85 research outputs found
Suzaku observation of the Phoenix Galaxy
In recent years, several Seyfert 2 galaxies have been discovered that change
state when observed in X-rays a few years apart, switching from Compton-thin to
reflection-dominated or viceversa. We observed a member of this class of
"Changing-look" sources, the Phoenix Galaxy, with Suzaku, with the aim of
better understanding the nature of the variations. The Suzaku spectrum was
analyzed, and the results compared with previous ASCA and XMM-Newton
observations. The source was caught in a Compton-thin state, as in XMM-Newton,
but differently from ASCA. Comparing the Suzaku and XMM-Newton observations, a
variation in the column density of the absorber on a time scale of years is
discovered. A similar change, but on much shorter time scales (i.e. ks) may
also explain the count-rate variations during the Suzaku observations. A soft
excess is also present, likely due to continuum and line emission from
photoionized circumnuclear matter.Comment: Accepted for publication in Astronomy & Astrophysic
The Suzaku X-ray spectrum of NGC 3147. Further insights on the best "true" Seyfert 2 galaxy candidate
NGC 3147 is so far the most convincing case of a "true" Seyfert 2 galaxy,
i.e. a source genuinely lacking the Broad Line Regions. We obtained a Suzaku
observation with the double aim to study in more detail the iron line complex,
and to check the Compton-thick hypothesis for the lack of observed optical
broad lines.
The Suzaku XIS and HXD/PIN spectra of the source were analysed in detail. The
line complex is composed of at least two unresolved lines, one at about 6.45
keV and the other one at about 7 keV, most likely identified with Fe XVII/XIX,
the former, and Fe XXVI, the latter. The high-ionization line can originate
either in a photoionized matter or in an optically thin thermal plasma. In the
latter case, an unusually high temperature is implied. In the photoionized
model case, the large equivalent width can be explained either by an extreme
iron overabundance or by assuming that the source is Compton-thick. In the
Compton-thick hypothesis, however, the emission above 2 keV is mostly due to a
highly ionized reflector, contrary to what is usually found in Compton-thick
Seyfert 2s, where reflection from low ionized matter dominates. Moreover, the
source flux varied between the XMM-Newton and the Suzaku observations, taken
3.5 years apart, confirming previous findings and indicating that the size of
the emitting region must be smaller than a parsec.
The hard X-ray spectrum is also inconclusive on the Compton-thick hypothesis.
Weighting the various arguments, a "true" Seyfert 2 nature of NGC 3147 seems to
be still the most likely explanation, even if the "highly ionized reflector"
Compton-thick hypothesis cannot at present be formally rejected.Comment: 6 pages, accepted for publication in Astronomy & Astrophysic
Comparing Vegetation Monitoring Methods in Shrublands: How Valuable is Grantâs Method in Shrub Communities?
Faced with managing extensive rangelands, land managers need reliable monitoring methods. Grant and others (2004) proposed a monitoring method that uses a floristic classification of dominant vegetation and assesses species frequency. The method was developed in native grass prairies with limited woody vegetation and is designed to provide more efficient repetitive monitoring. Our objective for this study was to determine if Grantâs method is useful in shrubland systems for a newly implemented baseline inventory and monitoring program. We conducted a study to compare Grantâs method and its efficacy to more commonly used line-point intercept in grass and shrub alliances at Brownâs Park National Wildlife Refuge, Colorado. In the summer of 2007 we conducted baseline inventory of vegetation characteristics on 39 permanent bottomland plots in six pre-determined grass and shrub vegetation alliances on the refuge. Within each plot, we monitored vegetation with line-point intercept and Grantâs method on three 50 m line transects. We compare data collected using these methods from a grassland (smooth brome dominated) and a shrubland (greasewood dominated) alliance. Results of this study indicate that data collected using Grantâs method is more variable than line-point intercept data in shrub systems. Bare ground was detected less using Grantâs than line-point intercept; however, Grantâs detected more herbaceous and invasive species overall than did the line-point intercept method. A complementary use of the methods for long-term monitoring is recommended that accommodates tradeoffs between incorporating detail versus efficiency of data collection efforts
The Structure and X-ray Recombination Emission of a Centrally Illuminated Accretion Disk Atmosphere and Corona
We model an accretion disk atmosphere and corona photoionized by a central
X-ray continuum source. We calculate the opacity and radiation transfer for an
array of disk radii, to obtain the two-dimensional structure of the disk and
its X-ray recombination emission. The atmospheric structure is insensitive to
the viscosity alpha. We find a feedback mechanism between the disk structure
and the central illumination, which expands the disk and increases the solid
angle subtended by the atmosphere. We model the disk of a neutron star X-ray
binary. We map the temperature, density, and ionization structure of the disk,
and we simulate the high resolution spectra observable with the Chandra and
XMM-Newton grating spectrometers. The X-ray emission lines from the disk
atmosphere are detectable, especially for high-inclination binary systems. The
grating observations of two classes of X-ray binaries already reveal important
spectral similarities with our models. The line spectrum is very sensitive to
the structure of each atmospheric layer, and it probes the heating mechanisms
in the disk. The model spectrum is dominated by double-peaked lines of H-like
and He-like ions, plus weak Fe L. Species with a broad range of ionization
levels coexist at each radius: from Fe XXVI in the hot corona, to C VI at the
base of the atmosphere. The choice of stable solutions affects the spectrum,
since a thermal instability is present in the regime where the X-ray
recombination emission is most intense.Comment: 32 pages, incl. 26 figures, accepted for publication in Ap
Partially Absorbed Comptonization Spectrum from the Nearly Edge-on Source X 1822-371
We report the results of a spectral analysis over the range 0.1-200 keV
performed on the dipping source X 1822-371 observed by BeppoSAX. We find the
best fit to the continuum using a partially covered Comptonization model, due
to scattering off soft seed photons by electrons at a temperature of ~4.8 keV,
without the presence of any soft blackbody emission. The equivalent hydrogen
column obtained for the absorbed component is ~4.5 10^{22} cm^{-2}, an order of
magnitude larger than the Galactic absorption for this source, and the covering
fraction is ~71%. Because the inclination angle of X 1822-371 to the line of
sight is ~85^\circ, this model gives a reasonable scenario for the source: the
Comptonized spectrum could come from an extended accretion disk corona (ADC),
probably the only region that can be directly observed due to the high
inclination. The excess of matter producing the partial covering could be close
to the equatorial plane of the system, above the outer disk, occulting the
emission from the inner disk and the inner part of the ADC. An iron emission
line is also present at ~6.5 keV with an equivalent width of ~150 eV. We argue
that this strong iron line cannot be explained as reflection of the Comptonized
spectrum by the accretion disk. It is probably produced in the ADC. An emission
line at ~1.9 keV (with an equivalent width of ~54 eV) and an absorption edge at
\~8.7 keV (with an optical depth of ~0.1) are also required to fit this
spectrum. These features are probably produced by highly ionized iron (Fe XXIV)
present in the outer part of the ADC, where the plasma density is
\~10^{11}-10^{12} cm^{-3} and ionized plasma is present.Comment: 15 pages, including 3 figures. Accepted by ApJ. Corrected typos and
Figure
RNA Sequencing Reveals Differential Expression of Mitochondrial and Oxidation Reduction Genes in the Long-Lived Naked Mole-Rat When Compared to Mice
The naked mole-rat (Heterocephalus glaber) is a long-lived, cancer resistant rodent and there is a great interest in identifying the adaptations responsible for these and other of its unique traits. We employed RNA sequencing to compare liver gene expression profiles between naked mole-rats and wild-derived mice. Our results indicate that genes associated with oxidoreduction and mitochondria were expressed at higher relative levels in naked mole-rats. The largest effect is nearly 300-fold higher expression of epithelial cell adhesion molecule (Epcam), a tumour-associated protein. Also of interest are the protease inhibitor, alpha2-macroglobulin (A2m), and the mitochondrial complex II subunit Sdhc, both ageing-related genes found strongly over-expressed in the naked mole-rat. These results hint at possible candidates for specifying species differences in ageing and cancer, and in particular suggest complex alterations in mitochondrial and oxidation reduction pathways in the naked mole-rat. Our differential gene expression analysis obviated the need for a reference naked mole-rat genome by employing a combination of Illumina/Solexa and 454 platforms for transcriptome sequencing and assembling transcriptome contigs of the non-sequenced species. Overall, our work provides new research foci and methods for studying the naked mole-rat's fascinating characteristics
Cryogenic 160-GHz MMIC Heterodyne Receiver Module
A cryogenic 160-GHz MMIC heterodyne receiver module has demonstrated a system noise temperature of 100 K or less at 166 GHz. This module builds upon work previously described in Development of a 150-GHz MMIC Module Prototype for Large-Scale CMB Radiation (NPO-47664), NASA Tech Briefs, Vol. 35, No. 8 (August 2011), p. 27. In the original module, the local oscillator signal was saturating the MMIC low-noise amplifiers (LNAs) with power. In order to suppress the local oscillator signal from reaching the MMIC LNAs, the W-band (75 110 GHz) signal had to be filtered out before reaching 140 170 GHz. A bandpass filter was developed to cover 120 170 GHz, using microstrip parallel-coupled lines to achieve the desired filter bandwidth, and ensure that the unwanted W-band local oscillator signal would be sufficiently suppressed. With the new bandpass filter, the entire receiver can work over the 140 180-GHz band, with a minimum system noise temperature of 460 K at 166 GHz. The module was tested cryogenically at 20 K ambient temperature, and it was found that the receiver had a noise temperature of 100 K over an 8-GHz bandwidth. The receiver module now includes a microstrip bandpass filter, which was designed to have a 3-dB bandwidth of approximately 120-170 GHz. The filter was fabricated on a 3-mil-thick alumina substrate. The filter design was based on a W-band filter design made at JPL and used in the QUIET (Q/U Imaging ExperimenT) radiometer modules. The W-band filter was scaled for a new center frequency of 150 GHz, and the microstrip segments were changed accordingly. Also, to decrease the bandwidth of the resulting scaled design, the center gaps between the microstrip lines were increased (by four micrometers in length) compared to the gaps near the edges. The use of the 150-GHz bandpass filter has enabled the receiver module to function well at room temperature. The system noise temperature was measured to be less than 600 K (at room temperature) from 154 to 168 GHz. Additionally, the use of a W-band isolator between the receiver module and the local oscillator source also improved the noise temperature substantially. This may be because the mixer was presented with a better impedance match with the use of the isolator. Cryogenic testing indicates a system noise temperature of 100 K or less at 166 GHz. Prior tests of the MMIC amplifiers alone have resulted in a system noise temperature of 65.70 K in the same frequency range (.160 GHz) when cooled to an ambient temperature of 20 K. While other detector systems may be slightly more sensitive (such as SIS mixers), they require more cooling (to 4 K ambient) and are not as easily scalable to build a large array, due to the need for large magnets and other equipment. When cooled to 20 K, this receiver module achieves approximately 100 K system noise temperature, which is slightly higher than single-amplifier module results obtained at JPL (65.70 K when an amplifier is corrected for back-end noise contributions). If this performance can be realized in practice, and a scalable array can be produced, the impact on cosmic microwave background experiments, astronomical and Earth spectroscopy, interferometry, and radio astronomy in general will be dramatic
The disk-bearing young star IM Lup
Classical T Tauri stars (CTTS) differ in their X-ray signatures from older
pre-main sequence stars, e.g. weak-lined TTS (WTTS). CTTS show a soft excess
and deviations from the low-density coronal limit in the He-like triplets. We
test whether these features correlate with accretion or the presence of a disk
by observing IM Lup, a disk-bearing object apparently in transition between
CTTS and WTTS without obvious accretion. We analyse a Chandra grating spectrum
and additional XMM-Newton data of IM Lup and accompanying optical spectra, some
of them taken simultaneously to the X-ray observations. We fit the X-ray
emission lines and decompose the Ha emission line in different components. In
X-rays IM Lup has a bright and hot active corona, where elements of low
first-ionisation potential are depleted. The He-like Ne IX triplet is in the
low-density state, but due to the small number of counts a high-density
scenario cannot be excluded on the 90% confidence level. In all X-ray
properties IM Lup resembles a main-sequence star, but it is also compatible
with CTTS signatures on the 90% confidence level, thus we cannot decide if the
soft excess and deviations from the low-density coronal limit in the He-like
triplets in CTTS require accretion or only the presence of a disk. IM Lup is
chromospherically active, which explains most of the emission in Ha. Despite
its low equivalent width, the complexity of the Ha line profile is reminiscent
of CTTS. We present an estimate for the mass accretion rate of 10e-11 Msun/yr.Comment: 10 pages, 6 figures, accepted by A&
A systematic analysis of the broad Fe Kalpha line in neutron star LMXBs with XMM-Newton
We analysed the XMM-Newton archival observations of 16 neutron star (NS)
low-mass X-ray binaries (LMXBs) to study the Fe K emission in these objects.
The sample includes all the observations of NS LMXBs performed in EPIC pn
Timing mode with XMM-Newton publicly available until September 30, 2009. We
performed a detailed data analysis considering pile-up and background effects.
The properties of the iron lines differed from previous published analyses due
to either incorrect pile-up corrections or different continuum
parameterization. 80% of the observations for which a spectrum can be extracted
showed significant Fe line emission. We found an average line centroid of 6.67
0.02 keV and a finite width, , of 0.33 0.02 keV. The
equivalent width of the lines varied between 17 and 189 eV, with an average
weighted value of 42 eV. For sources where several observations were
available the Fe line parameters changed between observations whenever the
continuum changed significantly. The line parameters did not show any
correlation with luminosity. Most important, we could fit the Fe line with a
simple Gaussian component for all the sources. The lines did not show the
asymmetric profiles that were interpreted as an indication of relativistic
effects in previous analyses of these LMXBs.Comment: Accepted for publication in A&
Unilateral versus coordinated effects:comparing the impact on consumer welfare of alternative merger outcomes
The nature of tacitly collusive behaviour often makes coordination unstable, and this may result in periods of breakdown, during which consumers benet from reduced prices. This is allowed for by adding demand uncertainty to the Compte et al. (2002) model of tacit collusion amongst asymmetric rms. Breakdowns occur when a rm cannot exclude the possibility of a deviation by a rival. It is then possible that an outcome with collusive behaviour, subject to long/frequent break downs, can improve consumer welfare compared to an alternative with sustained unilateral conduct. This is illustrated by re-examining the Nestle/Perrier merger analyzed by Compte et al., but now also taking into account the potential for welfare losses arising from unilateral behaviour
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