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Post Info TOPIC: Geminga & PSR B0656+14


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Geminga Supernova Remnant

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.

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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.

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Title: The Geminga Fraction
Authors: Alice K. Harding, Isabelle A. Grenier, Peter L. Gonthier

Radio-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.

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Astronomers have discovered a new feature in the Geminga pulsar.
The closest pulsars to Earth travels through space at 120 kilometres per second and leaves in its wake a comet-like trail of high-energy electrons.

A team led by Dr. Patrizia Caraveo of the Italian National Institute for Astrophysics (INAF) in Milan discovered this cometary trail with data from NASA's Chandra X-ray Observatory Archive. The discovery follows the team's discovery in 2003 using ESA's XMM-Newton of Geminga's twin X-ray tails stretching for billions of kilometres.
Together, these observations provide unique insight into the contents and density of the interstellar "ocean" Geminga is ploughing through, as well as the physics of Geminga itself. Not only is Geminga close, only about 550 light-years from Earth, it is cutting across our line of sight, offering a spectacular view of a pulsar in motion.

"Geminga is the only isolated pulsar we know of showing both a small comet-like trail and a larger tail structure. This jettison from Geminga's journey through interstellar space provides unprecedented information about the physics of pulsars." - Dr. Andrea De Luca of INAF's Istituto di Astrofisica Spaziale e Fisica Cosmica, lead author on an article about this discovery in Astronomy & Astrophysics.

Expand (324kb, 753 x 738)
Position(2000) RA = 06 33 54.20 Dec = +17 46 12.0
Composite image of Geminga, from Chandra and XMM-Newton.
The nature of this curious stellar object in Gemini was quite unknown for 20 years after its discovery by the SAS-2 satellite. Finally, in March 1991 the ROSAT satellite detected a periodicity of 0.237 s in soft x-ray emission. Thus, it is supposed that Geminga is a sort of neutron star: the decaying core of a behemoth star that went supernova about 100,000 years ago.

A pulsar is a type of rapidly spinning neutron star that emits steady pulses of radiation with each rotation, funnelled along strong magnetic field lines, much like a lighthouse beam sweeping across space. A neutron star is the core remains of an exploded star once at least eight times as massive as the sun.
These dense stars, only about 20 kilometres across, still contain roughly the mass of the sun. Neutron stars contain the densest material known. Like many neutron stars, Geminga got a "kick" from the explosion that created it and has been flying through space like a cannonball ever since.
Geminga's complex phenomenology of tails and a trail must be from high-energy electrons escaping the pulsar magnetosphere following paths clearly driven by the pulsar's motion in the interstellar medium.
Most pulsars emit radio waves. Yet Geminga is "radio quiet" and was discovered 30 years ago as a unique "gamma-ray only" source (only later was Geminga seen in the X-ray and optical light wavebands). Geminga generates gamma rays by accelerating electrons and positrons, a type of antimatter, to high speeds as it spins like a dynamo four times per second.

"Astronomers have known that only a fraction of these accelerated particles produce gamma rays, and they have wondered what happens to the remaining ones. Thanks to the combined capabilities of Chandra and XMM-Newton, we now know that such particles can escape. Once they reach the shock front, created by the supersonic motion of the star, the particles lose their energy radiating X-rays." - Caraveo, co-author on the Astronomy & Astrophysics article.

Meanwhile, an equal number of particles (with a different electric charge) should move in the opposite direction, aiming back at the star. Indeed, when they hit the star's crust they create tiny hotspots, which have been detected through their varying X-ray emission.
The next generation of high-energy gamma-ray instruments - namely, the planned Italian Space Agency's AGILE mission and NASA's GLAST mission - will explore the connection between the X-ray and gamma ray behaviour of pulsars to provide clues to the nature of unknown gamma-ray sources, according to Prof. Giovanni Bignami, a co-author and director of the Centre d'Etude Spatiale des Rayonnements (CESR) in Toulouse, France. Of the 271 higher-energy gamma-ray objects detected by a NASA telescope called EGRET, 170 remained unidentified in other wavebands.

These unidentified objects could be "gamma-ray pulsars" like Geminga, whose optical and X-ray light might be visible only because of its nearness to Earth.
Only about a dozen other radio-quiet isolated neutron stars are known, and Geminga is the only one with tails and trails and copious gamma-ray emission. Bignami named Geminga for "Gemini gamma-ray source" in 1973. In his local Milan dialect, the name is a pun on "ghè minga," which means "it is not there." Indeed, Geminga was unidentified in other wavelengths until 1993, twenty years after its discovery.

The discovery team also includes Drs. Fabio Mattana and Alberto Pellizzoni of the INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica.




Posts: 131433
Geminga & PSR B0656+14

Title: Subaru optical observations of the two middle-aged pulsars PSR B0656+14 and Geminga
Authors: 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: 00909
400` 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+14
Position(2000): RA 6h59m48 s1472 Dec 14°14021: 00160
2000` 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)

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