484 research outputs found
Inverse Compton Scattering as the Source of Diffuse EUV Emission in the Coma Cluster of Galaxies
We have examined the hypothesis that the majority of the diffuse EUV flux in
the Coma cluster is due to inverse Compton scattering of low energy cosmic ray
electrons (0.16 < epsilon < 0.31 GeV) against the 3K black-body background. We
present data on the two-dimensional spatial distribution of the EUV flux and
show that these data provide strong support for a non-thermal origin for the
EUV flux. However, we show that this emission cannot be produced by an
extrapolation to lower energies of the observed synchrotron radio emitting
electrons and an additional component of low energy cosmic ray electrons is
required.Comment: 11 pages, 5 figure
Determination of Potassium Ion Concentration using Paper-Based Devices and Electrochemical Methods
poster abstractRapid quantification of ions in bodily fluids can be important for guiding an individualâs nutrition to prevent illness. The level of micronutrients (magnesium, calcium, potassium) is also relevant for diagnostics, such as identifying a health condition. For example, potassium levels in blood below 3 mM can be indicative of abnormal heart rhythms. We are currently working on the detection of potassium on a paper-based device as a proof-of-concept of a novel electrochemical micronutrient sensing platform. Paper-based platforms are useful in bioanalysis for point-of-care measurements because of their simplicity, low cost, portability and disposability. These advantages make them a valid alternative to conventional ionselective electrodes, which are fragile, subject to interference from biological samples, often expensive and require careful calibration and maintenance.
Our platform is based on an unusual electrochemical method employing the measurement of the shift in potential of a redox reaction. For potassium quantification, we measured the redox reaction of an electrodeposited Prussian blue layer. The shift in potential is proportional to the concentration of the targeted ion (potassium). We explored the best conditions for electrodepositing Prussian blue using commercial screen-printed electrodes and successfully tested aqueous solutions containing potassium ions in the range of 0 to 1 M. The results in this range show a reliable and reproducible trend correlating the shift in potential and the concentration of potassium. We also verified that sodium ions at high concentration in blood have a negligible interference. The next steps of the project include the validation of the assay on paper-based electrodes, tests of human serum samples throughout the relevant health range (3.5-5 mM) and assessment of the reproducibility and specificity of the platform by considering other potentially interfering ions
RXTE and ASCA Constraints on Non-thermal Emission from the A2256 Galaxy Cluster
An 8.3 hour observation of the Abell 2256 galaxy cluster using the Rossi
X-ray Timing Explorer proportional counter array produced a high quality
spectrum in the 2 - 30 keV range. Joint fitting with the 0.7 - 11 keV spectrum
obtained with the Advanced Satellite for Astrophysics and Cosmology gas imaging
spectrometer gives an upperlimit of 2.3x10^-7 photons/cm^2/sec/keV for
non-thermal emission at 30 keV. This yields a lower limit to the mean magnetic
field of 0.36 micro Gauss (uG) and an upperlimit of 1.8x10^-13 ergs/cm^3 for
the cosmic-ray electron energy density. The resulting lower limit to the
central magnetic field is ~1 - 3 uG While a magnetic field of ~0.1 - 0.2 uG can
be created by galaxy wakes, a magnetic field of several uG is usually
associated with a cooling flow or, as in the case of the Coma cluster, a
subcluster merger. However, for A2256, the evidence for a merger is weak and
the main cluster shows no evidence of a cooling flow. Thus, there is presently
no satisfactory hypothesis for the origin of an average cluster magnetic field
as high as >0.36 uG in the A2256 cluster.Comment: 8 pages, Astrophysical Journal (in press
Contamination of Cluster Radio Sources in the Measurement of the Thermal Sunyaev-Zel'dovich Angular Power Spectrum
We present a quantitative estimate of the confusion of cluster radio halos
and galaxies in the measurement of the angular power spectrum of the thermal
Sunyaev-Zel'dovich (SZ) effect. To achieve the goal, we use a purely analytic
approach to both radio sources and dark matter of clusters by incorporating
empirical models and observational facts together with some theoretical
considerations. It is shown that the correction of cluster radio halos and
galaxies to the measurement of the thermal SZ angular power spectrum is no more
than 20% at for observing frequencies GHz. This eliminates
the concern that the SZ measurement may be seriously contaminated by the
existence of cluster radio sources.Comment: 15 pages, 3 figures, accepted for publication in Ap
Hard X-ray emission from the galaxy cluster A3667
We report the results of a long BeppoSAX observation of Abell 3667, one of
the most spectacular galaxy cluster in the southern sky. A clear detection of
hard X-ray radiation up to ~ 35 keV is reported, while a hard excess above the
thermal gas emission is present at a marginal level that should be considered
as an upper limit to the presence of nonthermal radiation. The strong hard
excesses reported by BeppoSAX in Coma and A2256 and the only marginal detection
of nonthermal emission in A3667 can be explained in the framework of the
inverse Compton model. We argue that the nonthermal X-ray detections in the PDS
energy range are related to the radio index structure of halos and relics
present in the observed clusters of galaxie.Comment: 15 pages, 1 figure, ApJL in pres
On The Origin of Radio Halos in Galaxy Clusters
Previously it has been recognized that radio halos in galaxy clusters are
preferentially associated with merging systems as indicated by substructure in
the X-ray images and temperature maps. Since, however, many clusters without
radio halos also possess substructure, the role of mergers in the formation of
radio halos has remained unclear. By using power ratios to relate gravitational
potential fluctuations to substructure in X-ray images, we provide the first
quantitative comparison of the dynamical states of clusters possessing radio
halos. A correlation between the 1.4 GHz power (P_{1.4}) of the radio halo (or
relic) and the magnitude of the dipole power ratio (P_1/P_0) is discovered such
that approximately P_{1.4} ~ P_1/P_0; i.e., the strongest radio halos appear
only in those clusters currently experiencing the largest departures from a
virialized state. From additional consideration of a small number of highly
disturbed clusters without radio halos detected at 1.4 GHz, and recalling that
radio halos are more common in clusters with high X-ray luminosity (Giovannini,
Tordi, & Feretti), we argue that radio halos form preferentially in massive
(L_x >~ 0.5 x 10^{45} erg/s) clusters experiencing violent mergers (P_1/P_0 >~
0.5 x 10^{-4}) that have seriously disrupted the cluster core. The association
of radio halos with massive, large-P_1/P_0, core-disrupted clusters is able to
account for both the vital role of mergers in accelerating the relativistic
particles responsible for the radio emission as well as the rare occurrence of
radio halos in cluster samples.Comment: 4 pages, 1 figure, Accepted for Publication in The Astrophysical
Journal Letters, updated reference
Off-Center Mergers of Clusters of Galaxies and Nonequipartition of Electrons and Ions in Intracluster Medium
We investigate the dynamical evolution of clusters of galaxies and their
observational consequences during off-center mergers, explicitly considering
the relaxation process between ions and electrons in intracluster medium by
N-body and hydrodynamical simulations. In the contracting phase a bow shock is
formed between the two subclusters. The observed temperature between two peaks
in this phase depends on the viewing angle even if the geometry of the system
seems to be very simple like head-on collisions. Around the most contracting
epoch, when we observe merging clusters nearly along the collision axis, they
look like spherical relaxed clusters with large temperature gradients. In the
expanding phase, spiral bow shocks occur. As in head-on mergers, the electron
temperature is significantly lower than the plasma mean one especially in the
post-shock regions in the expanding phase. When the systems have relatively
large angular momentum, double-peak structures in the X-ray images can survive
even after the most contracting epoch. Morphological features in both X-ray
images and electron temperature distribution characteristic to off-center
mergers are seriously affected by the viewing angle. When the clusters are
observed nearly along the collision axis, the distribution of galaxies'
line-of-sight (LOS) velocities is a good indicator of mergers. In the
contracting phase, an negative kurtosis and a large skewness are expected for
nearly equal mass collisions and rather different mass ones, respectively. To
obtain statistically significant results, about 1000 galaxies' LOS velocities
are required. For nearby clusters (), large redshift surveys such as
2dF will enable us to study merger dynamics.Comment: 21 pages, 7 figures. Accepted for publication in Ap
Nonthermal Bremsstrahlung and Hard X-ray Emission from Clusters of Galaxies
We have calculated nonthermal bremsstrahlung (NTB) models for the hard X-ray
(HXR) tails recently observed by BeppoSAX in clusters of galaxies. In these
models, the HXR emission is due to suprathermal electrons with energies of
about 10-200 keV. Under the assumption that the suprathermal electrons form
part of a continuous spectrum of electrons including highly relativistic
particles, we have calculated the inverse Compton (IC) extreme ultraviolet
(EUV), HXR, and radio synchrotron emission by the extensions of the same
populations. For accelerating electron models with power-law momentum spectra
(N[p] propto p^{- mu}) with mu <~ 2.7, which are those expected from strong
shock acceleration, the IC HXR emission exceeds that due to NTB. Thus, these
models are only of interest if the electron population is cut-off at some upper
energy <~1 GeV. Similarly, flat spectrum accelerating electron models produce
more radio synchrotron emission than is observed from clusters if the ICM
magnetic field is B >~ 1 muG. The cooling electron model produces vastly too
much EUV emission as compared to the observations of clusters. We have compared
these NTB models to the observed HXR tails in Coma and Abell 2199. The NTB
models require a nonthermal electron population which contains about 3% of the
number of electrons in the thermal ICM. If the suprathermal electron population
is cut-off at some energy above 100 keV, then the models can easily fit the
observed HXR fluxes and spectral indices in both clusters. For accelerating
electron models without a cutoff, the electron spectrum must be rather steep >~
2.9.Comment: Accepted for publication in the Astrophysical Journal. 10 pages with
5 embedded Postscript figures in emulateapj.sty. An abbreviated abstract
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