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تسجيل مستخدم جديدLong-term timing and emission behavior of the young Crab-like pulsar PSR B0540-69
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We present timing solutions and spin properties of the young pulsar PSR B0540-69 from analysis of 15.8 yr of data from the Rossi X-Ray Timing Explorer. We perform a partially phase-coherent timing analysis in order to mitigate the pronounced effects of timing noise in this pulsar. We also perform fully coherent timing over large subsets of the data set in order to arrive at a more precise solution. In addition to the previously reported first glitch undergone by this pulsar, we find a second glitch, which occurred at MJD 52927 $pm$ 4, with fractional changes in spin frequency $Delta u/ u = (1.64 pm 0.05) times 10^{-9}$ and spin-down rate $Deltadot{ u}/dot{ u} = (0.930 pm 0.011) times 10^{-4}$ (taken from our fully coherent analysis). We measure a braking index that is consistent over the entire data span, with a mean value $n = 2.129 pm 0.012$, from our partially coherent timing analysis. We also investigated the emission behavior of this pulsar, and have found no evidence for significant flux changes, flares, burst-type activity, or pulse profile shape variations. While there is strong evidence for the much-touted similarity of PSR B0540-69 to the Crab pulsar, they nevertheless differ in several aspects, including glitch activity, where PSR B0540-69 can be said to resemble certain other very young pulsars. It seems clear that the specific processes governing the formation, evolution, and interiors of this population of recently born neutron stars can vary significantly, as reflected in their observed properties.
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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
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In Dec. 2011 PSR B0540-69 experienced a spin-down rate transition (SRT), after which the spin-down power of the pulsar increased by ~36%. About 1000 days after the SRT, the X-ray luminosity of the associated pulsar wind nebula (PWN) was found to brig
hten by 32+/-8%. After the SRT, the braking index n of PSR B0540-69 changes from n=2.12 to n=0.03 and then keeps this value for about five years before rising to n=0.9 in the following years. We find that most of the current models have difficulties in explaining the measured braking index. One exceptive model of the braking index evolution is the increasing dipole magnetic field of PSR B0540-69. We suggest that the field increase may result from some instabilities within the pulsar core that enhance the poloidal component at the price of toroidal component of the magnetic field. The increasing dipole magnetic field will result in the X-ray brightening of the PWN. We fit the PWN X-ray light curve by two models: one assumes a constant magnetic field within the PWN during the brightening and the other assumes an enhanced magnetic field proportional to the energy density of the PWN. It appears that the two models fit the data well, though the later model seems to fit the data a bit better. This provides marginal observational evidence that magnetic field in the PWN is generated by the termination shock. Future high-quality and high-cadence data are required to draw a solid conclusion.
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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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