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[en] Results obtained using a polarimeter-electronographic system to record images of astronomical objects in various orientations of plane polarisation are discussed. By combining the various observations the degree of polarisation and the orientation of the polarisation can be determined. Measurements were made in conditions of clear dark skies at the 1-m telescope at the Wise Observatory, Israel. Polarisation maps obtained by this system, coupled to a computer to handle the vast amount of data, are shown and discussed for the Crab Nebulae, the Galaxy M82, the Orion Nebula and the Sombrero Galaxy M104. (U.K.)
[en] An analysis is made of the available pulse arrival-time data of the Crab pulsar (10.4 yr) and it is found that the large glitch of 1975 February has had quite unusual consequences. Previous reports that it was accompanied by a persistent shift in the time derivative of the rotation frequency are confirmed and the result (2.15 +- 0.16) x 10-4 is obtained. As a consequence of this change, the pulsar now rotates much slower than it would if the glitch had not occurred. This can be explained by a sudden increase in the external braking torque, which would require changes in the structure and/or contents of the corotating magnetosphere. It is found that the second frequency derivative was also changed at the same time. The post-glitch relaxation of the rotation frequency is also studied, and it is concluded that a significant fraction (0.59 <= Q <= 0.77) of the initial frequency-jump decayed with a time constant of about 10 days. (author)
[en] We present a multifrequency radio investigation of the Crab-like pulsar wind nebula (PWN) G54.1+0.3 using the Very Large Array. The high resolution of the observations reveals that G54.1+0.3 has a complex radio structure which includes filamentary and loop-like structures that are magnetized, a diffuse extent similar to the associated diffuse X-ray emission. But the radio and X-ray structures in the central region differ strikingly, indicating that they trace very different forms of particle injection from the pulsar and/or particle acceleration in the nebula. No spectral index gradient is detected in the radio emission across the PWN, whereas the X-ray emission softens outward in the nebula. The extensive radio polarization allows us to image in detail the intrinsic magnetic field, which is well-ordered and reveals that a number of loop-like filaments are strongly magnetized. In addition, we determine that there are both radial and toroidal components to the magnetic field structure of the PWN. Strong mid-infrared (IR) emission detected in Spitzer Space Telescope data is closely correlated with the radio emission arising from the southern edge of G54.1+0.3. In particular, the distributions of radio and X-ray emission compared with the mid-IR emission suggest that the PWN may be interacting with this interstellar cloud. This may be the first PWN where we are directly detecting its interplay with an interstellar cloud that has survived the impact of the supernova explosion associated with the pulsar's progenitor.
[en] An interim report on the linear polarization observation of Crab nebula (Tau A) at the wave-length of 7.2 cm, from 1971 to 1975 except 1972, is presented, when the source is occulted by the southern solar corona in the middle of June every year. Two kinds of the observational procedures for 1971 and 1973, and those for 1974 and 1975 to avoid errors caused by the reception of the solar radiation by the side lobes are described. In 1971 and 1973, the variations of position angle were in an oscillatory way with a typical time scale of 3-5 hours. In 1974, however, the variation was rather systematic and the position angle gradually decreased down to 1300 till June 15, then it suddenly increased up to 1600, and again gradually decreased returning to its intrinsic value, 1420. In 1975, no significant variation exceeding 100 was observed except for a sudden decrease at around 07h UT, June 15. Besides the variations of position angle, other additional data are also presented. (auth.)
[en] We report new observations of the intermediate-frequency peaked BL Lacertae object 3C 66A with the MAGIC telescopes. The data sample we use were taken in 2009 December and 2010 January, and comprises 2.3 hr of good quality data in stereoscopic mode. In this period, we find a significant signal from the direction of the blazar 3C 66A. The new MAGIC stereoscopic system is shown to play an essential role for the separation between 3C 66A and the nearby radio galaxy 3C 66B, which is at a distance of only 6'. The derived integral flux above 100 GeV is 8.3% of the Crab Nebula flux and the energy spectrum is reproduced by a power law of photon index 3.64 ± 0.39stat ± 0.25sys. Within errors, this is compatible with the one derived by VERITAS in 2009. From the spectra corrected for absorption by the extragalactic background light, we only find small differences between the four models that we applied, and constrain the redshift of the blazar to z < 0.68.
[en] Over a past decade very high energy (VHE) gamma ray astronomy has emerged as a major astronomical discipline. In India, we have a long tradition of experiments in this field. Few years ago, multi-institutional Himalayan Gamma Ray Observatory (HiGRO) collaboration was formed to set up VHE gamma rays experiments at Hanle, a high altitude location in Himalayas. HAGAR, the first phase of this collaboration is operational since 2008. HAGAR has successfully detected VHE gamma ray emission from some of the extragalactic objects like Mrk 421, Mrk 501 as well as galactic sources including Crab nebula/pulsar. Details of HAGAR telescope system and results obtained will be discussed. HiGRO is now gearing up for the next phase, i.e. 21 m diameter MACE telescope, which is being installed at Hanle at present. Details of MACE telescope system and future plans will be discussed. (author)