1,721,707 research outputs found
Extreme BL Lacs: probes for cosmology and UHECR candidates
Get PDFHigh-energy observations of extreme BL Lac objects, such as 1ES0229+200 or 1ES 0347-121, recently focused interest both for blazar and jet physics and for the implication on the extragalactic background light and intergalactic magnetic field estimate. Moreover, their enigmatic properties have been interpreted in a scenario in which their primary high- energy output is through a beam of high-energy hadrons. However, despite their possible important role in all these topics, the number of these extreme highly peaked BL Lac objects (EHBL) is still rather small. Aiming at increase their number, we selected a group of EHBL candidates considering those undetected (or only barely detected) by the LAT onboard Fermi and characterized by a high X-ray versus radio flux ratio. We assembled the multi-wavelength spectral energy distribution of the resulting 9 sources, using available archival data of Swift, GALEX, and Fermi satellites, confirming their nature. Through a simple one-zone synchrotron self-Compton model we estimate the expected very high energy flux, finding that in the majority of cases it is within the reach of present generation of Cherenkov arrays or of the forthcoming CTA
On the distribution of fluxes of gamma-ray blazars: hints for a stochastic process?
Get PDFWe examine a model for the observed temporal variability of powerful blazars in the γ-ray band in which the dynamics is described in terms of a stochastic differential equation, including the contribution of a deterministic drift and a stochastic term. The form of the equation is motivated by the current astrophysical framework, accepting that jets are powered through the extraction of the rotational energy of the central supermassive black hole mediated by magnetic fields supported by a so-called magnetically arrested accretion disc. We apply the model to the γ-ray light curves of several bright blazars and we infer the parameters suitable to describe them. In particular, we examine the differential distribution of fluxes (dN/dFγ) and we show that the predicted probability density function for the assumed stochastic equation naturally reproduces the observed power-law shape at large fluxes dN/dFγ∝F−αγ with α > 2
Inter Galactic Magnetic field constraints through the gamma ray observations of the Extreme High-frequency-peaked BL Lac candidate HESS 1943+213
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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
The dual nature of blazar fast variability: Space and ground observations of S5 0716+714
No full textFull list of authors: Raiteri, C. M.; Villata, M.; Carosati, D.; Benítez, E.; Kurtanidze, S. O.; Gupta, A. C.; Mirzaqulov, D. O.; D'Ammando, F.; Larionov, V. M.; Pursimo, T.; Acosta-Pulido, J. A.; Baida, G. V.; Balmaverde, B.; Bonnoli, G.; Borman, G. A.; Carnerero, M. I.; Chen, W. -P.; Dhiman, V.; Di Maggio, A.; Ehgamberdiev, S. A.; Hiriart, D.; Kimeridze, G. N.; Kurtanidze, O. M.; Lin, C. S.; Lopez, J. M.; Marchini, A.; Matsumoto, K.; Mujica, R.; Nakamura, M.; Nikiforova, A. A.; Nikolashvili, M. G.; Okhmat, D. N.; Otero-Santos, J.; Rizzi, N.; Sakamoto, T.; Semkov, E.; Sigua, L. A.; Stiaccini, L.; Troitsky, I. S.; Tsai, A. L.; Vasilyev, A. A.; Zhovtan, A. V.Blazar S5 0716+714 is well-known for its short-term variability, down to intraday time-scales. We here present the 2-min cadence optical light curve obtained by the TESS space telescope in 2019 December-2020 January and analyse the object fast variability with unprecedented sampling. Supporting observations by the Whole Earth Blazar Telescope Collaboration in B, V, R, and I bands allow us to investigate the spectral variability during the TESS pointing. The spectral analysis is further extended in frequency to the UV and X-ray bands with data from the Neil Gehrels Swift Observatory. We develop a new method to unveil the shortest optical variability time-scales. This is based on progressive de-trending of the TESS light curve by means of cubic spline interpolations through the binned fluxes, with decreasing time bins. The de-trended light curves are then analysed with classical tools for time-series analysis (periodogram, autocorrelation, and structure functions). The results show that below 3 d there are significant characteristic variability time-scales of about 1.7, 0.5, and 0.2 d. Variability on time-scales d is strongly chromatic and must be ascribed to intrinsic energetic processes involving emitting regions, likely jet substructures, with dimension less than about 10-3 pc. In contrast, flux changes on time-scales d are quasi-achromatic and are probably due to Doppler factor changes of geometric origin. © 2020 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical SocietyThe Astronomical Observatory of the University of Siena thanks the friend amateur astronomers Massimo Conti and Claudio Vallerani for their invaluable and unceasing contribution, essential for the performance of the observatory. This paper includes data collected by the TESS mission. Funding for the TESS mission is provided by the NASA Explorer Program. This research has made use of NASA's Astrophysics Data System and of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Based on observations made with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. Based (partly) on data obtained with the STELLA robotic telescopes in Tenerife, an AIP facility jointly operated by AIP and IAC. This work is partly based upon observations carried out at the Observatorio Astronomico Nacional on the Sierra San Pedro Martir (OAN-SPM), Baja California, Mexico. This research was partially supported by the Bulgarian National Science Fund of the Ministry of Education and Science under grantsDN18-13/2017, KP06-H28/3, and KP-06-PN38/4. KM acknowledges JSPS KAKENHI grant no. JP19K03930. SOK acknowledges financial support by Shota Rustaveli National Science Foundation of Georgia under contract PHDF-18-354. EB acknowledges support from DGAPAPAPIIT GRANT IN113320. GB acknowledges financial support from the State Agency for Research of the Spanish MCIU through the 'Center of Excellence Severo Ochoa' award to the Instituto de Astrofisica de Andalucia (SEV-2017-0709).Peer reviewe
Estimating the redshift of PKS 0447-439 through its GeV-TeV emission
Get PDFContext. Blazars are radio-loud active galactic nuclei (AGN) with a jet pointing at small angles towards the observer. The overall emitted spectrum is typically non-thermal, and in some cases the emission and/or absorption lines are so faint as to prevent the determination of the redshift based on optical spectroscopy methods. PKS 0447-439 is a bright blazar recently detected at very high energy. The redshift of the source is still disputed: a recent spectral analysis reports only a lower limit of z > 1.246, which contradicts the previous measure of z = 0.205 reported in the literature. Aims. We aim to give a redshift estimate of the blazar PKS 0447-439 based on combined GeV (Fermi/LAT) and TeV (HESS) observations. Methods. Taking into account the absorption of TeV photons by the interaction with the extragalactic background light (EBL), we applied the method we developed in a previous work to derive the redshift of PKS 0447-439. Moreover, we compiled the overall spectral energy distribution (SED) using optical-UV, soft X-ray, and γ-ray data, nearly simultaneous to the HESS observations at TeV energies. Finally we modelled the spectral energy distribution (SED) within the framework of a homogeneous, leptonic synchrotron self-Compton (SSC) model. Results. Using the recent TeV spectrum measured by HESS we obtain for PKS 0447-439 a redshift of zrec = 0.20 ± 0.05, which is our estimate on the source distance. This value agrees very well with the value reported in the literature and confirms that our method can be successfully used to constrain blazars distances. Assuming this distance, the SED can be well fitted with the above mentioned model. The physical parameters that we find suggest a strongly matter-dominated jet. Conclusions. Our analysis confirms that the redshift of PKS 0447-439 is likely 0.2, and supports the result present in the literature. © 2012 ESO
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