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HST multi-epoch imaging of the PSR B0540-69 system unveils a highly dynamic synchrotron nebula

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 نشر من قبل Andrea De Luca
 تاريخ النشر 2007
  مجال البحث فيزياء
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PSR B0540-69 is the Crab twin in the Large Magellanic Cloud. Age, energetic and overall behaviour of the two pulsars are very similar. The same is true for the general appearance of their pulsar wind nebulae (PWNe). Analysis of Hubble Space Telescope images spanning 10 years unveiled significant variability in the PWN surrounding PSR B0540-69, with a hot spot moving at ~0.04c. Such behaviour, reminiscent of the variability observed in the Crab nebula along the counter-jet direction, may suggest an alternative scenario for the geometry of the system. The same data were used to assess the pulsar proper motion. The null displacement recorded over 10 y allowed us to set a 3sigma upper limit of 290 km/s to the pulsar velocity.



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The study of the younger, and brighter, pulsars is important to understand the optical emission properties of isolated neutron stars. PSRB0540-69, the second brightest (V~22) optical pulsar, is obviously a very interesting target for these investigat ions. The aim of this work is threefold: constraining the pulsar proper motion and its velocity on the plane of the sky through optical astrometry, obtaining a more precise characterisation of the pulsar optical spectral energy distribution (SED) through a consistent set of multi-band, high-resolution, imaging photometry observations, measuring the pulsar optical phase-averaged linear polarisation, for which only a preliminary and uncertain measurement was obtained so far from ground-based observations. We performed high-resolution observations of PSRB0540-69 with the WFPC2 aboard the HST, in both direct imaging and polarimetry modes. From multi-epoch astrometry we set a 3sigma upper limit of 1 mas/yr on the pulsar proper motion, implying a transverse velocity <250 km/s at the 50 kpc LMC distance. Moreover, we determined the pulsar absolute position with an unprecedented accuracy of 70 mas. From multi-band photometry we characterised the pulsar power-law spectrum and we derived the most accurate measurement of the spectral index (0.70+/-0.07) which indicates a spectral turnover between the optical and X-ray bands. Finally, from polarimetry we obtained a new measurement of the pulsar phase-averaged polarisation degree (16+/-4%),consistent with magnetosphere models depending on the actual intrinsic polarisation degree and depolarisation factor, and we found that the polarisation vector (22+/-12deg position angle) is possibly aligned with the semi-major axis of the pulsar-wind nebula and with the apparent proper motion direction of its bright emission knot.
83 - M. Y. Ge , F. J. Lu , L. L. Yan 2019
It is believed that an isolated pulsar loses its rotational energy mainly through a relativistic wind consisting of electrons, positrons and possibly Poynting fluxcite{Pacini1973,Rees1974,Kennel1984}. As it expands, this wind may eventually be termin ated by a shock, where particles can be accelerated to energies of X-ray synchrotron emission, and a pulsar wind nebula (PWN) is usually detectable surrounding a young energetic pulsarcite{Pacini1973,Rees1974,Kennel1984}. However, the nature and/or energetics of these physical processes remain very uncertain, largely because they typically cannot be studied in a time-resolved fashion. Here we show that the X-ray PWN around the young pulsar PSR B0540--69 brightens gradually up to 32$pm8%$ over the mean previous flux, after a sudden spin-down rate ($dot{ u}$) transition (SRT) by $sim36%$ in December 2011, which has very different properties from a traditional pulsar glitchcite{Marshall2015}. No evidence is seen for any change in the pulsed X-ray emission. We conclude that the SRT results from a sudden change in the pulsar magnetosphere that increases the pulsar wind power and hence the PWN X-ray emission. The X-ray light curve of the PWN suggests a mean life time of the particles of $397pm374$,days, corresponding to a magnetic field strength of $0.78_{-0.28}^{+4.50}$,mG in the PWN.
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PSR B0540-69 is a young pulsar in the Large Magellanic Cloud that has similar properties with respect to the Crab Pulsar, and is embedded in a Pulsar Wind Nebula. We have analyzed the complete archival RXTE dataset of observations of this source, tog ether with new Swift-XRT and INTEGRAL-IBIS data. Accurate lightcurves are produced in various energy bands between 2 and 60 keV, showing no significant energy variations of the pulse shape. The spectral analysis shows that the pulsed spectrum is curved, and is best fitted up to 100 keV by a log-parabolic model: this strengthens the similarities with the Crab pulsar, and is discussed in the light of a phenomenologic multicomponent model. The total emission from this source is studied, the relative contributions of the pulsar and the PWN emission are derived, and discussed in the context of other INTEGRAL detected pulsar/PWN systems.
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