arXiv subjects

Yoel Rephaeli

Publications and source records attributed to Yoel Rephaeli.

64 records · Page 4Linked to original sources

arXiv2002

Results from a Second RXTE Observation of the Coma Cluster

The RXTE satellite observed the Coma cluster for 177 ksec during November and December 2000, a second observation motivated by the intriguing results from the first 87 ksec observation in 1996. Analysis of the new dataset confirms that thermal emission from isothermal gas does not provide a good fit to the spectral distribution of the emission from the inner 1 degree radial region. While the observed spectrum may be fit by emission from gas with a substantial temperature gradient, it is more likely that the emission includes also a secondary non-thermal component. If so, non-thermal emission comprises ~8% of the total 4--20 keV flux. Interpreting this emission as due to Compton scattering of relativistic electrons (which produce the known extended radio emission) by the cosmic microwave background radiation, we determine that the mean, volume-averaged magnetic field in the central region of Coma is B = 0.1-0.3 microgauss.

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arXiv2002

Quantification of Uncertainty in the Measurement of Magnetic Fields in Clusters of Galaxies

We assess the principal statistical and physical uncertainties associated with the determination of magnetic field strengths in clusters of galaxies from measurements of Faraday rotation (FR) and Compton-synchrotron emissions. In the former case a basic limitation is noted, that the relative uncertainty in the estimation of the mean-squared FR will generally be at least one third. Even greater uncertainty stems from the crucial dependence of the Faraday-deduced field on the coherence length scale characterizing its random orientation; we further elaborate this dependence, and argue that previous estimates of the field are likely to be too high by a factor of a few. Lack of detailed spatial information on the radio emission--and the recently deduced nonthermal X-ray emission in four clusters--has led to an underestimation of the mean value of the field in cluster cores. We conclude therefore that it is premature to draw definite quantitative conclusions from the previously-claimed seemingly-discrepant values of the field determined by these two methods.

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arXiv2002

RXTE View of the Starburst Galaxies M82 and NGC 253

The two nearby starburst galaxies M82 and NGC 253 were observed for 100 ksec over a 10-month period in 1997. An increase of the M82 flux by a factor ~2 was measured during the period July-November, when compared with the flux measured earlier in 1997. The flux measured in the field centered on M82 includes ~38 of the emission from the Seyfert 1 galaxy M81. The best-fitting model for the earlier emission from M82 is thermal with kT = 6.7 +/- 0.1 keV. In the high flux state, the emission additionally includes either an absorbed second thermal component or absorbed power-law component, with the former providing a much better fit. A likely origin for the temporal variability is a single source in M82. The flux of NGC 253, which did not vary significantly during the period of observations, can be well fit by either a thermal spectrum with kT ~ 3.8 +/- 0.3 keV, or by a power law with photon index of 2.7 +/- 0.10. We have also attempted fitting the measurements to more realistic composite models with thermal and power-law components, such as would be expected from a dominant contribution from binary systems, or Compton scattering of (far) IR radiation by radio emitting electrons. However, the addition of any amount of a power-law component, even with cutoff at 20 keV, only increases chi-square. The 90% confidence upper limit for power law emission with (photon) index 1.5 is only 2.4% of the 2 -- 10 keV flux of M82; the corresponding limit for NGC 253, with index 2.0, is 48%.

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arXiv2002

CMB Comptonization by Energetic Nonthermal Electrons in Clusters of Galaxies

Use of the Sunyaev-Zeldovich effect as a precise cosmological probe necessitates a realistic assessment of all possible contributions to Comptonization of the cosmic microwave background in clusters of galaxies. We have calculated the additional intensity change due to various possible populations of energetic electrons that have been proposed in order to account for measurements of intracluster radio, nonthermal X-ray and (possibly also) EUV emission. Our properly normalized estimates of (the highly model dependent value of) the predicted intensity change due to these electrons is well below $\sim 6%$ and $\sim 35%$ of the usual Sunyaev-Zeldovich effect due to electrons in the hot gas in Coma and A2199, respectively. These levels constitute high upper limits since they are based on energetic electron populations whose energy densities are {\it comparable} to those of the thermal gas. The main impact of nonthermal Comptonization is a shift of the crossover frequency (where the thermal effect vanishes) to higher values. Such a shift would have important consequences for our ability to measure cluster peculiar velocities from the kinematic component of the Sunyaev-Zeldovich effect.

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arXiv2001

The Sunyaev-Zeldovich Effect and Its Cosmological Significance

Comptonization of the cosmic microwave background (CMB) radiation by hot gas in clusters of galaxies - the Sunyaev-Zeldovich (S-Z) effect - is of great astrophysical and cosmological significance. In recent years observations of the effect have improved tremendously; high signal-to-noise images of the effect (at low microwave frequencies) can now be obtained by ground-based interferometric arrays. In the near future, high frequency measurements of the effect will be made with bolomateric arrays during long duration balloon flights. Towards the end of the decade the PLANCK satellite will extensive S-Z surveys over a wide frequency range. Along with the improved observational capabilities, the theoretical description of the effect and its more precise use as a probe have been considerably advanced. I review the current status of theoretical and observational work on the effect, and the main results from its use as a cosmological probe.

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arXiv2001

RXTE Spectrum of A2319

The cluster of galaxies A2319 was observed in 1999 for 160 ks by the PCA and HEXTE instruments aboard the RXTE satellite. No noticeable variability is seen in the emission measured by either instrument over the 8 week observation. Fitting the RXTE data by a single thermal component we obtain $kT = 8.6 \pm 0.1$ (90% confidence limits), a low iron abundance $Z_{F_e} \sim 0.16 \pm 0.02$, and large positive residuals below 6 keV and between 15 to 30 keV. The quality of the fit is drastically improved if a second component is added. A two-temperature model yields $kT_{1} \simeq 10.1 \pm 0.6$, $kT_{2}\simeq 2.8 \pm 0.6$, and of $Z_{F_e} \sim 0.23 \pm 0.03$. An equally good fit is obtained by a combination of a primary thermal and a secondary nonthermal component, with $kT \simeq 8.9 \pm 0.6$, and power-law index $α\simeq 2.4 \pm 0.3$. We have repeated the analysis by performing joint fits to both these RXTE measurements and archival ASCA data. At most 25% of the RXTE secondary component could be present in the ASCA data. Allowing for this difference, very similar results were obtained, with only somewhat different values for the temperature and power-law index in the latter model. The deduced value of $α$ is consistent with the measured spectrum of extended radio emission. Identifying the power-law emission as Compton scattering of the radio-emitting electrons by the CMB, we obtain $B \sim0.1-0.3$ $μG$ for the volume-averaged magnetic field, and $\sim 4 \times 10^{-14}(R/2 Mpc)^{-3}$ erg cm$^{-3}$ for the mean energy density of the emitting electrons in the central region (radius R) of A2319.

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arXiv2001

Nonthermal Phenomena in Clusters of Galaxies

Recent observations of high energy (> 20 keV) X-ray emission in a few clusters extend and broaden our knowledge of physical phenomena in the intracluster space. This emission is likely to be nonthermal, probably resulting from Compton scattering of relativistic electrons by the cosmic microwave background radiation. Direct evidence for the presence of relativistic electrons in some 30 clusters comes from measurements of extended radio emission in their central regions. I first review the results from RXTE and BeppoSAX measurements of a small sample of clusters, and then discuss their implications on the mean values of intracluster magnetic fields and relativistic electron energy densities. Implications on the origin of the fields and electrons are briefly considered.

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arXiv2001

The Sunyaev-Zeldovich Effect: Current Status and Future Prospects

The detailed spectral and spatial characteristics of the signature imprinted on the cosmic microwave background (CMB) radiation by Compton scattering of the radiation by electrons in the hot gas in clusters of galaxies - the Sunyaev-Zeldovich (S-Z) effect - are of great astrophysical and cosmological significance. In recent years observations of the effect have improved tremendously; high signal-to-noise images of the effect (at low microwave frequencies) can now be obtained by interferometric arrays. In the near future, high frequency measurements of the effect will be made with ground based and balloon-borne telescopes equipped with bolometeric arrays. Towards the end of the decade the PLANCK satellite will carry out an extensive S-Z survey over a wide frequency range. Along with the improved observational capabilities, the theoretical description of the effect, and its use as a precise cosmological probe, have been considerably advanced. In this review, I briefly discuss the nature and significance of the effect, its exact theoretical description, the current observational status, and prospects for the near future.

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arXiv2001

High Energy X-Ray Emission in Clusters of Galaxies

Observations with the RXTE and SAX satellites have recently led to the measurement of a second component in the spectra of several clusters of galaxies which are known to have regions of extended radio emission. This new component is quite likely nonthermal emission resulting from Compton scattering of relativistic electrons by the cosmic microwave background. The nonthermal X-ray and radio measurements yield the values of the mean intracluster magnetic field, and relativistic electron density, for the first time in extragalactic environments. These results have important consequences on issues such as the origin of cosmic ray electrons and protons, their propagation modes in clusters, and the effects of these particles and magnetic fields on the intracluster gas. The observational results are reviewed, and some of their direct implications are discussed, along with near-future prospects for improved spectral and spatial measurements of nonthermal emission in clusters.

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arXiv2000

The Sunyaev-Zeldovich MITO Project

Compton scattering of the cosmic microwave background radiation by electrons in the hot gas in clusters of galaxies - the Sunyaev-Zeldovich effect - has long been recognized as a uniquely important feature, rich in cosmological and astrophysical information. We briefly describe the effect, and emphasize the need for detailed S-Z and X-ray measurements of nearby clusters in order to use the effect as a precise cosmological probe. This is the goal of the MITO project, whose first stage consisted of observations of the S-Z effect in the Coma cluster. We report the results of these observations.

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