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Post Info TOPIC: Geminga & PSR B0656+14

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Posts: 131433
Date:
 Geminga Supernova Remnant Permalink Title: VERITAS Observations of the Geminga Supernova Remnant Author: Andy Flinders for the VERITAS collaboration Geminga was first detected as a gamma-ray point source by the SAS-2 gamma-ray satellite observatory and the COS-B X-ray satellite observatory. Subsequent observations have identified Geminga as a heavily obscured radio-quiet pulsar associated with a nearby (250 pc) late Sedov phase (300,000 year) supernova remnant. The Geminga pulsar is the second brightest source detected by the Large Area Telescope aboard the Fermi gamma-ray satellite (Fermi-LAT) and has been frequently advanced as a source of the anomalous excess of cosmic ray positrons reported by PAMELA, Fermi-LAT, and AMS-2. It is surrounded by a compact X-ray pulsar wind nebula. Observations above 10 TeV by the water Cherenkov observatory Milagro have also revealed a diffuse gamma-ray halo around Geminga extending over several square degrees. Since 2007 the VERITAS IACT observatory has performed observations of Geminga and the surrounding halo region. However, the standard methods of source detection in VERITAS data have insufficient sensitivity to angularly extended sources (>0.5 degrees) to reveal a source on the scale of the Milagro detection. In this talk, we describe two approaches being developed to search for angularly extended very high energy gamma-ray emission surrounding the Geminga pulsar. Read more (368kb, PDF) __________________

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Posts: 131433
Date:
 Geminga Permalink Title: The gamma-ray spectrum of Geminga and the inverse Compton model of pulsar high energy emission Authors: Maxim Lyutikov (Purdue University) We reanalyze the Fermi spectra of the Geminga and Vela pulsars. We find that the spectrum of Geminga above the break is exceptionally well approximated by a simple power law without the exponential cut-off, making Geminga's spectrum similar to that of Crab. Vela's broadband gamma-ray spectrum is equally well fit with both the exponential cut-off and the double power law shapes. In the broadband double power-law fits, for a typical Fermi spectrum of a bright \gamma-ray pulsar, most of the errors accumulate due to the arbitrary parametrization of the spectral roll-off. In addition, a power law with an exponential cut-off gives an acceptable fit for the underlying double power-law spectrum for a very broad range of parameters, making such fitting procedures insensitive to the underlying Fermi photon spectrum. Our results have important implications for the mechanism of pulsar high energy emission. A number of observed properties of \gamma-ray pulsars, i.e., the broken power law spectra without exponential cut-offs and stretching in case of Crab beyond the maximal curvature limit, spectral breaks close to or exceeding the maximal breaks due to curvature emission, a Crab patterns of relative intensities of the leading and trailing pulses repeated in the X-ray and \gamma-ray regions, all point to the inverse Compton origin of the high energy emission from majority of pulsars. Read more (780kb, PDF) __________________

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Posts: 131433
Date:
 Permalink Title: The Geminga FractionAuthors: Alice K. Harding, Isabelle A. Grenier, Peter L. GonthierRadio-quiet gamma-ray pulsars like Geminga may account for a number of the unidentified EGRET sources in the Galaxy. The number of Geminga-like pulsars is very sensitive to the geometry of both the gamma-ray and radio beams. Recent studies of the shape and polarization of pulse profiles of young radio pulsars have provided evidence that their radio emission originates in wide cone beams at altitudes that are a significant fraction (1 -10%) of their light cylinder radius. Such wide radio emission beams will be visible at a much larger range of observer angles than the narrow core components thought to originate at lower altitude. Using 3D geometrical modelling that includes relativistic effects from pulsar rotation, we study the visibility of such radio cone beams as well as that of the gamma-ray beams predicted by slot gap and outer gap models. From the results of this study one can obtain revised predictions for the fraction of Geminga-like, radio quiet pulsars present in the gamma-ray pulsar population. Read more (204kb, PDF) __________________

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Posts: 131433
Date:
 Geminga & PSR B0656+14 Permalink Title: Subaru optical observations of the two middle-aged pulsars PSR B0656+14 and GemingaAuthors: Yuri A. Shibanov (1), Sergei V. Zharikov (2), Viktoria N. Komarova (3), Nobuyuki Kawai (4), Yuji Urata (5), Alexey B. Koptsevich (1), Vladimir V. Sokolov (3), Shinpei Shibata (6), Noriaki Shibazaki (7) ((1) Ioffe Inst., Russia, (2) OAN SMP UNAM, Mexico, (3) SAO RAS, Russia, (4) Tokyo Inst. of Techn., Japan, (5) RIKEN, Japan, (6) Yamagata Univ., Japan, (7) Rikkyo Univ., Japan)Researchers have carried out deep subarcsecond BRI imaging of two middle-aged pulsars to establish their properties in the optical range. Both pulsars are detected at >10sigma level in all bands. Geminga is for the first time reliably detected in the I band with a magnitude of 25.10±0.14. Geminga (SN 437) (14'1 x 14'1)PSR B0656+14 (14'1 x 14'1)They also reanalyse archival ESO/NTT and Hubble Space Telescope broadband data and found that some published fluxes for Geminga were estimated inaccurately. The resulting de-reddened broadband spectra in the near-IR-UV range are analysed and compared with available data from the radio through gamma-rays. Geminga (SN 437)Position(2000): RA 6h33m54 s1530 Dec 17°46012: 00909400 x 400 images of Geminga in the F110W ( left) and F160W ( right) bands. The circle annulus in the left panel mark the expected Geminga positions at the epochs of I-band observations with the NTT (1) and Subaru (2), respectively, while the long arrow shows the pulsar path calculated based on the proper motion measurements of Caraveo et al. (1996). Contours of the Subaru image in I band are overlaid in the right panel where the arrow shows the shift of Geminga due to its proper motion over 3.2 yr between the HST and Subaru observations; "+" and 1o- ellipse mark the expected positions of the pulsar at the epochs of the HST and Subaru observations, respectively.PSR B0656+14Position(2000): RA 6h59m48 s1472 Dec 14°14021: 001602000 x 2000 fragments of the Subaru images of the PSR B0656+14 field in the B, R, and I bands. Units of RA (horizontal axis) are hours, minutes and seconds, and units of Dec (vertical one) are degrees, arcminutes and arcseconds. A box in this image marks the region which is magnified below.The de-reddened spectra of both pulsars are remarkably similar to each other and show significant flux increases towards the far-UV and near-IR, and a wide flux excess in V-I bands. This suggested a multicomponent structure of the optical emission. The nonthermal power law component of the pulsar magnetospheric origin dominates in the most part of the optical range. For PSR B0656+14 it is compatible with a low energy extension of the power law tail seen in hard X-rays. For Geminga the respective extension overshoots by a factor of 100 the nonthermal optical flux, which has a less steep spectral slope than in X-rays. This implies a spectral break at a photon energy of about 1 keV. The flux increases towards the far-UV are compatible with contributions of the Rayleigh-Jeans parts of the blackbody components from whole surfaces of the neutron stars dominating in soft X-rays. The V-I excess, which is most significant for PSR B0656+14, suggests a third spectral component of still unidentified origin. Faint, a few arcseconds in size nebulae extended perpendicular to the proper motion directions of the pulsars, are seen around both objects in our deepest I band images. They can be optical counterparts of the bow-shock head of Geminga and of the tentative pulsar wind nebula of PSR B0656+14 observed in X-rays. The paper with high resolution images and figures can be obtained HERE: map.pdf (1.04 Mb) __________________