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Register a new userAGILE and Fermi Observations of the Soft Gamma-Ray Pulsar B1509-58
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We present the results of 2.5 years of AGILE observations of PSR B1509-58 and of the same interval of Fermi observations. The modulation significance of AGILE light curve above 30 MeV is at a 5 sigma confidence level and the light curve shows a broad asymmetric first peak reaching its maximum $0.39 pm 0.02$ cycles after the radio peak plus a second peak at$0.94 pm 0.03$. The gamma-ray spectral energy distribution of pulsed flux is well described by a power-law (photon index $alpha=1.87pm0.09$) with a remarkable cutoff at below 100 MeV, representing the softest spectrum observed among gamma-ray pulsars so far. The unusual soft break in the spectrum of PSR B1509-58 has been interpreted in the framework of polar cap models as a signature of the exotic photon splitting process in the strong magnetic field of this pulsar. In the case of an outer-gap scenario, or the two pole caustic model, better constraints on the geometry of the emission would be needed from the radio band in order to establish whether the conditions required by the models to reproduce AGILE light curves and spectra match the polarization measurements.
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We present the results of new Agile observations of PSR B1509-58 performed over a period of 2.5 years following the detection obtained with a subset of the present data. The modulation significance of the lightcurve above 30 MeV is at a 5$sigma$ confidence level and the lightcurve is similar to those found earlier by Comptel up to 30 MeV: a broad asymmetric first peak reaching its maximum 0.39 +/- 0.02 cycles after the radio peak plus a second peak at 0.94 +/- 0.03. The gamma-ray spectral energy distribution of the pulsed flux detected by Comptel and Agile is well described by a power-law (photon index alpha=1.87+/-0.09) with a remarkable cutoff at E_c=81 +/- 20 MeV, representing the softest spectrum observed among gamma-ray pulsars so far. The pulsar luminosity at E > 1 MeV is $L_{gamma}=4.2^{+0.5}_{-0.2} times10^{35}$ erg/s, assuming a distance of 5.2 kpc, which implies a spin-down conversion efficiency to gamma-rays of $sim 0.03$. The unusual soft break in the spectrum of PSR B1509-58 has been interpreted in the framework of polar cap models as a signature of the exotic photon splitting process in the strong magnetic field of this pulsar. In this interpretation our spectrum constrains the magnetic altitude of the emission point(s) at 3 km above the neutron star surface, implying that the attenuation may not be as strong as formerly suggested because pair production can substitute photon splitting in regions of the magnetosphere where the magnetic field becomes too low to sustain photon splitting. In the case of an outer-gap scenario, or the two pole caustic model, better constraints on the geometry of the emission would be needed from the radio band in order to establish whether the conditions required by the models to reproduce Agile lightcurves and spectra match the polarization measurements.
The gamma-ray pulsar PSR B1509-58 and its surrounding nebulae have been observed with the CANGAROO 3.8m imaging atmospheric Cherenkov telescope. The observations were performed from 1996 to 1998 in Woomera, South Australia, under different instrumental conditions with estimated threshold energies of 4.5 TeV (1996), 1.9 TeV (1997) and 2.5 TeV (1998) at zenith angles of ~30 deg. Although no strong evidence of the gamma-ray emission was found, the lowest energy threshold data of 1997 showed a marginal excess of gamma-ray--like events at the 4.1 sigma significance level. The corresponding gamma-ray flux is calculated to be (2.9 +/- 0.7) * 10^{-12}cm^{-2}s^{-1} above 1.9 TeV. The observations of 1996 and 1998 yielded only upper limits (99.5% confidence level) of 1.9 * 10^{-12}cm^{-2}s^{-1} above 4.5 TeV and 2.0 * 10^{-12}cm^{-2}s^{-1} above 2.5 TeV, respectively. Assuming that the 1997 excess is due to Very High-Energy (VHE) gamma-ray emission from the pulsar nebula, our result, when combined with the X-ray observations, leads to a value of the magnetic field strength ~5 micro G. This is consistent with the equipartition value previously estimated in the X-ray nebula surrounding the pulsar. No significant periodicity at the 150ms pulsar period has been found in any of the three years data. The flux upper limits set from our observations are one order of magnitude below previously reported detections of pulsed TeV emission.
Pulsars are known to power winds of relativistic particles that can produce bright nebulae by interacting with the surrounding medium. These pulsar wind nebulae (PWNe) are observed in the radio, optical, x-rays and, in some cases, also at TeV energies, but the lack of information in the gamma-ray band prevents from drawing a comprehensive multiwavelength picture of their phenomenology and emission mechanisms. Using data from the AGILE satellite, we detected the Vela pulsar wind nebula in the energy range from 100 MeV to 3 GeV. This result constrains the particle population responsible for the GeV emission, probing multivavelength PWN models, and establishes a class of gamma-ray emitters that could account for a fraction of the unidentified Galactic gamma-ray sources.
The pulsar wind nebula (PWN) 3C 58 is one of the historical very-high-energy (VHE; E>100 GeV) gamma-ray source candidates. It is energized by one of the highest spin-down power pulsars known (5% of Crab pulsar) and it has been compared to the Crab Nebula due to their morphological similarities. This object was previously observed by imaging atmospheric Cherenkov telescopes (Whipple, VERITAS and MAGIC), although not detected, with an upper limit of 2.4% Crab Unit (C.U.) at VHE. It was detected by Fermi-LAT with a spectrum extending beyond 100 GeV. We analyzed 81 hours of 3C 58 data taken with the MAGIC telescopes and we detected VHE gamma-ray emission with a significance of 5.7 sigma and an integral flux of 0.65% C.U. above 1 TeV. The differential energy spectrum between 400 GeV and 10 TeV is well described by a power-law function dphi/dE=f_0(E/1TeV)^{-Gamma} with f_0=(2.0pm0.4_{stat}pm0.6_{sys})times10^{-13}cm^{-2}s^{-1}TeV^{-1} and Gamma=2.4pm0.2_{stat}pm0.2_{sys}. The skymap is compatible with an unresolved source. We report the first significant detection of PWN 3C 58 at TeV energies. According to our results 3C 58 is the least luminous VHE gamma-ray PWN ever detected at VHE and the one with the lowest flux at VHE to date. We compare our results with the expectations of time-dependent models in which electrons up-scatter photon fields. The best representation favors a distance to the PWN of 2 kpc and Far Infrared (FIR) comparable to CMB photon fields. If we consider an unexpectedly high FIR density, the data can also be reproduced by models assuming a 3.2 kpc distance. A low magnetic field, far from equipartition, is required to explain the VHE data. Hadronic contribution from the hosting supernova remnant (SNR) requires unrealistic energy budget given the density of the medium, disfavoring cosmic ray acceleration in the SNR as origin of the VHE gamma-ray emission.
The pulsar wind nebula (PWN) 3C 58 has been proposed as a good candidate for detection at VHE (VHE; E>100 GeV) for many years. It is powered by one of the highest spin-down power pulsars known (5% of Crab pulsar) and it has been compared to the Crab Nebula due to its morphology. This object was previously observed by imaging atmospheric Cherenkov telescopes (Whipple, VERITAS and MAGIC), and upper limit of 2.4% Crab Unit (C.U.) at VHE. It was detected by Fermi-LAT with a spectrum extending beyond 100 GeV. We analyzed 81 hours of 3C 58 data taken with the MAGIC telescopes and we detected VHE gamma-ray emission with a significance of 5.7 sigma and an integral flux of 0.65% C.U. above 1 TeV. We report the first significant detection of PWN 3C 58 at TeV energies. According to our results 3C 58 is the least luminous VHE gamma-ray PWN ever detected at VHE and the one with the lowest flux at VHE to date. We compare our results with the expectations of time-dependent models in which electrons up-scatter photon fields. The best representation favors a distance to the PWN of 2 kpc and Far Infrared (FIR) comparable to CMB photon fields. If we consider an unexpectedly high FIR density according to GALPROP, the data can also be reproduced by models assuming a 3.2 kpc distance. A low magnetic field, far from equipartition, is required to explain the VHE data. Hadronic contribution from the hosting supernova remnant (SNR) requires an unrealistic energy budget given the density of the medium, disfavoring cosmic ray acceleration in the SNR as origin of the VHE gamma-ray emission.
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