71 research outputs found
A significant hardening and rising shape detected in the MeV/GeV nuFnu spectrum from the recently-discovered very-high-energy blazar S4 0954+65 during the bright optical flare in 2015 February
We report on Fermi Large Area Telescope (LAT) and multi-wavelength results on
the recently-discovered very-high-energy (VHE, 100 GeV) blazar S4 0954+65
() during an exceptionally bright optical flare in 2015 February.
During the time period (2015 February, 13/14, or MJD 57067) when the MAGIC
telescope detected VHE -ray emission from the source, the Fermi-LAT
data indicated a significant spectral hardening at GeV energies, with a
power-law photon index of ---compared with the 3FGL value
(averaged over four years of observation) of . In contrast,
Swift/XRT data showed a softening of the X-ray spectrum, with a photon index of
(compared with averaged during the flare from
MJD 57066 to 57077), possibly indicating a modest contribution of synchrotron
photons by the highest-energy electrons superposed on the inverse Compton
component. Fitting of the quasi-simultaneous ( day) broadband spectrum with
a one-zone synchrotron plus inverse-Compton model revealed that GeV/TeV
emission could be produced by inverse-Compton scattering of external photons
from the dust torus. We emphasize that a flaring blazar showing high flux of
photons cm s ( 100 MeV) and a
hard spectral index of detected by Fermi-LAT on daily
time scales is a promising target for TeV follow-up by ground-based Cherenkov
telescopes to discover high-redshift blazars, investigate their temporal
variability and spectral features in the VHE band, and also constrain the
intensity of the extragalactic background light.Comment: 15 pages, 3 figures, 2 tables. Accepted by PAS
Multiband optical variability of the blazar OJ 287 during its outbursts in 2015 -- 2016
We present recent optical photometric observations of the blazar OJ 287 taken
during September 2015 -- May 2016. Our intense observations of the blazar
started in November 2015 and continued until May 2016 and included detection of
the large optical outburst in December 2016 that was predicted using the binary
black hole model for OJ 287. For our observing campaign, we used a total of 9
ground based optical telescopes of which one is in Japan, one is in India,
three are in Bulgaria, one is in Serbia, one is in Georgia, and two are in the
USA. These observations were carried out in 102 nights with a total of ~ 1000
image frames in BVRI bands, though the majority were in the R band. We detected
a second comparably strong flare in March 2016. In addition, we investigated
multi-band flux variations, colour variations, and spectral changes in the
blazar on diverse timescales as they are useful in understanding the emission
mechanisms. We briefly discuss the possible physical mechanisms most likely
responsible for the observed flux, colour and spectral variability.Comment: 11 pages, 6 figures, 4 tables; Accepted for publication in MNRA
Star Formation in the Central 400 pc of the Milky Way: Evidence for a Population of Massive YSOs
The central kpc of the Milky Way might be expected to differ significantly
from the rest of the Galaxy with regard to gas dynamics and the formation of
YSOs. We probe this possibility with mid-infrared observations obtained with
IRAC and MIPS on Spitzer and with MSX. We use color-color diagrams and SED fits
to explore the nature of YSO candidates (including objects with 4.5 micron
excesses possibly due to molecular emission). There is an asymmetry in the
distribution of the candidate YSOs, which tend to be found at negative Galactic
longitudes; this behavior contrasts with that of the molecular gas,
approximately 2/3 of which is at positive longitudes. The small scale height of
these objects suggests that they are within the Galactic center region and are
dynamically young. They lie between two layers of infrared dark clouds and may
have originated from these clouds. We identify new sites for this recent star
formation. The methanol masers appear to be associated with young, embedded
YSOs characterized by 4.5 micron excesses. We use the SEDs of these sources to
estimate their physical characteristics. Within the central 400x50 pc
(|l|<1.3\degr and |b|<10') the star formation rate based on the identification
of Stage I evolutionary phase of YSO candidates is about 0.14 solar mass/yr. We
suggest that a recent burst of star formation took place within the last 10^5
years. This suggestion is also consistent with estimates of star formation
rates within the last ~10^7 years showing a peak around 10^5 years ago. Lastly,
we find that the Schmidt-Kennicutt Law applies well in the central 400 pc of
the Galaxy. This implies that star formation does not appear to be dramatically
affected by the extreme physical conditions in the Galactic center region.Comment: 96 pages, ten tables, 35 figures, ApJ (in press), replaced by a
revised versio
X-ray emission from the giant molecular clouds in the Galactic Center region and the discovery of new X-ray sources
We report the results of X-ray (2-10 keV) observations of the giant molecular
clouds SgrB, SgrC and SgrD in the Galactic Center region, together with the
discovery of the point-like source SAXJ1748.2-2808. The data have been obtained
with the MECS instrument on the BeppoSAX satellite. The core of SgrB2 has an
X-ray luminosity of 6x10^34 erg/s and its spectrum is characterized by a strong
Fe emission line at 6.5 keV with an equivalent width of 2 keV. Faint diffuse
X-ray emission is detected from SgrC and from the SNR G1.05-0.15 (SgrD). A new,
unresolved source with a strong Fe line has been discovered in the SgrD region.
This source, SAXJ1748.2-2808, is probably associated with a SiO and OH maser
source at the Galactic Center distance. If so, its luminosity is 10^34 erg/s.
We propose that the X-ray emission from SAX J1748.2-2808 is produced either by
protostars or by a giant molecular cloud core. Emission from sources similar to
SAX J1748.2-2808 could have an impact on the expected contribution on the
observed Fe line emission from the Galactic ridge.Comment: 12 pages, 8 figures, accepted for publication in Astronomy &
Astrophysics Main Journa
Characterization of the nonlinear behavior of a F-16 aircraft using discrete-time Volterra series
The linearity assumption in the structural dynamics analysis is a severe practical limitation. Further, in the investigation of mechanisms presented in fighter aircrafts, as for instance aeroelastic nonlinearity, friction or gaps in wing-load-payload mounting interfaces, is mandatory to use a nonlinear analysis technique. Among different approaches that can be used to this matter, the Volterra theory is an interesting strategy, since it is a generalization of the linear convolution. It represents the response of a nonlinear system as a sum of linear and nonlinear components. Thus, this paper aims to use the discrete-time version of Volterra series expanded with Kautz filters to characterize the nonlinear dynamics of a F-16 aircraft. To illustrate the approach, it is identified and characterized a non-parametric model using the data obtained during a ground vibration test performed in a F-16 wing-to-payload mounting interfaces. Several amplitude inputs applied in two shakers are used to show softening nonlinearities presented in the acceleration data. The results obtained in the analysis have shown the capability of the Volterra series to give some insight about the nonlinear dynamics of the F-16 mounting interfaces. The biggest advantage of this approach is to separate the linear and nonlinear contributions through the multiple convolutions through the Volterra kernels
Seismic Turbidites in Lake Biwa, Japan
[15th International Sedimentological Congress] 12-17 April, 1998: Alicante, Spain
Turbiditic trench deposits at the South-Chilean active margin: A PleistoceneHolocene record of climate and tectonics
The active plate margin of South America is characterized by a frequent occurrence of large and devastating subduction earthquakes. Here we focus on marine sedimentary records off Southern Chile that are archiving the regional paleoseismic history over the Holocene and Late Pleistocene. The investigated records - Ocean Drilling Program (ODP) Site 1232 and SONNE core 50SL - are located at similar to 40 degrees S and similar to 38 degrees S, within the Peru-Chilc trench, and are characterized by frequent interbedded strata of turbiditic and hemipelagic origin. On the basis of the sedimentological characteristics and the association with the active margin of Southern Chile, we assume that the turbidites are mainly scismically triggered, and may be considered as paleo-megaearthquake indicators. However, the long-term changes in turbidite recurrence times appear to be strongly influenced by climate and sea level changes as well. During sea level highstands in the Holocene and Marine Isotope Stage (MIS) 5, recurrence times of turbiditic layers are substantially higher, primarily reflecting a climate-induced reduction of sediment availability and enhanced slope stability. In addition, segmented tectonic uplift changes and related drainage inversions likely influenced the postglacial decrease in turbidite frequencies. Glacial turbidite recurrence times (including MIS 2, MIS 3, cold substages of MIS 5, and MIS 6), on the other hand, are within the same order of magnitude as earthquake recurrence times derived from the historical record and other terrestrial paleoseismic archives of the region. Only during these cold stages sediment availability and slope instability were high enough to enable recording of the complete sequence of large earthquakes in Southern Chile. Our data thus suggest that earthquake recurrence times on the order of 100 to 200 years are a persistent feature at least during the last glacial period
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