arXiv subjects

R. Beck

Publications and source records attributed to R. Beck.

370 records · Page 21Linked to original sources

arXiv2000

Regular magnetic fields in the dwarf irregular galaxy NGC 4449

We present a high-resolution VLA study of the total power and polarized radio continuum emission at 8.46 and 4.86 GHz of the irregular galaxy NGC 4449, known for its weak rotation and non-systematic gas motions. We found strong galaxy-scale regular magnetic fields, which is surprising because of a lack of ordered rotation required for the dynamo action. The strength of the regular field reaches 8 $μ$G and that of the total field 14 $μ$G, comparable to that of the total magnetic field strength in radio-bright spirals. The magnetic vectors in NGC 4449 form radial ``fans'' in the central region and fragments of a spiral pattern in the galaxy's outskirts. These structures are associated with large regions of systematic Faraday rotation, implying genuine galaxy-scale magnetic fields rather than random ones compressed and stretched by gas flows. The observed pattern of polarization B-vectors is similar to dynamo-type fields in normal spirals. Nonstandard, fast dynamo concepts are required to explain the observed field strengths, though it is unknown what kind of magnetic field geometry can be produced in slowly and chaotically rotating objects. The so far neglected role of magnetic fields for the dynamics and star formation in dwarf irregulars also needs to be revised.

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arXiv1999

Determination of the E2/M1 Ratio in the γN \to Δ(1232) Transition from a Simultaneous Measurement of p(\vecγ,p)π^0 and p(\vecγ,π^+)n

Tagged linearly polarized photons have been used at the Mainz Microtron MAMI for simultaneous measurements of the p(\vecγ,p)π^0 and p(\vecγ,π^+)n reaction channels to study the γN \to Δ(1232) transition. The energy dependence of the magnetic dipole M_{1+}^{3/2} and electric quadrupole E_{1+}^{3/2} amplitudes have been extracted from these data in the photon energy range from 270 to 420 MeV. The E2/M1 ratio for the γN \to Δ(1232) transition has been determined to be - (2.5+-0.1_{stat}+-0.2_{sys}) % at the resonance position delta_{33}=90^0.

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arXiv1999

Coherent π^\circ photoproduction from ^4He

Differential cross sections and beam asymmetries for coherent π^\circ photoproduction from ^4He in the Δenergy-range have been measured with high statistical and systematic precisions using both decay photons for identifying the process.The experiment was performed at the MAinz MIcrotron using the TAPS photon spectrometer and the Glasgow/Mainz tagged photon facility. The differential cross sections are in excellent agreement with predictions based on the DWIA if an appropriate parametrization of the Δ-nuclear interaction is applied. The beam asymmetries are interpreted in terms of degrees of linear polarization of collimated coherent bremsstrahlung. The expected increase of the degree of linear polarization with decreasing collimation angle is confirmed. Agreement with calculations is obtained on a few-percent level of precision in the maxima of the coherent peaks.

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arXiv1999

Magnetic fields and strong density waves in the interacting galaxy NGC 3627

We present 10.55 GHz observations of strong density-wave spiral NGC 3627. Total power and polarization maps with a resolution of 1.13', sensitive to a weak extended emmision were obtained. In the analysiz we used also available data in the CO ans H_alpha lines. We present simple modelling results showing the imprtance of large-scale dynamo-type magnetic field present despite the strong density-wave action in this galaxy.

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arXiv1998

New clues to the magnetic field structure of M31

We observed 21 polarized background radio sources in the field of M31 at 1.365 GHz and 1.652 GHz, and determined their rotation measures (RMs). The RM data show that the regular magnetic field of the disk probably extends from about 5 kpc to 25 kpc from the center with similar structure. The RMs obtained from the polarized emission from M31 at $λ$6 cm and $λ$11 cm indicate that M31 might have a weak poloidal field in its tenuous halo. Observational features of the odd and even dynamo modes in a galaxy are discussed. An even mode (S0) dynamo may operate in M31.

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arXiv1998

A Measurement of the Interference Structure Function, R_LT, for the 12C(e,e'p) reaction in the Quasielastic Region

The coincidence cross-section and the interference structure function, R_LT, were measured for the 12C(e,e'p) 11B reaction at quasielastic kinematics and central momentum transfer of q=400 MeV/c. The measurement was at an opening angle of theta_pq=11 degrees, covering a range in missing energy of E_m = 0 to 65 MeV. The R_LT structure function is found to be consistent with zero for E_m > 50 MeV, confirming an earlier study which indicated that R_L vanishes in this region. The integrated strengths of the p- and s-shell are compared with a Distorted Wave Impulse Approximation calculation. The s-shell strength and shape are compared with a Hartree Fock-Random Phase Approximation calculation. The DWIA calculation overestimates the cross sections for p- and s-shell proton knockout as expected, but surprisingly agrees with the extracted R_LT value for both shells. The HF-RPA calculation describes the data more consistently, which may be due to the inclusion of 2-body currents in this calculation.

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arXiv1997

ROSAT PSPC X-ray observations of the nearby spiral galaxy M83

The nearly face-on SBc galaxy M83 (NGC 5236) was observed for 25 ksec with the ROSAT PSPC. We detected 13 point-like sources in this galaxy, 10 of which were previously unknown. We measured extended X-ray radiation from almost the whole optically visible galaxy. Comparing the diffuse soft and hard X-ray emission components, we observed a different asymmetric distribution and a slower radial decrease of the intensity profile of the soft X-ray emission. Both these results support the existence of a huge spherical gas halo of 10-15 kpc radius. On the other hand, the radial scale lengths of the hard X-ray radiation, that of the thermal radio emission and the profile of the optical surface brightness are similar, favouring the idea that all these emission processes are connected to star formation in the galaxy's disk. M83 is the first face-on galaxy where the diffuse X-ray emission spectrum can be characterized by a two-temperature thermal plasma: a soft X-ray emitting warm `halo component' and an internally absorbed hot `disk component' which is dominating the emission in the hard (0.5-2.0 keV) ROSAT energy range. The combination of X-ray and radio polarization observations allows an estimate of the plasma parameter beta = U_therm/U_magn which is found to be 0.2+/-0.1. This result supports the hypothesis that magnetic fields play an important role for the evolution and structure of galactic gas haloes. The high energy input rate in the active star-forming disk of M83 seems to be responsible for the outflow of hot gas and the halo formation.

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arXiv1997

Constraining the $(γ, π)$ amplitudes for E2 $N \to Δ$

We reply to the Comment of A.M. Sandorfi et al. [nucl-th/9707028] on the E2/M1 ratio from the Mainz photoproduction data, nucl-th/9707028 v2. We support our published result for the E2/M1 ratio of REM = -(2.5 +- 0.2 +- 0.2)%.

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arXiv1997

E2/M1 ratio from the Mainz $p(\vecγ, p)π^0$

We reply to the Comment of R.L. Workman on the E2/M1 ratio from the Mainz photoproduction data, nucl-th/9702052. We support our published result for the E2/M1 ratio of REM = -(2.5 +- 0.2 +- 0.2)%.

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arXiv1996

Observation of a unitary cusp in the threshold γp -> π^{0}p reaction

A rigorous multipole analysis of the recent γp -> π^{0}p cross section measurement is presented. The data were taken using the photon spectrometer TAPS at the tagged photon beam of the Mainz microtron. The s and p wave multipoles were extracted using minimal model assumptions. The predicted unitary cusp for the s wave multipole E_{0+} due to the two step γp -> π^{+}n -> π^{0}p reaction was observed. The results are consistent with one loop chiral perturbation theory calculations for which three low energy constants have been determined by a fit to the data. The uncertainties in the analysis and the need for polarization observables are discussed.

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