509 research outputs found
Letters to the Editor
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66389/1/j.1600-0447.1989.tb03017.x.pd
On the absence of radio halos in clusters with double relics
Pairs of radio relics are believed to form during cluster mergers, and are
best observed when the merger occurs in the plane of the sky. Mergers can also
produce radio halos, through complex processes likely linked to turbulent
re-acceleration of cosmic-ray electrons. However, only some clusters with
double relics also show a radio halo. Here, we present a novel method to derive
upper limits on the radio halo emission, and analyse archival X-ray Chandra
data, as well as galaxy velocity dispersions and lensing data, in order to
understand the key parameter that switches on radio halo emission. We place
upper limits on the halo power below the
correlation for some clusters, confirming that clusters with double relics have
different radio properties. Computing X-ray morphological indicators, we find
that clusters with double relics are associated with the most disturbed
clusters. We also investigate the role of different mass-ratios and
time-since-merger. Data do not indicate that the merger mass ratio has an
impact on the presence or absence of radio halos (the null hypothesis that the
clusters belong to the same group cannot be rejected). However, the data
suggests that the absence of radio halos could be associated with early and
late mergers, but the sample is too small to perform a statistical test. Our
study is limited by the small number of clusters with double relics. Future
surveys with LOFAR, ASKAP, MeerKat and SKA will provide larger samples to
better address this issue.Comment: 12 pages, 7 figures, MNRAS accepte
Dark matter line emission constraints from NuSTAR observations of the Bullet Cluster
Line emission from dark matter is well motivated for some candidates e.g.
sterile neutrinos. We present the first search for dark matter line emission in
the 3-80keV range in a pointed observation of the Bullet Cluster with NuSTAR.
We do not detect any significant line emission and instead we derive upper
limits (95% CL) on the flux, and interpret these constraints in the context of
sterile neutrinos and more generic dark matter candidates. NuSTAR does not have
the sensitivity to constrain the recently claimed line detection at 3.5keV, but
improves on the constraints for energies of 10-25keV.Comment: 7 pages, 5 figures, submitted to Ap
GRB 000418: A Hidden Jet Revealed?
We report on optical, near-infrared and centimeter radio observations of
GRB000418 which allow us to follow the evolution of the afterglow from 2 to 200
days after the gamma-ray burst. In modeling these broad-band data, we find that
an isotropic explosion in a constant density medium is unable to simultaneously
fit both the radio and optical data. However, a jet-like outflow with an
opening angle of 10-20 degress provides a good description of the data. The
evidence in favor of a jet interpretation is based on the behavior of the radio
light curves, since the expected jet break is masked at optical wavelengths by
the light of the host galaxy. We also find evidence for extinction, presumably
arising from within the host galaxy, with A(V)=0.4 mag, and host flux densities
of F_R=1.1 uJy and F_K=1.7 uJy. These values supercede previous work on this
burst due to the availability of a broad-band data set allowing a global
fitting approach. A model in which the GRB explodes into a wind-stratified
circumburst medium cannot be ruled out by these data. However, in examining a
sample of other bursts (e.g. GRB990510, GRB000301C) we favor the jet
interpretation for GRB000418.Comment: ApJ, submitte
Turbulence and Radio Mini-halos in the Sloshing Cores of Galaxy Clusters
A number of relaxed, cool-core galaxy clusters exhibit diffuse,
steep-spectrum radio sources in their central regions, known as radio
mini-halos. It has been proposed that the relativistic electrons responsible
for the emission have been reaccelerated by turbulence generated by the
sloshing of the cool core gas. We present a high-resolution MHD simulation of
gas sloshing in a galaxy cluster coupled with subgrid simulations of
relativistic electron acceleration to test this hypothesis. Our simulation
shows that the sloshing motions generate turbulence on the order of 50-200 km s on spatial scales of 50-100 kpc and below in the
cool core region within the envelope of the sloshing cold fronts, whereas
outside the cold fronts, there is negligible turbulence. This turbulence is
potentially strong enough to reaccelerate relativistic electron seeds (with
initial ) to via damping of
magnetosonic waves and non-resonant compression. The seed electrons could
remain in the cluster from, e.g., past AGN activity. In combination with the
magnetic field amplification in the core, these electrons then produce diffuse
radio synchrotron emission that is coincident with the region bounded by the
sloshing cold fronts, as indeed observed in X-rays and the radio. The result
holds for different initial spatial distributions of preexisting relativistic
electrons. The power and the steep spectral index () of the
resulting radio emission are consistent with observations of minihalos, though
the theoretical uncertainties of the acceleration mechanisms are high. We also
produce simulated maps of inverse-Compton hard X-ray emission from the same
population of relativistic electrons.Comment: 28 pages, 29 figures, in emulateapj format. Revised version accepted
by the referee, conclusions unchange
Anti-correlation between X-ray luminosity and pulsed fraction in the Small Magellanic Cloud pulsar SXP 1323
We report the evidence for the anti-correlation between pulsed fraction (PF)
and luminosity of the X-ray pulsar SXP 1323, found for the first time in a
luminosity range -- erg s from observations spanning
15 years. The phenomenon of a decrease in X-ray PF when the source flux
increases has been observed in our pipeline analysis of other X-ray pulsars in
the Small Magellanic Cloud (SMC). It is expected that the luminosity under a
certain value decreases as the PF decreases due to the propeller effect. Above
the propeller region, an anti-correlation between the PF and flux might occur
either as a result of an increase in the un-pulsed component of the total
emission or a decrease of the pulsed component. Additional modes of accretion
may also be possible, such as spherical accretion and a change in emission
geometry. At higher mass accretion rates, the accretion disk could also extend
closer to the neutron star (NS) surface, where a reduced inner radius leads to
hotter inner disk emission. These modes of plasma accretion may affect the
change in the beam configuration to fan-beam dominant emission.Comment: It has been accepted for publication in Monthly Notices of the Royal
Astronomical Society Letter
The Impact of Galaxy Cluster Mergers on Cosmological Parameter Estimation from Surveys of the Sunyaev-Zel'dovich Effect
Sensitive surveys of the Cosmic Microwave Background will detect thousands of
galaxy clusters via the Sunyaev-Zel'dovich (SZ) effect. Two SZ observables, the
central or maximum and integrated Comptonization parameters y_max and Y, relate
in a simple way to the total cluster mass, which allow the construction of mass
functions (MFs) that can be used to estimate cosmological parameters such as
Omega_M, sigma_8, and the dark energy parameter w. However, clusters form from
the mergers of smaller structures, events that can disrupt the equilibrium of
intracluster gas upon which SZ-M relations rely. From a set of
N-body/hydrodynamical simulations of binary cluster mergers, we calculate the
evolution of Y and y_max over the course of merger events and find that both
parameters are transiently "boosted," primarily during the first core passage.
We then use a semi-analytic technique developed by Randall et al. (2002) to
estimate the effect of merger boosts on the distribution functions YF and yF of
Y and y_max, respectively, via cluster merger histories determined from
extended Press-Schechter (PS) merger trees. We find that boosts do not induce
an overall systematic effect on YFs, and the values of Omega_M, sigma_8, and w
were returned to within 2% of values expected from the nonboosted YFs. The
boosted yFs are significantly biased, however, causing Omega_M to be
underestimated by 15-45%, sigma_8 to be overestimated by 10-25%, and w to be
pushed to more negative values by 25-45%. We confirm that the integrated SZ
effect, Y, is far more robust to mergers than y_max, as previously reported by
Motl et al. (2005) and similarly found for the X-ray equivalent Y_X, and we
conclude that Y is the superior choice for constraining cosmological
parameters.Comment: 16 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
Computer-Aided Design and Analysis of Spectrally Aligned Hybrid Plasmonic Nanojunctions for SERS Detection of Nucleobases
Hybrid plasmonic nanojunctions with optimal surface-enhanced Raman scattering (SERS) activity are designed via a computer-aided approach, and fabricated via time-controlled aqueous self-assembly of core@shell gold@silver nanoparticles (Au@Ag NPs) with cucurbit[7]uril (CB7) upon simple mixing. The authors showed that SERS signals can be significantly boosted by the incorporation of a strong plasmonic metal and the spectral alignment between the maximal localized surface plasmon resonance (LSPR) and a laser wavelength used for SERS excitation. In a proof-of-concept application, SERS detection of nucleobases with a 633-nm laser has been demonstrated by positioning them within the nanojunctions via formation of hostâguest complexes with CB7, achieving rapid response with a detection limit down to sub-nanomolar concentration and an enhancement factor (EF) up to â109â1010, i.e., the minimum required EF for single-molecule detection. Furthermore, machine-learning-driven multiplexing of nucleobases is demonstrated, which shows promise in point-of-care diagnosis of diseases related to oxidative damage of DNA and wastewater-based epidemiology
Astro 2020 Science White Paper: Time Domain Studies of Neutron Star and Black Hole Populations: X-ray Identification of Compact Object Types
What are the most important conditions and processes governing the growth of
stellar-origin compact objects? The identification of compact object type as
either black hole (BH) or neutron star (NS) is fundamental to understanding
their formation and evolution. To date, time-domain determination of compact
object type remains a relatively untapped tool. Measurement of orbital periods,
pulsations, and bursts will lead to a revolution in the study of the
demographics of NS and BH populations, linking source phenomena to accretion
and galaxy parameters (e.g., star formation, metallicity). To perform these
measurements over sufficient parameter space, a combination of a wide-field
(>5000 deg^2) transient X-ray monitor over a dynamic energy range (~1-100 keV)
and an X-ray telescope for deep surveys with <5 arcsec PSF half-energy width
(HEW) angular resolution are required. Synergy with multiwavelength data for
characterizing the underlying stellar population will transform our
understanding of the time domain properties of transient sources, helping to
explain details of supernova explosions and gravitational wave event rates.Comment: 9 pages, 2 figures. Submitted to the Astro2020 Decadal Surve
Hard X-ray emitting Active Galactic Nuclei selected by the Chandra Multi-wavelength Project
We present X-ray and optical analysis of 188 AGN identified from 497 hard
X-ray (f (2.0-8.0 keV) > 2.7x10^-15 erg cm^-2 s^-1) sources in 20 Chandra
fields (1.5 deg^2) forming part of the Chandra Multi-wavelength Project. These
medium depth X-ray observations enable us to detect a representative subset of
those sources responsible for the bulk of the 2-8 keV Cosmic X-ray Background.
Brighter than our optical spectroscopic limit, we achieve a reasonable degree
of completeness (77% of X-ray sources with counter-parts r'< 22.5 have been
classified): broad emission line AGN (62%), narrow emission line galaxies
(24%), absorption line galaxies (7%), stars (5%) or clusters (2%). We find that
most X-ray unabsorbed AGN (NH<10^22 cm^-2) have optical properties
characterized by broad emission lines and blue colors, similiar to
optically-selected quasars from the Sloan Digital Sky Survey but with a slighly
broader color distribution. However, we also find a significant population of
redder (g'-i'>1.0) AGN with broad optical emission lines. Most of the X-ray
absorbed AGN (10^22<NH<10^24 cm^-2) are associated with narrow emission line
galaxies, with red optical colors characteristically dominated by luminous,
early type galaxy hosts rather than from dust reddening of an AGN. We also find
a number of atypical AGN; for instance, several luminous AGN show both strong
X-ray absorption (NH>10^22 cm^-2) and broad emission lines. Overall, we find
that 81% of X-ray selected AGN can be easily interpreted in the context of
current AGN unification models. Most of the deviations seem to be due to an
optical contribution from the host galaxies of the low luminosity AGN.Comment: 26 pages; 13 figures (7 color); accepted for publication in the
Astrophysical Journa
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