHz

We present new measurements of the diffuse radio emission from the Coma cluster of galaxies at 1.4 GHz using the Effelsberg 100-m-telescope. The halo source Coma C has an extent down to noise of about 80 arcmin corresponding to 3 Mpc (H_0 = 50 km/s/Mpc) in Coma. The radio map reveals clear similarities with images of the extended X-ray halo of the Coma cluster. After subtracting the contributions from point sources we obtained an integrated diffuse flux density of S = 640 +- 35 mJy from Coma C. We derive relations between the various observationally determined spectral indices and the spectral index of the synchrotron emissivity, which allow one to achieve a rough estimate concerning the consistency of the presently available data at different frequencies and to place constraints on the emissivity index distribution. In the halo's core region, the inferred emissivity index between 0.3 GHz and 1.4 GHz appears to be in the range 0.4 - 0.75 implying that there must be some very effective mechanism for particle acceleration operating in the intracluster medium.

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arXiv1994

Anatomy of the Sagittarius A complex: IV. Sgr A* and the Central Cavity revisi- ted

We present submm images of Sgr A* and its surroundings obtained at 800, 600 and 450 $μ$m with the JCMT and derive flux densities of Sgr A* at all three wave- lengths. Combined with upper limits by Gezari and associates at MIR wavelengths a time averaged radio spectrum is obtained which increases $\propto ν^{1/3}$, attains a maximum at $ν_max \ga 600 GHz$ and must decrease rapidly at fre- quencies $\ga 10^4$GHz. This spectrum allows for about $3 M}_\odot$ of 50K dust and associated hydrogen in the telescope beam. While variations on time-scales of a few months are now well established for the frequency range $ν\la 100 GHz$, our investigation of variability at higher frequencies still yields only marginal results. The Circum-Nuclear Disk (CND) extends over the central 12 pc. At the galactocentric radius $R \sim 1 pc$ dust and hydrogen column densities drop to low values and form the Central Cavity. Our submm images show that the bottom of this cavity is rather flat. Variations in the dust emission are just consistent with the detection of a `Tongue' of $\sim 200 M_\odot$ of atomic hydrogen reported by Jackson et al. (1993) to be located between the Northern and the Eastern Arm of the Minispiral. We present a revised submm/IR spectrum of the central $30\arcsec$ (R$\la 0.6 pc$) with flux densities corrected for an interstellar extinction of $A_v \sim 31 mag$. This spectrum attains its maximum at $λ\sim 20 μm$ and comes from dust with temperatures $\sim 170 - 400 K$ which is associated with the Eastern Arm and the East-West Bar. The in- tegrated luminosity is $\sim 5 10^6 L_\odot$ to which emission at $λ\la 30 μm$ contributes $\sim 80\%$. Heating of this dust is not provided by a central source but rather by a cluster of hot ($T eff \sim 3 - 3.5 10^4 K$) and luminous stars which could include the He{\sc i}/H{\sc i}-stars detected by

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