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Register a new userRapid X-ray variations of the Geminga pulsar wind nebula
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Added by
Chung Yue Hui David
Publication date
2017
fields
Physics
and research's language is
English
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A recent study by Posselt et al. (2017) reported the deepest X-ray investigation of the Geminga pulsar wind nebula (PWN) by using emph{Chandra X-ray Observatory}. In comparison with previous studies of this system, a number of new findings have been reported and we found these suggest the possible variabilities in various components of this PWN. This motivates us to carry out a dedicated search for the morphological and spectral variations of this complex nebula. We have discovered variabilities on timescales from a few days to a few months from different components of the nebula. The fastest change occurred in the circumstellar environment at a rate of 80 per cent of the speed of light. One of the most spectacular results is the wiggling of a half light-year long tail as an extension of the jet, which is significantly bent by the ram pressure. The jet wiggling occurred at a rate of about 20 per cent of the speed of light. This twisted structure can possibly be a result of a propagating torsional Alf`{v}en wave. We have also found evidence of spectral hardening along this tail for a period of about nine months.
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Previous observations of the middle-aged pulsar Geminga with XMM-Newton and Chandra have shown an unusual pulsar wind nebula (PWN), with a 20 long central (axial) tail directed opposite to the pulsars proper motion and two 2 long, bent lateral (outer) tails. Here we report on a deeper (78 ks) Chandra observation and a few additional XMM-Newton observations of the Geminga PWN. The new Chandra observation has shown that the axial tail, which includes up to three brighter blobs, extends at least 50 (i.e., 0.06 d_{250} pc) from the pulsar. It also allowed us to image the patchy outer tails and the emission in the immediate vicinity of the pulsar with high resolution. The PWN luminosity, L_{0.3-8 keV} ~ 3times 10^{29} d_{250}^2 erg/s, is lower than the pulsars magnetospheric luminosity by a factor of 10. The spectra of the PWN elements are rather hard (photon index ~ 1). Comparing the two Chandra images, we found evidence of PWN variability, including possible motion of the blobs along the axial tail. The X-ray PWN is the synchrotron radiation from relativistic particles of the pulsar wind; its morphology is connected with the supersonic motion of Geminga. We speculate that the outer tails are either (1) a sky projection of the limb-brightened boundary of a shell formed in the region of contact discontinuity, where the wind bulk flow is decelerated by shear instability, or (2) polar outflows from the pulsar bent by the ram pressure from the ISM. In the former case, the axial tail may be a jet emanating along the pulsars spin axis, perhaps aligned with the direction of motion. In the latter case, the axial tail may be the shocked pulsar wind collimated by the ram pressure.
The superb spatial resolution of Chandra has allowed us to detect a 20-long tail behind the Geminga pulsar, with a hard spectrum (photon index 1.0+/-0.2) and a luminosity (1.3+/-0.2) 10^{29} ergs/s in the 0.5 - 8 keV band, for an assumed distance of 200 pc. The tail could be either a pulsar jet, confined by a toroidal magnetic field of about 100 microGauss, or it can be associated with the shocked relativistic wind behind the supersonically moving pulsar confined by the ram pressure of the oncoming interstellar medium. We also detected an arc-like structure 5 - 7 ahead of the pulsar, extended perpendicular to the tail, with a factor of 3 lower luminosity. We see a 3-sigma enhancement in the Chandra image apparently connecting the arc with the southern outer tail that has been possibly detected with XMM-Newton. The observed structures imply that the Gemingas pulsar wind is intrinsically anisotropic.
We present results from the Hitomi X-ray observation of a young composite-type supernova remnant (SNR) G21.5$-$0.9, whose emission is dominated by the pulsar wind nebula (PWN) contribution. The X-ray spectra in the 0.8-80 keV range obtained with the Soft X-ray Spectrometer (SXS), Soft X-ray Imager (SXI) and Hard X-ray Imager (HXI) show a significant break in the continuum as previously found with the NuSTAR observation. After taking into account all known emissions from the SNR other than the PWN itself, we find that the Hitomi spectra can be fitted with a broken power law with photon indices of $Gamma_1=1.74pm0.02$ and $Gamma_2=2.14pm0.01$ below and above the break at $7.1pm0.3$ keV, which is significantly lower than the NuSTAR result ($sim9.0$ keV). The spectral break cannot be reproduced by time-dependent particle injection one-zone spectral energy distribution models, which strongly indicates that a more complex emission model is needed, as suggested by recent theoretical models. We also search for narrow emission or absorption lines with the SXS, and perform a timing analysis of PSR J1833$-$1034 with the HXI and SGD. No significant pulsation is found from the pulsar. However, unexpectedly, narrow absorption line features are detected in the SXS data at 4.2345 keV and 9.296 keV with a significance of 3.65 $sigma$. While the origin of these features is not understood, their mere detection opens up a new field of research and was only possible with the high resolution, sensitivity and ability to measure extended sources provided by an X-ray microcalorimeter.
We report on new NuSTAR and archival Chandra observations of the pulsar wind nebula (PWN) 3C 58. Using the X-ray data, we measure energy-dependent morphologies and spatially-resolved spectra of the PWN. We find that the PWN size becomes smaller with increasing energy and that the spectrum is softer in outer regions. In the spatially integrated spectrum of the PWN, we find a hint of a spectral break at $sim$25 keV. We interpret these findings using synchrotron-radiation scenarios. We attribute the size change to the synchrotron burn-off effect. The radial profile of the spectral index has a break at $Rsim80$, implying a maximum electron energy of $sim$200 TeV which is larger than a previous estimate, and the 25-keV spectral break corresponds to a maximum electron energy of $sim$140 TeV for an assumed magnetic field strength of 80 $mu$G. Combining the X-ray data and a previous radio-to-IR SED, we measure a cooling break frequency to be $sim 10^{15}$ Hz, which constrains the magnetic-field strength in 3C 58 to be 30-200$mu$G for an assumed age range of 800-5000 years.
We obtained six observations of PSR J1741-2054 using the $Chandra$ ACIS-S detector totaling $sim$300 ks. By registering this new epoch of observations to an archival observation taken 3.2 years earlier using X-ray point sources in the field of view, we have measured the pulsar proper motion at $mu =109 pm 10 {rm mas yr}^{-1}$ in a direction consistent with the symmetry axis of the observed H$alpha$ nebula. We investigated the inferred past trajectory of the pulsar but find no compelling association with OB associations in which the progenitor may have originated. We confirm previous measurements of the pulsar spectrum as an absorbed power law with photon index $Gamma$=2.68$pm$0.04, plus a blackbody with an emission radius of (4.5$^{+3.2}_{-2.5})d_{0.38}$ km, for a DM-estimated distance of $0.38d_{0.38}$ kpc and a temperature of $61.7pm3.0$ eV. Emission from the compact nebula is well described by an absorbed power law model with a photon index of $Gamma$ = 1.67$pm$0.06, while the diffuse emission seen as a trail extending northeast of the pulsar shows no evidence of synchrotron cooling. We also applied image deconvolution techniques to search for small-scale structures in the immediate vicinity of the pulsar, but found no conclusive evidence for such structures.
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