2,734 research outputs found
Spectral Analysis and the Dynamic Response of Complex Networks
The eigenvalues and eigenvectors of the connectivity matrix of complex
networks contain information about its topology and its collective behavior. In
particular, the spectral density of this matrix reveals
important network characteristics: random networks follow Wigner's semicircular
law whereas scale-free networks exhibit a triangular distribution. In this
paper we show that the spectral density of hierarchical networks follow a very
different pattern, which can be used as a fingerprint of modularity. Of
particular importance is the value , related to the homeostatic
response of the network: it is maximum for random and scale free networks but
very small for hierarchical modular networks. It is also large for an actual
biological protein-protein interaction network, demonstrating that the current
leading model for such networks is not adequate.Comment: 4 pages 14 figure
Variable sequence of events during the past seven terminations in two deep-sea cores from the Southern Ocean
The relationships among internally consistent records of summer sea-surface temperature (SSST), winter sea
ice (WSI), and diatomaceous stable isotopes were studied across seven terminations over the last 660 ka in
sedimentary cores from ODP sites 1093 and 1094. The sequence of events at both sites indicates that SSST
and WSI changes led the carbon and nitrogen isotopic changes in three Terminations (TI, TII and TVI) and followed
them in the other four Terminations (TIII, TIV, TV and TVII). In both TIII and TIV, the leads and lags between
the proxies were related to weak glacial mode, while in TV and TVII they were due to the influence of
the mid-Pleistocene transition. We show that the sequence of events is not unique and does not follow the
same pattern across terminations, implying that the processes that initiated climate change in the Southern
Ocean has varied through time
The luminous late-time emission of the type Ic supernova iPTF15dtg - evidence for powering from a magnetar?
iPTF15dtg is a Type Ic supernova (SN) showing a broad light curve around
maximum light, consistent with massive ejecta if we assume a
radioactive-powering scenario. We study the late-time light curve of iPTF15dtg,
which turned out to be extraordinarily luminous for a stripped-envelope (SE)
SN. We compare the observed light curves to those of other SE SNe and also with
models for the Co decay. We analyze and compare the spectra to nebular
spectra of other SE SNe. We build a bolometric light curve and fit it with
different models, including powering by radioactivity, magnetar powering, as
well as a combination of the two. Between 150 d and 750 d past explosion,
iPTF15dtg's luminosity declined by merely two magnitudes instead of the six
magnitudes expected from Co decay. This is the first
spectroscopically-regular SE SN showing this behavior. The model with both
radioactivity and magnetar powering provides the best fit to the light curve
and appears to be the more realistic powering mechanism. An alternative
mechanism might be CSM interaction. However, the spectra of iPTF15dtg are very
similar to those of other SE SNe, and do not show signs of strong CSM
interaction. iPTF15dtg is the first spectroscopically-regular SE SN whose light
curve displays such clear signs of a magnetar contributing to the powering of
the late time light curve. Given this result, the mass of the ejecta needs to
be revised to a lower value, and therefore the progenitor mass could be
significantly lower than the previously estimated 35 .Comment: 9 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
Validity and practical utility of accelerometry for the measurement of in-hand physical activity in horses
Background:
Accelerometers are valid, practical and reliable tools for the measurement of habitual physical activity (PA). Quantification of PA in horses is desirable for use in research and clinical settings. The objective of this study was to evaluate a triaxial accelerometer for objective measurement of PA in the horse by assessment of their practical utility and validity.
Horses were recruited to establish both the optimal site of accelerometer attachment and questionnaire designed to explore owner acceptance. Validity and cut-off values were obtained by assessing PA at various gaits. Validation study- 20 horses wore the accelerometer while being filmed for 10 min each of rest, walking and trotting and 5 mins of canter work. Practical utility study- five horses wore accelerometers on polls and withers for 18 h; compliance and relative data losses were quantified.
Results:
Accelerometry output differed significantly between the four PA levels (P <0•001) for both wither and poll placement. For withers placement, ROC analyses found optimal sensitivity and specificity at a cut-off of <47 counts per minute (cpm) for rest (sensitivity 99.5 %, specificity 100 %), 967–2424 cpm for trotting (sensitivity 96.7 %, specificity 100 %) and ≥2425 cpm for cantering (sensitivity 96.0 %, specificity 97.0 %). Attachment at the poll resulted in optimal sensitivity and specificity at a cut-off of <707 counts per minute (cpm) for rest (sensitivity 97.5 %, specificity 99.6 %), 1546–2609 cpm for trotting (sensitivity 90.33 %, specificity 79.25 %) and ≥2610 cpm for cantering (sensitivity 100 %, specificity 100 %) In terms of practical utility, accelerometry was well tolerated and owner acceptance high.
Conclusion:
Accelerometry data correlated well with varying levels of in-hand equine activity. The use of accelerometers is a valid method for objective measurement of controlled PA in the horse
The Structure, Kinematics and Physical Properties of the Molecular Gas in the Starburst Nucleus of NGC 253
We present 5.2" x 2.6" resolution interferometry of CO J=1-0 emission from
the starburst galaxy NGC 253. The high spatial resolution of these new data, in
combination with recent high resolution maps of 13CO, HCN and near-infrared
emission, allow us for the first time to link unambiguously the gas properties
in the central starburst of NGC 253 with its bar dynamics. We confirm that the
star formation results from bar-driven gas flows as seen in "twin peaks"
galaxies. Two distinct kinematic features are evident from the CO map and
position-velocity diagram: a group of clouds rotating as a solid body about the
kinematic center of the galaxy, and a more extended gas component associated
with the near-infrared bar. We model the line intensities of CO, HCN and 13CO
to infer the physical conditions of the gas in the nucleus of NGC 253. The
results indicate increased volume densities around the radio nucleus in a
twin-peaks morphology. Compared with the CO kinematics, the gas densities
appear highest near the radius of a likely inner Linblad resonance, and
slightly lead the bar minor axis. This result is similar to observations of the
face-on, twin-peaks galaxy NGC 6951, and is consistent with models of starburst
generation due to gas inflow along a bar.Comment: To appear in the ApJ, 28 pages, 12 figure file
Supernova PTF 09uj: A possible shock breakout from a dense circumstellar wind
Type-IIn supernovae (SNe), which are characterized by strong interaction of
their ejecta with the surrounding circumstellar matter (CSM), provide a unique
opportunity to study the mass-loss history of massive stars shortly before
their explosive death. We present the discovery and follow-up observations of a
Type IIn SN, PTF 09uj, detected by the Palomar Transient Factory (PTF).
Serendipitous observations by GALEX at ultraviolet (UV) wavelengths detected
the rise of the SN light curve prior to the PTF discovery. The UV light curve
of the SN rose fast, with a time scale of a few days, to a UV absolute AB
magnitude of about -19.5. Modeling our observations, we suggest that the fast
rise of the UV light curve is due to the breakout of the SN shock through the
dense CSM (n~10^10 cm^-3). Furthermore, we find that prior to the explosion the
progenitor went through a phase of high mass-loss rate (~0.1 solar mass per
year) that lasted for a few years. The decay rate of this SN was fast relative
to that of other SNe IIn.Comment: Accepted to Apj, 6 pages, 4 figure
The Rising Light Curves of Type Ia Supernovae
We present an analysis of the early, rising light curves of 18 Type Ia
supernovae (SNe Ia) discovered by the Palomar Transient Factory (PTF) and the
La Silla-QUEST variability survey (LSQ). We fit these early data flux using a
simple power-law to determine the time of first
light , and hence the rise-time from first light to
peak luminosity, and the exponent of the power-law rise (). We find a mean
uncorrected rise time of days, with individual SN rise-times
ranging from to days. The exponent n shows significant
departures from the simple 'fireball model' of (or ) usually assumed in the literature. With a mean value of , our data also show significant diversity from event to event. This
deviation has implications for the distribution of 56Ni throughout the SN
ejecta, with a higher index suggesting a lesser degree of 56Ni mixing. The
range of n found also confirms that the 56Ni distribution is not standard
throughout the population of SNe Ia, in agreement with earlier work measuring
such abundances through spectral modelling. We also show that the duration of
the very early light curve, before the luminosity has reached half of its
maximal value, does not correlate with the light curve shape or stretch used to
standardise SNe Ia in cosmological applications. This has implications for the
cosmological fitting of SN Ia light curves.Comment: 19 pages, 19 figures, accepted for publication in MNRA
A flattening in the Optical Light Curve of SN 2002ap
We present the broad band optical photometry of the Type Ic
supernova SN 2002ap obtained during 2002 February 06 -- March 23 in the early
decline phases and also later on 2002 15 August. Combining these data with the
published ones, the general light curve development is studied. The time and
luminosity of the peak brightness and the peak width are estimated. There is a
flattening in the optical light curve about 30 days after the maximum. The
flux decline rates before flattening are 0.1270.005, 0.0820.001,
0.0740.001, 0.0620.001 and 0.0400.001 mag day in ,
, , and passbands respectively, while the corresponding
values after flattening are about 0.02 mag day in all the passbands. The
maximum brightness of SN 2002ap mag, is comparable to that of
the type Ic 1997ef, but fainter than that of the type Ic hypernova SN 1998bw.
The peak luminosity indicates an ejection of 0.06 M
Ni mass.
We also present low-resolution optical spectra obtained during the early
phases.
The SiII absorption minimum indicates that the photospheric velocity
decreased from
21,360 km s to 10,740 km s during a period of 6
days.Comment: 7 pages, 5 figures, Submitted to MNRA
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