* Astronomy

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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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: 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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'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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