arXiv subjects

S. Heinz

Publications and source records attributed to S. Heinz.

108 records · Page 6Linked to original sources

arXiv2002

Cosmic Rays from Microquasars: a Narrow Component to the CR Spectrum?

We propose that relativistic Galactic jets like those observed in GRS 1915+105 and GRO J1655-40 may produce small but measurable contributions to the cosmic ray (CR) spectrum. If these jets contain cold protons and heavy ions (as in SS433), it is plausible that this component will consist of a narrow spectral feature, with a mean particle energy corresponding roughly to the bulk kinetic particle energy in the beam, Gamma_jet m c^2, which, based on current estimates of Gamma_jet, falls into the range of 3-10 GeV. The presence of several sources with different Gamma_jet will lead to superposition of several such peaks. Additionally, diffusive particle acceleration should produce a powerlaw whose low energy cutoff at or above Gamma_jet^2 m c^2 would be visible as another spectral feature. The large metallicities measured in several microquasar companions suggest that this CR component could have an anomalous composition compared to the bulk Galactic CRs. We provide estimates of the effects of adiabatic losses, which pose the greatest challenge to models of narrow band CR production in microquasar jets. While the total energy contained in the microquasar CR component is highly uncertain, the CR spectrum in the vicinity of microquasars would be severely affected. AMS02 will be able to probe the low energy CR spectrum for such components and composition anomalies. The spectrally peculiar gamma-ray emission produced by interaction of the ISM with CRs surrounding microquasars might be detectable by GLAST. If the presence of a microquasar CR proton component can be ruled out observationally, this argument could be turned around in favor of electron-positron jets. We show that existing OSSE/GRO and future INTEGRAL data on the Galactic 511 keV line flux put interesting constraints on the particle content of microquasar jets.

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arXiv2002

Radio Lobe Dynamics and the Environment of Microquasars

We argue that, when compared to AGNs in dynamical terms, microquasars are found in low density, low pressure environments. Using a simple analytic model, we discuss radio lobe dynamics and emission. Dynamical considerations for GRS 1915+105 and GRO J1655-40 show that they are located in ISM densities well below the canonical n_ISM ~ 1 cm^{-3} unless the jets are unusually narrow or much more powerful than currently believed.

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arXiv2002

The hydrodynamics of dead radio galaxies

We present a numerical investigation of dead, or relic, radio galaxies and the environmental impact that radio galaxy activity has on the host galaxy or galaxy cluster. We perform axisymmetric hydrodynamical calculations of light, supersonic, back-to-back jets propagating in a beta-model galaxy/cluster atmosphere. We then shut down the jet activity and let the resulting structure evolve passively. The dead source undergoes an initial phase of pressure driven expansion until it achieves pressure equilibrium with its surroundings. Thereafter, buoyancy forces drive the evolution and lead to the formation of two oppositely directed plumes that float high into the galaxy/cluster atmosphere. These plumes entrain a significant amount of low entropy material from the galaxy/cluster core and lift it high into the atmosphere. An important result is that a large fraction (at least half) of the energy injected by the jet activity is thermalized in the ISM/ICM core. The whole ISM/ICM atmosphere inflates in order to regain hydrostatic equilibrium. This inflation is mediated by an approximately spherical disturbance which propagates into the atmosphere at the sound speed. The fact that such a large fraction of the injected energy is thermalized suggests that radio galaxies may have an important role in the overall energy budget of rich ISM/ICM atmospheres. In particular, they may act as a strong and highly time-dependent source of negative feedback for galaxy/cluster cooling flows.

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arXiv2000

$ϕ$ and $ω$ Meson Production in pp Reactions at $p_{lab}$=3.67 GeV/c

The exclusive production cross sections for $ω$ and $ϕ$ mesons have been measured in proton-proton reactions at $p_{lab}=3.67$ GeV/c. The observed $ϕ/ω$ cross section ratio is $(3.8\pm0.2^{+1.2}_{-0.9})\times 10^{-3}$. After phase space corrections, this ratio is enhanced by about an order of magnitude relative to naive predictions based upon the Okubo-Zweig-Iizuka (OZI) rule, in comparison to an enhancement by a factor $\sim 3$ previously observed at higher beam momenta. The modest increase of this enhancement near the production threshold is compared to the much larger increase of the $ϕ/ω$ ratio observed in specific channels of $\bar pp$ annihilation experiments. Furthermore, differential cross section results are also presented which indicate that although the $ϕ$ meson is predominantly produced from a $^3P_1$ proton-proton entrance channel, other partial waves contribute significantly to the production mechanism at this beam momentum.

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arXiv2000

Shocks and sonic booms in the intracluster medium: X-ray shells and radio galaxy activity

Motivated by hydrodynamic simulations, we discuss the X-ray appearance of radio galaxies embedded in the intracluster medium (ICM) of a galaxy cluster. We distinguish three regimes. In the early life of a powerful source, the entire radio cocoon is expanding supersonically and hence drives a strong shock into the ICM. Eventually, the sides of the cocoon become subsonic and the ICM is disturbed by the sonic booms of the jet's working surface. In both of these regimes, X-ray observations would find an X-ray shell. In the strong shock regime, this shell will be hot and relatively thin. However, in the weak shock (sonic-boom) regime, the shell will be approximately the same temperature as the undisturbed ICM. If a cooling flow is present, the observed shell may even be cooler than the undisturbed ICM due to the lifting of cooler material into the shell from the inner (cooler) regions of the cluster. In the third and final regime, the cocoon has collapsed and no well-defined X-ray shell will be seen. We discuss ways of estimating the power and age of the source once its regime of behavior has been determined.

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arXiv2000

Positron spectra from internal pair conversion observed in {238}U + {181}Ta collisions

We present new results from measurements and simulations of positron spectra, originating from 238U + 181Ta collisions at beam energies close to the Coulomb barrier. The measurements were performed using an improved experimental setup at the double-Orange spectrometer of GSI. Particular emphasis is put on the signature of positrons from Internal-Pair-Conversion (IPC) processes in the measured e+ energy spectra, following the de-excitation of electromagnetic transitions in the moving Ta-like nucleus. It is shown by Monte Carlo simulations that, for the chosen current sweeping procedure used in the present experiments, positron emission from discrete IPC transitions can lead to rather narrow line structures in the measured energy spectra. The measured positron spectra do not show evidence for line structures within the statistical accuracy achieved, although expected from the intensities of the observed $γ$ transitions (E$_γ~1250-1600$ keV) and theoretical conversion coefficients. This is due to the reduced detection efficiency for IPC positrons, caused by the limited spatial and momentum acceptance of the spectrometer. A comparison with previous results, in which lines have been observed, is presented and the implications are discussed.

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arXiv2000

Production of $η\prime$ Mesons in the $pp \to ppη\prime$ Reaction at 3.67 GeV/c

The ratio of the total exclusive production cross sections for $η\prime$ and $η$ mesons has been measured in the $pp$ reaction at $p_{beam}=3.67$ GeV/c. The observed $η\prime/η$ ratio is $(0.83\pm{0.11}^{+0.23}_{-0.18})\times 10^{-2}$ from which the exclusive $η\prime$ meson production cross section is determined to be $(1.12\pm{0.15}^{+0.42}_{-0.31})μb$. Differential cross section distributions have been measured. Their shape is consistent with isotropic $η\prime$ meson production.

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arXiv2000

The Flared Disc Project: RXTE and ASCA Observations of X 1822-371

We present archival RXTE and simultaneous ASCA data of the eclipsing low mass X-ray binary (LMXB) X 1822-371. Our spectral analysis shows that a variety of simple models can fit the spectra relatively well. Of these models, we explore two in detail through phase resolved fits. These two models represent the case of a very optically thick and a very optically thin corona. While systematic residuals remain at high energies, the overall spectral shape is well-approximated. The same two basic models are fit to the X-ray light curve, which shows sinusoidal modulations interpreted as absorption by an opaque disc rim of varying height. The geometry we infer from these fits is consistent with previous studies: the disc rim reaches out to the tidal truncation radius, while the radius of the corona (approximated as spherical) is very close to the circularization radius. Timing analysis of the RXTE data shows a time lag from hard to soft consistent with the coronal size inferred from the fits. Neither the spectra nor the light curve fits allow us to rule out either model, leaving a key ingredient of the X 1822-371 puzzle unsolved. Furthermore, while previous studies were consistent with the central object being a 1.4 Msol neutron star, which has been adopted as the best guess scenario for this system, our light curve fits show that a white dwarf or black hole primary can work just as well. Based on previously published estimates of the orbital evolution of X 1822-371, however, we suggest that this system contains either a neutron star or a low mass (~ 2.5 Msol) black hole and is in a transitional state of duration shortward of 10^7 years.

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arXiv1999

Jet Acceleration by Tangled Magnetic Fields

We explore the possibility that extragalactic radio jets might be accelerated by highly disorganized magnetic fields that are strong enough to dominate the dynamics until the terminal Lorentz factor is reached. Following the twin-exhaust model by Blandford & Rees (1974), the collimation under this scenario is provided by the stratified thermal pressure from an external medium. The acceleration efficiency then depends on the pressure gradient of that medium. In order for this mechanism to work there must be continuous tangling of the magnetic field, changing the magnetic equation of state away from pure flux freezing (otherwise conversion of Poynting flux to kinetic energy flux is suppressed). This is a complementary approach to models in which the plasma is accelerated by large scale ordered fields. We include a simple prescription for magnetic dissipation, which leads to tradeoffs among conversion of magnetic energy into bulk kinetic energy, random particle energy, and radiation. We present analytic dynamical solutions of such jets, assess the effects of radiation drag, and comment on observational issues, such as the predicted polarization and synchrotron brightness. Finally, we try to make the connection to observed radio galaxies and gamma-ray bursts.

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arXiv1999

A Shotgun Model for Gamma Ray Bursts

We propose that gamma ray bursts (GRBs) are produced by a shower of heavy blobs running into circumstellar material at highly relativistic speeds. The gamma ray emission is produced in the shocks these bullets drive into the surrounding medium. The short term variability seen in GRBs is set by the slowing-down time of the bullets while the overall duration of the burst is set by the lifetime of the central engine. A requirement of this model is that the ambient medium be dense, consistent with a strong stellar wind. The efficiency of the burst can be relatively high.

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arXiv1999

K$^-$ Meson Production in the Proton-Proton Reaction at 3.67 GeV/c

The total cross section of the reaction $pp\to ppK^+K^-$ has been determined for proton--proton reactions with $p_{beam}=3.67 GeV/c$. This represents the first cross section measurement of the $pp \to ppK^-K^+$ channel near threshold, and is equivalent to the inclusive $pp\to ppK^-X$ cross section at this beam momentum. The cross section determined at this beam momentum is about a factor 20 lower than that for inclusive $pp\to ppK^+X$ meson production at the same CM energy above the corresponding threshold. This large difference in the $K^+$ and $K^-$ meson inclusive production cross sections in proton-proton reactions is in strong contrast to cross sections measured in sub-threshold heavy ion collisions, which are similar in magnitude at the same energy per nucleon below the respective thresholds.

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arXiv1999

Spin Transfer in Exclusive Lambda Production from pp Collisions at 3.67 GeV/c

We report the first polarization transfer measurements for exclusive hyperon production reactions. The normal spin transfer coefficient DNN for pp -> p K+ Lambda is large and negative for forward Lambda production at a beam momentum of 3.67 GeV/c, a result qualitatively consistent with expectations for a mechanism dominated by kaon-exchange and rescattering. The sign of DNN is opposite to that observed in the fragmentation regime for inclusive Lambda production at much higher energies.

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arXiv1999

An RXTE study of M87 and the core of the Virgo cluster

We present hard X-ray observations of the nearby radio galaxy M87 and the core of the Virgo cluster using the Rossi X-ray Timing Explorer. These are the first hard X-ray observations of M87 not affected by contamination from the nearby Seyfert 2 galaxy NGC4388. Thermal emission from Virgo's intracluster medium is clearly detected and has a spectrum indicative of kT=2.5keV plasma with approximately 25% cosmic abundances. No non-thermal (power-law) emission from M87 is detected in the hard X-ray band, with fluctuations in the Cosmic X-ray Background being the limiting factor. Combining with ROSAT data, we infer that the X-ray spectrum of the M87 core and jet must be steep (Gamma_core>1.90$ and Gamma_jet>1.75), and we discuss the implications of this result. In particular, these results are consistent with M87 being a mis-aligned BL-Lac object.

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arXiv1998

First Energy and Angle differential Measurements of e^+e^- -pairs emitted by Internal Pair Conversion of excited Heavy Nuclei

We present the first energy and angle resolved measurements of e+e- pairs emitted from heavy nuclei (Z>=40) at rest by internal pair conversion (IPC) of transitions with energies of less than 2MeV as well as recent theoretical results using the DWBA method, which takes full account of relativistic effects, magnetic substates and finite size of the nucleus. The 1.76MeV E0 transition in Zr90 (Sr source) and the 1.77MeV M1 transition in Pb207 (Bi source) have been investigated experimentally using the essentially improved set-up at the double-ORANGE beta-spectrometer of GSI. The measurements prove the capability of the setup to cleanly identify the IPC pairs in the presence of five orders of magnitude higher beta- and gamma background from the same source and to yield essentially background-free sum spectra despite the large background. Using the ability of the ORANGE setup to directly determine the opening angle of the e+e- pairs, the angular correlation of the emitted pairs was measured. In the Zr90 case the correlation could be deduced for a wide range of energy differences of the pairs. The Zr90 results are in good agreement with recent theory. The angular correlation deduced for the M1 transition in Pb207 is in strong disagreement with theoretical predictions derived within the Born approximation and shows almost isotropic character. This is again in agreement with the new theoretical results.

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arXiv1997

Weak e+e- lines from internal pair conversion observed in collisions of 238U with heavy nuclei

We present the results of a Doppler-shift correction to the measured e+e- sum-energy spectra obtained from e+e- coincidence measurements in 238U + 206Pb and 238U + 181Ta collisions at beam energies close to the Coulomb barrier, using an improved experimental setup at the double-Orange spectrometer of GSI. Internal-Pair-Conversion (IPC) e+e- pairs from discrete nuclear transitions of a moving emitter have been observed following Coulomb excitation of the 1.844 MeV (E1) transition in 206Pb and neutron transfer to the 1.770 MeV (M1) transition in 207Pb. In the collision system 238U + 181Ta, IPC transitions were observed from the Ta-like as well as from the U-like nuclei. In all systems the Doppler-shift corrected e+e- sum-energy spectra show weak lines at the energies expected from the corresponding $γ$-ray spectra with cross sections being consistent with the measured excitation cross sections of the $γ$ lines and the theoretically predicted IPC coefficients. No other than IPC e+e- sum-energy lines were found in the measured spectra. The transfer cross sections show a strong dependence on the distance of closest approach (R_min), thus signaling also a strong dependence on the bombarding energy close to the Coulomb barrier.

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arXiv1997

Analysis of the Synchrotron Emission from the M87 Jet

We propose that the intensity changes and spectral evolution along the M87 jet can be explained by adiabatic changes to the particle momentum distribution function and the magnetic field. This is supported by the lack of any significant variation in the radio-to-optical spectral index along the jet and the moderate changes in radio brightness. Assuming a simple scaling law between magnetic field and density, we use the deprojection of a 2 cm VLA intensity map by Sparks, Biretta, & Macchetto (1996) to predict the spectral evolution along the jet. We derive limits for the magnetic field and the total pressure by comparing our results with the spatially resolved fit to spectral data by Neumann, Meisenheimer, & Roeser (1997) of a model spectrum that cuts off at approx 10^15 Hz. To explain the weakness of synchrotron cooling along the jet, the magnetic field strength must lie below the equipartition value. Although the inferred pressure in the limit of nonrelativistic bulk flow lies far above the estimated pressure of the interstellar matter in the center of M87, bulk Lorentz factors Gamma_jet in the range of 3 - 5 and inclination angles approx less than 25 deg lead to pressure estimates close to the ISM pressure. The average best fit magnetic fields we derive fall in the range of 20 - 40 microG, departing from equipartition by a factor approx 1.5 - 5. This model is consistent with the proposal by Bicknell & Begelman (1996) that the knots in the M87 jet are weak, oblique shocks. First-order Fermi acceleration will then have a minimal effect on the slope of the radio-- to-optical spectrum while possibly accounting for the X-ray spectrum.

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arXiv1996

e+e- pairs from 238U + 206Pb collisions

We present the results from e+e- coincidence measurements in 238U + 206Pb collisions at a beam energy of 5.93 MeV/u, using an improved experimental setup at the double-Orange spectrometer of GSI. The capability of our device to detect Internal-Pair-Conversion (IPC) e+e- pairs from discrete nuclear transitions of a moving emitter is demonstrated by investigating the known 1.844 MeV (E1) transition in 206Pb and the 1.770 MeV (M1) transition in 207Pb, excited via Coulomb excitation and a neutron transfer reaction, respectively. The Doppler-shift corrected e+- sum-energy spectra show weak lines at the expected energies with cross sections being consistent with the measured excitation cross sections of the corresponding $γ$ lines and the theoretically predicted IPC coefficients. No other e+e- sum-energy lines were found in the measured spectra. The observed transfer cross sections show a strong dependence on the distance of closest approach (R_min), thus signaling also a strong dependence on the bombarding energy close to the Coulomb barrier.

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arXiv1996

New Results on e+e- Line Emission in U+Ta Collisions

We present new results obtained from a series of follow-up e+e- coincidence measurements in heavy-ion collisions, utilizing an improved experimental set-up at the double-Orange beta-spectrometer of GSI. The collision system U+Ta was reinvestigated in three independent runs at beam energies in the range (6.0-6.4)xA MeV and different target thicknesses, with the objective to reproduce a narrow sum-energy e+e- line at ~635 keV observed previously in this collision system. At improved statistical accuracy, the line could not be found in these new data. For the ''fission'' scenario, an upper limit (1 sigma) on its production probability per collision of 1.3x10^{-8} can be set which has to be compared to the previously reported value of [4.9 +- 0.8 (stat.) +- 1.0 (syst)]x10^{-7}. In the light of the new results, a reanalysis of the old data shows that the continuous part of the spectrum at the line position is significantly higher than previously assumed, thus reducing the production probability of the line by a factor of two and its statistical significance to < 3.4sigma.

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