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Register a new userThe young pulsar PSR B0540-69.3 and its synchrotron nebula in the optical and X-rays
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Added by
Natalia Serafimovich I.
Publication date
2004
fields
Physics
and research's language is
English
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The young PSR B0540-69 (B0540) in the LMC is the only pulsar (except the Crab pulsar) for which a near-UV spectrum has been obtained. However, the absolute flux and spectral index of previously published HST/FOS data are significantly higher than suggested by broadband groundbased UBVRI photometry. Using our ESO/VLT/FORS1 spectral observations and HST/WFPC2 archival images we show that the old HST and new VLT spectral data are >50% contaminated by the Pulsar Wind Nebula (PWN) and that this is the reason for the above mentioned difference. We find that the broadband HST spectrum for the range 3300-8000 A is clearly non- thermal and has a negative spectral index of 1.07(+0.20/-0.19). This is dif- ferent from the almost flat spectrum of the Crab pulsar. The PWN of B0540 shows a clear asymmetry of the surface brightness distribution along the major axis of the PWN torus-like structure with respect to the pulsar position, also seen in Chandra X-ray images. This can be linked to the asymmetry of the surrounding SN ejecta. We find no significant spectral index variation over the PWN. Using HST archival images we estimate the proper motion of B0540 to be 4.9+/-2.3 mas/yr, i.e. a transverse velocity of 1190+/-560 km/s along the southern jet of the PWN. This can make PSR B0540 the third pulsar with a proper motion aligned with the jet axis of its PWN, which poses constraints on pulsar kick models. We discuss the interstellar absorption toward B0540 including the contributions from the Milky Way, LMC and the supernova ejecta, and compare unabsorbed multi- wavelength spectra of B0540 and the Crab pulsar, and their PWNs. Compared with the Crab, B0540 and its PWN show a significant depression in the optical range.
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We present high spatial resolution optical imaging and polarization observations of the PSR B0540-69.3 and its highly dynamical pulsar wind nebula (PWN) performed with HST, and compare them with X-ray data obtained with the Chandra X-ray Observatory. We have studied the bright region southwest of the pulsar where a bright blob is seen in 1999. We show that it may be a result of local energy deposition around 1999, and that the emission from this then faded away. Polarization data from 2007 show that the polarization properties show dramatic spatial variations at the 1999 blob position arguing for a local process. Several other positions along the pulsar-blob orientation show similar changes in polarization, indicating previous recent local energy depositions. In X-rays, the spectrum steepens away from the blob position, faster orthogonal to the pulsar-blob direction than along this axis of orientation. This could indicate that the pulsar-blob orientation is an axis along where energy in the PWN is mainly injected, and that this is then mediated to the filaments in the PWN by shocks. We highlight this by constructing an [S II]-to-[O III]-ratio map. We argue, through modeling, that the high [S II]/[O III] ratio is not due to time-dependent photoionization caused by possible rapid Xray emission variations in the blob region. We have also created a multiwavelength energy spectrum for the blob position showing that one can, to within 2sigma, connect the optical and X-ray emission by a single power law. We obtain best power-law fits for the X-ray spectrum if we include extra oxygen, in addition to the oxygen column density in the interstellar gas of the Large Magellanic Cloud and the Milky Way. This oxygen is most naturally explained by the oxygen-rich ejecta of the supernova remnant. The oxygen needed likely places the progenitor mass in the 20 - 25 Msun range.
We have used the ESO NTT/EMMI and VLT/FORS1 instruments to examine the LMC supernova remnant 0540-69.3 as well as its pulsar (PSR B0540-69) and pulsar-powered nebula in the optical range.Spectroscopic observations of the remnant covering the range of 3600-7350 A centered on the pulsar produced results consistent with those of Kirshner et al. (1989), but also revealed many new emission lines. The most important are [Ne III] 3869, 3967 and Balmer lines of hydrogen. In both the central part of the remnant, as well as in nearby H II regions, the [O III] temperature is higher than about 20 000 K, but lower than previously estimated. For PSR B0540-69, previous optical data are mutually inconsistent: HST/FOS spectra indicate a significantly higher absolute flux and steeper spectral index than suggested by early time-resolved groundbased UBVRI photometry. We show that the HST and VLT spectroscopic data for the pulsar have more then about 50% nebular contamination, and that this is the reason for the previous difference. Using HST/WFPC2 archival images obtained in various bands from the red part of the optical to the NUV range we have performed an accurate photometric study of the pulsar, and find that the spectral energy distribution of the pulsar emission has a negative slope with 1.07 +/- 0.2. This is steeper than derived from previous UBVRI photometry, and also different from the almost flat spectrum of the Crab pulsar. We also estimate that the proper motion of the pulsar is 4.9 +/- 2.3 mas/yr, corresponding to a transverse velocity of 1190 +/- 560 km/s, projected along the southern jet of the pulsar nebula.
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, together 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.
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 terminated 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.
We observed the young pulsar J1357--6429 with the {it Chandra} and {it XMM-Newton} observatories. The pulsar spectrum fits well a combination of absorbed power-law model ($Gamma=1.7pm0.6$) and blackbody model ($kT=140^{+60}_{-40}$ eV, $Rsim2$ km at the distance of 2.5 kpc). Strong pulsations with pulsed fraction of $42%pm5%$, apparently associated with the thermal component, were detected in 0.3--1.1 keV. Surprisingly, pulsed fraction at higher energies, 1.1--10 keV, appears to be smaller, $23%pm4%$. The small emitting area of the thermal component either corresponds to a hotter fraction of the neutron star (NS) surface or indicates inapplicability of the simplistic blackbody description. The X-ray images also reveal a pulsar-wind nebula (PWN) with complex, asymmetric morphology comprised of a brighter, compact PWN surrounded by the fainter, much more extended PWN whose spectral slopes are $Gamma=1.3pm0.3$ and $Gamma=1.7pm0.2$, respectively. The extended PWN with the observed flux of $sim7.5times10^{-13}$ erg s$^{-1}$ cm$^{-2}$ is a factor of 10 more luminous then the compact PWN. The pulsar and its PWN are located close to the center of the extended TeV source HESS J1356--645, which strongly suggests that the VHE emission is powered by electrons injected by the pulsar long ago. The X-ray to TeV flux ratio, $sim0.1$, is similar to those of other relic PWNe. We found no other viable candidates to power the TeV source. A region of diffuse radio emission, offset from the pulsar toward the center of the TeV source, could be synchrotron emission from the same relic PWN rather than from the supernova remnant.
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