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PSR B1259-63/LS 2883
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Title: VHE Emission from PSR B1259-63/LS 2883 around 2010/2011 Periastron Passage observed with H.E.S.S
Authors: I. Sushch, M. de Naurois, U. Schwanke, G. Spengler, P. Bordas (the H.E.S.S. Collaboration)

PSR B1259-63/LS 2883 is a binary system consisting of a 48 ms pulsar orbiting around a Be star with an orbital period of ~3.4 years. The system was detected at very high energies (VHE; E > 100 GeV) by the High Energy Stereoscopic System (H.E.S.S.) during its periastron passages in 2004 and 2007. Here we present new H.E.S.S. observations corresponding to its last periastron passage, which occurred on December 15th 2010. These new observations partially overlap with the beginning of a spectacular gamma-ray flare reported by the Fermi-LAT. The H.E.S.S. observations show both flux and spectral properties similar to those reported in previous periastron passages, without any signature of the emission enhancement seen at GeV energies. A careful statistical study based on the Fermi and H.E.S.S. lightcurves leads to the conclusion that the GeV and TeV emission during the flare have a different physical origin. This conclusion, in turn, allows to use Fermi-LAT measurements of the GeV flux as upper limits for the modelling of the VHE emission.

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Title: H.E.S.S. Observations of the Binary System PSR B1259-63/LS 2883 around the 2010/2011 Periastron Passage
Authors: H.E.S.S. Collaboration: A. Abramowski, F. Acero, F. Aharonian, A. G. Akhperjanian, G. Anton, S. Balenderan, A. Balzer, A. Barnacka, Y. Becherini, J. Becker Tjus, K. Bernlöhr, E. Birsin, J. Biteau, A. Bochow, C. Boisson, J. Bolmont, P. Bordas, J. Brucker, F. Brun, P. Brun, T. Bulik, S. Carrigan, S. Casanova, M. Cerruti, P. M. Chadwick, R. C. G. Chaves, A. Cheesebrough, S. Colafrancesco, G. Cologna, J. Conrad, C. Couturier, M. Dalton, M. K. Daniel, I. D. Davids, B. Degrange, C. Deil, P. deWilt, H. J. Dickinson, A. Djannati-Ataï, W. Domainko, L. O'C. Drury, G. Dubus, K. Dutson, J. Dyks, M. Dyrda, K. Egberts, P. Eger, P. Espigat, L. Fallon, C. Farnier, S. Fegan, F. Feinstein, M. V. Fernandes, D. Fernandez, A. Fiasson, G. Fontaine, A. Förster, M. FüBling, M. Gajdus, et al. (145 additional authors not shown)

Aim. In this paper we present very high energy (VHE; E>100 GeV) data from the \gamma-ray binary system PSR B1259-63/LS 2883 taken around its periastron passage (15th of December 2010) with the High Energy Stereoscopic System (H.E.S.S.) of Cherenkov Telescopes. We aim to search for a possible TeV counterpart of the GeV flare detected by the Fermi LAT. In addition, we aim to study the current periastron passage in the context of previous observations taken at similar orbital phases, testing the repetitive behaviour of the source.
Methods. Observations at VHE were conducted with H.E.S.S. from 9th to 16th of January 2011. The total dataset amounts to around 6 h of observing time.
Results. The source is detected in the 2011 data at a significance level of 11.5\sigma\ revealing an averaged integral flux above 1 TeV of (1.01 ±0.18_{stat} ±0.20_{sys}) x 10^{-12} cm^{-2}s^{-1}. The differential energy spectrum follows a power-law shape with a spectral index \Gamma = 2.92 ±0.30_{stat} ±0.20_{sys} and a flux normalisation at 1 TeV of N_{0} = 1.95 ±0.32_{stat} ±0.39_{sys}) x 10^{-12} TeV^{-1} cm^{-2} s^{-1}. The measured lightcurve does not show any evidence for variability of the source on the daily scale.
Conclusions. The measured integral flux and the spectral shape of the 2011 data are compatible with the results obtained around previous periastron passages. The absence of variability in the H.E.S.S. data indicates that the GeV flare observed by Fermi LAT in the time period covered also by H.E.S.S. observations originates in a different physical scenario than the TeV emission. Additionally, new results compared to those obtained in the observations which were performed in 2004 at a similar orbital phase, further support the hypothesis of the repetitive behaviour of the source.

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RE: PSR B1259-63/SS2883
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Title: Detailed studies of the PSR B1259-63 spectrum evolution and classification of radio pulsar spectra
Authors: M. Dembska, J. Kijak, W. Lewandowski

Kijak et al. (2011a) studied the radio spectrum of PSR B1259-63 in an unique binary with Be star LS 2883 and showed that this pulsar undergoes a spectrum evolution due to orbital motion. They proposed a qualitative model which explains this evolution. They considered two mechanisms that might influence the observed radio emission: free-free absorption and cyclotron resonance. Using the same database we constructed spectra for chosen observing days and obtained different types of spectra. Comparing to current classification of pulsar spectra, there occurs a suggestion that the appearance of various spectra shapes, different from a simple power law which is typical for radio pulsars, is possibly caused by environmental conditions around neutron stars. Therefore, the case of B1259-63 can be treated as a key factor to explain not only the GPS phenomenon observed for the solitary pulsars with interesting environments and also another types of spectra (e.g. with break).

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PSR B1259-63
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Title: Binary Pulsar B1259-63 Spectrum Evolution and Classification of Pulsar Spectra
Authors: M. Dembska, J. Kijak, W. Lewandowski

Recently published results (Kijak et al. 2011a) indicated the evidence for a new aspect in radio pulsars spectra. We studied the radio spectrum of PSR B1259-63 in an unique binary with Be star LS 2883 and showed that this pulsar undergoes a spectrum evolution due to orbital motion. We proposed a qualitative model which explains this evolution. We considered two mechanisms that might influence the observed radio emission: free-free absorption and cyclotron resonance. According to published results (Kijak et al. 2011b), there were found objects with a new type of pulsar radio spectra, called gigahertz-peaked spectra (GPS) pulsars. Most of them were found to exist in very interesting environments. Therefore, it is suggested that the turnover phenomenon is associated with the environment than being related intrinsically to the radio emission mechanism. Having noticed the apparent resemblance between the B1259-63 spectrum and the GPS, we suggested that the same mechanisms should be responsible for both cases. Thus, we believe that this binary system can hold the clue to the understanding of gigahertz-peaked spectra of isolated pulsars. Using the same database we constructed spectra for chosen observing days and obtained different types of spectra. Comparing to current classification of pulsar spectra, there occurs a suggestion that the appearance of various spectra shapes, different from a simple power law which is typical for radio pulsars, is possibly caused by environmental conditions around neutron stars.

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Title: Anisotropic scattering from the circumstellar disc in PSR B1259-63
Authors: B. van Soelen, P. J. Meintjes

The gamma-ray binary system PSR B1259-63 has recently passed through periastron and has been of particular interest as it was observed by Fermi near the December 2010 periastron passage. The system has been detected at very high energies with H.E.S.S. The most probable production mechanism is inverse Compton scattering between target photons from the optical companion and disc, and relativistic electrons in the pulsar wind. We present results of a full anisotropic inverse Compton scattering model of the system, taking into account the IR excess from the extended circumstellar disc around the optical companion.

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RE: PSR B1259-63/SS2883
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Title: GeV Gamma-ray Emission from the Binary PSR B1259-63/SS2883 During the 2010 Periastron Passage
Authors: Masaki Mori, Akiko Kawachi, Shigehiro Nagataki, Jumpei Takata

PSR B1259-63/SS2883 is a binary system which consists of a 48-ms radio pulsar and a massive star in a highly eccentric orbit with a period of about 3.4 years. Non-pulsed and non-thermal emissions from this binary have been reported in the radio, X-ray and TeV gamma-ray energy ranges. Light curves in the radio and X-ray bands showed characteristic double-peaked features which can be attributed to the interactions of the pulsar wind and the Be disk during the crossings by the pulsar. The TeV light curves around periastron differ between 2004 and 2007 observations, and the feature is not conclusive. We report a detection of GeV gamma-ray emission around the periastron passage in December 2010 with Fermi-LAT.

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Binary pulsar B1259-63/LS 2883 Be star
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Title: Spectrum evolution in binary pulsar B1259-63/LS 2883 Be star and gigahertz-peaked spectra
Authors: J. Kijak, M. Dembska, W. Lewandowski, G. Melikidze, M. Sendyk

We study the radio spectrum of PSR B1259-63 orbiting around the Be star LS 2883 and show that the shape of the spectrum depends on the orbital phase. At frequencies below 3 GHz PSR B1259-63 flux densities are lower when measured near the periastron passage than those measured far from periastron. We suggest that an interaction of the radio waves with the Be star environment accounts for this effect. While it is quite natural to explain the pulsar eclipse by the presence of an equatorial disk around LS 2883, this disk alone cannot be responsible for the observed spectral evolution of PSR B1259-63 and we, therefore, propose a qualitative model which explains this evolution. We consider two mechanisms that might influence the observed radio emission: free-free absorption and cyclotron resonance. We believe that this binary system can hold the clue to the understanding of gigahertz-peaked spectra of pulsars.

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PSR B1259-63
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'Odd Couple' Binary Makes Dual Gamma-ray Flares
 
In December 2010, a pair of mismatched stars in the southern constellation Crux whisked past each other at a distance closer than Venus orbits the sun. The system possesses a so-far unique blend of a hot and massive star with a compact fast-spinning pulsar. The pair's closest encounters occur every 3.4 years and each is marked by a sharp increase in gamma rays, the most extreme form of light.
The unique combination of stars, the long wait between close approaches, and periods of intense gamma-ray emission make this system irresistible to astrophysicists. Now, a team using NASA's Fermi Gamma-ray Space Telescope to observe the 2010 encounter reports that the system displayed fascinating and unanticipated activity.

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PSR B1259-63/LS 2883
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Discovery of Structure of Radio Source from a Pulsar Orbiting a Massive Star

In work led by researchers from the University of Barcelona, for the first time the morphology of an extended radio source in a binary system formed of a pulsar and a massive star has been determined. In a few such systems, the strong interactions of the stellar winds produces high-energy gamma radiation, up to 10 million times more energetic than visible light. The results, published in Astrophysical Journal Letters, show for the first time the effect of the winds colliding and support existing theoretical models of radiation emitted by this type of high-energy binary systems, known as gamma-ray binaries.
The research was carried out by Javier Moldón, Marc Ribó and Josep Maria Paredes, of the Department of Astronomy and Meteorology at the University of Barcelona and the UB Institute of Cosmos Sciences, together with Simon Johnston, of the Australia Telescope National Facility (Australia) and Adam Deller, of the National Radio Astronomy Observatory (USA), and in it they studied the only gamma-ray binary that is known to be formed of a pulsar (PSR B1259-63; that is, a neutron star with a radius of some 10 km that is spinning extremely fast) and a massive star (LS 2883), which is 30 times the mass of the Sun.

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RE: PSRB1259-63
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Title: Suzaku Observations of PSR B1259-63: A New Manifestation of Relativistic Pulsar Wind
Authors: Yasunobu Uchiyama (SLAC/KIPAC), Takaaki Tanaka (SLAC/KIPAC), Tadayuki Takahashi (ISAS/JAXA), Koji Mori (Miyazaki U.), Kazuhiro Nakazawa (U. Tokyo)

We observed PSR B1259-63, a young non-accreting pulsar orbiting around a Be star SS 2883, eight times with the Suzaku satellite in 2007, to characterize the X-ray emission arising from the interaction between a pulsar relativistic wind and Be star outflows. The X-ray spectra showed a featureless continuum in 0.6-10 keV, modelled by a power law with a wide range of photon index 1.3-1.8. When combined with the Suzaku PIN detector which allowed spectral analysis in the hard 15-50 keV band, X-ray spectra show a break at 5 keV in a certain epoch. Regarding the system as a compactified pulsar wind nebula, in which e+e- pairs are assumed to be accelerated at the inner shock front of the pulsar wind, we attribute the X-ray spectral break to the low-energy cutoff of the synchrotron radiation associated with the Lorentz factor of the relativistic pulsar wind gamma_1 = 4x10^5. Our result indicates that Comptonization of stellar photons by the unshocked pulsar wind will be accessible (or tightly constrained) by observations with the Fermi Gamma-ray Space Telescope during the next periastron passage. The PSR B1259-63 system allows us to probe the fundamental properties of the pulsar wind by a direct means, being complementary to the study of large-scale pulsar wind nebulae.

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