619 research outputs found
The Impact of Non-Equipartition on Cosmological Parameter Estimation from Sunyaev-Zel'dovich Surveys
The collisionless accretion shock at the outer boundary of a galaxy cluster
should primarily heat the ions instead of electrons since they carry most of
the kinetic energy of the infalling gas. Near the accretion shock, the density
of the intracluster medium is very low and the Coulomb collisional timescale is
longer than the accretion timescale. Electrons and ions may not achieve
equipartition in these regions. Numerical simulations have shown that the
Sunyaev-Zel'dovich observables (e.g., the integrated Comptonization parameter
Y) for relaxed clusters can be biased by a few percent. The Y-mass relation can
be biased if non-equipartition effects are not properly taken into account.
Using a set of hydrodynamical simulations, we have calculated three potential
systematic biases in the Y-mass relations introduced by non-equipartition
effects during the cross-calibration or self-calibration when using the galaxy
cluster abundance technique to constraint cosmological parameters. We then use
a semi-analytic technique to estimate the non-equipartition effects on the
distribution functions of Y (Y functions) determined from the extended
Press-Schechter theory. Depending on the calibration method, we find that
non-equipartition effects can induce systematic biases on the Y functions, and
the values of the cosmological parameters Omega_8, sigma_8, and the dark energy
equation of state parameter w can be biased by a few percent. In particular,
non-equipartition effects can introduce an apparent evolution in w of a few
percent in all of the systematic cases we considered. Techniques are suggested
to take into account the non-equipartition effect empirically when using the
cluster abundance technique to study precision cosmology. We conclude that
systematic uncertainties in the Y-mass relation of even a few percent can
introduce a comparable level of biases in cosmological parameter measurements.Comment: 10 pages, 3 figures, accepted for publication in the Astrophysical
Journal, abstract abridged slightly. Typos corrected in version
XMM-Newton Observation of the Northwest Radio Relic Region in Abell 3667
Abell 3667 is the archetype of a merging cluster with radio relics. The NW
radio relic is the brightest cluster relic or halo known, and is believed to be
due to a strong merger shock. We have observed the NW relic for 40 ksec of net
XMM time. We observe a global decline of temperature across the relic from 6 to
1 keV, similar to the Suzaku results. Our new observations reveal a sharp
change of both temperature and surface brightness near the position of the
relic. The increased X-ray emission on the relic can be equivalently well
described by either a thermal or nonthermal spectral model. The parameters of
the thermal model are consistent with a Mach number M~2 shock and a shock speed
of ~1200 km s^-1. The energy content of the relativistic particles in the radio
relic can be explained if they are (re)-accelerated by the shock with an
efficiency of ~0.2%. Comparing the limit on the inverse Compton X-ray emission
with the measured radio synchrotron emission, we set a lower limit to the
magnetic field in the relic of 3 muG. If the emission from the relic is
non-thermal, this lower limit is in fact the required magnetic field.Comment: 11 pages, ApJ in pres
Predictions of local ground geomagnetic field fluctuations during the 7-10 November 2004 events studied with solar wind driven models
The 7-10 November 2004 period contains two events for which the local ground magnetic field was severely disturbed and simultaneously, the solar wind displayed several shocks and negative <i>B<sub>z</sub></i> periods. Using empirical models the 10-min RMS and at Brorfelde (BFE, 11.67&deg; E, 55.63&deg; N), Denmark, are predicted. The models are recurrent neural networks with 10-min solar wind plasma and magnetic field data as inputs. The predictions show a good agreement during 7 November, up until around noon on 8 November, after which the predictions become significantly poorer. The correlations between observed and predicted log RMS is 0.77 during 7-8 November but drops to 0.38 during 9-10 November. For RMS the correlations for the two periods are 0.71 and 0.41, respectively. Studying the solar wind data for other L1-spacecraft (WIND and SOHO) it seems that the ACE data have a better agreement to the near-Earth solar wind during the first two days as compared to the last two days. Thus, the accuracy of the predictions depends on the location of the spacecraft and the solar wind flow direction. Another finding, for the events studied here, is that the and models showed a very different dependence on <i>B<sub>z</sub></i>. The model is almost independent of the solar wind magnetic field <i>B<sub>z</sub></i>, except at times when <i>B<sub>z</sub></i> is exceptionally large or when the overall activity is low. On the contrary, the model shows a strong dependence on <i>B<sub>z</sub></i> at all times
The Swift BAT Perspective on Non-thermal Emission in HIFLUGCS Galaxy Clusters
The search for diffuse non-thermal, inverse Compton (IC) emission from galaxy
clusters at hard X-ray energies has been underway for many years, with most
detections being either of low significance or controversial. In this work, we
investigate 14-195 keV spectra from the Swift Burst Alert Telescope (BAT)
all-sky survey for evidence of non-thermal excess emission above the
exponentially decreasing tail of thermal emission in the flux-limited HIFLUGCS
sample. To account for the thermal contribution at BAT energies, XMM-Newton
EPIC spectra are extracted from coincident spatial regions so that both thermal
and non-thermal spectral components can be determined simultaneously. We find
marginally significant IC components in six clusters, though after closer
inspection and consideration of systematic errors we are unable to claim a
clear detection in any of them. The spectra of all clusters are also summed to
enhance a cumulative non-thermal signal not quite detectable in individual
clusters. After constructing a model based on single-temperature fits to the
XMM-Newton data alone, we see no significant excess emission above that
predicted by the thermal model determined at soft energies. This result also
holds for the summed spectra of various subgroups, except for the subsample of
clusters with diffuse radio emission. For clusters hosting a diffuse radio
halo, a relic, or a mini-halo, non-thermal emission is initially detected at
the \sim5-sigma confidence level - driven by clusters with mini-halos - but
modeling and systematic uncertainties ultimately degrade this significance. In
individual clusters, the non-thermal pressure of relativistic electrons is
limited to \sim10% of the thermal electron pressure, with stricter limits for
the more massive clusters, indicating that these electrons are likely not
dynamically important in the central regions of clusters.Comment: 25 pages, 15 figures; some figure and table numbering differs from
published ApJ version: please see that for superior formattin
Hard X-ray Properties of the Merging Cluster Abell 3667 as Observed with Suzaku
Wide-band Suzaku data on the merging cluster Abell 3667 were examined for
hard X-ray emission in excess to the known thermal component. Suzaku detected
X-ray signals in the wide energy band from 0.5 to 40 keV. The hard X-ray (> 10
keV) flux observed by the HXD around the cluster center cannot be explained by
a simple extension of the thermal emission with average temperature of ~7 keV.
The emission is most likely an emission from a very hot (kT > 13.2 keV) thermal
component around the cluster center, produced via a strong heating process in
the merger. In the north-west radio relic, no signature of non-thermal emission
was observed. Using the HXD, the overall upper-limit flux within a 34'x34'
field-of-view around the relic is derived to be 5.3e-12 erg s-1 cm-2 in the
10-40 keV band, after subtracting the ICM contribution estimated using the XIS
or the XMM-Newton spectra. Directly on the relic region, the upper limit is
further tightened by the XIS data to be less than 7.3e-13 erg s-1 cm-2, when
converted into the 10--40 keV band. The latter value suggest that the average
magnetic field within the relic is higher than 1.6 uG. The non-thermal pressure
due to magnetic fields and relativistic electrons may be as large as ~20% of
the thermal pressure in the region.Comment: 18 pages, 13 figures, to be appeared in PASJ 200
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