606 research outputs found
Liftable vector fields over corank one multigerms
In this paper, a systematic method is given to construct all liftable vector
fields over an analytic multigerm of
corank at most one admitting a one-parameter stable unfolding.Comment: 34 pages. In ver. 2, several careless mistakes for calculations in
Section 6 were correcte
Letters to the Editor
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66389/1/j.1600-0447.1989.tb03017.x.pd
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
A shock front in the merging galaxy cluster Abell 754: X-ray and radio observations
We present new Chandra X-ray and Giant Meterwave Radio Telescope (GMRT) radio
observations of the nearby merging galaxy cluster Abell 754. Our X-ray data
confirm the presence of a shock front by obtaining the first direct measurement
of a gas temperature jump across the X-ray brightness edge previously seen in
the imaging data. A754 is only the fourth galaxy cluster with confirmed merger
shock fronts, and it has the weakest shock of those, with a Mach number
M=1.57+0.16-0.12. In our new GMRT observation at 330 MHz, we find that the
previously-known centrally located radio halo extends eastward to the position
of the shock. The X-ray shock front also coincides with the position of a radio
relic previously observed at 74 MHz. The radio spectrum of the post-shock
region, using our radio data and the earlier results at 74 MHz and 1.4 GHz, is
very steep. We argue that acceleration of electrons at the shock front directly
from thermal to ultrarelativistic energies is problematic due to energy
arguments, while reacceleration of preexisting relativistic electrons is more
plausible.Comment: 10 pages, 8 figures, "emulateapj" format. 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
A Hard X-ray Study of the Normal Star-Forming Galaxy M83 with NuSTAR
We present results from sensitive, multi-epoch NuSTAR observations of the
late-type star-forming galaxy M83 (d=4.6 Mpc), which is the first investigation
to spatially resolve the hard (E>10 keV) X-ray emission of this galaxy. The
nuclear region and ~ 20 off-nuclear point sources, including a previously
discovered ultraluminous X-ray (ULX) source, are detected in our NuSTAR
observations. The X-ray hardnesses and luminosities of the majority of the
point sources are consistent with hard X-ray sources resolved in the starburst
galaxy NGC 253. We infer that the hard X-ray emission is most likely dominated
by intermediate accretion state black hole binaries and neutron star low-mass
X-ray binaries (Z-sources). We construct the X-ray binary luminosity function
(XLF) in the NuSTAR band for an extragalactic environment for the first time.
The M83 XLF has a steeper XLF than the X-ray binary XLF in NGC 253, consistent
with previous measurements by Chandra at softer X-ray energies. The NuSTAR
integrated galaxy spectrum of M83 drops quickly above 10 keV, which is also
seen in the starburst galaxies NGC253, NGC 3310 and NGC 3256. The NuSTAR
observations constrain any AGN to be either highly obscured or to have an
extremely low luminosity of 10 erg/s (10-30 keV), implying it
is emitting at a very low Eddington ratio. An X-ray point source consistent
with the location of the nuclear star cluster with an X-ray luminosity of a few
times 10 erg/s may be a low-luminosity AGN but is more consistent with
being an X-ray binary.Comment: Accepted for publication in ApJ (25 pages, 17 figures
NuSTAR observations of the powerful radio-galaxy Cygnus A
We present NuSTAR observations of the powerful radio galaxy Cygnus A,
focusing on the central absorbed active galactic nucleus (AGN). Cygnus A is
embedded in a cool-core galaxy cluster, and hence we also examine archival
XMM-Newton data to facilitate the decomposition of the spectrum into the AGN
and intracluster medium (ICM) components. NuSTAR gives a source-dominated
spectrum of the AGN out to >70keV. In gross terms, the NuSTAR spectrum of the
AGN has the form of a power law (Gamma~1.6-1.7) absorbed by a neutral column
density of N_H~1.6x10^23 cm^-2. However, we also detect curvature in the hard
(>10keV) spectrum resulting from reflection by Compton-thick matter out of our
line-of-sight to the X-ray source. Compton reflection, possibly from the outer
accretion disk or obscuring torus, is required even permitting a high-energy
cutoff in the continuum source; the limit on the cutoff energy is E_cut>111keV
(90% confidence). Interestingly, the absorbed power-law plus reflection model
leaves residuals suggesting the absorption/emission from a fast
(15,000-26,000km/s), high column-density (N_W>3x10^23 cm^-2), highly ionized
(xi~2,500 erg cm/s) wind. A second, even faster ionized wind component is also
suggested by these data. We show that the ionized wind likely carries a
significant mass and momentum flux, and may carry sufficient kinetic energy to
exercise feedback on the host galaxy. If confirmed, the simultaneous presence
of a strong wind and powerful jets in Cygnus A demonstrates that feedback from
radio-jets and sub-relativistic winds are not mutually exclusive phases of AGN
activity but can occur simultaneously.Comment: 13 pages; accepted for publication in The Astrophysical Journa
The Chandra Multi-Wavelength Project: Optical Spectroscopy and the Broadband Spectral Energy Distributions of X-ray Selected AGN
From optical spectroscopy of X-ray sources observed as part of ChaMP, we
present redshifts and classifications for a total of 1569 Chandra sources from
our targeted spectroscopic follow up using the FLWO, SAAO, WIYN, CTIO, KPNO,
Magellan, MMT and Gemini telescopes, and from archival SDSS spectroscopy. We
classify the optical counterparts as 50% BLAGN, 16% NELG, 14% ALG, and 20%
stars. We detect QSOs out to z~5.5 and galaxies out to z~3. We have compiled
extensive photometry from X-ray to radio bands. Together with our spectroscopic
information, this enables us to derive detailed SEDs for our extragalactic
sources. We fit a variety of templates to determine bolometric luminosities,
and to constrain AGN and starburst components where both are present. While
~58% of X-ray Seyferts require a starburst event to fit observed photometry
only 26% of the X-ray QSO population appear to have some kind of star formation
contribution. This is significantly lower than for the Seyferts, especially if
we take into account torus contamination at z>1 where the majority of our X-ray
QSOs lie. In addition, we observe a rapid drop of the percentage of starburst
contribution as X-ray luminosity increases. This is consistent with the
quenching of star formation by powerful QSOs, as predicted by the merger model,
or with a time lag between the peak of star formation and QSO activity. We have
tested the hypothesis that there should be a strong connection between X-ray
obscuration and star-formation but we do not find any association between X-ray
column density and star formation rate both in the general population or the
star-forming X-ray Seyferts. Our large compilation also allows us to report
here the identification of 81 XBONG, 78 z>3 X-ray sources and 8 Type-2 QSO
candidates. Also we have identified the highest redshift (z=5.4135) X-ray
selected QSO with optical spectroscopy.Comment: 17 pages, 16 figures, accepted for publication in ApJS. Full data
table and README file can be found online at
http://hea-www.harvard.edu/~pgreen/Papers.htm
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