2,617 research outputs found
Revealing two radio active galactic nuclei extremely near PSR J04374715
Newton's gravitational constant may vary with time at an extremely low
level. The time variability of will affect the orbital motion of a
millisecond pulsar in a binary system and cause a tiny difference between the
orbital period-dependent measurement of the kinematic distance and the direct
measurement of the annual parallax distance. PSR J04374715 is the nearest
millisecond pulsar and the brightest at radio. To explore the feasibility of
achieving a parallax distance accuracy of one light-year, comparable to the
recent timing result, with the technique of differential astrometry, we
searched for compact radio sources quite close to PSR J04374715. Using
existing data from the Very Large Array and the Australia Telescope Compact
Array, we detected two sources with flat spectra, relatively stable flux
densities of 0.9 and 1.0 mJy at 8.4 GHz and separations of 13 and 45 arcsec.
With a network consisting of the Long Baseline Array and the Kunming 40-m radio
telescope, we found that both sources have a point-like structure and a
brightness temperature of 10 K. According to these radio inputs and
the absence of counterparts in the other bands, we argue that they are most
likely the compact radio cores of extragalactic active galactic nuclei rather
than Galactic radio stars. The finding of these two radio active galactic
nuclei will enable us to achieve a sub-pc distance accuracy with the in-beam
phase-referencing very-long-baseline interferometric observations and provide
one of the most stringent constraints on the time variability of in the
near future.Comment: 9 pages, 3 tables, 3 figures. Accepted for publication in MNRA
A broadband X-ray study of the Rabbit pulsar wind nebula powered by PSR J1418-6058
We report on broadband X-ray properties of the Rabbit pulsar wind nebula
(PWN) associated with the pulsar PSR J1418-6058 using archival Chandra and
XMM-Newton data, and a new NuSTAR observation. NuSTAR data above 10 keV allowed
us to detect the 110-ms spin period of the pulsar, characterize its hard X-ray
pulse profile, and resolve hard X-ray emission from the PWN after removing
contamination from the pulsar and other overlapping point sources. The extended
PWN was detected up to 20 keV and is well described by a power-law model
with a photon index 2. The PWN shape does not vary significantly
with energy, and its X-ray spectrum shows no clear evidence of softening away
from the pulsar. We modeled the spatial profile of X-ray spectra and broadband
spectral energy distribution in the radio to TeV band to infer the physical
properties of the PWN. We found that a model with low magnetic field strength
( G) and efficient diffusion ( cm s)
fits the PWN data well. The extended hard X-ray and TeV emission, associated
respectively with synchrotron radiation and inverse Compton scattering by
relativistic electrons, suggests that particles are accelerated to very high
energies ( TeV), indicating that the Rabbit PWN is a Galactic
PeVatron candidate.Comment: 21 pages, 10 figures. ApJ accepte
Broadband X-ray Imaging and Spectroscopy of the Crab Nebula and Pulsar with NuSTAR
We present broadband (3 -- 78 keV) NuSTAR X-ray imaging and spectroscopy of
the Crab nebula and pulsar. We show that while the phase-averaged and spatially
integrated nebula + pulsar spectrum is a power-law in this energy band,
spatially resolved spectroscopy of the nebula finds a break at 9 keV in
the spectral photon index of the torus structure with a steepening
characterized by . We also confirm a previously reported
steepening in the pulsed spectrum, and quantify it with a broken power-law with
break energy at 12 keV and . We present spectral
maps of the inner 100\as\ of the remnant and measure the size of the nebula as
a function of energy in seven bands. These results find that the rate of
shrinkage with energy of the torus size can be fitted by a power-law with an
index of , consistent with the predictions of Kennel
and Coroniti (1984). The change in size is more rapid in the NW direction,
coinciding with the counter-jet where we find the index to be a factor of two
larger. NuSTAR observed the Crab during the latter part of a -ray
flare, but found no increase in flux in the 3 - 78 keV energy band
X-ray studies of the pulsar PSR J1420-6048 and its TeV pulsar wind nebula in the Kookaburra region
We present a detailed analysis of broadband X-ray observations of the pulsar
PSR J1420-6048 and its wind nebula (PWN) in the Kookaburra region with Chandra,
XMM-Newton, and NuSTAR. Using the archival XMM-Newton and new NuSTAR data, we
detected 68 ms pulsations of the pulsar and characterized its X-ray pulse
profile which exhibits a sharp spike and a broad bump separated by ~0.5 in
phase. A high-resolution Chandra image revealed a complex morphology of the
PWN: a torus-jet structure, a few knots around the torus, one long (~7') and
two short tails extending in the northwest direction, and a bright diffuse
emission region to the south. Spatially integrated Chandra and NuSTAR spectra
of the PWN out to 2.5' are well described by a power law model with a photon
index 2. A spatially resolved spectroscopic study, as well
as NuSTAR radial profiles of the 3--7 keV and 7--20 keV brightness, showed a
hint of spectral softening with increasing distance from the pulsar. A
multi-wavelength spectral energy distribution (SED) of the source was then
obtained by supplementing our X-ray measurements with published radio,
Fermi-LAT, and H.E.S.S. data. The SED and radial variations of the X-ray
spectrum were fit with a leptonic multi-zone emission model. Our detailed study
of the PWN may be suggestive of (1) particle transport dominated by advection,
(2) a low magnetic-field strength (B ~ 5G), and (3) electron
acceleration to ~PeV energies.Comment: 18 pages and 8 figures. Accepted for publication in Ap
Reconstructions of deltaic environments from Holocene palynological records in the Volga delta, northern Caspian Sea
This article was made available through open access by the Brunel Open Access Publishing Fund.New palynological and ostracod data are presented from the Holocene Volga delta, obtained from short cores and surface samples collected in the Damchik region, near Astrakhan, Russian Federation in the northern Caspian Sea. Four phases of delta deposition are recognized and constrained by accelerated mass spectrometry (AMS) radiocarbon ages. Palynological records show that erosive channels, dunes (Baer hills) and inter-dune lakes were present during the period 11,500–8900 cal. BP at the time of the Mangyshlak Caspian lowstand. The period 8900–3770 cal. BP was characterized regionally by extensive steppe vegetation, with forest present at times with warmer, more humid climates, and with halophytic and xerophytic vegetation present at times of drought. The period 3770–2080 cal. BP was a time of active delta deposition, with forest or woodland close to the delta, indicating relatively warm and humid climates and variable Caspian Sea levels. From 2080 cal. BP to the present-day, aquatic pollen is frequent in highstand intervals and herbaceous pollen and fungal hyphae frequent in lowstand intervals. Soils and incised valley sediments are associated with the regional Derbent regression and may be time-equivalent with the ‘Medieval Warm Period’. Fungal spores are an indicator of erosional or aeolian processes, whereas fungal hyphae are associated with soil formation. Freshwater algae, ostracods and dinocysts indicate mainly freshwater conditions during the Holocene with minor brackish influences. Dinocysts present include Spiniferites cruciformis, Caspidinium rugosum, Impagidinium caspienense and Pterocysta cruciformis, the latter a new record for the Caspian Sea. The Holocene Volga delta is a partial analogue for the much larger oil and gas bearing Mio-Pliocene palaeo-Volga delta.Funding for the data collection and field work was provided from the following sources: 1 – IGCP-UNESCO 2003–2008 (Project 481 CASPAGE, Dating Caspian Sea Level Change); 2 – NWO, Netherlands Science Foundation and RFFI, Russian Science Foundation 2005–2008 (Programme: ‘VHR Seismic Stratigraphy and Paleoecology of the Holocene Volga Delta’); and 3 – BP Exploration (Caspian Sea) Sea Ltd. (Azeri-Chirag-Gunashli) 2005–2008 (‘Unravelling the Small-Scale Stratigraphy and Sediment Dynamics of the Modern Volga Delta Using VHR Marine Geophysics’). The palynological work was funded jointly by BP Exploration (Caspian Sea) Ltd., Delft University of Technology and KrA Stratigraphic Ltd. Ostracod analyses were funded by StrataData Ltd. and funding for two additional radiocarbon dates provided by Deltares
High-Energy X-ray Imaging of the Pulsar Wind Nebula MSH~15-52: Constraints on Particle Acceleration and Transport
We present the first images of the pulsar wind nebula (PWN) MSH 15-52 in the
hard X-ray band (>8 keV), as measured with the Nuclear Spectroscopic Telescope
Array (NuSTAR). Overall, the morphology of the PWN as measured by NuSTAR in the
3-7 keV band is similar to that seen in Chandra high-resolution imaging.
However, the spatial extent decreases with energy, which we attribute to
synchrotron energy losses as the particles move away from the shock. The
hard-band maps show a relative deficit of counts in the northern region towards
the RCW 89 thermal remnant, with significant asymmetry. We find that the
integrated PWN spectra measured with NuSTAR and Chandra suggest that there is a
spectral break at 6 keV which may be explained by a break in the
synchrotron-emitting electron distribution at ~200 TeV and/or imperfect cross
calibration. We also measure spatially resolved spectra, showing that the
spectrum of the PWN softens away from the central pulsar B1509-58, and that
there exists a roughly sinusoidal variation of spectral hardness in the
azimuthal direction. We discuss the results using particle flow models. We find
non-monotonic structure in the variation with distance of spectral hardness
within 50" of the pulsar moving in the jet direction, which may imply particle
and magnetic-field compression by magnetic hoop stress as previously suggested
for this source. We also present 2-D maps of spectral parameters and find an
interesting shell-like structure in the NH map. We discuss possible origins of
the shell-like structure and their implications.Comment: 15 pages, 9 figures, accepted for publication in Ap
Observational Evidence for Young Radio Galaxies are Triggered by Accretion Disk Instability
The bolometric luminosities and black hole (BH) masses are estimated by
various methods for a sample of young radio galaxies with known ages. We find
that the ages are positively correlated with the bolometric luminosities in
these young radio galaxies, which is consistent with theoretical prediction
based on radiation pressure instability of accretion disk in Czerny et al. The
ages of young radio galaxies are also found to be consistent with the
theoretical durations of outbursts in BH mass and accretion rate (in Eddington
unit) plane, where the outbursts are assumed to be triggered by the radiation
pressure instabilities. Our results provide the observational evidence for the
radiation pressure instability, which causes limit-cycle behavior, as a
physical mechanism that may be responsible for these short-lived young radio
galaxies.Comment: 5 pages, 3 figures, accepted by ApJ Letter, emulateapj styl
The Hard X-Ray View of the Young Supernova Remnant G1.9+0.3
NuSTAR observed G1.9+0.3, the youngest known supernova remnant in the Milky
Way, for 350 ks and detected emission up to 30 keV. The remnant's X-ray
morphology does not change significantly across the energy range from 3 to 20
keV. A combined fit between NuSTAR and CHANDRA shows that the spectrum steepens
with energy. The spectral shape can be well fitted with synchrotron emission
from a power-law electron energy distribution with an exponential cutoff with
no additional features. It can also be described by a purely phenomenological
model such as a broken power-law or a power-law with an exponential cutoff,
though these descriptions lack physical motivation. Using a fixed radio flux at
1 GHz of 1.17 Jy for the synchrotron model, we get a column density of N = cm, a spectral index of
, and a roll-off frequency of Hz. This can be explained by particle
acceleration, to a maximum energy set by the finite remnant age, in a magnetic
field of about 10 G, for which our roll-off implies a maximum energy of
about 100 TeV for both electrons and ions. Much higher magnetic-field strengths
would produce an electron spectrum that was cut off by radiative losses, giving
a much higher roll-off frequency that is independent of magnetic-field
strength. In this case, ions could be accelerated to much higher energies. A
search for Ti emission in the 67.9 keV line results in an upper limit of
assuming a line width of 4.0 keV (1 sigma).Comment: 9 pages, 6 figures, accepted Ap
Fluid Particle Accelerations in Fully Developed Turbulence
The motion of fluid particles as they are pushed along erratic trajectories
by fluctuating pressure gradients is fundamental to transport and mixing in
turbulence. It is essential in cloud formation and atmospheric transport,
processes in stirred chemical reactors and combustion systems, and in the
industrial production of nanoparticles. The perspective of particle
trajectories has been used successfully to describe mixing and transport in
turbulence, but issues of fundamental importance remain unresolved. One such
issue is the Heisenberg-Yaglom prediction of fluid particle accelerations,
based on the 1941 scaling theory of Kolmogorov (K41). Here we report
acceleration measurements using a detector adapted from high-energy physics to
track particles in a laboratory water flow at Reynolds numbers up to 63,000. We
find that universal K41 scaling of the acceleration variance is attained at
high Reynolds numbers. Our data show strong intermittency---particles are
observed with accelerations of up to 1,500 times the acceleration of gravity
(40 times the root mean square value). Finally, we find that accelerations
manifest the anisotropy of the large scale flow at all Reynolds numbers
studied.Comment: 7 pages, 4 figure
A Spatially Resolved Study of the Synchrotron Emission and Titanium in Tycho's Supernova Remnant with NuSTAR
We report results from deep observations (~750 ks) of Tycho's supernova
remnant (SNR) with NuSTAR. Using these data, we produce narrow-band images over
several energy bands to identify the regions producing the hardest X-rays and
to search for radioactive decay line emission from 44Ti. We find that the
hardest (>10 keV) X-rays are concentrated in the southwest of Tycho, where
recent Chandra observations have revealed high emissivity "stripes" associated
with particles accelerated to the knee of the cosmic-ray spectrum. We do not
find evidence of 44Ti, and we set limits on its presence and distribution
within the SNR. These limits correspond to a upper-limit 44Ti mass of M44 <
2.4x10^-4 M_sun for a distance of 2.3 kpc. We perform spatially resolved
spectroscopic analysis of sixty-six regions across Tycho. We map the best-fit
rolloff frequency of the hard X-ray spectra, and we compare these results to
measurements of the shock expansion and ambient density. We find that the
highest energy electrons are accelerated at the lowest densities and in the
fastest shocks, with a steep dependence of the roll-off frequency with shock
velocity. Such a dependence is predicted by models where the maximum energy of
accelerated electrons is limited by the age of the SNR rather than by
synchrotron losses, but this scenario requires far lower magnetic field
strengths than those derived from observations in Tycho. One way to reconcile
these discrepant findings is through shock obliquity effects, and future
observational work is necessary to explore the role of obliquity in the
particle acceleration process.Comment: 12 pages, 12 figures, ApJ in pres
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