408 research outputs found
The SMC SNR 1E0102.2-7219 as a Calibration Standard for X-ray Astronomy in the 0.3-2.5 keV Bandpass
The flight calibration of the spectral response of CCD instruments below 1.5
keV is difficult in general because of the lack of strong lines in the on-board
calibration sources typically available. We have been using 1E 0102.2-7219, the
brightest supernova remnant in the Small Magellanic Cloud, to evaluate the
response models of the ACIS CCDs on the Chandra X-ray Observatory (CXO), the
EPIC CCDs on the XMM-Newton Observatory, the XIS CCDs on the Suzaku
Observatory, and the XRT CCD on the Swift Observatory. E0102 has strong lines
of O, Ne, and Mg below 1.5 keV and little or no Fe emission to complicate the
spectrum. The spectrum of E0102 has been well characterized using
high-resolution grating instruments, namely the XMM-Newton RGS and the CXO
HETG, through which a consistent spectral model has been developed that can
then be used to fit the lower-resolution CCD spectra. We have also used the
measured intensities of the lines to investigate the consistency of the
effective area models for the various instruments around the bright O (~570 eV
and 654 eV) and Ne (~910 eV and 1022 eV) lines. We find that the measured
fluxes of the O VII triplet, the O VIII Ly-alpha line, the Ne IX triplet, and
the Ne X Ly-alpha line generally agree to within +/-10 % for all instruments,
with 28 of our 32 fitted normalizations within +/-10% of the RGS-determined
value. The maximum discrepancies, computed as the percentage difference between
the lowest and highest normalization for any instrument pair, are 23% for the O
VII triplet, 24% for the O VIII Ly-alpha line, 13% for the Ne IX triplet, and
19% for the Ne X Ly-alpha line. If only the CXO and XMM are compared, the
maximum discrepancies are 22% for the O VII triplet, 16% for the O VIII
Ly-alpha line, 4% for the Ne IX triplet, and 12% for the Ne X Ly-alpha line.Comment: 16 pages, 11 figures, to be published in Proceedings of the SPIE
7011: Space Telescopes and Instrumentation II: Ultraviolet to Gamma Ray 200
The accretion flow in the discless intermediate polar V2400 Ophiuchi
RXTE observations confirm that the X-ray lightcurve of V2400 Oph is pulsed at
the beat cycle, as expected in a discless intermediate polar. There are no
X-ray modulations at the orbital or spin cycles, but optical line profiles vary
with all three cycles. We construct a model for line-profile variations in a
discless accretor, based on the idea that the accretion stream flips from one
magnetic pole to the other, and show that this accounts for the observed
behaviour over the spin and beat cycles. The minimal variability over the
orbital cycle implies that 1) V2400 Oph is at an inclination of only ~10 deg,
and 2) much of the accretion flow is not in a coherent stream, but is circling
the white dwarf, possibly as a ring of denser, diamagnetic blobs. We discuss
the light this sheds on disc formation in intermediate polars.Comment: 10 pages, 12 figures, To appear in MNRAS, includes low-res figures to
reduce siz
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: I. Early X-ray emission from the shocked ejecta and red giant wind
RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts
have indicated that high velocity ejecta interact with a pre-existing red giant
wind, setting up shock systems analogous to those seen in Supernova Remnants.
However, in the previous outburst in 1985, X-ray observations did not commence
until 55 days after the initial explosion. Here we report on Swift observations
covering the first month of the 2006 outburst with the Burst Alert (BAT) and
X-ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14-25
keV band from t=0 to days. XRT observationsfrom 0.3-10 keV, started at
3.17 days after outburst. The rapidly evolving XRT spectra clearly show the
presence of both line and continuum emission which can be fitted by thermal
emission from hot gas whose characteristic temperature, overlying absorbing
column, , and resulting unabsorbed total flux decline monotonically
after the first few days. Derived shock velocities are in good agreement with
those found from observations at other wavelengths. Similarly, is in
accord with that expected from the red giant wind ahead of the forward shock.
We confirm the basic models of the 1985 outburst and conclude that standard
Phase I remnant evolution terminated by days and the remnant then
rapidly evolved to display behaviour characteristic of Phase III. Around t=26
days however, a new, luminous and highly variable soft X-ray source began to
appear whose origin will be explored in a subsequent paper.Comment: 20 pages, 4 figures (2 updated), accepted by Ap
An Efficient Molecular Dynamics Scheme for the Calculation of Dopant Profiles due to Ion Implantation
We present a highly efficient molecular dynamics scheme for calculating the
concentration depth profile of dopants in ion irradiated materials. The scheme
incorporates several methods for reducing the computational overhead, plus a
rare event algorithm that allows statistically reliable results to be obtained
over a range of several orders of magnitude in the dopant concentration.
We give examples of using this scheme for calculating concentration profiles
of dopants in crystalline silicon. Here we can predict the experimental profile
over five orders of magnitude for both channeling and non-channeling implants
at energies up to 100s of keV.
The scheme has advantages over binary collision approximation (BCA)
simulations, in that it does not rely on a large set of empirically fitted
parameters. Although our scheme has a greater computational overhead than the
BCA, it is far superior in the low ion energy regime, where the BCA scheme
becomes invalid.Comment: 17 pages, 21 figures, 2 tables. See: http://bifrost.lanl.gov/~reed
GRB 050911: a black hole - neutron star merger or a naked GRB
GRB 050911, discovered by the Swift Burst Alert Telescope, was not seen 4.6
hr later by the Swift X-ray Telescope, making it one of the very few X-ray
non-detections of a Gamma-Ray Burst (GRB) afterglow at early times. The
gamma-ray light-curve shows at least three peaks, the first two of which (~T_0
- 0.8 and T_0 + 0.2 s, where T_0 is the trigger time) were short, each lasting
0.5 s. This was followed by later emission 10-20 s post-burst. The upper limit
on the unabsorbed X-ray flux was 1.7 x 10^-14 erg cm^-2 s^-1 (integrating 46 ks
of data taken between 11 and 18 September), indicating that the decay must have
been rapid. All but one of the long bursts detected by Swift were above this
limit at ~4.6 hr, whereas the afterglows of short bursts became undetectable
more rapidly. Deep observations with Gemini also revealed no optical afterglow
12 hr after the burst, down to r=24.0 (5-sigma limit). We speculate that GRB
050911 may have been formed through a compact object (black hole-neutron star)
merger, with the later outbursts due to a longer disc lifetime linked to a
large mass ratio between the merging objects. Alternatively, the burst may have
occured in a low density environment, leading to a weak, or non-existent,
forward shock - the so-called 'naked GRB' model.Comment: 4 pages using emulateapj; 2 figures. Accepted for publication in ApJ
Letter
The Far-Ultraviolet Spectrum and Short Timescale Variability of AM Herculis from Observations with the Hopkins Ultraviolet Telescope
Using the Hopkins Ultraviolet Telescope (HUT), we have obtained 850-1850
angstrom spectra of the magnetic cataclysmic variable star AM Her in the high
state. These observations provide high time resolution spectra of AM Her in the
FUV and sample much of the orbital period of the system. The spectra are not
well-modelled in terms of simple white dwarf (WD) atmospheres, especially at
wavelengths shortward of Lyman alpha. The continuum flux changes by a factor of
2 near the Lyman limit as a function of orbital phase; the peak fluxes are
observed near magnetic phase 0.6 when the accreting pole of the WD is most
clearly visible. The spectrum of the hotspot can be modelled in terms of a 100
000 K WD atmosphere covering 2% of the WD surface. The high time resolution of
the HUT data allows an analysis of the short term variability and shows the UV
luminosity to change by as much as 50% on timescales as short as 10 s. This
rapid variability is shown to be inconsistent with the clumpy accretion model
proposed to account for the soft X-ray excess in polars. We see an increase in
narrow line emission during these flares when the heated face of the secondary
is in view. The He II narrow line flux is partially eclipsed at secondary
conjunction, implying that the inclination of the system is greater than 45
degrees. We also present results from models of the heated face of the
secondary. These models show that reprocessing on the face of the secondary
star of X-ray/EUV emission from the accretion region near the WD can account
for the intensities and kinematics of most of the narrow line components
observed.Comment: 19 pp., 12 fig., 3 tbl. To appear in The Astrophysical Journal. Also
available at http://greeley.pha.jhu.edu/papers/amherpp.ps.g
The early and late-time spectral and temporal evolution of GRB 050716
We report on a comprehensive set of observations of Gamma Ray Burst 050716,
detected by the Swift satellite and subsequently followed-up rapidly in X-ray,
optical and near infra-red wavebands. The prompt emission is typical of
long-duration bursts, with two peaks in a time interval of T90 = 68 seconds (15
- 350 keV). The prompt emission continues at lower flux levels in the X-ray
band, where several smaller flares can be seen, on top of a decaying light
curve that exhibits an apparent break around 220 seconds post trigger. This
temporal break is roughly coincident with a spectral break. The latter can be
related to the extrapolated evolution of the break energy in the prompt
gamma-ray emission, and is possibly the manifestation of the peak flux break
frequency of the internal shock passing through the observing band. A possible
3 sigma change in the X-ray absorption column is also seen during this time.
The late-time afterglow behaviour is relatively standard, with an electron
distribution power-law index of p = 2 there is no noticable temporal break out
to at least 10 days. The broad-band optical/nIR to X-ray spectrum indicates a
redshift of z ~> 2 for this burst, with a host-galaxy extinction value of
E(B-V) ~ 0.7 that prefers an SMC-like extinction curve.Comment: Accepted to MNRAS. 8 pages, 5 figure
Ribosome-binding antibiotics increase bacterial longevity and growth efficiency
This is the final version. Available on open access from the National Academy of Sciences via the DOI in this recordData, Materials, and Software Availability:
Phenotypic data in all figures from the main text can be downloaded from Zenodo repository available via the link https://zenodo.org/record/8334696Antibiotics, by definition, reduce bacterial growth rates in optimal culture conditions; however, the real-world environments bacteria inhabit see rapid growth punctuated by periods of low nutrient availability. How antibiotics mediate population decline during these periods is poorly understood. Bacteria cannot optimize for all environmental conditions because a growth-longevity tradeoff predicts faster growth results in faster population decline, and since bacteriostatic antibiotics slow growth, they should also mediate longevity. We quantify how antibiotics, their targets, and resistance mechanisms influence longevity using populations of Escherichia coli and, as the tradeoff predicts, populations are maintained for longer if they encounter ribosome-binding antibiotics doxycycline and erythromycin, a finding that is not observed using antibiotics with alternative cellular targets. This tradeoff also predicts resistance mechanisms that increase growth rates during antibiotic treatment could be detrimental during nutrient stresses, and indeed, we find resistance by ribosomal protection removes benefits to longevity provided by doxycycline. We therefore liken ribosomal protection to a "Trojan horse" because it provides protection from an antibiotic but, during nutrient stresses, it promotes the demise of the bacteria. Seeking mechanisms to support these observations, we show doxycycline promotes efficient metabolism and reduces the concentration of reactive oxygen species. Seeking generality, we sought another mechanism that affects longevity and we found the number of doxycycline targets, namely, the ribosomal RNA operons, mediates growth and longevity even without antibiotics. We conclude that slow growth, as observed during antibiotic treatment, can help bacteria overcome later periods of nutrient stress.Engineering and Physical Sciences Research Council (EPSRC)Biotechnology and Biological Sciences Research Council (BBSRC
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