611 research outputs found
Empirical multi-wavelength prediction method for very high energy gamma-ray emitting BL Lacs
Aim: We have collected the most complete multi-wavelength () dataset of very high energy (VHE) -ray emitting (TeV) BL
Lacs, which are the most numerous extragalactic VHE sources. Using significant
correlations between different bands, we aim to identify the best TeV BL Lac
candidates that can be discovered by the current and next generation of imaging
air Cherenkov telescopes.
Methods: We formed five datasets from lower energy data, i.e. radio,
mid-infrared, optical, X-rays, and GeV -ray, and five VHE -ray
datasets to perform a correlation study between different bands and to
construct the prediction method. The low energy datasets were averaged for
individual sources, while the VHE -ray data were divided into subsets
according to the flux state of the source. We then looked for significant
correlations and determined their best-fit parameters. Using the best-fit
parameters we predicted the level of VHE -ray flux for a sample of 182
BL Lacs, which have not been detected at TeV energies. We identified the most
promising TeV BL Lac candidates based on the predicted VHE -ray flux
for each source.
Results: We found 14 significant correlations between radio, mid-infrared,
optical, -ray, and VHE -ray bands. The correlation between
optical and VHE -ray luminosity is established for the first time. We
attribute this to the more complete sample and more accurate handling of host
galaxy flux in our work. We found nine BL Lac candidates whose predicted VHE
-ray flux is high enough for detection in less than 25 hours with
current imaging air Cherenkov telescopes.Comment: 10 pages, 2 figures, 4 Table
Quasi-periodicities of BL Lac Objects
We review the reports of possible year-long quasi-periodicities of BL Lac
objects in the -ray and optical bands, and present a homogeneous time
analysis of the light curves of PKS2155304, PG1553+113, and BL Lac. Based on
results from a survey covering the entire Fermi -ray sky we have
estimated the fraction of possible quasi-periodic BL Lac objects. We compared
the cyclical behaviour in BL Lac objects with that derived from the search of
possible optical periodicities in quasars, and find that at z1 the
cosmic density of quasi-periodic BL Lac objects is larger than that of
quasi-periodic quasars. If the BL Lac quasi-periodicities were due to a
supermassive binary black hole (SBBH) scenario, there could be a tension with
the upper limits on the gravitational wave background measured by the pulsar
timing array. The argument clearly indicates the difficulties of generally
associating quasi-periodicities of BL Lac objects with SBBHs.Comment: In publication on A&A, 6 pages, 4 figure (11 plots). Minor
corrections adde
Phase field simulation of dendritic microstructure in additively manufactured titanium alloy
Additive manufacturing (AM) processes for metals, such as selective laser sintering and electron beam melting, involve rapid solidification process. The microstructure of the fabricated material and its properties strongly depend on the solidification. Therefore, in order to control and optimize the AM process, it is important to understand the microstructure evolution. In this work, using Ti-6Al-4V as a model system, the phase field method is applied to simulate the microstructure evolution in additively manufactured metals. First, the fundamental governing equations are presented. Then the effects of various processing related parameters, including local temperature gradient, scan speed and cooling rate, on dendrites’ morphology and growth velocity are studied. The simulated results show that the dendritic arms grow along the direction of the heat flow. Higher temperature gradient, scan speed and cooling rate will result in small dendritic arm spacing and higher growth velocity. The simulated dendritic morphology and arm spacings are in good agreement with experimental data and theoretical predictions
Measurement of the EBL through a combined likelihood analysis of gamma-ray observations of blazars with the MAGIC telescopes
The extragalactic background light (EBL) is the radiation accumulated through
the history of the Universe in the wavelength range from the ultraviolet to the
far infrared. Local foregrounds make the direct measurement of the diffuse EBL
notoriously difficult, while robust lower limits have been obtained by adding
up the contributions of all the discrete sources resolved in deep infrared and
optical galaxy observations. Gamma-ray astronomy has emerged in the past few
years as a powerful tool for the study of the EBL: very-high-energy (VHE)
photons traversing cosmological distances can interact with EBL photons to
produce ee pairs, resulting in an energy-dependent depletion of the
gamma-ray flux of distant sources that can be used to set constraints on the
EBL density. The study of the EBL is one of the key scientific programs
currently carried out by the MAGIC collaboration. We present here the results
of the analysis of 32 VHE spectra of 12 blazars in the redshift range 0.03 -
0.94, obtained with over 300 hours of observations with the MAGIC telescopes
between 2010 and 2016. A combined likelihood maximization approach is used to
evaluate the density and spectrum of the EBL most consistent with the MAGIC
observations. The results are compatible with state-of-the-art EBL models, and
constrain the EBL density to be roughly within of the nominal
value in such models. The study reveals no anomalies in gamma-ray propagation
in the large optical depth regime - contrary to some claims based on
meta-analyses of published VHE spectra.Comment: Proceedings of the 35th International Cosmic Ray Conference (ICRC
2017), Bexco, Busan, Korea (arXiv:1708.05153
- …