319 research outputs found
Stimulated Emission from a single excited atom in a waveguide
We study stimulated emission from an excited two-level atom coupled to a
waveguide containing an incident single-photon pulse. We show that the strong
photon correlation, as induced by the atom, plays a very important role in
stimulated emission. Additionally, the temporal duration of the incident photon
pulse is shown to have a marked effect on stimulated emission and atomic
lifetime.Comment: 6 pages, 3 figure
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.Comment: 10 pages, 1 figure; APJ, in pres
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.Comment: Invited review; in proceedings of the Erice NATO/ASI `Astrophysical
Sources of High Energy Particles and Radiation'; 11 pages, 3 figure
Intracluster Comptonization of the CMB: Mean Spectral Distrortion and Cluster Number Counts
The mean sky-averaged Comptonization parameter, y, describing the scattering
of the CMB by hot gas in clusters of galaxies is calculated in an array of flat
and open cosmological and dark matter models. The models are globally
normalized to fit cluster X-ray data, and intracluster gas is assumed to have
evolved in a manner consistent with current observations. We predict values of
y lower than the COBE/FIRAS upper limit. The corresponding values of the
overall optical thickness to Compton scattering are < 10^{-4} for relevant
parameter values. Of more practical importance are number counts of clusters
across which a net flux (with respect to the CMB) higher than some limiting
value can be detected. Such number counts are specifically predicted for the
COBRAS/SAMBA and BOOMERANG missions.Comment: 23 pages, Latex, 11 PostScript figures, 5 PostScript tables, to
appear in Ap
2-10 keV luminosity of high-mass binaries as a gauge of ongoing star-formation rate
Based on recent work on spectral decomposition of the emission of
star-forming galaxies, we assess whether the integrated 2-10 keV emission from
high-mass X-ray binaries (HMXBs), L_{2-10}^{HMXB}, can be used as a reliable
estimator of ongoing star formation rate (SFR). Using a sample of 46 local (z <
0.1) star forming galaxies, and spectral modeling of ASCA, BeppoSAX, and
XMM-Newton data, we demonstrate the existence of a linear SFR-L_{2-10}^{HMXB}
relation which holds over ~5 decades in X-ray luminosity and SFR. The total
2-10 keV luminosity is not a precise SFR indicator because at low SFR (i.e., in
normal and moderately-starbursting galaxies) it is substantially affected by
the emission of low-mass X-ray binaries, which do not trace the current SFR due
to their long evolution lifetimes, while at very high SFR (i.e., for very
luminous FIR-selected galaxies) it is frequently affected by the presence of
strongly obscured AGNs. The availability of purely SB-powered galaxies - whose
2-10 keV emission is mainly due to HMXBs - allows us to properly calibrate the
SFR-L_{2-10}^{HMXB} relation. The SFR-L_{2-10}^{HMXB} relation holds also for
distant (z ~ 1) galaxies in the Hubble Deep Field North sample, for which we
lack spectral information, but whose SFR can be estimated from deep radio data.
If confirmed by more detailed observations, it may be possible to use the
deduced relation to identify distant galaxies that are X-ray overluminous for
their (independently estimated) SFR, and are therefore likely to hide strongly
absorbed AGNs.Comment: Astronomy & Astrophysics, in press (15 pages, 7 figures, 4 tables
Supermodel Analysis of the Hard X-Ray Excess in the Coma Cluster
The Supermodel provides an accurate description of the thermal contribution
by the hot intracluster plasma which is crucial for the analysis of the hard
excess. In this paper the thermal emissivity in the Coma cluster is derived
starting from the intracluster gas temperature and density profiles obtained by
the Supermodel analysis of X-ray observables: the XMM-Newton temperature
profile and the Rosat brightness distribution. The Supermodel analysis of the
BeppoSAX/PDS hard X-ray spectrum confirms our previous results, namely an
excess at the c.l. of ~4.8sigma and a nonthermal flux of 1.30+-0.40x 10^-11 erg
cm^-2 s^-1 in the energy range 20-80 keV. A recent joint XMM-Newton/Suzaku
analysis reports an upper limit of ~6x10^-12 erg cm^-2 s^-1 in the energy range
20-80 keV for the nonthermal flux with an average gas temperature of 8.45+-0.06
keV, and an excess of nonthermal radiation at a confidence level above 4sigma,
without including systematic effects, for an average XMM-Newton temperature of
8.2 keV in the Suzaku/HXD-PIN FOV, in agreement with our earlier PDS analysis.
Here we present a further evidence of the compatibility between the Suzaku and
BeppoSAX spectra, obtained by our Supermodel analysis of the PDS data, when the
smaller size of the HXD-PIN FOV and the two different average temperatures
derived by XMM-Newton and by the joint XMM-Newton/Suzaku analysis are taken
into account. The consistency of the PDS and HXD-PIN spectra reaffirms the
presence of a nonthermal component in the hard X-ray spectrum of the Coma
cluster. The Supermodel analysis of the PDS data reports an excess at c.l.
above 4sigma also for the higher average temperature of 8.45 keV thanks to the
PDS FOV considerably greater than the HXD-PIN FOV.Comment: 18 pages, 7 figures, accepted for publication in Ap
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.Comment: 31 pages including 3 figures; accepted for publication in the
Astrophysical Journal as paper #5497
Spectral Analysis of RXTE Observations of A3667
X-ray emission from the cluster of galaxies A3667 was measured by the PCA and
HEXTE experiments aboard the RXTE satellite during the period December 2001 -
July 2002. Analysis of the ~141 ks RXTE observation and lower energy ASCA/GIS
data, yields only marginalevidence for a secondary power-law emission component
in the spectrum. The 90% confidence upper limit on nonthermal emission in the
15-35 keV band is determined to be 2.6x10^{-12} erg/(cm^{2}s). When combined
with the measured radio flux and spectral index of the dominant region of
extended radio emission, this upper limit implies a lower limit of ~0.4
microgauss on the mean, volume-averaged intracluster magnetic field in A3667.Comment: 7 pages, 1 figure; ApJ, in pres
Clusters of galaxies in the microwave band: influence of the motion of the Solar System
In this work we consider the changes of the SZ cluster brightness, flux and
number counts induced by the motion of the Solar System with respect to the
frame defined by the cosmic microwave background (CMB). These changes are
connected with the Doppler effect and aberration and exhibit a strong spectral
and spatial dependence. The correction to the SZ cluster brightness and flux
has an amplitude and spectral dependence, which is similar to the first order
cluster peculiar velocity correction to the thermal SZ effect. Due to the
change in the received cluster CMB flux the motion of the Solar System induces
a dipolar asymmetry in the observed number of clusters above a given flux
level. Similar effects were discussed for -ray bursts and radio
galaxies, but here, due to the very peculiar frequency-dependence of the
thermal SZ effect, the number of observed clusters in one direction of the sky
can be both, decreased or increased depending on the frequency band. A
detection of this asymmetry should be possible using future full sky CMB
experiments with mJy sensitivities.Comment: 7 pages, 6 figures, submitted to Astronomy and Astrophysics,
corrected pdf-proble
Cosmic Microwave Background Temperature at Galaxy Clusters
We have deduced the cosmic microwave background (CMB) temperature in the Coma
cluster (A1656, ), and in A2163 () from spectral
measurements of the Sunyaev-Zel'dovich (SZ) effect over four passbands at radio
and microwave frequencies. The resulting temperatures at these redshifts are
K and K, respectively. These values confirm the expected
relation , where K is the value
measured by the COBE/FIRAS experiment. Alternative scaling relations that are
conjectured in non-standard cosmologies can be constrained by the data; for
example, if or , then
and (at 95% confidence). We
briefly discuss future prospects for more precise SZ measurements of at
higher redshifts.Comment: 13 pages, 1 figure, ApJL accepted for publicatio
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