477 research outputs found
Analysis of the X-ray Emission of Nine Swift Afterglows
The X-ray light-curves of 9 Swift XRT afterglows (050126, 050128, 050219A,
050315, 050318, 050319, 050401, 050408, 050505) display a complex behaviour: a
steep t^{-3.0 \pm 0.3} decay until ~400 s, followed by a significantly slower
t^{-0.65+/-0.20} fall-off, which at 0.2--2 d after the burst evolves into a
t^{-1.7+/-0.5} decay. We consider three possible models for the geometry of
relativistic blast-waves (spherical outflows, non-spreading jets, and spreading
jets), two possible dynamical regimes for the forward shock (adiabatic and
fully radiative), and we take into account a possible angular structure of the
outflow and delayed energy injection in the blast-wave, to identify the models
which reconcile the X-ray light-curve decay with the slope of the X-ray
continuum for each of the above three afterglow phases. By piecing together the
various models for each phase in a way that makes physical sense, we identify
possible models for the entire X-ray afterglow. The major conclusion of this
work is that a long-lived episode of energy injection in the blast-wave, during
which the shock energy increases at t^{1.0+/-0.5}, is required for five
afterglows and could be at work in the other four as well. Optical observations
in conjunction with the X-ray can distinguish among these various models. Our
simple tests allow the determination of the location of the cooling frequency
relative to the X-ray domain and, thus, of the index of the electron power-law
distribution with energy in the blast-wave. The resulting indices are clearly
inconsistent with an universal value.Comment: 10 pages, minor changes, to be published in the MNRA
Mitigating Charge Transfer Inefficiency in the Chandra X-ray Observatory's ACIS Instrument
The ACIS front-illuminated CCDs onboard the Chandra X-ray Observatory were
damaged in the extreme environment of the Earth's radiation belts, resulting in
enhanced charge transfer inefficiency (CTI). This produces a row dependence in
gain, event grade, and energy resolution. We model the CTI as a function of
input photon energy, including the effects of de-trapping (charge trailing),
shielding within an event (charge in the leading pixels of the 3X3 event island
protect the rest of the island by filling traps), and non-uniform spatial
distribution of traps. This technique cannot fully recover the degraded energy
resolution, but it reduces the position dependence of gain and grade
distributions. By correcting the grade distributions as well as the event
amplitudes, we can improve the instrument's quantum efficiency. We outline our
model for CTI correction and discuss how the corrector can improve
astrophysical results derived from ACIS data.Comment: Accepted by ApJ Letters; see
http://www.astro.psu.edu/users/townsley/cti
Performance of single photon-counting X-ray charge coupled devices
Results of intial performance tests on X-ray sensing properties of charge-coupled devices (CCDs) are presented. CCDs have demonstrated excellent spatial resolution and good spectral resolution, superior to that of non-imaging proportional counters
Are orchid bees useful indicators of the impacts of human disturbance?
Biodiversity and ecosystem functions are threatened by human disturbance, and tropical forests are one the most vulnerable habitats. Monitoring the impacts of disturbance and the success of conservation projects is crucial, and to do this effectively it is important to identify suitable measures that are sensitive to ecosystem disturbance. Orchid bees (Euglossini) are a specialist group with mutualistic relationships with many plant species and can fly long distances, making them important pollinators of widely dispersed plant species. A loss of specialist pollinators such as these could have severe consequences for the plants that rely on their services. We therefore aimed to answer the following question: are orchid bees useful indicators of the impacts of human disturbance? If so, what measures of orchid bee diversity are most sensitive? And do orchid bees provide any indication of changes in pollination services along a disturbance gradient? Orchid bees were collected from 18 sites across a gradient of disturbance in a tropical forest region in southeast Peru. Alpha diversity across the gradient was compared using Hills numbers. Beta diversity was assessed using community composition, species contributions to beta diversity, beta diversity partitioning and novel measures of redundancy and representativeness. The potential pollination services available at each site were measured using artificial flowers and counts of pollinator visits. Alpha diversity of orchid bees showed low sensitivity to disturbance. Beta diversity measures were more informative, with disturbed sites found to be highly redundant in the ecosystem compared to the less disturbed sites. However, the most sensitive measure across the gradient was abundance – there was a significant decrease in the number of bees caught as disturbance increased, with likely consequences for pollination services. These results suggest that orchid bees may be useful indicators of the impacts of human disturbance, but alpha diversity is a poor metric for this purpose. In order to understand how human disturbance is affecting biodiversity, multiple diversity indices should be considered, and in the case of orchid bees, redundancy and abundance could be useful for detecting sensitive responses to forest disturbance. © 2019 Elsevier Lt
Large-Scale Outflows in Edge-on Seyfert Galaxies. III. Kiloparsec-Scale Soft X-ray Emission
We present ROSAT PSPC and HRI images of eight galaxies selected from a
distance-limited sample of 22 edge-on Seyfert galaxies. Kiloparsec-scale soft
X-ray nebulae extend along the galaxy minor axes in three galaxies (NGC 2992,
NGC 4388 and NGC 5506). The extended X-ray emission has 0.2-2.4 keV X-ray
luminosities of . The X-ray nebulae are
roughly co-spatial with the large-scale radio emission, suggesting that both
are produced by large-scale galactic outflows. Assuming pressure balance
between the radio and X-ray plasmas, the X-ray filling factor is \gapprox
10^4 times larger than the radio plasma filling factor, suggesting that
large-scale outflows in Seyfert galaxies are predominantly winds of thermal
X-ray emitting gas. We favor an interpretation in which large-scale outflows
originate as AGN-driven jets that entrain and heat gas on kpc scales as they
make their way out of the galaxy. AGN- and starburst-driven winds are also
possible explanations in cases where the winds are oriented along the rotation
axis of the galaxy disk.Comment: 24 pages, 7 ps figures, AASTEX 4.0, accepted for ApJ April 1, 199
Evidence for chromatic X-ray light-curve breaks in Swift GRB afterglows and their theoretical implications
The power-law decay of the X-ray emission of GRB afterglows 050319, 050401,
050607, 050713A, 050802 and 050922C exhibits a steepening at about 1--4 hours
after the burst which, surprisingly, is not accompanied by a break in the
optical emission. If it is assumed that both the optical and X-ray afterglows
arise from the same outflow then, in the framework of the standard forward
shock model, the chromaticity of the X-ray light-curve breaks indicates that
they do not arise solely from a mechanism related to the outflow dynamics (e.g.
energy injection) or the angular distribution of the blast-wave kinetic energy
(structured outflows or jets). The lack of a spectral evolution accompanying
the X-ray light-curve breaks shows that these breaks do not arise from the
passage of a spectral break (e.g. the cooling frequency) either. Under these
circumstances, the decoupling of the X-ray and optical decays requires that the
microphysical parameters for the electron and magnetic energies in the forward
shock evolve in time, whether the X-ray afterglow is synchrotron or
inverse-Compton emission. For a steady evolution of these parameters with the
Lorentz factor of the forward shock and an X-ray light-curve break arising from
cessation of energy injection into the blast-wave, the optical and X-ray
properties of the above six Swift afterglows require a circumburst medium with
a r^{-2} radial stratification, as expected for a massive star origin for long
GRBs. Alternatively, the chromatic X-ray light-curve breaks may indicate that
the optical and X-ray emissions arise from different outflows. Neither feature
(evolution of microphysical parameters or the different origin of the optical
and X-ray emissions) were clearly required by pre-Swift afterglows.Comment: 6 pages, sumbitted to MNRA
0103-72.6: A New Oxygen-Rich Supernova Remnant in the Small Magellanic Cloud
010372.6, the second brightest X-ray supernova remnant (SNR) in the Small
Magellanic Cloud (SMC), has been observed with the {\it Chandra X-Ray
Observatory}. Our {\it Chandra} observation unambiguously resolves the X-ray
emission into a nearly complete, remarkably circular shell surrounding bright
clumpy emission in the center of the remnant. The observed X-ray spectrum for
the central region is evidently dominated by emission from reverse shock-heated
metal-rich ejecta. Elemental abundances in this ejecta material are
particularly enhanced in oxygen and neon, while less prominent in the heavier
elements Si, S, and Fe. We thus propose that 010372.6 is a new
``oxygen-rich'' SNR, making it only the second member of the class in the SMC.
The outer shell is the limb-brightened, soft X-ray emission from the swept-up
SMC interstellar medium. The presence of O-rich ejecta and the SNR's location
within an H{\small II} region attest to a massive star core-collapse origin for
010372.6. The elemental abundance ratios derived from the ejecta suggest an
18 M progenitor star.Comment: 6 pages (ApJ emulator format), including 5 figures and 2 tables. For
high quality Figs.1,2, & 3, contact [email protected]. Accepted by the ApJ
Letter
The Radial Structure of SNR N103B
We report on the results from a Chandra ACIS observation of the young,
compact, supernova remnant N103B. The unprecedented spatial resolution of
Chandra reveals sub-arcsecond structure, both in the brightness and in spectral
variations. Underlying these small-scale variations is a surprisingly simple
radial structure in the equivalent widths of the strong Si and S emission
lines. We investigate these radial variations through spatially resolved
spectroscopy using a plane-parallel, non-equilibrium ionization model with
multiple components. The majority of the emission arises from components with a
temperature of 1 keV: a fully ionized hydrogen component; a high ionization
timescale (n_e*t > 10^12 s cm^-3) component containing Si, S, Ar, Ca, and Fe;
and a low ionization timescale (n_e*t ~ 10^{11} s cm^-3) O, Ne, and Mg
component. To reproduce the strong Fe Kalpha line, it is necessary to include
additional Fe in a hot (> 2 keV), low ionization (n_e*t ~ 10^10.8 s cm^-3)
component. This hot Fe may be in the form of hot Fe bubbles, formed in the
radioactive decay of clumps of 56Ni. We find no radial variation in the
ionization timescales or temperatures of the various components. Rather, the Si
and S equivalent widths increase at large radii because these lines, as well as
those of Ar and Ca, are formed in a shell occupying the outer half of the
remnant. A shell of hot Fe is located interior to this, but there is a large
region of overlap between these two shells. In the inner 30% of the remnant,
there is a core of cooler, 1 keV Fe. We find that the distribution of the
ejecta and the yields of the intermediate mass species are consistent with
model prediction for Type Ia events.Comment: 34 pages, including 7 tables and 7 figures, Accepted by Ap
Joint Astrophysics Nascent Universe Satellite:. utilizing GRBs as high redshift probes
The Joint Astrophysics Nascent Universe Satellite (JANUS) is a multiwavelength cosmology mission designed to address fundamental questions about the cosmic dawn. It has three primary science objectives: (1) measure the massive star formation rate over 5 ≤ z ≤ 12 by discovering and observing high-z gamma-ray bursts (GRBs) and their afterglows, (2) enable detailed studies of the history of reionization and metal enrichment in the early Universe, and (3) map the growth of the first supermassive black holes by discovering and observing the brightest quasars at z ≥ 6. A rapidly slewing spacecraft and three science instruments – the X-ray Coded Aperture Telescope (XCAT), the Near InfraRed Telescope (NIRT), and the GAmma-ray Transient Experiment for Students (GATES) – make-up the JANUS observatory and are responsible for realizing the three primary science objectives. The XCAT (0.5–20 keV) is a wide field of view instrument responsible for detecting and localizing ∼60 z ≥ 5 GRBs, including ∼8 z ≥ 8 GRBs, during a 2-year mission. The NIRT (0.7–1.7 µm) refines the GRB positions and provides rapid (≤ 30 min) redshift information to the astronomical community. Concurrently, the NIRT performs a 20, 000 deg2 survey of the extragalactic sky discovering and localizing ∼300 z ≥ 6 quasars, including ∼50 at z ≥ 7, over a two-year period. The GATES provides high-energy (15 keV −1.0 MeV) spectroscopy as well as 60–500 keV polarimetry of bright GRBs. Here we outline the JANUS instrumentation and the mission science motivations
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