11,706 research outputs found
The Global Implications of the Hard X-ray Excess in Type 1 AGN
Recent evidence for a strong 'hard excess' of flux at energies > 20 keV in
some Suzaku observations of type 1 Active Galactic Nuclei (AGN) has motivated
an exploratory study of the phenomenon in the local type 1 AGN population. We
have selected all type 1 AGN in the Swift Burst Alert Telescope (BAT) 58-month
catalog and cross-correlated them with the holdings of the Suzaku public
archive. We find the hard excess phenomenon to be a ubiquitous property of type
1 AGN. Taken together, the spectral hardness and equivalent width of Fe K alpha
emission are consistent with reprocessing by an ensemble of Compton-thick
clouds that partially cover the continuum source. In the context of such a
model, ~ 80 % of the sample has a hardness ratio consistent with > 50% covering
of the continuum by low-ionization, Compton-thick gas. More detailed study of
the three hardest X-ray spectra in our sample reveal a sharp Fe K absorption
edge at ~ 7 keV in each of them, indicating that blurred reflection is not
responsible for the very hard spectral forms. Simple considerations place the
distribution of Compton-thick clouds at or within the optical broad line
region.Comment: Accepted for publication in Ap
Potential Terrorist Uses of Highway-Borne Hazardous Materials, MTI Report 09-03
The Department of Homeland Security (DHS) has requested that the Mineta Transportation Institutes National Transportation Security Center of Excellence (MTI NTSCOE) provide any research it has or insights it can provide on the security risks created by the highway transportation of hazardous materials. This request was submitted to MTI/NSTC as a National Transportation Security Center of Excellence. In response, MTI/NTSC reviewed and revised research performed in 2007 and 2008 and assembled a small team of terrorism and emergency-response experts, led by Center Director Brian Michael Jenkins, to report on the risks of terrorists using highway shipments of flammable liquids (e.g., gasoline tankers) to cause casualties anywhere, and ways to reduce those risks. This report has been provided to DHS. The teams first focus was on surface transportation targets, including highway infrastructure, and also public transportation stations. As a full understanding of these materials, and their use against various targets became revealed, the team shifted with urgency to the far more plentiful targets outside of surface transportation where people gather and can be killed or injured. However, the team is concerned to return to the top of the use of these materials against public transit stations and recommends it as a separate subject for urgent research
Global energetic neutral atom (ENA) measurements and their association with the Dst index
We present a new global magnetospheric index that measures the intensity of the Earth\u27s ring current through energetic neutral atoms (ENAs). We have named it the Global Energetic Neutral Index (GENI), and it is derived from ENA measurements obtained by the Imaging Proton Spectrometer (IPS), part of the Comprehensive Energetic Particle and Pitch Angle Distribution (CEPPAD) experiment on the POLAR satellite. GENI provides a simple orbit-independent global sum of ENAs measured with IPS. Actual ENA measurements for the same magnetospheric state look different when seen from different points in the POLAR orbit. In addition, the instrument is sensitive to weak ion populations in the polar cap, as well as cosmic rays. We have devised a method for removing the effects of cosmic rays and weak ion fluxes, in order to produce an image of “pure” ENA counts. We then devised a method of normalizing the ENA measurements to remove the orbital bias effect. The normalized data were then used to produce the GENI. We show, both experimentally and theoretically the approximate proportionality between the GENI and the Dst index. In addition we discuss possible implications of this relation. Owing to the high sensitivity of IPS to ENAs, we can use these data to explore the ENA/Dst relationship not only during all phases of moderate geomagnetic storms, but also during quiescent ring current periods
The XMM-Newton Iron Line Profile of NGC 3783
We report on observations of the iron K line in the nearby Seyfert 1 galaxy,
NGC 3783, obtained in a long, 2 orbit (240 ks) XMM-Newton observation. The line
profile obtained exhibits two strong narrow peaks at 6.4 keV and at 7.0 keV,
with measured line equivalent widths of 120 and 35 eV respectively. The 6.4 keV
emission is the K-alpha line from near neutral Fe, whilst the 7.0 keV feature
probably originates from a blend of the neutral Fe K-beta line and the H-like
line of Fe at 6.97 keV. The relatively narrow velocity width of the K-alpha
line (<5000 km/s), its lack of response to the continuum emission on short
timescales and the detection of a neutral Compton reflection component are all
consistent with a distant origin in Compton-thick matter such as the putative
molecular torus. A strong absorption line from highly ionized iron (at 6.67
keV) is detected in the time-averaged iron line profile, whilst the depth of
the feature appears to vary with time, being strongest when the continuum flux
is higher. The iron absorption line probably arises from the highest ionization
component of the known warm absorber in NGC 3783, with an ionization of logxi=3
and column density of 5x10^{22}cm{-2} and may originate from within 0.1pc of
the nucleus. A weak red-wing to the iron K line profile is also detected below
6.4 keV. However when the effect of the highly ionized warm absorber on the
underlying continuum is taken into account, the requirement for a relativistic
iron line component from the inner disk is reduced.Comment: 34 pages, including 11 figures. Accepted for publication in Ap
The Merredin Early strain of Wimmera ryegrass
WITH the exception of subterranean clover, no introduced pasture species has a wider application in Western Australia than Wimmera ryegrass.
This popular pasture plant is believed to have originated as a natural cross between two ryegrass species and first became prominent in the Wimmera district of Victoria
Direct Measurement of the X-ray Time-Delay Transfer Function in Active Galactic Nuclei
The origin of the observed time lags, in nearby active galactic nuclei (AGN),
between hard and soft X-ray photons is investigated using new XMM-Newton data
for the narrow-line Seyfert I galaxy Ark 564 and existing data for 1H0707-495
and NGC 4051. These AGN have highly variable X-ray light curves that contain
frequent, high peaks of emission. The averaged light curve of the peaks is
directly measured from the time series, and it is shown that (i) peaks occur at
the same time, within the measurement uncertainties, at all X-ray energies, and
(ii) there exists a substantial tail of excess emission at hard X-ray energies,
which is delayed with respect to the time of the main peak, and is particularly
prominent in Ark 564. Observation (i) rules out that the observed lags are
caused by Comptonization time delays and disfavors a simple model of
propagating fluctuations on the accretion disk. Observation (ii) is consistent
with time lags caused by Compton-scattering reverberation from material a few
thousand light-seconds from the primary X-ray source. The power spectral
density and the frequency-dependent phase lags of the peak light curves are
consistent with those of the full time series. There is evidence for
non-stationarity in the Ark 564 time series in both the Fourier and peaks
analyses. A sharp `negative' lag (variations at hard photon energies lead soft
photon energies) observed in Ark 564 appears to be generated by the shape of
the hard-band transfer function and does not arise from soft-band reflection of
X-rays. These results reinforce the evidence for the existence of X-ray
reverberation in type I AGN, which requires that these AGN are significantly
affected by scattering from circumnuclear material a few tens or hundreds of
gravitational radii in extent.Comment: Accepted for publication in Ap
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